Chapter 2 High-Temperature Superconducting Tapes ... - OhioLINK ETD

DIRECT DEPOSITION OF c-AXIS TEXTURED HIGH-TC YBCO SUPERCONDUCTING THICK FILMS ON UNORIENTED METALLIC SUBSTRATES

A thesis submitted to the Division of Research and Advanced Studies of the University of Cincinnati

in partial fulfillment of the requirements for the degree of

MASTER OF SCIENCE in the Department of Material Science & Engineering of the College of Engineering

2000 by Xuejun Wen Master of Engineering, Zhejiang University 1997 Bachelor of Medicine, Henan Medical University 1994

Committee Chair: Dr. Donglu Shi

ABSTRACTS

In this thesis, first two chapters give brief introduction to several aspects of superconductivity, including the concepts, history and applications of superconductivity, and fabrication techniques for long and uniform high-temperature superconductors tapes/wires. Last four chapters present detail information about my research in developing c-axis textured high-Tc YBCO superconducting thick films on unoriented metallic substrates

Previous work in the development of YBCO superconducting wires and tapes has been focused on the deposition of YBCO on buffered metallic substrates. Although such an approach has proved successful in terms of achieving grain texturing and high transport current density, critical issues involving continuous processing of long-length conductors and stabilization of the superconductor have not yet been entirely settled.

In this work, a novel process Direct Peritectic Growth (DPG) approach was used to directly deposit c-texture YBCO thick film onto an unoriented silver alloy. No buffer layer is employed between the YBCO superconducting film and the metallic substrate. The textured YBCO grains have been obtained through peritectic solidification over a wide range of temperature and time on Ag10%Pd alloy substrate. No observable reaction of the Ag10%Pd substrate was found with the YBCO melt at the maximum processing temperatures near the peritectic point (from 950 °C to 1030°C).

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In order to investigating YBCO thick film growth dynamics, quenching experiments were carried out near the peritectic temperature for YBCO thick films on the Ag10%Pd substrate using DPG method. The initial YBCO morphology exhibits a column-like grain structure as a result of rapid a-growth when quenched from 1000 °C (the sample was pre-melted at 1030°C). A waffle-like structure was observed on the surface of the silver alloy substrate as the quenching temperature was lowered to 950°C providing a much greater driving force. We found that a grain-oriented substrate may not be required to achieve the grain texturing in the peritectic-reaction controlled process. During solidification, the YBCO grains will parallely nucleate on the surface of the silver alloy to minimize its surface energy, and grow along the a-axis rapidly resulting in a textured film.

To identify the underlying mechanism of grain texturing, extensive transmission electron microscopy (TEM) experiments were carried out in this study. A thin “buffer” of 300 nm thickness was observed between the textured YBCO and the silver alloy substrate. This “buffer” was identified to be essentially the YBCO structure, however lacking of the superlattice. Initially randomly oriented, the grains of the buffer gradually self-organize into a preferred orientation and eventually grow into a single crystal YBCO.

The

underlying mechanism of crystal evolution on an unoriented substrate is discussed in chapter 5.

To conclude, we have found a structural evolution from the randomly oriented silver alloy substrate that eventually leads to a large grain of YBCO. Within each large grain

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the in-plane texture is nearly perfect. The transport Jc has reached a respectable value of 8×104 A/cm2 at 77 K and in zero magnetic field. This self-orientation mechanism suggests that the formation of ordered YBCO crystal lattices may not require a single crystal substrate in the peritectic solidification.

Such a film growth mechanism is

fundamentally different from the epitaxial growth in the vapor deposition. Based on such a phenomenon, it is possible to induce growth along a specific orientation that is essential in the development of textured YBCO film on randomly oriented substrates. Based on the experimental results in this work, We show that the DPG method offers an effective alternative for the fabrication of long-length YBCO conductors. Also reported is a physical explanation of the texturing mechanism on the metal substrate.

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Acknowledgements

Firstly, I would like to express my sincere thanks to my advisor, Dr. Donglu Shi, for his patient guidance in research skill and in living experience as well. The main research results of this thesis have been published and will be published as research articles, either in journals or the conference symposiums. The achievements are mainly owing to Dr. Donglu Shi’s excellent supervisory.

Many thanks are also go to my supervisory committee for their generous help, reading through thesis and discussions. Dr. Iroh and Dr. Roseman gave valuable input during the whole period of study and spent time working with me on special details regarding the research report here.

I also enjoyed working with the research group under Dr. Donglu Shi’s guidance, Mr. Dehui Qu, Mr. Fang Mei, and Ms. Gengwei Jiang. Their friendly help, discussions and their understandings have made the research environment unforgettable. I am very proud to be one of them.

I am grateful to Dr. M. Tomsic for providing the all metallic substrates; Drs. S.X. Wang and L.M. Wang at the Electron Microbeam Analysis Laboratory at University of Michigan for conducting the TEM work; M. White for the Phi scan work, and S. Mukhopadhyay for some of the SEM work.

Thanks also go to sponsors supporting my research. This work was supported by a grant from National Science Foundation-SBIR program and a grant from ARPA/DSO under the contact No. P881-030-D027-1042.

Special thanks must go to my wife Ms. Ning Zhang and my whole family. Their concern, love, support and help have accompanied me throughout this study.

Xuejun Wen July, 2000

Table of Contents ACKNOWLEDGEMENTS LIST OF FIGURES CHAPTER 1. BRIEF INTRODUCTION TO CONCEPTS, HISTORY AND APPLICATIONS OF SUPERCONDUCTIVITY.......................................................... 1 ABSTRACT .................................................................................................................... 1 1.1 SUPERCONDUCTOR AND SUPERCONDUCTIVITY...................................... 2 1.2 BRIEF HISTORY OF SUPERCONDUCTORS.................................................... 4 1.2.1

Before 1986 ...................................................................................................... 4

1.2.2

1986 and later ................................................................................................ 12

1.3

APPLICATIONS OF SUPERCONDUCTORS. .................................................. 14

1.3.1

Transmission Line .......................................................................................... 14

1.3.2

Electric Motors............................................................................................... 14

1.3.3

High Temperature Superconducting Transformers ....................................... 15

1.3.4

Fault Current Limiters ................................................................................... 15

1.3.5

Super Fast Computer Chips ........................................................................... 15

1.3.6

Levitation........................................................................................................ 15

1.3.7

Accelerator Magnets ...................................................................................... 16

1.3.8 Superconducting Quantum Interference Devices (SQUID) ........................... 17 1.3.9

Power Electronics .......................................................................................... 17

1.3.10 Uniform Long length tapes or wires............................................................. 18 REFERENCES .............................................................................................................. 19

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CHAPTER 2 HIGH-TEMPERATURE SUPERCONDUCTING TAPES/WIRES: MATERIALS AND MANUFACTURE TECHNOLOGY.......................................... 20 ABSTRACT .................................................................................................................. 20 2.2

SUBSTRATE SELECTIONS .............................................................................. 24

2.2.1

Criteria for substrate selection....................................................................... 25

2.2.2 Substrate Materials ........................................................................................ 33 2.3 SUPERCONDUCTING TAPES AND WIRES MANUFACTURE ......................................... 44 2.3.1

Thin Film........................................................................................................ 44

2.3.2

Thick Film Fabrication Techniques ............................................................... 53

2.4 PROJECT OBJECTIVE .............................................................................................. 56 REFERENCES .............................................................................................................. 58 CHAPTER 3 EFFECT OF MAXIMUM PROCESSING TEMPERATURE, PROCESSING TIME AND SUBSTRATES ON THE YBCO THICK FILM BY DPG DEPOSITION ON UNORIENTED METALLIC SUBSTRATES . ................. 63 ABSTRACT .................................................................................................................. 63 3.1 INTRODUCTION ................................................................................................ 65 3.2

MATERIALS AND METHODS ......................................................................... 67

3.3

RESULTS AND DISCUSSION........................................................................... 69

3.4

CONCLUSION .................................................................................................... 81

REFERENCES .............................................................................................................. 82 CHAPTER 4 GROWTH DYNAMICS OF DIRECT PERITECTIC GROWTH YBCO THICK FILM ON AG10%PD SUBSTRATE. ................................................ 83

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ABSTRACT .................................................................................................................. 83 4.1 INTRODUCTION ................................................................................................ 84 4.2

MATERIALS AND METHODS ......................................................................... 85

4.3 RESULTS AND DISCUSSIONS ........................................................................ 87 4.4

CONCLUSION .................................................................................................. 100

REFERENCES ............................................................................................................ 101 CHAPTER 5 STRUCTURE EVOLUTION OF YBCO FROM UNORIENTED SILVER ALLOY SUBSTRATE ................................................................................. 102 ABSTRACT ................................................................................................................ 102 5.1 INTRODUCTION .............................................................................................. 103 5.2

MATERIALS AND METHODS ....................................................................... 103

5.3 RESULTS AND DISCUSSIONS ...................................................................... 104 5.4 CONCLUSIONS ................................................................................................ 111 CHAPTER 6

FUTURE WORK................................................................................ 115

APPENDIX:

MY PUBLICATIONS AT UNIVERSITY OF CINCINNATI........ 116

JOURNAL PAPERS.......................................................................................................... 116 CONFERENCE PAPERS ................................................................................................... 116

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List of Figures Figure 1.1 The Kamerlingh Onnes' Orignal Temperature/Resistance Curve of

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Superconducting Transition in Mercury

Figure 1.2 Experiments showing the difference between a superconductor and a

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normal conductor. Experiment 1: sample cooled in zero magnetic field after which a field is applied. Experiment 2: sample cooled in applied magnetic field

Figure 1.3 The demonstration and schematic graph of Meissner Effect. A

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permanent magnet floats above a superconductor bath in very low temperature liquid gas because its magnetic field is completely repulsed at the surface of the superconductor.

Figure 1.4 The temperature dependence of the critical field Hc(T)

10

Figure 3.1 Heating Schedule for DPG processing

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Figure 3.2 DPG method processed long tape

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Figure 3.3 SEM photograph showing the porous grain structure when the film

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was heated at 980°C for 0.2 hours.

Figure 3.4 SEM photograph showing the c-axis textured grain structure when the

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film was heated at 1010°C for 0.2 hours.

Figure 3.5 SEM photograph showing the 'spiral' growth pattern locally textured

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grain structure when the film was heated at 990 °C for 0.2 hours.

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Figure 3.6 SEM photograph showing the interface between the YBCO thick film

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and the Ag-10%Pd substrate when the film was heated at 1015 oC for 60 min. Note that the interface is sharp and clean without observable reaction layer.

Figure 3.7 SEM photograph showing the interface between the YBCO thick film

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and the nickel substrate. Note that there is an interface reaction layer as indicated by the arrows.

Figure 3.8 XRD spectra for the thick films textured at temperatures indicated.

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Note that the (004) and (006) peaks steadily increase as the temperature has increased from 980°C to 1010°C indicating an enhanced grain texturing. Figure 3.9 XRD spectra for the thick films textured at 1010 oC, holding time

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indicated. Note that the (006) peaks decrease as the treated time has increased from 0.2h to 1.4h.

Figure 3.10 (103) Phi Scan of YBCO thick film on silver alloy (1015oC, 0.6h).

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Figure 3.11 A detail peak of (103) Phi Scan of YBCO thick film in figure 3.10.

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Figure 3.12 A typical sharp Tc of YBCO on silver alloy (Notes: Treated at 1010

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o

C for 0.2hr).

Figure 4.1 Heating Schedule of Group I.

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Figure 4.2 Heating Schedule of Group II for the quenching experiments.

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Quenched from 1030 oC to a given temperature, held for certain time, and quenched again to room temperature.

Figure 4.3 Heating Schedule of Group III for the quenching experiments.

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Quenched from 1030 oC to a given temperature, held for certain time, and cooled at 200 oC/h to room temperature.

Figure 4.4 SEM photographs showing the microstructure of group I samples,

92

Tmax = 950 oC for 20 min., showing a untextured porous grain structure.

Figure 4.5 SEM photographs showing the microstructure of group I samples,

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Tmax = 990 oC for 20 min., showing a small island of locally textured grain structure.

Figure 4.6 SEM photographs showing the microstructure of group I samples.

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Tmax = 1020 oC for 20 min., showing a textured grain structure.

Figure 4.7 The SEM photographs showing the microstructures of group II

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samples, as quenched from 1030 oC.

Figure 4.8 The SEM photographs showing the microstructures of group II

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samples, as quenched from 1000 oC.

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Figure 4.9 The SEM photographs showing the microstructures of group II

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samples, as quenched from 950 oC.

Figure 4.10 SEM micrographs showing the microstructures of group III samples.

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They were quenched from 1030 oC to 950 oC and held for 10 min.

Figure 4.11 SEM micrographs showing the microstructures of group III samples.

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They were quenched from 1030 oC to 950 oC and held for 20 min.

Figure 4.12 SEM micrographs showing the microstructures of group III samples.

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They were quenched from 1030 oC to 950 oC and held for 120 min. followed by a cooling at 200 oC/h to room temperature.

Figure 4.13 Growth rate along (a) aS and (b) aL.

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Figure 4.14 XRD spectra for the YBCO film quenched from the temperatures

98

indicated.

Figure 4.15 XRD spectra for the films textured for 20 min. at the temperatures

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indicated.

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Figure 5.1 (a) Bright field TEM image showing the “buffer” layer with a 300 nm

107

thickness; (b) HREM image of the YBCO-buffer-substrate (from left to right); (c) HREM image of buffer (left) and the substrate (right), note that the buffer is identified to be essentially the YBCO without the superlattice structure with a dspacing: a=b=c=3.8 Å, and (d) HREM image showing the transition zone of the buffer (right) to YBCO (left), this image shows structural evolution and the selforganized texturing effect.

Figure 5.2 Bright field image of the interface between the silver alloy and

112

YBCO. The insets are the corresponding diffractions of each layer.

Figure 5.3 Schematic diagram showing the structural evolution at the interface.

113

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Chapter 1.

Brief Introduction to Concepts, History and Applications of Superconductivity

ABSTRACT This chapter will discuss several aspects of superconductivity. It will discuss what a superconductor is and what superconductivity is. It will review the history of the superconductor. It will also briefly discuss some important properties come with the superconductors. Finally, it will conclude noting some potential applications of superconductors.

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1.1

SUPERCONDUCTOR AND SUPERCONDUCTIVITY

What is superconductor? To explain this concept, we need to understand what is normal conductor and semiconductor first. For most materials, which are normal conductors, whenever electrical current flows, there is some resistance to the motion of electrons through the material. It is necessary to apply a voltage to keep the current going, to replace the energy dissipated by the resistance. For example, ordinary copper wire in a house is a good conductor, with only a little resistance; the filament in a light bulb has a high resistance, and generates so much heat that light is given off. It is a fact that the resistance through any regular wire is not negligible, and over long distances, it is a significant factor in determining the efficiency of a power source. Electronics is based on components in which the resistance changes under control of an input voltage; these components are made of semiconductors. A superconductor, in contrast, is a compound that conducts an electrical current with no resistance at all, i.e., zero resistance at certain temperature [1]. In other words, one could introduce an electrical current into a circuit made from a superconductor and observe that this current would continue to flow for billions of years based on the very-very small drop in voltage over a five year data collection period.

What is superconductivity? Many materials, some metals, alloys, intermetallic compounds, some ceramics and even some polymers, undergo a phase transition at some (generally low, or very low) temperature, Tc (The "critical temperature". Note: the symbol "Tc" is used to express the empirically derived maximum temperature at which a

2

specific compound will begin to exhibit superconducting.), to a state which having zero electrical resistance (figure 1.1) and extremely low losses, a phenomenon which is called superconductivity [2]. This ability to carry large amounts of current can be applied to electric power devices such as motors and generators, and to electricity transmission in power lines.

0.0020

R (Ω)

0.0015

Hg

0.0010

0.0005 90 K and Jc>5×106 A/cm2 at 77 K. The films are typically highly epitaxial with strong c-axis orientation.

NdGaO3 With an orthorhombic crystal structure and a =0.5417 nm, b =0.5499 nm, and c0.7717 nm, and the dielectric constant is 20. It has smaller lattice mismatches than either LaGaO3 or LaA1O3. The coefficient of thermal expansion is 5.8×106/K [17], making this mismatch somewhat larger than that of some of the other perovskite materials. It also has the advantage of having no phase transitions between its melting point and room temperature; so twin-free substrates are available for YBCO growth. Nd3+ is a magnetic

35

ion, however, which precludes its use as a substrate for microwave devices. NdGaO3 is quite an attractive substrate for the growth of a-axis-oriented YBCO films because of the low mismatch in this orientation: 0.28% for the c axis and 0.10% for the b axis [17]. Some of the better a-axis-oriented films that have been reported have been grown on this substrate [5]. NdGaO3 can be grown by the Czochralski technique, and the process has been scaled up to about 50-cm-diam boules. Further scale-up should be possible given sufficient demand, although there have been some problems with crystal cracking during growth [5].

NdAlO3 It has somewhat larger lattice mismatches with high-temperature superconducting compounds than the other materials, although still smaller than some other heteroepitaxial systems of current interest, e.g., GaAs on Si [18]. Its main potential advantage is the very high temperature of its second-order phase transition, 1820 K.

YAIO3 Its dielectric constant of 16 is smaller than that of most of the perovskites. It has a large mismatch with YBCO, given its pseudocubic lattice parameters of a =0.3662 nm, b = 0.3768 nm, and c =0.3685 nm. Only small substrate sizes are available[5]. KTaO3

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It is cubic with a = 0.399 nm.The lattice mismatches are somewhat larger than those of some of the other perovskite substrates [19]. YbFeO3 and other magnetic orthoferrite materials Their magnetic properties and structural compatibility with the films are of interest as substrates [20]. YbFeO3 has lattice constants a = 0.536 nm, b = 0.550 nm, and c = 0.777 nm. The dielectric constant is much lower than that of most of the other high-temperature superconductor substrate candidates.

2.2.2.2 Non-perovskite-structure oxides

A number of problems have emerged with high-temperature superconductor substrate materials with perovskite-related crystal structures. These include high substrate cost, dielectric constant values that are higher than desired for many applications, twinning transitions within the high-temperature superconductor processing range in a number of the materials, questions of the scalability of substrate size to arbitrary diameter, and large dielectric losses in many of the substrates due to the presence of magnetic ions. These issues have led to extensive investigations of substrate materials that have other crystal structures. Many of these materials are oxides and, as such, their chemical compatibility with high-temperature superconducting films may still be good. In general, however, the structural matching between film and substrate is poorer for non-perovskite substrates, making the growth of films with multiple orientations and many grain boundaries more likely. On the other hand, a number of these substrates are already available in large sizes

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at reasonable cost. They may also have more favorable dielectric properties and be known to be compatible with other technologically important materials such as semiconductors. MgO MgO, with the NaCl crystal structure, has received a good deal of interest in light of its ready availability and its modest dielectric constant (ε=9.65). It has a 9% lattice mismatch with YBCO, but with proper substrate preparation high-quality epitaxial films can be grown. Substrate preparation has been shown to have a major effect on the properties of YBCO films on MgO. Thermal annealing of the substrates produces films with good structural and superconducting properties at reasonable growth temperatures, whereas alternative chemical and mechanical treatments lead to the formation of largeangle tilt boundaries and degraded superconducting properties. Even the best films, however, tend to have some high-angle grain boundaries, which is probably responsible for the fact that such films have poorer high-frequency characteristics than do the best films grown on other substrates. MgO also reacts with water vapor, which precludes its use as a substrate for films grown by methods that require the presence of water. MgO is one of the substrates that has been used to support a-axis-oriented YBCO layers. The best a-axis films 100-200 nm thick have Tc =90 K and Jc = 3~5 × 106 A/cm2. Sapphire (Al 2O3) Al2O3 is of considerable interest as a substrate in view of its modest dielectric constant (ε=9.34) and its commercial availability in large-diameter wafers as well as its demonstrated compatibility with silicon in the silicon-on-sapphire technology. Sapphire

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reacts with high-temperature superconducting materials, making growth on unbuffered substrates challenging. The best results for YBCO films on unbuffered sapphire give Tc= 85-88 K, and Jc(67 K)=l × l06 A/cm2 [21]. A buffer layer (such as SrTiO3, MgO, or LaAlO3) must be used in order to obtain high-quality films. Because the crystal structure is not cubic, the dielectric properties of sapphire are anisotropic, which makes the modeling of microwave device performance difficult. ZrO, has served as a buffer layer for YBCO growth sapphire substrates. In this structure there is evidence of Al in the ZrO2 layer but not in the YBCO. Yttria-stabilized ZrO2 (YSZ) It has been investigated extensively both as a substrate and as a buffer layer between other substrates and films [15]. It has the cubic fluorite crystal. Advantages for this material are availability, fair lattice match with both YBCO and Si, and the ability to grow it in thin-film form. CeO2 It has the same crystal structure as that of YSZ. It has also received attention as a buffer layer, although not as a bulk substrate. The properties of films are similar to those on YSZ. Oxides with the tetragonal K2NiF4 structure Oxides with the tetragonal K2NiF4 structure have been investigated as potential substrate materials. In general, they have dielectric properties that are rather similar to those of the perovskite-based oxides and reasonable lattice matches with the high-temperature superconducting materials. One advantage is the lack of twinning. One of these materials 39

is CaNdAlO4. It has a dielectric constant ε=19. Its lattice constants of a =0.369 nm and c = 1.215 nm give a reasonable lattice match to the high-temperature superconducting materials [22]. One problem with CaNdAlO4, is the sharp increase in microwave loss at temperatures below 100 K. This is explained by magnetic ordering of the Nd ions and limits the appeal of the substrate for many HTS film applications [5].

CaYAlO4 is another potential substrate material with the K2NiF4 structure. It has lattice parameters a = b=0.3648 nm and c = 1.189 nm. The dielectric constant is 20. YBCO and TBCCO (2212) films on this substrate have worse properties than those on CaNdAlO4.[5]

LaSrA1O4 has a 3% lattice mismatch with YBCO, with a = b = 0.3754 nm and c = 1.1263 nm. Its dielectric properties are quoted as being “similar” to those of LaAlO5, while the coefficient of thermal expansion is less than that of the perovskites.

LaSrGaO4 has lattice parameters a =0.384 nm and c = 1.2676 nm. The coefficient of thermal expansion is 10×10-6/K. The dielectric constant is 22. Only YBCO films have been reported on this substrate [5].

Sr2RuO4 This material is of interest because it exhibits metallic conductivity in two dimensions, with ρab= 10-4 Ω at 300 K. It is semiconducting in the third dimension. The other oxide substrate materials discussed in this subsection are all insulators or semiconductors. The

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metallic nature of the conductivity, coupled with the potential chemical and structural compatibility of the material with high-temperature superconducting compounds, is a very attractive combination of properties for some applications. Sr2RuO4 has a tetragonal crystal structure with a =0.387 nm and c = 1.274 nm [23].

Mg2TiO4 It has been grown on MgO by solid-state reaction and then investigated as a substrate for YBCO. It is a cubic material with a =0.844 nm. The coefficient of thermal expansion is 12× 10-6/K, matching quite well with the high-temperature superconducting materials. The dielectric constant is 12 [24].

2.2.2.3

Semiconductors

In spite of serious problems with chemical reactivity, Si has received considerable attention as a substrate material due to the possibilities of integrating semiconductors with superconductors. Some of the potential applications of high-temperature superconducting films require intimate contact between semiconducting and superconducting layers. One natural way of accomplishing this is to have the semiconductor serve as the substrate. Si is by far the most common semiconductor in use today. It also offers a reasonable lattice match with the HTS compounds. The reactivity between Si and high-temperature superconducting materials is extremely severe. In addition, the coefficients of thermal expansion are very badly matched, being 3-4 times less in Si than in the high-temperature superconducting compounds. For these reasons,

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the results on YBCO films grown on Si have been extremely disappointing. The better results can be achieved by using of YSZ and other materials as a buffer layer [25].

Growth of high-temperature superconducting films on GaAs has proven to be even more difficult than similar depositions on Si because of the lower temperatures tolerated by the compound semiconductor. A number of buffer layers has been tried, mostly with limited success. Examples include CaF2 , AlGaO3, indium tin oxide, Al 2O3, MgO, YSZ, and a YSZ/Si 3N4 double-buffer layer [26].

2.2.2.4

Metals

Attempts to grow high-quality high-temperature superconducting films on metallic substrates have been proceeding almost as long as high-temperature superconducting films have been grown on any substrate. This interest has been spurred by the desirability of metallic substrates as an alternative current-carrying path, which is desirable for a number of applications. The flexibility of thin metal substrates is also attractive for many purposes. These applications usually require thick films so that the techniques used for film deposition tend to be different from those used on nonmetallic substrates. Whereas physical vapor deposition techniques of various sorts tend to be favored for growth on nonmetallic substrates of high-temperature films that are perhaps a few hundred nm thick, various chemical and spray deposition techniques have been studied most extensively for films on metal substrates. This by itself tends to result in a difference in the film quality that has been reported, since physical deposition techniques have been more highly developed to give optimized superconducting properties. Whether this will

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remain an obstacle to high-quality high-temperature superconducting films on metals remains to be seen. Metallic substrates pose many challenges as high-temperature superconducting substrates. First, the crystal structures of metals is very different from that of the high-temperature superconducting compounds, so that epitaxy is not likely on most metal substrates. Second, many metals react strongly with the HTS materials, degrading the superconducting properties substantially. Finally, the coefficient of thermal expansion of metals tends to be quite different from that of high-temperature superconducting films, giving problems with film cracking, etc. Nevertheless, the growth of high-temperature superconducting films on metal substrates continues to be an very active area of research that has met with some success [27]. In this thesis, We are mainly dealing with growth YBCO thick film on flexible metal substrates. Silver is one of the metals that has received the most attention as a high-temperature superconducting substrate because it does not react with high-temperature superconducting compounds in a detrimental way and is used to improve the transport properties of bulk hightemperature superconducting material.

Hastelloy (Ni-Cr-Mo alloy) has been studied extensively as a potential high-temperature superconducting substrate. A buffer layer is essential to prevent harmful film-substrate reactions. The buffer layers that have been studied include YSZ, Pt, SrTiO3, TiN, and BaTiO3, SrTiO3 [28].

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Gold and copper have also been used as substrates. Gold has a more detrimental effect on the superconducting properties of the film. Copper tends to react with high-temperature superconducting materials unless a buffer layer is used [29].

2.3

Superconducting tapes and wires manufacture

2.3.1

Thin Film

Soon after the discovery of high-temperature superconductor above liquid-nitrogen temperature, Chaudhari et al firstly growed this new materials in thin film form[14]. From that time, thin-film superconductor research has been motivated by three tantalizing possibilities [5]:

•

High-temperature superconductor thin films have the potential to be used as tools which can give insight into the fundamental mechanism governing high-temperature superconductivity.

•

High quality thin films might enable various applications, such as low-loss microwave cavities and filters, bolometers, flux transformers, and dc and rf superconducting quantum interface devices (SQUIDs).

•

There is also need for high-temperature superconductor interconnects to wire together various active devices.

The epitaxial growth of any high-temperature superconducting compound involves a specific optimization of the synthesis process in order to achieve optimum

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superconducting properties for that phase. Since high-temperature superconducting materials are multi-cation oxides with rather complex crystal structures, the general requirements for the formation of high-temperature superconducting films with little or no impurity phase include stringent control of the composition during the deposition process. Even with the correct cation composition, the formation of a specific hightemperature superconducting oxide phase requires an optimization of both the temperature and the partial pressure of the chosen oxidizing species consistent with the phase stability of the compound. Because the electronic properties of the superconducting cuprates show a significant dependence on oxygen content, specific oxidation conditions after film growth are generally required in order to achieve optimal doping for superconductivity. Of course, most of the interest is in epitaxial films, which require maintenance of a specific crystallographic orientation between the film and crystalline substrate. For the synthesis of multilayer device structures, control of the film surface morphology is also important [5].

Numerous film growth techniques have been investigated for the epitaxial growth of high-temperature superconducting thin films. 2.3.1.1

in situ growth techniques

The correct crystallographic phase is formed as the material is being deposited. For in situ growth, the kinetics of epitaxial film growth, along with the thermodynamic requirements for proper phase formation, require deposition at elevated temperatures (650–800°C) in an oxidizing ambient. The ability to produce relatively smooth film

45

surfaces and synthesize multilayer film structures are obvious advantages with in situ film growth.

In situ film growth techniques that have been successfully employed in the synthesis of epitaxial high-temperature superconducting thin film include:

1) physical deposition techniques: With physical deposition of high-temperature superconducting cuprates, the phase constituents are delivered as a flux of individual atoms or simple oxide species. Atomic-level control of the film growth process is possible with most physical deposition approaches, thus enabling the formation of novel multilayer structures. •

co-evaporation [30],

•

molecular beam epitaxy (MBE) [31],

•

pulsed-laser deposition [32], and

•

sputtering [33].

2) metal-organic chemical vapor deposition (MOCVD) and 3) liquid phase epitaxy (LPE)s.

3.3.1.1.1 Co-evaporation and molecular beam epitaxy (MBE) [30, 31]

In the growth of high-temperature superconducting thin films by co-evaporation or MBE, the flux is delivered by e-beam or thermal evaporation sources. A separate source is required for each element due to the differences in vapor pressures for various elements

46

or oxides. The flux from each source must be carefully controlled to ensure proper stoichiometry of the film. In situ monitoring of the flux from each source can be accomplished with the use of multiple crystal-quartz monitors. Optical techniques have been developed in which the optical absorption coefficient of each element is used to monitor the flux. Film deposition by evaporation generally requires a background pressure less than 10-4 torr. This presents somewhat of a hindrance to the in situ growth of high-temperature superconducting thin films because nearly all these compounds require oxygen pressures much higher than this to form. To overcome this limitation, highly oxidizing gases, such as NO2 or O3, as well as atomic oxygen created by a plasma source, are utilized. With O3, the oxidation of the HTS films can be enhanced by irradiating the growing film with ultraviolet light. The UV photons produce excited-state O and O2 species from the ozone, thereby increasing the activity by an order of magnitude. With any of these oxidizing species, background pressures less than 10-4 torr can be maintained while growing epitaxial HTS film [30, 31].

Film growth by evaporation can occur by the simultaneous co-evaporation of all the components or by sequentially shuttering the delivery of each component. The latter is often associated with MBE, which offers atomic-level control of the film-growth process, and has proven useful in the formation of multilayered structures. For some hightemperature superconducting compounds, MBE can be used to tailor the formation of specific phases through layer-by-layer growth of the various components of the layered high-temperature superconducting compounds. The low background pressure used in MBE also permits the in situ monitoring of film growth with electron beam techniques,

47

including reflection high-energy electron diffraction (RHEED), which can be used to characterize the crystallinity of a surface as well as to monitor and characterize the growth mode of epitaxial films. This not only gives insight into how film growth proceeds but also gives unique opportunities to control film growth at the atomic level [30, 31].

3.3.1.1.2

Pulsed-laser deposition

To a large extent, pulsed-laser deposition (PLD) was popularized as an oxide film growth technique through its success in growing in situ epitaxial high-temperature superconducting thin films. It involves a process commonly called laser ablation - a high power, pulsed, ultra-violet wavelength laser is used to evaporate materials. The exact mechanisms that are involved in the process are still unclear. In PLD a pulsed laser is focused onto a target of the material to be deposited. For sufficiently high laser energy density, each laser pulse vaporizes or ablates a small amount of the material that is ejected from the target in a forward-directed plume. The ablation plume provides the material flux for film growth. Pulsed-laser deposition has several advantages, including stoichiometric transfer of material from the target, generation of energetic species, and compatibility with background pressures ranging from UHV to 1 torr. Epitaxial HTS films can be deposited with PLD using single, stoichiometric targets of the material of interest, or with multiple targets for each element. With PLD, the thickness distribution is quite non-uniform due to the highly forward-directed nature of the ablation plume. However, raster scanning of the ablation beam over the target and/or rotating the substrate can produce uniform films over large areas. As with evaporation, the filmgrowth process can be controlled at the atomic level using PLD. In addition, deposition

48

rates higher than 100 Å/s have also been demonstrated with this technique. One drawback of PLD is the ejection of micron-size particulates in the ablation process. If these particles are deposited onto the substrate, they present obvious problems in the formation of multilayer device structures. The use of highly dense ablation targets tends to reduce particulate formation but does not eliminate this problem completely. Several techniques have been developed to further reduce particle density, although each has its limitations. Approaches that focus on preventing the particles from reaching the substrate surface include velocity filters , off-axis laser deposition, and line-of-sight shadow masks [32].

3.3.1.1.3

Sputtering

Several sputtering techniques have been used in the growth of high-temperature superconducting thin films including:

1)

on-axis DC magnetron sputtering,

2)

cylindrical magnetron sputtering,

3)

ion beam sputtering, and

4)

off-axis sputtering.

In sputter deposition, energetic ions created in a plasma bombard a metal or oxide target surface. This process ejects atoms from the target that subsequently deposit on a nearby substrate surface. In an on-axis configuration, the substrate and target are facing one another. Although this is the optimal geometry for maximum deposition rate, the on-axis

49

configuration can result in film damage due to the bombardment of the film surface with energetic species from the plasma. An alternative is the off-axis approach, in which the substrate surface is oriented perpendicular to the surface of the sputter target. This removes the film from the plasma region, eliminates sputter damage, and generally results in better films. Unfortunately the off-axis approach also significantly reduces the growth rate that can be achieved by sputter deposition. One disadvantage with sputter deposition is that the stoichiometry of a multicomponent target material is not necessarily reproduced in the deposited film due to differences in sputtering yields for different elements [33].

3.3.1.1.4

Metal-organic chemical vapor deposition (MOCVD)

For large-scale production of high-temperature superconducting thin films MOCVD is very attractive. It is routinely utilized in the electronics industry and is quite amenable to large-area deposition with high throughput. It is independent of line-of-site deposition and can be used for in situ growth at oxygen pressures near 1 atm. With MOCVD, the cations necessary for film growth are delivered as constituents of organometallic molecules. If the organometallic molecules are sufficiently volatile, they can be delivered to the heated substrate via a carrier gas. For nonvolatile precursors, the reactants are delivered as a condensed phase. The molecules thermally decompose at the heated substrate surface, resulting in film growth. For oxide film growth, oxygen is included within the gas flow. A key challenge in the synthesis of HTS thin films using MOCVD has been the development of volatile precursor molecules that are stable in storage and

50

transport and that decompose at elevated temperatures to yield good films with no contamination from the organic ligand. One interesting modification of MOCVD for high-temperature superconducting thin films involves a photo-assisted technique. In this approach, a tungsten halogen lamp is used to irradiate the substrate surface with UV photons during growth, providing both substrate heating and photostimulation of the chemical processes involved in the reaction [34]. . 3.3.1.1.5

Liquid phase epitaxy

With Liquid phase epitaxy (LPE), film growth occurs from a melt in contact with a substrate surface. LPE has proven to be quite useful in growing relatively thick films with near-perfect crystallinity. Superior crystallinity is possible with LPE as film growth from the melt takes place very near thermodynamic equilibrium. However, the structural and chemical complexities of the high-temperature superconducting materials have made it difficult to determine conditions for film growth using LPE. The epitaxial growth of hightemperature superconducting thin films with near-single crystal-like properties has been achieved using this technique [35].

3.3.1.2

ex situ techniques

A film that is either amorphous or an assemblage of polycrystalline phases is deposited and subsequently annealed to form the desired high-temperature superconducting phase.

ex situ post-annealing Despite the stated success in using in situ techniques to grow high-temperature superconducting thin films, there are numerous limitations to these approaches. In situ

51

growth requires significant and uniform substrate heating in an oxygen ambient during film deposition. This proves challenging for large-area, double-sided, or continuous length deposition of high-temperature superconducting thin films. In contrast, ex situ processing requires no substrate heating during precursor deposition, greatly simplifying the film growth apparatus. The desired crystallographic phase is formed through bulk diffusion and solid phase epitaxy by annealing the precursor film at elevated temperatures. Annealing can be performed as a batch process of multiple substrates. In addition, several important high-temperature superconducting compounds consisting of cations with high vapor pressures have not been successfully grown by in situ film growth techniques but can be synthesized by ex situ annealing of precursor films. Of course, the use of solid phase epitaxy virtually eliminates the possibility of fabricating multilayered thin-film structures [36].

52

2.3.2

Thick Film Fabrication Techniques

2.3.2.1 Disadvantages of thin film techniques

High-temperature superconducting thin films are a challenge to grow. A major contributor to the difficulties that arise is the substrate complexity. And it is almost impossible to grow high temperature conducting thin film to a large area and large scale under even the most ideal circumstances. Many of problems that have plagued thin film growers in other materials systems have also been encountered in high temperature superconducting thin film, exacerbated by the special material properties of the hightemperature supercondcting materials that present their own challenges. For examples, each high temperature superconductor contains oxygen, an element that is incompatible with many aspects of traditional thin-film growth methods, especially the substrate heating necessary for producing films that are superconducting upon removal from growth charmber as well as hot filaments for evaporators. The role of oxygen is also critical in determining the properties of various film imperfections such as grain boundaries. The crystal structure of the materials is highly anisotropic, as are the resultant electrical and superconducting properties, meaning that crystallographic alignment in the film is critical. For the thin film development, highly uniformed surfaces are very necessary to insure a uniform, homogenous film formation. For thick film deposition, uniform surface is good for film development, but not as critical as for thin film [4, 5, 36].

53

2.3.2.2

Thick film techniques

It is important to define precisely what is meant by the term thick film in order to avoid confusion. The term thick film relates not so much to the thickness of films but to the mode of deposition. In the electronics industry a thick-film conductor of silver, copper or gold, for example, is prepared by taking a powder of the metal and mixing the powder with a vehicle. The vehicle is a mixture of polymers and solvents which are thoroughly mixed with the chosen powder, until a homogeneous mixture is formed. This mixture is known as an ink. The ink is then deposited onto a suitable substrate, for example polycrystalline alumina or polymer circuit board, by screen printing and then, in the case of ceramic substrates, fired at high temperature in order to sinter the particles in the ink so that a homogeneous electrical circuit is obtained. This differs from thin-film methods such as sputtering, laser ablation, chemical vapour deposition, evaporation etc in three very important respects. First, thin-film deposition usually requires the use of singlecrystal substrates, second thin films are usually deposited with the intention of achieving a degree of epitaxy with respect to the crystallographic orientation of the substrate and third thin films usually require the use of vacuum technology [4].

One most often used method is encapsulating the powder in a metal or alloy substrate such as silver and drawn into a tape or wire. One good example of this method are powder-in-tube (PIT) method in which a superconductor powder is placed inside a silver tube and swaged and/or rolled into the desired conductor shape. Another technique which is also used is the deposition of a superconductor thick film paste or slurry onto a metallic (silver) substrate. In a variant, the superconductor may be deposited onto a

54

barrier layer such as zirconia which itself is on a metallic substrate, e.g. a high temperature nickel alloy. This is classed as a thick film and conductors made by these methods are referred to as having been prepared by ‘open’ methods as opposed to ‘closed’ methods such as PIT. Such conductors have been shown to possess excellent properties [4, 6, 7].

55

2.4

Project Objective

Following the discovery of High Tc superconductors (HTS), an intense research effort has been aimed at finding practical applications for these new materials. Of these applications, the most promising include power system components such as electrical motors, generators, power transmission cable, transformers, and superconducting magnetic energy storage devices [2, 37]; The other large scale application has been the construction of high-field magnets [4]. These applications requires the conductors to carry high critical current densities (> 104 A/cm2) [4, 38, 39]. Another important requirement is that the conductors possess high ductility, mechanical strength, and chemical stability[2, 37]. In particular, the conductors must be processed into long wires or tapes with a uniform microstructure. Extensive investigations have been devoted to optimizing the fabrication and processing techniques of superconducting wires and tapes.

Generally, there are two common processing routes to manufacture the superconducting wires and tapes: thin film synthesis which includes ion-beam-assisted deposition (IBAD) [40, 41] and rolling-assisted biaxially textured substrates (RABiTS) [38, 39, 42], and thick film methods [4]. The primary advantage of RABiTS processing techniques is that they produce a highly textured superconducting film with superb transport properties [38, 39, 42]. The drawback to RABiTS techniques, however, is that they usually require the use of one or more buffer layers, which in turn produces a complex buffer architecture [38, 39, 42]. The physical dimension of the thin film also substantially limits the absolute amount of current carrying abilities of the finished materials [6, 7]. Although the critical

56

current densities obtained at present in thick films do not match those obtained by thin films, thick film technology may offer important advantages for some applications, especially where large areas or complex shapes are required.

In this project, we are trying to explore an economical route to fabricate uniformly long superconducting tapes or wires on unoriented flexible metal substrate by direct peritectic growth (DPG) approach. We are going to use several unoriented pure metals and alloys as candidate substrates and deposit uniformly superconducting YBCO thick film on substrates by doctor blading method. By adjusting maximum processing temperature and holding time, to exploring the direct peritectic growth processing parameter for growth caxis textured YBCO films. Further, we are going to using quenching techniques to investigate YBCO crystal growth behavior on unoriented metallic substrates. Finally, XRD and TEM techniques will be employed to investigate interface structure evolution of YBCO from randomly oriented metallic alloy substrates.

Successful implementation of this novel approach will have a significant impact on the commercialization of YBCO compounds for large-scale applications.

57

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X. Wen, D. Qu, B.A. Tent, and D. Shi, Direct deposition of c-axis textured YBCO thick film on unoriented metallic substrate for the development of long superconducting tapes. IEEE Transactions on Applied Superconductivity,. 9: p.1506 (1999)

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D. Shi, D. Qu, X. Wen, B.A. Tent, and M. Tomsic, Direct Peritectic Growth of cAxis Textured YBa2Cu3Ox on a flexible metallic substrate. J. of Superconductivity,. 11: p.575 (1998)

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K. Char, N. N., G.S. M., R.W. Barton, R.C. Taber, S.S. Laderman, and R.D. Jacowitz, Microwave surface resistance of epitaxial YBa2Cu3O7 thin films on sapphire. Appl. phys. Lett.,. 57: p.409 (1990)

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A. Goyal, D.P. Norton, D.M. Kroeger, D.K. Christen, M. Paranthaman, E.D. Specht, J.D. Budai, Q. He, B. Saffian, F.A. List, D.F. Lee, E. Hatfield, P.M. Martin, C.E. Klabunde, J. Mathis, and C. Park, Conductors with controlled grain boundaries: An approach to the next generation, high temperature superconducting wire. J. Mater. Res.,. 12: p.2924 (1997)

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R.P. Reade, P. Berdahl, R.E. Russo, and S.M. Garrison, Laser deposition of biaxially textured yttria-stabilized zirconia buffer layers on polycrystalline metallic alloys for high critical current Y-Ba-Cu-O thin-films. Appl. Phys. Lett.,. 61: p.2231 (1992)

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X.D. Wu, S.R. Foltyn, P. Arendt, J. Townsend, C. Adams, I.H. Campbell, P. Tiwari, C. Y., and D.E. Peterson, High-current YBa2Cu3O7-delta thick-films on flexible nickel substrates with textured buffer layers. Appl. Phys. Lett.,. 15: p.1961 (1994)

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Chapter 3.

Effect of Maximum Processing Temperature, Processing Time and Substrates on the YBCO Thick Film by DPG deposition on Unoriented Metallic Substrates ξ .

ABSTRACT Direct peritectic growth (DPG) is a novel process in which textured YBCO thick films are deposited directly onto unoriented metallic substrates. No buffer layer is employed between the YBCO superconducting film and the metallic substrate. Four kinds of metals and alloys, pure silver, Ag3%Pd, Ag10%Pd and pure Nickel, were used as YBCO thick film substrate. By adjusting processing temperature and processing time, YBCO thick films were formed on these substrates. The textured YBCO grains can be achieved through peritectic solidification on Ag10%Pd and Nickel substrates over a wide range of temperature and time. Highly textured YBCO films can not be obtained on pure silver and Ag3%Pd substrates owing to lower melting points of these two substrate than YBCO peritectic point. No observable reaction of Ag10%Pd substrate was found with the YBCO melt at the maximum processing temperatures near the YBCO peritectic point (from 950 °C to 1030°C). However, there is a thick reaction layer at the YBCO and Nickel interface. The transport Jc has reached a respectable value of ~104 A/cm2 at 77 K and in

ξ

Part of this chapter was published in: IEEE Transactions on Applied Superconductivity; Vol. 9; p15061509 and Journal of Superconductivity; Vol. 11; p575-580

63

zero magnetic field. In this chapter, I shows that the DPG method is capable of producing highly textured YBCO thick films, which are scalable to long lengths with low cost.

64

3.1 INTRODUCTION There are a variety of methods employed to obtain thick films; for example, sol-gel, screen printing, doctor blading, electrodeposition, solution spraying, spray pyrolysis and electrophoretic deposition [1]. Now, several methods to produce long wires and tapes are available for commercial use, such as Powder-in-tube (PIT) [1], Powder/ wire-in-tube (PWIT) [2], Continuous-tube-forming/filling (CTFF) [3] and ‘open’-deposition (‘open’ as opposed to ‘closed’ methods such as PIT, PWIT and CTFF) [1]of superconductor thick films onto a metallic substrate.

The work presented in this chapter is specifically aimed at the development of long YBCO conductors on flexible metallic substrates with high transport critical current densities. Several kinds of untextured pure metals and alloys are used as candidate substrates. Silver and its alloys and nickel are the most often used substrates for superconducting thick film fabrication in previous works by other people. Silver and its alloys are very attractive thick film substrates due to their chemical compatibility with YBa2Cu3Ox (YBCO), reasonable oxygen diffusivity, and oxidation resistance at elevated temperatures. Silver alloy supplies ductility and protection for the tape or wire for use in potential applications such as long power transmission lines and superconducting coil [1]. A non-vapor approach has been taken in order to produce a textured YBCO structure. In this novel approach, unoriented metallic substrates are used in the growth of textured YBCO. A thin layer of c-axis textured YBCO has been formed with a fast processing rate capable of potentially fabricating thousand-meter-long conductors. The preliminary

65

results have shown that the transport Jc has approached values on the order of 104 A/cm2 at 77 K and in zero field. Successful implementation of this novel approach will have a significant impact on the commercialization of YBCO compounds for large-scale applications. We use a doctor-blade thick film technique to deposit green-state YBCO film on the metallic substrates. When melt processed near the peritectic temperature, well textured YBCO grains have been obtained in the form of long tapes. X-ray techniques (2 theta, Phi Scan, Omega Scan etc.), Scanning electron microscopy (SEM), Tc and Jc measurements were used to characterize the degree of texture and superconducting properties of the tapes.

66

3.2 MATERIALS AND METHODS The YBCO powders were supplied by Superconductive Components, Inc. (Columbus, Ohio, USA) and as rolled untextured pure silver, Ag3%Pd, Ag10%Pd and pure Nickel were provided by Plastronic Inc. (Tipp City, Ohio, USA). The specimens used in this study were melt-textured thick films of YBCO (123) on four kinds of metallic substrates. The films were prepared using doctor-blade technique. The painted specimens had a final thickness up to 20 µm.

These coated specimens were then subjected to the melt processing. They were heated from room temperature at a rate 200 oC/h and held at 500 oC for 5 hours. This procedure was used to burn out the polymer binder. The same rate was used to heat the samples to peritectic temperature (from 950 oC to 1030 oC) where they were held for a range of times (from 0.2 h to 1.4 h). The samples were then quickly cooled (~500 oC/h) to 900 oC, and further cooled at a rate of 100 oC/h to room temperature. The samples were given a post-growth oxygenation at 400 oC for 48 h, and then cooled to room temperature at 100 o

C/h. The heating schedule of this experiment is shown in Figure 3.1.

Philips X'Pert MPD X-ray diffractometer was used to determine the texture of the YBCO film. The surface and interface microstructures were characterized by SEM. The samples were evaluated for their superconducting properties by resistance versus temperature (RT) and transport critical current density measurements in liquid nitrogen. A 1 µV/cm voltage criterion was used to determine Jc in the four-probe measurements.

67

1100

Final Temperature and time 500 o C/hr

1000

C/h r

200

o

800 700 600 500 oC for 5 hrs

200 o C/h r

200

r /h

300

oC

400

0

500

10

Temperature (oC)

900

100 0 0

2

4

6

8

10

12

14

16

18

20

22

Time (hours) Figure 3.1 Heating Schedule for DPG processing.

68

3.3 RESULTS AND DISCUSSION Figure 3.2 shows a type coil shape tape fabricated by DPG technique. The microstructure of the films was studied with the use of an optical microscope and a SEM. Polarized optical microscopy showed that the main contents of the films are YBCO (123) structure. Figure 3.3, 3.4 and 3.5 show the microstructure of the YBCO thick film surfaces at various maximum processing temperatures (Tmax). Figure 3.6 is a transverse crosssectional SEM micrograph of the YBCO thick film processed at 1010 oC, for 0.2 h. From these micrographs, we can see that the Tmax appears to have a strong effect on the degree of texturing. At low temperatures such as 980°C, we have found that stoichiometric YBCO was not entirely decomposed into the liquid phase. Therefore subsequent crystallization during cooling did not result in large, c-axis oriented grains (Figure 3.3). However, the partial melting at 1010°C appeared to be the optimum temperature at which only a short holding time was required. As can be seen in Figure 3.4, well textured YBCO grains with a high density were obtained at 1010°C. From Figure 3.4 and 3.5, it can be seen that the grain morphology exhibits a "spiral" pattern indicating a unique growth mechanism during the rapid cooling. Figure 3.5 shows this growth pattern in a localized area as indicated by the arrows. At the lower temperature of 990°C, the rapid grain growth was limited as a result of low driving forces. Therefore, the majority of the grains formed with a random structure such as that seen in the matrix of Figure 3.5. Only in some localized areas where liquids concentrate, rapid grain growth was promoted resulting in a special growth pattern. As indicated in Figure 3.5, a group of small grains appears to join together within a short period of time and form a large grain. Comparing this growth to that seen in Figure 3.4, where the majority of the matrix consists of well 69

connected grains, the grain growth exhibits two steps in which the grain morphology is distinctly different.

For the direct peritectic growth on a metallic substrate, the interface reaction between YBCO and the substrate is of great importance. As evidenced in many previous works reference, YBCO is reactive with many crucible materials including platinum and alumina [4]. Although pure silver has been found to be non-reactive with YBCO, its melting point (962°C) is well below the peritectic temperature of YBCO (~1010°C). As we observed in this experiment, pure silver substrates can not withstand Tmax above 960 o

C and Ag3%Pd can not withstand 1000 oC. In this research, the YBCO thick films have

been melt textured at a maximum temperature of 1020°C on Ag10%Pd alloy substrate for at least several hours in air. First, we found that, there was no evidence of melting even at the interface where possible reactions may induce early melting of the Ag10%Pd substrate. Second, according to our preliminary SEM examination, there was no observable reaction at the interfaces as can be seen in Figure 3.6. As indicated by arrows, the interface between the YBCO and the Ag10%Pd substrate appears to be sharp and clean without any secondary phases. Within the YBCO matrix, we found that the textured grains formed a dense layer parallel with the surface of the substrate. This result is consistent with both SEM and XRD data that the YBCO grains can be well textured on an unoriented substrate. However, the surface exhibits a somewhat irregular grain structure, which may have been caused by sample mounting. For comparison, the interface structure of YBCO thick films on a nickel substrate is shown in Figure 3.7. As can be seen in this figure, not only have we seen a 5 µm-thick reaction layer at the

70

interface as indicated by arrows, but also a porous matrix of YBCO, which, we believe, was attributed to the reaction with the substrate.

2 cm

Figure 3.2 DPG method processed long tape

10 µm

Figure 3.3 SEM photograph showing the porous grain structure when the film was heated at 980°C for 0.2 hours.

71

10 µ m

Figure 3.4 SEM photograph showing the c-axis textured grain structure when the film was heated at 1010°C for 0.2 hours.

Figure 3.5 SEM photograph showing the 'spiral' growth pattern locally textured grain structure when the film was heated at 990 °C for 0.2 hours.

72

YBCO thick film

Ag alloy substrate

10 µm Figure 3.6. SEM photograph showing the interface between the YBCO thick film and the Ag-10%Pd substrate when the film was heated at 1015 oC for 60 min. Note that the interface is sharp and clean without observable reaction layer.

10 µm

Figure 3.7 SEM photograph showing the interface between the YBCO thick film and the nickel substrate. Note that there is an interface reaction layer as indicated by the arrows

73

(103) (110) •

600

YBCO on Silver Alloy • YBCO ° Silver Alloy

550 500

(003) •

(006) (020) •°

(005) (014) ° •

(004) •

980 ° C

(007) •

(116) (123) •

ntensity (Arbitrary Unit)

450 400 350 990 °C

300 250

I

200 150

1000 °C

100 50 1010 °C

0

20

25

30

35

40

45

50

55

60

2θ Figure 3.8 XRD spectra for the thick films textured at temperatures indicated. Note that the (004) and (006) peaks steadily increase as the temperature has increased from 980°C to 1010°C indicating an enhanced grain texturing.

74

500 (103) (110) •

450

ntensity (Arbitrary Unit)

(003) •

(006) (020) •

(005) (014) •

400

350

YBCO on Silver Alloy • YBCO ° Silver Alloy (116)

(004) •

°

(123) •

°

0.2hr

(007) •

300

250 0.6hr

I

200

150 1.0hr 100

50 1.4 hr 0

20

25

30

35

40

45

50

55

60

2θ Figure 3.9 XRD spectra for the thick films textured at 1010 oC, holding time indicated. Note that the (006) peaks decrease as the treated time has increased from 0.2h to 1.4h

75

1000000 Center = 26.7578 Intensity = 909630 Gauss FWHM = 0.2893 Pearson FWHM = 0.2827

900000 800000

Center = 205.4583 Intensity = 873680 Gauss FWHM = 0.2680 Pearson FWHM = 0.2872

700000 Average Gauss FWHM=0.3223 Average Pearson VII FWHM=0.2841

Counts

600000

2Theta(o): 32.7366 Omega(o): 16.3602 Psi Tilt (o): 45.44

500000 400000

Center = 115.3397 Intensity = 157850 Gauss FWHM = 0.3351 Pearson FWHM = 0.2800

300000

Center = 296.5640 Intensity = 98462 Gauss FWHM = 0.3968 Pearson FWHM = 0.2866

200000 100000 0 0

45

90

135

180 o

225

270

315

360

Phi

Figure 3.10 (103) Phi Scan of YBCO thick film on silver alloy (1015oC, 0.6h)

76

1000000 900000 800000 Center = 26.7578 Intensity = 909630 Gauss FWHM = 0.2893 Pearson FWHM = 0.2827

Counts

700000 600000 500000 400000 300000 200000 100000 0 20

22

24

26

28

30

32

34

o

Phi

Figure 3.11 A detail peak of (103) Phi Scan of YBCO thick film in figure 3.10

77

Figure 3.12 A typical sharp Tc of YBCO on silver alloy (Notes: Treated at 1010 oC for 0.2hr)

78

The x-ray diffraction patterns(XRD) of the thick films at different temperatures (980 oC ~ 1010 oC) for 0.2 h to 1.4 h are shown in Figure 3.8, and the XRD patterns of different holding times at 1010 oC are shown in Figure 3.9. XRD patterns of the films showed highly textured grains with the c-axis normal to the film plane. The XRD spectra indicate clear differences in out-plane texturing among the samples melt textured at different temperatures as indicated. They showed that the phase-pure, c-axis oriented 123 phase (i.e., good out-plane texturing) formed on these thick films. As evidenced in Figure 3.8, the degree of texturing is steadily enhanced as the temperature is increased from 980°C to 1010°C and held for 0.2 hours. Most noticeably, the intensities of the (004) and (006) peaks are doubled when melt textured at 1010°C indicating an increased texturing process. Figure 3.8 and 3.9 also showed that I(006)/I(103) ratio increased with increasing temperature from 980 oC to 1010 oC, and decreased with extending the holding time from 0.2 h to 1.4 h. I(006)/I(103) reflects the grain alignment, the higher the I(006)/I(103), the better the grain alignment. It is clear from the Figure 3.8 and 3.9 that the maximum texturing occurs at about 1010 oC and 0.2 h in our investigation. These experimental data suggest that the out-plane texturing can be further improved as the processing parameters are optimized.

In-plane texturing was also characterized by x-ray Phi scan. A (103) Phi scan of 1010 oC, 0.6 h is shown in Figure 3.10. One detail peak of Figure 3.10 (103) is shown in Figure 3.11. The full width at half maximum (FWHM) value for the (103) c-axis peak is extremely sharp (only about 0.3o), which is significant considering the majority of the

79

grains have a grain-boundary angle of less than 5o, i.e. high in-plane texture was achieved by DPG processing.

Based on our SEM and XRD data, we conclude that a textured substrate may not be required to achieve the out-plane and in-plane grain texturing in the peritectic solidification process. In vapor route synthesis, a single crystal or textured buffer is often used since the subsequent crystallization of YBCO in the solid state must take place according to the lattice structure of the substrate. However, in non-vapor route processing involving a large amount of liquids such as melt texturing, the YBCO phase is entirely decomposed above the peritectic point. The superconducting phase re-nucleates and grows from the liquid through the peritectic reaction. During cooling, YBCO grains will nucleate on a solid surface such as the silver alloy to minimize the surface energy. Since YBCO tends to grow more rapidly along the a-axis, grain growth will proceed in a direction along the surface of the metal substrate. In this situation, the crystal structure of the substrate is not important since the YBCO grain growth is rapid and cannot follow the atomic arrangement of the substrate lattice. Therefore, direct peritectic growth is possible on an untextured metal substrate. We must note here that the advantages of this novel approach are multi-fold: (1) there is virtually no reaction between YBCO and silver alloy; (2) silver alloy is an excellent conductor making it an ideal candidate as a stabilizer; (3) the silver alloy does not severely oxidize in air, which makes mass production simple and economical, and (4) the substrate is highly ductile and therefore well suitable for making long flexible tapes.

80

The superconducting properties have been measured using a standard four-probe method in zero magnetic field. One typical Tc plot of YBCO thick film on Ag10%Pd is shown in Figure 3.12. The transport critical current density, Jc, measurements at 77 K and zero magnetic field have shown respectable Jc values up to ~104 A/cm2, which is comparable to most of the reported data on YBCO thick films on ceramic substrates [1, 5, 6].

3.4 CONCLUSION From this work, it can be concluded that the Ag10%Pd alloy can serve as an industrially viable substrate for the fabrication of long-length YBCO conductors. First, such a substrate can withstand the melt processing temperature near the peritectic point. Second, well-textured YBCO grains can be obtained directly on a metal substrate without textured buffer layers. Third, using the silver alloy substrate, the net-shape processing will become possible as the YBCO thick films can be readily deposited on a long-length substrate. This is a critical step in the making of industrial-scale conductors. In this work, not only is highly textured YBCO films obtainable on a metal substrate without a textured buffer, but also a respectable Jc value which shows promise for this unique approach in making long YBCO conductors.

81

REFERENCES 1.

N.M. Alford, S.J. Penn, and T.W. Button, High-temperature superconducting thick films. Supercond. Sci. Technol.,. 10: p.169 (1997)

2.

R. Zeng, H.K. Liu, and S.X. Dou, A new structural powder/wire-in-tube (PWIT) Ag-sheathed multifilamentary Bi-2223 tape and its superconducting properties. Physica C.,. 300: p.49 (1998)

3.

Y.C. Guo, H.K. Liu, S.X. Dou, and E.W. Collings, Microstructure and electromagnetic properties of Bi2223/Ag superconducting wires and tapes prepared by the `continuous tube forming/filling (CTFF)' technique. Physica C: Superconductivity,. 282-287: p.2597 (1997)

4.

J.M. Phillips, Substrate selection for high-temperature superconducting thin films. J. Appl. Phys.,. 79: p.1829 (1996)

5.

A. Goyal, D.P. Norton, J.D. Budai, M. Paranthaman, E.D. Specht, D.M. Kroeger, D.K. Christen, Q. He, B. Saffian, F.A. List, D.F. Lee, P.M. Martin, C.E. Klabunde, E. Hartfield, and V.K. Sikka, High critical current density superconducting tapes by epitaxial deposition of YBa2Cu3Ox thick films on biaxially textured metals. Appl. Phys. Lett.,. 69: p.1795 (1996)

6.

A. Goyal, D.P. Norton, D.M. Kroeger, D.K. Christen, M. Paranthaman, E.D. Specht, J.D. Budai, Q. He, B. Saffian, F.A. List, D.F. Lee, E. Hatfield, P.M. Martin, C.E. Klabunde, J. Mathis, and C. Park, Conductors with controlled grain boundaries: An approach to the next generation, high temperature superconducting wire. J. Mater. Res.,. 12: p.2924 (1997)

82

Chapter 4

Growth Dynamics of Direct Peritectic Growth YBCO Thick Film on Ag10%Pd Substrateξ .

ABSTRACT Quenching experiments were carried out near the peritectic temperature for the growth dynamics of YBCO thick films on the Ag10%Pd substrate using DPG method. The initial YBCO morphology exhibits a column-like grain structure as a result of rapid agrowth when quenched from 1000 °C (the sample was pre-melted at 1030°C). A wafflelike structure was observed on the surface of the silver alloy substrate as the quenching temperature was lowered to 950°C providing a much greater driving force. We found that a grain-oriented substrate may not be required to achieve the grain texturing in the peritectic-reaction controlled process. During solidification, the YBCO grains will parallely nucleate on the surface of the silver alloy to minimize its surface energy, and grow along the a-axis rapidly resulting in a textured film.

ξ

Part of this chapter was accepted for publishing at Cryogenics and posted on 1999 ICMC Conference.

83

4.1

INTRODUCTION

There are a variety of methods employed to obtain thick films, for example, sol-gel, screen printing, doctor blading, electrodeposition, solution spraying, spray pyrolysis, and electrophoretic deposition [1]. YBCO thick films on unoriented silver alloy (e.g. Ag10%Pd) substrate are actually most amenable to the techniques currently available to manufacture of long-length tape/wire with good performance. Remarkable progress has been made in obtaining the c-axis YBCO films on silver alloy with a textured grain structure by a so-called direct peritecic growth (DPG) method [2, 3]. The results in chapter 3 have shown that the in-plane alignment can be obtained using an unbuffered metallic substrate.

In DPG, a unoriented silver alloy substrate is used in the growth of textured YBCO. The unoriented silver alloy is used as the substrate due to its chemical compatibility with YBCO, reasonable oxygen diffusivity, and oxidation resistance at elevated temperatures. After solidification near the peritectic point, a 10 µm-thick film of c-axis textured YBCO has been formed on the silver substrate. The preliminary results have shown that the transport Jc has approached values on the order of 104 A/cm2 at 77 K and in zero field. Successful implementation of this novel approach will have a significant impact on the commercialization of YBCO compounds for large-scale applications. Based on our previous work, several critical issues about the YBCO thick film growth dynamics need to be addressed. These include maximum a-growth rate during solidification and the mechanisms of nucleation and growth of YBCO. In this chapter, we attempt to address

84

these issues by conducting a series of quenching experiments from near the peritectic temperature.

4.2 MATERIALS AND METHODS The YBCO powders were supplied by Superconductive Components, Inc. (Columbus, Ohio, USA) and as rolled untextured Ag10%Pd alloy was provided by Plastronic Inc. (Tipp City, Ohio, USA). The specimens used in this study were thick films of YBCO (123) on Ag-10%Pd alloy substrate. The specimens were prepared using a doctor-blade technique same as the approach in chapter 3. In an initial experiment, we simply heated the specimens at different temperatures between 900 oC and 1050 oC at 200°C/h and held at these temperatures for 20 min. in air, followed by a cooling at the same rate. These samples were designated as Group I (Heating schedule of group is shown in Figure 4.1).

In the second experiment, the specimens after coating were heated to 1030 oC at 200 o

C/h. Then two different cooling schedules, as shown in Figure 4.2 and 4.3, were used.

After holding at 1030°C for 60 min., it was quenched from 1030°C to several temperatures between 950 oC and 1025oC. Holding at these temperatures for various time intervals from 10 mim. to 3600 min., they were quenched according to schedule (a) (Group II) and air cooled at 200°C/h according to for schedule (b) (Group III) to room temperature. The quenching experiment was designed to preserve the high temperature structures for the subsequent scanning electron microscopy (SEM) and x-ray diffraction (XRD) studies. All samples were given a post-annealing in flowing oxygen at 400 oC for 48 h. Philips X'Pert MPD X-ray diffractometer was used to determine the texture of

85

the YBCO film. The surface and interface microstructures were characterized by SEM. The samples were evaluated for their transport superconducting properties by a fourprobe method.

1100

Final Temperature and time 500 oC/hr

1000

/hr 200

oC

800 700 600 500 oC for 5 hrs

/hr

C/

200

oC

hr

200

o

400 300

0

500

10

Temperature (o C)

900

100 0 0

2

4

6

8

10

12

14

16

18

20

22

Time (hours)

Figure 4.1 Heating Schedule of Group I.

1200 1100

1030 oC, 1hr

∆t

/hr

900

∆T

200

oC

800 700 600 500 oC for 5 hrs

/hr

500 300 200

oC

400

200

Temperature (o C)

1000

Air Quench

100 0 0

2

4

6

8

10

12

14

16

18

20

22

Time (hours)

Figure 4.2 Heating Schedule of Group II for the quenching experiments. Quenched from 1030 oC to a given temperature, held for certain time, and quenched again to room temperature

86

1200 1100 1030 oC, 1hr

∆t

/hr

900

∆T

200

oC

800 700 600

o

200

oC

hr

/hr

C/

400 200

0

500 oC for 5 hrs

500 300

20

Temperature ( oC)

1000

100 0 0

2

4

6

8

10

12

14

16

18

20

22

Time (hours)

Figure 4.3 Heating Schedule of Group III for the quenching experiments. Quenched from 1030 oC to a given temperature, held for certain time, and cooled at 200 oC/h to room temperature.

4.3 RESULTS AND DISCUSSIONS The samples in Group I heated at 950 oC for 20 min. appear to be untextured sintered state as shown in Figure 4.4. When the sample was heated above 1010 oC, however, YBCO melted and decomposed. The amount of liquid phase produced increases with the increasing temperature, and promotes the densification of the film. As shown in Figure 4.5, at 990oC, small dense islands have formed in the film, within which the grains are well textured. At a higher temperature of 1020 oC, these islands join each other forming a textured, tense matrix of the film as shown in Figure 4.6. From these micrographs, we can see the same results as in Chapter 3. At even a higher temperature of 1035 oC, the Ag10%Pd alloy began to melt. However, the substrate remained “wet” by the YBCO liquid due to its high viscosity and surface tension. These results indicate that the YBCO/Ag10%Pd can be processed in a wide temperature range even well above the peritectic point. In this way, better substrates with higher melting temperatures can be utilized in the development of the YBCO tapes.

87

The group II samples that were quenched from various temperatures are shown in Figure 4.7, 4.8, and 4.9. As shown in Figure 4.7, the sample quenched directly from 1030°C, which is above the peritectic point of 1010°C, exhibits entirely an amorphous structure. This was later confirmed by the x-ray diffraction study. However, as indicated by Figure 4.7, small crystallinities of YBCO could present in the amorphous matrix. As quenching took place from 1000°C, below the peritectic point, crystallization occurred during holding although for a short time of 10 min. We were able to preserve this structure by quenching the sample in air as shown in Figure 4.8. As can be seen in this figure, the crystallized YBCO grains exhibit elongated column-like grains indicating a rapid growth rate along, presumably, the a-direction. Note that, as described before, the YBCO/Ag10%Pd green-state sample was preheated to 1030°C first to decompose the YBCO phase. It was then quenched to a given temperature allowing YBCO to precipitate and grow from the liquid. We realize that, as quenched from 1030°C to 1000°C, there is a significant driving force involved in the precipitation process. As we see in Figure 4.8, the resulting microstructure suggests a rapid a-growth leading to a column-like grain morphology. The situation drastically changed as we provided even higher driving force by quenching the sample from 1030°C to 950°C. The undercooling now is 80°C instead of 30°C. Such a large driving force enabled a more aggressive grain growth and resulted in an entirely different microstructure as shown in Figure 4.9. In this figure, we can see that the YBCO grains now crystallized in a two-dimensional form which is significantly different from that shown in Figure 4.8.

Such a microstructural difference suggests a new growth kinetics which is currently

88

under investigation. However, we are able to see that the YBCO grains appear to form in the liquid relatively fast causing a volume reduction in areas where the majority of liquid has transformed to YBCO. This can be easily understood that the low density liquid is consumed by the solid YBCO in a two-dimensional platform. Wherever the YBCO precipitates more rapidly, the shrinkage of the liquid is large. As a result, we see a waffle-like structure. Compared to Figure 4.7 where the amorphous structure is smooth with an even thickness, the waffle structure gives us a clear picture of grain growth during the solidification.

Note that the samples shown in Figure 4.7, 4.8 and 4.9 were held at intermediate temperatures for very short time and quenched in air. Thus we are only seeing the initial stage of the crystallization (i.e., the crystallization proceeded for short time and the growth of YBCO was terminated by quenching). In the following, we show the group III samples that were held at the intermediate temperature for a prolonged time up to 120 min., followed by a gradual cooling (200°C/h) to room temperature. This allowed a sufficient grain growth thus produced significantly different microstructure. In Figure 4.10, 4.11 and 4.12, we show three samples that were heated to the maximum temperature of 1030°C first as described before. They were then quenched to 950°C and held there for 10 min. (Figure. 4.10), 20 min. (Figure 4.11), and 120 min. (Figure 4.12). After holding at 950°C, they were slowly cooled to room temperature at a rate of 200°C/h. The grain growth evolution can be clearly seen in these figures. For the 10 min. sample, the grains are column-like indicating that, at the initial stage of the crystallization, a-growth is rapid. As the liquid fully crystallized, growth continues along another a-direction as indicated in Figure 4.11. The a-growth enlarged the grains

89

into a two-dimensional form squeezing the grains tightly against each other. Further holding at this temperature resulted in much tense, c-axis textured grain structure as shown in Figure 4.12.

The growth rate along the a-axis has been determined based on the results of a series quenching experiments with the typical ones shown in Figure 4.10, 4.11 and 4.12. As has been well established, the YBCO exhibits a tetragonal structure above 700°C with a = b in the basal plane. However, as observed in Figure 4.10, 4.11 and 4.12, the grain structure starts as a narrow rods and gradually grows into a rectangular shape. Based on the x-ray diffraction data, we have determined that the flat surface is the ab-plane. Therefore the rectangular grains must grow along one a-direction first initially (see Figure 4.10), which we call the long-a-direction, and then expands from the other adirection (the short-a-direction, see Figure 4.11), leading to an enhanced c-axis texturing on the silver alloy substrate (see Figure 4.12). The growth rates along the short-adirection, as, and the long-a-direction, aL, are presented in Figure 4.13. As can be seen in these figures, the growth rates along aS and aL are both rapid initially up to 20 min., and then saturate afterwards due to grain impingement. However, the growth rate along al is much more rapid about four times faster than that along aS. X-ray diffraction (XRD) data are consistent with the SEM observation. As can be seen in Figure 4.14, the XRD spectra indicate a gradual microstructural evolution of the YBCO grain structure. The group II samples quenched from 1030°C and 1020°C has entirely the liquid structure while some crystallinity starts to appear as the quenching temperature is lowered to 1020°C. This is consistent with the well-established peritectic point of

90

1010°C. Quenching from 950°C has resulted well crystallized YBCO phase as evidenced in Figure 4.14. Note that the x-ray patterns shown in Figure 4.14 is only for the quenched grain structures. Presumably, there crystallinity is far less established from those wellcrystallized Group I and III samples.

In Figure 4.15 we show the x-ray diffraction patterns of well crystallized YBCO grains from group I. As shown in this figure, the degree of texturing is steadily enhanced as the temperature is increased from 980°C to 1010°C (note they were held at these temperatures for 20 min.). Most noticeably, the intensities of the (004) and (006) peaks are doubled when heated at 1010°C indicating an increased degree of texturing.

91

10 µm

Figure 4.4 SEM photographs showing the microstructure of group I samples, T max = 950 o

C for 20 min., showing a untextured porous grain structure.

10µ µm

m Figure 4.5 SEM photographs showing the microstructure of group I samples, T max = 990 o

C for 20 min., showing a small island of locally textured grain structure.

92

6 µm Figure 4.6 SEM photographs showing the microstructure of group I samples. Tmax = 1020 oC for 20 min., showing a textured grain structure.

6 µm Figure 4.7 The SEM photographs showing the microstructures of group II samples, as quenched from 1030 oC;

93

12 µm Figure 4.8 The SEM photographs showing the microstructures of group II samples, as quenched from 1000 oC

12 µ m Figure 4.9 The SEM photographs showing the microstructures of group II samples, as quenched from 950 oC.

94

6 µm Figure 4.10 SEM micrographs showing the microstructures of group III samples. They were quenched from 1030 oC to 950 oC and held for 10 min.

6 µm Figure 4.11 SEM micrographs showing the microstructures of group III samples. They were quenched from 1030 oC to 950 oC and held for 20 min.

95

6 µm Figure 4.12 SEM micrographs showing the microstructures of group III samples. They were quenched from 1030 oC to 950 oC and held for 120 min. followed by a cooling at 200 oC/h to room temperature.

96

a S -axis (µm)

9 8 7 6 5 4 3 2 1 0

a. 0.025 µ m/min Tmax = 1030 oC ∆ T = 80 oC 0.53 µ m/min

0

50

100

150

Time (min) 35

b.

a L -axis ( µm)

30 25

0.045 µ m/min

20 15

Tmax=1030 oC

10

∆ T = 80 oC 2.156 µ m/min

5 0 0

50

100

150

Time (min.) Figure 4.13. Growth rate along (a) aS and (b) aL.

97

140

YBCO on silver alloy o: Silver alloy

120

o

1030 C Intensity (Arbitrary Unit)

100

o

80

1020 C

60

o

1000 C

(103) (110)

40

(005)

ο

(006) (116) (123)

ο

20 o

950 C 0 30

35

40

45

50

55

60

2θ θ Figure 4.14. XRD spectra for the YBCO film quenched from the temperatures indicated.

98

(103) (110) •

600

YBCO on Silver Alloy • YBCO ° Silver Alloy

550

500

400

350

300

250

200

(003) •

Intensity (Arbitrary Unit)

450

(005) (014) ° •

(004) ° •

(006) (020) • °

(116) (123) •

980 ° C (007) •

990 °C

1000 °C °

150

100

50

1010 °C ° 0

20

25

30

35

40

45

50

55

60

2θ Figure 4.15. XRD spectra for the films textured for 20 min. at the temperatures indicated.

99

4.4

CONCLUSION

The quenching experiments have been carried out to investigate the nucleation and growth of YBCO from the liquid state. After melting at well above the peritectic point (1010°C), the sample was brought to a lower temperature for the YBCO phase to precipitate from the liquid followed by a rapid quenching in air. The structure quenched in were examined by SEM and XRD. Our experimental results have shown that the initial nucleation takes place on the surface of the substrate and preferably parallel with it. This can be understood by the minimization of the surface energy during nucleation. The initial grain morphology exhibits a column-like rod due to rapid growth along the aaxis. Grain growth proceeds along the other a-axis after certain time period leading to a rectangular-shaped geometry. Prolonged holding at the temperature has resulted in well textured YBCO grains which are confirmed by the XRD data. Therefore, we conclude that the grain oriented substrate may not be needed in obtaining a well-textured YBCO on the metallic substrate. The buffered substrate may only be required in the thin film synthesis.

100

REFERENCES 1.

N.M. Alford, S.J. Penn, and T.W. Button, High-temperature superconducting thick films. Supercond. Sci. Technol.,. 10: p.169 (1997)

2.

D. Shi, D. Qu, X. Wen, B.A. Tent, and M. Tomsic, Direct Peritectic Growth of cAxis Textured YBa2Cu3Ox on a flexible metallic substrate. J. of Superconductivity,. 11: p.575 (1998)

3.

X. Wen, D. Qu, B.A. Tent, and D. Shi, Direct deposition of c-axis textured YBCO thick film on unoriented metallic substrate for the development of long superconducting tapes. IEEE Transactions on Applied Superconductivity,. 9: p.1506 (1999)

101

Chapter 5

Structure Evolution of YBCO from Unoriented Silver Alloy Substrate ξ

ABSTRACT In previous chapter, we have obtained textured YBCO thick films on silver alloy substrate via the peritectic solidification. To identify the underlying mechanism of grain texturing, extensive transmission electron microscopy (TEM) experiments were carried out in this study. A thin “buffer” of 300 nm thickness was observed between the textured YBCO and the silver alloy substrate. This “buffer” was identified to be essentially the YBCO structure, however lacking of the superlattice. Initially randomly oriented, the grains of the buffer gradually self-organize into a preferred orientation and eventually grow into a single crystal YBCO. The underlying mechanism of crystal evolution on an unoriented substrate is discussed.

ξ

Part of this chapter was submitted to Physica C.

102

5.1

INTRODUCTION

Without using a textured buffer layer on the alloy substrate, peritectic solidification was employed to induce the grain texturing. Previous chapters have shown that c-axis textured YBCO obtained via this unique process has resulted in a respectable Jc of 104 A/cm2 at 77 K and zero magnetic field. However, it is critical to identify the mechanism based on which highly textured YBCO can be readily obtained on an unbuffered and unoriented polycrystalline substrate. In this chapter, we present transmission electron microscopy (TEM) experimental results on the YBCO thick films. Based on these results we show the structural lattice evolution at the interface that reveals the underlying texturing mechanism.

5.2

MATERIALS AND METHODS

The YBCO powders were supplied by Superconductive Components, Inc. (Columbus, Ohio, USA) and as rolled untextured pure silver, Ag3%Pd, Ag10%Pd and pure Nickel were provided by Plastronic Inc. (Tipp City, Ohio, USA). The specimens used in this study were melt-textured thick films of YBCO (123) on four kinds of metallic substrates. The films were prepared using doctor-blade technique. The painted specimens had a final thickness up to 20 µm.

These coated specimens were then subjected to the melt processing. They were heated from room temperature at a rate 200 oC/h and held at 500 oC for 5 hours. This procedure was used to burn out the polymer binder. The same rate was used to heat the samples to peritectic temperature (from 950 oC to 1030 oC) where they were held for a range of

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times (from 0.2 h to 1.4 h). The samples were then quickly cooled (~500 oC/h) to 900 oC, and further cooled at a rate of 100 oC/h to room temperature. The samples were given a post-growth oxygenation at 400 oC for 48 h, and then cooled to room temperature at 100 o

C/h. The heating schedule of this experiment is shown in Figure 3.1.

Philips X'Pert MPD X-ray diffractometer was used to determine the texture of the YBCO film. The surface and interface microstructures were characterized by SEM. The transmission electron microscopy (TEM) experiments were performed on a JEM 4000EX TEM.

5.3 RESULTS AND DISCUSSIONS As a result of large driving force during the fast cooling, YBCO grains tended to nucleate and grow rapidly along the surface of the substrate. The film thickness is 10 mm as indicated in the scanning electron microscopy results in Figure 3.6. We have found that holding the sample at different Tmax has resulted in different texturing effects as indicated by x-ray diffraction (XRD) spectra shown in Figure 3.8. As can be seen in this figure, the (004) and (006) peaks steadily increase with the maximum holding temperature indicating an increased grain texturing. At 1010ºC and 1015 ºC (not shown in Fig. 1), we have found that their XRD spectra essentially overlap with each other. The phi-scan of (103) of the 1015 ºC sample is shown in Figure 3.10. As can be seen, sharp and evenly distributed (103) peaks indicate well-aligned YBCO grains along the a-axis. The full width at half maximum (FWHM) is only about 0.3º further showing an excellent in-plane texturing (see Figure. 3.11). It must be noted that the same phi-scan experiment

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was repeated on different areas of the sample and the same results were obtained indicating a general behavior of the in-plan texturing.

As indicated in Figure 3.8, texturing is significantly enhanced at higher maximum processing temperatures. This suggests that, at the temperature range close to peritectic point, more liquids are formed promoting the precipitation of large platelet YBCO grains. Although these grains may not have the perfect out-plane texture as in the thin films, there is a tendency for them to align along the substrate surface normal as indicated in Figure 3.8. Since these grains can be quite large extending to several millimeters, the inplane texture is nearly perfect within each large grain as evidenced in Figure 3.10 and 3.11. But it is realized that each of these large platelet may grow at certain angle on the substrate, as will be seen in the TEM results.

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Ag alloy YBCO

“buffer”

10 nm

500 nm

(a)

(b)

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3.8 Å

3.8 Å

3 nm

(c)

3 nm

(d)

Figure 5.1 (a) Bright field TEM image showing the “buffer” layer with a 300 nm thickness; (b) HREM image of the YBCO-buffer-substrate (from left to right); (c) HREM image of buffer (left) and the substrate (right), note that the buffer is identified to be essentially the YBCO without the superlattice structure with a d-spacing: a=b=c=3.8 Å, and (d) HREM image showing the transition zone of the buffer (right) to YBCO (left), this image shows structural evolution and the self-organized texturing effect.

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To investigate such a unique texturing process, we have carried out extensive TEM analysis on the interface layer between the YBCO and the Ag10%Pd alloy. Figure 5.1a shows the bright field image of the interface in the 1010 ºC-sample. As can be seen in this figure, there appears to be a “buffer” layer of 300 nm thickness between Ag10%Pd substrate and YBCO thick film even though there was no real buffer deposited on Ag10%Pd substrate prior to the film synthesis. A high-resolution electron microscopy (HREM) image of the interface is shown in 5.1b. The left side is YBCO with a buffer in the middle and Ag10%Pd on the right. A close-up image of the buffer is shown in Figure 5.1c. The buffer is on the left side and Ag10%Pd on the right. The lattice structure of the buffer is a cubic with a=b=c=3.8 Å. Its structure exhibits a provskite variation of YBCO without the superlattice. Figure 5.1d shows the transition zone of the buffer (right) to YBCO (left).

From these TEM data we can see an interesting structural evolution that leads to a large YBCO grain on the polycrystalline Ag10%Pd. First, this buffer has a randomly oriented structure, which is understandable since it grows directly on the untextured substrate and it has no crystalline correlation with Ag10%Pd. The grain structure of the silver alloy substrate can be clearly seen from Figure 5.2. Diffraction patterns from different grains have shown the random nature of the substrate. Therefore, the initial YBCO growth (the buffer) must have taken place from the randomly oriented substrate. As mentioned above, this buffer layer is essentially YBCO without superlattice. As the buffer layer grows, the grains gradually orient themselves as evidenced in Figure 5.1c. At certain thickness (~300nm), these crystals in the buffer have grown into a large grain (or a single crystal) with its [001] direction 45° toward the surface normal. The direction of growth is

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determined to be along the [111] direction of the provskite structure. In the YBCO structure with the superlattice this direction would be [331]. The cations are ordered in the c-direction so that the unit cell edge of c-axis is tripled. Although it is difficult to see from HREM the extension of this large grain, the XRD data shown in Figures 3.8, 3.10 and 3.11 indicate a tendency of grain texturing along the c-axis in the thick film, particularly in the 1010ºC and 1015ºC samples.

In previous chapter 3, we have found that the silver alloy does not react with the YBCO nor does it oxidize in air at the top processing temperature. Therefore, the buffer can be regarded as the early growth of YBCO, although lacking of the superlattice structure. Above the peritectic point (1010ºC), the YBCO phase decomposes into liquid and Y2BaCuO5. Upon rapid cooling, stoichiometric YBCO must precipitate from the liquid as a line compound. Near the interface where the heat is being conducted away most efficiently, the YBCO precipitation should take place first. However, energetically, as there is a large lattice parameter difference between YBCO and silver (the lattice constant of Ag is essentially the same as that of its alloy), coherent growth is difficult. Instead, YBCO nucleates in a form with a provskite variation in order to reduce its surface energy. As can be seen in Figure 5.1, the superlattice is absent which may be attributed the cation disordering. Induced by the untextured Ag10%Pd, the newly grown buffer assumes unoriented polycrystalline structure. However, as the buffer continues to grow, it is energetically more favorable for the YBCO lattices to orient themselves and grow along a preferred direction (see Figure 5.3). As a consequence, these crystals grow into a large single crystal and the superlattice has appeared in the structure as evidenced in Figure 5.1.

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In the vapor deposition, the single crystal substrate or textured buffer layer is essential since the growth of YBCO is via the slow diffusion and arrangement of atoms according to the lattice structure of the substrate. Therefore, the growth is epitaxial. A typical deposition rate of molecular beam epitaxy (MBE) is about 1 mm/h [1, 2] . Another feature in vapor deposition is that the growth takes place entirely in the solid state. This can cause considerable strain field at the growth front. In contrast, in non-vapor deposition processes such as peritectic solidification, the grain growth involves a large amount of liquid. The diffusion is much more rapid compared to that in the solid state. As a consequence, the growth rate in YBCO has reached 1mm/h [3] , which is much faster than that in MBE. For such a rapid growth, the atoms can no longer follow the structure of the substrate, therefore impossible to grow epitaxially. Furthermore, the growth front interfaces with a liquid providing certain freedom for lattice rotation as the strain energy is significantly reduced. Therefore on a polycrystalline substrate, peritectic reaction allows the crystals to quickly “find” a preferred orientation and grow without the “guide” from the substrate. However, as can be seen in Figure 3.8, although the overall out-plane texture is improving as the maximum processing temperature increases, certain fraction of the grains may not be well aligned with the surface normal of the substrate. We believe that there is a correlation between the structural self-organization and the overall outplane texture, although the detailed mechanism is not yet identified.

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5.4 CONCLUSIONS To conclude, we have found a structural evolution from the randomly oriented silver alloy substrate that eventually leads to a large grain of YBCO. Within each large grain the in-plane texture is nearly perfect. This self-orientation mechanism suggests that the formation of ordered YBCO crystal lattices may not require a single crystal substrate in the peritectic solidification. Such a film growth mechanism is fundamentally different from the epitaxial growth in the vapor deposition. Based on such a phenomenon, it is possible to induce growth along a specific orientation that is essential in the development of textured YBCO film on randomly oriented substrates.

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Figure 5.2 Bright field image of the interface between the silver alloy and YBCO. The insets are the corresponding diffractions of each layer.

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YBCO crystal with the supper lattice

buffer

Ag alloy

Figure 5.3 Schematic diagram showing the structural evolution at the interface .

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REFERENCES 1.

A.R. Jones, D.A. Cardwell, N.J.C. Ingle, S.P. Ashworth, A.M. Campbell, N.M. Alford, T.W. Button, W. F., and J.S. Abell, Correlation of transport and magnetic critical currents in melt-processed YBa2Cu3O7–delta thick films. J. Appl. Phys.,. 76: p.1720 (1994)

2.

Y. Yamada, J. Kawashima, Y. Niiori, and I. Hirabayashi, Liquid phase epitaxy for the production of YBa2Cu3O7-delta coated conductor. Appl. Supercon.,. 4: p.497 (1996)

3.

X. Wen, D. Qu, and D. Shi, Development of c-axis textured YBCO on unoriented metallic substrate. Cryogenisis, (in press)

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Chapter 6 Future Work From our results using Direct Peritectic Growth (DPG) method fabricating a YBCO thick film on unoriented flexible silver alloy, We have shown that the both out-plane and inplane alignment of YBCO thick film can be achieved by using an unbuffered metallic substrate. If such a process can be further optimized to result in high transport Jc , it may offer an effective alternative method to the RABiTS and IBAD process. The way to further improve the superconducting properties is to optimize YBCO thick film microstructure, which may include:

1. Optimize DPG process parameters through maximum temperature, supercooling, and processing times, and so on.

2. Improve YBCO alignment in thick film though single crystal seeds guidance.

3. Modify substrate surface to achieve uniform surface conditions.

After achieved desirable superconducting properties, the most important future work is dealing with scale-up issues, i.e. Design and setup the equipments that capable of fabricating unlimited length and uniform structure YBCO long tapes.

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Appendix: My Publications at University of Cincinnati Journal Papers 1. Xuejun Wen, Dehui Qu, Brian A. Tent, Donglu Shi, Micheal Tomsic, Lisa Cowey, Marvis White; “Direct deposition of c-axis textured YBCO thick film on unoriented metallic substrate for the development of long superconducting tapes”; IEEE Transactions on Applied Superconductivity; 9: (2); 1506-1509; (1999) 2. D. Shi, D. Qu, X. Wen, B. A. Tent, and M. Tomsic; “Direct Peritectic Growth of c-Axis Textured YBCO for Development of Long Conductors;” J. of Superconductivity; 11; 575580; (1998). 3. Xuejun Wen, Dehui Qu, and Donglu Shi; “Development of C-Axis Textured YBCO on Unoriented Metallic Substrate;” Crygenics, 1999, in press 4. D. Shi, X. Wen, D. Qu, L. Wang and S. Wang; “Interface Structure and Texturing Mechenism in YBCO Thick Films on Silver Alloy Substrate;” Submitted to Physica C. 1999 5. D. Shi, X. Wen, Ron Birkhahn, and A. J. Steckl ; “Effects of Er Doping on Structural Transition and Emission Behavior of GaN Thin Film;” Submitted to J. of Appl. Phys, 1999

6. Donglu Shi, Gengwei Jiang, and Xuejun Wen; “in vitro Bioactive Behavior of Hydroxylaptite-Coated Porous Alumina;” Applied Biomaterials, 1999, in press

Conference papers 1. Xuejun Wen, Dehui Qu, Brian A. Tent, Donglu Shi, Michael Tomsic and Marvis White; “Direct Deposition of c-Axis Textured YBCO Thick Film on Unoriented Metallic Substrate for the Development of Long Superconducting Tapes;” 1998 Applied Superconductivity Conference;13 to 18 September 1998; Palm Desert, California. U.S.A. 2. Donglu Shi, Dehui Qu, Xuejun Wen, Brian A. Tent, Mike Tomsic; “Deposition of c-axis Textured YBa2 Cu3 Ox on a Flexible Metallic Substrate through Direct Peritectic Solidification;” The 128th TMS ANNUAL MEETING; February 28-March 4; 1999 · San Diego, California. U.S.A. 3. Xuejun Wen and Donglu Shi; “Direct Peritectic Deposition of YBCO on Silver Alloy Substrate;” International Cryogenic Materials Conference, July 12-19, 1999, Montreal, Cananda. 4. Donglu Shi, Gengwei Jiang, and Xuejun Wen; “in vitro Bahavior of Hydroxylapatite Prepared by a Thermal Deposition Method;” Invited paper submitted to International Conf. on Surface Modification Tech. Singapore, Sept. 1999 5. Dehui Qu, Brian Tent, Xuejun Wen, Donglu Sh, Shih-Lin Lu, Altan Ferendecii, Ebru Selcuki, David Mast; “Development Of High Frequency Cavity Filters Using SeededMelt Growth Of Single-Domain YBa2Cu3Ox Materials;” American Physical Society Centennial Meeting; March 20-26, 1999, Atlanta, GA

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