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Expansion of BiSrCaCuO pellets
MaterialsScienceand Engineering, B3 (1989) 501-505
501
Expansion of BiSrCaCuO Pellets C. J. KIM, H. G. LEE and D. Y. WON Korea AdvancedEnergy'Research Institute, P.O.Box 7, Daeduk Danfi Daejun 302-353 (Korea) H. S. SHIN Departrnent of ChemicalEngineering, ChonbukNational University, Chunju, Chonbuk 560-756 (Korea) (Received May 1, 1989)
Abstract 7he pellet expansion was found during the sintering of the BiSrCaCuO oxide. The lead doping in BiSrCaCuO oxide accelerates the expansion as well as the promotion of the high T, phase. The apparent density increased at the early stage of sintering of about 20 h, but abruptly decreased until 80 h and then stabilized. The release of gasifying elements such as lead and bismuth from the PbBiSrCaCuO pellet is considered to be responsible for the pellet expansion. We discussed the causes for the expansion in relation to the formation mechanism of the high Tc phase and the growth characteristics of the superconducting phases.
system, sintering for tong period of time seems to be necessary to increase the volume fraction of the high Tc phase, implying that the formation of the high Tc phase is controlled by reactions among various phases formed at early stages. Furthermore, the pellet was considerably expanded during the prolonged sintering of the lead-doped sample [5]. The expansion of the sintered pellet seems to be related to the formation of the high Tc phase. In this study, we report the sintering behaviour, especially the pellet expansion, in the lead-doped sample. The possible causes for the pellet expansion were discussed in relation to the formation of the high Tc phase, the gas release from the sample during sintering and the growth characteristics of the high Tc phase.
1. Introduction One of the most important results of recent investigations on oxide superconductors is the introduction of the B i - S r - C a - C u - O system [1]. This system consists of a low T~ phase (80 K) and a high T~ (110 K) phase. The separation of the high T~ single phase has, thus, been a current research subject in this system. A most effective method for increasing the volume fraction of the high Tc phase is known to be partial substitution of lead for bismuth. Recently, Takano et al. [2] have fabricated a superconductor at 107 K by prolonged sintering of samples containing lead. In this sample, the volume fraction of the high T~ phase was about 90%. In our previous reports [3, 4], it was found that several phases such as a low T~ phase, Sr 3 xCaxCusO,~ PbCa204 and CuO existed in the initial stage of sintering but the volume fraction of these phases was reduced and the growth of the high T~ phase was enhanced as sintering time was extended. For this lead-doped 0921-5107/89/$3.50
2. Experiments Specimens were prepared by the solid state reaction of Bi203, PbO, SrCO 3, CaO and CuO powders. The powders were weighed and well mixed in an aluminar mortar and pestle. The cation ratio of starting materials was Bi:Pb: Sr: Ca: Cu = 0.7 : 0.3 : 1 : 1 : 1.8. The powder mixture was calcined at 830°C for 24 h in air. After grinding, the powder mixture was pressed into pellets with a diameter of 15.2 mm and a thickness of 3 mm under 900 k g c m -2. The pellets were then sintered in the temperature range from 840 to 870°C for various time periods up to 200 h and cooled in air. The electrical resistances were measured by an a.c. fourprobe method for bar-shaped samples cut from the sintered samples. The phases present in the calcined powders and sintered specimens were identified by means of X-ray powder diffraction © ElsevierSequoia/Printed in The Netherlands
502
using CuKa~ radiation, an optical microscope and SEM EDS (scanning electron microscope energy-dispersive X-ray spectra). 3.
Results
and discussion
Figure 1 shows the green pellet (a) and the sintered specimen (b) at 855 °C for 160 h. It can be seen that the green pellet expanded after sintering. The diameters of specimens (a) and (b) were 15.2 mm and 17.6 mm respectively. The thickness of the specimen was also lengthened from 3.5 mm to 3.7 mm. The thickness and diameter tend to be gradually lengthened as sintering time is extended. Figure 2 shows the relationship between the apparent density and the sintering time. The density of the green pellet was 3.25 g cm -3. Contrary to normal expectation, increasing the sintering time did not result in increasing densification. In fact, the sample with a long sintering time had a lower density than those at the early stages of sintering. The density
reached a maximum for the specimen sintered for about 20 h and then rapidly decreased until 80 h. However. further sintering over 80 h did not result in a considerable decrease in the denstty. This expansion of the sintered specimen is considered to be related to several phenomena caused by lead doping. When the lead was introduced into the sample, the formation of the high 7~, superconducting phase was enhanced [3--5]. The formation of the high Tc phase is very sluggish [4]. Figure 3 shows the relationship between Tc CR=0 point) and sintering time. The specimens sintered at 840°C and 855°C for 24 h showed 7~.sof 85 K and 87 K respectively. 7 c has gradually increased with increasing sintering time. To obtain zero resistance at 105 K using the conventional solid state reaction, long-termsintering for about 200 h is required. Figure 4 shows the XRD IX-ray diffraction patterns t for the calcined powder mixture and the specimen sintered at 855 °C for 168 h. As seen in Fig. 3(a), the calcined powder mixture consists of a large amount of the low Tc phase having a tetragonal structure with a c parameter of 30 A [6, 7], together with a small amount of the high Tc phase with a c parameter of 37 A, PbCa20~, CuO and Sr3_xCaxCu5Oy [3-5]. When the calcined powder was sintered at 855°C for 160 h. as shown in Fig. 3(a), a nearly single high 7~, phase was produced. One interesting feature is the existence of PbCa204. This phase is made by lead doping and
120r
• 8 5 5 °C [] 840~C
Fig. 1. Photograph of the as-compacted (a) and sintered specimen at 855 °C for 168 h and air cooled (b).
110F 100~
g
4~
go~-
/
a.
E
7 2
0
l
,
40
,
80
120
,~0 Sintering
200
2~,. . . . .
'o
o
T
I
I
I
[
40
80
120
160
200
360
time{hi
Fig. 2. Variation of apparent density with sintering time (sintering temperature was 855 °C).
Sintering
time (hrs.)
Fig. 3. Variation of zero resistance temperatur e (To) with sintering time (sintering temperatures were 840 and 855 *C).
503
0 ,
o
0
i 0.5"
1.C (b)
# ,g
Ij o=
o :z
4
0.5 :~
8:
_=
i
I,i °
04
10
20
30
40
2e/deg.
Fig. 4. X-ray diffraction patterns of lead-doped BiSrCaCuO (subscripts L, H and • indicate the low Tc phase, the high T~ phase and PbCazO4, respectively). Preparation conditions were (a) calcined at 830 °C for 24 h and (b) sintered at 855 °C for 168 h. Fig. 5. Scanning electron micrographs for the polished surface of lead-doped BiSrCaCuO (a) sintered at 855 °C for 24 h and(b)for 168 h.
may partially melt during sintering because its melting point is 822 °C [8, 9]. This partially melted phase is believed to play an important role in mass transfer to form the high Tc phase [6, 10]. The scanning electron micrographs for the specimens sintered at 855 °C for 24 h and 168 h are shown in Fig. 5. It was observed that a liquid phase was formed by sintering of the lead-doped specimen. This liquid phase was generated by the partial melting of PbCa204. As seen in Fig. 5(a), the liquid phase formed near the high Tc phase. As the formation reaction proceeded, the fraction of the liquid phase considerably reduced and the formation of the high Tc phase was enhanced (see Fig. 5(a)). The presence of the liquid phase is concentrated near PbCa204 [4] and the narrow space between the high T~ plates. As a large amount of the liquid phase begins to be produced at the initial stage of sintering, the low T~ phase formed at the calcining stage would dissolve into the liquid phase and the high Tc phase may nucleate from the liquid phase. These observations indicate that the formation of the high T~ phase may proceed through the liquid phase sintering proposed by Hatano e t al, [5].
Another interesting feature of this system is the evaporation of lead, first reported by Takano e t a/. [2]. The total lead content decreases quickly in the early stage (less than 60 h) of sintering and more slowly after. This time schedule is consistant with the variation of apparent density as demonstrated in Fig. 2. The increase in density can be explained by formation of the liquid phase and subsequent particle rearrangement by liquid phase sintering, while the abrupt decrease in density might be due to lead evaporation. The gas is thought to be released from the liquid phase which contains a large amount of lead and bismuth [5]. As the lead evaporated, the gas pressure may be evolved in the liquid phase. The high Tc plates nucleated in the initial stage of sintering would be pushed by the gas pressure and separated from the other plates. As these plates grow, the liquid phase moves into the interspaces between them by capillary force. As a result of the gas release from the liquid phase, the growing plates might be continuously separated from each other.
504
To determine the relationship between gas release and the pellet expansion, specimens were sealed in quartz tubes in air or in a vacuum of 10 -3 Torr and then sintered at 855 °C for 72 h. These specimens were compared with unsealed specimens. It could be seen that both the sealed specimens expanded less than the unsealed one but the rate of formation of the high Tc phases in these specimens was considerably faster owing to the low oxygen partial pressure [10]. The evaporation of lead was also observed as in the unsealed sample. The evaporated lead was deposited onto the inner wall of the quartz tube and reacted to form a silicon compound. This means that the suppression of the evaporation of the gasifying elements such as lead and bismuth suppresses the pellet expansion. To confirm the compositional effect on the pellet expansion, Bil.4Pb0.6Sr 4_ ~CaxCu3.sO s specimens were prepared. The pellet expansion was observed in all the specimens but the degree of expansion was increased as the calcium content increases up to x = 2.5. For this composition, the diameter of the specimen sintered at 855 °C for 72 h was 19.5 ram. This value is comparable with the 17.5 mm diameter of the specimen sintered at 855°C for 168 h with x=2.0. The XRD patterns for the specimen with x = 2 . 5 showed that a large amount of PbCa20~ was formed by excess calcium addition. A lead- or calcium-rich composition enhances the formation of PbCa204 [11 ]. This means that increasing the fraction of the lead-containing phase promotes the possibility for the pellet expansion. As a confirmation of the effect of lead doping on the expansion, the microstructures of undoped and lead-doped BiSrCaCuO are compared. Figure 6 shows scanning electron micrographs for undoped and lead-doped BiSrCaCuO. The starting compositions of the compounds were B i : S i : C a : C u = 2 : 2 : 2 : 3 and Bi:Pb:Sr:Ca:Cu = 1.4:0.6:2:2:3.6 respectively. BiSrCaCuO and undoped BiSrCaCuO were sintered at 870 °C for 72 h and at 855 °C for 168 h respectively, and then air cooled. The XRD data for the undoped and the lead-doped samples mainly consist of the low 7/'~ and high Tc phases respectively. All the undoped and the lead-doped specimens showed a pellet expansion but the degree of expansion in the lead-doped specimen is larger than that in the undoped specimen. The grain morphologies of both phases are thin plates with a thickness of a few micrometres. However, the aspect ratio of the
Fig. 6. Scanning electron fractographs ior (a) BiSrCaCuO and (b) lead-doped BiSrCaCuO. Preparation conditions were (a) sintered at 870 °C for 72 h and (b) sintered at 855 °C for 168 h and air cooled.
high ]~, phase is larger than that of the low 7~ phase. The growth direction of the plate is parallel to the c plane of the unit celt. However, the microstructure of the undoped specimen is more packed than that of the lead-doped sample. As shown in Fig. 6(b), the high T~. plates are piled up randomly and connected weakly to each other in most regions of the specimen. From the grain shape of low and high Tc phases, it is thought that the surface energy of the c plane is very lowl When these plates grow and impinge each other, they make a pore with the low energy surfaces of the plates. These pores are difficult to fill because of this special geometry. This geometrical difficulty contributes to the stability of density after the pellet expansion owing to lead evaporation. Thus densification cannot occur even though the sintering time is extended. However, undoped BiSrCaCuO has a texture owing to the appropriate close packing of the low T~plates as shown in Fig. 6(a). It is believed that this close packing
5O5
contributes less to the expansion of undoped BiSrCaCuO than it does to the lead-doped specimen. 4. Conclusion The pellet expansion of the lead-doped BiSrCaCuO system was investigated with its related microstructure concerning the formation of the high T~ superconducting phase. It was found that the pellet expansion was observed in both BiSrCaCuO and lead-doped BiSrCaCuO. The pellet expansion was considerably enhanced by lead doping in BiSrCaCuO. It is considered that the expansion occurred during the formation of the high T< superconducting phase and is influenced by gas release such as lead and bismuth from the specimen. Preventing the evaporation of the gasifying elements causes a specimen to expand less. Anisotropic grain growth of the superconducting phases seems to also contribute to the de-densification. Acknowledgments This work was supported by the Ministry of Science and Technology, Korea. The authors
wish to thank Ki B. Kim and Kyung S. Lim for help in the experiments.
References 1 H. Maeda, T. Tanaka, M. Fukutomi and T. Asano, Jpn. J. AppL Phys., 27(198811,209. 2 M. Takano, J. Takada, K. Oda, H. Kitaguehi, Y. Miura, Y. lkeda, Y. Tomii and H. Mazaki. Jim. J. AppL Phys., 27 (1988) LI041. 3 H. G. Lec, C. J. Kim, K. H. Lee and I). Y. Won. AppL Phys. Lett., 54 (1989) 391. 4 C.J. Kim, C. K. Rhee, H. G. Lee, (7. T. Lee, S. J-L. Kang and D. Y. Won, .ltm. J. AppL l'hys., 28 (1988) L45. 5 T. Hatano, K. Aota, S. Ikeda, K. Nakamura and K. Ogawa, Jpn. J. AppL Ph3w., 27(1988) k2(i55. 6 J. M. Tarascon, Y. Lepage, R Barboux, 13. G. Begley, L. H. Green, W. R. Mckinnon, G. W. Hull, M. Giround and D. M. Hwang, Phys. Rev. B, 371198819383. 7 C. J. D. Hetherington. R. Ramcsh, M. A. O'Keefe, R. Kilaas, G. Thomas, S. M. Green and H, L. Luo. Appl. Pto,s. Lett., 5,7( 198811016. 8 U. Kuxmann and P. Fisher, Erzmemll., 27(1974) 533. 9 T. Uzumaki, K. Yamanaka, N. Kamehara and K. Niwa, ,lpn. J. Appl. Phys., 28(19891 L75. 10 U. Endo, S. Koyama and T. Kawai, JpH. J. AppL l'hvs., 27 (19881 L1476. 11 S. Koyama, U. Endo and T. Kawai, dlm. ,1. AppI. l'hys., 27 (19881 L861.
501
Expansion of BiSrCaCuO Pellets C. J. KIM, H. G. LEE and D. Y. WON Korea AdvancedEnergy'Research Institute, P.O.Box 7, Daeduk Danfi Daejun 302-353 (Korea) H. S. SHIN Departrnent of ChemicalEngineering, ChonbukNational University, Chunju, Chonbuk 560-756 (Korea) (Received May 1, 1989)
Abstract 7he pellet expansion was found during the sintering of the BiSrCaCuO oxide. The lead doping in BiSrCaCuO oxide accelerates the expansion as well as the promotion of the high T, phase. The apparent density increased at the early stage of sintering of about 20 h, but abruptly decreased until 80 h and then stabilized. The release of gasifying elements such as lead and bismuth from the PbBiSrCaCuO pellet is considered to be responsible for the pellet expansion. We discussed the causes for the expansion in relation to the formation mechanism of the high Tc phase and the growth characteristics of the superconducting phases.
system, sintering for tong period of time seems to be necessary to increase the volume fraction of the high Tc phase, implying that the formation of the high Tc phase is controlled by reactions among various phases formed at early stages. Furthermore, the pellet was considerably expanded during the prolonged sintering of the lead-doped sample [5]. The expansion of the sintered pellet seems to be related to the formation of the high Tc phase. In this study, we report the sintering behaviour, especially the pellet expansion, in the lead-doped sample. The possible causes for the pellet expansion were discussed in relation to the formation of the high Tc phase, the gas release from the sample during sintering and the growth characteristics of the high Tc phase.
1. Introduction One of the most important results of recent investigations on oxide superconductors is the introduction of the B i - S r - C a - C u - O system [1]. This system consists of a low T~ phase (80 K) and a high T~ (110 K) phase. The separation of the high T~ single phase has, thus, been a current research subject in this system. A most effective method for increasing the volume fraction of the high Tc phase is known to be partial substitution of lead for bismuth. Recently, Takano et al. [2] have fabricated a superconductor at 107 K by prolonged sintering of samples containing lead. In this sample, the volume fraction of the high T~ phase was about 90%. In our previous reports [3, 4], it was found that several phases such as a low T~ phase, Sr 3 xCaxCusO,~ PbCa204 and CuO existed in the initial stage of sintering but the volume fraction of these phases was reduced and the growth of the high T~ phase was enhanced as sintering time was extended. For this lead-doped 0921-5107/89/$3.50
2. Experiments Specimens were prepared by the solid state reaction of Bi203, PbO, SrCO 3, CaO and CuO powders. The powders were weighed and well mixed in an aluminar mortar and pestle. The cation ratio of starting materials was Bi:Pb: Sr: Ca: Cu = 0.7 : 0.3 : 1 : 1 : 1.8. The powder mixture was calcined at 830°C for 24 h in air. After grinding, the powder mixture was pressed into pellets with a diameter of 15.2 mm and a thickness of 3 mm under 900 k g c m -2. The pellets were then sintered in the temperature range from 840 to 870°C for various time periods up to 200 h and cooled in air. The electrical resistances were measured by an a.c. fourprobe method for bar-shaped samples cut from the sintered samples. The phases present in the calcined powders and sintered specimens were identified by means of X-ray powder diffraction © ElsevierSequoia/Printed in The Netherlands
502
using CuKa~ radiation, an optical microscope and SEM EDS (scanning electron microscope energy-dispersive X-ray spectra). 3.
Results
and discussion
Figure 1 shows the green pellet (a) and the sintered specimen (b) at 855 °C for 160 h. It can be seen that the green pellet expanded after sintering. The diameters of specimens (a) and (b) were 15.2 mm and 17.6 mm respectively. The thickness of the specimen was also lengthened from 3.5 mm to 3.7 mm. The thickness and diameter tend to be gradually lengthened as sintering time is extended. Figure 2 shows the relationship between the apparent density and the sintering time. The density of the green pellet was 3.25 g cm -3. Contrary to normal expectation, increasing the sintering time did not result in increasing densification. In fact, the sample with a long sintering time had a lower density than those at the early stages of sintering. The density
reached a maximum for the specimen sintered for about 20 h and then rapidly decreased until 80 h. However. further sintering over 80 h did not result in a considerable decrease in the denstty. This expansion of the sintered specimen is considered to be related to several phenomena caused by lead doping. When the lead was introduced into the sample, the formation of the high 7~, superconducting phase was enhanced [3--5]. The formation of the high Tc phase is very sluggish [4]. Figure 3 shows the relationship between Tc CR=0 point) and sintering time. The specimens sintered at 840°C and 855°C for 24 h showed 7~.sof 85 K and 87 K respectively. 7 c has gradually increased with increasing sintering time. To obtain zero resistance at 105 K using the conventional solid state reaction, long-termsintering for about 200 h is required. Figure 4 shows the XRD IX-ray diffraction patterns t for the calcined powder mixture and the specimen sintered at 855 °C for 168 h. As seen in Fig. 3(a), the calcined powder mixture consists of a large amount of the low Tc phase having a tetragonal structure with a c parameter of 30 A [6, 7], together with a small amount of the high Tc phase with a c parameter of 37 A, PbCa20~, CuO and Sr3_xCaxCu5Oy [3-5]. When the calcined powder was sintered at 855°C for 160 h. as shown in Fig. 3(a), a nearly single high 7~, phase was produced. One interesting feature is the existence of PbCa204. This phase is made by lead doping and
120r
• 8 5 5 °C [] 840~C
Fig. 1. Photograph of the as-compacted (a) and sintered specimen at 855 °C for 168 h and air cooled (b).
110F 100~
g
4~
go~-
/
a.
E
7 2
0
l
,
40
,
80
120
,~0 Sintering
200
2~,. . . . .
'o
o
T
I
I
I
[
40
80
120
160
200
360
time{hi
Fig. 2. Variation of apparent density with sintering time (sintering temperature was 855 °C).
Sintering
time (hrs.)
Fig. 3. Variation of zero resistance temperatur e (To) with sintering time (sintering temperatures were 840 and 855 *C).
503
0 ,
o
0
i 0.5"
1.C (b)
# ,g
Ij o=
o :z
4
0.5 :~
8:
_=
i
I,i °
04
10
20
30
40
2e/deg.
Fig. 4. X-ray diffraction patterns of lead-doped BiSrCaCuO (subscripts L, H and • indicate the low Tc phase, the high T~ phase and PbCazO4, respectively). Preparation conditions were (a) calcined at 830 °C for 24 h and (b) sintered at 855 °C for 168 h. Fig. 5. Scanning electron micrographs for the polished surface of lead-doped BiSrCaCuO (a) sintered at 855 °C for 24 h and(b)for 168 h.
may partially melt during sintering because its melting point is 822 °C [8, 9]. This partially melted phase is believed to play an important role in mass transfer to form the high Tc phase [6, 10]. The scanning electron micrographs for the specimens sintered at 855 °C for 24 h and 168 h are shown in Fig. 5. It was observed that a liquid phase was formed by sintering of the lead-doped specimen. This liquid phase was generated by the partial melting of PbCa204. As seen in Fig. 5(a), the liquid phase formed near the high Tc phase. As the formation reaction proceeded, the fraction of the liquid phase considerably reduced and the formation of the high Tc phase was enhanced (see Fig. 5(a)). The presence of the liquid phase is concentrated near PbCa204 [4] and the narrow space between the high T~ plates. As a large amount of the liquid phase begins to be produced at the initial stage of sintering, the low T~ phase formed at the calcining stage would dissolve into the liquid phase and the high Tc phase may nucleate from the liquid phase. These observations indicate that the formation of the high T~ phase may proceed through the liquid phase sintering proposed by Hatano e t al, [5].
Another interesting feature of this system is the evaporation of lead, first reported by Takano e t a/. [2]. The total lead content decreases quickly in the early stage (less than 60 h) of sintering and more slowly after. This time schedule is consistant with the variation of apparent density as demonstrated in Fig. 2. The increase in density can be explained by formation of the liquid phase and subsequent particle rearrangement by liquid phase sintering, while the abrupt decrease in density might be due to lead evaporation. The gas is thought to be released from the liquid phase which contains a large amount of lead and bismuth [5]. As the lead evaporated, the gas pressure may be evolved in the liquid phase. The high Tc plates nucleated in the initial stage of sintering would be pushed by the gas pressure and separated from the other plates. As these plates grow, the liquid phase moves into the interspaces between them by capillary force. As a result of the gas release from the liquid phase, the growing plates might be continuously separated from each other.
504
To determine the relationship between gas release and the pellet expansion, specimens were sealed in quartz tubes in air or in a vacuum of 10 -3 Torr and then sintered at 855 °C for 72 h. These specimens were compared with unsealed specimens. It could be seen that both the sealed specimens expanded less than the unsealed one but the rate of formation of the high Tc phases in these specimens was considerably faster owing to the low oxygen partial pressure [10]. The evaporation of lead was also observed as in the unsealed sample. The evaporated lead was deposited onto the inner wall of the quartz tube and reacted to form a silicon compound. This means that the suppression of the evaporation of the gasifying elements such as lead and bismuth suppresses the pellet expansion. To confirm the compositional effect on the pellet expansion, Bil.4Pb0.6Sr 4_ ~CaxCu3.sO s specimens were prepared. The pellet expansion was observed in all the specimens but the degree of expansion was increased as the calcium content increases up to x = 2.5. For this composition, the diameter of the specimen sintered at 855 °C for 72 h was 19.5 ram. This value is comparable with the 17.5 mm diameter of the specimen sintered at 855°C for 168 h with x=2.0. The XRD patterns for the specimen with x = 2 . 5 showed that a large amount of PbCa20~ was formed by excess calcium addition. A lead- or calcium-rich composition enhances the formation of PbCa204 [11 ]. This means that increasing the fraction of the lead-containing phase promotes the possibility for the pellet expansion. As a confirmation of the effect of lead doping on the expansion, the microstructures of undoped and lead-doped BiSrCaCuO are compared. Figure 6 shows scanning electron micrographs for undoped and lead-doped BiSrCaCuO. The starting compositions of the compounds were B i : S i : C a : C u = 2 : 2 : 2 : 3 and Bi:Pb:Sr:Ca:Cu = 1.4:0.6:2:2:3.6 respectively. BiSrCaCuO and undoped BiSrCaCuO were sintered at 870 °C for 72 h and at 855 °C for 168 h respectively, and then air cooled. The XRD data for the undoped and the lead-doped samples mainly consist of the low 7/'~ and high Tc phases respectively. All the undoped and the lead-doped specimens showed a pellet expansion but the degree of expansion in the lead-doped specimen is larger than that in the undoped specimen. The grain morphologies of both phases are thin plates with a thickness of a few micrometres. However, the aspect ratio of the
Fig. 6. Scanning electron fractographs ior (a) BiSrCaCuO and (b) lead-doped BiSrCaCuO. Preparation conditions were (a) sintered at 870 °C for 72 h and (b) sintered at 855 °C for 168 h and air cooled.
high ]~, phase is larger than that of the low 7~ phase. The growth direction of the plate is parallel to the c plane of the unit celt. However, the microstructure of the undoped specimen is more packed than that of the lead-doped sample. As shown in Fig. 6(b), the high T~. plates are piled up randomly and connected weakly to each other in most regions of the specimen. From the grain shape of low and high Tc phases, it is thought that the surface energy of the c plane is very lowl When these plates grow and impinge each other, they make a pore with the low energy surfaces of the plates. These pores are difficult to fill because of this special geometry. This geometrical difficulty contributes to the stability of density after the pellet expansion owing to lead evaporation. Thus densification cannot occur even though the sintering time is extended. However, undoped BiSrCaCuO has a texture owing to the appropriate close packing of the low T~plates as shown in Fig. 6(a). It is believed that this close packing
5O5
contributes less to the expansion of undoped BiSrCaCuO than it does to the lead-doped specimen. 4. Conclusion The pellet expansion of the lead-doped BiSrCaCuO system was investigated with its related microstructure concerning the formation of the high T~ superconducting phase. It was found that the pellet expansion was observed in both BiSrCaCuO and lead-doped BiSrCaCuO. The pellet expansion was considerably enhanced by lead doping in BiSrCaCuO. It is considered that the expansion occurred during the formation of the high T< superconducting phase and is influenced by gas release such as lead and bismuth from the specimen. Preventing the evaporation of the gasifying elements causes a specimen to expand less. Anisotropic grain growth of the superconducting phases seems to also contribute to the de-densification. Acknowledgments This work was supported by the Ministry of Science and Technology, Korea. The authors
wish to thank Ki B. Kim and Kyung S. Lim for help in the experiments.
References 1 H. Maeda, T. Tanaka, M. Fukutomi and T. Asano, Jpn. J. AppL Phys., 27(198811,209. 2 M. Takano, J. Takada, K. Oda, H. Kitaguehi, Y. Miura, Y. lkeda, Y. Tomii and H. Mazaki. Jim. J. AppL Phys., 27 (1988) LI041. 3 H. G. Lec, C. J. Kim, K. H. Lee and I). Y. Won. AppL Phys. Lett., 54 (1989) 391. 4 C.J. Kim, C. K. Rhee, H. G. Lee, (7. T. Lee, S. J-L. Kang and D. Y. Won, .ltm. J. AppL l'hys., 28 (1988) L45. 5 T. Hatano, K. Aota, S. Ikeda, K. Nakamura and K. Ogawa, Jpn. J. AppL Ph3w., 27(1988) k2(i55. 6 J. M. Tarascon, Y. Lepage, R Barboux, 13. G. Begley, L. H. Green, W. R. Mckinnon, G. W. Hull, M. Giround and D. M. Hwang, Phys. Rev. B, 371198819383. 7 C. J. D. Hetherington. R. Ramcsh, M. A. O'Keefe, R. Kilaas, G. Thomas, S. M. Green and H, L. Luo. Appl. Pto,s. Lett., 5,7( 198811016. 8 U. Kuxmann and P. Fisher, Erzmemll., 27(1974) 533. 9 T. Uzumaki, K. Yamanaka, N. Kamehara and K. Niwa, ,lpn. J. Appl. Phys., 28(19891 L75. 10 U. Endo, S. Koyama and T. Kawai, JpH. J. AppL l'hvs., 27 (19881 L1476. 11 S. Koyama, U. Endo and T. Kawai, dlm. ,1. AppI. l'hys., 27 (19881 L861.