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Effect of infiltration temperature on the properties of infiltration growth processed YBCO superconductor
Physica C 487 (2013) 72–76
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Effect of infiltration temperature on the properties of infiltration growth processed YBCO superconductor S. Pavan Kumar Naik a, N. Devendra Kumar a, P. Missak Swarup Raju a, T. Rajasekharan b, V. Seshubai a,⇑ a b
School of Physics, University of Hyderabad, Hyderabad 500046, India Rajiv Gandhi University of Knowledge Technologies, Vindhya C4, IIIT Campus, Hyderabad 500032, India
a r t i c l e
i n f o
Article history: Received 24 August 2012 Received in revised form 5 January 2013 Accepted 8 January 2013 Available online 30 January 2013 Keywords: Infiltration temperature Refinement in Y-211 particle High critical current density Hardness
a b s t r a c t The importance of optimizing the fabrication of the Y2BaCuO5 (Y-211) preform in achieving high current densities to high magnetic fields has recently been established. We report the effect of the choice of infiltration temperature 1040 °C (sample A) and 1100 °C (sample B) on the microstructure, magnetic properties and mechanical strength. Both the samples showed [1 0 3] texture after slow cooling through peritectic temperature. Infiltration at higher temperature is found to yield highly dense composites with minimal macrodefects and higher hardness of 18.73 GPa in sample B. Both the samples show uniform distribution of Y-211and comparable zero-field critical current density. High current densities are retained to a higher field of 7 T in sample B, unlike 2 T in sample A. The occurrence of [1 0 3] texture promoting higher hardness, simultaneous with retention of considerably high current density to high fields in sample B has definite advantages for trapped field applications. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction REBa2Cu3O7d (REBCO) is one of the best studied High Temperature Superconductors (HTS); they are investigated with a large number of practical applications such as fault current limiters, trapped field magnets, magnetic bearings, and fly wheels in mind. Their critical temperatures (Tc) are higher than the boiling point of liquid N2 (Tc 92 K, for YBCO), and they have the best flux creep characteristics among all HTS. Sintering is the commonly used technique in ceramic processing and allows formation of various shapes needed for various practical applications. However, it has been recognized that, although good Tc values can be easily achieved, critical current density (Jc) values are very small in bulk sintered oxide superconductors [1–3]. Melt textured REBCO materials [4–6] have superior current carrying capacities due to the elimination of most of the weak links and alignment of the grains. Even though the properties of YBa2Cu3O7-d (Y-123) processed by melt texturing are substantially better than those of sintered Y-123, the melt texturing technique poses several problems which are severe enough to block many applications. The incongruent melting of REBCO produces liquid phases, which tend to diffuse out to the exterior of the components as a result of their low viscosity. In the case of samples exceeding a few centimeters in dimensions, the problem is very severe and leads to products with defects like distortions, cracks, and
⇑ Corresponding author. Tel.: +91 40 23134365; fax: +91 40 23010227. E-mail address: [email protected] (V. Seshubai). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.01.008
macro and microporosities. The depletion of liquid phases from the interior of the material will also alter the stoichiometry and hinder grain growth and texturing process. These drawbacks therefore limit the fabrication of large and complex shapes and the only shapes reported so far by melt processing in the literature, are either those of disks or bars [4–6]. Due to their ceramic nature, and due to various kinds of defects mentioned above, the melt processed Y-123 materials pose difficulties in machining into suitable shapes for applications. The possibilities of using the bulk RE-123 material for applications would be aided by the availability of a process which allows the fabrication of near-net-shaped and large-sized components without internal defects and with superconducting properties at least comparable to those of existing in melt processed samples. With the aim of overcoming the difficulties associated with the MG process, the Infiltration Growth (IG) process was developed [7]. It allows near-net shape fabrication of REBCO with microstructures that support high current density [8]. The IG process involves the infiltration of liquid phases (BaCuO2 and CuO) into a porous preform of the primary RE2BaCuO5 (RE-211) phase and subsequently allowing reaction between them to form RE-123 on cooling below the peritectic formation temperature [9,10]. The reaction occurring between RE-211 and the liquid phases is shown below.
RE2 BaCuO5 þ 3BaCuO2 þ 2CuO ! 2REBa2 Cu3 O7d Though the IG process has many advantages compared to the MG process [8,11] like the minimization of shrinkage, cracks and distortions in the final product, various reports discuss the occurrence of inhomogeneity in the distribution of Y2BaCuO5 (Y-211)
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particles in the YBCO matrix [12,13]. This causes a wide variation, across the sample volume, in properties such as microstructure and current densities [12,14,15]. The problem of inhomogeneity in the RE-211 distribution and hence of the non-uniformity in Jc(H), occurs even in MG processed samples [16–18]. Both Mahmood et al. and Iida et al. [19,15] have reported a considerable amount of residual porosity in the IG processed samples. Modifications have been made to the infiltration growth process paying special attention to the stability of the preform used in the process. This modified process was named as ‘‘Preform Optimized Infiltration Growth Process (POIGP)’’. The Y-211 preforms used were made under relatively higher compaction pressures (460 MPa). It was also demonstrated that one needs to sinter the preforms at 950 °C for an optimum duration prior to the liquid phase infiltration, for them to offer enough mechanical rigidity during infiltration. Thus POIGP led to microstructures with minimal porosity and a homogenous distribution of Y-211 inclusions in the superconducting Y-123 matrix [20]. The POIG process involves heat treatment to 1100 °C to render efficient infiltration and was found to yield good samples with flat Jc to high fields. However, the Electron Back Scattered Diffraction (EBSD) results showed [1 0 3] texture in these samples [20]. Since the source pellet Y-123 melts at 1008 °C and the Nd-123 seed melts at 1065 °C, it is worth investigating the pros and cons of selecting a lower infiltration temperature (Ti) during the POIG process. In the present work we compare the result of subjecting the sample assembly to a Ti of 1040 °C with those subjected to Ti of 1100 °C. In this paper, we present the effect of lowering the infiltration temperature on the final microstructures, the critical current density and hardness of the samples. 2. Experimental details Y-123 and Y-211 powders were synthesized following a chemical route employing citrate precursors. In order to prepare powders of Y-123 and Y-211, Y2O3 of Indian Rare Earth make, BaCO3 and CuO of E-Merck make, each of them of 99.99% purity, were used. X-ray diffractograms were recorded using Co Ka radiation to confirm the phase formation of Y-123 and Y-211 powders. Sintered powders of Y-123 and Y-211 were taken in 2:1 ratio and were compacted into pellets under an optimum pressure of 460 MPa. The Y-211 and Y-123 pellets were assembled as shown in Fig. 1 for the POIG process. The pellets were supported on inert substrates to prevent liquid phase loss and to avoid contamination from the alumina crucible used [20]. Two different samples A and B are prepared by POIG process following heat-treatment schedules with infiltration temperatures 1040 °C and 1100 °C as shown in Fig. 2a and b respectively. The POIG process involves heat treating the sample assembly to Ti
Fig. 1. The sample assembly used in POIGP. Y-123, which is the liquid phase source, is kept on top of a Y-211 pellet prepared under the optimum pressure of 460 MPa. They are supported on top of a thin Yttria pellet and dense plates of Yttria stabilized Zirconia and alumina as shown.
Fig. 2. (a and b) Depict the heat treatment schedules followed to synthesize the samples A and B respectively. The only difference between the heat treatments followed for the two samples was the infiltration temperature (Ti). For sample A, Ti was 1040 °C, and for sample B, it was 1100 °C.
where infiltration of liquid phases takes place into Y-211 preform, followed by cooling to 1010 °C, and slow cooing from 1010 °C to 980 °C. Then the samples were furnace cooled. The samples thus obtained were oxygenated in a tubular furnace at 460 °C in flowing oxygen for 100 h [21]. The temperature of the furnace was controlled and monitored using a Eurotherm-make temperature controller (model 2404) and thyristor (TE10A). Photographs of the IG processed samples A and B are shown in Fig. 3. Low speed diamond saw (Isomet – 1000) was used to slice the samples for ac susceptibility (v), microstructural and magnetization measurements. Temperature dependence of ac susceptibility was measured to detect if any low Tc phases were present in the samples. Keithley make ac/dc current source (model 6221) and a Stanford make lock-in amplifier (model SR830) were used in the experiment. The temperature of the sample was measured and controlled using a cryogenic temperature controller (model DRC-93C, Lakeshore). The data was acquired automatically using ‘LabVIEW’ software. Thin sections from different regions of the samples A and B were mounted in bakelite and polished under kerosene on a Buehler auto-polisher using diamond paste down to 0.25 lm size. The polished surface of each of the sample was studied using a field-emission scanning electron microscope (FESEM; Zeiss-make Ultra 55 model). An in-lens detector was used to observe Y-211 particles in the matrix of Y-123. Quantitative analysis of various phases present was carried out using ‘AxioVision’ software.
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Fig. 3. (a) IG processed sample A prepared starting with a preform infiltrated at 1040 °C and (b) IG processed sample B prepared starting with a preform infiltrated at 1100 °C. The arrows point to the final products.
M–H loops were recorded in the samples at different temperatures sweeping the magnetic field in the range 0–8 Tesla and 0– 14 Tesla for sample A and sample B respectively using a physical property measurement system (PPMS). Jc was calculated following extended Bean’s Critical State model [22,23], using the formula J C ¼ 20dDM A/cm2. Here DM (in emu/cc) is the difference in the hysteretic magnetization between the curves obtained while increasing and decreasing the magnetic fields (DM = M+ M) and b d ¼ b 1 3a is the reduced dimension, where ‘a’ and ‘b’ are the planar dimensions (in cm) of the sample with a P b. The dimensions of the specimens used in magnetization measurements were a = 2 mm, b = 1.5 mm, c = 5 mm, for samples A and B. Nano-indentation (TI 900 TriboIndenter, Hysitron made) measurements were carried out at different points on sectioned polished samples to estimate hardness of the material. 3. Results and discussion The temperature dependence of the in-phase components of ac susceptibility (v0 ) measured at 33 Hz for the two samples are shown in Fig. 4. Fig. 4a reveals onset of Tc to be at 91.6 K in sample A with a small tail region close to Tc (zero) indicating possible existence of minor amounts of lower Tc phases which could be oxygen deficient phases [15,24]. Fig. 4b shows onset of a sharp superconducting transition at 92.1 K in sample B. The transition width is nearly 1.5 K in both the samples indicating the samples to be of good quality. Analysis of the microstructures was carried out in samples A and B to assess the distribution and size of the non-superconducting Y-211 particles in the superconducting Y-123 matrix using Carl Zeiss-AxioVision image processing software on the micrographs obtained from FESEM. FESEM images obtained in samples A and B are shown in Fig. 5a and b respectively. The Y-211 particles can be seen to be fine, spherical and uniformly distributed in the Y-123 matrix in both the samples. From histograms it is clear that the distribution of Y-211 shows that majority of the particles are in the range of 0.5–1.5 lm in both the samples. The fact that the Y-211 particles are fine and spherical is expected to contribute to considerable flux pinning and enhancement of Jc due to Y-123/Y-211 interfacial defects [25]. Compared to sample B, sample A shows presence of a large number of fine particles <0.5 lm and absence of many particles larger than 2 lm, probably since lower temperature of operation hinders the grain growth of Y-211 particles. The above
Fig. 4. (a and b) The temperature dependence of ac susceptibility in samples A and B respectively.
microstructural studies demonstrate a clear improvement in producing uniform distribution of fine Y-211 particles in both the samples A and B processed by POIGP, even without adding any grain refiners like Pt and CeO2, when compared to various reports in literature on samples made using MG and IG processes [16,17,14,15].
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Fig. 5. (a and b) Y-211 particles are distributed uniformly in the Y-123 matrix of the samples A and B respectively. The number of Y-211 particles in each sample in different size ranges was estimated from the micrographs and the respective histograms are shown on the right side of the micrographs. It can be observed that a substantial number of Y-211 particles have size below 1 lm in both the samples, and in the sample A for which the infiltration temperature was lower, the particles are finer in size.
Fig. 6. XRD pattern from sample B showing [1 0 3] texture. The region where the lines are crowded is expanded in the inset.
Processing to 1100 °C does not permit seeded growth generally used to achieve [0 0 1] texture. Both the samples processed by POIGP showed [1 0 3] texture. The XRD pattern recorded in sample B is shown in Fig. 6. It can be seen that the sample B shows strong texture in [1 0 3] direction. To estimate the critical current densities, magnetic hysteresis loops were recorded for which the magnetic field (H) was applied normal to the pressed surface of the sample and was varied up to 8 T and 14 T at different temperatures for samples A and B respectively. The field dependences of Jc at different temperatures are shown in Fig. 7.
At temperatures of 50 K and below, high Jc values were retained up to the maximum applied fields of 8 T and 14 T for samples A and B respectively. On the other hand at 77 K, the Jc is found to decrease rapidly beyond 2 T for sample A and beyond 7 T for sample B. Sample A showed a zero-field Jc of 126.4 kA/cm2 at 25 K which decreased to 18.1 kA/cm2 when warmed up to 77 K. Sample B showed Jc(0) of 316 kA/cm2 at 15 K which reduced to 18.2 kA/ cm2 at 77 K. It is evident from Figs. 7a and b that the performance of sample B which exhibited a flat Jc better than 1 kA/cm2 at 77 K up to 7 T is superior to that of sample A. Moreover sample A is very brittle, while sample B is quite hard as observed while cutting sections for various measurements. Higher Ti appears to have resulted in sufficient amount of liquid phase infiltration into Y-211 preform reducing the extent of macrodefects to 0.2% in sample B compared to 2.7% in sample A, as measured from microstructures recorded at low magnification (not shown). In order to assess the effect of infiltration temperature on the mechanical strength, nano-indentation measurements have been carried out at 16 different points on polished samples A and B and average values of hardness have been determined. Sample A, which was processed at an infiltration temperature, Ti of 1040 °C showed a hardness of 8.2 GPa, while sample B showed much higher hardness of 18.7 GPa. This value of sample B is higher compared to not only sample A but also the MGP and IG processed samples reported in literature by Joan Josep Roa Rovira [26]. We attribute higher hardness of the sample B to resultant [1 0 3] texture obtained through slow cooling during POIG process and absence of macrodefects after facilitating infiltration at 1100 °C. Enhancements in mechanical strength simultaneous with high Jc to high fields is important for applications based on trapped field.
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and it also exhibits higher hardness. Though YBCO samples with [0 0 1] texture are reported to yield higher zero field Jc values when field is applied along c-axis, heat treatment at 1100 °C is found from the present work to promote [1 0 3] texture, higher hardness and retention of high Jc to high fields. These results are of importance from application point of view. Acknowledgements Authors thank Center for Nanotechnology for PPMS facility. Grant from XI plan for FE-SEM is gratefully acknowledged. SPKN thanks UGC for Rajiv Gandhi National Fellowship. Department of Science and Technology (DST) is acknowledged by PMSR and NDK for financial assistance. VS acknowledges support in the form of research projects (SR/S2/CMP-47/2004 and IR/S5/IU-01/2006) from DST. References
Fig. 7. (a and b) shows magnetic field dependence of Jc at different temperatures for samples A and B respectively. It can be observed that the Jc of sample A (Ti = 1040 °C) is not sustained to very high fields as for the sample B for which the liquid phases were infiltrated at 1100 °C. This is in spite of the fact that the Y211 distribution is finer in the former case. This has to do with the fact that the former sample is more prone to developing micro-cracks.
4. Conclusion The results obtained on the two samples A and B subjected to infiltration temperatures, Ti of 1040 °C and 1100 °C respectively are compared. Though both the samples A and B show equally good distribution of Y-211 in Y-123 matrix and zero field Jc, sample B shows better field dependence of Jc which is flat up to 7 T at 77 K
[1] M. Murakami, Supercond. Sci. Technol. 5 (1992) 185–203. [2] D.C. Larbalestier, S.E. Babcock, X. Cai, M. Daeumling, D.P. Hampshire, T.F. Kelly, L.A. Lavanier, P.J. Lee, J. Seuntjens, Physica C 153-155 (1988) 1580. [3] F. Stucki, P. Brüesch, T. Baumann, Physica C 153155 (1988) 200. [4] R.L. Meng, C. Kinalidis, Y.Y. Sun, L. Gao, Y.K. Tao, P.H. Hor, C.W. Chu, Nature 345 (1990) 326. [5] J.G. Larsen, N.E. Dam, C.S. Jacobsen, H.D. Baerendtsen, J.G. Sommerschield, Physica C 217 (1993) 135. [6] D. Shi, K. Lahiri, J.R. Hull, D. LeBlanc, M.A.R. LeBlanc, A. Dabkowski, Y. Chang, Y. Jiang, Z. Zhang, H. Fan, Physica C 246 (1995) 253. [7] E. Sudhakar Reddy, T. Rajasekharan, J. Mater. Res. 13 (1998) 2472. [8] A. Mahmood, Y.S. Chu, T.H. Sung, Supercond. Sci. Technol. 25 (2012) 045008. [9] R. Cloots, T. Koutzarova, J.P. Mathieu, M. Ausloos, Supercond. Sci. Technol. 18 (2005) R9. [10] E. Sudhakar Reddy, T. Rajasekharan, Supercond. Sci. Technol. 11 (1998) 523. [11] S. Umakoshi, Y. Ikeda, A. Wongsatanawarid, C.J. Kim, M. Murakami, Physica C 471 (2011) 843–845. [12] S.Y. Chen, Y.S. Hsiao, C.L. Chen, D.C. Yan, I.G. Chen, M.K. Wu, Mater. Sci. Eng. B 151 (2008) 31. [13] K. Iida, Hari N. Babu, T.D. Withnell, Y. Shi, S. Haindl, H.W. Weber, D.A. Cardwell, Physica C 445–448 (2006) 277. [14] K. Iida, N. Hari Babu, Y. Shi, T. Miyazaki, N. Sakai, M. Murakami, D.A. Cardwell, J. Phys. Conf. Ser. 97 (2008) 012105. [15] K. Iida, N. Hari Babu, Y. Shi, D.A. Cardwell, Supercond. Sci. Technol. 18 (2005) 1421. [16] Y.L. Jiao, L. Xiao, H.T. Ren, M.H. Zheng, Y.X. Chen, Physica C 386 (2003) 266. [17] J.C.L. Chow, H.T. Leung, W. Lo, D.A. Cardwell, J. Mater. Sci. 33 (1998) 1083. [18] A. Endo, H.S. Chauhan, Y. Shiohara, Physica C 273 (1996) 107. [19] A. Mahmood, B.H. Jun, H.W. Park, C.J. Kim, Physica C 468 (2008) 1350. [20] N. Devendra Kumar, T. Rajasekharan, K. Muraleedharan, A. Banerjee, V. Seshubai, Supercond. Sci. Technol. 23 (2010) 105020. 8pp. [21] N. Devendra Kumar, T. Rajasekharan, C. Ravi Gundakaram, V. Seshubai, IEEE Trans. Appl. Supercond. 21 (2011) 6. [22] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [23] E.M. Gyorgy, R.B. van Dover, K.A. Jackson, L.F. Schneemeyer, J.V. Waszczak, Appl. Phys. Lett. 55 (1989) 283. [24] M. Daeumling, J.M. Seutjens, D.C. Larbalestier, Nature 346 (1990) 332. [25] D.F. Lee, V. Selvamanickam, K. Salama, Physica C 202 (1992) 83. [26] D. Joan Josep Roa Rovira, Ph.D. thesis, Mechanical Properties of HTSC at Micron/Nanometric Scale, University of Barcelona, 2010, pp. 58 (Chapter 1).
Contents lists available at SciVerse ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Effect of infiltration temperature on the properties of infiltration growth processed YBCO superconductor S. Pavan Kumar Naik a, N. Devendra Kumar a, P. Missak Swarup Raju a, T. Rajasekharan b, V. Seshubai a,⇑ a b
School of Physics, University of Hyderabad, Hyderabad 500046, India Rajiv Gandhi University of Knowledge Technologies, Vindhya C4, IIIT Campus, Hyderabad 500032, India
a r t i c l e
i n f o
Article history: Received 24 August 2012 Received in revised form 5 January 2013 Accepted 8 January 2013 Available online 30 January 2013 Keywords: Infiltration temperature Refinement in Y-211 particle High critical current density Hardness
a b s t r a c t The importance of optimizing the fabrication of the Y2BaCuO5 (Y-211) preform in achieving high current densities to high magnetic fields has recently been established. We report the effect of the choice of infiltration temperature 1040 °C (sample A) and 1100 °C (sample B) on the microstructure, magnetic properties and mechanical strength. Both the samples showed [1 0 3] texture after slow cooling through peritectic temperature. Infiltration at higher temperature is found to yield highly dense composites with minimal macrodefects and higher hardness of 18.73 GPa in sample B. Both the samples show uniform distribution of Y-211and comparable zero-field critical current density. High current densities are retained to a higher field of 7 T in sample B, unlike 2 T in sample A. The occurrence of [1 0 3] texture promoting higher hardness, simultaneous with retention of considerably high current density to high fields in sample B has definite advantages for trapped field applications. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction REBa2Cu3O7d (REBCO) is one of the best studied High Temperature Superconductors (HTS); they are investigated with a large number of practical applications such as fault current limiters, trapped field magnets, magnetic bearings, and fly wheels in mind. Their critical temperatures (Tc) are higher than the boiling point of liquid N2 (Tc 92 K, for YBCO), and they have the best flux creep characteristics among all HTS. Sintering is the commonly used technique in ceramic processing and allows formation of various shapes needed for various practical applications. However, it has been recognized that, although good Tc values can be easily achieved, critical current density (Jc) values are very small in bulk sintered oxide superconductors [1–3]. Melt textured REBCO materials [4–6] have superior current carrying capacities due to the elimination of most of the weak links and alignment of the grains. Even though the properties of YBa2Cu3O7-d (Y-123) processed by melt texturing are substantially better than those of sintered Y-123, the melt texturing technique poses several problems which are severe enough to block many applications. The incongruent melting of REBCO produces liquid phases, which tend to diffuse out to the exterior of the components as a result of their low viscosity. In the case of samples exceeding a few centimeters in dimensions, the problem is very severe and leads to products with defects like distortions, cracks, and
⇑ Corresponding author. Tel.: +91 40 23134365; fax: +91 40 23010227. E-mail address: [email protected] (V. Seshubai). 0921-4534/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2013.01.008
macro and microporosities. The depletion of liquid phases from the interior of the material will also alter the stoichiometry and hinder grain growth and texturing process. These drawbacks therefore limit the fabrication of large and complex shapes and the only shapes reported so far by melt processing in the literature, are either those of disks or bars [4–6]. Due to their ceramic nature, and due to various kinds of defects mentioned above, the melt processed Y-123 materials pose difficulties in machining into suitable shapes for applications. The possibilities of using the bulk RE-123 material for applications would be aided by the availability of a process which allows the fabrication of near-net-shaped and large-sized components without internal defects and with superconducting properties at least comparable to those of existing in melt processed samples. With the aim of overcoming the difficulties associated with the MG process, the Infiltration Growth (IG) process was developed [7]. It allows near-net shape fabrication of REBCO with microstructures that support high current density [8]. The IG process involves the infiltration of liquid phases (BaCuO2 and CuO) into a porous preform of the primary RE2BaCuO5 (RE-211) phase and subsequently allowing reaction between them to form RE-123 on cooling below the peritectic formation temperature [9,10]. The reaction occurring between RE-211 and the liquid phases is shown below.
RE2 BaCuO5 þ 3BaCuO2 þ 2CuO ! 2REBa2 Cu3 O7d Though the IG process has many advantages compared to the MG process [8,11] like the minimization of shrinkage, cracks and distortions in the final product, various reports discuss the occurrence of inhomogeneity in the distribution of Y2BaCuO5 (Y-211)
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particles in the YBCO matrix [12,13]. This causes a wide variation, across the sample volume, in properties such as microstructure and current densities [12,14,15]. The problem of inhomogeneity in the RE-211 distribution and hence of the non-uniformity in Jc(H), occurs even in MG processed samples [16–18]. Both Mahmood et al. and Iida et al. [19,15] have reported a considerable amount of residual porosity in the IG processed samples. Modifications have been made to the infiltration growth process paying special attention to the stability of the preform used in the process. This modified process was named as ‘‘Preform Optimized Infiltration Growth Process (POIGP)’’. The Y-211 preforms used were made under relatively higher compaction pressures (460 MPa). It was also demonstrated that one needs to sinter the preforms at 950 °C for an optimum duration prior to the liquid phase infiltration, for them to offer enough mechanical rigidity during infiltration. Thus POIGP led to microstructures with minimal porosity and a homogenous distribution of Y-211 inclusions in the superconducting Y-123 matrix [20]. The POIG process involves heat treatment to 1100 °C to render efficient infiltration and was found to yield good samples with flat Jc to high fields. However, the Electron Back Scattered Diffraction (EBSD) results showed [1 0 3] texture in these samples [20]. Since the source pellet Y-123 melts at 1008 °C and the Nd-123 seed melts at 1065 °C, it is worth investigating the pros and cons of selecting a lower infiltration temperature (Ti) during the POIG process. In the present work we compare the result of subjecting the sample assembly to a Ti of 1040 °C with those subjected to Ti of 1100 °C. In this paper, we present the effect of lowering the infiltration temperature on the final microstructures, the critical current density and hardness of the samples. 2. Experimental details Y-123 and Y-211 powders were synthesized following a chemical route employing citrate precursors. In order to prepare powders of Y-123 and Y-211, Y2O3 of Indian Rare Earth make, BaCO3 and CuO of E-Merck make, each of them of 99.99% purity, were used. X-ray diffractograms were recorded using Co Ka radiation to confirm the phase formation of Y-123 and Y-211 powders. Sintered powders of Y-123 and Y-211 were taken in 2:1 ratio and were compacted into pellets under an optimum pressure of 460 MPa. The Y-211 and Y-123 pellets were assembled as shown in Fig. 1 for the POIG process. The pellets were supported on inert substrates to prevent liquid phase loss and to avoid contamination from the alumina crucible used [20]. Two different samples A and B are prepared by POIG process following heat-treatment schedules with infiltration temperatures 1040 °C and 1100 °C as shown in Fig. 2a and b respectively. The POIG process involves heat treating the sample assembly to Ti
Fig. 1. The sample assembly used in POIGP. Y-123, which is the liquid phase source, is kept on top of a Y-211 pellet prepared under the optimum pressure of 460 MPa. They are supported on top of a thin Yttria pellet and dense plates of Yttria stabilized Zirconia and alumina as shown.
Fig. 2. (a and b) Depict the heat treatment schedules followed to synthesize the samples A and B respectively. The only difference between the heat treatments followed for the two samples was the infiltration temperature (Ti). For sample A, Ti was 1040 °C, and for sample B, it was 1100 °C.
where infiltration of liquid phases takes place into Y-211 preform, followed by cooling to 1010 °C, and slow cooing from 1010 °C to 980 °C. Then the samples were furnace cooled. The samples thus obtained were oxygenated in a tubular furnace at 460 °C in flowing oxygen for 100 h [21]. The temperature of the furnace was controlled and monitored using a Eurotherm-make temperature controller (model 2404) and thyristor (TE10A). Photographs of the IG processed samples A and B are shown in Fig. 3. Low speed diamond saw (Isomet – 1000) was used to slice the samples for ac susceptibility (v), microstructural and magnetization measurements. Temperature dependence of ac susceptibility was measured to detect if any low Tc phases were present in the samples. Keithley make ac/dc current source (model 6221) and a Stanford make lock-in amplifier (model SR830) were used in the experiment. The temperature of the sample was measured and controlled using a cryogenic temperature controller (model DRC-93C, Lakeshore). The data was acquired automatically using ‘LabVIEW’ software. Thin sections from different regions of the samples A and B were mounted in bakelite and polished under kerosene on a Buehler auto-polisher using diamond paste down to 0.25 lm size. The polished surface of each of the sample was studied using a field-emission scanning electron microscope (FESEM; Zeiss-make Ultra 55 model). An in-lens detector was used to observe Y-211 particles in the matrix of Y-123. Quantitative analysis of various phases present was carried out using ‘AxioVision’ software.
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Fig. 3. (a) IG processed sample A prepared starting with a preform infiltrated at 1040 °C and (b) IG processed sample B prepared starting with a preform infiltrated at 1100 °C. The arrows point to the final products.
M–H loops were recorded in the samples at different temperatures sweeping the magnetic field in the range 0–8 Tesla and 0– 14 Tesla for sample A and sample B respectively using a physical property measurement system (PPMS). Jc was calculated following extended Bean’s Critical State model [22,23], using the formula J C ¼ 20dDM A/cm2. Here DM (in emu/cc) is the difference in the hysteretic magnetization between the curves obtained while increasing and decreasing the magnetic fields (DM = M+ M) and b d ¼ b 1 3a is the reduced dimension, where ‘a’ and ‘b’ are the planar dimensions (in cm) of the sample with a P b. The dimensions of the specimens used in magnetization measurements were a = 2 mm, b = 1.5 mm, c = 5 mm, for samples A and B. Nano-indentation (TI 900 TriboIndenter, Hysitron made) measurements were carried out at different points on sectioned polished samples to estimate hardness of the material. 3. Results and discussion The temperature dependence of the in-phase components of ac susceptibility (v0 ) measured at 33 Hz for the two samples are shown in Fig. 4. Fig. 4a reveals onset of Tc to be at 91.6 K in sample A with a small tail region close to Tc (zero) indicating possible existence of minor amounts of lower Tc phases which could be oxygen deficient phases [15,24]. Fig. 4b shows onset of a sharp superconducting transition at 92.1 K in sample B. The transition width is nearly 1.5 K in both the samples indicating the samples to be of good quality. Analysis of the microstructures was carried out in samples A and B to assess the distribution and size of the non-superconducting Y-211 particles in the superconducting Y-123 matrix using Carl Zeiss-AxioVision image processing software on the micrographs obtained from FESEM. FESEM images obtained in samples A and B are shown in Fig. 5a and b respectively. The Y-211 particles can be seen to be fine, spherical and uniformly distributed in the Y-123 matrix in both the samples. From histograms it is clear that the distribution of Y-211 shows that majority of the particles are in the range of 0.5–1.5 lm in both the samples. The fact that the Y-211 particles are fine and spherical is expected to contribute to considerable flux pinning and enhancement of Jc due to Y-123/Y-211 interfacial defects [25]. Compared to sample B, sample A shows presence of a large number of fine particles <0.5 lm and absence of many particles larger than 2 lm, probably since lower temperature of operation hinders the grain growth of Y-211 particles. The above
Fig. 4. (a and b) The temperature dependence of ac susceptibility in samples A and B respectively.
microstructural studies demonstrate a clear improvement in producing uniform distribution of fine Y-211 particles in both the samples A and B processed by POIGP, even without adding any grain refiners like Pt and CeO2, when compared to various reports in literature on samples made using MG and IG processes [16,17,14,15].
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Fig. 5. (a and b) Y-211 particles are distributed uniformly in the Y-123 matrix of the samples A and B respectively. The number of Y-211 particles in each sample in different size ranges was estimated from the micrographs and the respective histograms are shown on the right side of the micrographs. It can be observed that a substantial number of Y-211 particles have size below 1 lm in both the samples, and in the sample A for which the infiltration temperature was lower, the particles are finer in size.
Fig. 6. XRD pattern from sample B showing [1 0 3] texture. The region where the lines are crowded is expanded in the inset.
Processing to 1100 °C does not permit seeded growth generally used to achieve [0 0 1] texture. Both the samples processed by POIGP showed [1 0 3] texture. The XRD pattern recorded in sample B is shown in Fig. 6. It can be seen that the sample B shows strong texture in [1 0 3] direction. To estimate the critical current densities, magnetic hysteresis loops were recorded for which the magnetic field (H) was applied normal to the pressed surface of the sample and was varied up to 8 T and 14 T at different temperatures for samples A and B respectively. The field dependences of Jc at different temperatures are shown in Fig. 7.
At temperatures of 50 K and below, high Jc values were retained up to the maximum applied fields of 8 T and 14 T for samples A and B respectively. On the other hand at 77 K, the Jc is found to decrease rapidly beyond 2 T for sample A and beyond 7 T for sample B. Sample A showed a zero-field Jc of 126.4 kA/cm2 at 25 K which decreased to 18.1 kA/cm2 when warmed up to 77 K. Sample B showed Jc(0) of 316 kA/cm2 at 15 K which reduced to 18.2 kA/ cm2 at 77 K. It is evident from Figs. 7a and b that the performance of sample B which exhibited a flat Jc better than 1 kA/cm2 at 77 K up to 7 T is superior to that of sample A. Moreover sample A is very brittle, while sample B is quite hard as observed while cutting sections for various measurements. Higher Ti appears to have resulted in sufficient amount of liquid phase infiltration into Y-211 preform reducing the extent of macrodefects to 0.2% in sample B compared to 2.7% in sample A, as measured from microstructures recorded at low magnification (not shown). In order to assess the effect of infiltration temperature on the mechanical strength, nano-indentation measurements have been carried out at 16 different points on polished samples A and B and average values of hardness have been determined. Sample A, which was processed at an infiltration temperature, Ti of 1040 °C showed a hardness of 8.2 GPa, while sample B showed much higher hardness of 18.7 GPa. This value of sample B is higher compared to not only sample A but also the MGP and IG processed samples reported in literature by Joan Josep Roa Rovira [26]. We attribute higher hardness of the sample B to resultant [1 0 3] texture obtained through slow cooling during POIG process and absence of macrodefects after facilitating infiltration at 1100 °C. Enhancements in mechanical strength simultaneous with high Jc to high fields is important for applications based on trapped field.
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and it also exhibits higher hardness. Though YBCO samples with [0 0 1] texture are reported to yield higher zero field Jc values when field is applied along c-axis, heat treatment at 1100 °C is found from the present work to promote [1 0 3] texture, higher hardness and retention of high Jc to high fields. These results are of importance from application point of view. Acknowledgements Authors thank Center for Nanotechnology for PPMS facility. Grant from XI plan for FE-SEM is gratefully acknowledged. SPKN thanks UGC for Rajiv Gandhi National Fellowship. Department of Science and Technology (DST) is acknowledged by PMSR and NDK for financial assistance. VS acknowledges support in the form of research projects (SR/S2/CMP-47/2004 and IR/S5/IU-01/2006) from DST. References
Fig. 7. (a and b) shows magnetic field dependence of Jc at different temperatures for samples A and B respectively. It can be observed that the Jc of sample A (Ti = 1040 °C) is not sustained to very high fields as for the sample B for which the liquid phases were infiltrated at 1100 °C. This is in spite of the fact that the Y211 distribution is finer in the former case. This has to do with the fact that the former sample is more prone to developing micro-cracks.
4. Conclusion The results obtained on the two samples A and B subjected to infiltration temperatures, Ti of 1040 °C and 1100 °C respectively are compared. Though both the samples A and B show equally good distribution of Y-211 in Y-123 matrix and zero field Jc, sample B shows better field dependence of Jc which is flat up to 7 T at 77 K
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