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Fabrication and characterization of bulk BSCCO(2212)-SrSO4 composites by melt casting process
Physica C 445–448 (2006) 447–450 www.elsevier.com/locate/physc
Fabrication and characterization of bulk BSCCO(2212)-SrSO4 composites by melt casting process K.T. Kim a, S.H. Jang a, E.C. Park a, J. Joo a,*, H. Kim a, G.-W. Hong b, C.-J. Kim c, H.-R. Kim d, O.-B. Hyun d b
a School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440 746, Republic of Korea The Department of Electronic Engineering, Korea Polytechnic University, Siheung, GyungGi-Do 429 793, Republic of Korea c Nuclear Material Development Team, Korea Atomic Energy Research Institute, Daejeon 305 353, Republic of Korea d Power System Laboratory, Korea Electric Power Research Institute, Daejeon 305 380, Republic of Korea
Available online 12 June 2006
Abstract We fabricated 2212-SrSO4 composites by the melt casting process and evaluated the effect of melt flowing on the critical current (Ic) by using two different pouring methods, i.e., the conventional vertical casting and the tilt casting methods. It was observed that the 2212SrSO4 rod processed by the tilt casting method had a higher Ic value than that processed by the vertical casting method. This improved Ic is considered to be related to the more homogenous microstructure due to the laminar flow of the melt. In addition, we evaluated the variation of the Ic with the SrSO4 content. The Ic value was found to be significantly dependent on the SrSO4 content: the Ic of the 2212 increased as the SrSO4 content increased and reached a peak value (260 A at 77 K) at an SrSO4 content of 6 wt.%, and then decreased when the SrSO4 content was further increased. This increase is probably due to the lower porosity, larger grain size and stronger texture, resulting from the enhanced diffusion kinetics. The possible causes of the variation in the Ic with the SrSO4 content based on the microstructural observations were discussed in detail. Ó 2006 Elsevier B.V. All rights reserved. PACS: 74.72.Hs; 74.81.Bd Keywords: BSCCO-2212; Critical current; Melt casting process; Microstructure; SrSO4
1. Introduction High-Tc bulk superconductors are used in fault current limiters, magnetic shield systems and current lead and bus bars in various superconducting systems, etc. BSCCO-2212 (2212) is known to be one of the most promising materials for these applications, because of its highcritical temperature (Tc), the wide temperature range in which the superconductor phase forms, and its reversible phase formability during melting, solidification and annealing. Thus, the melt casting process (MCP) can be used to
*
Corresponding author. Tel.: +82 31 290 7385; fax: +82 31 290 7371. E-mail address: [email protected] (J. Joo).
0921-4534/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.04.035
obtain bulk 2212 of complex shape and large size with a strong texture and high critical current (Ic). Substantial research has been conducted into the use of MCP process to improve the Ic value of bulk 2212. The MCP process includes powder melting, casting, solidification, and heat treatment and the critical properties vary with the processing conditions. In addition, several alloying elements, such as Ag, MgO, BaSO4, SrSO4, etc., can be added to improve both the electrical properties and mechanical integrity of bulk 2212, and the addition of SrSO4 was reported to have beneficial effects on its Ic value and machinability [1,2]. However, the effect of melt flowing on the Ic was not evaluated and the SrSO4 content was not systematically optimized for improving the Ic and mechanical properties.
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K.T. Kim et al. / Physica C 445–448 (2006) 447–450
In this study, we fabricated pure 2212 and 2212-(3– 15 wt.%) SrSO4 composite rods by the MCP process and then evaluated the effect of melt flowing on the Ic value by using two different pouring methods, i.e., the conventional vertical casting and the tilt casting methods. In addition, we determined the optimum content of SrSO4 required to improve the Ic and the possible causes of the variation of the Ic with the SrSO4 content based on the microstructural observations.
2212 2201 SrSO4 CaCuO2 (Sr,Ca)CuOx
Intensity (A.U.)
annealed rod
ingot
powder
2. Experimental Bi2Sr2Ca1Cu2O8+d (2212) and SrSO4 powders were supplied by Nexans and Kojundo chemical Co. The powders were mixed and heated at 1200 °C for 10 min in an alumina crucible. The melt was then poured into a quartz-tube mold, which was pre-heated to 500 °C, and then cooled to room temperature. The casting process was performed using two different methods, i.e., vertical and tilt casting. In the vertical casting method, the mold was vertically located inside the heater and the melt was poured into the mold. In the tilt casting method, on the other hand, the mold was placed inside the heater, tilted at an angle of 60° and then slowly returned to the upright position during the melt pouring. The specimens were 10 mm in diameter and 85 mm in length. The ingot was heat treated using two separate steps: annealing at 800 °C for 120 h in an oxygen atmosphere to form the 2212 phase, followed by annealing at 650 °C for 20 h in a nitrogen atmosphere to optimize the oxygen content. The microstructures were observed by scanning electron microscopy (HITACHI, S-3000 H). Phase identification was performed by X-ray diffraction (RIGAKU, 12KW) and electron probe microanalysis (SHIMADZU, EPMA1600). The porosity was measured by the Archimedes method. The hardnesses of the specimens were measured by Vickers hardness testing machine (MITUTOYO, MVK-H2). The Tc was measured by a four probe resistive method with an applied current of 10 mA. The Ic was measured by the same method with a 1 lV/cm criterion at 77 K in a self field.
22
26
28 30 32 2 theta (2 Θ )
34
36
38
Fig. 1. XRD patterns for the 2212 powder, 2212-10 wt.% SrSO4 ingot, and 2212-10 wt.% SrSO4 rod after the heat treatments.
gen content was reduced to be in the range of 8.16–8.2 at a nitrogen atmosphere during the second heat treatment [4]. To evaluate the effect of the casting method on the Ic of the 2212-10 wt.% SrSO4 rod, the casting was performed by the vertical and tilt casting methods. Fig. 2 shows the annealed 2212-10 wt.% SrSO4 rod and the voltage–current characteristics of the samples. In order to analyze the variation of the Ic at various positions on the rod, four voltage probes were connected: the a–b, b–c, and c–d taps to measure the Ic at the top, center and bottom of the rod and the a–d tap to measure that of the whole rod. As shown in Fig. 2(b), the Ic of the rods processed by the vertical and tilt casting methods were 57 A and 87 A, respectively,
(a) 0.010
Vertical casting (large ΔT) Tilt casting (large ΔT) Tilt casting (small ΔT)
0.008
Voltage (mV)
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns for the 2212 powder, 2212-10 wt.% SrSO4 ingot, and annealed rod. The as-received 2212 powder consisted of mainly 2212 phase together with small amounts of unknown phases, while the solidified ingot had 2201 as the major phase and SrSO4, (Sr,Ca)CuOx, and CaCuO2 as minor phases. After annealing, the 2201 phase disappeared and the 2212 phase became the major one, suggesting that the 2201 reacted with the Ca- and Cu-rich phases to form the 2212 phase during the heat treatment in an oxygen atmosphere [3]. The Tczero was measured to be 88 K and 91 K after the first and second heat treatments, respectively, and this improvement in the Tc is believed to be due to the fact that the oxy-
24
0.006 0.004 0.002 0.000 0
50
100
150
200
Current (A) (b) Fig. 2. (a) The 2212 rod with the four voltage probe and (b) the voltage– current characteristics of the rods processed with the different casting methods.
K.T. Kim et al. / Physica C 445–448 (2006) 447–450
condition is related to the formation of dense and uniform microstructures. To evaluate the dependence of the Ic on the SrSO4 content, the pure 2212 and 2212-3, 6, 10, 12, and 15 wt.% SrSO4 composite rods were made by the tilt casting and their Ic values are shown in Fig. 4. The Ic was found to be 177 A for the pure 2212 rod, and this value improved as the SrSO4 content increased, reaching a peak value of 256 A at an SrSO4 content of 6 wt.%. The Ic subsequently decreased to 186 A at an SrSO4 content of 10 wt.%, and continued to decrease significantly as the SrSO4 content was further increased. An initial increase in the Ic with increasing SrSO4 content was also observed in other studies [2], but the role of the SrSO4 content in 2212 has not been systematically studied. The possible causes for the variation in the Ic with the SrSO4 content were correlated with the SEM observations. Fig. 5 shows the SEM micrographs of the pure 2212, 2212-6 wt.%, and 2212-15 wt.% SrSO4 rods. For the 2212 rod, there were many pores and the grains were not aligned parallel to each other. On the other hand, it was observed that the 2212-6 wt.% SrSO4 rod had less pores, larger grains, and a stronger texture than the pure 2212 rod, and some of the dark-colored particles were confirmed to be SrSO4 by EPMA analysis. The DTA analysis indicated that the addition of 6 wt.% SrSO4 decreased the melting point of 2212 by 6 °C. A lower melting point will increase 300 350
Critical current (A)
250
300
200
250
150
200 150
100
100 50
50
2
Critical current density (A/cm )
and this disparity is probably due to the difference in the pouring methods. Because the melt moves vertically into the mold, the vertical casting method results in the so called ‘‘turbulent flow’’ of the melt and, consequently, produces many pores that can be trapped during solidification [5]. The existence of pores can degrade the texture formability of the plate-like 2212 grains and, hence, reduce the Ic value. On the other hand, in the tilt casting method, the melt flows along the mold-wall and induces ‘‘laminar flow’’, thereby restricting the formation of pores. The porosities were found to be 3.54% and 2.98% for the rods processed by the vertical and tilt casting methods, respectively, which is in good agreement with the above explanation. It is also to be noted that the Ic varied significantly between the different regions of the rod. The Ic at the top, center and bottom of the rod were 102, 64 and 51 A for the rod processed by the vertical casting method and 87, 79 and 107 A by the tilt casting method, respectively, indicating that the Ic of the whole rod was primarily affected by the lowest Ic region. The large variation of the Ic in the rod is probably due to the temperature difference in the mold: the temperature at the top and bottom of the mold differed from each other by approximately 50 °C. In a previous study, we observed that the preheating temperature was one of the parameters which influenced the microstructure and Ic. Using a specially designed heating system which assured a temperature variation of less than 3 °C within the mold, we obtained an improved Ic with less deviation in the tilt casting method. The Ic valued were 155, 153, and 163 A for the top, center, and bottom of the rod, respectively, and that of the whole rod was 153 A. Fig. 3 shows the SEM micrographs of the fracture surface of the rods processed by the vertical and tilt casting methods. In the case of the tilt casting method with a uniform pre-heating temperature, the rod had less pores and a stronger texture than that processed by the vertical casting with a large temperature difference. From additional SEM observations, we found that the former method led to the alignment of more grains in the direction of the length of the rod and the development of a more uniform microstructure throughout the rod than in the case of the latter method. Thus, it is considered that the improved Ic obtained in the tilt casting with the uniform preheating
449
0
0 0
3
6
9
12
15
SrSO4 content (wt.%) Fig. 4. Variation of the Ic value of the 2212 rods with the SrSO4 content.
Fig. 3. SEM micrographs of the fracture surface of the rods processed by (a) the vertical casting method and (b) the tilt casting method : The arrow indicated the longitudinal direction of the rod.
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K.T. Kim et al. / Physica C 445–448 (2006) 447–450
Fig. 5. SEM micrographs of the (a) pure 2212, (b) 2212-6 wt.%, and (c) 2212-15 wt.% SrSO4 rods.
the diffusion kinetics at a given heat treatment temperature, leading to an improvement in the density, grain growth and alignment. This improved microstructure is one of the possible reasons for the observed increase in the Ic of the 2212SrSO4 rod. For the 2212-15 wt.% SrSO4 rod, we observed that the grain was not well aligned and that dendrites formed on the surface. In addition, the dendrites became larger and their density became higher with increasing depth inside the rod. The EPMA analysis indicated that the dendrite consisted mainly of SrSO4 phase. The reason for the formation of dendrite at a higher SrSO4 content is not clearly understood, however, it is well known that its presence interferes with the formation of texture. It is also to be noted that the fraction of 2212 phase decreased with increasing SrSO4 content and that the grain connectivity of the 2212 was considerably degraded when the SrSO4 phases began to be cross-linked with one another. Therefore, the observed decrease in the Ic at a higher SrSO4 content is partly related to the degradation of the texture and grain connectivity of the 2212 phase. The hardnesses of the pure 2212 and 2212-SrSO4 composites were measured. The hardness of pure 2212 was found to be 92.8 and this value linearly increased as the SrSO4 content increased. The hardnesses of the rods with SrSO4 contents of 3, 6, 10, 12, and 15 wt.% were 123.3, 131.8, 141.7 148.7 and 177.2, respectively. This improvement in the hardness was attributed to the strengthening mechanism afforded by the addition of the alloying material and the fact that the hardness of the SrSO4 (193) was higher than that of 2212 [6]. 4. Conclusions We evaluated the effects of the melt flowing process and the SrSO4 content on the Ic value of 2212-SrSO4 superconductor. It was observed that the 2212-SrSO4 rod processed
by the tilt casting method had a higher Ic than that processed by the vertical casting method. This improved Ic value is considered to be related to the more homogenous microstructure associated with the lower porosity and stronger texture afforded by the laminar flow of the melt. In addition, the Ic value was significantly dependent on the SrSO4 content. The Ic of 2212 increased with increasing SrSO4 content, reached a peak value (260 A at 77 K) at an SrSO4 content of 6 wt.%, and then decreased when the SrSO4 content was further increased. The initial improvement in the Ic value with increasing SrSO4 content is probably due to the lower porosity, larger grain size, and stronger texture, resulting from the enhanced diffusion kinetics. On the other hand, it is believed that the decrease in the Ic at a higher SrSO4 content is partly related to the degraded texture and poor grain connectivity of the 2212 phase. The addition of SrSO4 also had a beneficial effect on the hardness of 2212. Acknowledgements This research was supported by Electric Power Industry R&D program funded by the Ministry of Commerce, Industry, and Energy, Republic of Korea. References [1] S. Pavard, C. Vollard, D. Bourgault, R. Tournier, Supercond. Sci. Technol. 11 (1998) 1359. [2] J. Bock, H. Bestgen, S. Elchner, E. Preisler, IEEE Trans. Appl. Supercond. 3 (1993) 1659. [3] J. Bock, E. Preisler, Solid State Commun. 72 (1989) 453. [4] D. Buhl, T. Lang, L.J. Gauckler, Supercond. Sci. Technol. 10 (1997) 32. [5] J. Mi, R.A. Harding, J. Campbell, Int. J. Cast Metals Res. 14 (2002) 325. [6] B.-H. Kim, K.-H. Ko (Eds.), Handbook of Chemistry and Physics, KDR, Korea, 2002, p. 1199.
Fabrication and characterization of bulk BSCCO(2212)-SrSO4 composites by melt casting process K.T. Kim a, S.H. Jang a, E.C. Park a, J. Joo a,*, H. Kim a, G.-W. Hong b, C.-J. Kim c, H.-R. Kim d, O.-B. Hyun d b
a School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440 746, Republic of Korea The Department of Electronic Engineering, Korea Polytechnic University, Siheung, GyungGi-Do 429 793, Republic of Korea c Nuclear Material Development Team, Korea Atomic Energy Research Institute, Daejeon 305 353, Republic of Korea d Power System Laboratory, Korea Electric Power Research Institute, Daejeon 305 380, Republic of Korea
Available online 12 June 2006
Abstract We fabricated 2212-SrSO4 composites by the melt casting process and evaluated the effect of melt flowing on the critical current (Ic) by using two different pouring methods, i.e., the conventional vertical casting and the tilt casting methods. It was observed that the 2212SrSO4 rod processed by the tilt casting method had a higher Ic value than that processed by the vertical casting method. This improved Ic is considered to be related to the more homogenous microstructure due to the laminar flow of the melt. In addition, we evaluated the variation of the Ic with the SrSO4 content. The Ic value was found to be significantly dependent on the SrSO4 content: the Ic of the 2212 increased as the SrSO4 content increased and reached a peak value (260 A at 77 K) at an SrSO4 content of 6 wt.%, and then decreased when the SrSO4 content was further increased. This increase is probably due to the lower porosity, larger grain size and stronger texture, resulting from the enhanced diffusion kinetics. The possible causes of the variation in the Ic with the SrSO4 content based on the microstructural observations were discussed in detail. Ó 2006 Elsevier B.V. All rights reserved. PACS: 74.72.Hs; 74.81.Bd Keywords: BSCCO-2212; Critical current; Melt casting process; Microstructure; SrSO4
1. Introduction High-Tc bulk superconductors are used in fault current limiters, magnetic shield systems and current lead and bus bars in various superconducting systems, etc. BSCCO-2212 (2212) is known to be one of the most promising materials for these applications, because of its highcritical temperature (Tc), the wide temperature range in which the superconductor phase forms, and its reversible phase formability during melting, solidification and annealing. Thus, the melt casting process (MCP) can be used to
*
Corresponding author. Tel.: +82 31 290 7385; fax: +82 31 290 7371. E-mail address: [email protected] (J. Joo).
0921-4534/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2006.04.035
obtain bulk 2212 of complex shape and large size with a strong texture and high critical current (Ic). Substantial research has been conducted into the use of MCP process to improve the Ic value of bulk 2212. The MCP process includes powder melting, casting, solidification, and heat treatment and the critical properties vary with the processing conditions. In addition, several alloying elements, such as Ag, MgO, BaSO4, SrSO4, etc., can be added to improve both the electrical properties and mechanical integrity of bulk 2212, and the addition of SrSO4 was reported to have beneficial effects on its Ic value and machinability [1,2]. However, the effect of melt flowing on the Ic was not evaluated and the SrSO4 content was not systematically optimized for improving the Ic and mechanical properties.
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K.T. Kim et al. / Physica C 445–448 (2006) 447–450
In this study, we fabricated pure 2212 and 2212-(3– 15 wt.%) SrSO4 composite rods by the MCP process and then evaluated the effect of melt flowing on the Ic value by using two different pouring methods, i.e., the conventional vertical casting and the tilt casting methods. In addition, we determined the optimum content of SrSO4 required to improve the Ic and the possible causes of the variation of the Ic with the SrSO4 content based on the microstructural observations.
2212 2201 SrSO4 CaCuO2 (Sr,Ca)CuOx
Intensity (A.U.)
annealed rod
ingot
powder
2. Experimental Bi2Sr2Ca1Cu2O8+d (2212) and SrSO4 powders were supplied by Nexans and Kojundo chemical Co. The powders were mixed and heated at 1200 °C for 10 min in an alumina crucible. The melt was then poured into a quartz-tube mold, which was pre-heated to 500 °C, and then cooled to room temperature. The casting process was performed using two different methods, i.e., vertical and tilt casting. In the vertical casting method, the mold was vertically located inside the heater and the melt was poured into the mold. In the tilt casting method, on the other hand, the mold was placed inside the heater, tilted at an angle of 60° and then slowly returned to the upright position during the melt pouring. The specimens were 10 mm in diameter and 85 mm in length. The ingot was heat treated using two separate steps: annealing at 800 °C for 120 h in an oxygen atmosphere to form the 2212 phase, followed by annealing at 650 °C for 20 h in a nitrogen atmosphere to optimize the oxygen content. The microstructures were observed by scanning electron microscopy (HITACHI, S-3000 H). Phase identification was performed by X-ray diffraction (RIGAKU, 12KW) and electron probe microanalysis (SHIMADZU, EPMA1600). The porosity was measured by the Archimedes method. The hardnesses of the specimens were measured by Vickers hardness testing machine (MITUTOYO, MVK-H2). The Tc was measured by a four probe resistive method with an applied current of 10 mA. The Ic was measured by the same method with a 1 lV/cm criterion at 77 K in a self field.
22
26
28 30 32 2 theta (2 Θ )
34
36
38
Fig. 1. XRD patterns for the 2212 powder, 2212-10 wt.% SrSO4 ingot, and 2212-10 wt.% SrSO4 rod after the heat treatments.
gen content was reduced to be in the range of 8.16–8.2 at a nitrogen atmosphere during the second heat treatment [4]. To evaluate the effect of the casting method on the Ic of the 2212-10 wt.% SrSO4 rod, the casting was performed by the vertical and tilt casting methods. Fig. 2 shows the annealed 2212-10 wt.% SrSO4 rod and the voltage–current characteristics of the samples. In order to analyze the variation of the Ic at various positions on the rod, four voltage probes were connected: the a–b, b–c, and c–d taps to measure the Ic at the top, center and bottom of the rod and the a–d tap to measure that of the whole rod. As shown in Fig. 2(b), the Ic of the rods processed by the vertical and tilt casting methods were 57 A and 87 A, respectively,
(a) 0.010
Vertical casting (large ΔT) Tilt casting (large ΔT) Tilt casting (small ΔT)
0.008
Voltage (mV)
3. Results and discussion Fig. 1 shows the X-ray diffraction patterns for the 2212 powder, 2212-10 wt.% SrSO4 ingot, and annealed rod. The as-received 2212 powder consisted of mainly 2212 phase together with small amounts of unknown phases, while the solidified ingot had 2201 as the major phase and SrSO4, (Sr,Ca)CuOx, and CaCuO2 as minor phases. After annealing, the 2201 phase disappeared and the 2212 phase became the major one, suggesting that the 2201 reacted with the Ca- and Cu-rich phases to form the 2212 phase during the heat treatment in an oxygen atmosphere [3]. The Tczero was measured to be 88 K and 91 K after the first and second heat treatments, respectively, and this improvement in the Tc is believed to be due to the fact that the oxy-
24
0.006 0.004 0.002 0.000 0
50
100
150
200
Current (A) (b) Fig. 2. (a) The 2212 rod with the four voltage probe and (b) the voltage– current characteristics of the rods processed with the different casting methods.
K.T. Kim et al. / Physica C 445–448 (2006) 447–450
condition is related to the formation of dense and uniform microstructures. To evaluate the dependence of the Ic on the SrSO4 content, the pure 2212 and 2212-3, 6, 10, 12, and 15 wt.% SrSO4 composite rods were made by the tilt casting and their Ic values are shown in Fig. 4. The Ic was found to be 177 A for the pure 2212 rod, and this value improved as the SrSO4 content increased, reaching a peak value of 256 A at an SrSO4 content of 6 wt.%. The Ic subsequently decreased to 186 A at an SrSO4 content of 10 wt.%, and continued to decrease significantly as the SrSO4 content was further increased. An initial increase in the Ic with increasing SrSO4 content was also observed in other studies [2], but the role of the SrSO4 content in 2212 has not been systematically studied. The possible causes for the variation in the Ic with the SrSO4 content were correlated with the SEM observations. Fig. 5 shows the SEM micrographs of the pure 2212, 2212-6 wt.%, and 2212-15 wt.% SrSO4 rods. For the 2212 rod, there were many pores and the grains were not aligned parallel to each other. On the other hand, it was observed that the 2212-6 wt.% SrSO4 rod had less pores, larger grains, and a stronger texture than the pure 2212 rod, and some of the dark-colored particles were confirmed to be SrSO4 by EPMA analysis. The DTA analysis indicated that the addition of 6 wt.% SrSO4 decreased the melting point of 2212 by 6 °C. A lower melting point will increase 300 350
Critical current (A)
250
300
200
250
150
200 150
100
100 50
50
2
Critical current density (A/cm )
and this disparity is probably due to the difference in the pouring methods. Because the melt moves vertically into the mold, the vertical casting method results in the so called ‘‘turbulent flow’’ of the melt and, consequently, produces many pores that can be trapped during solidification [5]. The existence of pores can degrade the texture formability of the plate-like 2212 grains and, hence, reduce the Ic value. On the other hand, in the tilt casting method, the melt flows along the mold-wall and induces ‘‘laminar flow’’, thereby restricting the formation of pores. The porosities were found to be 3.54% and 2.98% for the rods processed by the vertical and tilt casting methods, respectively, which is in good agreement with the above explanation. It is also to be noted that the Ic varied significantly between the different regions of the rod. The Ic at the top, center and bottom of the rod were 102, 64 and 51 A for the rod processed by the vertical casting method and 87, 79 and 107 A by the tilt casting method, respectively, indicating that the Ic of the whole rod was primarily affected by the lowest Ic region. The large variation of the Ic in the rod is probably due to the temperature difference in the mold: the temperature at the top and bottom of the mold differed from each other by approximately 50 °C. In a previous study, we observed that the preheating temperature was one of the parameters which influenced the microstructure and Ic. Using a specially designed heating system which assured a temperature variation of less than 3 °C within the mold, we obtained an improved Ic with less deviation in the tilt casting method. The Ic valued were 155, 153, and 163 A for the top, center, and bottom of the rod, respectively, and that of the whole rod was 153 A. Fig. 3 shows the SEM micrographs of the fracture surface of the rods processed by the vertical and tilt casting methods. In the case of the tilt casting method with a uniform pre-heating temperature, the rod had less pores and a stronger texture than that processed by the vertical casting with a large temperature difference. From additional SEM observations, we found that the former method led to the alignment of more grains in the direction of the length of the rod and the development of a more uniform microstructure throughout the rod than in the case of the latter method. Thus, it is considered that the improved Ic obtained in the tilt casting with the uniform preheating
449
0
0 0
3
6
9
12
15
SrSO4 content (wt.%) Fig. 4. Variation of the Ic value of the 2212 rods with the SrSO4 content.
Fig. 3. SEM micrographs of the fracture surface of the rods processed by (a) the vertical casting method and (b) the tilt casting method : The arrow indicated the longitudinal direction of the rod.
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K.T. Kim et al. / Physica C 445–448 (2006) 447–450
Fig. 5. SEM micrographs of the (a) pure 2212, (b) 2212-6 wt.%, and (c) 2212-15 wt.% SrSO4 rods.
the diffusion kinetics at a given heat treatment temperature, leading to an improvement in the density, grain growth and alignment. This improved microstructure is one of the possible reasons for the observed increase in the Ic of the 2212SrSO4 rod. For the 2212-15 wt.% SrSO4 rod, we observed that the grain was not well aligned and that dendrites formed on the surface. In addition, the dendrites became larger and their density became higher with increasing depth inside the rod. The EPMA analysis indicated that the dendrite consisted mainly of SrSO4 phase. The reason for the formation of dendrite at a higher SrSO4 content is not clearly understood, however, it is well known that its presence interferes with the formation of texture. It is also to be noted that the fraction of 2212 phase decreased with increasing SrSO4 content and that the grain connectivity of the 2212 was considerably degraded when the SrSO4 phases began to be cross-linked with one another. Therefore, the observed decrease in the Ic at a higher SrSO4 content is partly related to the degradation of the texture and grain connectivity of the 2212 phase. The hardnesses of the pure 2212 and 2212-SrSO4 composites were measured. The hardness of pure 2212 was found to be 92.8 and this value linearly increased as the SrSO4 content increased. The hardnesses of the rods with SrSO4 contents of 3, 6, 10, 12, and 15 wt.% were 123.3, 131.8, 141.7 148.7 and 177.2, respectively. This improvement in the hardness was attributed to the strengthening mechanism afforded by the addition of the alloying material and the fact that the hardness of the SrSO4 (193) was higher than that of 2212 [6]. 4. Conclusions We evaluated the effects of the melt flowing process and the SrSO4 content on the Ic value of 2212-SrSO4 superconductor. It was observed that the 2212-SrSO4 rod processed
by the tilt casting method had a higher Ic than that processed by the vertical casting method. This improved Ic value is considered to be related to the more homogenous microstructure associated with the lower porosity and stronger texture afforded by the laminar flow of the melt. In addition, the Ic value was significantly dependent on the SrSO4 content. The Ic of 2212 increased with increasing SrSO4 content, reached a peak value (260 A at 77 K) at an SrSO4 content of 6 wt.%, and then decreased when the SrSO4 content was further increased. The initial improvement in the Ic value with increasing SrSO4 content is probably due to the lower porosity, larger grain size, and stronger texture, resulting from the enhanced diffusion kinetics. On the other hand, it is believed that the decrease in the Ic at a higher SrSO4 content is partly related to the degraded texture and poor grain connectivity of the 2212 phase. The addition of SrSO4 also had a beneficial effect on the hardness of 2212. Acknowledgements This research was supported by Electric Power Industry R&D program funded by the Ministry of Commerce, Industry, and Energy, Republic of Korea. References [1] S. Pavard, C. Vollard, D. Bourgault, R. Tournier, Supercond. Sci. Technol. 11 (1998) 1359. [2] J. Bock, H. Bestgen, S. Elchner, E. Preisler, IEEE Trans. Appl. Supercond. 3 (1993) 1659. [3] J. Bock, E. Preisler, Solid State Commun. 72 (1989) 453. [4] D. Buhl, T. Lang, L.J. Gauckler, Supercond. Sci. Technol. 10 (1997) 32. [5] J. Mi, R.A. Harding, J. Campbell, Int. J. Cast Metals Res. 14 (2002) 325. [6] B.-H. Kim, K.-H. Ko (Eds.), Handbook of Chemistry and Physics, KDR, Korea, 2002, p. 1199.