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Melt textured Y123 bulk and thick film
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Physica C 341-348 (2000) 2485 2486 www.elsevier.nl/Iocate/physc
Melt t e x t u r e d Y123 bulk and thick film T. Yamashita", A. Y. Ilyushechkina, J.A. Alarcoa'b, J. Richesc P. Talbot" and I. D. R. Mackinnon" "Advanced Ceramics Development, UniQuest Pty. Ltd., University of Queensland, St. Lucia, Queensland 4072, Australia bcentre for Microscopy and Microanalysis, University of Queensland, St.Lucia, Queensland 4072, Australia CDepartment of Physics, University of Queensland, St.Lucia, Queensland 4072, Australia Y B a 2 C B 3 0 7 . 8 - 25mol%Y2BaCuO5 bars and thick films have been melt textured using a stationary furnace with a temperature gradient of 3 or 6 "C/cm. Samples are heated above the peritectic reaction temperature and quenched to just above the solidification temperature and then slowly cooled below the solidification temperature. All bar shaped samples consist of 2-5 mm grains though the grain orientations strongly depend on the heat treatment conditions. The bar shows the maximum Jc of above 3000 A/era2, whereas the maximum Jc of 200 A/cm2 and T~ze~o of 88 K are obtained for the thick film on (100) LaAIO3 single crystal.
1. INTRODUCTION Since the discovery of high-temperature superconducting ceramics, many attempts have been made to commercialise YBa2Cu307.s (Y123) bulk materials. Though it is known that Y123 has intrinsically superior current carrying capability to Bi- or TI- based superconductors, the weak link problem in Y123 makes commercial application difficult. To overcome this problem a melt texturing process was introduced [1]. Recently this group reported on the fabrication of long lengths of wires using Y123 and developed a melt texturing process using a moving fiu'nace [2]. In the present study, short Y123 bars and thick films have been melt-textured using a stationary furnace and the effect of heat treatment on the critical current density (Jc) has been studied.
shaped samples were then cut from the discs. For thick film preparation, the same powder was used. A detailed description of the thick film preparation can be found elsewhere [3]. For the melt texturing heat treatment, a 2-zone controllable horizontal tube furnace was utilised in air. The schematic firing scheme was shown in Figure 1. The samples were heated rapidly to the temperature TI (°C) and held at the temperature for tl (h). The samples were quenched to the temperature/'2 (oc) and then slowl y cooled to 915 oC at rate R1 (°C/h). The slow cooling between/'2 and 915°C was accomplished in a horizontal temperature gradient of 3 or 6°C/cm. The samples were finally oxygenated in flowing oxygen at 475°C for 60h. The microstructures were characterised using a JEOL 6400F scanning electron microscope. The ac resistivity was measured using a LakeShore Model 7000 AC
2. EXPERIMENTAL Stoichiometric Y123 and Y2BaCuOs (Y211) powders were prepared by a co-precipitation method. 25 tool% of Y211 was added into Y123 powder and mixed using a rod mill with A1203. The mixed powder was then pressed into discs and fired at 950°C for 10h. About 5ram x 5mm x 30mm bar-
Figure 1. Heat treatment scheme.
0921-4534/00/$ - see front matter ~ 2000 Elsevier Science B.V. All rights reserved. PII S0921-4534(00)01275-2
2486
T. Yamashita et aL /Physica C 341-348 (2000) 2485-2486
susceptomcter. J~ of the tapes was measured in liquid nitrogen using a four-point probe technique.
cooling rate, governs grain alignment during solidification. The same heat treatment condition used for Samples 2 and 7 was applied to texture thick films on LaA103 single crystal substrates. The film did not show zero resistivity at 77K after heat treatment. The peak height ratio (006)/(013) was about 2.5. When /'2 was increased to 990 °C from 970 °C, the film showed zero resistivity below 88 K and was much more oriented, ie. the peak height ratio increased to 12.0. Though the thick film has smaller volume of Y123 to be textured, it requires higher T2 temperature and longer period between/'2 and 915 °C to texture the film. This characteristic may be attributed to a reaction between film and the substrate. The film composition may have changed and it requires a higher temperature to texture.
3. RESULTS AND DISCUSSION Details of the partial melt process employed for each sample are given in Table 1. The grain size and orientation were studied after the surface is slightly etched [2]. All bar samples consisted of large grains after the melt texturing process. Most of the grains had sizes between 2 mm and 5 mm and the grain orientation depends strongly on the heat treatment conditions. When a temperature gradient of 6"C/cm was applied between T2 and 915 "C, J~'s above I000 A/ClTI2 w e r e obtained for Sample 2 and 7. Sample 1 was partially melted at 1100 *C for 0.3 h, while Samples 2 and 7 were kept at the temperature for 1 h. This difference in the holding period had a significant effect in grain alignment. It indicates that the liquid in the sample that was partially melted at 1100 "C for 0.3 h was viscous and the grain could not easily rotate along the temperature gradient. When the sample was quenched to 1000 °C instead of 970 *C, the grains could orient more easily even after partial melt heat treatment at 1100 *C for 0.3 h as Sample 3 has a higher Jc than Sample 1. With a horizontal temperature gradient 3 "C/em, Samples 4 and 5 show almost the same Jc value. In these cases, 40"C difference in/'1 temperatures does not affect J~ very much. On the other hand, R1 ramp rate has a significant influence on Jc. To obtain high J~ using a furnace with a 3 "C/cm temperature gradient, a smaller R1 is required. This is consistent with the fact that G/R ratio, where G is a temperature gradient and R is a
5. CONCLUSION Bars and thick films of Y123 with 25mo1% Y211 have been melt textured using a horizontal furnace with a horizontal temperature gradient. Though all samples contain large grains, the orientation strongly depends on the heat treatment conditions. Highest Jc values above 3000 A/cm2 and 200 A/cm2 are obtained for the bars and thick film, respectively. REFERENCES
1. S. Jill et al., Appl. Phys. Lett. 52 (1988) 2074. 2. T. Yamashita et al., Materials Lett. 30 (1997) 223. 3. A. Y. Ilyushechkin, et.al., Supercond. Sci. and Teclmol. 10, (1997) 330.
Table 1 Summary of the melt texturing heat treatment. sample#
Temperature gradient ('C/cm)
1 2 3 4 5 6 7 8(film) 9(film)
6 6 6 3 3 3 6 6 6
1100 1100 1100 1140 1100 1100 I100 1100 ll00
/'1
h (h) 0.3 1 0.3 1 1 1 1 1 1
R; (*C/h)
/'2 (*C) 970 970 1000 960 960 965 970 970 990
3 3 3 3 3 1 3 3 3
Jc (A/cm2)
150 >1700 500 130 90 >3000 >2600 p>0 ~ 77K 200
lc (A)
20 >245 75 20 14 >460 >390 /3>0 ~ 77K 0.2
Physica C 341-348 (2000) 2485 2486 www.elsevier.nl/Iocate/physc
Melt t e x t u r e d Y123 bulk and thick film T. Yamashita", A. Y. Ilyushechkina, J.A. Alarcoa'b, J. Richesc P. Talbot" and I. D. R. Mackinnon" "Advanced Ceramics Development, UniQuest Pty. Ltd., University of Queensland, St. Lucia, Queensland 4072, Australia bcentre for Microscopy and Microanalysis, University of Queensland, St.Lucia, Queensland 4072, Australia CDepartment of Physics, University of Queensland, St.Lucia, Queensland 4072, Australia Y B a 2 C B 3 0 7 . 8 - 25mol%Y2BaCuO5 bars and thick films have been melt textured using a stationary furnace with a temperature gradient of 3 or 6 "C/cm. Samples are heated above the peritectic reaction temperature and quenched to just above the solidification temperature and then slowly cooled below the solidification temperature. All bar shaped samples consist of 2-5 mm grains though the grain orientations strongly depend on the heat treatment conditions. The bar shows the maximum Jc of above 3000 A/era2, whereas the maximum Jc of 200 A/cm2 and T~ze~o of 88 K are obtained for the thick film on (100) LaAIO3 single crystal.
1. INTRODUCTION Since the discovery of high-temperature superconducting ceramics, many attempts have been made to commercialise YBa2Cu307.s (Y123) bulk materials. Though it is known that Y123 has intrinsically superior current carrying capability to Bi- or TI- based superconductors, the weak link problem in Y123 makes commercial application difficult. To overcome this problem a melt texturing process was introduced [1]. Recently this group reported on the fabrication of long lengths of wires using Y123 and developed a melt texturing process using a moving fiu'nace [2]. In the present study, short Y123 bars and thick films have been melt-textured using a stationary furnace and the effect of heat treatment on the critical current density (Jc) has been studied.
shaped samples were then cut from the discs. For thick film preparation, the same powder was used. A detailed description of the thick film preparation can be found elsewhere [3]. For the melt texturing heat treatment, a 2-zone controllable horizontal tube furnace was utilised in air. The schematic firing scheme was shown in Figure 1. The samples were heated rapidly to the temperature TI (°C) and held at the temperature for tl (h). The samples were quenched to the temperature/'2 (oc) and then slowl y cooled to 915 oC at rate R1 (°C/h). The slow cooling between/'2 and 915°C was accomplished in a horizontal temperature gradient of 3 or 6°C/cm. The samples were finally oxygenated in flowing oxygen at 475°C for 60h. The microstructures were characterised using a JEOL 6400F scanning electron microscope. The ac resistivity was measured using a LakeShore Model 7000 AC
2. EXPERIMENTAL Stoichiometric Y123 and Y2BaCuOs (Y211) powders were prepared by a co-precipitation method. 25 tool% of Y211 was added into Y123 powder and mixed using a rod mill with A1203. The mixed powder was then pressed into discs and fired at 950°C for 10h. About 5ram x 5mm x 30mm bar-
Figure 1. Heat treatment scheme.
0921-4534/00/$ - see front matter ~ 2000 Elsevier Science B.V. All rights reserved. PII S0921-4534(00)01275-2
2486
T. Yamashita et aL /Physica C 341-348 (2000) 2485-2486
susceptomcter. J~ of the tapes was measured in liquid nitrogen using a four-point probe technique.
cooling rate, governs grain alignment during solidification. The same heat treatment condition used for Samples 2 and 7 was applied to texture thick films on LaA103 single crystal substrates. The film did not show zero resistivity at 77K after heat treatment. The peak height ratio (006)/(013) was about 2.5. When /'2 was increased to 990 °C from 970 °C, the film showed zero resistivity below 88 K and was much more oriented, ie. the peak height ratio increased to 12.0. Though the thick film has smaller volume of Y123 to be textured, it requires higher T2 temperature and longer period between/'2 and 915 °C to texture the film. This characteristic may be attributed to a reaction between film and the substrate. The film composition may have changed and it requires a higher temperature to texture.
3. RESULTS AND DISCUSSION Details of the partial melt process employed for each sample are given in Table 1. The grain size and orientation were studied after the surface is slightly etched [2]. All bar samples consisted of large grains after the melt texturing process. Most of the grains had sizes between 2 mm and 5 mm and the grain orientation depends strongly on the heat treatment conditions. When a temperature gradient of 6"C/cm was applied between T2 and 915 "C, J~'s above I000 A/ClTI2 w e r e obtained for Sample 2 and 7. Sample 1 was partially melted at 1100 *C for 0.3 h, while Samples 2 and 7 were kept at the temperature for 1 h. This difference in the holding period had a significant effect in grain alignment. It indicates that the liquid in the sample that was partially melted at 1100 "C for 0.3 h was viscous and the grain could not easily rotate along the temperature gradient. When the sample was quenched to 1000 °C instead of 970 *C, the grains could orient more easily even after partial melt heat treatment at 1100 *C for 0.3 h as Sample 3 has a higher Jc than Sample 1. With a horizontal temperature gradient 3 "C/em, Samples 4 and 5 show almost the same Jc value. In these cases, 40"C difference in/'1 temperatures does not affect J~ very much. On the other hand, R1 ramp rate has a significant influence on Jc. To obtain high J~ using a furnace with a 3 "C/cm temperature gradient, a smaller R1 is required. This is consistent with the fact that G/R ratio, where G is a temperature gradient and R is a
5. CONCLUSION Bars and thick films of Y123 with 25mo1% Y211 have been melt textured using a horizontal furnace with a horizontal temperature gradient. Though all samples contain large grains, the orientation strongly depends on the heat treatment conditions. Highest Jc values above 3000 A/cm2 and 200 A/cm2 are obtained for the bars and thick film, respectively. REFERENCES
1. S. Jill et al., Appl. Phys. Lett. 52 (1988) 2074. 2. T. Yamashita et al., Materials Lett. 30 (1997) 223. 3. A. Y. Ilyushechkin, et.al., Supercond. Sci. and Teclmol. 10, (1997) 330.
Table 1 Summary of the melt texturing heat treatment. sample#
Temperature gradient ('C/cm)
1 2 3 4 5 6 7 8(film) 9(film)
6 6 6 3 3 3 6 6 6
1100 1100 1100 1140 1100 1100 I100 1100 ll00
/'1
h (h) 0.3 1 0.3 1 1 1 1 1 1
R; (*C/h)
/'2 (*C) 970 970 1000 960 960 965 970 970 990
3 3 3 3 3 1 3 3 3
Jc (A/cm2)
150 >1700 500 130 90 >3000 >2600 p>0 ~ 77K 200
lc (A)
20 >245 75 20 14 >460 >390 /3>0 ~ 77K 0.2