gc university lahore - HEC

PLASMA PROCESSING OF MATERIALS USING PLASMA FOCUS

MUHAMMAD HASSAN SESSION 2002–2006 REG. NO. 03-Ph.D-GCU-Phy-03 DEPARTMENT OF PHYSICS

GC UNIVERSITY LAHORE

Thesis entitled

PLASMA PROCESSING OF MATERIALS USING PLASMA FOCUS Submitted to GC University Lahore in partial fulfillment of the requirements for the award of degree of

DOCTOR OF PHILOSOPHY IN

PHYSICS By MUHAMMAD HASSAN SESSION 2002–2006 REG. NO. 03-Ph.D-GCU-Phy-03 DEPARTMENT OF PHYSICS

GC UNIVERSITY LAHORE ii

DECLARATION I, Mr. Muhammad Hassan, Reg. no. 03-Ph.D-GCU-Phy-03 student of Prof. Dr. G. Murtaza and Dr. Riaz Ahmad in the subject of Physics session 2002 – 2006, hereby declare that the matter printed in the thesis titled “Plasma Processing of Materials using Plasma Focus” is my own work and has not been printed, published and submitted as thesis or in any form in any University, Research Institution etc. in Pakistan or abroad.

Muhammad Hassan

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RESEARCH COMPLETION CERTIFICATE Certified that the research work contained in this thesis titled “Plasma Processing of Materials using Plasma Focus” has been carried out and completed by Mr. Muhammad Hassan, Reg. no. 03 – Ph.D – GCU – Phy – 03 under our supervision.

(Prof. Dr. G. Murtaza) Supervisor Salam Chair in Physics GC University Lahore

(Dr. Riaz Ahmad) Co-supervisor Department of Physics GC University Lahore

Submitted Through:

(Prof. Dr. Hassan. A. Shah) Chairperson Department of Physics GC University Lahore

Controller of Examinations GC University Lahore

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To

My Parents

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ACKNOWLEDGEMENTS All praise to Almighty ALLAH, the most Merciful and Benevolent Who enabled me to complete my research work successfully. I also offer my humblest thanks to His last Prophet MUHAMMAD (peace be upon him) Who is a source of guidance and knowledge for the humanity. First of all, I would like to express my deepest gratitude and obligation to my worthy supervisor Prof. Dr. G. Murtaza for his kind and inspiring guidance throughout the research work. His sympathetic and encouraging behavior enabled me in broadening and improving my capabilities not only in the field of physics but in other aspects of life as well. I am very grateful to have such a rewarding and enriching experience of working with him. My heartfelt gratitude goes to my co-supervisor Dr. Riaz Ahmad, for giving me the insight to conduct the research work. His guidance was a complete source of learning. Special thanks are due to Prof. Dr. Hassan A. Shah, Chairman Department of Physics, for providing research facilities to conduct the research work. I would like to submit my regards to my teacher Prof. Dr. M. Zakaullah for his guidance and support, without whose cooperation it would have been an uphill task to complete the thesis. I am also thankful to my fellows Dr. Abdul Qayyum and Dr. Sarfraz Ahmad for their constant support. Infact, Dr. Qayyum has always been very kind to me in both academic and personal matters. I am very much thankful to Dr. Paul Lee and Dr. Rajdeep Singh Rawat for providing me opportunities to work at the Plasma Radiation Sources Lab. National Institute of Education, Nanyang Technological University, Singapore twice in the years 2006 and 2007, where I executed few experiments on the repetitive plasma focus NX2 device and performed sufficient data analysis. Dr. Rajdeep has always been very generous to help me during my research work. I must acknowledge the financial support of the National Engineering and Scientific Commission (NESCOM), Pakistan throughout my research work within Pakistan and abroad. Here, I must also acknowledge the Higher Education Commission of Pakistan for providing me financial support during my visits to the Al-Azhar University Cairo, Egypt and Kathmandu University Kathmandu, Nepal for papers presentation and training.

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I would like to acknowledge all of my lab fellows, especially Ijaz Ahmad Khan for help during research work. To all my friends and well-wishers, I owe a lot and extend heartiest thanks, especially to Syed Murtaza Hassan and Malik Ijaz Qamar who rendered their assistance during my studies. Finally, I owe my academic success and progress in life to my parents whose endless prayers are a source of encouragement and determination for me throughout the long years of my education. I reserve my sincerest salutations to my dear brothers and sisters for their prayers, affection and endurance that can never be paid back.

Muhammad Hassan

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ABSTRACT The work presented in this thesis addresses the parametric study of ion beams emitted from Mather type plasma focus devices and their flourishing utilization in materials processing. Experiments have been performed by using two different plasma focus devices; a conventional 2.3 kJ plasma focus device developed under the joint venture of the United Nations University (UNU) and the Abdus Salam International Centre for Theoretical Physics (ICTP) designated as the UNU/ICTP device operational at the GC University Lahore and a modified version called the Nanyang X-ray source-2 designated as the NX2 device (a repetitive plasma focus) operational at the National Institute of Education (NIE),

Nanyang Technological

University (NTU),

Singapore.

The

measurements of ion parameters such as energy, energy distribution, number density and current density are carried out in the ambient gas pressure by employing a BPX65 photodiode and a Faraday cup (FC) using time of flight technique. A major motivation is to establish the optimum processing conditions for ion nitriding, surface modification, phase changes and carburizing of materials of industrial interest like Ti, AlFe1.8Zn0.8 alloy and SS-321 in plasma environment. The processed samples are characterized for structural and morphological changes, compositional profile and surface hardness by employing X-ray diffraction (XRD) at GC University Lahore, scanning electron microscopy (SEM) at University of Peshawar, field emission SEM (FESEM) and energy dispersive X-ray spectroscopy (EDX) at the NIE NTU Singapore, X-ray photoelectron spectroscopy (XPS) at the National University of Singapore (NUS) Singapore, Raman spectroscopy and Vickers microhardness test at Quaid-i-Azam University Islamabad, Pakistan. The SRIM code and microindentation measurements are used to estimate the depth profile of the modified layers. Nanocrystalline spatially uniform TiN thin films with petal like features are developed on Ti substrates exposed to 30 focus shots at various axial positions. The surface roughness and the relative proportion of the TiN films are strongly influenced by the ion beam energy flux. The film acquires eminent appearance with maximum relative proportion of nitrogen at 7 cm axial position. The probable energy of the ions reaching this position is 64 keV with the maximum ion number density of 5.9´1013 cm-3. The corresponding energy flux and current density are 2.69´1013 keV cm-3 nsec-1 and 1142 A cm-2

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respectively. The grain size of the film is estimated to be about 90 nm while the compound layer thickness is about 0.66 µm. The surface microhardness is also maximum at this axial position with typical value of 7650±10 MPa. The SEM images of a typical microcracked TiN thin film and the SRIM code estimations of ion penetration help in understanding the growth mechanism of the film in terms of ion dose. The granular nanostructures appearing on the substrate surface are grown from nucleates of a few nm size developed by the energetic ions induced collision cascades. The predeposited nitride layer or nitrogen ions interstitially implanted into the substrate surface are also redistributed by the successive pulses of the ion beams leading to layer densification along with possible resputtering. Moreover, the temperature evolution during the DPF ions irradiation also enhances the reactivity of the nitrogen already introduced during the preceding pulses. The residual tensile stresses on the sample surface are transformed to the compressive stresses after DPF ion irradiation. Nitrogen ions induced surface changes in AlFe1.8Zn0.8 alloy are investigated as functions of axial and angular positions for 30 shots. The expanded fcc phase of Al is evolved owing to the incorporation of nitrogen along with Fe and Zn into the Al lattice. A comparatively smooth and crack free nitride layer is formed on the sample treated at 7 cm axial and 100 angular position with 4- to 5-fold increase in Vickers hardness. TiN0.9 and (Fe,Cr)2N are deposited on SS-321 along with formation of non-stoichiometric (Fe,Cr)xN phase by exposing the samples to multiple focus shots in nitrogen plasma at different axial and radial positions. The transformation from (Fe,Cr)xN to (Fe,Cr)2N is attributed to an increased nitrogen ion dose. The point-like structures of flakes reveal the nucleation of crystal growth with the increased ion doses. The nitride layer is golden in colour and is spatially uniform with improved surface hardness. Multiphase nanocrystalline titanium oxycarbide TiCxOy thin films composed of TiC2, TiO0.325, Ti2O3 and carbon phases are deposited on titanium substrate in CH4 discharges by the UNU/ICTP and the NX2 devices. The nanocomposite films are non-porous and microcrack-free with grain-like surface morphology having spatially uniform carbon distribution. XRD, Raman and XPS results reveal the favorable evolution of multiphase coatings having a stoichiometric TiC2 phase and graphitic carbon adsorbates along with

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the residual oxide (TiO0.325, Ti2O3) phases with the lower energy flux and lower repetition rate in the UNU/ICTP treatment. Whereas, the deposition of carbon and a nonstoichiometric TiO0.325 phase is favored due to the improved oxide removal and enhanced disorder in the substrate surface during the NX2 treatment. In addition, TiC2 phase is also suppressed, possibly due to the enhanced substrate temperature caused by the higher energy flux of the ion beams and the higher repetition rate. The granular profile of the films attains a definite coagulation pattern. The energy flux of the ion beam and the repetition rate are found to be critical parameters which influence the preferred evolution of a particular phase during the restructuring of various phases.

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TABLE OF CONTENTS ACKNOWLEDGEMENTS

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ABSTRACT

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LIST OF PUBLICATIONS

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LIST OF TABLES AND FIGURES

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CHAPTER # 1

1

1.1

1.2

1.3

1.4

INTRODUCTION

Materials Processing – An Overview

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1.1.1 Ion Solid Interaction

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1.1.2 Ion Implantation

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1.1.3 Ion-Beam-Assisted Deposition (IBAD) of Thin Films

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1.1.4 Ion-Beam Mixing (IBM)

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Applications of Plasma Processing of Materials

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1.2.1 Tribological Applications

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1.2.2 Ultra-Pure Cleaning

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1.2.3 Medical Applications

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1.2.4 Applications in Semiconductor Industry

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Plasma Focus -A Potential Candidate for Materials Processing

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1.3.1 Dense Plasma Focus as an Ion Source

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1.3.2 Advantages of Plasma Focus based Materials Processing

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Layout of the Thesis

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CHAPTER # 2

LITERATURE SURVEY

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CHAPTER # 3

EXPERIMENTAL SETUP AND DIAGNOSTICS

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3.0

Introduction

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3.1

The Plasma Focus Device at GCU Lahore, Pakistan

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3.1.1 Plasma Focus Tube

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3.1.2 The driver for DPF

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3.2

3.1.3 Sparkgap and Triggering

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3.1.4 The DPF operation

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The Repetitive Plasma Focus Device-NX2 machine at the NIE,

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NTU, Singapore

3.3

3.4

3.5

3.2.1 Introduction

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3.2.2 Novel Features of the Device

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3.2.3 Description of the NX2 Configuration

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Preliminary Electrical Diagnostics

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3.3.1 Rogowski Coil

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3.3.2 High Voltage Probe

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3.3.3 BPX65 Photodiode Detector

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3.3.4 Faraday Cup

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Sample Preparation and Treatment

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3.4.1 Sample Selection

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3.4.2 Rough Grinding

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3.4.3 Mounting

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3.4.4 Polishing

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3.4.5 Etching/Cleaning

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Diagnostic Techniques for Processed Materials

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3.5.1 X-Ray Diffractometer (XRD)

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3.5.2 Scanning Electron Microscopy (SEM) with

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Energy Dispersive X-Ray Spectroscopy (EDS) 3.5.3 X-ray Photoelectron Spectroscopy (XPS)

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3.5.4 Raman Spectroscopy

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3.5.5 Microhardness Measurements

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CHAPTER # 4 4.1

RESULTS AND DISCUSSIONS

Ion Beam Characterization and Nitriding

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4.2

4.1.1 Introduction

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4.1.2 Experimental Setup

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4.1.3 Results and Discussion

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Plasma Focus Assisted Synthesis of Nanocrystalline TiN

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Thin Films

4.3

4.4

4.5

4.6

4.2.1. Introduction

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4.2.2 Experimental Details

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4.2.3 Results and Discussions

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Surface Modification of Al-Alloy

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4.3.1 Introduction

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4.3.2 Experimental Setup

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4.3.3 Results and Discussion

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Co-deposition of Titanium and Iron Nitrides on SS-321

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4.4.1 Introduction

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4.4.2 Experimental Setup

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4.4.3 Results and Discussion

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Carburizing of Titanium using UNU/ICTP and NX2 DPF Devices 113 4.5.1 Introduction

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4.5.2 Experimental Arrangements

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4.5.3 Results and Discussion

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Summary of the Work and Future Suggestions

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4.6.1 Future Suggestions

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4.6.2 Concluding Remarks

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BIBLIOGRAPHY

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REFERENCES

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LIST OF PUBLICATIONS (In refereed journals of international repute)

1. Study of lateral spread of ions emitted from 2.3 kJ plasma focus with hydrogen and nitrogen gases R. Ahmad, M. Hassan, G. Murtaza, A. Waheed and M. Zakaullah, J. Fusion Energy, 21(3/4) (2003) 217-220 2. Surface modification of AlFe1.8Zn0.8 alloy by using dense plasma focus M. Hassan, R. Ahmad, A. Qayyum, G. Murtaza, A. Waheed and M. Zakaullah, Vacuum, 81 (2006) 291-298 3. Co-deposition of titanium and iron nitrides on SS-321 by using plasma focus R. Ahmad, M. Hassan, G. Murtaza, J.I. Akhter, A. Qayyum, A. Waheed and M. Zakaullah, Rad. Eff. & Def. in solids, 161(2) (2006) 121-129 4. Nitriding of titanium by using ion beam delivered by a plasma focus M. Hassan, A. Qayyum, R. Ahmad, G. Murtaza and M. Zakaullah, J. Phys. D: Appl. Phys. 40 (2007) 769-777 5. Synthesis of nanocrystalline multiphase titanium oxycarbide (TiCxOy) thin films by UNU/ICTP and NX2 plasma focus devices M. Hassan, R.S. Rawat, P. Lee, S.M. Hassan, A. Qayyum, R. Ahmad, G. Murtaza and M. Zakaullah, (Accepted in Appl. Phys. A) 6. Plasma focus assisted synthesis of nanocrystalline TiN thin film M. Hassan, A. Qayyum, R. Ahmad, R.S. Rawat, P. Lee, S.M. Hassan, G. Murtaza and M. Zakaullah, (Submitted in Surf. Coat. Technol.) 7. Nitridation of zirconium disks using ion beam delivered by plasma focus discharges I.A. Khan, M. Hassan, R. Ahmad A. Qayyum, G. Murtaza, M. Zakaullah and R.S. Rawat, (Revised in Thin Solids Films) 8. Nano-phase titanium dioxide thin film deposited by repetitive plasma focus: Ions dose and annealing based agglomeration R.S. Rawat, V. Aggarwal, M Hassan, P Lee, S.V. Springham, T.L. Tan and S. Lee (Submitted in Plasma Sources Sci. Technol.)

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(In international conference proceedings)

9. AlN thin film deposition on Al alloy by using dense plasma focus M. Hassan, R. Ahmad, G. Murtaza and M. Zakaullah, Al-Azhar Bull. Sci. 16(2) (2005) 9-13 10. Effect of plasma focus discharge currents and repetition rates on evolution of nanocrystalline multiphase titanium carbide/a-C thin films M. Hassan, R.S. Rawat, P. Lee, S.M. Hassan, A. Qayyum, R. Ahmad, G. Murtaza and M. Zakaullah, J. Nepal Phys. Soc. 23(1) (2007) 19-23

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LIST OF TABLES AND FIGURES TABLES Table 3.1: The operating parameters of the NX2 device

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Table 4.1: XRD data showing the relative intensity (%) and the broadening (2q0) 78 of typical peaks of TiN film deposited on titanium at different axial positions Table 4.2: EDS data showing the nitrogen and titanium concentration (at. and

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wt. %) in nitrided titanium at different axial positions Table 4.3: EDS data showing the nitrogen and titanium concentrations (at. %

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and wt. %) for various ratios of micro-crack to film surface areas Table 4.4: Technical parameters of the UNU/ICTP and the NX2 device used

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for the carburizing of titanium samples Table 4.5: EDS data showing the carbon and oxygen concentrations (at. % and

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wt. %) for typical samples surfaces carburized by both the devices FIGURES Figure 1.1: The schematic of ion implantation phenomenone

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Figure 1.2: The schematic of ion beam assisted deposition process

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Figure 1.3: The schematic of ion beam mixing during ion bombardment

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Figure 1.4: Schematics of (a) Mather type and (b) Fillippov type plasma focus

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geometries Figure 3.1: Schematic diagram of the DPF device working at GCU Lahore

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Pakistan used for materials processing Figure 3.2: A schematic of plasma focus electrode system

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Figure 3.3: A schematic of triggertron-type pressurized spark gap

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Figure 3.4: The circuit diagram of trigger unit

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Figure 3.5: Schematic diagram of the dumping system

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Figure 3.6: A Schematic of the NX2 machine at the NIE NTU, Singapore,

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used for materials processing Figure 3.7: Equivalent circuit of DPF device

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Figure 3.8: A schematic of (a) design (b) working and (c) Passive integrator

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circuit of Rogowski coil Figure 3.9: The schematic diagram of a typical high voltage probe

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Figure 3.10: (a) A BPX65 photodiode and (b) Schematic arrangement of biasing 44 circuit for the photodiode Figure 3.11: Schematic arrangement of Faraday cup used in experiment

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Figure 3.12: The conditions for the beams reflected from successive planes to be 51 in phase, and hence reinforcing each other, as given by Bragg’s Law Figure 3.13: Schematic representation of a sample mounted on a goniometer

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stage, which can be rotated about one or more axis and the detector which travels along the focusing circle in the Bragg-Brentano geometry Figure 3.14: A schematic diagram of the electron beam collimator in SEM

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Figure 3.15: A schematic diagram of electron source used in the electron beam

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collimator Figure 3.16: (a) Typical EDS spectrum of SS and (b) Elemental map of SS

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showing Fe lines Figure 3.17: The schematic of an XPS physical mechanism

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Figure 3.18: The schematic arrangement of an XPS system

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Figure 3.19: Energy level diagram for Raman scattering; (a) Stokes Raman

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scattering (b) Anti-Stokes Raman scattering

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Figure 3.20: Vickers indentation on an SS specimen with 50 gf

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Figure 3.21: (a) The schematic of indentation force and (b) Test cycle profile

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Figure 4.1: Typical ion beam signals recorded by (a) BPX65 photodiode

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(b) Faraday cup Figure 4.2: Ion energy (keV) as a function of time evolution (nsec) estimated

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with BPX65 photodiode and Faraday cup Figure 4.3: Distribution of ion number density (cm-3) as a function of ion energy 73 (keV) estimated with BPX65 photodiode and Faraday cup Figure 4.4: Ion energy (keV) and corresponding max. ion number density

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(cm-3) as a function of axial positions (cm) evaluated with (a) BPX65 photodiode and (b) Faraday cup Figure 4.5: Comparison of ion current density (A cm-2) as a function of axial

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positions (cm) recorded with BPX65 photodiode and Faraday cup Figure 4.6: XRD patterns of titanium sample surfaces treated at different axial

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positions Figure 4.7: Typical SEM micrographs of TiN films deposited at different axial

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positions; (a) 3cm, ´40000, (b) 5cm, ´40000, (c) 7cm, ´10000 and (d) 9cm, ´20000 Figure 4.8: Typical SEM micrographs of (a) titanium substrate, and TiN film

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deposited at 7cm at (b) ´10000, (c) ´20000 and (d) ´50000 magnifications Figure 4.9: Typical SEM micrograph with its EDS maps showing titanium and

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nitrogen elemental distribution on TiN thin film deposited at 7 cm Figure 4.10: Variation of micro-hardness (MPa) as a function of indentation

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depth (µm) with different test loads (10, 25, 50, 100, 200, 300 and 500g) on sample surfaces treated at different axial positions Figure 4.11: Typical ion beam signal recorded with BPX65 photodiode detector

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Figure 4.12: SEM micrographs of (a) The substrate surface, (b) TiN film surface 88 at ´750, (c) ´20,000 and (d) ´50,000 magnifications Figure 4.13: (a) Typical SEM image of the TiN film at ´37,000 magnification

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showing a micro-crack and (b) a simple geometry illustrating the micro-crack Figure 4.14: Typical SEM micrograph along with EDS maps showing the

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titanium and nitrogen distribution on the TiN thin film having a micro-crack Figure 4.15: X-ray diffractograms of the TiN thin film obtained with X’Pert

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PRO MPD (q-q scan mode) and SIEMENS (q-2q scan mode) XRD machines Figure 4.16: Variation of microhardness (MPa) as a function of indentation

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depth (mm) at various imposed loads (10, 25, 50, 100, 200, 300 and 500 g) Figure 4.17: X-ray diffraction patterns of Al alloy samples exposed with 30

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shots placed at different axial positions at the anode axis Figure 4.18: X-ray diffraction patterns of Al alloy samples exposed at

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Z = 7 cm with 30 shots, placed at different angular positions Figure 4.19: The XRD data showing relative change in d-spacing and

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FWHM of the (111) plane reflection at different (a) axial and (b) angular positions for a sample exposed with 30 shots Figure 4.20: EDX spectra of the samples (a) unexposed, and exposed at

100

Z = 7cm with 30 shots for angular position; (b) 00 and (c) 100 Figure 4.21: SEM micrographs of the samples (a) unexposed, and exposed at

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101

Z = 7cm with 30 shots at angular positions (b) 00, (c) 100 and (d) 200 Figure 4.22: Variation of Vickers hardness (MPa) as a function of indentation

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depth (µm) for the samples exposed with 30 shots at different axial positions Figure 4.23: Variation of Vickers hardness (MPa) as a function of indentation

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depth (µm) for the samples exposed at Z = 7cm with 30 shots at different angular positions Figure 4.24: The XRD diffractograms of stainless steel samples treated with

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10 and 15 shots at axial position of Z = 15 cm Figure 4.25: XRD diffractograms of stainless steel samples treated at Z = 15 cm 108 with 15 shots at different radial positions Figure 4.26: EDX spectra of stainless steel samples (a) unexposed, (b) exposed

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with 10 shots and (c) exposed with 15 shots Figure 4.27: SEM micrograph of a typical stainless steel sample treated at

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Z = 15cm with 15 shots (a) with 1000x magnification and (b) with 5000x magnification Figure 4.28: Variation of surface hardness (HV) and the indentation depth (µm)

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as a function of imposed load (g) for the unexposed and the exposed samples with 10 and 15 shots at axial position of Z = 15 cm Figure 4.29: X-ray diffraction patterns of the multiphase thin films deposited by

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the UNU/ICTP DPF at different axial and angular positions; The legend notations follow the pattern as: 5cm represents the sample treated at axial position of 5cm along the anode axis (00 angular position with respect to the anode axis), and so on; 5cm, 100 represents the one treated at 5 cm axial position and 100 angular position with respect to the anode axis and so on. Figure 4.30: X-ray diffraction patterns of the multiphase thin films deposited by the NX2 machine at different axial and angular positions (at 0.5 and 1.0 Hz repetition rates); The legend notations follow the pattern as:

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7cm, 0.5Hz represents the sample treated at 7 cm axial position along the anode axis (00 angular position), and so on; 9cm, 1.0Hz, 100 represents the one treated at 9 cm axial position and 100 angular position with respect to the anode axis when the device is operated at 1.0 Hz repetition rate and so on. Figure 4.31: Crystallite size estimated from carbon phase of the films deposited

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at (a) 5 cm by the UNU/ICTP DPF and (b) 9cm,1.0 Hz by the NX2 device Figure 4.32: SEM micrographs of (a) untreated substrate surface, (b) composite

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film surface deposited with the UNU/ICTP DPF at 5cm, 00, (c) 100 and (d) 200 angular positions Figure 4.33: SEM micrographs of (a) composite film surface deposited by

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the NX2 machine at 9cm,1.0 Hz repetition rate when sample is at 00 and (b) 100 angular positions Figure 4.34: Typical EDS images along with elemental maps of the films

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deposited with (a) the UNU/ICTP device at 5cm and (b) the NX2 device at 9cm,1.0Hz repetition rate Figure 4.35: XPS spectra of the film deposited at 5cm by the UNU/ICTP

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device; (a) Ti 2p and (b) Cs1 line shapes with Gaussian fit, and those of the film deposited at 9cm,by the NX2 device operated at 1.0 Hz repetition rate; (c) Ti 2p and (d) Cs1 line shapes with Gaussian fit Figure 4.36: Typical Raman spectra of the multiphase composite films deposited 127 at different axial positions using UNU/ ICTP DPF device Figure 4.37: Variation of microhardness (MPa) as a function of indentation 128 depth (mm) of the films deposited at typical samples by the UNU/ICTP and the NX2 device

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CHAPTER # 1

INTRODUCTION 1.1

Materials Processing- An Overview

The growing commercial demands of components and tools having necessary load bearing capacity and required tribological properties, have increased to such an extent that they cannot be met in many cases with conventional metals and alloys. The desired properties of the materials can only be achieved by resorting to composite materials, new fabrication methods or sophisticated surface treatments. In recent years, many novel techniques have been developed for processing of materials such as laser surface treatment, ion implantation, plasma immersion ion implantation, chemical vapour deposition, physical vapour deposition, ion assisted surface modification, phase changes of thin films, and thin film deposition such as diamond like coatings. It has become imperative that the surface treatment of various materials can provide better and tailored properties of cheap substrates having better performance, longer lifetime and higher reliability with more economical production and operation. The use of plasma and ion-beam-modified surfaces and surface coatings is continually expanding in engineering disciplines. The purpose of these modifications and treatments is to impart favourable properties, such as wear resistance and lubricity, to the surfaces, while at the same time retaining the strength or toughness of the bulk materials. The bombardment of energetic ion beams can modify the structural and chemical properties of surfaces or applied coatings. In the process of selecting a material, a compromise must often be made between bulk mechanical properties and surface tribological properties, with neither property being at its optimal value. Metals and alloys are often the materials of choice when properties such as strength, ductility, and toughness are needed. Hard compounds are frequently found to be the best when excellent tribological properties, such as high-temperature hardness and corrosion resistance, are required. Thus there is an ongoing interest in technologies that permit to retain the desirable properties of a bulk material while

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imparting favourable tribological and chemical properties to its surface. In particular, the development of tribological systems for spacecraft [1], advanced heat engine [2], and aeronautical applications [3] present special obstacles. Plasma processes such as RF and DC magnetron sputtering and plasma-assisted chemical vapor deposition (CVD) have long been used for the application of surface coatings that improve the wear resistance or frictional properties of materials. Recently, ion-beam-based techniques have been studied and employed for modifying or coating the surfaces of electronic, biomedical, and engineering materials. The area of ion assisted processing is broad, and encompasses such techniques as ion plating, activated reactive evaporation, ionized cluster beam deposition, and plasmaenhanced chemical vapour deposition (PECVD). However, the more general techniques that employ ions beams with well-defined energy and flux include ion implantation (II), ion-beam-assisted deposition (IBAD) of thin films and ion-beam mixing (IBM). When a surface is bombarded with energetic ions, the chemical, microstructural, and physical properties of the surface can be significantly changed. The nature of the changes depends on the ion species, mass, energy, flux and temperature of the substrate. Energetic ions beams can penetrate into the near surface region and produce chemical changes. Collisions of the ions with specimen atoms may disorder and displace large numbers of atoms from their equilibrium lattice sites. Numerous combinations of ion and film deposition parameters are possible, and significant advances have been made in decreasing friction and wear. Some approaches are extensions of conventional methods of tribology. For example, when a steel rolling element is implanted with N atoms, the nitrides thus formed increase the wear resistance [4]. Other approaches make use of ion bombardment to provide novel and perhaps unexpected means of reducing friction or wear. Steels implanted with Ti, C, and N become amorphous and experience considerably less dry-sliding friction [5]. The ion-beam assisted processes have several advantages: (i) the energy and flux of the ions can easily be controlled, (ii) low background pressures can be maintained, minimizing the introduction of unwanted gases into films, (iii) low substrate temperatures can be used because the driving force for atomic rearrangement is supplied by the kinetic

2

energy of the ions rather than by thermal activation, (iv) reactive ions may be used to produce compound films of controlled composition and/or structure, (v) the dimensional accuracy of the treated part is preserved because coatings are thin, and (vi) the adhesion of thin films to substrates can be significantly improved by bombarding the substrate prior to and during deposition. Before discussing the general ion beam processing techniques, it will be beneficial to describe the ion interaction with the solid surface. 1.1.1

Ion Solid Interaction

When energetic ion beams strike a material specimen, momentum is transferred to atoms of the specimen. Highly energetic ions (»10 keV to several MeV) can penetrate deeply into a specimen (»1mm at 1 MeV, depending on the ion species) to produce collision cascades causing large number of atomic displacements in a small region. Numerous interstitials and vacancies are produced, and many atomic replacements occur. Owing to the considerable vibrational energy of the atoms in the cascade region for a short time, a thermal spike is developed. A crystalline material may be rendered amorphous, or an ordered metallic alloy may become disordered in the region of the cascade. The collision of ions with substrate atoms is a non-equilibrium ballistic process, equilibrium thermodynamic considerations play an important role to determine which phases and microstructures will form during irradiation [6]. Ion beams produce small collision cascades that rearrange atoms in the first several monolayers of the specimen, but do not penetrate to large depths. When a film under development is bombarded, low-energy ions collide with surface atoms and may drive these atoms into pores and vacancies. Open clusters of atoms may be compacted to form a film with a dense, equiaxed microstructure [7]. When chemically reactive ions are used, compounds can be formed as a result of the increased chemical activity of the ionized species and the implantation of the ions just below the surface e.g., N bombardment of elemental B to produce BN [8]. When a collision cascade occurs in the near surface zone of the material, sufficient energy may be transferred to one or more atoms. Eventually, the atoms are dislodged or sputtered from the surface [9]. Major changes can also occur in final particle distributions by varying the type of gas used in the plasma, the target-

3

substrate distance, and the target material. These changes, in turn, can produce variations in composition, stress and grain size as well as the surface morphology of the film. The most common materials processing techniques using the ion beams emitted from the plasmas or other sources are as follows: 1.1.2

Ion Implantation

Ion Implantation is the direct implantation or injection of high-energy ions into the substrate [10], as depicted schematically in figure 1.1.

Figure 1.1: The schematic of ion implantation phenomenone

During ion implantation, ions may be injected upto varying depths depending upon their energy. Eventually, the possibility of delamination at an interface is eliminated. When energetic ions collide with the substrate surface and displace the atoms from their equilibrium lattice sites, displacement damage occurs, and near-surface regions may be extensively modified [11]. As a result of atomic rearrangements and accumulation of implanted atoms, second-phase compounds may form [12], dislocations and point defects may be produced, or the intrinsic stress and hardness of films may be changed [4]. Improvements may arise from oxidation reduction, the introduction of compressive stresses, or surface hardening. Ion implantation can be used to amorphize a crystalline material, markedly improving mechanical [5] and corrosion properties [13]. However, it is limited in terms of the quantities of atoms that can be injected into the treated parts. The major advantage of the ion implantation is to develop a progressive interface between the implanted surface layer and the bulk material without modifying the original dimensions of the material specimen. Eventually, the adherence problems are avoided 4

with ion implantation. The ion implantation, which causes microstructural and chemical changes in the near surface zone of the substrate, has proven successful for a wide variety of applications [14]. For examples, introducing alloying elements in a material may enhance its lubricity for corrosional use. 1.1.3

Ion-Beam-Assisted Deposition (IBAD) of Thin Films

In ion beam assisted deposition of thin films, an energetic ion beam obtained from an ion source is used to deposit a film on the substrate. Because directed ion beams can be easily neutralized, problems arising from substrate charging are avoided. IBAD of a thin film is

Substrate

Compound layer m)

depicted in figure 1.2.

Figure 1.2: The schematic of ion beam assisted deposition process

The depositing material can originate from an evaporation source, or a second ion gun can be used to sputter material onto the substrate. Prior to deposition, the ion bombardment both cleans the substrate by sputtering and changes the chemical bonding at the interface. Energetic reactive ions, such as N, react strongly with substrate atoms to form compounds that would otherwise require high substrate temperatures. Thus, IBAD with reactive ions is a low-temperature process, sometimes referred to as reactive IBAD or RIBAD. Compound films can be developed when reactive gases, such as O2 or N2, are used. For example, cubic BN [8, 15] and TiN [16] have been produced when evaporated Ti or B metal reacts with N ions during IBAD process. IBAD has been used to produce optical coatings with superior film packing at low temperatures [17]. The energy of bombarding ions must be selected with care. The film microstructural residual stresses and packing density can often be controlled by suitable choice of ion parameters.

5

Thin films are generally not the same as bulk materials since, in most cases, they contain impurities, internal and external surfaces/interfaces, density variations with thickness, short range density fluctuations, etc., which are highly dependent upon the particular deposition technique parameters and fundamental processes such as energetic particle bombardment. The result is a virtual infinity of thin film materials covering a broad continuum of free energy states and macroscopic and microscopic structures. It is this variability which leads to the wide range of possible characteristics and resulting properties that make thin films such extensively used materials both scientifically and technologically. At the same time, quantitative preparation-characterization-property relations remain an elusive goal, in large part due to the seeming simplicity and yet actual complexity of vapor-deposited films [18]. 1.1.4

Ion-Beam Mixing (IBM)

Ion beam mixing is a process in which one or more thin films are grown on a substrate and then subsequently mixed with high-energy ions [19]. Ion beam mixing process is mostly taking place in parallel with the ion beam assisted deposition of thin films, and is schematically illustrated in figure 1.3.

Figure 1.3: The schematic of ion beam mixing during ion bombardment

When energetic ions collide with the atoms in the substrate, a collision cascade occurs. Atomic mixing results from the motion associated with the production of large numbers of interstitials and vacancies near the cascade and from the large amount of disorder in the region of the displacement cascade. The amount of mixing depends strongly on temperature, and is generally larger than would be expected from simple ballistic mixing

6

of atoms. When the collisional damage occurs at the interface between the film and the substrate, the film adhesion can be markedly improved. Thus, the technique of ion beam mixing along with the ion beam assisted deposition of thin film could be a better choice with respect to the formation of the film having enhanced adhesion.

1.2

Applications of Plasma Processing of Materials

Plasma processing of materials has applications in diverse disciplines such as aerospace, automotive, steel, biomedical, and toxic waste management industries. Recently, plasma processing technology has been utilized increasingly in the emerging technologies of diamond film and superconducting film growth. The major applications of the plasma based ion treatment are discussed as follows. 1.2.1

Tribological Applications

1.2.1.1 Surface Lubricity During the operation of a tribosystem, mechanical and chemical interactions may occur between the sliding surfaces, the lubricants, the reaction products, and the environment. The lubricant applied to the sliding or rolling surfaces may be an oil, a vapor, a solid compound, a chemical, a soft metal, or a combination of these. Various elements or compounds incorporated into the sliding surfaces can promote or inhibit chemical reactions that affect lubrication. For example, friction may be reduced by the application of a thin coating that reacts with the environment to form a lubricious film during operation. The corrosion of a material may be inhibited by producing an amorphous surface layer. Surface hardening by ion implantation may be the most effective route for severe loading. Ion beam processing will be beneficial if the surface can be engineered to improve lubricity or wear resistance. A solid lubricant is a material that can shear or slide easily between rubbing parts and prevent the parts from coming into direct contact [20]. If a thin layer of a low-shear-strength compound is deposited on a substrate, this film will deform first during sliding, protecting the underlying substrate. 1.2.1.2 Surface Wear Reduction Surface wear depends critically on the hardness of the rubbing surfaces. Ion implantation has been extensively used to improve the wear resistance in various Fe-, Ti- and Cr-base

7

alloys and ceramics, such as Si3N4, by surface hardening and/or production of compressive surface stresses [21]. Implantation of N into steels has been extensively used to impart wear resistance. The mechanism is believed to be the formation of hard nitrides, g'-Fe4N, e-Fe3N, and ζ-Fe2N, with the g' phase being the most effective [22]. The beneficial effects of N implantation are found to persist even after the modified layer has been worn away. The penetration depth of ions extends much deeper (>10 mm) than expected by thermal diffusion, and excellent wear resistance can be achieved. The interstitials and vacancies that are produced in the near-surface region can significantly accelerate diffusion of N atoms into the bulk. The deposition of hard compound films such as TiN for wear resistance can be produced by ion beam assisted plasma processing. Evaporation of Ti by concurrent bombardment with energetic N ions produces TiN films with greater adhesion. Other hard nitride coatings (BN, SixNy, and TixNy) showed low friction (m » 0.1) against steel [23]. Alternately, ion implantation of N having MeV energy directly into Ti produces significant surface hardening [12]. Much of the work done on wear reduction in tribological systems is directed toward improving the mechanical properties of sliding surfaces. However, corrosion or oxidative wear can be a serious problem. The tribology and chemical behaviour of various alloys are receiving attention owing to the need to extend the operating life of medical prostheses in the body. Incorporation of nitrogen into Ti and its alloys has been found to dramatically extend their wear life [24], due to inherently less electrochemical corrosion and hardening of the Ti surface, thus protecting it from wear debris. Surface coatings have long been used to protect substrates from attack by forming a protective scale, or by reducing the corrosion rate. A good coating should resist penetration of agents that could cause corrosion at an interface. 1.2.2

Ultra-Pure Cleaning

The high reaction rates and low temperatures in ion beam assisted plasma processing make this a very attractive tool for cleaning such materials as polymers and glasses, which may be thermally sensitive. Because the zone of interaction of a plasma with a surface is restricted to the top few molecular layers, plasma processing is primarily a

8

technique for ultra cleaning and modification of the surface chemistry of materials. This can be accomplished either through functionalization or passivation of an existing surface or through creation of a new surface by plasma polymerization of a thin film. Since primarily organic materials are removed during typical plasma processes, a precleaning step to remove heavy inorganic soils may be required in some instances. The plasma process is then utilized for an ultra-pure cleaning step. As a cleaning process, plasma cleaning is most effectively used for complete removal of thin organic soils. 1.2.3

Medical Applications

For many applications in the medical and optical industries, it is necessary to ensure that products or instruments are extremely clean. For a surface to be clean, it must be not only free of visible debris (e. g. dust), but also free from molecular-level contamination, such as hydrocarbons. A recent interesting application requiring a hydrocarbon contamination free surface involved a cast ceramic part to be adhesively bonded to another assembly. Plasma processing helps to provide better adhesion and cleaning of the surface as compared with the conventional chemical solvent methods which are unable to improve the solvent adhesion to an acceptable level. Moreover, harsher solvents and acidic or caustic cleaners are environmentally undesirable alternatives and require an additional drying step in the production process. 1.2.4

Applications in Semiconductor Industry

Plasma processing technologies are of vital importance in the electronics industry, in which plasma-based processes are indispensable for the manufacturer at large-scale. Decreasing device dimensions in integrated circuits (IC) fabrication has put forth many new demands on processing semiconductors. Decreasing cost of ICs also necessitates plasma treatment, being cost effective with high productivity options. Shallow junction implantation, deep trench doping, manufacturing the silicon-on-insulator (SOI) substrates, selective electroless copper deposition, thin film transistor hydrogenation, etc. are the major applications Amongst the various plasma based materials processing techniques, the dense plasma focus device can be a promising candidate for tailoring the surface properties of various materials. 9

1.3

Plasma Focus - A Potential Candidate for Materials Processing

The dense plasma focus (DPF) is a magneto-hydro-dynamic coaxial plasma accelerator [25], which utilizes self-generated magnetic field for efficiently compressing the gas to a high density (ne ~1019–1020 cm-3) and high temperature (Te~1-2 keV) pinched plasma for a short duration (~100 nsec) [26]. It is an excellent source for the generation of highly energetic high fluence ions, relativistic electrons, X-rays and neutrons [27-31]. The dense plasma focus flourished as a nuclear fusion device in the 70’s and 80’s, based on the pinched phenomenon occurring during the course of high electric currents through the working gas. The dense plasma focus was invented independently by Filippov in Russia [32] and by Mather in USA [25] in two different geometries as shown in figure 1.4. The Mather type and Filippov type geometries differ from each other on the basis of aspect ratio (ratio of anode diameter to its length). The aspect ratio is less than one in the Mather type design (figure 1.4a), and is greater than one in the Filippov type design (figure 1.4b). Although both the devices behave identically, the Mather type is preferred due to its simple design, convenient access to various diagnostics, distinguishable phases of current sheath dynamics and high neutron yield for the same driver energy. It consists of an electrode assembly placed inside a vacuum chamber, an energy storage system (capacitor bank) with control electronics. The working principle of this device is, first, to convert the stored electrical energy in the capacitor to magnetic energy that appears in the plasma focus as well as in the external circuit, then a part of the magnetic energy is converted into the plasma energy. Therefore, the design of transmission line from the capacitor bank to the load (plasma focus chamber) plays an essential role in efficient transfer of energy. Hence, it is essential to have very low inductance line in between the capacitor bank and the plasma focus chamber.

(a)

10

Fig. 1.4 Schematics of (a) Mather type and (b) Fillippov type Plasma Focus geometries

The DPF device is among the cheapest available radiation sources with unique features of extremely short pulses (hundreds of ns) suitable for a number of applications. There are interesting possibilities to take advantage of X-rays (1 – 100 keV), electrons and ion beams emitted during the DPF discharge. The plasma focus phenomenon occurs at the open end of co-axial electrodes when an electric pulse is applied across the electrodes by discharging a low inductance capacitor. The coaxial electrodes are located inside a vacuum chamber filled with a gas at low pressure. A charged capacitor is connected to the closed end of the electrodes through a switch. After closing the switch, a gas discharge starts in the gap between the electrodes forming an umbrella-like plasma layer called current sheath. The azimuthal magnetic field located in the volume enclosed by the current, produces a Lorentz force that pushes the sheath towards the open end of the electrodes. The run down of the current sheath is a sweeping supersonic shock that propagates collecting the gas particles ahead of the front. On its arrival at the open end (few msec after triggering the discharge), the magnetic field starts to contract, accelerating the plasma towards the axis. Finally, the current sheath collapses on the axis in the form of a small dense plasma column (focus), that exists for a very short duration of about ~300 nsec. The disruption of this plasma column takes place owing to the

11

microinstabilities and turbulence [33], leading to the generation of powerful bursts of radiation. 1.3.1

Dense Plasma Focus as an Ion Source

Dense plasma focus emits intense X-rays, relativistic electrons, neutrons and energetic ion beams (having energy up to few MeV). In the present work, ion beams emitted from dense plasma focus are utilized for processing of different materials such as metals and alloys. The disruption of plasma column during the radial phase induces an electric field, which accelerates the ions and electrons in opposite direction with very high velocities. The ions are accelerated during the progress of m = 0 instabilities towards the top of the chamber in a conical fashion with energies ranging from tens of keV to few MeV. Various mechanisms have been proposed for ion acceleration [34, 35]. It has been empirically found that the energy spectrum of ion beam obeys the empirical power law

dN dE

µ E

- x

where E is the ion energy, N is the ion number and x ranges from 2 to 5 [36, 37]. Gullickson et al. [38] reported that the maximum ion emission occurs along the Z-axis and the ion fluence decreases gradually with increasing angle inside a conic geometry. On the contrary, Sadowski et al. [36] observed a distinct drop of ion fluence at 00. These ion beams accelerated down the coaxial accelerator in a conical fashion, have been used for the treatment of materials specimens. When a substrate is subject to these accelerated ion beams, they interact with the substrate in two ways. Firstly, the flux of energetic ion beams impinging on the substrate changes the physical properties of the outer layer of the substrate and secondly, it increases the surface temperature abruptly without changing its bulk temperature due to energy conversion. Thus, the ions penetrate to a depth of the surface layer well beyond the ions energy range. No external heating arrangement is required for ion diffusion in this device. The whole process takes a few hundred nano-seconds. These features make the dense plasma focus device unique for material processing work compared to the other conventional devices. An understanding of the basic physical and chemical processes underlying the interaction of ions with the substrate is vital for the development of many technologies. In order to have a clear 12

insight into the interaction mechanism, one needs to have a precise and accurate measurement of the ion flux as well as the ion energy that are impinging on the substrate. Recently, the use of dense plasma focus devices for material processing have attracted good attention because several experiments have shown good results in surface modification and thin film deposition. The dense plasma focus has been used for nitrogen ion implantation on stainless steel, titanium and high carbon steel [37]. Similarly, Nayak et al., [39] used nitrogen ions of the dense plasma focus for fabrication of carbon nitride coating on graphite substrate. Others have reported ion induced structural and morphological changes of a variety of thin films [40-42]. 1.3.2

Advantages of Plasma Focus based Materials Processing

When compared with other surface modification techniques, plasma processing with dense plasma focus has several attractive features. ·

It is an inherently dry process.

·

The reactants are generally inexpensive gases (e. g. nitrogen, methane, oxygen, argon etc.) and the gas consumption is appreciably small as compared with that in the conventional techniques.

·

Micro- to nano-second pulse power sources are used in glow discharge plasma reactors to overcome sheath barriers. As plasma focus has a pulse duration of tens of nano-seconds, the above difficulty is eliminated.

·

The nitrogen ions coming from the focus region are much more energetic than the nitrogen ions present in other processing reactors, so that the specimens are heated during ion beam treatment. Hence, additional heating of the specimen is not usually required.

·

Plasma focus assisted modification of a few micron thick layer of a material specimen can be performed within several minutes, which represents very good time savings compared to dc pulse or rf glow discharge.

·

Plasma focus device is operated under pressure conditions which can easily be maintained.

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1.4

Layout of the Thesis

The present thesis is compiled in four chapters. Chapter 1 describes the introduction of the problem encompassing the basic materials processing techniques and the use of the dense plasma focus as a promising source in this respect. Chapter 2 presents an overview of the research work conducted worldwide in connection with the present thesis. Chapter 3 discusses the experimental arrangements of the UNU/ICTP and the NX2 dense plasma focus devices used for materials processing along with the preliminary diagnostics as well as the materials characterization techniques. Chapter 4 describes the details of the experimental investigations, covering ion implantation of Al alloy, phase changes of SS321, and nitriding and carburizing of titanium. A summary of the work is also presented in this chapter along with some suggestions for future studies.

14

CHAPTER # 2

LITERATURE SURVEY The materials processing by utilizing energetic ion beams emitted from a dense plasma focus (DPF) dates back to the early experiments conducted in Instituto de Fisica Rosario, Buenos Aires, Argentina, followed by the major research groups in New Dehli University India, and the Andrzej Soltan Institute for Nuclear Studies, Otwock-Swierk, Poland. Before reviewing the DPF based plasma processing of different materials, it seems essential to review the ion beams parametric studies in order to optimize the plasma generation processes for possible applications. Kelly et al. [43] investigated the energy and flux distribution of nitrogen ions emanated from a low energy (4.75 kJ) PF II device by using a Faraday cup in secondary electron emission (SEE) mode. A kinetic energy threshold of about 50 keV, much lower than that obtained with a Thomson spectrometer in the previous work, was reported. The total maximum particle flux was found to be 3.2´1013 ions/sr with energy distribution of 0.74 J/sr, respectively. Jakubowski et al. [44] studied the energetic ion beams and their correlation with pulsed relativistic electron beams emitted from plasma focus discharge. Time integrated measurements of ions were performed using a pinhole camera equipped with solid state nuclear track detectors, while time resolved investigations were carried out with a scintillation detector. Time of flight technique was adopted to determine the energy spectrum of the ion beams. The observed ion beams with energies higher than 1.3 MeV were found to be usually emitted within a narrow cone oriented along the axis of the DPF discharge. The ion current density at the measuring diaphragm reached several mA and the particle flux density amounted to 2.5´1012 deutrons/sr. The FWHM of the pulse was ~20 nsec. The most probable energy of the ions was found to appear in the range of 400500 keV. Bhuyan et al. [45] investigated the nitrogen ions from 2.2 kJ plasma focus by employing a nanosecond response Faraday cup in bias ion collector mode, along the electrode axis

15

(0o) of the device. A lower energy threshold of ~7 keV was registered with maximum ion density corresponding to 17 keV ions. The correlation of the ion beam with the filling gas pressure was also established. Szydlowski et al. [46] investigated neutron and fast ions emitted from PF-1000 facility equipped with modified electrode assembly and operated for the first time at an energy level upto 1 MJ. The ion pinhole pictures, as taken with the camera equipped with CR-39 track detectors, revealed a complex structure of ion beams having different sources inside the plasma column. Taking into account the calibration characteristics of the detectors, the registered ion energies ranged from 100 keV to 1.5 MeV. Bhuyan et al. [47] investigated methane ion beams emitted from a low energy (1.8 kJ) plasma focus device. Graphite collectors, operating in the bias ion collector (BIC) mode, were used to estimate the energy and flux of the ions along the anode axis of PF, using time of flight technique. The ion beam energy and flux correlation for methane discharge indicated that the dominant charge states of carbon ions were C+4 and C+5. The maximum ion energies for H+, C+4 and C+5 were in the range of 200-400 keV, 400-600 keV and 900-1100 keV, whereas their densities were maximum for the energy range 60-100 keV, 150-250 keV and 350-450 keV respectively. A multiple Faraday cup assembly capable of BIC mode of operation was developed by Mohanty et al. [48] for investigating the pulsed nitrogen ion beams of a 2.2 kJ Mather type plasma focus device. Time of flight technique based measurements revealed that the energy of the ion beams ranged between ~5 keV to ~700 keV, with the most probable energy of ~25 keV. The ion flux was found strongly depending on the filling gas pressure. Measurements showed a strong anisotropy in the angular distribution of ions. The maximum ion flux was investigated at an angle of 5o and minimum at an angle of 0o with respect to anode axis. Sadowski et al. [49] studied the applications of intense ion streams emitted from high current pulsed discharges produced with different DPF facilities for materials engineering. Results of the experiments demonstrated that the intense pulses of electromagnetic and corpuscular radiation could be used not only for basic plasma studies, but also for research on the interaction with various constructional materials. The

16

pulsed ions beams and plasma streams could be temporally separated by the placement of material specimens at an appropriate distance from the electrode outlet. With multiple shot exposures, the surface of the specimens could be restructured by the combined effect of radiation and thermal processes, eventually changing the concentration of the material elements, and thus modifying various surface characteristics. Ion implantation and surface modification of different materials using energetic ions of DPF has been studied by various research groups worldwide in the last decade. Feugeas et al. [50] implanted AISI 304 stainless steel with nitrogen ions by employing a 1 kJ dense plasma focus device. The nitrogen ion implanted samples showed a reduction in wear of 42 times as compared to the unimplanted ones. X-ray diffraction and X-ray photoelectron spectroscopy analyses of nitrided samples showed the Fe2N formation in an almost homogeneous fashion, with about 0.4 mm thick superficial layer. The use of DPF device as an ion implanter was found to be feasible with the submissions of designing and constructing similar systems that could perform nitriding in short time intervals. Pulsed ion implantation of nitrogen into pure titanium was studied by Feugeas et al. [51] by employing high current, short length (300 and 400 nsec) ion beam pulses. They investigated that total accumulated ion fluences of 7´1014 and 7´1016 cm-2 showed a heating effect with significant compositional and physical changes in the near surface region of the titanium samples. XPS studies revealed the formation of TiN0.8 independent of the total range of fluences, with an increase in the superficial microhardness, when short pulse lengths were used. For the case of long pulse length, a correlation between the N/Ti concentration ratio, the binding energy difference (Ti2p3/2 – N1s) and the x value in the stoichiometry of the TiNx compound was established. Sanchez et al. [52] investigated the thermal effect of ion implantation into pure titanium, stainless steel and copper with ultra-short duration ion beams by employing a coaxial plasma gun. The temperature profiles and their evolution during and after nitrogen ion implantation were investigated by finite difference method. Nitrogen ion implantation (fluence of 1013 cm-2 and pulse time of 400 nsec) in pure titanium showed a melting layer of 20 nm after the first 200 nsec of implantation, followed by a fast cooling rate. Thermal gradient of 1000 K mm-1 and a heating rate of 5 K nsec-1 were reported.

17

Feugeas et al. [53] reported the significant improvement in wear performance of stainless steel and titanium owing to the ion implantation with DPF. A reduction in the wear rate of stainless steel by a factor of 10 and of high speed steel by a factor of 2 along with the reduction in friction coefficient was reported. XPS and XRD measurements also established the presence of high nitrogen concentration in the surface layers with implantation providing desired stoichiometry for tribological purposes. Results showed that heating rate of ~15 K nsec-1, temperature gradients of 1500 K mm-1 and peak temperatures near the melting point played an important role in the surface modification process. Piekoszewski et al. [54] employed intense nitrogen ion pulses of about 1 msec duration having energy density of ~5 Jcm-2, for the surface modification of pure iron. Nuclear reaction analysis (NRA), XRD and conversion electron Mossbauer spectroscopy (CEMS) were selected for the characterization of the treated specimens. The results revealed the gradual transformation of original ferrite a-phase of iron into austenitic g-phase, along with the presence of expanded austenite phase gN owing to the incorporation of nitrogen. A treatment of 20 pulses resulted in almost complete phase transformation with a retained nitrogen content of 5.5´1017 cm-2. The analysis of the ratio of g to gN phases as a function of the nitrogen content confirmed the presence of strong repulsive forces between the first and the second nearest neighbor nitrogen atoms in the fcc austenitic structure formed as result of nitriding of ferrite iron. Sartowska et al. [55] employed high intensity (5-6 Jcm-2), short duration (~msec) argon mixed nitrogen plasma pulses for the surface modification of carbon steels. Conversion electron Mossbauer spectroscopy (CEMS), scanning electron microscopy (SEM) and Vickers micro-indentation measurements were employed for the characterization of the modified layers. The paramagnetic phases were detected in the modified surface layers along with the nitrogen content in the depth depending on the carbon concentration in the material. The nitrogen expanded austenite phase (gN) was found in the surface region of Armco and carbon steels when exposed to nitrogen discharge, despite the depletion of Cr and Ni caused by the precipitation of their nitrides.

18

Marques et al. [56] studied the effect of ion implantation on austenitic stainless steel AISI 304 with argon admixed nitrogen discharges. XRD measurements in terms of phase analysis, crystallographic texture, and in depth residual stress revealed that the ion implantation modify the surface properties without producing a new phase and affecting the crystallographic texture. However, the initial compression residual stress profile was changed to a tensile residual stress profile after ion implantation. It was investigated that a residual stress gradient was induced in the implanted surfaces and became more significant with the increase of ion beam fluence. Kelly et al. [57] deposited TiN thin films on AISI 1010 and AISI 304 stainless steel samples by utilizing ion beams from a low energy (10.5 mF, 30 kV) PF II device having a titanium insert at the anode tip. The ambient gas pressure of nitrog en was in the range from 0.1 – 0.3 mbar. The ion nitrided samples were characterized by X-ray diffraction, scanning electron microscopy equipped with energy dispersive X-ray spectroscopy, electron probe X-ray micro-analysis and X-ray photoelectron spectroscopy. The results confirmed the formation of thin and well-adhered TiN coatings. It was investigated that the ion induced strong heating of the superficial layers was favorable for the diffusion of subsequent impinging ions. Thin carbon films were deposited on glass, silicon and quartz substrate by ablation of graphite target using highly energetic ions of a 3.3 kJ dense plasma focus by Kant et al. [40]. The films were characterized for their surface profile, structure and chemical composition using a surface profilometer, XRD, Raman spectrometer and electron spectroscopic composition analyzer (ESCA). The results recognized the deposition of crystalline graphite films on silicon and quartz substrates but amorphous carbon films on glass substrates. ESCA investigations revealed that the relative proportion of carbon content in the film decreases with the increase of the target exposure height. The C1s ESCA as well as Raman spectra confirmed the presence of sp2 hybridized carbon bonding along with the presence of oxygen owing to the oxidation of carbon film. Nayak et al. [39] executed the surface nitriding of graphite in order to synthesize carbon nitride coatings by employing nitrogen ions of 100 nsec short duration pulses coming from a 2.2 kJ Mather type dense plasma focus device. Circular specimens of 2.5 cm

19

diameter and 1 cm thickness were exposed to 20 and 30 focus shots for nitriding. XRD investigations showed the presence of CNx and C3N4 phases. SEM and optical microscopic studies of the nitrided graphite surface revealed a rounded and island like morphology of the surface. The C1s and N1s scans of XPS spectra confirmed carbon nitrogen bonding in the film. From the absorption peak at 1274.4 cm-1 in IR spectra, the presence of C3N4 (sp3) phase was confirmed. Microhardness measurements showed an approximately three- to four-fold increase in the surface hardness of the nitrided surfaces. Rawat et al. [42] deposited titanium carbide (TiC) thin films on 304-stainless steel substrate at room temperature by using the energetic ions emanated from a 3.3 kJ plasma focus discharge in argon-acetylene mixture (7:3 ratio). Substrates were positioned at axial distance of 11.5 cm from the tip of a titanium anode. TiC thin films deposited with 10, 20 and 30 focus shots, were analyzed for their microstructure, surface morphology, surface profile, elemental composition and distribution by using XRD, SEM, surface profiling and EDS techniques. XRD patterns confirmed the successful growth of as-deposited polycrystalline films of TiC. SEM images demonstrated a smooth film surface with increasing clustering of TiC grains when the ion doses were increased. EDS analyses confirmed the carbon as a constituent of the deposited films, while the elemental mapping recognized a uniform distribution of TiC over the steel substrates. Rawat et al. [58] also deposited TiN thin films on the AISI 304–stainless steel substrates at room temperature by using nitrogen ions emitted from a 3.3 kJ plasma focus hollow Cu anode. TiN films were deposited with different number of shots (10, 20 and 30) at repetition rate of one shot per 3 minute, at different axial and angular positions with respect to the anode axis. Surface microstructure, surface morphology, elemental composition and distribution, and surface hardness profile of the deposited films was investigated by employing XRD, SEM, EDS and nanoindentation measurements. XRD patterns confirmed the growth of as-deposited polycrystalline TiN thin films along with an iron chromium nickel phase for the samples surfaces treated along the axis only. Crack free films having uniform distribution of the grains were presented by the SEM images. Conglomeration of the TiN grains was attributed to the increased ion flux. EDS spectra confirmed the constituent elements of the film whereas the Ti Ka/Fe Ka ratio was found

20

to decrease with the increase of axial as well as angular position and vise versa. EDS maps exhibited a uniform spatial distribution of TiN on the samples surfaces. The microhardness of the TiN deposited samples was significantly improved. TiC thin films were deposited on titanium by Gupta et al. [59] using a 3.3 kJ Mather dense plasma focus device equipped with a graphite insert at the anode tip. The titanium samples placed at the axial distance of 2.5 cm with respect to anode tip were irradiated by argon and carbon ions of DPF with 10, 15, 20, 25 and 30 focus shots. XRD spectra revealed the deposition of a multiphase TiC film along with the carbon ion implantation. The average grain size estimated from the broadening of a typical diffraction peak of TiC films deposited with 10 and 30 shots was found to be 45 and 66 nm respectively. The surface morphology of the films was investigated by the SEM, which illustrated a uniform distribution of TiC grains of sizes 40 – 60 nm and 50 – 90 nm on the samples treated with 10 and 30 shots respectively. The growth of TiC and, hence the surface hardness of the substrates was found to be increased by increasing the number of shots. Sadiq et al. [60] carried out room temperature nitriding of aluminum by utilizing short pulses of ion beams emitted from a low energy (1.8 kJ) Mather type plasma focus device. Mechanically polished polycrystalline aluminum specimens were mounted axially above the anode tip at 8 cm and were irradiated with 5, 10, 15, 20, 25 and 30 shots. XRD spectra confirmed the evaluation of fcc phase of AlN which increases with ion doses and eventually attained the maximum proportion for ion doze of 20 shots. The chemical composition of specimens was analyzed using electron probe microanalysis (EPMA). The results showed that the nitrogen content in the film increased with the increase of the ion doze. SEM images revealed that the higher ion doses produced the damage of the surfaces, leading to crack formation and surface roughness. Vickers microhardness of the ion implanted samples was improved significantly (300%) owing to the formation of cubic AlN phase. Rawat et al. [61] employed the energetic argon ions emitted from a 3.3 kJ Mather type DPF device for the crystallization of an amorphous lead zirconate titanate (PZT) thin films. The as-grown PZT thin films of different thickness were exposed for a single focus shot at different axial positions. XRD patterns demonstrated the crystallization of the

21

amorphous PZT films having dependence on axial positions (4.0, 5.0 and 5.8 cm). The grain size estimated from the width of typical diffraction peaks was ranging from 13 nm to 30 nm corresponding to axial positions of 5.8 and 4.0 cm respectively. SEM micrographs showed reasonably smooth surface of ion treated PZT films. It was suggested that the phase transformation of PZT thin films was induced by the bombardment of argon ions imparting energy and stress. Amorphization of crystalline CdS thin films was performed by Sagar et al. [62] by irradiation of energetic argon ions emanated from a low energy (3.3 kJ) Mather type DPF device. The systematic XRD analysis of ion irradiated CdS films showed significant structural changes in the films. SEM micrographs demonstrated a large number of pitholes in the form of black spots on the ion irradiated films because of melting and resolidification of the CdS films. The ESCA of amorphized films showed no stoichiometric change at any depths. Agarwala et al. [63], reported a complete phase transformation from hematite to magnetite phases in iron oxide thin films by utilizing energetic argon ions emitted from a 3.3 kJ Mather type plasma focus with a single shot. The hematite or iron oxide (a-Fe2O3) films were prepared by spin coating on glass substrate, using the gel obtained from iron (III) nitrate as precursor and 2-methoxy ethanol as solvent. The films were ion irradiated at different axial distances from the tip of the anode. XRD patterns of the films irradiated at a typical axial distance revealed the transformation from a-Fe2O3 to Fe3O4 phase. SEM micrographs of the Fe3O4 films exhibited columnar grains having dimensions of the order of 50 to 90 nm. The transformation from non-magnetic a-Fe2O3 to magnetic Fe3O4 phase was also confirmed by the magnetization curves. Kant et al. [41] deposited crystalline fullerene films on Si(111) substrates using highly energetic and high fluence argon ions emitted from a low energy (3.3 kJ) Mather type dense plasma focus. A graphite target ablated by the impingement of argon ions was deposited onto the Si substrate. XRD, high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, FTIR spectroscopy and SEM investigations of the films were carried out. XRD, Raman and FTIR spectra showed the peaks corresponding to fullerenes, mainly a mixture of C60 and C70. The TEM micrographs exhibited crystalline

22

fullerene structures whereas the selected area diffraction (SAD) analysis showed single crystalline spot patterns exhibiting the (110) and (006) planes of the hcp C60 clusters. SEM micrographs also confirmed the presence of spherical clusters of fullerenes. The changes in antimony telluride (Sb2Te3) thin films induced by the argon ions from a low energy plasma focus device were analyzed by Lam et al. [64]. XPS and SEM studies of the ion irradiated films at different axial positions demonstrated that at smaller distances, the formation of antimony rich non-stoichiometric SbxTe1-x with x = 0.6 was favorable along with Sb2O3. The exposure at higher distances was found to result in negligible oxidation with complete stoichiometry. The crystallinity of the Sb2Te3 films was enhanced by increasing the axial distance from the anode tip. Moreover, AFM and SEM analyses showed the smoother surface for the films irradiated at larger axial distances. Agarwala et al. [65] investigated the enhancement in Tc of superconducting BPSCCO thick films for the first time by employing highly energetic high fluence argon ions emitted from 3.3 kJ dense plasma focus. The BPSCCO films of about 20 mm thickness were prepared by screen printing technique on single crystal (100) MgO substrate, and were exposed at different axial positions for two focus shots. The films obtained after ion treatment are a mixture of phases 2212 (Tc = 85 K) and 2223 (Tc = 110 K), a high Tc phase developed as a results of ion induced melting, atomic mixing and resolidification. The Tc of these films was found to increase by maximum of 15 K. XRD measurements also confirmed the evolution of the high Tc (2223) phase of BPSCCO only, for the films with Tc rise of 15 K. Rawat et al. [66] studied the effect of argon ions irradiation on vacuum-evaporatd asgrown Sb2Te3 films in a dense plasma focus device. XRD, XPS and SEM techniques were adopted for the characterization of structural, compositional and morphological changes.

The

as-grown

films

were

composed

of

both

stoichiometric

and

nonstoichiometric phases with traces of Te. After ion irradiation, the films were transformed to a single stoichiometric Sb2Te3 phase. Investigations revealed that the ion irradiation at axial distances less than 8.0 cm (ion energy ³ 1 MeV) endorsed the formation of nonstoichiometric SbxTe1-x phase and the oxidation of Sb. While the ion

23

irradiation at distances greater than 8.5 cm (ion energy £ 1 MeV) favoured the formation of a single stoichiometric Sb2Te3 phase having homogeneous grain size distribution with some preferred orientations. Amorphization of crystalline Si by using energetic argon ions emanated from a low energy (1.4 kJ) dense plasma focus was accomplished by Sadiq et al. [67]. Raman spectroscopy of crystalline and irradiated samples revealed the disappearance of c-Si LO mode and the appearance of a-Si TO-like mode after exposure to multiple pulses of argon ions, suggesting a complete transformation from crystalline to a-Si. XRD analysis of the treated samples also confirmed the complete amorphization of Si when the samples are exposed with four or more focus shots. The surface morphology of the samples was examined with Jeol-JSM-5910 SEM, and the results revealed that with increasing ion doses the ion implantation process led to the shallow ion-impact crater formation surrounded by lattice disorders owing to the melting and resolidification.

24

CHAPTER # 3

EXPERIMENTAL SETUP AND DIAGNOSTICS 3.0

Introduction

This chapter describes the experimental setup along with the diagnostic techniques employed in the experiments. The experimental setup is comprised of both the UNU/ICTP plasma focus device operational at the GC University Lahore Pakistan and the NX2 repetitive plasma focus device working at the National Institute of Education (NIE), Nanyang Technological University (NTU), Singapore. The preliminary diagnostics are high voltage probe, Rogowski coil, solid state nuclear track detector (SSNTD), BPX65 photodiode and Faraday cup whereas the diagnostics techniques used for the surface characterization of the plasma processed materials are X-ray diffractometer (XRD), scanning electron microscopy (SEM) with energy dispersive Xray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Raman Spectroscopy and microhardness tester.

3.1

The Plasma Focus Device at GCU Lahore, Pakistan

The design of Mather type Plasma Focus device at GC University Lahore is based on UNU/ICTP facility [26], developed by United Nation University fellows at Plasma Physics Laboratory University of Malaya, Malaysia. The plasma focus consists of the following subsystems: ·

Focus tube (electrode system and discharge chamber)

·

Vacuum and filling gas systems (vacuum pumps, valves, gauges, working gas containers etc.)

·

Drivers (high voltage chargers, capacitors, switches and high voltage cables)

·

Control and triggering electronics (pulse generators, silicon controlled rectifier, trigger unit and pulse transformers)

25

·

Diagnostics (Rogowski coil, high voltage probe, SSNTDs, BPX65 photodiode and Faraday cup)

·

Data acquisition system (digital storage oscilloscopes, computers and interface/data acquisition softwares)

The schematic arrangement of the Mather type DPF device working at GCU Lahore, Pakistan [28], powered by a single Maxwell 32 mF, 15 kV capacitor and used as a source

Coaxial cable

of energetic ion beams for the treatment of material samples, is shown in the figure 3.1.

Figure 3.1: Schematic diagram of the DPF device working at GCU Lahore, Pakistan used for materials processing

3.1.1

Plasma Focus Tube

The Plasma focus electrode system is similar to one reported earlier [68]. It is a coaxial system consisting of 152 mm long, 18 mm diameter Cu rod, which serves as a central electrode and it extends to a 200 mm diameter Cu plate called anode header. The anode is surrounded by six 9 mm diameter Cu rods with equal length and placed equidistant from the anode forming a cathode of internal diameter 50 mm. These outer electrode rods are

26

screwed to another Cu plate, of diameter 130 mm, which in turn is tightened to 10 mm thick Cu cathode header of 285 mm diameter with a hole of 100 mm at its center. A Pyrex glass insulator sleeve [28, 68] with external diameter 24.5 mm, wall thickness 2 mm and effective length 25 mm is fitted in between the electrodes to support the low inductance breakdown. A 13 mm thick and 100 mm diameter circular rubber pad having a hole of diameter 24 mm at its center is placed to tightly grip the sleeve at its position, so that it does not touch either anode or the cathode base plate.

Figure 3.2: A schematic of plasma focus electrode system

The optimum choice of insulator sleeve length [69, 70] gives an azimuthally symmetric and uniform current sheath and any departure from the optimum value leads to spokes formation on the sleeve surface. For necessary electrical insulation, a thick Perspex disc of 188 mm in diameter along with sufficient layers of Mylar and Polythene is sandwiched between the cathode and anode headers. For transfer of energy from the main energy storage system to the coaxial electrode of the focus system, 18 coaxial cables of 110 cm length, connected in parallel with the cathode and anode headers are used. A view of the plasma focus electrodes arrangement is shown in figure 3.2. The electrode system is housed inside a stainless steel chamber that has ten openings called ports. These ports may be used for evacuation, gas inlet and diagnostic purposes. The plasma focus chamber is evacuated down to 10-2 mbar using a rotary vane pump. A

27

capsule type dial gauge is used to measure the working gas pressure inside the vacuum chamber. The plasma focus is operated in a static filing pressure mode and is evacuated for refilling after five discharges to remove the impurities due to material erosion from the anode surface and the chamber wall. 3.1.2

The Driver for DPF

For the generation of high temperature plasma in a plasma focus device, a sufficiently high current pulse is required. Such type of current pulse can be achieved by discharging a capacitor across a desired load. To generate high current, charging voltage and capacitance of the capacitor should be high whereas its internal inductance should be low. Moreover, it should be connected to the load through a path of minimum circuit inductance. The driver for the plasma focus system at GCU is a single 32 mF, 15 kV (3.6 kJ) Maxwell capacitor, charged at 12 kV (2.3 kJ), giving peak discharge current of about 190 kA. To charge the capacitor, a power supply of ±25 kV is used which can provide a current of 50 mA. The equivalent series inductance of this capacitor is rated at 40 nH. 3.1.3

Sparkgap and Triggering

The energy transfer from the low inductance capacitor to the load in high voltage pulsed power applications requires special types of fast response, low inductance and high voltage switches. Commonly used switches for such purpose are triggertron [70], laser triggered and field distortion sparkgaps [71]. The sparkgap switch used in the experiment is triggertron-type, which is coupled with 32 mF, capacitor. It is machined from nylon rod of 100 mm diameter and the electrodes are made from 50 mm diameter copper rod. A motorbike spark plug with slight modification is used as a trigger pin. To get the low inductance path for the current, six brass bolts tightened coaxially out side the spark gap body provide the return current path. The spark gap is pressurized with compressed air. A schematic view of the triggertron-type pressurized spark gap is shown in the figure 3.3.

28

Figure 3.3: A schematic of triggertron-type pressurized spark gap

A trigger pulse is used to initiate discharge in the spark gap, which in turn acts as a switch to transfer energy from the capacitor bank to electrode assembly. The circuit description of the trigger unit is shown in figure 3.4. A step up transformer T1 is employed, which provides the two outputs 3 kV and 600 V. A chain of 10 diodes rectifies the 3 kV voltage, which is used to charge the capacitor C1 while 600 V is rectified by a bridge circuit to obtain positive dc voltage. This voltage is used to charge the capacitor C2. A manually operated switch is used to discharge a small capacitor, which can give a pulse of 4.5 V. This pulse is applied at the Gate of SCR. SCR conducts and capacitor C2 is discharged through SCR across a step up transformer T2, which then generates a pulse of 25 kV. As this pulse appears across the spark gap S1, the capacitor C1 is discharged through this spark gap across another step up transformer T3, which provides –50 kV pulse in return of –3.5 kV capacitor discharge pulse. This pulse of –50 kV is applied to the trigger electrode of the main spark gap, which serves as a switch to transfer the energy from the capacitor to the electrode system. A 10 V pulse is also obtained by means of a resistor divider from SCR circuit to trigger the oscilloscope.

29

Figure 3.4: The circuit diagram of trigger unit

It is highly unsafe to get close to the charged capacitor to discharge it manually (if it fails to be discharged through the sparkgap). Thus, arrangements are made to ground the capacitor indirectly by means of dumping system. The schematic of the dumping system is shown in figure 3.5. It consists of a pneumatically controlled plunger, which is connected to a pneumatic solenoidal valve. An electric switch controls the operation of the dumping system. The plunger is fitted with a Cu plate that connects or disconnects the charged capacitor to the ground through a resistor depending upon the direction of flow of air pressure behind the plunger. The air flow can be directed by the switch, either ON or OFF.

Supply

Compressed air inlet

Solenoidal control unit P l u n g e r

Copper strip

+

Figure 3.5: Schematic diagram of the dumping system

3.1.4

The DPF Operation

30

The plasma focus device works in the following way. The discharge chamber is evacuated down to 10-2 mbar by a vacuum pup (rotary vane pump) and filled with desired gas at an optimum filling pressure. When the capacitor voltage is applied across the electrodes, a low inductance gas breakdown occurs. A current sheath is developed across the insulator sleeve surface. The resultant current sheath is accelerated axially up the chamber in the shape of paraboloid by the J ´ B force and undergoes radial collapse on reaching the top of the anode. Thus, an extremely hot/dense pinched plasma column is formed with the ion density 1025 – 1026 m-3, which is then disrupted as a result of instabilities producing energetic ions moving towards the top of the chamber owing to the intense electric fields [72]. A resistor divider is used as high voltage probe to monitor the focussing of plasma, and a Rogowski coil to record the discharge current. The electrical signals are recorded by using a TDS 500MHz Digital Phosphor Oscilloscope.

3.2

The Repetitive Plasma Focus Device-NX2 machine at the NIE, NTU, Singapore

3.2.1

Introduction:

So far, most of the efforts were concentrated towards optimizing a particular device for operation as a single shot radiation source, with the hope that in this way the efficiency of the device can be increased. After intensive development work, done both on the UNU/ICTP plasma focus device and the NX2 device in its original configuration, based on experiments and measurements done using ultrafast time-resolved multi-frame CCDbased laser picture capturing system, as well as numerical simulations, the NX2 was completely redesigned [73]. Subsequent tests and measurements proved that in the new configuration, the NX2 device is the most powerful multiple shot radiation source in its class, as well as the most efficient X-ray and neutron source (when it is seen as a single radiation source). The schematic diagram of the repetitive plasma focus-NX2 machine working at the NIE, NTU, Singapore, used for materials treatment, is shown in the figure 3.6. The subsystems of the NX2 machines have different design parameters and specifications, discussed in the following sections. The sample treatment with NX2 also utilizes a sample holder arranged with the help of a Wilson seal as show in the schematic diagram. 31

Figure 3.6: A Schematic of the NX2 machine at the NIE NTU, Singapore, used for materials processing

3.2.2

Novel Features of the Device

The optimized electrode design, with a cylindrical part followed by a tapered region, enables the use of the machine in a very wide range of conditions, without an important decrease of the emission parameters. The fact that the tapered section remains unchanged (in shape and dimensions), regardless of the experimental conditions, simplifies the construction of the anodes; the only changing parameters of the electrode system are the length of the cylindrical part of the anode and the length of the insulator sleeve. The device can be tuned for single shot operation, or it can operate as a radiation source for more than one of the following modes simultaneously: X-rays, electrons, ions and neutrons (depending on the operating conditions). 3.2.3

Description of the NX2 Configuration

32

Every part of the device as well as the technical solutions used were intensively studied and modified in order to combine cost effectiveness, reliability, maximized output and low maintenance. The high voltage capacitor bank makes use of 46 small capacitors connected in parallel to replace the six big capacitors used in a classic arrangement. The advantages of this configuration are: ·

lower inductance, which leads to a higher value of discharge current

·

lower maintenance cost and shorter run down time

·

possibility to operate even if a number of capacitors fail, due to the small change in parameters

·

increased lifetime for each capacitor, since they are operated far from the maximum constructive parameters

The capacitor bank can be charged by any commercially-available high-voltage current charger, able to give at least 15 kV charging and minimum 10 kW electric power. The high voltage switching (trigger) system makes use of pseudo-spark switches (PSS). These innovative types of switches have an extremely high lifetime – more than 106 shots. A pseudo-spark switch comprises of a gas filled chamber with anode, cathode, trigger electrode and an injector of electrons. The last one needs a heater of a low voltage, high current power supply. The NX2 system employs four PSSs. For reliable switching of the PSSs in repetitive mode, an original generator device – the nanosecond master trigger (NMT) is employed, which can control the PSS with the repetition rate of at least 10 Hz and a jitter of about 3 ns. It consists of a thyristor pulse generator that fires up a spark generator consisting of one pair of cables for each PSS (so four pairs in the present case; four charging cables and four discharging cables), as well as two electrodes positioned in a spark chamber. In this case, a rising voltage pulse of appropriate amplitude is delivered from the generator to the charging electrode and also to the cable. The gap between the spark electrodes can be tuned to obtain self-breakdown between the electrodes at a desired voltage. So in this scheme, the electronic generator plays two roles at the same moment; it works both as a charger for the cables and as a trigger for the sparkgap. Another way to achieve the triggering is to charge the cables

33

upto a voltage of about 15 – 20 kV, while the gap between the electrodes is tuned in such a manner that no breakdown occurs unless the gap is triggered using a negative pulse of appropriate amplitude coming from an auxiliary generator, through a triggering electrode in the spark chamber. Regardless of the method used (with or without triggering circuit), after the breakdown the charging cables transfer their energy to the discharging cables as a pulse with duration equal to a double transient time of the cables. The pulse duration determines the energy delivered to the PSS firing electrode. The length of the pulse is determined by the length of the cables, while the rise time and the sloping of the pulse are determined by the characteristics of the spark gap, configuration of the spark chamber and quality of the coaxial cables. The length of the charging and discharging cables has to be equal for all cables with the accuracy better than 10 cm to ensure the jitter of firing signals at the PSS less than 1 ns. In order to provide safety to the heaters and the NMT used in PSS from a returning high voltage spikes after the firing of the discharge, we use: ·

the separation transformers (in transformer oil) and

·

the high permeability ceramics magnetic material for the cables of the NMT

The second one works in the following manner: a high frequency pulse generated by the NMT travels freely inside the coaxial cables to the PSSs and fires them. This happens because the current flows in opposite direction in the two conductors of the transmission line and thus the magnetic field is confined in the region between the conductors. Therefore, the voltage pulse is unaffected by the high permeability ceramic. However, the voltage spike generated later in the device because of the current rupture phenomenon cannot propagate along the paths formed by the earth parts of the cables due to the inductive impedance of this path, where the current travels in the same direction in both conductors of the transmission line. Another very important feature of the NMT is the protection of the electronic circuits against the high voltage pulses generated in the PF discharge. This is done, apart from winding all the high voltage cables (including the ones from the charger and NMT generator) around high permeability ceramics and the isolation transformers used in the heaters, by a special safeguard chain, used in the scheme of the NMT. It operates in the 34

following manner: a high current, high voltage diode chain is connected in parallel across the output of the NMT. The NMT is operated so that the polarity of its output is opposite as compared to the high voltage pulse produced by the PF. The diode chain is thus connected is reverse bias for the NMT and therefore presenting no load to the NMT. However, the chain is forward biased with respect to the pulses generated by the PF and thus presents a short circuit to these pulses, preventing them from reaching the NMT. The main chamber of the NX2 machine has eight radial ports, for electromagnetic radiation extraction, and a top port on which axial windows can be mounted. The chamber materials are resistant to any gas intended to be used in the experiments, and the construction is suitable for ultra-high vacuum. The small size of the main chamber helps to reduce the gas consumption and the radiation/particle absorption. The focus tube consists of two coaxial electrodes separated by an insulator sleeve. The central electrode or anode is made of copper surrounded by brass cathode built in an open squirrel cage structure, and is able to withstand repeated mechanical and thermal shocks. The Pyrex glass insulator sleeve can also withstand such shocks. For repetitive operation, the anode is cooled using circulating de–ionized water and can dissipate heat without releasing an important amount of impurities in the chamber atmosphere. The anode is tapered towards the open end, for higher final velocity of the collapsing plasma sheath. An advantage of this geometry as compared to other devices is that the tapered region of the anode has fixed dimensions, regardless of the operating conditions. The length of the cylindrical part varies, depending on the operating gas. The operating parameters of the NX2 machine used in the experiment are summarized in the table 3.1. Table 3.1: The operating parameters of the NX2 device Capacitance

C0 = 27.6 mF (0.6 mF ´ 46)

Operating charging voltage

V0 = 13.5 kV

Maximum charging voltage

V0 = 15 kV

Maximum stored energy

E = 3.1 kJ

Inductance of circuit

L0 = 26 nH

35

Impedance

Z0 = (L0/C0)1/2 = 31 mW

Circuit resistance

3.28 mW

Maximum current

I0 = 430 kA

T/4 time of short circuit

t = 1.33 ms

Operating repetition rate

f = 0.5 – 1.0 Hz

Maximum repetition rate

f = 16 Hz

Anode (copper) length

la = 45 mm

Anode radius

a = 15.5 mm

3.3

Preliminary Electrical Diagnostics

In the DPF device, plasma heating is achieved by the passage of intense current pulses through the plasma. This heating mechanism involves the magnetic compression that is inductive in nature and the Joule heating that is resistive in nature [74]. In any case, one may consider the plasma as an active element in the discharge circuit with its electrical properties represented by the combination of a variable resistor (Rp) and a variable inductor (Lp), which is illustrated in the equivalent circuit of the DPF device (figure 3.7) where Re and Le are the external circuit resistance and inductance respectively. When the switch S is closed, the energy is initially stored in the capacitor C and then discharge through the circuit.

Figure 3.7: Equivalent circuit of DPF device

36

This discharge current flowing through the circuit is mainly affected by the conditions of the plasma. Thus a measurement of the discharge current reveals information concerning the plasma. The transient voltage across the plasma is directly related to the plasma conditions. The combined interpretation of the measured current and voltage waveforms is usually sufficient for basic dynamics study. In the present experiment, a Rogowski coil and a resistive voltage divider are used to display the current and voltage waveform respectively, which are explained subsequently. 3.3.1

Rogowski Coil

High voltage applied across the electrodes causes discharge along the surface of cylindrical insulator sleeve. During this period, the internal resistance of the system becomes very small and high transient plasma current of the order of several hundred kA is passed through the device. Conventional current measuring equipment cannot measure such a large transient current. The most simple and effective device commonly used to measure such a high current is Rogowski coil [75], an indirect current measuring device. It works on the principle of Faraday’s law of electromagnetic induction. It is an air cored toroidal coil placed around the current carrying conductor. The time dependent magnetic field produced by the varying current induces voltage in the coil, which is proportional to the rate of change of current. The combination of the coil and an integrator provides an exceptionally versatile current-measuring system. One of the most important properties of a Rogowski coil measuring system is that it is inherently linear. The coil contains no saturable component and the output increases linearly in proportion to current right up to the operating limit determined by voltage breakdown. The integrator is approximately linear for periods much less than its RC time constant. Linearity makes Rogowski coil easy to calibrate because a transducer can be calibrated at any convenient current level and the calibration will be accurate for all current values. The output from the integrator can be used with any form of electronic indicating device that has input impedance greater than about 10 kW such as a voltmeter, oscilloscope, transient recorder etc. The direct output from the coil is given by Vout =MdI/dt, where M is the mutual inductance of the coil and dI/dt is the rate of change of

37

current. To complete the transducer, the voltage is integrated electronically so that the output from the integrator is a voltage that accurately reproduces the current waveform. The Rogowski coil used in experiments is a small multi turn solenoid, which is bent, in a toroidal form as shown in figure 3.8.

Figure 3.8: A schematic of (a) design (b) working and (c) Passive integrator circuit of Rogowski coil

The output voltage is proportional to the time derivative of the total current passing through the coil, irrespective of the spatial distribution of the current. The two conditions, which must be satisfied, are:

·

The length of the solenoid must be larger than the minor radius of the toroidal coil,

·

The variation in the magnetic field over the spacing of one turn must be negligible [76], i. e. DB B

> 0 2 . Thus, the equation (2.9) becomes L0 C 0 4L0

I(t) =I0 sin [

t L0 C 0

] exp (-

R0 t ) 2 Lo

(3.5)

The above equation shows the sinusoidal profile of the current having .exponential decay with time.

39

Let "T" be the time period of oscillations. Taking the first positive peak as a reference point, the voltage decreases from peak value V0 to V1 during the first half cycle. The amount of charge flowing through the circuit during the first half cycle is given by

where “f =

Q = C0V0 +C0V1

(3.6)

= CoV0 (1+f )

(3.7)

V1 ” is called reversal ratio which is always less than unity. V0

The peak value of the current can be calculated from the equation

I0 = (

I0 =

w )C V ( 2 0 0 1+

f)

pC0V0 (1 + f ) T

(3.8)

(3.9)

The external inductance can also be calculated by using the formula w=

Since w =

1 L0 C 0

(3.10)

2p , then T

T= 2p L0 C 0

(3.11)

where C0 is the capacitance of the capacitor bank, L0 is the parasitic inductance and T is the oscillation period observed on the oscilloscope. 3.3.2

High Voltage Probe

High voltage of the order of several hundred kilovolts is developed due to a rapid change in inductance during the radial collapse of the current sheath beyond the face of anode. The voltage developed is several times the charging voltage, which cannot be measured by the conventional voltage measuring equipment. However, a simple resistor divider, called high voltage probe, may be employed to record such a high voltage. The schematic diagram of a typical high voltage probe is shown in figure 3.9.

40

The high voltage (HV) probe is constructed for voltage measurement of plasma focus system, by resistive dividing technique. It is assured that the error in values of resistors is less than 5% and response time is lower than 15 ns. The main error is related directly to resistor quality but the error is in the acceptable limits. It consists of a series of low inductance resistors, 10 pieces of 560 W resistors and a shunting resistor of 51 W, each with a power rating of one watt. The use of 10 pieces of 560 W resistors instead of a single 5.6 kW resistor is for the safety of the oscilloscope. During operation, in case of single 5.6 kW resistor, it may be damaged, resulting either short-circuited or open circuited. If the resistor is short-circuited, the high voltage will appear to the oscilloscope, which may be damaged. However, there is a little chance that all of the 560 W resistors are damaged and short-circuited simultaneously. The whole configuration is enclosed in a copper tube of about half inch diameter, which is at ground potential. The electrical insulation between the high voltage point and the grounded copper casing is very crucial and therefore special attention is paid in this regard. The high voltage probe cannot be connected directly to the plasma so we connect it to the anode and cathode headers. The output signal across 51 W is monitored by transmitting it to oscilloscope through a coaxial cable of the same characteristic impedance. Observing the signal displayed on the oscilloscope at the time of radial collapse, one can monitor the status of the focus. The signal may also provide information about the delay in breakdown. While employing high voltage probe with the system, the scope should not be placed directly in series with the higher resistors. The first resistor should be in parallel with the scope and ground with the scope chassis. If the connection to the scope becomes open then there will be no shock hazard. However, a very good reason to put a 51 W termination resistor is across the input to the scope is to avoid the reflections in the cable.

41

Figure 3.9: The schematic diagram of a typical high voltage probe

3.3.3

BPX65 Photodiode Detector

Photodiodes are light sensitive p-n or p-i-n semiconductor devices manufactured in essentially the same way as semiconductor diodes which are frequently used for the detection of light. The device with an intrinsic layer is called p-i-n or PIN photodiode. Light absorbed in the depletion region or the intrinsic region generates electron-hole pairs, most of which contribute to a photocurrent. Photodiodes can be operated in two different modes: Photovoltaic mode: The illuminated photodiode generates a voltage across a load

1.

resistance, which is measured. However, the dependence of this voltage on the light power is rather nonlinear, and the dynamic range is quite small. Photoconductive mode: A reverse bias voltage is applied to the diode and resultant

2.

photocurrent is measured. The dependence of photocurrent on the light power can be very linear over six or more orders of magnitude of the light power. The reverse voltage does not affect the linear behavior of photocurrent and only tends to make the response of the diode faster. Photodiodes offer many conveniences and advantages that make them very realistic for a wide range of applications: ·

They can easily measure optical power from picowatts to milliwatts

·

They are available in package that can be tooled to fit any application

·

They can detect wavelengths (190 to > 2,000 nm ) 42

·

They are small and light weight and inexpensive

·

Almost any photosensitive shape can be fabricated

·

They have very reproducible sensitivity

·

Very large areas can be fabricated (> 10cm2, but with cost increasing with area)

·

They can be very responsive, with risetimes as fast as 10 picoseconds

Photodiodes are widely used in applications ranging from sensors for door openings, assembly line controls, load levelers in luxury cars, to personal blood sugar meters for diabetics, sun-tan exposure meters, smoke detectors, x-ray baggage inspection systems and even cranial pressure sensors for head injury patients. A PBX 65 photodiode detector is used for the detection of ion induced current, for the first time. Figure 3.10 depicts the BPX65 diode detector along with the schematic of the biasing network used in the experiment. The protective glass of the diode is removed enabling the ions and soft Xrays photons to reach its active area. The diode has a response time of 12 nsec with an active layer of thickness 10 mm. A 600 µm diameter aperture with area of 0.28 mm2 limits the ion beam flux striking the detection area of the diode. The ion induced electrical signal is coupled to the oscilloscope via a 50 Ω coaxial transmission line. (a)

Figure 3.10: (a) A BPX65 photodiode and (b): Schematic arrangement of biasing circuit for the photodiode

3.3.4

Faraday Cup

A Faraday Cup (FC) is a device for measuring the current in a beam of charged particles. In its simplest form, it consists of a conducting metallic chamber or cup, which intercepts a particle beam. An electrical lead is attached which conducts the current to a measuring

43

instrument. Detection can be as simple as an ammeter in the conducting lead to ground or a voltmeter or oscilloscope displaying the voltage developed across a resistor from the conducting lead to ground. A bias voltage is applied to the cup to prevent secondary electron emission from distorting the signal. The design can be significantly more complicated when it is necessary to make measurements of very short pulses or very high energy beams which may not be fully stopped in the thickness of the detector. The name of the device is intended to honor Michael Faraday. The FC has been employed for measuring the nitrogen ion beams of the UNU/ICTP DPF device in the present work. The FC collector in different configurations (flat plate, deep cup and honey comb) was attempted by Pearlman et al. [77], who concluded that the deep cup configuration showed the least effect of secondary electron emission. Therefore, a deep cup collector is used for the estimation of ion parameters. Figure 3.11 shows the schematic arrangement of the FC. The dimensions of the electrodes and the insulation material between them are chosen such that a characteristic impedance of 50 Ω is achieved according to the expression [78] Z=

138.2 K

log10

D where D = 28 mm is the internal diameter of anode, d = 8 mm is the d

external diameter of cathode and K=2.269 is dielectric constant of Teflon. An electrically insulated perforated circular disc of brass with cross-sectional area of 3.92 mm2 acts as an entrance window, and also avoids escaping of charged particles once they entered the FC.

80 mm

BNC

Brass NW40

100 mm Axially moveable port

50 mm

Anode D = 28 mm Cathode d = 8 mm

Teflon

38 mm

Copper

6 mm

Entrance window

44

Figure 3.11: Schematic arrangement of Faraday cup used in the experiment

The FC is operated in the biased ion collector (BIC) mode with a bias voltage of -60V using the same differentiator circuit. When the ion stream hits the cathode serving as ion collector, a current is generated in the circuit and a voltage V proportional to the incoming ion flux, is developed across the resistor R . TOF technique [79] is employed for the evaluation of ion parameters. The nitrogen ion velocity is estimated by taking the ratio of the distance to the flight time of ions from the source to the detector. The flight time of the ions is evaluated by correlating the ion beam pulse with the X-ray pulse emitted due to the intense electric fields at the time of maximum compression during the pinch phase. The ion velocity v thus estimated, is used in the calculation of the energy of the ions reaching the detector at different instants of time by using the expression E=

1 2 mv 2

(3.12)

where m is the atomic mass of nitrogen ions. The peak ion number density N d having velocity v and charge q can be written

Nd =

V RqAv

(3.13)

where V is the maximum voltage of the ion pulse developed across the resistor R and A is the area of the entrance window. The ion beam current density J is also estimated from the BPX65 diode and the FC signals using simple expression

J= where I =

I A

(3.14)

dQ is the ion current and Q = V dt is the charge collected at the collector. òR dt

To check the efficacy of the results evaluated by both the detectors, the ion beam parameters are also evaluated by employing the empirical relation [57]

45

dN = aE - k dE

(3.15)

where N is the number of ions having energy E of a few keV to several MeV with exponent k in the range 2-5 and the proportionality constant a = 1.12´1019 a.u. Different values of k have been tried for the estimation of the ion number density in the probable energy range. For k = 5, the numerical results reasonably agree with the experiment. Thus, the equation (4) yields the number of ions as: N (E) =

a -4 [ E1 - E2- 4 ] 4

(3.16)

where E1 and E2 are the low and high energy limits corresponding to an energy interval. The mean energy per ion E is calculated by the equation as 1 E = N

N ( E2 )

ò

(3.17)

E ( N ) dN

N ( E1 )

The energy flux (energy deliverance per unit volume per unit time) delivered to the sample by the ions with a certain energy interval is calculated [52] by using the equation Q=

where

Nd =

N Dhs l

Nd E NE = Dt DtDhs l

(3.18)

is the ion number density, N is the number of ions, E is the mean

energy per ion, Dt represents the time of interactions between the ions of this energy interval and the target, Dh is the mean layer thickness, which is estimated with the help of ion penetration range by using the SRIM code [80] and s l is the cross-section of the conic beam at a distance l from the focus given as s l = p (l tan 20 0 ) 2 . After characterization, the ion beams are utilized for the surface treatment of various materials. Different substrates have been used for ion implantation, surface modification, thin film deposition and phase changes. The choice of a material depends upon the growing interest in it and chemically affinity of the material as substrate to be deposited with a thin film. Ti, AlFeZn alloy and SS-321 were selected because of the potential tribological applications of these materials. 46

3.4

Sample Preparation and Treatment

Before treatment of a material substrate, it must be ensured that the substrate sample is suitably prepared for placing it in the plasma focus processing chamber. Especially, the size of the sample must be appropriate for the analysis by the diagnostics like XRD, SEM, XPS etc. Furthermore, the sample surface must be flat, scratch-free and mirror like in order to get the better treatment as well as characterization results. The procedure to be followed in the preparation of a sample mainly involves the following steps.

3.4.1

Sample Selection

The choice of the sample for surface analysis is very important. If the material is soft, such as nonferrous metals or alloys and non-heat-treated steels, the cross-section of the sample may be obtained by manual hacksawing. If the material is hard, the section may be obtained by use of an abrasive cutoff wheel. Low speed precision diamond cutter equipped with a thin disk coated by diamond abrasive, is used for this purpose. The sample is kept cooled by the coolant (machine oil) during the cutting process. 3.4.2

Rough Grinding

If possible, the sample should be of a size convenient to handle. A soft sample may be made flat by slowly moving it up and back across the surface of a flat smooth file. The soft or hard samples may be rough ground on a belt sander, with the sample kept cooling frequent dropping in water during the grinding operation. In all the grinding and polishing procedures, the samples should be moved perpendicular to the existing scratches until the surface is flat and free of nicks, burrs etc. 3.4.3

Mounting

Small and awkwardly shaped samples should be mounted to facilitate intermediate and final polishing. Wires, small rods, sheet metal specimens, thin sections etc. must be appropriately mounted in a suitable material or rigidly clamped in a mechanical mount. Synthetic plastic materials applied in a special mounting press will yield mounts of a

47

uniform convenient size for handling in subsequent polishing operations. The most common thermosetting resin for mounting is Bakelite. 3.4.4

Polishing

After mounting, the sample is polished on a series of emery papers containing successively finer abrasives. In certain cases, the intermediate polishing is also accomplished by using silicon carbide abrasive that has greater particles removal rate. The final approximation to a flat scratch-free surface is obtained by using a wet rotating wheel covered with a special cloth that is charged with carefully sized abrasive particles. Usually g-aluminium oxide for ferrous and copper-based materials, and cerium oxide for aluminium, magnesium and their alloys are preferred abrasives. Other final polishing abrasives are diamond paste, chromium oxide and magnesium oxide. Synthetic polishing cloths (Gamal and Microcloth) are also available. 3.4.5

Etching/Cleaning

Etching of a sample makes visible many structural features of metals and alloys. It is accomplished by the use of an appropriate reagent which subjects the polished surface to chemical action. In alloys, the phase components are revealed during the etching by a preferential reaction of one or more of these constituents by the reagent, because of difference in chemical composition of the phases. In metals or single phase alloys, contrast is obtained and grain boundaries are made visible because of differences in the reaction rate. The prepared samples are ultrasonically cleaned in distilled water, acetone or alcohol environment to remove any residual particulates. The finally prepared samples are mounted at different axial and angular positions in the DPF chamber, to be processed with ion beams. An axially moveable sample holder is fitted with the top plate of the chamber to support the substrate samples. A Wilson seal along with Al shutter is arranged to avoid the exposure of the samples until proper focusing is achieved, which is confirmed by the HV probe signal on the DSO.

3.5

Diagnostics Techniques for Plasma Processed Materials

The surface characterization of materials includes the identification of surface microstructure, surface and near surface atoms and compounds, as well as their spatial

48

profile quantitatively and qualitatively. Although numerous analytical techniques are available, however the most common ones employed for surface treated samples are XRD, SEM & FESEM equipped with EDS, XPS, Raman spectroscopy and microhardness measurements, which are described as below: 3.5.1

X-Ray Diffractometer (XRD)

X-ray diffraction is a versatile, non-destructive analytical technique for the identification and quantitative determination of the various crystalline compounds, known as ‘phases’, present in the solid materials and powders. In X-ray diffraction (XRD), a collimated beam of X-rays is made incident on a specimen and is diffracted by the crystalline phases present in the specimen. The intensity of the diffracted X-rays is measured as a function of the diffraction angle, and the specimen’s orientation. This diffraction pattern is used to identify the specimen’s crystalline phases and other structural properties. XRD can be employed to identify unknown substances, determine crystal structures and phases. It can analyze trace elements, determine phase transformation, detect crystal imperfections, determine compound layer thickness and can evaluate grain size and measure mechanical stresses. A crystal lattice is a regular three-dimensional periodic arrangement (cubic, rhombic, etc.) of atoms in space. These are arranged so that they form a series of parallel planes separated from one another by a distance d, which varies according to the nature of the material. For any polycrystalline material, planes can exist in a number of different orientations-each with its specific d spacing. The condition for a crystalline material to yield a discrete diffraction pattern is that the wavelength of the radiation should be comparable to, or less than the interatomic spacing in the lattice. In practice, this means that only X-rays, high-energy electrons and neutrons can be used to extract structural information of the crystal lattice. 3.5.1.1 Bragg’s law Bragg’s law relates the interplaner distance d hkl and the Bragg angle q hkl ( hkl are the miller indices) as follows: nl = 2d hkl sin q hkl

49

This relation is a convenient form of the geometrical relationship determining the angular distribution of the peak intensities in the diffraction pattern, where ‘n’ is an integer, ‘l’ is the wavelength of the radiation, ‘d’ is the spacing of the crystal lattice and q is the angle, the incident beam makes with the lattice plane as shown in figure 3.12. The assumption made in deriving the Bragg’s equation is that the planes of atoms responsible for a diffraction peak behave as a specular mirror, so that angle of incidence is equal to the angle of reflection. The path difference between the incident beam and the beams reflected from the consecutive planes is (x-y). The angle between the incident and the transmitted beams is 2q, and y = x cos2q. However, cos2q = 1-2Sin2q, while x sinq = d, the interplaner spacing, so that (x-y) = 2dsinq.

y

q 2q d x

q

Figure 3.12: The conditions for the beams reflected from successive planes to be in phase, and hence reinforcing each other, as given by Bragg’s Law

The distance ‘d’ between the lattice planes is a function of the Miller indices of the planes and the lattice parameters of the crystal lattice. For an orthorhombic lattice, for which a ¹ b ¹ c , a = b = g = 900 , the equation for Bragg lattice is [81]:

d 2 hkl =

1 2

2

æhö ækö æl ö ç ÷ +ç ÷ +ç ÷ èaø èbø ècø

2

For cubic lattice, for which a = b = c , a = b = g = 900 the equation for Bragg lattice takes the form:

d hkl =

a (h + k + l 2 ) 1 / 2 2

2

50

In the Bragg equation, l = 2d hkl sin q hkl , the integer n is referred to as the order of reflection. A first-order hkl reflection, (n=1), corresponds to a path difference of a single wavelength between the incident and the reflected beams from the (hkl) planes, a secondorder reflection corresponds to a path difference of twice wavelengths. 3.5.1.2 Working principle of XRD An X-ray diffractrometer is comprised of an X-ray source, a diffractrometer assembly, and X-ray data collection and analysis systems. The diffractrometer assembly controls the alignment of the beam, as well as the position and orientation of both the specimen and the X-ray detector. A schematic of the Bragg-Brentano geometry is shown in figure 3.13. The system requires a monochromatic radiation, generated by exciting K-radiation from a pure metal target and then filtering the beam by interposing a foil, which strongly absorbs the b-component of the K-radiation without significant reduction of the intensity of the a-component. This can be accomplished by choosing a filter, which has an absorption edge falling exactly between the Ka and Kb. A good example is the use of a nickel filter with absorption edge at E=8.33 keV for copper radiation, which transmits the CuKα beam (8.05 keV), but not the CuKβ (8.90 keV). More complete selection of a monochromatic beam can be achieved by interposing a single crystal oriented to diffract at the characteristic Kα peak. This monochromatic beam can then be used as the source of radiation. By varying angle q, the Bragg’s law conditions are satisfied for different dspacing in the polyphase materials. The plot of the angular positions and intensities of the resultant diffracted peaks of radiation produces a characteristic pattern of the sample.

51

Focal circle

Detector

Source

q rotation of specimen

2q rotation of detector

q

2q Sample

Figure 3.13: Schematic representation of a sample mounted on a goniometer stage, which can be rotated about one or more axis and the detector which travels along the focusing circle in the Bragg-Brentano geometry

The result of an XRD measurement is a diffractrogram, showing phases present (peaks’ positions), phase concentration (peaks’ height) and crystalline size/strain (peaks’ widths). For a crystal, planes can exist in a number of different orientations- each with its own specific d-spacing. 3.5.2

Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray spectroscopy (EDX)

The scanning electron microscopy is one of the most versatile and widely used diagnostic of modern science that allows the study of both surface morphology and composition of biological and physical materials. In SEM, a very fine ‘probe’ of electrons with energies up to 40 keV is focused at the surface of the specimen in the microscope and scanned across it in a ‘raster’ or pattern of parallel lines. A number of phenomena occur at the surface under electrons impact: most important for scanning microscopy is the emission of secondary electrons with energies of a few tens eV and re-emission or reflection of the high energy backscattered electrons from the primary beam. The intensity of emission of both secondary and backscattered electrons is very sensitive to the angle at which the electron beam strikes the surface, i.e. to topographical features on the specimen. The emitted electron current is collected and amplified. There is thus a direct positional correspondence between the electron beam scanning across the specimen and the fluorescent image on the cathode ray tube.

52

The magnification produced by scanning microscope is the ratio between the dimensions of the final image display and the field scanned on the specimen. Magnification from 10 to 400,000´, and the resolution (resolving power) between 10 to 4 nm is possible [82, 83]. There are many different types of SEM designed for specific purposes ranging from morphological studies, to compositional analysis. 3.5.2.1 Working principle of SEM A schematic arrangement of the electron beam collimator is shown in figure 3.14. The electron source at the top represents the electron gun, producing a stream of monochromatic electrons. The first condenser lens (usually controlled by the coarse probe current knob condenses the stream. This lens is used both to form the beam and limit the amount of current in the beam. It works in conjunction with the condenser aperture to eliminate the high-angle electrons from the beam. Electron source

First condenser lens

Condenser aperture Second condenser lens

Objective aperture

Scan coils Objective lens

Sample

Figure 3.14: A schematic diagram of the electron beam collimator in SEM

The second condenser lens forms the electrons into a thin, tight, coherent beam and is usually controlled by the fine probe current knob. An objective aperture further eliminates high-angle electrons from the beam. A set of coils then “scan” the beam in a grid fashion, dwelling on points for a period of time determined by the scan speed (usually in the microseconds range). The final lens, the objective, focuses the scanning beam onto the part 53

of the specimen desired to be analyzed. When the beam strikes the sample, interactions occur on the sample surface, which are detected. 3.5.2.2 Electron source (Gun) All microscopes utilize an electron source mostly a thermionic gun as shown in figure 3.15. The gun works as follows: A positive electrical potential is applied to the anode. The filament (cathode) is heated until a stream of electrons is produced. The electrons are then accelerated by the positive potential down to the column. A negative electrical potential (~500 V) is applied to the Whenelt cap. As the electrons move towards the anode after being emitted from the filament, they are repelled by the Whenelt cap toward the optic axis (horizontal center). Filament

Wehnelt cap (negative potential)

Space charge Electron beam Anode plate (positive potential)

Figure 3.15: A schematic diagram of electron source used in the electron beam collimator

A collection of electrons occurs in the space between the filament tip and Whenelt cap. This collection is called a space charge. These electrons then move down the column to be later used in imaging. The process ensures that the electrons used for imaging are emitted from a nearly perfect point (space charge), have similar energies (monochromatic) and are nearly parallel to the optic axis. 3.5.2.3 JSM-6700F field emission SEM (FESEM)

54

The JSM-6700F is a field emission scanning electron microscope (FESEM) incorporating a cold cathode field emission gun, ultra high vacuum, and sophisticated digital technologies for high resolution, high quality imaging of micro structures. The machine is equipped with a conical field emission gun and a semi-in-lens objective lens, and can provide high quality real time image display at all scan speeds, enabling observation and recording of superior images even in a bright room. The system is able to handle samples up to 8 inches in diameter. The JSM-6700F has unique graphical user interface that controls condition setup, motor stage drive, imaging, and data filing, assuring stable and reliable operation. Using superior networking with a host computer, the system facilitates the process from imaging to data processing under optimum conditions, to display real time images, data transfer to an external PC, retrieve and manipulate the data. Difference between field emission and tungsten microscopes Tungsten (or thermionic) SEM uses a tungsten wire filament heated to a high temperature to emit electrons. It has a lower inherent brightness, a larger source size and energy spread, which limit their ultimate resolving power especially at lower accelerating voltages. Typical magnifications are up to 100,000x. FESEM uses a room temperature emitter in ultra-high vacuum, generating an electron beam with a very small size and energy spread. FESEM has very high resolution with magnifications up to >500,000x, and maintains it even at very low accelerating voltages (