Superconducting properties of Pb-free and Pb-substituted bulk ceramics of Bi-2212 cuprates

Mllgl

Physica C 219 (1994) 167-175 North-Holland

Superconducting properties of Pb-free and Pb-substituted bulk ceramics of Bi-2212 cuprates T. R e n t s c h l e r a, S. K e m m l e r - S a c k a, P. Kessler b a n d H. Lichte b • lnstitutflir Anorganische Chemic der Universitlit, Aufder Morgenstelle 18, 72076 Tf~bingen, Germany b lnstitutflir Angewandte Physik der Universitlit, Aufder Morgenstelle 10, 72076 Ti~bingen, Germany

Received 9 August 1993 Revised manuscript received 4 October i 993

Superconducting properties of Pb-free and partially Pb-substituted bulk ceramics of the Bi2_~Pb~Sr2CaCu2Ozsystemare studied under the influence of various methods of post-treatment. It is shown that the shift of Tcfrom the maximum of about 95 K to lowervalues is due to an intercalation of oxygenin the perovskiteslabs, most probablyresulting in the creation oflocafizedcarriers in form ofCu s+. The substitution Pb--.Bi displays a beneficial role in the improvement of the superconducting properties of the intergrain material of ceramics as well as in the creation of pinning centres with an optimal Pb concentration ofxffiO.4.For this value the averagedistance of the Pb pinning centres of about 10/~ correlates with the coherencelength of 2212 in the a-b plane. As possiblepinning centres lattice defects created by the intergrowth of 2223 in 2212 are considered.

1. Introduction Bi-based high-T¢ superconductors of type 2212 are of interest for technical application in form of wires and bulk ceramics. The 2212 phase exhibits a T~ up to 98 K [ 1,2] and, therefore, competes with the highTo phase of YBa2Cu3OT_~ (123 phase). For both types of superconductors, 2212 as well as 123, the position of T~ is influenced by the conditions of preparation and the knowledge of this correlation enables the researcher to control the microstructure of the superconducting material with the aim to improve the transport critical current densities under high magnetic fields. The Bi-based 2212 phase is obtained in the leadfree system B i - S r - C a - C u - O as well as for lead-substituted compositions B i - P b - S r - C a - C u - O . Interestingly, the highest values for T~ (98 K ) were reported exclusively for Pb-containing systems (a monoclinic distorted N2-treated material of composition BiL6Pbo.4Sr2-CaCu2Oz [ 1] and an orthorhombic Ar-treated sample B i t . 7 5 P b o . 2 5 - S r 2 C a C u 2 0 z [ 2 ] ), whereas for Pb-free 2212 materials slightly inferior values up to ~ 93 K are typical [ 3-5 ]. From these observations a correlation between the Pb con-

tent of the 2212 phase and the superconducting properties can be deduced. To investigate this effect more thoroughly the present study deals with bulk 2212 ceramics from the Pb-free and Pb-substituted systems with respect to a possible correlation between microstructure and superconducting properties. It is shown that by proper post-treatment of ceramics T¢ values of 95 K are obtainable for both types, Pb-free and Pb-substituted 2212 materials. However, the Pb--,Bi substitution has a very important advantage of combining a high Tc with relatively clean grain boundaries and improved pinning properties.

2. Experimental Materials of nominal composition Bi2Sr2CaCu20= Bi2_xPb~Sr2-CaCu2Oz (x=0.2; 0.4; 0.6) and BiLsPbo.4Sr2Cao.sCu2Oz were prepared from Bi(NO3)a'5H20 (DAB6; Merck), PbO, Sr(NO3)2, CaCO3 and CuO (all p.A.; Merck) via solid state reaction or sol/gel method. Both methods gave very similar results and the materials are not further dis-

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168

T. Rentsctder et al. / Pb-free and Pb-substituted Bi-2212 cuprates

tingnished. The air-synthesis was done in alumina crucibles (Degussit A123) at 845-855°C. The total firing time of 150-200 h was interrupted several times for regrindings and for X-ray analysis (Philips powder diffractometer, CuKa radiation, Au standard). The air-quenched materials were post-treated in form of ceramics (pellets of 13 mm in diameter and about 1.15 mm in thickness; density ~ 85% of the X-ray density) and powders (Heating in air at about 850°C for 3 h (T-3) or 70 h (T-70) and subsequently air-quenched). Post-treatment in flowing Ar (99.9%) for 15 h at 450°C (Ar-450), 500°C (Ar500), 650°C (At-650), 750*C (Ar-750) or 800°C with subsequently cooling down in Ar at a rate of ~ 1o min- ~. Some samples were annealed in flowing 02 at 5500C/6 h and furnace cooled (0-5500C). Quenching with liquid N2 was done for pellets after heating 3 h at about 8500C (T-3-Q). The samples were characterized by XRD and chemical analysis. The oxygen content was determined via potentiometric redoxtitration and calculated with the fixed valencies of 3 + for Bi and 2 + for Pb, Sr and Ca. Weight controls for some materials indicated weight losses during annealing of ~ 1%. For TEM investigations with the Philips EM420 transmission electron microscope (100 keV) the materials were ground in an agate mortar and transferred onto a copper grid covered with a hole carbon foil. Superconducting properties were studied by both resistance and susceptibility measurements using standard four-probe DC method and a SQUID magnetometer (Quantum Design).

3. Results and discussion 3.1. X R D , T E M a n d oxygen content

The as-prepared and post-treated materials contain the 2212 phase in nearly pure form. The only exception is the Pb-rich sample with x--0.6. In this case the strongest reflections of (Sr, Ca)2PbO4 are visible in the XRD. The Pb-free 2212 material Bi2Sr2CaCu20z crystallizes with a tetrngonal subcell, whereas a partial substitution Pb--,Bi introduces an orthorhombic distortion (table 1 ). This observation is in agreement with the results from literature [ 6 ]. Interestingly, the substitution of Bi3+ (r= 1.03 A,

CN6 [ 7 ] ) by the larger Pb 2+ ion ( 1.19 A) results in an overall shrinkage of the unit cell volume, whereas the simultaneous formal substitution pb2+--~Ca2+ ( 1.00 A) yields a dilatation (table 1). Additionally, the lattice constants are influenced by all presently applied methods of post-annealing. The possible explanation for these effects can be deduced from the literature data, clearly indicating an expansion of the c-axis with either increasing Sr: Ca ratio or increasing Bi content [ 3 ] and the present observations are similarly interpreted by the dependence of the 2212 composition on the experimental conditions. The employment of different gas atmospheres influences the cell dimensions without affecting the purity of the materials. From the data of table I some general trends can be deduced: Post-treatment in flowing oxygen reduces the lattice constants, whereas for Ar post-treatment with increasing reaction temperature an expansion of the cell volume takes place, reaching in some cases (e.g. Bi2Sr2CaCu20,) a clear maximum at 650°C. At 800°C/Ar a melt is uniformly obtained. Furthermore, in the Pb-containing samples the employment of Ar reduces the symmetry of the subeell from tetragonal to orthorhombic (x=0.2) and from orthorhombic to monoclinic (x>~0.4 and Bil.sPbo.2Sr2-Cao.gPbo.2Cu2Oz). In ease of x=0.4 this effect is in agreement with the observation of Kambe et al. [ 1 ]. However, for the present material the position of Tc is not exceptional high. Furthermore, for x=0.6 the admixed impurity phase (Sr, Ca)2PbO4 did neither improve the properties of the grain boundaries nor the pinning forces [ 8 ]. The experimental oxygen content z (table 1) always exceeds the ideal value 8.00 and the excess can be correlated with the wavelength of the incommensurate superstructure by the assumption that the insertion of extra oxygen in the BiO layer is the decisive factor for the development of the modulation [ 9 ]. For the Pb-free case the wavelength 2 = 4.7 [ 9 ] indicates the insertion of oxygen in the BiO layers of every 4.7 subcell and the value of z--8.21. For Pbcontaining materials the wavelength increases to ~8.7 [10] and this value is combined with a total oxygen content of 8.11. From a comparison between the calculated values and the data of table 1 it follows that the analytical values are slightly higher. This observation may be explained by either an uptake of additional oxygen in the rocksalt layers or an inser-

T. Rentschler et al. / Pb-fiee and Pb-substituted Bi-2212 cuprates

169

Table 1 Coml~ition, post-treatment, lattice comtants of the average 2212 unit cell 1.), average formal valence of copper (Ox) b), To(X) c), T©(o)d), T©(,~) d), temperature coefficient of resi~vity (TCR) and reei~ivity at 290 K ~ K) for the systems Bi2Sr2CaCu20=, Bi2_~Pb~Sr2CaCu20, and Bit.sPbe~Sr2Ca~0P1~2Cu20~ Post-treatment

a (]~)

b (A)

c (A)

V (~')

Ox

z

T©(X) (K)

To(o) Tc(,.~) (K) (K)

TCR( 160 K) (ppmK -t )

p(290 K) (nffl cm)

5.416 5.411 5.422 5.423 5.411 5.405 5.414 5.414 5.395

-

30.83 30.91 30.94 30.90 30.92 30.90 30.96 30.90 30.77

904.3 905.0 909.6 908.7 905.3 902.7 907.5 905.7 895.6

+2.27 +2.25 + 2.25 +2.23 +2.19 + 2.23 +2.23 +2.17 +2.33

8.27 8.25 8.25 8.23 8.19 8.23 8.23 8.17 8.33

84 94 84 95 94 90 96 92 68

<77 85 < 77 88 77 < 77 <77 <77 <77

79 93 80 95 88 < 77 83 86. <77

+4940 +3410 + 5160 +4600 +660 + 760 +1100 +2100 +2280

1.3 3.0 1.1 1.9 14.7 18.5 -

Bil.sPbo.2Sr2CaCu2Oz as-prepared 5.398 T-70 5.390 T-30-Q 5.399 Ar-450 5.378 Ar-500 5.392 Ar-650 5.377 Ar-750 5.388

5.408 5.400 5.408 5.404

30.83 30.85 30.86 30.79 30.82 30.82 30.82

898.3 899.3 899.5 894.2 896.1 896.2 897.4

+2.31 + 2.26 +2.25 +2.29 +2.26 +2.21 +2.18

8.21 8.16 8.15 8.19 8.16 8.11 8.08

76 75 84 95 92

57 < 77 87 <77 <77 87 89

62 < 77 89 <77 <77 92 91

+4750 + 5600 +4520 semiconductin8 +240 +2130 +2220

BiLePbo.4Sr2CaCu2Oz as-prepared 5.367 T-3-Q 5.368 At-650 5.380 Ar-750 5.364

5.397 5.400 5.414 5.417

30.75 30.80 30.82 30.83 *)

890.7 892.8 897.7 895.8

+2.41 +2.39 +2.40 +2.21

8.21 8.19 8.20 8.01

76 94 92

62 86 83 85

65 87 89 90

+5000 +4520 +2500 +2560

1.2 1.4 8.0 -

Bit.4Pbo.6Sr2CaCu2Oz as-prepared 5.387 T-3-Q 5.383 T-70 5.383 Ar-650 5.380

5.414 5.420 5.420 5.436

30.79 30.86 30.89 30.90 °)

898.9 900.4 901.2 903.7

-

80 86 88 90

69 85 86 82

72 87 88 88

+4090 + 3830 + 5440 + 1860

1.2 2.1 1.1 15.0

76 94 92

< 77 <77 82 79 77

81 83 85 87 85

- 1830 -2900 +3500 +290 -

9.6 19.5 1.4 10.0 -

Bi2Sr2C.aCu20: as-prepared T-3-Q T-70 T-70-Q T-70-Ar At-500 At-650 Ar-750 0-550

Bil.sPbo.2Sr2Cao.sPbo.2Cu2Oz as-prepared 5.390 5.416 T-3-Q 5.388 5.419 T-70 5.387 5.411 Ar-650 5.394 5.441 At-750 5.379 5.422

30.85 900,6 30.88 901.6 30.83 898.7 30.89 *) 906.6 30.83 -899.2

+2.31 +2.30 +2.29 +2.30 +2.16

8.21 8.20 8.19 8.20 8.06

0.9 1.6 12.0 4.0

o) +0.005 A (a, b); +0.05 A (c). b) Calculated with the fixed values Bi: 3+; Pb, Sr, Ca: 2 +. ©) Fromzvs. 7". d) Fromp vs. T. *) Slightly monoclinic distorted.

t i o n i n t o t h e p e r o v s k i t e slabs. S i n c e i n c a s e o f o x y g e n i n s e r t i o n i n t o t h e r o c k s a l t layer a r e d u c t i o n o f t h e w a v e l e n g t h o f t h e m o d u l a t i o n is e x p e c t e d , a d e c i s i o n can be made by TEM investigations. Accordingly, E D p a t t e r n s o f s e v e r a l c r y s t a l s w e r e s t u d i e d . A s in-

d i c a t e d f o r t h e example o f Bi2Sr2CaCu2Oz i n fig. 1 ( a ) we obtained for the as-prepared material the value of 4=4.65 in excellent agreement with the data from the literature. Thus, the oxygen content o f the rocksalt-like l a y e r s r e m a i n s u n a f f e c t e d a n d t h e e x c e s s o f

170

T. Rentschler et al. /Pb-free and Pb-substimted Bi-2212 cuprates

J Fig. 1. ED pattern ofBi2Sr2CaCu20,: (a)as-prepared; (b)Ar/650°C in the [001 ] pole.

171

T. Rentschler et al. / Pb-free and Pb-substitutedBi-2212 cuprates

oxygen is present in the perovskite blocks. It is assumed that this oxygen directly creates a corresponding amount of Cu 3+, thus resulting in a localiTation of positive charge (see below). Furthermore, Bi2Sr2CaCu20 z crystals with the highest Tc (Ar-650; table 1, sections 3.2. and 3.3.) were studied by ED. From the ED pattern in fig. 1 (b) it follows that the modulation wavelength (k=4.65) is conserved. Consequently, the oxygen content of the rocksalt-like layers rests unaffected. Moreover, the present conservation of k is in agreement with the observations from the literature, reporting that the modulation wavelength does not correlate with the superconducting properties [ 11 ]. With the knowledge of an identical modulation wavelength but different oxygen content for the asprepared as well as the At-650 material it follows that the reduction of z in the At-650 sample is due to a depletion of oxygen from the perovskite slabs. Furthermore, this removal is combined with an considerable increase of Tc by about 12 IC Consequently, the presence of extra oxygen in the perovskite slabs of the as-prepared material has a detrimental influence on the superconducting properties. This finding is explained by the assumption that the extra oxygen does not create mobile carriers (holes) but provokes a charge localization by formation of Cu 3+. This assumption is further supported by the analytical data for the 02 post-treated (0-550) material. The low Tc of 68 K is combined with z=8.33. The high excess of oxygen is synonymous with the high concentration of localized carriers in form of Cu 3+. The plot of T~ vs. Ox for Bi2Sr2CaCu20~ in fig. 2 reveals a general correlation between Tc and the average oxidation state of copper ( O x ) . For the series Bi2Sr2CaCu20~ the highest T~ is situated near O x = +2.21. This value corresponds to an oxygen content of 8.21. Interestingly, an identical oxygen content of 8.21 is calculated as ideal z for a modulated structure with A=4.7 (see above). From the correspondence of both values it follows that an excess in z above 8.21 results in the insertion of oxygen in the perovskite slabs and reduces T~. The data for the Pb-containing materials of composition BiLsPbo.2Sr2CaCu20~ follow basically an identical trend (fig. 2). For the pure Bi samples additionally, a correlation between T~ and the dimensions of the c-axis is likely.

tO0 Tc (x) *

0

[KI

0

0

0

0

gO

0

0 u

0

80 ¸

~4

70

60 2,16

I 2.18

I 2.2

I 2.22

l 2.24

I 2.26

l ~'.28

I 2.3

( :'.32

I 2.34 OX

Fig. 2. To(A) vs. Ox for the system Bi2Sr2CaCu2Oz(0) and Bil.sPbo.2Sr2CaCu20, ( * ). However, in the Pb-containing systems this effect cannot clearly be seen. 3.2. Magnetic susceptibility o f powders

In the susceptibility vs. temperature signal of the as-prepared 2212 powder samples the transition temperature (onset of the Meissner signal) is situated at 84 K for the lead-free material and slightly inferior (76 K) in all Pb-substituted samples (table 1 ). The superconducting volume fraction at 20 K (calculated with Z= - M / H ~ V ; H ~ = ( 1 / 1 - nM)Ho; n ~ ~, and compared with a perfect diamagnet with X= - 1/4~) is about 50%. From that value it follows that the superconductivity is undoubtedly bulk and results from the 2212 majority phase. The main difference between the Pb-free and Pb-containing materials in the as-prepared state consists of the steeper transition to superconductivity in the latter case (fig. 3). This behavior is indicative for a more uniform material distribution in the Pb-containing 2212 phase. Moreover, the well known features of numerous Pb-containing 2212 materials get visible: Due to the Pb stabilization of the Bi-2223 phase the present Bi2_xPb~Sr2CaCu2Oz materials reveal a small 2223 content by the weak diamagnetic signal at about 110 K (not considered for the determination of To(Z) ). Interestingly this 2223 fraction was not detected in the XRD, thus demonstrating the reduced sensitivity of the latter method for the analysis of

172

T. Rentschler et aL / Pb-fiee and Pb-substituted Bi-2212 cuprates 20 0

,

.

40

60

80

iO0

- "

T

70

120

,

[K]

75

80

B5

90

95

100

°1

-0.001

-0.008

,-~-0.002

,~-0.004

~-0.003

-0.005

-.-,~-0.004-0.008

FC

=/ "~a--O.O05.

//

.,-,o-,,

-

.e- 1-70-0

-0.01 Lr)

u') -0.006-0.0~8

ZFC

-0.007-0.014

-0.008

Fig. 3. X vs. T for as-prepared powders of composition Bi2Sr2CaCu=O, (O) and BiL=Pbo.2Sr2CaCu=O,(+); FC: fieldcooled;ZFC: zero-field-cooled. mixtures of superconducting phases with different transition temperatures. From the present experiments it follows additionally, that for the stabilization of 2223 a comparatively high Ca content is likewise necessary, because in the 2212 material with a simultaneous formal Pb-~Ca and Pb-~Bi substitution of composition Bil.sPbo.2Sr2Cao.sPbo.2Cu20= no 2223 content could be detected either magnetically or by XRD. Post-treatment of powders in flowing Ar yields generally an increase of T¢ (table 1 ). At 650°C values of about 95-96 K are easily obtained for the Pbfree as well as Pb-containing materials. Additionally, T~ is not only influenced by the Ar atmosphere but also by the reaction temperature. A maximal value of T¢ is obtained after post-annealing at 650°C, whereas higher (750°C) as well as lower temperatures (500°C) are less suitable. Moreover, the superconducting volume fraction is clearly dependent on the conditions of post-treatment. However, this value does not necessarily correlate with the position of To. 3. 3. Magnetic susceptibility and resistivity o f ceramics

The influence of post-treatment on the superconducting properties of Pb-free ceramics can be deduced from fi~ 4. Prolonged heating (70 h) at 860°C ('1"-70) results in Tc ~ 84 K; an identical value is oh-

Fig. 4 Sectionof the Xvs. Tdiagram for Pb-freeceramicsof composition Bi=Sr2CaCu,O,after several post-treatments;FC: field cooled;ZFC: zero-field-cooled. rained for the as-prepared powder. By quenching the T-70 material with liquid N2 (T-70-Q) the hightemperature situation is obviously frozen and Tc increases to 95 K. From a second experiment, cooling down of a T-70 material in flowing Ar (T-70-Ar) with To--94 K, we learn that obviously the protection of 2212 from access of oxygen from the air during the cooling process is important and not the velocity of the temperature decrease because both experiments produce superconductors with very similar T~ and corresponding very similar intragrain properties. These observations can be correlated with the oxygen content (table 1 ), being reduced in comparison with the value of the T-70 material. The decrease of z in the T-70-Q sample is the result of the increasing oxygen partial pressure of the material with increasing temperature. This high-temperature situation can be frozen by quenching (T-70-Q) or stabilized by protecting from access of oxygen (T-70At). However, by cooling in Ar from 860°C additional oxygen is removed and z decreases beneath the ideal value for the Pb-free modulated structure of 8.21 (of. section 3.1.). As can be seen in fig. 5 this decrease has a detrimental effect on the grain boundaries. From the comparison of the ZFC signal of the 1"-70 and T-70-Ar material it follows that in the latter case flux easily penetrates into the intergraln material and the behavior of the ceramic resembles the properties of powders (decoupled grains), whereas

173

T. Rentschler et aL / Pb-fiee and Pb-substituted Bi-2212 cuprates

20 '

40 '

60

, , ~

,

,

.

80 ' ---= ....

t00 ::

:

:

=

120 =

0

•

,

,

20 I

,

,

,

40 ;

.

,

.

60 ;

,

,

~ ~ ~ ~ ~:.~x~f FC

x

x

....

.

80 ',

100 ,

120

='~ 2

T

----'~

[K]

-0.005.

-0.005-

ZFC

~

-0.0~.

-0.01~

-0.015.

~_ -0.015-

55 --0.025

o

-0.02-

o3

tO

-0.03

ZFC

j

-0.025-0.035 ZFC -0.03

Fig. 5. Z vs. T for ceramics of eoml~ition Bi2Sr2CaCu20, after post-treatment at 860"C/70 h ('1"-70) and additional eooEng down in flowing Ar (T-70-Ar); FC: field-cooled; ZFC: zero-fieldcooled.

the original T-70 material shows much better intergrain properties. Conformable, the RT resistivity increases from T-70 to T-70-Ar by a factor of ~ 10 (1.1-14.7 m ~ cm) and the temperature coefficient of resistivity, TCR, is likewise drastically reduced (table 1 ). The main differences between Pb-free and Pb-containing 2212 ceramics of the system Bi2_~Pb~Sr2CaCu20~ are the improved properties of the grain boundaries in the latter ease. This effect can be deduced from the development of p (290 K ) and TCR (160 K) in table 1. Moreover, the microstructure of the grain boundaries of the Pb-containing 2212 ceramics can be further improved by employing optimized conditions of post-treatment. Figure 6 gives an example for Xvs. T o f a 2212 material with x=0.2 after Ar-treatment at 650 or 7500C. From the ZFC curve of both ceramics follows a drastical improvement of the intergrain properties for the 750°C case. Furthermore, the electrical properties in the normal conducting regime have improved (table 1 ). In the corresponding powders the superconducting volume fraction increases from about 50 to 65%. Interestingly, the beneficial role of Pb is more pronounced for a simple Pb--.Bi substitution than for a combined nominal Pb--* Bi and Pb--,Ca substitution. This is caused by the fact that the material of composition Bi,.sPbo.2Sr2Cao.sPbo.2Cu20: is in the normal conducting regime in the as-prepared state as weU

-0.0

Fig. 6. Zvs. Tfor ceramics of composition Bit.sPbo.2Sr2-CaCu20: (x--O.2) after post-treatment in flowing Ar at 650"C ( × ) and 7500C ( 0 ) .

as after quenching a semiconductor with relative high P290K values (table 1 ). This behavior can be explained by a double function of Ca in the process of 2212 formation, consisting not only in the role as constituent of the 2212 phase but also in the intermediate participation as a low-melting sintering aid. Consequently, in materials with a relatively low Ca content longer firing times (e.g. T-70) are necessary for the development of improved intergrain properties. The critical current density j== has been determined by m'agnetie hysteresis loop measurements. The results for the post-annealed (T-70) ceramics Bi2Sr2CaCu20= (x = 0) and the Pb-eontaining series Bi2_xPb~Sr2CaCu20~ for 5 and 30 K are shown in fig. 7. The hysteresis loop widths 2M, for the Pb-e(mtaining materials are very similar at 5 K, whereas for the Pb-free sample 2M, is slightly lower. At 30 K the loop widths of xffi0.2 and especially 0.4 are larger than in the Bi material and xffi0.6 is lying between. This observation shows that j== ocAM (according to the Bean model [14]) of x=0.2 and 0.4 has been improved most probably according to the introduction of pinning centres. This effect is absent at 5 K since thermal excitation of flux lines is not effective. The critical current density j~= has been calculated by using Bean's model j¢= = 3M,/(2rX V) [ 15] and an average sample radius of 4X 10 -6 m (according to SEM investigations). From the magnetic field dependence of it= at 30 K in fig. 8 it follows that the

174

T. Rentschler et al. /Pb-free and Pb-substituted Bi-2 212 cuprates lO,

BOO000~

~

~ x =0

8, 6. '-~

a.

700000 ~

-w-x= 0,2

600000

-s- x = _

~ 500000 O.

._~

=

0.4 ,

~v~400000 300000 200000

tO0000

-B 1

....+ -IOH -12

I 10000

I 20000

I 30000

I 40000

[G]

o

5~o ,o'oo ,~'oo ~o'oo ~'oo ~o'oo 3~'oo ,o'oo ,5'00 ~o'oo

I 50000

H

(G}

Fig. 8. Calculated/== (using Bean'smodel) as a functionofHfor Bi2Sr2CaCu20=(x= 0 ) and Bi2_~b~Sr2CaCu20=(x= 0.2;0.4 and 0.6) at 30 K.

-2-

-3J -4

I

1000

I

2000

I

3000

I

4000

I

I

5000

Fig. 7. Magnetic hysteresisloop of ceramics of BizSr=CaCu~O, (+) and Biz_~Pb~SrzCaCuzO~(xffi0.2 (X), 0.4 (O) and 0.6 (.)) at 5 K (a) and 30K (b). values of the Pb-containing materials and especially for the sample with x = 0 . 4 are larger than for the pure Bi material. Although, the conditions of preparation are not comparable with the present study, an improved pinning in partially Pb-substituted Bi-2212 materials was observed in ref. [ 16 ] as well. In search for possible pinning centres we compared additionally the critical current density j=m of materials with and without oxygen intercalated in the perovskite slabs (e.g. as-prepared and At-650 samples; cf. section 3.1.), but no significant differences were observed. Consequently, we conclude that intercalated oxygen in the perovskite slabs is unlikely to act as pinning centre. Since the improvement of Jm is up to x = 0 . 4 clearly connected with the Pb con-

tent the main contribution must be due to the substitution Pb~Bi. For the series Bi2_~Pb~Sr2CaCu20= the average distance of the Pb pinning centres is about 10 k for x--0.4 (calculated according to ref. [ 17] ). This value correlates with the coherence length of 2212 in the a - b plane. One possible reason for the action of Pb as pinning centre is the creation of local charge unbalance by the predominantly presence of Pb in the 2 + oxidation state. However, more likely is the function of Pb as the well known promotor for the formation of the 2223 phase, finally resulting in the formation of defect centres via local intergrowth of 2223 and 2212.

4. Conclusions We have shown that in the system Bi2_xPbxSr2CaCu20= by suitable post-treatment the temperature of about 95 K for Tc is simultaneously obtained in the Pb-free as well as the Pb-containing materials with x ~ 0.4. Furthermore, for the shift of Tc to lower values an intercalation of oxygen in the perovskite slabs is responsible, most probably resuiting in the creation of localized carriers in form of Cu 3+. The substitution P b ~ B i exerts a beneficial role by (i) a more uniform material distribution, ( ii ) the improvement of the superconducting properties of the intergrain material and (iii) the creation of pinning centres. In the third case the optimal Pb con-

T. Rentschler et al. / Pb-free and Pb-substituted Bi-2212 cuprates

centration is situated near x=0.4. This value corresponds to the average distance of Pb pinning centres of about 10 A, properly situated in the range of the coherence length in the a-b plane of 2212. Since one important difference between the Pb-free and Pb-substituted 2212 materials is the uniform presence of small amounts of 2223 in the latter case, it is speculated that the pinning centres are created by lattice defects due to a local intergrowth of 2223 and 2212.

Acknowledgements

This work was supported by the Bundesministerium flir Forschung und Technologic (FKZ 13N5842), the Forschungsschwerpunkt "Supralcitung" des Landes Baden-Wfirttemberg and the Vetband der Chemischen Industrie. The authors are indepted to the Rh6ne Poulcnc G m b H for supplying the yttrium and rare earth oxides. The help of A. Ehmann and E. Niquet is much appreciated.

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