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Processing effects on mechanical and superconducting properties of Bi2201 and Bi2212 glass ceramics
PHYSICA ELSEVIER
Physica C 275 (1997) 337-345
Processing effects on mechanical and superconducting properties of Bi2201 and Bi2212 glass ceramics C o r n e l i a M t i l l e r * , P e t e r M a j e w s k i , Gt~nter T h u r n , F r i t z A l d i n g e r Max-Planck-lnstitut ffir Metallforschung, Institut fiir Werkstoffwissenschafl, Pulvermetallurgisches Laboratorium, Heisenbergstr. 5, D-70569 Stuttgart, Germany
Received 3 September 1996; revised manuscriptreceived 12 December 1996
Abstract
Bi2212 and Bi2201 were prepared by annealing of glass with the composition Bi2.taSr2CaCu2Os+a. Using this preparation technique, structural and microstructural properties such as phase formation, grain size and oxygen content can easily be affected by different annealing conditions, i.e. by variation of time and temperature. Hence, the deformation behaviour and the critical temperature (To) are a function of the annealing conditions. This could be shown by creep experiments and AC susceptibility measurements. Samples with nanocrystalline microstructures exhibit substantially better plasticity under stress than samples with larger grains. Good superconducting properties could only be achieved in nearly single-phase Bi2212 samples with grains extending over several hundred nanometers to a few micrometers in the direction of the c axis. Keywords: High-To superconductor;Bi-Sr-Ca-Cu-O system;Crystallization;Glass ceramics;Deformation
1. Introduction Bi oxides are known as conditional glass formers, i. e. they will form glasses when mixed with a suitable amount of a second oxide. Hence, high-Tc superconductors of the system Bi-Sr-Ca-Cu-O exhibit the advantage that they can be processed via a glass intermediate product. By variation of crystallization conditions, structural and microstructural properties such as phase formation, oxygen content, and grain size can easily be affected. In order to optimize the superconducting and mechanical properties of glass ceramics in the system Bi-Sr-Ca-Cu-O (BSCCO), it is important to investigate in detail the changes in these properties during the crystallization process. Since the first prepa* Corresponding author. Fax: +49 711 6861 131; e-mail: [email protected].
ration of BSCCO glass ceramics [ 1 ] many authors have studied the crystallization mechanism [2-12]. Because of many different starting compositions and preparation routes it is difficult to compare the results of these studies, as noted in an overview by WongNg and Freiman [ 13]. Therefore, no general rule for the phase formation mechanism of Bi2Sr2CaCu2Os+d (Bi2212) could be given. The dependence of the electrical properties of highTc superconductors on structure and microstructure is well known. For instance, the number of CuO layers determines the critical temperature (Tc) and the oxygen content determines the charge carrier concentration [ 14]. On the microstructural scale the connectivity at grain boundaries (weak links) [ 15] is important especially in the presence of a magnetic field. Still the shaping of BSCCO superconducting ma-
0921-4534/97/$17.00 Copyright (~) 1997 Elsevier Science B.V. All rights reserved. PII S0921-4534(97)00026-9
338
C. Mailer et al./Physica C 275 (1997) 337-345
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Angle 20 (deg) Fig. i. XRD pattern of BSCCO samples, annealed at 500°C (lower curve), 550°C (middle curve), and 600°C (upper curve) for l0 minutes (1 Bi2201, ~ Cu20, + CuO, * Bi203).
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C. M~iller et al,/Physica C 275 (1997) 337-345
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Angle 20 (deg) Fig. 3. XRD pattern of BSCCO samples,annealedat ?00°C (lower curve), 780°C (middle curve), and 850°C (upper curve) for 10 minutes and 5 minutes (1 Bi2201, 2 Bi2212, ~ Cu20, + CuO, * Bi20~).
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340
C. Mailer et al./Physica C 275 (1997) 337-345
terial is a problem, because fully crystalline BSCCO does not deform easily due to its big, mica-like grains. Therefore changing grain size and shape might improve the deformability of BSCCO. In this report we investigate the dependence of structural and microstructural changes on annealing conditions for BSCCO high-temperature superconductors with a cation ratio Bi : Sr : Ca : Cu = 2.18 : 2 : 1 : 2. This composition was chosen, because it is situated in the center of the homogeneity area for Bi2212 in the phase diagram [ 16]. The structural properties are determined by X-ray powder diffraction (XRD) for different crystallization temperatures and times. Also, the variation of oxygen content is observed by means of chemical analysis. Microstructural changes, i.e. variation in grain size and shape, are investigated in detail by transmission electron microscopy (TEM). From these results we try to interpret the change of critical temperature (To) measured by AC susceptibility and also the deformation behaviour under stress during creep experiments.
2. Experimental procedure The preparation of the Bi glass ceramics was carried out as follows. Commercially available Bi203, SrCO3, CaCO3 and CuO (99.9%) powders were weighed to the nominal composition of Bi2.tsSr2CaCu2Ox, mixed and calcined at 750°C, 780°C, and 820°C for 96 hours in total. After milling, the powder was melted in a ZrO2 crucible at 1200°C for 30 minutes. In order to obtain glass, the melt was poured onto a copper plate or was quenched in a quartz glass tube with an internal diameter of 2 mm. Thus we had two different sample shapes: plate-like and cylindrical. The platelike samples were used for XRD measurements, TEM investigations, and chemical analysis, whereas cylinders proved to be a suitable shape for AC susceptibility measurements and creep tests. The samples were annealed at temperatures between 400°C and 850°C in air for times ranging from I0 minutes to 160 hours. For that purpose they were placed in a preheated electrical furnace for a specified time and then quenched onto a copper plate. The phase formation of the powdered samples was recorded by an X-ray diffractometer (Model D5000, Siemens) with a position sensitive proportional
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Fig. 5. Grain size versus crystallization temperature in BSCCO glass ceramics,annealed for 10 minutes or 5 minutes (825°C and 850°C). counter (PSPC) from 28=10 ° to 80 °. The oxygen
content was determined by carrier gas hot extraction followed by an IR detection of CO2. As a function of temperature, the real part of the intrinsic susceptibility was measured by an AC susceptometer with a frequency of 1 kHz. TEM investigations were carried out on samples which were prepared according to the method of Strecker, Salzberger, and Mayer, which involves the embedding of 300-/zm thick slices in ceramic holders [ 17]. From these cylindrical holders, disks with a diameter of 3 mm were cut. After grinding and dimpling to a thickness of approximately 25 /zm, electron transparency was achieved by ion milling at 6 kV with an incident sputtering angle of 12 °. Micrographs were taken using a transmission electron microscope (JEM-200 CX, JEOL) where the electrons were accelerated from a LaB6 cathode by a voltage of 200 kV. To obtain the average grain size, the grains in the micrographs were measured manually. Compression creep tests were carried out in a creep testing machine (AMSLER). A constant stress of 30 MPa was applied on the cylindrical sample at a constant temperature of 600°C.
3. Results To distinguish between effects of temperature and annealing time, two types of annealing conditions were used: fixed annealing time (10 minutes)
C. Miiller et al./Physica C 275 (1997) 337-345
at temperatures between 400°C and 850°C (called "short-time annealing") and different annealing time (10 minutes to 160 hours) at the same temperature (780°C or 850°C). The crystallization of different phases in short-time annealed samples was observed by X-ray diffraction measurements. At temperatures below 500°C the amorphous structure remains. At temperatures up to 630°C small grains with the Bi2Sr2CuOs+d (Bi2201) structure develop, as shown in Fig. 1 by broad X-ray peaks. The peaks become more narrow with increasing temperature, representing larger grains. At 630°C, the first Bi2212 peaks emerge from the background (Fig. 2). As the temperature becomes higher the amount of Bi2212 phase increases at the expense of Bi2201. Secondary phases like Bi203, Cu20, and CuO disappear at higher annealing temperatures (above 630°C) as well as longer annealing time (see Figs. 3 and 4). As already shown in the XRD patterns, the grain size changes significantly with higher annealing temperature. The results of TEM investigations for short annealing times are summarized in Fig. 5. Samples annealed at 500°C to 600°C are nanocrystalline and the grains have the same size in the direction of the a-, b- and c-axes. The grains grow with different rates for the two marked directions at higher annealing temperatures. At 850°C, the average grain extension along the a- and b-axes is 10 times higher than along the caxis. Figs. 6 to 8 show examples ofTEM micrographs for different annealing temperatures. Because parts of the larger grains are projecting beyond the border of the micrograph, the measured grain size of samples annealed at higher temperatures is a lower limit of the average grain size. The error bars indicate the statistical error associated with measurements of the grain size on micrographs. For crystallization over longer time, Takei et al. [6] showed that for temperatures around 500°C no significant change in grain size is expected. In the present work also no significant narrowing of XRD peaks could be observed for longer annealing time even at higher temperatures (780°C and 850°C) (Fig. 4). The amount of oxygen in the glassy intermediate product yields x = 7.45, if x is the molar ratio of oxygen in Bi2.18Sr2CaCu2Ox. The oxygen content increases to x = 8.25 at a crystallization temperature of 780°C after 96 hours. No impurity pick-up from the ZrO2 crucible or
341
the copper plate used for quenching was found by chemical analysis or by energy-dispersive X-ray spectroscopy in the scanning electron microscope (SEM/EDX). Concerning the superconducting properties of sampies which were annealed at temperatures below 780°C, no drop in the AC susceptibility signal was observed regardless of annealing time. Hence, there is no continuous screening superconducting surface layer in samples crystallized below this temperature. The dependence of the critical temperature on the annealing time for samples annealed at 780°C and 850°C is shown in Fig. 9. At 780°C there is a significant increase in the critical temperature in the first 20 hours of crystallization. This behaviour was not observed during annealing at 850°C. The mechanical properties were tested by compression creep experiments. The results are presented in Fig. 10 where the true strain is plotted versus time. A negative change in length is considered as positive strain. Nanocrystalline samples, i.e. samples annealed at 600°C, showed the highest strain rate, reaching the maximum length change allowed by the experimental conditions. In Fig. 11 the corresponding strain rates are plotted versus time. They show a difference of almost one order of magnitude between samples with nanosized grains and samples annealed at temperatures of 750°C. In addition, the slope of the strain rate curves increases with higher annealing temperature.
4. Discussion
It is evident from the presented data that the crystallization conditions (time and temperature) play a significant role in structural and microstructural characteristics and therefore in the mechanical and superconducting properties. Crystallization of Bi2201 could be observed at temperatures above 500°C. The lowest annealing temperature at which Bi2212 peaks in XRD patterns were found was 630°C for the composition of Bi2.18Sr2CaCu2Ox. TEM investigations show that for this crystallization temperature, grain size exceeds 50 nm along c- axis. Therefore nanosized Bi2212 could not be obtained. The formation of the Bi2201 and Bi2212 phases can be explained by a mechanism similar to that proposed by Takei et al. [6]:
342
C. Miiller et al./Physica C 275 (1997) 337-345
Fig. 6. TEM bright field micrographof Bi2201 glass ceramics,annealedat 500°C for 10 minutes. {Bi2212}r~5°°, °c [Bi2201] + {(Sr, Ca)CuO2},
(1) [Bi2201 ] + {(Sr, Ca)CuO2}r>63°~ c [Bi2212],
(2) {Bi2212} r->63°°c [Bi2212],
(3)
where the parentheses { } and [ ] represent the amorphous and crystalline states, respectively. As mentioned above [Bi2201 ] is the first phase crystallizing from the glassy matrix. Here a glass with a composition close to Bi2212 (indicated by {Bi2212}) rather than {Bi2201} (Takei et al. [6] ) was chosen as an educt. This was revealed by SEM/EDX. Considering the XRD patterns at higher annealing temperature and short annealing time, it can be concluded that the ratio of the rate of reaction (3) to the rate of reaction ( 1) increases with temperature. Fig. 3 shows that almost no [Bi2201 ] is formed at 850°C after 5 minutes. Also, at temperatures above 630°C [Bi2212] forms at the expense of [Bi2201 ]. This is shown in long-time anneaiings (Fig. 4) where the amount of [Bi2201] decreases with time. The investigation of grain size shows a pronounced dependence of grain growth kinetics on annealing temperature. The mechanism for the formation of large grains at higher temperature and small grains at lower
temperature can be explained by a higher portion of liquid phase due to the softening of intermediate glass at higher temperatures. Usually the presence of a liquid phase promotes grain growth because of enhanced transport in the liquidphase compared to the bulk. The plate like growth behaviour is directly attributable to the layered structure of BSCCO superconductors. With respect to the mechanical properties, it is evident that samples with small, equiaxed grains in a glassy matrix deform more rapidly than samples with large grains and a high aspect ratio. This causes the significant decrease of strain rate with increasing grain size. The difference in strain rate slopes expresses the potential of samples with nanosized grains for enhanced deformation. Unfortunately, highly deformable samples with nanosized grains are not superconducting. But longer annealing time and higher temperature lead to the development of superconductivity. It is not yet understood why a drop in AC susceptibility is observed only at annealing temperatures above 780°C. Explanations for this behaviour may lie in the ratio of superconducting grains to non-superconducting glassy matrix and precipitates, or in the oxygen content. In this study chemical analysis indicated a significant change in oxygen content for annealed samples. But the difference of the dependence of Tc on annealing
C. Miiller et al./Physica C 275 (1997) 337-345
343
Fig. 7. TEM bright field micrograph of Bi2212/Bi2201 glass ceramics, annealed at 630°C for 10 minutes.
Fig. 8. TEM bright field micrograph of Bi2212 glass ceramics, annealed at 790°C for 10 minutes.
time between annealing at 780°C and 850°C could not be explained by these considerations. Further investigations are needed to clarify these questions. In summary a manufacturing program for bulk BSCCO superconductors can be proposed, which includes in the first step an annealing at lower temperature and immediate deformation and in the second
step a crystallization at higher temperature where the superconducting properties should develop.
344
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References
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change in critical temperature it could not be clearly explained whether it depends on the oxygen content. A manufacturing program for bulk Bi2212 superconductors is proposed.
.
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5. Conclusions
By annealing of high-Tc superconductors with a composition of Bi2.18Sr2CaCu2Ox from a glassy intermediate product at temperatures between 500°C and 600°C, nanosized Bi2201 grains are obtained. Samples annealed at 600°C show high deformation rates which could be explained by the microstructure. Bi2212 crystallizes at temperatures above 630°C with grain sizes up to several 100 nm along the a- and baxes. Samples annealed at temperatures above 780°C show good superconducting properties with a maximum critical temperature of 85 K. For the observed
[ 1 ] T. Komatsu, R. Sato, K. lmai, K. Matusita and T. Yamashita, Jpn. J. Appl. Phys. 27 (1988) L550. [2] T Komatsu, T. Ohki, C. Hirose and K. Matusita, J. NonCrystall. Sol. 113 (1989) 274. [3] K. Nassau, A. Miller, E. Gyorgy and T. Siegrist, J. Mater. Res. 4 (6) (1989) 1330. [4] Y. Oka, N. Yamamoto, H. Kitaguchi, K. Oda and J. Takada, Jpn. J. Appl. Phys. 28 (2) (1989) L213. [ 5 ] D. Shi, M. Tang, K. Vandervoort and H. Claus, Phys. Rev. B 39 (13) (1989) 9091. [6] H. Takei, M. Koike, H. Takeya, K. Suzuki and M. Ichihara, Jpn. J. Appl. Phys. 28 (7) (1989) Ll193. [7] N. Tohge, S. Tsuboi, M. Tatsumisago and T. Minami, Jpn. J. Appl. Phys. 28 (10) (1989) L1742. [8] M.R. DeGuire, N.P. Bansal and C.J. Kim, J. Am. Ceram. Soc. 75 (5) (1990) 1165. [9] D.P Matheis, R.L Snyder and C.R. Hubbard, in: Superconductivity and its Applications, eds. Y. Kao, P. Coppens and H. Kwok (American Institute of Physics, New York, 1991) pp. 582-588. [ 10] R. Sato, T. Komatsu and K. Matusita, J. Non-Crystall. Sol. 134 (1991) 270. [ i 1] T.G. Holesinger, D.J. Miller and L.S. Chumbley, J. Mater. Res. 7 (7) (1992) 1658. [12] A.Q. He, X.M. Meng, Y.Z. Wang, G.W. Qiao and H.Q. Ye, Physica C 204 (1993) 365. [ 13] W. Wong-Ng and S.W. Freiman, Appl. Supercond. 2 (3/4) (1994) 163.
C. Miiller et al./Physica C 275 (1997) 337-345
[141 P. Majewski, Adv. Mater. 6 (6) (1994) 460. [15] S.E. Babcock and J.L. Vargas, Ann. Rev. Mater. Sci. 25 (1995) 193.
345
[16] E M~ewski, H.-L. Suand B. He~ich, Adv. Mate~ 4(7/8) (1992) 508. [171A. StreckeL U. SMzbergerand J. MayeLPrakt. MetallogL 30 (10) (1993) 482.
Physica C 275 (1997) 337-345
Processing effects on mechanical and superconducting properties of Bi2201 and Bi2212 glass ceramics C o r n e l i a M t i l l e r * , P e t e r M a j e w s k i , Gt~nter T h u r n , F r i t z A l d i n g e r Max-Planck-lnstitut ffir Metallforschung, Institut fiir Werkstoffwissenschafl, Pulvermetallurgisches Laboratorium, Heisenbergstr. 5, D-70569 Stuttgart, Germany
Received 3 September 1996; revised manuscriptreceived 12 December 1996
Abstract
Bi2212 and Bi2201 were prepared by annealing of glass with the composition Bi2.taSr2CaCu2Os+a. Using this preparation technique, structural and microstructural properties such as phase formation, grain size and oxygen content can easily be affected by different annealing conditions, i.e. by variation of time and temperature. Hence, the deformation behaviour and the critical temperature (To) are a function of the annealing conditions. This could be shown by creep experiments and AC susceptibility measurements. Samples with nanocrystalline microstructures exhibit substantially better plasticity under stress than samples with larger grains. Good superconducting properties could only be achieved in nearly single-phase Bi2212 samples with grains extending over several hundred nanometers to a few micrometers in the direction of the c axis. Keywords: High-To superconductor;Bi-Sr-Ca-Cu-O system;Crystallization;Glass ceramics;Deformation
1. Introduction Bi oxides are known as conditional glass formers, i. e. they will form glasses when mixed with a suitable amount of a second oxide. Hence, high-Tc superconductors of the system Bi-Sr-Ca-Cu-O exhibit the advantage that they can be processed via a glass intermediate product. By variation of crystallization conditions, structural and microstructural properties such as phase formation, oxygen content, and grain size can easily be affected. In order to optimize the superconducting and mechanical properties of glass ceramics in the system Bi-Sr-Ca-Cu-O (BSCCO), it is important to investigate in detail the changes in these properties during the crystallization process. Since the first prepa* Corresponding author. Fax: +49 711 6861 131; e-mail: [email protected].
ration of BSCCO glass ceramics [ 1 ] many authors have studied the crystallization mechanism [2-12]. Because of many different starting compositions and preparation routes it is difficult to compare the results of these studies, as noted in an overview by WongNg and Freiman [ 13]. Therefore, no general rule for the phase formation mechanism of Bi2Sr2CaCu2Os+d (Bi2212) could be given. The dependence of the electrical properties of highTc superconductors on structure and microstructure is well known. For instance, the number of CuO layers determines the critical temperature (Tc) and the oxygen content determines the charge carrier concentration [ 14]. On the microstructural scale the connectivity at grain boundaries (weak links) [ 15] is important especially in the presence of a magnetic field. Still the shaping of BSCCO superconducting ma-
0921-4534/97/$17.00 Copyright (~) 1997 Elsevier Science B.V. All rights reserved. PII S0921-4534(97)00026-9
338
C. Mailer et al./Physica C 275 (1997) 337-345
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C. M~iller et al,/Physica C 275 (1997) 337-345
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340
C. Mailer et al./Physica C 275 (1997) 337-345
terial is a problem, because fully crystalline BSCCO does not deform easily due to its big, mica-like grains. Therefore changing grain size and shape might improve the deformability of BSCCO. In this report we investigate the dependence of structural and microstructural changes on annealing conditions for BSCCO high-temperature superconductors with a cation ratio Bi : Sr : Ca : Cu = 2.18 : 2 : 1 : 2. This composition was chosen, because it is situated in the center of the homogeneity area for Bi2212 in the phase diagram [ 16]. The structural properties are determined by X-ray powder diffraction (XRD) for different crystallization temperatures and times. Also, the variation of oxygen content is observed by means of chemical analysis. Microstructural changes, i.e. variation in grain size and shape, are investigated in detail by transmission electron microscopy (TEM). From these results we try to interpret the change of critical temperature (To) measured by AC susceptibility and also the deformation behaviour under stress during creep experiments.
2. Experimental procedure The preparation of the Bi glass ceramics was carried out as follows. Commercially available Bi203, SrCO3, CaCO3 and CuO (99.9%) powders were weighed to the nominal composition of Bi2.tsSr2CaCu2Ox, mixed and calcined at 750°C, 780°C, and 820°C for 96 hours in total. After milling, the powder was melted in a ZrO2 crucible at 1200°C for 30 minutes. In order to obtain glass, the melt was poured onto a copper plate or was quenched in a quartz glass tube with an internal diameter of 2 mm. Thus we had two different sample shapes: plate-like and cylindrical. The platelike samples were used for XRD measurements, TEM investigations, and chemical analysis, whereas cylinders proved to be a suitable shape for AC susceptibility measurements and creep tests. The samples were annealed at temperatures between 400°C and 850°C in air for times ranging from I0 minutes to 160 hours. For that purpose they were placed in a preheated electrical furnace for a specified time and then quenched onto a copper plate. The phase formation of the powdered samples was recorded by an X-ray diffractometer (Model D5000, Siemens) with a position sensitive proportional
1000
[] ab-direction * c-direction
--
, + -
..
ioo ._m .a
(.9
10
1450
550 650 750 Crystallization temperature (°C)
850
Fig. 5. Grain size versus crystallization temperature in BSCCO glass ceramics,annealed for 10 minutes or 5 minutes (825°C and 850°C). counter (PSPC) from 28=10 ° to 80 °. The oxygen
content was determined by carrier gas hot extraction followed by an IR detection of CO2. As a function of temperature, the real part of the intrinsic susceptibility was measured by an AC susceptometer with a frequency of 1 kHz. TEM investigations were carried out on samples which were prepared according to the method of Strecker, Salzberger, and Mayer, which involves the embedding of 300-/zm thick slices in ceramic holders [ 17]. From these cylindrical holders, disks with a diameter of 3 mm were cut. After grinding and dimpling to a thickness of approximately 25 /zm, electron transparency was achieved by ion milling at 6 kV with an incident sputtering angle of 12 °. Micrographs were taken using a transmission electron microscope (JEM-200 CX, JEOL) where the electrons were accelerated from a LaB6 cathode by a voltage of 200 kV. To obtain the average grain size, the grains in the micrographs were measured manually. Compression creep tests were carried out in a creep testing machine (AMSLER). A constant stress of 30 MPa was applied on the cylindrical sample at a constant temperature of 600°C.
3. Results To distinguish between effects of temperature and annealing time, two types of annealing conditions were used: fixed annealing time (10 minutes)
C. Miiller et al./Physica C 275 (1997) 337-345
at temperatures between 400°C and 850°C (called "short-time annealing") and different annealing time (10 minutes to 160 hours) at the same temperature (780°C or 850°C). The crystallization of different phases in short-time annealed samples was observed by X-ray diffraction measurements. At temperatures below 500°C the amorphous structure remains. At temperatures up to 630°C small grains with the Bi2Sr2CuOs+d (Bi2201) structure develop, as shown in Fig. 1 by broad X-ray peaks. The peaks become more narrow with increasing temperature, representing larger grains. At 630°C, the first Bi2212 peaks emerge from the background (Fig. 2). As the temperature becomes higher the amount of Bi2212 phase increases at the expense of Bi2201. Secondary phases like Bi203, Cu20, and CuO disappear at higher annealing temperatures (above 630°C) as well as longer annealing time (see Figs. 3 and 4). As already shown in the XRD patterns, the grain size changes significantly with higher annealing temperature. The results of TEM investigations for short annealing times are summarized in Fig. 5. Samples annealed at 500°C to 600°C are nanocrystalline and the grains have the same size in the direction of the a-, b- and c-axes. The grains grow with different rates for the two marked directions at higher annealing temperatures. At 850°C, the average grain extension along the a- and b-axes is 10 times higher than along the caxis. Figs. 6 to 8 show examples ofTEM micrographs for different annealing temperatures. Because parts of the larger grains are projecting beyond the border of the micrograph, the measured grain size of samples annealed at higher temperatures is a lower limit of the average grain size. The error bars indicate the statistical error associated with measurements of the grain size on micrographs. For crystallization over longer time, Takei et al. [6] showed that for temperatures around 500°C no significant change in grain size is expected. In the present work also no significant narrowing of XRD peaks could be observed for longer annealing time even at higher temperatures (780°C and 850°C) (Fig. 4). The amount of oxygen in the glassy intermediate product yields x = 7.45, if x is the molar ratio of oxygen in Bi2.18Sr2CaCu2Ox. The oxygen content increases to x = 8.25 at a crystallization temperature of 780°C after 96 hours. No impurity pick-up from the ZrO2 crucible or
341
the copper plate used for quenching was found by chemical analysis or by energy-dispersive X-ray spectroscopy in the scanning electron microscope (SEM/EDX). Concerning the superconducting properties of sampies which were annealed at temperatures below 780°C, no drop in the AC susceptibility signal was observed regardless of annealing time. Hence, there is no continuous screening superconducting surface layer in samples crystallized below this temperature. The dependence of the critical temperature on the annealing time for samples annealed at 780°C and 850°C is shown in Fig. 9. At 780°C there is a significant increase in the critical temperature in the first 20 hours of crystallization. This behaviour was not observed during annealing at 850°C. The mechanical properties were tested by compression creep experiments. The results are presented in Fig. 10 where the true strain is plotted versus time. A negative change in length is considered as positive strain. Nanocrystalline samples, i.e. samples annealed at 600°C, showed the highest strain rate, reaching the maximum length change allowed by the experimental conditions. In Fig. 11 the corresponding strain rates are plotted versus time. They show a difference of almost one order of magnitude between samples with nanosized grains and samples annealed at temperatures of 750°C. In addition, the slope of the strain rate curves increases with higher annealing temperature.
4. Discussion
It is evident from the presented data that the crystallization conditions (time and temperature) play a significant role in structural and microstructural characteristics and therefore in the mechanical and superconducting properties. Crystallization of Bi2201 could be observed at temperatures above 500°C. The lowest annealing temperature at which Bi2212 peaks in XRD patterns were found was 630°C for the composition of Bi2.18Sr2CaCu2Ox. TEM investigations show that for this crystallization temperature, grain size exceeds 50 nm along c- axis. Therefore nanosized Bi2212 could not be obtained. The formation of the Bi2201 and Bi2212 phases can be explained by a mechanism similar to that proposed by Takei et al. [6]:
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Fig. 6. TEM bright field micrographof Bi2201 glass ceramics,annealedat 500°C for 10 minutes. {Bi2212}r~5°°, °c [Bi2201] + {(Sr, Ca)CuO2},
(1) [Bi2201 ] + {(Sr, Ca)CuO2}r>63°~ c [Bi2212],
(2) {Bi2212} r->63°°c [Bi2212],
(3)
where the parentheses { } and [ ] represent the amorphous and crystalline states, respectively. As mentioned above [Bi2201 ] is the first phase crystallizing from the glassy matrix. Here a glass with a composition close to Bi2212 (indicated by {Bi2212}) rather than {Bi2201} (Takei et al. [6] ) was chosen as an educt. This was revealed by SEM/EDX. Considering the XRD patterns at higher annealing temperature and short annealing time, it can be concluded that the ratio of the rate of reaction (3) to the rate of reaction ( 1) increases with temperature. Fig. 3 shows that almost no [Bi2201 ] is formed at 850°C after 5 minutes. Also, at temperatures above 630°C [Bi2212] forms at the expense of [Bi2201 ]. This is shown in long-time anneaiings (Fig. 4) where the amount of [Bi2201] decreases with time. The investigation of grain size shows a pronounced dependence of grain growth kinetics on annealing temperature. The mechanism for the formation of large grains at higher temperature and small grains at lower
temperature can be explained by a higher portion of liquid phase due to the softening of intermediate glass at higher temperatures. Usually the presence of a liquid phase promotes grain growth because of enhanced transport in the liquidphase compared to the bulk. The plate like growth behaviour is directly attributable to the layered structure of BSCCO superconductors. With respect to the mechanical properties, it is evident that samples with small, equiaxed grains in a glassy matrix deform more rapidly than samples with large grains and a high aspect ratio. This causes the significant decrease of strain rate with increasing grain size. The difference in strain rate slopes expresses the potential of samples with nanosized grains for enhanced deformation. Unfortunately, highly deformable samples with nanosized grains are not superconducting. But longer annealing time and higher temperature lead to the development of superconductivity. It is not yet understood why a drop in AC susceptibility is observed only at annealing temperatures above 780°C. Explanations for this behaviour may lie in the ratio of superconducting grains to non-superconducting glassy matrix and precipitates, or in the oxygen content. In this study chemical analysis indicated a significant change in oxygen content for annealed samples. But the difference of the dependence of Tc on annealing
C. Miiller et al./Physica C 275 (1997) 337-345
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Fig. 7. TEM bright field micrograph of Bi2212/Bi2201 glass ceramics, annealed at 630°C for 10 minutes.
Fig. 8. TEM bright field micrograph of Bi2212 glass ceramics, annealed at 790°C for 10 minutes.
time between annealing at 780°C and 850°C could not be explained by these considerations. Further investigations are needed to clarify these questions. In summary a manufacturing program for bulk BSCCO superconductors can be proposed, which includes in the first step an annealing at lower temperature and immediate deformation and in the second
step a crystallization at higher temperature where the superconducting properties should develop.
344
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change in critical temperature it could not be clearly explained whether it depends on the oxygen content. A manufacturing program for bulk Bi2212 superconductors is proposed.
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5. Conclusions
By annealing of high-Tc superconductors with a composition of Bi2.18Sr2CaCu2Ox from a glassy intermediate product at temperatures between 500°C and 600°C, nanosized Bi2201 grains are obtained. Samples annealed at 600°C show high deformation rates which could be explained by the microstructure. Bi2212 crystallizes at temperatures above 630°C with grain sizes up to several 100 nm along the a- and baxes. Samples annealed at temperatures above 780°C show good superconducting properties with a maximum critical temperature of 85 K. For the observed
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