A.c. and d.c. susceptibility studies on silver-doped BPSCCO (2223) superconductors

Materials Science and Engineering, B26 (1994) 151-155

151

A.c. and d.c. susceptibility studies on silver-doped BPSCCO (2223) superconductors M . M u r a l i d h a r a, K. N a n d a K i s h o r e a, S. S a t y a v a t h i a, O. P e n a b a n d V. H a r i B a b u a* aDepartment of Physics, Osmania University, Hyderabad-500 007 (India) bLaboratoire Chimie de Solide et Inorganique Moleculaire, URA CNRS 1495, Universit~ de Rennes I, 35042 Rennes Cedex (France) (Received August 12, 1993; in revised form March 29, 1994)

Abstract A.c. and d.c. susceptibility studies were undertaken on Ag-doped Bil.7Pb0.3Ca2Sr2Cu3Oysamples to investigate the quality of the materials and the critical current density. X-ray diffraction (XRD) and resistance measurements were also performed to determine the phase of the samples and the influence of Ag addition on To(0). XRD results show that the (2223) high-Tc phase is retained and To(0) varies between 103 and 107 K. These results suggest that the superconducting properties are not destroyed by Ag addition. The a.c. susceptibility results show two troughs in the X' vs. T curves and two peaks in the X" vs. T plots. The first peak is smaller, is close to the transition temperature and corresponds to the midpoint of the first trough in the z - T curve. The second peak appears below 98 K and corresponds to the midpoint of the second trough. The positions of both sets of peaks remain almost the same. The low-temperature peaks are sharper in samples containing silver compared with the pure sample, and the sharpness increases with an increase in silver concentration. Jc was calculated from d.c. susceptibility data using Bean's critical state and plate-like models. Jc was calculated from d.c. susceptibility data using Bean's critical state and plate-like models. Jc increases with silver doping. The increase in sharpness of the low-temperature loss peak with an increase in Ag concentration and critical current density suggests that Ag probably precipitates along the grain boundaries and improves the intercormectivity between the grains.

1. Introduction

Since the discovery of new high-Tc superconducting Bi-based oxides by Maeda et al. [1], many attempts have been made to improve the quality of the samples. The BPSCCO 2223 phase is particularly interesting for energy transport and high-field magnets since its Tc value (110 K) lies 3 0 K above the boiling point of nitrogen. For many practical applications, these materials should be single phase and carry high critical current densities and power. They must be flexible, chemically stable and have good mechanical strength. Polycrystalline bulk superconductors usually have a low critical current density (Jc) and this has been explained by the weak coupling of the grains [2, 3]. It has been demonstrated that the addition of silver to YBCO and BISCCO superconductors improves their critical current densities and mechanical behaviour without adversely affecting their superconducting properties [4-6].

*Author to whom correspondence should be addressed. 0921-5107/94/$7.00

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More information on weak coupling and how it is related to Jc can be obtained from a.c. and d.c. susceptibility studies. Therefore we have undertaken a systematic study of the effect of silver addition on the Bil.7Pbo.3CazSr2Cu3Oy system using a.c. and d.c. magnetic susceptibility measurements. The critical current density was also calculated from d.c. susceptibility data using Bean's critical state and plate-like models and the results were correlated with a.c. loss peaks.

2. Experimental details

Samples with a nominal composition of Bi~.7Pb0.JAgxSr2Ca2Cu3Oy with x = 0.0, 1.4, 2.5 and 3.0 were prepared by the high-temperature precursor matrix reaction method, the details of which have been reported elsewhere [7]. Finally, these pellets were sintered at 845 °C for 100 h and then furnace cooled to room temperature. The samples with x = 0.0, 1.4, 2.5 and 3.0 were designated as A0, A1, A2 and A3 respectively. D.c. resistance measurements were carried out using the standard Vander Pauw technique, © 1994 - Elsevier Science S.A. All rights reserved

152

M. Muralidhar et al.

,4. ('. and d.c. susceptibility ()/'Ag-doped BI~S'CCO superconductor.s

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the details of which are described in ref. 8. The real (X') and imaginary (Z") parts of the a.c. complex susceptibility were measured in the range 70 ~< T~< 120 K; the details are reported elsewhere [9]. The diamagnetic signal was measured with a home-built a.c. susceptometer. D.c. magnetic susceptibility measurements were carried out at 5 K using an SHE SQUID magnetometer (VTS-906) in the field range 0-30 kOe.

3. Results and discussion

The powder X-ray diffraction (XRD) patterns in Fig. 1 show that pure (A0) and silver-added (A3) samples are essentially of single 2223 phase. Only a small amount of 2212 phase is detected in the pure sample. The lattice constants of the 2223 type were computed using a least-squares fit method and are a = 5.41 A a n d c = 37.10 A. Figure 2 shows the RT/R3o o w'. temperature plots of samples A0-A3. All the R-T plots show a single-step superconducting transition. The T~ (onset) value varies from 118 to 122 K after which there is a sharp fall in resistance. T~(0) varies from 103 to 107 K and the results are given in Table 1.

3.1. A.c. susceptibility The real part of the a.c. magnetic susceptibility is often used to determine the onset temperature of superconductivity, whereas the imaginary part (loss peaks) gives information on the grain boundaries and other defects which act as weak links. The real (Z') and imaginary (g") parts of the a.c. complex susceptibility were measured in samples A0, A1, A2 and A3 and are shown in Figs. 3(a) and 3(b). From these figures, we can see that, in all samples, the g " - T curve exhibits two

peaks and the x'-T curve exhibits two troughs. The first peak is close to the transition temperature and corresponds to the midpoint of the first trough in Z'- T. The second peak appears below 98 K with its maximum at 92 K. This peak maximum corresponds to the midpoint of the second trough in X'-71 The 7~, (onset) values and loss peak temperatures are summarized in Table 1. From the table, it can be seen that the temperatures of both peaks are at a maximum for sample A1. For sample A0, without silver, the A2 and A3, where the silver content is higher, the loss peak temperatures are lower. In granular YBCO (123) and BPSCCO (2223) superconductors, it has been suggested that the superconducting grains are coupled by Josephson currents via weak links [2, 10]. The high-temperature peaks in the X"-T curve shown in Fig. 3 are smaller and are probably induced by screening currents inside the superconducting grains. These peaks have also been called intrinsic peaks. The low-temperature peaks are probably due to the coupling signal induced by the macroscopic shielding current through grains and weak links [11]. Although the Ag-doped and undoped samples were prepared under similar conditions and consist mostly of the high-7~. (2223) phase, the X"-T curve of the pure sample is much broader compared with the silver-doped samples where the loss peaks are much sharper. The sharpness of the peaks increases with an increase in Ag concentration. These results show that Ag doping improves the interconnectivity

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A.c. and d.c. susceptibility of Ag-doped BPSCCO superconductors

TABLE 1. Resistance and a.c. susceptibility results of the silver-doped BiLTPb,.3Ca2Sr2Cu3Oysuperconductors Silver content

Tc (onset) from Z'

To(0)a (K)

X

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vs. T b

Peak temperature from Z"

vs. T b

High temperature

Low temperature

High temperature

Low temperature

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(K)

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106 107 103 103

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between the grains and thus improves the quality of the sample. From electron probe microanalysis (EPMA) studies, Ren et al. [12] have confirmed that silver atoms precipitate at the sites of grain boundries and help to

interconnect the grains. Earlier scanning electron microscopy (SEM) results [13] also indicated that Ag preferentially precipitates along the grain boundaries and decreases the effective number of weak links. Thus

M. Muralidhar et al.

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A.(I) and d.c. suscel)tibilio, of Ag-doped BPS('('O Sul)erconducmr.s

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the sharpness of the loss peaks suggests that Ag improves the quality of the samples. The diamagnetic signal for all samples were measured at 77 K by taking equal quantities and placing them at the same position in the secondary coil. The results are shown in Fig. 4. It can be seen that there is a clear increase in the diamagnetic signal with an increase in silver content (x = 3.0). These results and those of XRD confirm that Ag improves the quality of the samples and helps to form and stabilize the high-r/(2223) phase.

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M. Muralidhar et al.

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A.e. and d.c. susceptibility of Ag-doped BPSCCO superconductors

TABLE 2. Caiculated Jc values of Bil.7Pbo.3Ca2SrzCu3Oy-Ag x using Bean's model Sample

1 2 3 4 5 6 7 8 9 10

Field (kOe)

3 4 6 8 10.0 12.5 15.0 17.5 20.0 25.0

J~ from Bean's model (at 5 K)(A cm -2) Ag = 0.0

Ag = 1.4

Ag = 2.5

Ag = 3.0

289.86 262.03 232.03 212.12 209.20 162.84 125.48 115.76 98.55 70.21

300.24 279.08 358.24 215.44 230.36 193.14 169.48 144.50 139.90 129.21

350.12 312.20 278.68 258.16 235.21 225.20 200.80 181.76 161.20 132.21

375.82 345.21 310.11 278.22 258.00 232.01 211.02 193.12 181.17 161.00

of J~ are based on Bean's critical state model [14], which provides a mathematical relationship between a superconductor's magnetic behaviour and its J~ value. The basic idea of this approach is that, in a homogeneous type II superconductor, the magnetic hysteresis is related to the critical current density Jc. Thus the magnetic hysteresis can be used to derive J~. This assumes, of course, that the superconductor is homogeneous and not a collection of weakly coupled grains. In order to determine J~ using the magnetization method, thin flat specimens of A0, A1, A2 and A3 were taken (area, 5 x 14 mm2; thickness, d=0.31 mm; mass = 0.041 g) and the magnetization was measured using a SQUID magnetometer with H parallel to the slab plane. J~ was estimated by analysing the magnetic hysteresis data according to the flat plate model [15-17]. Jc (A cm -2) as a function of the applied magnetic field can be calculated by the expression 4I-I(M + - M - ) = (8FI/10)dJc where 4 H M + and 4 H M - are the magnetizations of the sample during increasing and decreasing fields respectively and d is the thickness (cm) of the plate. The hysteresis curves and the calculated J~(H)curves at 5 K for the A0, A1, A2 and A3 samples are shown in Figs. 5(a)-5(d) respectively. From these figures, it can be seen that J¢ decreases with increasing magnetic field and increases with silver concentration. The J¢ values of thin flat samples can also be calculated using Bean's critical state model [14] from the following relation J~=ZIM + - M - I / d

155

The calculated Jc values of different samples using this model are given in Table 2. It can be seen that Jc decreases with increasing magnetic field and increases with silver concentration, and the same behaviour was also observed in the flat plate model. The J~ results obtained from d.c. magnetization studies and the loss peaks obtained from a.c. susceptibility studies suggest that the effective number of grain boundaries which act as weak links are reduced on Ag addition and Jc is improved.

Acknowledgments The authors wish to thank the Department of Science and Technology, New Delhi for providing financial assistance, and the CSIR, New Delhi for the award of a Research Associateship and a Senior Research Fellowship (M. Muralidhar and K. N. Kishore).

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