- Email: [email protected]
Effect of silver additions on properties of (Bi,Pb)2Sr2Ca2Cu3O10Ag high-temperature superconducting tapes
Materials Letters 18 ( 1994) 336-340 North-Holland
Effect of silver additions on properties of (Bi,Pb ) &Ca2Cu30 high-temperature superconducting tapes
&Ag
Y.C. Guo, H.K. Liu and S.X. Dou School ofMaterials Science and Engineering, The University of New South Wales, P. 0. Box I, Kensington, NSW 2033, Australia Received 27 July 1993; in final form 8 November 1993; accepted 15 November 1993
A series of silver-sheathed ( Bi,Pb)2Sr2Ca2Cu3010tapes with various amounts of silver were prepared and the effect of the silver additions on the superconducting properties studied. The results indicate that silver additions accelerate the formation of the high-T, phase by lowering the sample’s partial melting temperature. No effect of silver on T, was found because the added silver is present as an isolated phase in the superconductor matrix without visible reaction or mutual diffusion between them. However, the silver additions do show a detrimental influence on J, which decreases with increasing silver content when the tapes are treated at the same temperature. The J, degradation is attributed to grain misalignment caused by the undesirable silver morphology. A slight improvement of &magnetic field behaviour in the tapes with low silver content (x< 1.0) and small degradation in the samples with high silver content (x2 2.4) is observed.
1. Introduction Silver has been successfully used to fabricate highquality high-temperature superconductor/metal composites in various forms such as silver-cored wires, silver-substrate films and silver-sheathed wires and multifilaments. In particular, the silver-sheathed BiPbSrCaCuO (2-2-2-3 ) superconducting wires with critical current density (J,) exceeding lo4 A/cm* at 77 K and 0 T have been achieved [ 1,2 1, which has already satisfied the requirements for some practical applications. However, the published results of silver effect on the superconductivity of BiPbSrCaCuO compound are very inconsistent. For instance, Ren and Wang [ 31 reported that silver additions depressed the critical temperature (T,) of BiPbSrCaCuO (2-2-2-3) bulk samples from 107 K to 6286 K while Oota et al. [ 41 claimed that silver caused no change in T,for the same material. Investigation by Ishida et al. [ 5 1, however, showed an increase in T, for samples with silver contents up to 10% volume and then a decrease with more silver. Conflicting results have also been reported about the influence of silver on critical current density (J,) and J, behaviour in magnetic fields [ 3,4,6]. Therefore, further detailed investigation is needed to understand 336
the behaviour of silver in BiPbSrCaCuO superconductors. In this Letter, we present the results of our investigation of the influence of silver additions on the superconducting properties of silver-sheathed ( Bi,Pb)2Sr2Ca2Cu30,0 composite tapes, including T,, .I,and J,-magnetic field characteristics. The observed results will be related to the microstructure of samples.
2. Experimental
procedure
The precursor powders with a stoichiometry of Bi,,8Pb,,4Sr2.0Ca2.2Cu~.0 were prepared by pyrolysis of a metal nitrate solution. The powders were mixed with appropriate amounts of line Ag,O powders in accordance with the formula Bi,.8Pb,,4Sr2.0Ca2.2Cu3.0Ag,O,, (x=0.5-3.5), pressed into pellets, and sintered at 830-840°C for 15 h. To ensure homogeneity, the grinding and sintering process was repeated. For comparison, a powder without silver (x=0.0) was also made by the same processing. The powders were then packed into silver tubes and the oxide/metal composites were cold-worked into thin wires and tapes by swaging, drawing and rolling (the tape made
0167-577x/94/.$07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDI 0167-577X(93)E0193-C
February 1994
MATERIALS LETTERS
Volume 18, number 5,6
from the powder without silver is hereinafter referred to as normal tape). The resultant tapes were subjected to a heat-treatment process consisting of several cycles of pressing and sintering. The sintering was conducted in air at 830-840°C for a period varying from 50 to 100 h for each step. X-ray diffraction (XRD) analyses and transport measurements were carried out to check the high-Tc 2-2-2-3 phase formation and .I, development of the samples during the process of heat treatment. T, and J, dependences on magnetic field were measured for the tapes after heat treatment. Microstructural characterisation was performed through scanning electron microscope (SEM) observations.
ditions. This non-poisoning behaviour of silver is essential for silver to be used as a proper stabiliser and sheath material in superconductor/metal composites such as films, ribbons and wires. The silver, however, was found to influence the formation rate of the high-Tc 2-2-2-3 phase and .I, values of tapes. As shown in fig. 2a, the volume fraction of high-T, 2-2-2-3 phase inside the silver-containing (x= 3.5 ) sample increases faster than that in normal sample (x=0.0) with increasing sintering time and becomes saturated with shorter sintering period, indicating that silver addition enhanced the formation rate of high-l; 2-2-2-3 phase. We have previously reported that silver additions lower the sample’s partial melting temperature by x 10°C in air [ 7 1. This means that the silver-containing sample was being sintered at a temperature closer to its
3. Results and discussion Shown in fig. 1 are the T, values as a function of silver content (x) for a series of tape samples with various levels of silver after sintering at 832°C for 300 h in air. The T, was determined by measuring the electrical resistivity versus temperature curve and defined as the temperature at which the sample’s resistivity reached zero. As can be seen in fig. 1, all samples have almost identical Tc value of 107 K independent of the silver content added, suggesting that silver did not significantly affect the T, of BiPbSrCaCuO (2-2-2-3) in the present range of silver ad-
0.5
Sintering
1201 1101
0
0
I
0
0
Time (hrs)
0
0
go0 E
2 90-
Silver-Clad
%b
at 0.0
Tapes
(Bi,Pb)2SrZCazCuJAg.0,~
1
2.0
1.0
Silver
Content
3.0
4.0
(x)
Fig. 1. Critical temperature (r,) versus silver content (x) for a series of Bi(2-2-2-3) tapes with various levels of silver (x=0.03.5) after sintering at 832°C for 380 h.
1.0 Silver
4.0
2.0
Content
(x”r
Fig. 2. (a) Sintering time dependence of the fraction of the highT, phase volume and critical current density, 3, (77 K), in the siiver~ontaining (x= 3.5) and normal (x=0.0) Bi(2-2-2-3) tapes; (b) maximum J, value versus silver content (x) for a series of Bi(2-2-2-3) tapes treated with the same sintering conditions.
337
Volume 18, number
5,6
MATERIALS
melting temperature and therefore better sintered than the normal sample when both were treated at the same temperature. It is well believed that the highT, 2-2-2-3 phase is formed most efficiently at a temperature just below the partial melting point [ 891. So, the lowering of sample’s melting point by silver is the reason for the enhancement of high-T, 2-2-23 phase formation rate. It is also noted that with sufficient sintering time both the silver-containing and normal samples can reach a high and almost identical volume fraction of high-T, 2-2-2-3 phase, suggesting that silver only accelerates the 2-2-2-3 phase formation rate, but does not affect the final phase assemblage of samples. Fig. 2a also gives a comparison of J, (77 K) dependence on sintering time for the silver-containing and normal tapes. The J, values for the silver-containing tape were calculated using the cross section area after deducting the part of area occupied by silver, which were measured from SEM micrograph. In consistency with the results of the phase analyses, the J, of the silver-containing tape goes up more quickly than the normal sample as the result of the enhancement of high-T, 2-2-2-3 phase formation rate by the silver addition. In spite of the initial enhancement of J,, the maximum J, value of silver-containing sample is lower than that of normal tape when both are sintered at the same temperature. This indicates that the silver additions have detrimental effect on tape’s J,, as presented in fig. 2b which shows that the J, decreases with increasing silver content (x). By comparing the high-T, 2-2-2-3 phase fractionsintering time curve with the J,-sintering time curve for the same sample, it is found that the initial rapid increase of the fraction of high-T, 2-2-2-3 phase does not lead to a large J, increase because the formed high-T, 2-2-2-3 particles are isolated by the major amount of the low-T, 2-2-l-2 phase; very few highT, 2-2-2-3 phase paths are formed at this stage. Then, as sintering proceeds, J, continuously increases with increasing fraction of high-T, 2-2-2-3 phase, indicating dominance of the high-T, phase in enhancement of J,. With further sintering, although the change in the fraction of high-T, 2-2-2-3 phase is small, J, increases substantially owing to the improvement of grain alignment, grain growth and intergrain connectivity. Finally, with more pressing and prolonged sintering J, begins to drop probably due 338
LETTERS
February
1994
to irreversible cracks, degradation of grain alignment, and Bi and Pb loss [ 6,101. Shown in figs. 3a and 3b are the SEM micrographs for normal (x=0.0) and silver-containing (x=3.5) tapes respectively. Although the silver particles show a very good connection with the high-T= 2-2-2-3 grains, the boundary between them is clear and clean and no reaction or inter-diffusion was observed. The silver is present as isolated particles in the matrix and does not enter the superconductor lattice, so it does not affect the sample’s T,. However, compared with the plate-like high-T, 2-2-2-3 grains which have an extremely large aspect-ratio morphology, the silver particles are thick and irregularly shaped and hence result in high angle boundaries between the surrounding superconductor grains. In contrast, the normal sample, as shown in fig. 3a, is dense and the grains are highly aligned with low angle boundaries. The average angle of grain misalignment was estimated to increase from about 5” to 20” when the silver content was increased from x=0.0 to x= 3.5. It is well known that the cuprate-based high-T, superconductors all have very short coherence lengths and large penetration depths, resulting in weak links at grain boundaries and therefore low critical current density. However, numerous results [ 1 l-l 71 have
Fig. 3. SEM micrographs of polished cross section of the core of (a) normal tape (x=0.0) and (b) silver-containing tape (x=3.5).
Volume
18, number
MATERIALS
5,6
shown that the weak links can be largely eliminated in well textured samples, e.g. silver-clad Bi (2-2-2-3 ) tapes, in which the plate-like grains are highly aligned with their conduction Cu-0 planes parallel to the tape’s wide surface, forming a “brick-wall” microstructure [ 181 which enables the current flows predominantly in the Cu-0 planes and transfers between grains across the large-area c-axis boundaries at low current density. So, a high degree of grain alignment is one of the essential requirements for the high-T, materials to carry a large current. The grain misalignment resulted from the undesirable morphology of silver particles may be the main reason for the J, degradation in the silver-containing tapes. The J, behaviour in magnetic field for the silvercontaining tape is compared with that for the normal tape in fig. 4. A slight improvement of the J,-magnetic field characteristic for the samples with low level of silver (x= 1.0) is obtained while similar J,-magnetic field behaviour as that for the normal tape is observed for the sample with a silver content of x= 1.6. But a small degradation of J, dependence on magnetic field for the samples with high level of silver (x= 3.5) is observed. From above discussion, it is known that silver causes grain misalignment and the degree of misalignment becomes more pronounced with increasing silver content. On the other hand, the good adhesion between silver particles and superconductor grains may improve the connectivity between grains and prevent crack producing during heating and cooling. The difference of J,-mag-
LETTERS
February
1994
netic field characteristics in samples with various silver contents is the combined effect of these two contradictory factors.
4. Conclusion The effect of silver additions on the properties of silver-sheathed BiPbSrCaCuO (2-2-2-3 ) superconducting tapes was studied through determination of the rate of formation of the high-T, 2-2-2-3 phase, T,, J, and J,-magnetic field characteristics. The results indicated that silver additions cause neither changes in the value of T, nor the decomposition of high-T, 2-2-2-3 phase. However, silver does influence the sample’s J,, which decreases with increasing silver content when the tapes are treated at the same temperature. The microstructural analyses revealed that silver is present as an isolated phase inside the tape without visible reaction and diffusion with the superconductor matrix, but the undesirable morphology of the added silver particles causes a degradation of grain alignment and boundary order. A slight improvement in the J,-magnetic field characteristics for the tapes with low levels of silver (x< 1.0) and a small degradation for the samples with high levels of silver (x> 2.4) were also observed.
Acknowledgement The authors wish to thank Metal Manufactures Ltd. and Commonwealth Department of Industry, Technology and Commerce for financial support. Thanks are also given to Dr. M.H. Appley for assistance in wire drawing.
References 088861
x=0.0
[ 1] K. Sato, N. Shibuta, H. Mukai, T. Hikata, M. Ueyama and
ooooo x=1.0 -x=3.5 Ag-Clad
(Bi,Pb)zSrzCazCuzAg.O,~
0.01
,
0.1
Magnetic
Field
Tapes
(T)
Fig. 4. Magnetic field dependence of nonnalised J, for the Bi (22-2-3) tapes with various silver contents (x=0.0, 1.0 and 3.5).
T. Kato, J. Appl. Phys. 70 ( 199 1) 6484. [2] Y.C. Guo, H.K. Liu and S.X. Dou, Appl. Supercond. l-2 (1993) 25. [3]Z.F.RenandJ.H. Wang, J.Mater.Sci. 10 (1991) 1139. [4] A. Oota, T. Horio, K. Ohba and K. Iwasaki, J. Appl. Phys. 71 (1992) 5997. [ 51 Y. Ishida, J. Matsuzaki, T. Kizuka and H. Ichinose, Physica C 19 (1991) 67.
339
Volume 18, number 56
MATERIALS LETTERS
[6] Y. Yamada, B. Obst and K. Flukiger, Supercond. Sci. Technol. 4 (1991) 165. [7] Y.C. Guo, H.K. Liu and S.X. Dou, J. Mater. Res. 8 (1993) 2187. [ 8 ] K. Aota, H. Hattori, T. Hatano, K. Nakamura and K. Ogawa, Japan. J. Appl. Phys. 28 (1989) L2169. [ 91 T. Hatano, K. Aota, H. Hattori, S. Ikeda, K. Nakamura and K. Ogawa, Cryogenics 30 ( 1990) 6 Il. [lo] S.X. Dou, H.K. Liu, Y.C. Guo, D.L. Shi, M.D. Sumption and E.D. Collings, to be published. [ 111 H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Japan. J. Appl. Phys. 27 (1988) L209.
340
February 1994
[ 12 ] S. Jin and J.E. Graebner, Mater. Sci. Eng. B 7 ( 1991) 243. [ 131 K.H. Sandhage, G.N. Reley and W.C. Carter, J. Metals 43 (1991) 21. [ 141 P. Haldar and L. Motowildo, J. Metals 44 (1992) 54. [ 151 E.E. Hellstrom, Mater. Res. Sot. Bull. XVII ( 1992) 45. [ 161 S.X. Dou and H.K. Liu, Supercond. Sci. Technol. 6 (1993) 297. [ 171 D.C. Larbalestier and M.P. Maley, Mater. Res. Sot. Bull. XVIII (1993) 50. [ 181 L.N. Bulaevskii, J.R. Glazman and A.P. Malozemoff, Phys. Rev. B 45 (1992) 2545.
Effect of silver additions on properties of (Bi,Pb ) &Ca2Cu30 high-temperature superconducting tapes
&Ag
Y.C. Guo, H.K. Liu and S.X. Dou School ofMaterials Science and Engineering, The University of New South Wales, P. 0. Box I, Kensington, NSW 2033, Australia Received 27 July 1993; in final form 8 November 1993; accepted 15 November 1993
A series of silver-sheathed ( Bi,Pb)2Sr2Ca2Cu3010tapes with various amounts of silver were prepared and the effect of the silver additions on the superconducting properties studied. The results indicate that silver additions accelerate the formation of the high-T, phase by lowering the sample’s partial melting temperature. No effect of silver on T, was found because the added silver is present as an isolated phase in the superconductor matrix without visible reaction or mutual diffusion between them. However, the silver additions do show a detrimental influence on J, which decreases with increasing silver content when the tapes are treated at the same temperature. The J, degradation is attributed to grain misalignment caused by the undesirable silver morphology. A slight improvement of &magnetic field behaviour in the tapes with low silver content (x< 1.0) and small degradation in the samples with high silver content (x2 2.4) is observed.
1. Introduction Silver has been successfully used to fabricate highquality high-temperature superconductor/metal composites in various forms such as silver-cored wires, silver-substrate films and silver-sheathed wires and multifilaments. In particular, the silver-sheathed BiPbSrCaCuO (2-2-2-3 ) superconducting wires with critical current density (J,) exceeding lo4 A/cm* at 77 K and 0 T have been achieved [ 1,2 1, which has already satisfied the requirements for some practical applications. However, the published results of silver effect on the superconductivity of BiPbSrCaCuO compound are very inconsistent. For instance, Ren and Wang [ 31 reported that silver additions depressed the critical temperature (T,) of BiPbSrCaCuO (2-2-2-3) bulk samples from 107 K to 6286 K while Oota et al. [ 41 claimed that silver caused no change in T,for the same material. Investigation by Ishida et al. [ 5 1, however, showed an increase in T, for samples with silver contents up to 10% volume and then a decrease with more silver. Conflicting results have also been reported about the influence of silver on critical current density (J,) and J, behaviour in magnetic fields [ 3,4,6]. Therefore, further detailed investigation is needed to understand 336
the behaviour of silver in BiPbSrCaCuO superconductors. In this Letter, we present the results of our investigation of the influence of silver additions on the superconducting properties of silver-sheathed ( Bi,Pb)2Sr2Ca2Cu30,0 composite tapes, including T,, .I,and J,-magnetic field characteristics. The observed results will be related to the microstructure of samples.
2. Experimental
procedure
The precursor powders with a stoichiometry of Bi,,8Pb,,4Sr2.0Ca2.2Cu~.0 were prepared by pyrolysis of a metal nitrate solution. The powders were mixed with appropriate amounts of line Ag,O powders in accordance with the formula Bi,.8Pb,,4Sr2.0Ca2.2Cu3.0Ag,O,, (x=0.5-3.5), pressed into pellets, and sintered at 830-840°C for 15 h. To ensure homogeneity, the grinding and sintering process was repeated. For comparison, a powder without silver (x=0.0) was also made by the same processing. The powders were then packed into silver tubes and the oxide/metal composites were cold-worked into thin wires and tapes by swaging, drawing and rolling (the tape made
0167-577x/94/.$07.00 0 1994 Elsevier Science B.V. All rights reserved. SSDI 0167-577X(93)E0193-C
February 1994
MATERIALS LETTERS
Volume 18, number 5,6
from the powder without silver is hereinafter referred to as normal tape). The resultant tapes were subjected to a heat-treatment process consisting of several cycles of pressing and sintering. The sintering was conducted in air at 830-840°C for a period varying from 50 to 100 h for each step. X-ray diffraction (XRD) analyses and transport measurements were carried out to check the high-Tc 2-2-2-3 phase formation and .I, development of the samples during the process of heat treatment. T, and J, dependences on magnetic field were measured for the tapes after heat treatment. Microstructural characterisation was performed through scanning electron microscope (SEM) observations.
ditions. This non-poisoning behaviour of silver is essential for silver to be used as a proper stabiliser and sheath material in superconductor/metal composites such as films, ribbons and wires. The silver, however, was found to influence the formation rate of the high-Tc 2-2-2-3 phase and .I, values of tapes. As shown in fig. 2a, the volume fraction of high-T, 2-2-2-3 phase inside the silver-containing (x= 3.5 ) sample increases faster than that in normal sample (x=0.0) with increasing sintering time and becomes saturated with shorter sintering period, indicating that silver addition enhanced the formation rate of high-l; 2-2-2-3 phase. We have previously reported that silver additions lower the sample’s partial melting temperature by x 10°C in air [ 7 1. This means that the silver-containing sample was being sintered at a temperature closer to its
3. Results and discussion Shown in fig. 1 are the T, values as a function of silver content (x) for a series of tape samples with various levels of silver after sintering at 832°C for 300 h in air. The T, was determined by measuring the electrical resistivity versus temperature curve and defined as the temperature at which the sample’s resistivity reached zero. As can be seen in fig. 1, all samples have almost identical Tc value of 107 K independent of the silver content added, suggesting that silver did not significantly affect the T, of BiPbSrCaCuO (2-2-2-3) in the present range of silver ad-
0.5
Sintering
1201 1101
0
0
I
0
0
Time (hrs)
0
0
go0 E
2 90-
Silver-Clad
%b
at 0.0
Tapes
(Bi,Pb)2SrZCazCuJAg.0,~
1
2.0
1.0
Silver
Content
3.0
4.0
(x)
Fig. 1. Critical temperature (r,) versus silver content (x) for a series of Bi(2-2-2-3) tapes with various levels of silver (x=0.03.5) after sintering at 832°C for 380 h.
1.0 Silver
4.0
2.0
Content
(x”r
Fig. 2. (a) Sintering time dependence of the fraction of the highT, phase volume and critical current density, 3, (77 K), in the siiver~ontaining (x= 3.5) and normal (x=0.0) Bi(2-2-2-3) tapes; (b) maximum J, value versus silver content (x) for a series of Bi(2-2-2-3) tapes treated with the same sintering conditions.
337
Volume 18, number
5,6
MATERIALS
melting temperature and therefore better sintered than the normal sample when both were treated at the same temperature. It is well believed that the highT, 2-2-2-3 phase is formed most efficiently at a temperature just below the partial melting point [ 891. So, the lowering of sample’s melting point by silver is the reason for the enhancement of high-T, 2-2-23 phase formation rate. It is also noted that with sufficient sintering time both the silver-containing and normal samples can reach a high and almost identical volume fraction of high-T, 2-2-2-3 phase, suggesting that silver only accelerates the 2-2-2-3 phase formation rate, but does not affect the final phase assemblage of samples. Fig. 2a also gives a comparison of J, (77 K) dependence on sintering time for the silver-containing and normal tapes. The J, values for the silver-containing tape were calculated using the cross section area after deducting the part of area occupied by silver, which were measured from SEM micrograph. In consistency with the results of the phase analyses, the J, of the silver-containing tape goes up more quickly than the normal sample as the result of the enhancement of high-T, 2-2-2-3 phase formation rate by the silver addition. In spite of the initial enhancement of J,, the maximum J, value of silver-containing sample is lower than that of normal tape when both are sintered at the same temperature. This indicates that the silver additions have detrimental effect on tape’s J,, as presented in fig. 2b which shows that the J, decreases with increasing silver content (x). By comparing the high-T, 2-2-2-3 phase fractionsintering time curve with the J,-sintering time curve for the same sample, it is found that the initial rapid increase of the fraction of high-T, 2-2-2-3 phase does not lead to a large J, increase because the formed high-T, 2-2-2-3 particles are isolated by the major amount of the low-T, 2-2-l-2 phase; very few highT, 2-2-2-3 phase paths are formed at this stage. Then, as sintering proceeds, J, continuously increases with increasing fraction of high-T, 2-2-2-3 phase, indicating dominance of the high-T, phase in enhancement of J,. With further sintering, although the change in the fraction of high-T, 2-2-2-3 phase is small, J, increases substantially owing to the improvement of grain alignment, grain growth and intergrain connectivity. Finally, with more pressing and prolonged sintering J, begins to drop probably due 338
LETTERS
February
1994
to irreversible cracks, degradation of grain alignment, and Bi and Pb loss [ 6,101. Shown in figs. 3a and 3b are the SEM micrographs for normal (x=0.0) and silver-containing (x=3.5) tapes respectively. Although the silver particles show a very good connection with the high-T= 2-2-2-3 grains, the boundary between them is clear and clean and no reaction or inter-diffusion was observed. The silver is present as isolated particles in the matrix and does not enter the superconductor lattice, so it does not affect the sample’s T,. However, compared with the plate-like high-T, 2-2-2-3 grains which have an extremely large aspect-ratio morphology, the silver particles are thick and irregularly shaped and hence result in high angle boundaries between the surrounding superconductor grains. In contrast, the normal sample, as shown in fig. 3a, is dense and the grains are highly aligned with low angle boundaries. The average angle of grain misalignment was estimated to increase from about 5” to 20” when the silver content was increased from x=0.0 to x= 3.5. It is well known that the cuprate-based high-T, superconductors all have very short coherence lengths and large penetration depths, resulting in weak links at grain boundaries and therefore low critical current density. However, numerous results [ 1 l-l 71 have
Fig. 3. SEM micrographs of polished cross section of the core of (a) normal tape (x=0.0) and (b) silver-containing tape (x=3.5).
Volume
18, number
MATERIALS
5,6
shown that the weak links can be largely eliminated in well textured samples, e.g. silver-clad Bi (2-2-2-3 ) tapes, in which the plate-like grains are highly aligned with their conduction Cu-0 planes parallel to the tape’s wide surface, forming a “brick-wall” microstructure [ 181 which enables the current flows predominantly in the Cu-0 planes and transfers between grains across the large-area c-axis boundaries at low current density. So, a high degree of grain alignment is one of the essential requirements for the high-T, materials to carry a large current. The grain misalignment resulted from the undesirable morphology of silver particles may be the main reason for the J, degradation in the silver-containing tapes. The J, behaviour in magnetic field for the silvercontaining tape is compared with that for the normal tape in fig. 4. A slight improvement of the J,-magnetic field characteristic for the samples with low level of silver (x= 1.0) is obtained while similar J,-magnetic field behaviour as that for the normal tape is observed for the sample with a silver content of x= 1.6. But a small degradation of J, dependence on magnetic field for the samples with high level of silver (x= 3.5) is observed. From above discussion, it is known that silver causes grain misalignment and the degree of misalignment becomes more pronounced with increasing silver content. On the other hand, the good adhesion between silver particles and superconductor grains may improve the connectivity between grains and prevent crack producing during heating and cooling. The difference of J,-mag-
LETTERS
February
1994
netic field characteristics in samples with various silver contents is the combined effect of these two contradictory factors.
4. Conclusion The effect of silver additions on the properties of silver-sheathed BiPbSrCaCuO (2-2-2-3 ) superconducting tapes was studied through determination of the rate of formation of the high-T, 2-2-2-3 phase, T,, J, and J,-magnetic field characteristics. The results indicated that silver additions cause neither changes in the value of T, nor the decomposition of high-T, 2-2-2-3 phase. However, silver does influence the sample’s J,, which decreases with increasing silver content when the tapes are treated at the same temperature. The microstructural analyses revealed that silver is present as an isolated phase inside the tape without visible reaction and diffusion with the superconductor matrix, but the undesirable morphology of the added silver particles causes a degradation of grain alignment and boundary order. A slight improvement in the J,-magnetic field characteristics for the tapes with low levels of silver (x< 1.0) and a small degradation for the samples with high levels of silver (x> 2.4) were also observed.
Acknowledgement The authors wish to thank Metal Manufactures Ltd. and Commonwealth Department of Industry, Technology and Commerce for financial support. Thanks are also given to Dr. M.H. Appley for assistance in wire drawing.
References 088861
x=0.0
[ 1] K. Sato, N. Shibuta, H. Mukai, T. Hikata, M. Ueyama and
ooooo x=1.0 -x=3.5 Ag-Clad
(Bi,Pb)zSrzCazCuzAg.O,~
0.01
,
0.1
Magnetic
Field
Tapes
(T)
Fig. 4. Magnetic field dependence of nonnalised J, for the Bi (22-2-3) tapes with various silver contents (x=0.0, 1.0 and 3.5).
T. Kato, J. Appl. Phys. 70 ( 199 1) 6484. [2] Y.C. Guo, H.K. Liu and S.X. Dou, Appl. Supercond. l-2 (1993) 25. [3]Z.F.RenandJ.H. Wang, J.Mater.Sci. 10 (1991) 1139. [4] A. Oota, T. Horio, K. Ohba and K. Iwasaki, J. Appl. Phys. 71 (1992) 5997. [ 51 Y. Ishida, J. Matsuzaki, T. Kizuka and H. Ichinose, Physica C 19 (1991) 67.
339
Volume 18, number 56
MATERIALS LETTERS
[6] Y. Yamada, B. Obst and K. Flukiger, Supercond. Sci. Technol. 4 (1991) 165. [7] Y.C. Guo, H.K. Liu and S.X. Dou, J. Mater. Res. 8 (1993) 2187. [ 8 ] K. Aota, H. Hattori, T. Hatano, K. Nakamura and K. Ogawa, Japan. J. Appl. Phys. 28 (1989) L2169. [ 91 T. Hatano, K. Aota, H. Hattori, S. Ikeda, K. Nakamura and K. Ogawa, Cryogenics 30 ( 1990) 6 Il. [lo] S.X. Dou, H.K. Liu, Y.C. Guo, D.L. Shi, M.D. Sumption and E.D. Collings, to be published. [ 111 H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Japan. J. Appl. Phys. 27 (1988) L209.
340
February 1994
[ 12 ] S. Jin and J.E. Graebner, Mater. Sci. Eng. B 7 ( 1991) 243. [ 131 K.H. Sandhage, G.N. Reley and W.C. Carter, J. Metals 43 (1991) 21. [ 141 P. Haldar and L. Motowildo, J. Metals 44 (1992) 54. [ 151 E.E. Hellstrom, Mater. Res. Sot. Bull. XVII ( 1992) 45. [ 161 S.X. Dou and H.K. Liu, Supercond. Sci. Technol. 6 (1993) 297. [ 171 D.C. Larbalestier and M.P. Maley, Mater. Res. Sot. Bull. XVIII (1993) 50. [ 181 L.N. Bulaevskii, J.R. Glazman and A.P. Malozemoff, Phys. Rev. B 45 (1992) 2545.