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Gd2BaCuO5 composites
January 1999
Materials Letters 38 Ž1999. 62–69
Effect of Ag on the melt processed GdBa 2 Cu 3 O yrGd 2 BaCuO5 composites E. Sudhakar Reddy ) , T.V.R.K. Sastry, T. Rajasekharan Defence Metallurgical Research Laboratory, P.O. Kanchanbagh, Hyderabad-500 058, India Received 5 January 1998; revised 22 June 1998; accepted 23 June 1998
Abstract The effect of Ag addition on the microstructural and critical current densities of melt processed GdBa 2 Cu 3 O yrGd 2 BaCuO5 ŽGd-123r211. composites as compared to the sample without Ag are studied. Ag is found to refine Gd-211 particles to some extent. The addition of Ag resulted in a non-uniform distribution of Gd-211 in the Gd-123 matrix. The width of the platelets and gap width was found to decrease with Ag addition. Coarse Ag particles were observed to associate with Gd-211 free regions. The role of Ag was found to be different compared to that of Gd-211 particles in modifying the microstructure of the composites. The presence of Ag reduced the normal state resistivity, and marginally increased the critical current density of the material. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Composites; Ag; Melt processing; GdBa 2 Cu 3 O y ; Microstructure
1. Introduction Composite materials have emerged as a good set of materials in increasing the various properties of the matrix materials, right from the materials used for structural applications to enhancing the critical current densities Ž Jc . of the superconducting materials. The reinforcing particles are thought to serve in different ways. In the case of REBa 2 Cu 3 O y ŽRE s Rare Earth, Y, GD, SM, etc., ŽRE-123.. materials, just aligning the grains and eliminating the weak-links itself is not found to be sufficient in enhancing the Jc s of the bulk material to practical levels, whereas the introduction of Y2 BaCuO5 Ž211. particles within the YBa 2 Cu 3 O y ŽY-123. matrix during melt processing was found to enhance the Jc s by an order of )
Corresponding author
magnitude more as compared to the pure material w1–4x. The 211 particles are found to generate secondary defects of the order of coherence length at the 123r211 interphase which are effective in pinning the magnetic flux w5–8x. The amount of pinning depends on the volume and morphology of 211 particles in the material. It has been reported that 40 vol.% or ; 30 mol% of 211 in the 123 matrix was the optimised ratio to result in better Jc w1–3,9,10x. Though a large amount of research has been directed towards understanding the effect of 211 on the microstructural and superconducting properties of the melt processed 123, there exist only a few reports w11–13x on the study of Ag in melt processed 123, and particularly in the optimised compositions with extra Gd-211 additions in the Gd-123 system. One of the few metals which have been found to be chemically inert and compatible with 123 without degrad-
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 1 3 3 - 5
E. Sudhakar Reddy et al.r Materials Letters 38 (1999) 62–69
63
effect of Ag on the microstructural features, particularly on the size and distribution of Gd 2 BaCuO5 ŽGd-211. and the critical current density Ž Jc . of the material is studied and compared with a sample without Ag addition.
2. Experimental
Fig. 1. The time–temperature profile used for melt processing the composites.
ing its superconducting properties is Ag. As the melt processing temperatures of RE-123 are approximately 1008C above the melting point of Ag, uniform distribution of Ag in the matrix was considered to be difficult. It is technologically important to process melt textured 123r211 composites with a uniform distribution of Ag in the matrix. Ag is expected to improve the mechanical properties of the matrix w14x and to refine its microstructural features w3x. In this paper, Gd-123 with 30 mol% of 211, which corresponds to an optimised ratio for better Jc w1–4,9,10x, and another sample with the same composition but with 5 wt.% Ag are melt processed. The
Precursor powder of GdBa 2 Cu 3 O y ŽGd-123. and Gd 2 BaCuO5 in the molar ratio of 70:30 were prepared from high purity Gd 2 O 3 , BaCO 3 and CuO by a chemical route, which involved the dissolution of Gd 2 O 3 and CuO in nitric acid, BaCO 3 in acetic acid and the slow decomposition of the mixed solutions on a hot plate and subsequent vacuum calcination at 9008C. Particle size analysis of the resulting powders showed a substantial amount of micron-sized particles. For processing of a Ag containing composite, GdBa 2 Cu 3 O y q 30 mol% Gd 2 BaCuO5 q Ag ŽGd-30 q Ag., high purity commercially available Ag powder of an average particle size of 5–10 microns was mixed with the above powder in an agate mortar. The powders were shaped into pellets of 15 mm = 15 mm and 7 mm height in a steel die under a pressure of 10 tons. The melt processing of the composites was carried out using thermal schedules as shown
Fig. 2. Optical micrograph of the melt processed Gd-123r211q Ag composite, taken under polarized light. The picture shows two differently oriented domains of Gd-123. The shining particles are Ag and the small dot like particles are Gd-211.
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schematically in Fig. 1. The pellets were first sintered to a density of 80% of the theoretical value, at 9308C for 6 h, so as to minimize the liquid phase loss during melting. They were rapidly heated to 11108C and held there for 20 min, cooled rapidly to 10158C then slow cooled to 9008C at a rate of
18Crh. Processing of the material in ambient atmosphere results in a material with low Tc w10x. This is due to the formation of a solid solution of the kind Gd 1q x Ba 2yx Cu 3 O y . The solid solubility range in the Gd-123 is around x s 2.5 w15x. To suppress the formation of the solid solution, the samples were
Fig. 3. SEM micrographs of the composites; Ža. without silver and Žb. with Ag, showing the distribution and morphology of the residual Gd-211 particles in the matrix.
E. Sudhakar Reddy et al.r Materials Letters 38 (1999) 62–69
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ally decreasing sizes, the last one being of 0.25 mm. The microstructural features were observed using a Leitz optical microscope equipped with a polarizer and a JEOL JSM-840 Scanning Electron Microscope. Grain sizes were measured directly from the micrographs by standard quantitative metallographic methods w17x. The transition temperature ŽTc . of the samples were measured by standard four probe technique. The magnetization loops were recorded using a VSM at 80 K, and up to an applied field of 1.3 T.
Fig. 4. Histograms of the size of the residual Gd-211 particles present in the Gd-123 matrix of the composites with and without Ag.
processed in high pure Ar gas Žoxygen impurity: 2 ppm. with a flow rate of 1 lrmin, in a similar manner as reported for the processing of Sm-123 and Nd-123 superconductors by the OCMG process w15,16x. The time-temperature schedule shown in Fig. 1 is the experimentally optimized schedule for thermal treatment in high pure Ar gas. The resulting textured tetragonal Gd-123 were transformed into the orthorhombic phase by annealing in flowing oxygen between 6008C–3008C according to the schedule shown in the Fig. 1. Samples for microscopic examination were prepared by mounting in a cold setting resin and polishing using diamond pastes of gradu-
3. Results and discussion Generally, the microstructure of melt textured 123 processed in the absence of temperature gradients consists of differently oriented domains, each domain containing parallel platelets of 123 with a common c-axis. If the starting composition is richer in 211 phase, small 211 particles are observed to be randomly distributed within the 123 grains. Fig. 2 shows an optical micrograph obtained using polarized light, of the melt processed Gd-123r211q Ag sample. Two domains with different shades reflecting their mutually different orientations can be seen. The bright particles in the microstructure are silver and the small grey particles distributed all over the microstructure are Gd-211. The Silver particles are observed to have average size of 10 mm. Another
Fig. 5. Optical micrograph of the Ag containing composite. Gd-211-free regions Žmarked. around the coarse Ag particles can be seen.
E. Sudhakar Reddy et al.r Materials Letters 38 (1999) 62–69
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Table 1 Estimated microstructural features and transition temperatures for Gd-30 and Gd-30q Ag Sample
Initial 211 Žmol%.
Tc Ž0.
DTc
Residual 211 Žvol.%.
Residual 211 size Žmm.
Platelet width Žmm.
Gd-30 Gd-30q Ag
30 30
90.2 91.4
0.6 0.8
41.1 42.1
1.9 1.0
1.9 1.1
important observation is the refinement of Gd-211 particles with Ag addition. Fig. 3Ža. and Žb. show the representative pictures of the residual Gd-211 particles after melt texturing in Gd-123r211 samples with and without Ag addition taken at a higher magnification using scanning electron microscope. The size distribution of Gd-211 particles in the melt processed microstructures are shown in Fig. 4. Ag addition is found to refine the average Gd-211 size from ; 2 mm to ; 1 mm. It has been reported in the literature that addition of other noble metals like Pt also refines 211 particles w18x. Ag is found not to be as effective in refining the Gd-211 particles as Pt w18,19x in the case of Y-211 particles. But undoubtedly the addition of Ag reduces the Gd-211 particle size. The possible mechanism by which Ag can reduce the Gd-211 particle size are the following. According to the theory of Ostwald ripening w20x the radius of the Gd-211 particles is given by the equation. R 3 y r 3 s Ž Õ 2s CD1rKT . t where t is the holding time, R the particle size at time t, r the initial particle size, Õ is the volume per atom in the particles, s the surface energy between 211 and liquid, C the concentration of the atom at the interface, D 1 the diffusion coefficient in liquid, k Boltzman constant and T is the temperature. The dissolution of some amount of Ag in the liquid can change the properties of liquid w21x and that can be expected to modify either s , C or D 1 and contribute to the refinement of Gd-211. Addition of extra 211, CeO 2 etc. are known to refine 211 particle size during melt texturing w22–25x. The mechanism of their doing so is by acting as additional nucleation sites for the 211 formed by decomposition of 123 w24x. The available flux would then be distributed among more number of particles, thus reducing the size of each particle. But such a mechanism is
unlikely in the case of Ag containing samples because the temperatures in question is about 1008C above the melting point of silver and silver remains a liquid at this temperature. A close observation of the Ag containing sample reveals a few more microstructural features. One of them is the occurrence of Gd-211-free regions around the coarse Ag particles as shown in Fig. 5. The smaller Ag particles are found to be rather uniformly distributed among the Gd-211 particles, whereas coarser Ag particles, of size ; 30 mm, are found in the Gd-211-free regions. The above observations can be explained in the following way. It has been observed that during the melting stage of 123, above the peritectic temperature, spherical pores are formed either due to oxygen evolution w26,27x, or due to the low viscosity of the liquid in the molten state w10x. These pores are later filled with liquid phases in the molten state. At this stage, as the temperature is nearly 1008C above the melting point of Ag, the liquid phases along with some amount of molten Ag will fill these pores. The 211 with their sluggish
Fig. 6. Temperature dependence of the electrical resistivity of the composites after oxygenation. Inset shows the magnified portion near Tc . Note the decrease in resistivity for Ag containing sample. Tc of )90 K with sharp transition width represents the absence of solid solution formation in the present composites.
E. Sudhakar Reddy et al.r Materials Letters 38 (1999) 62–69
movement, will not move into the pools. During the texturing stage, when cooling through Tp , these liquid phase pools will be converted into 123 by the diffusion of rare-earth ions from the neighbouring regions, with Ag solidifying in the centre of the pores resulting in Gd-211 free regions with silver at the centre. For an effective and homogeneous pin-
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ning of the flux and for better mechanical properties, the above microstructural defect is not desirable. The above microstructural defect has been eliminated by a modification of processing technique and is reported elsewhere w28x. Various quantities estimated from the microstructures are presented in Table 1. The platelet thickness
Fig. 7. Ža. Magnetization hysteresis loops recorded at 80 K for the composites. Žb. Critical current densities versus applied field at 80 K.
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E. Sudhakar Reddy et al.r Materials Letters 38 (1999) 62–69
is observed to decrease with the addition of Ag as also the width of the gap between the platelets. The width of the platelets have been reported to scale with the 211 particle size w3,29,30x. In the present case, the decrease in the platelet thickness and gap width can be attributed indirectly to Ag through the refinement of Gd-211 particles. Another observation is that the platelet size does not correlate to the size trapped Ag particles. Which affirms that the role of Ag during the formation of Gd-123 platelets is different from that of Gd-211 particles w30x. This follows that Ag is not acting as nucleating sites for Gd-123 or Ag particles are not contributing to the formation of gaps similar to 211 particles w30x. This is in contrast to the other additives like ZrO 2 , etc., where they are found to be favoured nucleation sites for the 123 phase and the amount of texturing is altered w31x. The reason for the different behaviour of Ag can be attributed to its physical state during the formation of Gd-123. Fig. 6 shows plots of resistivity versus temperature of the samples. Both samples exhibited Tc Ž0. of 92 K with a transition width of 1 K, representing the absence of solid solution formation in the samples. The decrease in normal state resistivity of the Ag containing sample can be noticed. A low resistivity in the normal state is desirable for making low resistance ohmic contacts in practical use. The magnetization loops, obtained using a VSM for Hrrc-axis of the samples are shown in Fig. 7Ža.. It can be observed that the magnetic hysteresis loops of the Ag containing sample with fine sized Gd-211 is larger in size than that of the undoped sample. The magnetic Jc , of the samples was calculated using the extended Bean’s critical model for a specimen of an orthorhombic shape w32x: Jc s 20D MraŽ1 y ar3b ., where D M is the difference in the magnetization between increasing and decreasing magnetic fields and aŽ1 y ar3b . is a geometric factor related to sample dimensions with b ) a. The calculated Jc of the sample with magnetic field was shown in Fig. 7Žb.. In the case of sample containing Ag, an increase in Jc can be observed, and also the fall in Jc with field is better. The observed increase in Jc s with Ag may be due to the small size of Gd-211 particles in the Gd-123 matrix as compared to the Ag free sample. The fine sized Gd-211 particles with small curvatures are more effective in generating
secondary defects at the interface of the order of coherence length and in pinning the flux w3–8x. Although, the Ag-free sample has equal amount of Gd-211 phase, the decrease in the Gd-211 particle size due to Ag is contributing for enhancement in Jc as compared to the Ag free sample with coarser Gd-211 particles. The Ag particles themselves with a particle size of few mm may not contribute for flux pinning. The Ag particles have a lower thermal expansion coefficient in comparison with 123 w33x and they are unlikely to create stresses which generate defects at the Gd-123rAg interfaces for flux pinning. In summary, the effect of Ag on the microstructural features and magnetic Jc s of Gd-123r211 composite in comparison with an equivalent composite, but without Ag are studied. The Ag is found to refine the Gd-211 particles in the material. The Ag is not found to alter the microstructural features of the melt textured material directly, whereas the observed microstructural changes are attributed indirectly to Ag due to the refinement of Gd-211 particles. Addition of Ag decreased the normal state resistivity of the composite. The magnetization measurements showed that addition of Ag improved Jc of the Gd-123r211 composite superconductor over the Ag free sample. Acknowledgements The authors are grateful to the Director, DMRL, for permission to publish this work. ESR thanks the UGC for a fellowship. References w1x M. Murakami, S. Gotoh, N. Koshizuka, S. Tanaka, T. Matsuishita, S. Kambe, K. Kitazawa, Cryogenics 30 Ž1990. 390. w2x P.J. Kung, M.P. Maley, M.E. McHenry, J.O. Willis, M. Murakami, S. Tanaka, Phys. Rev. 18 Ž1993. 13922. w3x R. Gopalan, T. Roy, T. Rajasekharan, G. Rangarajan, N. Hari Babu, Physica C 24 Ž1995. 106. w4x M. Chopra, S.W. Chan, R.L. Meng, C.W. Chu, J. Mater. Res. 11 Ž1996. 1616. w5x D.F. Lee, M. Mironova, V. Selvamanickam, K. Salama, Interface Sci. 1 Ž1994. 381. w6x M. Mironova, V. Selvamanickam, D.F. Lee, K. Salama, J. Mater. Res. 8 Ž1993. 2767. w7x Z.L. Wang, A. Goyal, M. Kroeger, Phys. Rev. B 47 Ž1993. 5373. w8x V. Selvamanickam, K. Salama, Physica C 202 Ž1992. 83.
E. Sudhakar Reddy et al.r Materials Letters 38 (1999) 62–69 w9x M. Murakami, T. Oyama, H. Fujimoto, T. Taguchi, S. Gotoh, Y.S. Shiohara, N. Koshizuka, S. Tanaka, Jpn. J. Appl. Phys. 29 Ž1990. L1991. w10x E.S. Reddy, Ph.D. Thesis, University of Hyderabad, Ž1997.. w11x A. Goyal, P.D. Funkengusch, D.M. Kroeger, S.J. Burns, Physica C 182 Ž1991. 203. w12x A. Xia, H.T. Ren, Y. Zhao, C. Andrikidis, H.K. Liu, S.X. Dou, Physica C 194–196 Ž1994. 1615. w13x N.L. Wu, H.H. Zern, C.L. Chen, Physica C 241 Ž1995. 198. w14x P.A. Curreri, P.N. Peters, R.C. Sisk, M.K. Wu, Y.C. Huang, Metall. Trans. A 21A Ž1990. 257. w15x M. Murakami, N. Sakai, T. Higuchi, S.I. Yoo, Supercond. Sci. Tech. 9 Ž1996. 2914. w16x S.I. Yoo, M. Murakami, N. Sakai, T. Higuchi, S. Tanaka, Jpn. J. Appl. Phys. 33 Ž1994. L1000. w17x The basis of quantitative metallography, in: F.B. Pickering ŽEd.., Institute of Metallurgical Technicians, Monograph No. 1, Ž1984.. w18x N. Ogawa, I. Hirabayashi, S. Tanaka, Physica C 177 Ž1991. 101. w19x T. Izumi, Y. Nakahara, T.H. Sung, Y. Shiohara, J. Mater. Res. 7 Ž1992. 801. w20x W. Ostwald, Z. Phys. Chem. 34 Ž1900. 495. w21x T. Oka, Y. Itoh, Y. Yanagi, H. Tanaka, S. Takashiara, Y. Yamado, U. Mizutani, Physica C 200 Ž1992. 55.
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w22x C.J. Kim, S.H. Lai, P.J. McGinn, Mater. Lett. 19 Ž1994. 185. w23x C.J. Kim, K.B. Kim, G.W. Hong, J. Mater. Res. 7 Ž1995. 2605. w24x P.J. McGinn, W. Chen, N. Zhu, L. Tan, C. Varanasi, S. Sengupta, Appl. Phys. Lett. 59 Ž1991. 120. w25x E. Sudhakar Reddy, T. Rajasekharan, Mat. Lett., in press. w26x C.J. Kim, H.G. Lee, B.B. Kim, G.W. Hong, J. Mater. Res. 9 Ž1995. 2235. w27x H.W. Park, K.B. Kim, K.W. Lee, H. Kuk, G.W. Hong, C.J. Kim, Supercond. Sci. and Tech. 9 Ž1996. 694. w28x E. Sudhakar Reddy, T. Rajasekharan, J. Mater. Res., in press. w29x S. Jin, G.W. Kammlott, T.H. Tiefel, T.T. Kodas, T.L. Ward, D.M. Kroeger, Physica C 181 Ž1991. 5. w30x G.J. Schmitz, J. Laakmann, Ch. Wolters, S. Rex, W. Gawalek, T. Habisreuther, G. Bruchlos, P. Gornert, J. Mater. Res. 8 Ž1993. 2774. w31x Y. Feng, L. Zhou, S. Shi, Y. Lu, X. Jin, Y. Zhang, J. Jin, X. Yao, Y. Zhang, J. Appl. Phys. 76 Ž1994. 2954. w32x A. Umezawa, G.W. Crabretee, J.Z. Liu, W. Weber, W.K. Kwok, L.H. Nunez, P.J. Moran, C.H. Sowers, H. Claus, Phys. Rev. B 36 Ž1987. 7151. w33x J.P. Singh, D.S. Kuperman, S. Majumdar, R.L. Hitterman, D.W. Schroeder, Proc. Int. Conf. on Micromechanics of Failure of Quasi-Brittle Materials, Albuquerque, NM, 7–8 June, 1990.
Materials Letters 38 Ž1999. 62–69
Effect of Ag on the melt processed GdBa 2 Cu 3 O yrGd 2 BaCuO5 composites E. Sudhakar Reddy ) , T.V.R.K. Sastry, T. Rajasekharan Defence Metallurgical Research Laboratory, P.O. Kanchanbagh, Hyderabad-500 058, India Received 5 January 1998; revised 22 June 1998; accepted 23 June 1998
Abstract The effect of Ag addition on the microstructural and critical current densities of melt processed GdBa 2 Cu 3 O yrGd 2 BaCuO5 ŽGd-123r211. composites as compared to the sample without Ag are studied. Ag is found to refine Gd-211 particles to some extent. The addition of Ag resulted in a non-uniform distribution of Gd-211 in the Gd-123 matrix. The width of the platelets and gap width was found to decrease with Ag addition. Coarse Ag particles were observed to associate with Gd-211 free regions. The role of Ag was found to be different compared to that of Gd-211 particles in modifying the microstructure of the composites. The presence of Ag reduced the normal state resistivity, and marginally increased the critical current density of the material. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Composites; Ag; Melt processing; GdBa 2 Cu 3 O y ; Microstructure
1. Introduction Composite materials have emerged as a good set of materials in increasing the various properties of the matrix materials, right from the materials used for structural applications to enhancing the critical current densities Ž Jc . of the superconducting materials. The reinforcing particles are thought to serve in different ways. In the case of REBa 2 Cu 3 O y ŽRE s Rare Earth, Y, GD, SM, etc., ŽRE-123.. materials, just aligning the grains and eliminating the weak-links itself is not found to be sufficient in enhancing the Jc s of the bulk material to practical levels, whereas the introduction of Y2 BaCuO5 Ž211. particles within the YBa 2 Cu 3 O y ŽY-123. matrix during melt processing was found to enhance the Jc s by an order of )
Corresponding author
magnitude more as compared to the pure material w1–4x. The 211 particles are found to generate secondary defects of the order of coherence length at the 123r211 interphase which are effective in pinning the magnetic flux w5–8x. The amount of pinning depends on the volume and morphology of 211 particles in the material. It has been reported that 40 vol.% or ; 30 mol% of 211 in the 123 matrix was the optimised ratio to result in better Jc w1–3,9,10x. Though a large amount of research has been directed towards understanding the effect of 211 on the microstructural and superconducting properties of the melt processed 123, there exist only a few reports w11–13x on the study of Ag in melt processed 123, and particularly in the optimised compositions with extra Gd-211 additions in the Gd-123 system. One of the few metals which have been found to be chemically inert and compatible with 123 without degrad-
00167-577Xr99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 9 8 . 0 0 1 3 3 - 5
E. Sudhakar Reddy et al.r Materials Letters 38 (1999) 62–69
63
effect of Ag on the microstructural features, particularly on the size and distribution of Gd 2 BaCuO5 ŽGd-211. and the critical current density Ž Jc . of the material is studied and compared with a sample without Ag addition.
2. Experimental
Fig. 1. The time–temperature profile used for melt processing the composites.
ing its superconducting properties is Ag. As the melt processing temperatures of RE-123 are approximately 1008C above the melting point of Ag, uniform distribution of Ag in the matrix was considered to be difficult. It is technologically important to process melt textured 123r211 composites with a uniform distribution of Ag in the matrix. Ag is expected to improve the mechanical properties of the matrix w14x and to refine its microstructural features w3x. In this paper, Gd-123 with 30 mol% of 211, which corresponds to an optimised ratio for better Jc w1–4,9,10x, and another sample with the same composition but with 5 wt.% Ag are melt processed. The
Precursor powder of GdBa 2 Cu 3 O y ŽGd-123. and Gd 2 BaCuO5 in the molar ratio of 70:30 were prepared from high purity Gd 2 O 3 , BaCO 3 and CuO by a chemical route, which involved the dissolution of Gd 2 O 3 and CuO in nitric acid, BaCO 3 in acetic acid and the slow decomposition of the mixed solutions on a hot plate and subsequent vacuum calcination at 9008C. Particle size analysis of the resulting powders showed a substantial amount of micron-sized particles. For processing of a Ag containing composite, GdBa 2 Cu 3 O y q 30 mol% Gd 2 BaCuO5 q Ag ŽGd-30 q Ag., high purity commercially available Ag powder of an average particle size of 5–10 microns was mixed with the above powder in an agate mortar. The powders were shaped into pellets of 15 mm = 15 mm and 7 mm height in a steel die under a pressure of 10 tons. The melt processing of the composites was carried out using thermal schedules as shown
Fig. 2. Optical micrograph of the melt processed Gd-123r211q Ag composite, taken under polarized light. The picture shows two differently oriented domains of Gd-123. The shining particles are Ag and the small dot like particles are Gd-211.
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E. Sudhakar Reddy et al.r Materials Letters 38 (1999) 62–69
schematically in Fig. 1. The pellets were first sintered to a density of 80% of the theoretical value, at 9308C for 6 h, so as to minimize the liquid phase loss during melting. They were rapidly heated to 11108C and held there for 20 min, cooled rapidly to 10158C then slow cooled to 9008C at a rate of
18Crh. Processing of the material in ambient atmosphere results in a material with low Tc w10x. This is due to the formation of a solid solution of the kind Gd 1q x Ba 2yx Cu 3 O y . The solid solubility range in the Gd-123 is around x s 2.5 w15x. To suppress the formation of the solid solution, the samples were
Fig. 3. SEM micrographs of the composites; Ža. without silver and Žb. with Ag, showing the distribution and morphology of the residual Gd-211 particles in the matrix.
E. Sudhakar Reddy et al.r Materials Letters 38 (1999) 62–69
65
ally decreasing sizes, the last one being of 0.25 mm. The microstructural features were observed using a Leitz optical microscope equipped with a polarizer and a JEOL JSM-840 Scanning Electron Microscope. Grain sizes were measured directly from the micrographs by standard quantitative metallographic methods w17x. The transition temperature ŽTc . of the samples were measured by standard four probe technique. The magnetization loops were recorded using a VSM at 80 K, and up to an applied field of 1.3 T.
Fig. 4. Histograms of the size of the residual Gd-211 particles present in the Gd-123 matrix of the composites with and without Ag.
processed in high pure Ar gas Žoxygen impurity: 2 ppm. with a flow rate of 1 lrmin, in a similar manner as reported for the processing of Sm-123 and Nd-123 superconductors by the OCMG process w15,16x. The time-temperature schedule shown in Fig. 1 is the experimentally optimized schedule for thermal treatment in high pure Ar gas. The resulting textured tetragonal Gd-123 were transformed into the orthorhombic phase by annealing in flowing oxygen between 6008C–3008C according to the schedule shown in the Fig. 1. Samples for microscopic examination were prepared by mounting in a cold setting resin and polishing using diamond pastes of gradu-
3. Results and discussion Generally, the microstructure of melt textured 123 processed in the absence of temperature gradients consists of differently oriented domains, each domain containing parallel platelets of 123 with a common c-axis. If the starting composition is richer in 211 phase, small 211 particles are observed to be randomly distributed within the 123 grains. Fig. 2 shows an optical micrograph obtained using polarized light, of the melt processed Gd-123r211q Ag sample. Two domains with different shades reflecting their mutually different orientations can be seen. The bright particles in the microstructure are silver and the small grey particles distributed all over the microstructure are Gd-211. The Silver particles are observed to have average size of 10 mm. Another
Fig. 5. Optical micrograph of the Ag containing composite. Gd-211-free regions Žmarked. around the coarse Ag particles can be seen.
E. Sudhakar Reddy et al.r Materials Letters 38 (1999) 62–69
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Table 1 Estimated microstructural features and transition temperatures for Gd-30 and Gd-30q Ag Sample
Initial 211 Žmol%.
Tc Ž0.
DTc
Residual 211 Žvol.%.
Residual 211 size Žmm.
Platelet width Žmm.
Gd-30 Gd-30q Ag
30 30
90.2 91.4
0.6 0.8
41.1 42.1
1.9 1.0
1.9 1.1
important observation is the refinement of Gd-211 particles with Ag addition. Fig. 3Ža. and Žb. show the representative pictures of the residual Gd-211 particles after melt texturing in Gd-123r211 samples with and without Ag addition taken at a higher magnification using scanning electron microscope. The size distribution of Gd-211 particles in the melt processed microstructures are shown in Fig. 4. Ag addition is found to refine the average Gd-211 size from ; 2 mm to ; 1 mm. It has been reported in the literature that addition of other noble metals like Pt also refines 211 particles w18x. Ag is found not to be as effective in refining the Gd-211 particles as Pt w18,19x in the case of Y-211 particles. But undoubtedly the addition of Ag reduces the Gd-211 particle size. The possible mechanism by which Ag can reduce the Gd-211 particle size are the following. According to the theory of Ostwald ripening w20x the radius of the Gd-211 particles is given by the equation. R 3 y r 3 s Ž Õ 2s CD1rKT . t where t is the holding time, R the particle size at time t, r the initial particle size, Õ is the volume per atom in the particles, s the surface energy between 211 and liquid, C the concentration of the atom at the interface, D 1 the diffusion coefficient in liquid, k Boltzman constant and T is the temperature. The dissolution of some amount of Ag in the liquid can change the properties of liquid w21x and that can be expected to modify either s , C or D 1 and contribute to the refinement of Gd-211. Addition of extra 211, CeO 2 etc. are known to refine 211 particle size during melt texturing w22–25x. The mechanism of their doing so is by acting as additional nucleation sites for the 211 formed by decomposition of 123 w24x. The available flux would then be distributed among more number of particles, thus reducing the size of each particle. But such a mechanism is
unlikely in the case of Ag containing samples because the temperatures in question is about 1008C above the melting point of silver and silver remains a liquid at this temperature. A close observation of the Ag containing sample reveals a few more microstructural features. One of them is the occurrence of Gd-211-free regions around the coarse Ag particles as shown in Fig. 5. The smaller Ag particles are found to be rather uniformly distributed among the Gd-211 particles, whereas coarser Ag particles, of size ; 30 mm, are found in the Gd-211-free regions. The above observations can be explained in the following way. It has been observed that during the melting stage of 123, above the peritectic temperature, spherical pores are formed either due to oxygen evolution w26,27x, or due to the low viscosity of the liquid in the molten state w10x. These pores are later filled with liquid phases in the molten state. At this stage, as the temperature is nearly 1008C above the melting point of Ag, the liquid phases along with some amount of molten Ag will fill these pores. The 211 with their sluggish
Fig. 6. Temperature dependence of the electrical resistivity of the composites after oxygenation. Inset shows the magnified portion near Tc . Note the decrease in resistivity for Ag containing sample. Tc of )90 K with sharp transition width represents the absence of solid solution formation in the present composites.
E. Sudhakar Reddy et al.r Materials Letters 38 (1999) 62–69
movement, will not move into the pools. During the texturing stage, when cooling through Tp , these liquid phase pools will be converted into 123 by the diffusion of rare-earth ions from the neighbouring regions, with Ag solidifying in the centre of the pores resulting in Gd-211 free regions with silver at the centre. For an effective and homogeneous pin-
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ning of the flux and for better mechanical properties, the above microstructural defect is not desirable. The above microstructural defect has been eliminated by a modification of processing technique and is reported elsewhere w28x. Various quantities estimated from the microstructures are presented in Table 1. The platelet thickness
Fig. 7. Ža. Magnetization hysteresis loops recorded at 80 K for the composites. Žb. Critical current densities versus applied field at 80 K.
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is observed to decrease with the addition of Ag as also the width of the gap between the platelets. The width of the platelets have been reported to scale with the 211 particle size w3,29,30x. In the present case, the decrease in the platelet thickness and gap width can be attributed indirectly to Ag through the refinement of Gd-211 particles. Another observation is that the platelet size does not correlate to the size trapped Ag particles. Which affirms that the role of Ag during the formation of Gd-123 platelets is different from that of Gd-211 particles w30x. This follows that Ag is not acting as nucleating sites for Gd-123 or Ag particles are not contributing to the formation of gaps similar to 211 particles w30x. This is in contrast to the other additives like ZrO 2 , etc., where they are found to be favoured nucleation sites for the 123 phase and the amount of texturing is altered w31x. The reason for the different behaviour of Ag can be attributed to its physical state during the formation of Gd-123. Fig. 6 shows plots of resistivity versus temperature of the samples. Both samples exhibited Tc Ž0. of 92 K with a transition width of 1 K, representing the absence of solid solution formation in the samples. The decrease in normal state resistivity of the Ag containing sample can be noticed. A low resistivity in the normal state is desirable for making low resistance ohmic contacts in practical use. The magnetization loops, obtained using a VSM for Hrrc-axis of the samples are shown in Fig. 7Ža.. It can be observed that the magnetic hysteresis loops of the Ag containing sample with fine sized Gd-211 is larger in size than that of the undoped sample. The magnetic Jc , of the samples was calculated using the extended Bean’s critical model for a specimen of an orthorhombic shape w32x: Jc s 20D MraŽ1 y ar3b ., where D M is the difference in the magnetization between increasing and decreasing magnetic fields and aŽ1 y ar3b . is a geometric factor related to sample dimensions with b ) a. The calculated Jc of the sample with magnetic field was shown in Fig. 7Žb.. In the case of sample containing Ag, an increase in Jc can be observed, and also the fall in Jc with field is better. The observed increase in Jc s with Ag may be due to the small size of Gd-211 particles in the Gd-123 matrix as compared to the Ag free sample. The fine sized Gd-211 particles with small curvatures are more effective in generating
secondary defects at the interface of the order of coherence length and in pinning the flux w3–8x. Although, the Ag-free sample has equal amount of Gd-211 phase, the decrease in the Gd-211 particle size due to Ag is contributing for enhancement in Jc as compared to the Ag free sample with coarser Gd-211 particles. The Ag particles themselves with a particle size of few mm may not contribute for flux pinning. The Ag particles have a lower thermal expansion coefficient in comparison with 123 w33x and they are unlikely to create stresses which generate defects at the Gd-123rAg interfaces for flux pinning. In summary, the effect of Ag on the microstructural features and magnetic Jc s of Gd-123r211 composite in comparison with an equivalent composite, but without Ag are studied. The Ag is found to refine the Gd-211 particles in the material. The Ag is not found to alter the microstructural features of the melt textured material directly, whereas the observed microstructural changes are attributed indirectly to Ag due to the refinement of Gd-211 particles. Addition of Ag decreased the normal state resistivity of the composite. The magnetization measurements showed that addition of Ag improved Jc of the Gd-123r211 composite superconductor over the Ag free sample. Acknowledgements The authors are grateful to the Director, DMRL, for permission to publish this work. ESR thanks the UGC for a fellowship. References w1x M. Murakami, S. Gotoh, N. Koshizuka, S. Tanaka, T. Matsuishita, S. Kambe, K. Kitazawa, Cryogenics 30 Ž1990. 390. w2x P.J. Kung, M.P. Maley, M.E. McHenry, J.O. Willis, M. Murakami, S. Tanaka, Phys. Rev. 18 Ž1993. 13922. w3x R. Gopalan, T. Roy, T. Rajasekharan, G. Rangarajan, N. Hari Babu, Physica C 24 Ž1995. 106. w4x M. Chopra, S.W. Chan, R.L. Meng, C.W. Chu, J. Mater. Res. 11 Ž1996. 1616. w5x D.F. Lee, M. Mironova, V. Selvamanickam, K. Salama, Interface Sci. 1 Ž1994. 381. w6x M. Mironova, V. Selvamanickam, D.F. Lee, K. Salama, J. Mater. Res. 8 Ž1993. 2767. w7x Z.L. Wang, A. Goyal, M. Kroeger, Phys. Rev. B 47 Ž1993. 5373. w8x V. Selvamanickam, K. Salama, Physica C 202 Ž1992. 83.
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