Materials Science Forum Vols. 702-703 (2012) pp 1011-1014 Online available since 2011/Dec/06 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.702-703.1011
Microstructural Characterization of Ferroelectric Bismuth Ferrite (BiFeO3) Ceramic by Electron Backscattered Diffraction Jayant Kolte1, a, Devidas Gulwade2, b, Aatish Daryapurkar1, c, Prakash Gopalan1, d 1
Department of Metallurgical Engineering & Materials Science, Indian Institute of Technology Bombay, Mumbai, India.
2
Department of Physics, VES College of Arts, Science & Commerce, Chembur Mumbai, India. a
[email protected],
[email protected],
[email protected], d
[email protected]
Keywords: Bismuth Ferrite, Ferroelectric Ceramic, EBSD, Texture and Microstructure
Abstract. Ferroelectric BiFeO3 (BFO) is potential candidate for future generation of FeRAM due to its large polarization. However, BFO is very sensitive to secondary phase formation during synthesis because of volatility issues related to Bismuth. Investigation of the microstructure for phase purity is the key as impurities can destroy the desired properties. We have used backscattered electron diffraction to study the microstructure of BFO ceramic. The EBSD results provide a direct evidence of the appearance of secondary phase that XRD could not be detected in XRD. Introduction Multiferroic materials can provide new dimensions to the existing applications and can operate with new functionalities such as data storage in which data can be written electrically and read magnetically, thereby increasing the lifetime of the memory device. However, there are very few intrinsic multiferroic materials which have magnetoelectric coupling. Bismuth ferrite is one of the few single phase room temperature multiferroic materials. Bismuth ferrite has been widely investigated and exhibits large spontaneous polarization as well as ferromagnetic property. This material exhibits additional advantage of a large Curie (TC = 8630C) and high Neel temperature (TN = 397 0C) [1] which makes it attractive for various applications with multifunctionalities. The roomtemperature phase of BFO is rhombohedral (point group R3C). [2] Two perovskite type unit cells with a lattice parameter of 3.965A0 form a unit cell of rhombohedral angle 89.30 at room temperature, with ferroelectric polarization along [111]pseudocubic owing to the shift of Bi ions along the [111] direction and distortion of FeO6 octahedra surrounding the [111] axis. The unit cell can also be described in a hexagonal frame of reference, with the hexagonal c-axis parallel to the diagonals of the perovskite cube, i.e., [001]hexagonal || [111]pseudocubic. The hexagonal lattice parameters are a = 5.58A0 and c = 13.90A0 [3]. BFO is very difficult to synthesize because Bi volatilizes during sintering of the material. There are several methods to synthesize of bismuth ferrite like solid state reaction, pechini method, xerogel method, co-precipitation method, PVA sol gel method [4-8]. The density and stoichiometry of the target plays a vital role in pulsed laser deposition, as it affects the microstructure and quality of thin film. Therefore it is necessary to characterize a sintered pellet before making thin films of the material. In this work, BFO is analysed for the first time by EBSD for detection of secondary phase present in the sample.
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Experimental procedure We have synthesized BFO by PVA sol gel method [8]. Starting materials Bi2O3 and Fe (NO3)3.9H2O were taken in stoichiometric amount. A 5% PVA solution was made by adding PVA (MW=130000) to deionized water. The polymer was dissolved by heating at 70°C and constant stirring. Bi2O3 were dissolved in nitric acid to make bismuth nitrate. Then Bi (NO3)3 and PVA were mixed at 70°C with stirring. Finally Fe (NO3)3.9H2O was added to the solution. In this process the ratio of positively charged valance to hydroxyl group were kept at 2:1 to obtain the desired product. With constant heating the liquid became viscous, and then a colloid formed with the evolution of NOx gases. Finally, it was kept on hot plate at 250°C for 2h. The fluffy powder was then calcined at 500°C for 2 hrs. and has been characterized using XRD. Further, calcined powder was mixed with 5% solution of PVA as binder and pressed uniaxially into cylindrical compact of diameter 10 mm and thickness 2 mm at 100MPa pressure. The resultant green compacts were slowly heated up to 500°C and maintained there for one hour to remove the organic binder. Thereafter, pellet was sintered at 750°C for 4 h. Furthermore polishing parameters were optimized to obtain good EBSD pattern. X-ray diffraction patterns were recorded on Philips X'Pert system using step size of 0.01°/10 Seconds. EBSD measurements were carried out on Quanta-3D Field Emission Gun (FEG) system which has low vacuum mode. Conductive coating of sample was avoided because there was no sign of charging. A TSL-OIM software was used for the analysis and a Confidence Index (CI) [9] of 0.1 was used which gives the 95% accuracy of the measured data. Result and Discussion
(110)
(220)
(024)
(116) (122) (018) (214)
(006)
Intensity (AU)
(202)
(012)
(104)
The XRD pattern of BFO is exhibited in Fig.1. The material exhibits a single phase perovskite structure and has been indexed using JCPDS file (01-086-1518) with R3C space group and lattice parameter a = 5.58A0 and c = 13.90A0, in agreement with the patterns reported in the literature [3]. The strongest diffraction peak at 2θ = 32.08° corresponds to the diffraction from the plane.
Sintered Pellet
Calcined Powder 20
40
60
80
2θ (degrees)
Figure 1 X-ray diffraction pattern of BFO calcined powder and sintered pellet. Fig. 2(a) depicts the orientation map of BFO which is indexed in R3C (161) space group having lattice parameters a = 5.58A0 and c = 13.90A0.There are several grains in the range between 10 and 25 µm. Figure 2(b) shows the pole figure which indicates the randomness of the texture.
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(a)
(b)
Figure 2 (a) Orientation map showing the Electron backscattered diffraction pattern of Bismuth ferrite, (b) Pole figure by EBSD showing random orientation in BFO ceramic. We were also able to detect secondary phase by EBSD. We have tried Bi25FeO40 and Bi2Fe4O9 phases to index impurity phases. The secondary phase is Bi25FeO40 which gives good indexing as compared to Bi2Fe4O9. A phase contrast map have been provided in Fig 3(a) clearly depicts that secondary phase is only present at grain-boundaries. The presence of secondary phase was also confirmed by EDX line scan across the grains. A grain size distribution has been exhibited in Figure 3(b). The average grain size is 22 µm.
Grain Size (Diameter)
0.40 0.35
Area Fraction
0.30 0.25
Average Grain Size 22 µm
0.20 0.15 0.10 0.05
BiFeO3 Bi25FeO40
0.00 0
5
10
15
20
25
30
Grain Size Diameter (Microns)
(a)
(b)
Figure 3(a) Micrograph showing two phases (Secondary phase Bi25FeO40 is marked by circles), (b) grain size distribution shows average grain size of about 22 microns.
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In summary, EBSD has emerged as a very novel technique for local characterization of ceramic materials. For the first time we have locally resolved and established the existence of secondary phase in BFO using electron backscattered diffraction pattern. The EBSD result provides a direct evidence of the appearance of secondary phase. This is a very useful finding as bismuth ferrite is very sensitive to secondary phase at grain boundary leading to artifacts in the electrical properties of the material. We plan to further investigate thin films of BiFeO3 by EBSD using high resolution FEG-EBSD instrument. Acknowledgement The authors gratefully acknowledgement support from National facility of OIM and Texture (a DST- IRPHA facility at IIT Bombay). References
[1] G A Smolenskii and I E Chupis, Ferroelectromagnets, Sov. Phys. Usp. 25 (1982) 475-493. [2] J. M. Moreau, C. Michel, R. Gerson and W. J. James, Ferroelectric BiFeO3 X-Ray and Neutron Diffraction Study, J. Phys. Chem. Solids 32 (1971) 1315-1320. [3] F. Kubel and H. Schmid, Structure of a Ferroelectric and Ferroelastic Monodomain Crystal of the Perovskite BiFeO3, Acta Cryst. B46 (1990) 698-702. [4] M. Mahesh Kumar ,V. R. Palkar, K. Srinivas and S. V. Suryanarayana, Ferroelectricity in a pure BiFeO3 ceramic, Appl. Phys. Lett. 76 2764 (2000). [5] Qing-hui Jiang & Ce-wen Nan & Yao Wang & Yu-heng Liu & Zhi-jian Shen, Synthesis and properties of multiferroic BiFeO3 ceramics, J Electroceram. 21 690 (2008). [6] O. D. Jayakumar,S. N. Achary,K. G. Girija, A. K. Tyagi, C. Sudakar, G. Lawes, R. Naik, J. Nisar, X. Peng, and R. Ahuja, Theoretical and experimental evidence of enhanced ferromagnetism in Ba and Mn cosubstituted BiFeO3, Appl. Phys. Lett. 96 032903 (2010). [7] S. Chattopadhyay, S. D. Kelly, V. R. Palkar, L.Fan and C. U. Segre, Investigation of Size Effects in Magnetoelectric BiFeO3, Physica Scripta. T115 709 (2005). [8] Ting Liu, Yebin Xu and Jingyuan Zhao, Low-Temperature Synthesis of BiFeO3 via PVA Sol– Gel Route, J. Am. Ceram. Soc., 93 3637 (2010). [9] M. M. Nowell, S. I. Wright, Orientation effects on indexing of electron backscatter diffraction patterns, Ultramicroscopy 103 41 (2005).
Textures of Materials - ICOTOM 16 10.4028/www.scientific.net/MSF.702-703
Microstructural Characterization of Ferroelectric Bismuth Ferrite (BiFeO3) Ceramic by Electron Backscattered Diffraction 10.4028/www.scientific.net/MSF.702-703.1011
