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Zero resistivity at 122 K in Bi-Pb-Sr-Ca-Cu-O system
~
0038-I098/9053.00+.00 Pergamon Press plc
Solid State Communications, Vol. 75, No. 6, pp. 499-502, 1990. Printed in Great Britain.
ZERO RESISTIVITY AT 1 2 2 K IN B i - P b - S r - C a - C u - O Dhananjai
Pandey,
R. M a h e s h ,
V.S. Tiwari,
A.K.
SYSTEM
Singh and S . K .
Kak*
School of M a t e r i a l s Science and Technology, *Department of E l e c t r o n i c s Engineering, Banaras Hindu University, V a r a n a s i - 2 2 1 0 0 5 (India). (Received
April 21,1990 by S. Amelinckx)
Zero resistivity at 122 K in samples of nominal composition Bi1.6Pbo.535Sr2.675Ca2.675Cu3Oy Pbo.2SrCa(CO3)2. 2 precursors 10 K enhancement
of Tc(R-O)
prepared
is reported. over
via a semi-wet route using Possible reasons for the
the commonly
reported
values are
suggested.
I.
Introduction
and Ca compounds using ammonium carbonate as the precipitating agent. XRD studies on this precursor confirmed it to be a solid solution with an orthorhombic structure with unit-cell parameters a - 5.15A °, b = 8.45A ° and c-6.09A ° .
Partial s u b s t i t u t i o n o f Bi by Pb in the 2223 phase of the polytypold family of superconductors Bi2Sr2Can_lCUnOy (n-i,2,3) is now well known to enhance the volume fraction of this phase [1-4]. The s u p e r c o n d u c t i n g transition temperature c o r r e s p o n d i n g to zero resistivity and/or onset of d i a m a g n e t i c signal for this phase is generally believed to be around 110 K [5-7]. Resistivity drops and diamagnetic signals in the 1 1 0 K to 120K range have, however, been reported by several workers in Pb substituted samples [8-13]. Flash heat treatment above the melting point f o r 3 to 5 minutes followed by 50 hour a n n e a l i n g at 1140 K have been recommended for getting Tc(R=O) of 120 K in thin specimens [14]. Recently, we have shown [15,16] the advantages of semi-wet routes to the synthesis of s u p e r c o n d u c t i n g YBa2Cu307_ v and f e r r o e l e c t r i c (Ba,Ca)TiO 3 ceramics~ The purpose of this c o m m u n i c a t i o n is to report stable zero resistivity above 120 K in bulk samples of nominal composition Bil.6Pbo.535Sr2.675Ca2.675Cu30y prepared vla a seml-wet route using Pbo.2SrCa(CO3)2. 2 precursor. Our samples were not subjected to any flash heat treatment either above or below the melting point.
This precursor carbonate was mixed w i t h ( B i O ) 2 C O 3 a n d CuO c o r r e s p o n d i n g to the nominal composition of Bi I 6Pbo 5 3 5 S r 2 e 6 7 5 C a 2 . 6 7 5 C u 3 O y This mixture "was then calcined at "1073 K, 1103 K and 1123 K for 8 hours duration at each temperature followed by intermediate grindings. Fig.1 depicts the XRD pattern of the calcined powder. Except for the C ~ P b O 4 line at 28=17.78, all the prominent lines in this pattern could be indexed with respect to the 2212 unit cell with a~bf5.397A ° and c=30.807A ° . Close agreement between ~ b s and dca I values, as given in Table 1, confirms the correctness of the indexing scheme.
The calcined powder was pelletised at a pressure of 80000 ibs/sq, inch and sintered at 1133±5 K in air for 48 and 72 hours; the samples were however furnace cooled after every 24 hours even for longer sintering durations. Resistivity was measured in the temperature range 77 K to 300 K in the standard four probe configuration at a constant current of 1 mA using Solaritron 7081 precision system multimeter with a precision of O.I~V. Temperature was measured using a calibrated Pt I00 RTD with an accuracy of !l K. Fig. 2 depicts the temperature dependence of resistivity of two samples sintered for 48 and 72 hours. It is evident from this
2. Results a n d D i s c u s s i o n In order to ensure uniform supply of Pb,Sr and Ca during the solid state thermochemical reaction, a precursor with nominal composition Pbo,2SrCa(CO3)2. 2 was prepared from a mixture of aqueous solutions of Pb, Sr
499
Vol. 75, NO. 6
ZERO RESISTIVITY IN B i - P b - S r - C a - C u - O SYSTEM
500
14
r.. .~.. • -
K)
5
"G ~c
4
n-
12
•/
3
r
a • .,..s
(Z
2
I
0
I 8O
T(K)
Fig.2. 35
45
55
28 (degree}
Fig,I.
XRD pattern of as-calcined powder; indexed with respect to 2212 tetragonal unit cell (CuKg radiation).
Table.l: Calculated and observed dspacings for calcined (pre-sintered) powder indexed with respect to the 2212 phase with a bfS.397F and c=30.807A ° (CuK~ radiation).
hkl
002 Oli 008 113 107 115
*001...00 117 200 001.._2.2 1Oll 211 214 208 2010 220 224 1115
313 315 308 317 0O2O 319
dcal
dobs
2Sobs
(A °)
(F)
(deg)
15.404 5.316 3.851 3.577 3.411 3.224 3.081
2.883 2.698 2.567 2.486 2.406 2.303 2.209 2.029 1.908 1.852 1.809 1.684 1.645 1.630 1.591 1.540 1.527
15.291 5.323 3.848 3.573 3.397 3.241 3.081 2.880 2.698 2.548 2.485 2.391 2.322 2.208 2.029 ~I~.907 1.858 1~808 1.709 1.659 1.643 1.592~1.540 1.528
I/Ima x
5.775 16.640 23.095 24.900 26.210 27.500 28.960 31.025 33.175 35.200 36.110 37.595 38.755 40.835 4~.625 47.650 48.975 50.445 53.590 55.320 55.905 57.865 60.035 60.530
Reflections used for calculating parameters.
(%) 3 3 17 33 4 89 23 100 82 17 5 4 6 3 41 39 11 15 7 11 16 12 8 12 cell
Temperature dependence of resistivity f o r samples sintered for 48 hrs and 72 h r s with T c (R=O) of IIO K and 122 K respectively.
figure that the small drop in resistivity just above 120 K after 48 hours of sintering indeed leads to zero resistivity at 122 K a f t e r an additional 24 hours of sintering. The distinct change in slope near 140 K is also quite noteworthy. Samples prepared under identical conditions but using the conventional dry route showed Tc(R-O) of 82 K only which confirms the advantages of our semi-wet route for the nucleation and growth of the 2223 phase. The XRD pattern from powders obtained after crushing the sample sintered for 72 hours shows that the sample contains mixture of 2212 and 2223 phases besides the Ca2PbO 4 impurity phase (see Fig. 3). Table 2 lists the dob s and dca I for this specimen. The close agreement between dob s and dc@ 1 not only confirms the correctness of the indexing scheme but also demonstrates the insensitivity of the formation of the 2223 and 2212 phases to the starting nominal composition. The microstructure of the pellets sintered for 72 hours recorded using Leitz Optical Metallux-III shows the presence of two fine fibrous phases (presumably 2212 and 2223) and looks akin to a typical eutectoidally decomposed microstructure (see Fig. 4). A higher magnification SEM picture taken using JEOL JSM 840-A of the same sample shows the presence of feathery 2223 phase in abundance (see Fig. 5).
While the factors responsible for the 10 K enhancement in Tc(R=O) over the values generally reported in the literature are being investigated, the following possibilities can be conjectured: (i) Pb partially occupying the Ca/Sr - sites rather than Bi-sites
Vol. 75, No. 6
ZERO
RESISTIVITY IN
Table.2: Calculated and observed d spacings for powders after 48 hrs sintering. Lines indexed with respect to 2212 (affibf5.384ff , c = 3 0 . 3 4 3 F ) and 2223 (a=bffi5.384ff , c=37.077ff) phases (CoK~ I radiation).
O
g
v 8 .........
;. . . . . . T,
•
= T
~:
I
T
12
14
16
18
20
8 (degree)
Fig.3.
dcal
dobs
(ff)
(A° )
(deg)
L-OO8 H-O010 H-II3 L-II3 H-115 *L-II5 H-f17
3.793 3.708 3.638 3.563 3.387 3.225
3.826 3.702 3.644 3.551 3.383 3.225
27.04 27.96 28.42 29.18 30.66 32.20
18 6 7 27 23 74
3.091
3.086
33.70
37
L-II7 ~H-II9
2.861 2.796
2.868 2.796
36.34 37.32
66 43
hkl
~
501
Bi-Pb-Sr-Ca-Cu-O SYST~4
XRD pattern of powder obtained after 72 hrs sintering (CoK~ radiation); all the major lines indexed with respect to the 2212/2223 tetragonal unit cells.
H-ooi_22
*L-200 *H-200 L-O012 H-1111 L-20IO H-2012 L-220 H-220
28obs
Optical m i c r o g r a p h of a pellet sintered for 72 hrs (X 650).
Fig.5.
Scanning electron micrograph of the pellet shown in Fig. 4 (X2400).
exclusively because of the use of Pbo.2SrCa(CO3)2. 2 precursor, (ii) different type of commensurate/ incommensurate m o d u l a t i o n s than those
(%)
2.692
2.692
38.82
lO0
2.529 2.524 2.014
2.529 2.534 2.020
40.86 41.34 52.56
3 ii 22
1.904
1.908
55.92
36
* Reflections used for calculating parameters.
Fig.4.
I/Ima x
cell
reported for the ii0 K phase, (iii) better chemical homogeneity and cleaner grain boundaries and (iv) good connectivity for electrical conduction due to the fine fibrous m i c r o s t r u c t u r e running throughout the sample (see Fig. 4). While detailed work on these lines is in progress, we have now observed zero resistivity above 120 K even in stoichiometric Bil.6Pb 0 4Sr2Ca2Cu30, samples prepared using Pbo.2SrCa(C03)2. ~ precursors [17] ruling out the possibility of the slightly off-stoichiometric composition being responsible for this high Tc(R=O ) in the Bi-Pb-Sr-Ca-Cu-O system. It will not be out of context to mention that nearly iO K enhancement in Tc as a result of better powder synthesis procedure has been reported by Wang et.al. [18] for the 2212 phase prepared via citrate route and Haung et.al. [19] for the 2223 phase prepared by pyrolysis of nitrate solutions; the latter was, however, not stable with respect to thermal cycling. Acknowledgements This work was supported by Programme Management Board on S u p e r c o n d u c t i v i t y of the Department of Science and Technology, G o v e r n m e n t of India. We thank Prof. S. Ranganathan, Prof. D.H. Sastry and Prof. K.M. Pai for XRD facilities and Shri G.M.K. Sharma for SEM work. Special thanks are due to Mr. Anjan Sil for the camera ready copy.
502
ZERO RESISTIVITY IN Bi-Pb-Sr-Ga-Cu-O SYSTEM
VOI. 75, NO. 6
References
1.
2.
3. 4. 5.
6. 7. 8.
M. T a k a n o , J. Takads, K. Oda, H. K i t a g u c h i , Y. M i u r a , Y. I k e d a , Y. Tomi and H. M a z a k i , J a p n . J . A p p l . P h y s . , 27, L1041, ( 1 9 8 8 ) . R.J. Cars, B.Batlogg, S.A.Sunshine, T. Siegrist, R.M. Flemming, K.Rabe, L. F. Schneemeyer, D.W. Murphy, R.B. V a n D o v e r , P . K . G a l l o g h e r , S . H . Glarum, S. N a k a h a r a , R.C. F a r r o w , J . J . K r a j e w s k i , S.M. Z a h u r a k , J.V. Waszczak, J.H. Marshall, P. M a r s h , L.W. Rupp, J r . , W.F. Peck and E.A. Rietman, Physica C 153-155, 560, (1988). S.M.Green, C.Jiang, Yu Mei, H.L.Luo and C.Polltis, Phys.Rev. B38, 5016, (1988). B. W. Statt, Z. Wang, M. J. G. Lee, J. V. Yakhmi and P. C. deCamagro, Physica C 156, 251, (1988). B. L. Ramakrlshna, J. C. Barry, Z. Iqbal, E. W. On8, A. Bose and H. Eckhardt, Physica C 158, 203, (1989). S.M. G r e e n , Yu Mei, A.E. Manzi and H.L. Luo, J . A p p l . P h y s . , 66~ 3 7 0 3 , (1989). A. K. S a r k a r , I. Maartense, T. L. P e t e r s o n and B. Kumar, J . A p p l . P h y s . 66, 3717, ( 1 9 5 9 ) . M. T a k a n o , J. Taksds, K. Oda, H. K i t a g u c h i , Y. M i u r a , ¥. Ikeda, Y. Tomi and H. M s z a k i , J a p n . J . A p p l . Phys. 27, LI041, (1988).
9. 10.
11. 12. 13. 14.
15. 16. 17. 18. 19.
N. Murayama, E. S u d o , M. Awano, K. K a n i and Y. T o r i i , J a p n . J . Appl. P h y s . , 27, 1629, ( 1 9 8 8 ) . T. K a n s i , T. K u m a g a i , A. S o e t a , T. Suzuki, K. A i h a r a , T. Kamo and S. Matsuda, Japn. J. Appl. Phys., 27, 1435, (1988). X. Zhu, S. F e n g , Zhang, Z. Gan, IEEE Trans. Magnetics 25, 2154, (1989). M. Wilhelm and O. Eibl, Solid State Comm., 70, 137, (1989). J. Rong-ying, S. Fan, Q. Ran, S. Ni-Chen8, S. Zhen-hus and Z. ShouZen8, Physica C 158, 255, (1989). P.V.S.S. Ssstry, I.K.Gopalkrishnan, J.V. Yakhml and R.M. Iyer, Physica C 157, 491, (1989). D.Pandey, V.S.Tiwari and A.K.Singh, J.Phys.D:Appl.Phys.,22, 182,(1989). V.S.Tiwari, P.Groves and D. Pandey, J.Phys. D:Appl.Phys.,22, 837,(1989). D. Pandey, R. Mahesh, A. K. Singh, V. S. Tiwarl and S. K. Kak, (to be published). N. H. Wang, C. M. Wang, H.C.I. Kao, D. C. Ling, H. C. Ku and K. H. Lii, Japn.J.Appl.Phys.,28, 1505, (1989). C. J. Haun8, T. ¥. Tseng, T.S. Heh, F.H. Chen, W.S. Jong, ¥.S. Fran and S.M. Shlau, Solid State Comm. 72, 563, (1989).
0038-I098/9053.00+.00 Pergamon Press plc
Solid State Communications, Vol. 75, No. 6, pp. 499-502, 1990. Printed in Great Britain.
ZERO RESISTIVITY AT 1 2 2 K IN B i - P b - S r - C a - C u - O Dhananjai
Pandey,
R. M a h e s h ,
V.S. Tiwari,
A.K.
SYSTEM
Singh and S . K .
Kak*
School of M a t e r i a l s Science and Technology, *Department of E l e c t r o n i c s Engineering, Banaras Hindu University, V a r a n a s i - 2 2 1 0 0 5 (India). (Received
April 21,1990 by S. Amelinckx)
Zero resistivity at 122 K in samples of nominal composition Bi1.6Pbo.535Sr2.675Ca2.675Cu3Oy Pbo.2SrCa(CO3)2. 2 precursors 10 K enhancement
of Tc(R-O)
prepared
is reported. over
via a semi-wet route using Possible reasons for the
the commonly
reported
values are
suggested.
I.
Introduction
and Ca compounds using ammonium carbonate as the precipitating agent. XRD studies on this precursor confirmed it to be a solid solution with an orthorhombic structure with unit-cell parameters a - 5.15A °, b = 8.45A ° and c-6.09A ° .
Partial s u b s t i t u t i o n o f Bi by Pb in the 2223 phase of the polytypold family of superconductors Bi2Sr2Can_lCUnOy (n-i,2,3) is now well known to enhance the volume fraction of this phase [1-4]. The s u p e r c o n d u c t i n g transition temperature c o r r e s p o n d i n g to zero resistivity and/or onset of d i a m a g n e t i c signal for this phase is generally believed to be around 110 K [5-7]. Resistivity drops and diamagnetic signals in the 1 1 0 K to 120K range have, however, been reported by several workers in Pb substituted samples [8-13]. Flash heat treatment above the melting point f o r 3 to 5 minutes followed by 50 hour a n n e a l i n g at 1140 K have been recommended for getting Tc(R=O) of 120 K in thin specimens [14]. Recently, we have shown [15,16] the advantages of semi-wet routes to the synthesis of s u p e r c o n d u c t i n g YBa2Cu307_ v and f e r r o e l e c t r i c (Ba,Ca)TiO 3 ceramics~ The purpose of this c o m m u n i c a t i o n is to report stable zero resistivity above 120 K in bulk samples of nominal composition Bil.6Pbo.535Sr2.675Ca2.675Cu30y prepared vla a seml-wet route using Pbo.2SrCa(CO3)2. 2 precursor. Our samples were not subjected to any flash heat treatment either above or below the melting point.
This precursor carbonate was mixed w i t h ( B i O ) 2 C O 3 a n d CuO c o r r e s p o n d i n g to the nominal composition of Bi I 6Pbo 5 3 5 S r 2 e 6 7 5 C a 2 . 6 7 5 C u 3 O y This mixture "was then calcined at "1073 K, 1103 K and 1123 K for 8 hours duration at each temperature followed by intermediate grindings. Fig.1 depicts the XRD pattern of the calcined powder. Except for the C ~ P b O 4 line at 28=17.78, all the prominent lines in this pattern could be indexed with respect to the 2212 unit cell with a~bf5.397A ° and c=30.807A ° . Close agreement between ~ b s and dca I values, as given in Table 1, confirms the correctness of the indexing scheme.
The calcined powder was pelletised at a pressure of 80000 ibs/sq, inch and sintered at 1133±5 K in air for 48 and 72 hours; the samples were however furnace cooled after every 24 hours even for longer sintering durations. Resistivity was measured in the temperature range 77 K to 300 K in the standard four probe configuration at a constant current of 1 mA using Solaritron 7081 precision system multimeter with a precision of O.I~V. Temperature was measured using a calibrated Pt I00 RTD with an accuracy of !l K. Fig. 2 depicts the temperature dependence of resistivity of two samples sintered for 48 and 72 hours. It is evident from this
2. Results a n d D i s c u s s i o n In order to ensure uniform supply of Pb,Sr and Ca during the solid state thermochemical reaction, a precursor with nominal composition Pbo,2SrCa(CO3)2. 2 was prepared from a mixture of aqueous solutions of Pb, Sr
499
Vol. 75, NO. 6
ZERO RESISTIVITY IN B i - P b - S r - C a - C u - O SYSTEM
500
14
r.. .~.. • -
K)
5
"G ~c
4
n-
12
•/
3
r
a • .,..s
(Z
2
I
0
I 8O
T(K)
Fig.2. 35
45
55
28 (degree}
Fig,I.
XRD pattern of as-calcined powder; indexed with respect to 2212 tetragonal unit cell (CuKg radiation).
Table.l: Calculated and observed dspacings for calcined (pre-sintered) powder indexed with respect to the 2212 phase with a bfS.397F and c=30.807A ° (CuK~ radiation).
hkl
002 Oli 008 113 107 115
*001...00 117 200 001.._2.2 1Oll 211 214 208 2010 220 224 1115
313 315 308 317 0O2O 319
dcal
dobs
2Sobs
(A °)
(F)
(deg)
15.404 5.316 3.851 3.577 3.411 3.224 3.081
2.883 2.698 2.567 2.486 2.406 2.303 2.209 2.029 1.908 1.852 1.809 1.684 1.645 1.630 1.591 1.540 1.527
15.291 5.323 3.848 3.573 3.397 3.241 3.081 2.880 2.698 2.548 2.485 2.391 2.322 2.208 2.029 ~I~.907 1.858 1~808 1.709 1.659 1.643 1.592~1.540 1.528
I/Ima x
5.775 16.640 23.095 24.900 26.210 27.500 28.960 31.025 33.175 35.200 36.110 37.595 38.755 40.835 4~.625 47.650 48.975 50.445 53.590 55.320 55.905 57.865 60.035 60.530
Reflections used for calculating parameters.
(%) 3 3 17 33 4 89 23 100 82 17 5 4 6 3 41 39 11 15 7 11 16 12 8 12 cell
Temperature dependence of resistivity f o r samples sintered for 48 hrs and 72 h r s with T c (R=O) of IIO K and 122 K respectively.
figure that the small drop in resistivity just above 120 K after 48 hours of sintering indeed leads to zero resistivity at 122 K a f t e r an additional 24 hours of sintering. The distinct change in slope near 140 K is also quite noteworthy. Samples prepared under identical conditions but using the conventional dry route showed Tc(R-O) of 82 K only which confirms the advantages of our semi-wet route for the nucleation and growth of the 2223 phase. The XRD pattern from powders obtained after crushing the sample sintered for 72 hours shows that the sample contains mixture of 2212 and 2223 phases besides the Ca2PbO 4 impurity phase (see Fig. 3). Table 2 lists the dob s and dca I for this specimen. The close agreement between dob s and dc@ 1 not only confirms the correctness of the indexing scheme but also demonstrates the insensitivity of the formation of the 2223 and 2212 phases to the starting nominal composition. The microstructure of the pellets sintered for 72 hours recorded using Leitz Optical Metallux-III shows the presence of two fine fibrous phases (presumably 2212 and 2223) and looks akin to a typical eutectoidally decomposed microstructure (see Fig. 4). A higher magnification SEM picture taken using JEOL JSM 840-A of the same sample shows the presence of feathery 2223 phase in abundance (see Fig. 5).
While the factors responsible for the 10 K enhancement in Tc(R=O) over the values generally reported in the literature are being investigated, the following possibilities can be conjectured: (i) Pb partially occupying the Ca/Sr - sites rather than Bi-sites
Vol. 75, No. 6
ZERO
RESISTIVITY IN
Table.2: Calculated and observed d spacings for powders after 48 hrs sintering. Lines indexed with respect to 2212 (affibf5.384ff , c = 3 0 . 3 4 3 F ) and 2223 (a=bffi5.384ff , c=37.077ff) phases (CoK~ I radiation).
O
g
v 8 .........
;. . . . . . T,
•
= T
~:
I
T
12
14
16
18
20
8 (degree)
Fig.3.
dcal
dobs
(ff)
(A° )
(deg)
L-OO8 H-O010 H-II3 L-II3 H-115 *L-II5 H-f17
3.793 3.708 3.638 3.563 3.387 3.225
3.826 3.702 3.644 3.551 3.383 3.225
27.04 27.96 28.42 29.18 30.66 32.20
18 6 7 27 23 74
3.091
3.086
33.70
37
L-II7 ~H-II9
2.861 2.796
2.868 2.796
36.34 37.32
66 43
hkl
~
501
Bi-Pb-Sr-Ca-Cu-O SYST~4
XRD pattern of powder obtained after 72 hrs sintering (CoK~ radiation); all the major lines indexed with respect to the 2212/2223 tetragonal unit cells.
H-ooi_22
*L-200 *H-200 L-O012 H-1111 L-20IO H-2012 L-220 H-220
28obs
Optical m i c r o g r a p h of a pellet sintered for 72 hrs (X 650).
Fig.5.
Scanning electron micrograph of the pellet shown in Fig. 4 (X2400).
exclusively because of the use of Pbo.2SrCa(CO3)2. 2 precursor, (ii) different type of commensurate/ incommensurate m o d u l a t i o n s than those
(%)
2.692
2.692
38.82
lO0
2.529 2.524 2.014
2.529 2.534 2.020
40.86 41.34 52.56
3 ii 22
1.904
1.908
55.92
36
* Reflections used for calculating parameters.
Fig.4.
I/Ima x
cell
reported for the ii0 K phase, (iii) better chemical homogeneity and cleaner grain boundaries and (iv) good connectivity for electrical conduction due to the fine fibrous m i c r o s t r u c t u r e running throughout the sample (see Fig. 4). While detailed work on these lines is in progress, we have now observed zero resistivity above 120 K even in stoichiometric Bil.6Pb 0 4Sr2Ca2Cu30, samples prepared using Pbo.2SrCa(C03)2. ~ precursors [17] ruling out the possibility of the slightly off-stoichiometric composition being responsible for this high Tc(R=O ) in the Bi-Pb-Sr-Ca-Cu-O system. It will not be out of context to mention that nearly iO K enhancement in Tc as a result of better powder synthesis procedure has been reported by Wang et.al. [18] for the 2212 phase prepared via citrate route and Haung et.al. [19] for the 2223 phase prepared by pyrolysis of nitrate solutions; the latter was, however, not stable with respect to thermal cycling. Acknowledgements This work was supported by Programme Management Board on S u p e r c o n d u c t i v i t y of the Department of Science and Technology, G o v e r n m e n t of India. We thank Prof. S. Ranganathan, Prof. D.H. Sastry and Prof. K.M. Pai for XRD facilities and Shri G.M.K. Sharma for SEM work. Special thanks are due to Mr. Anjan Sil for the camera ready copy.
502
ZERO RESISTIVITY IN Bi-Pb-Sr-Ga-Cu-O SYSTEM
VOI. 75, NO. 6
References
1.
2.
3. 4. 5.
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