Effect of Ca2PbO4 on the formation of the (2223) phase in the BiPbSrCaCuO system

Materials Science and Engineerl/~g, B20 1993 312- ~ ~ ?

312

Effect of Ca2PbO 4 o n the formation of the (2223) phase in the Bi-Pb-Sr-Ca-Cu-O system M. Muralidhar, S. Satyavathi and V. Hari Babu Department of Physics, Osmania University, Hyderabad-500 007 (India)

O. Pena and M. Sergent Laboratoire de Chimie du Solide et Inorganique Moleculaire, URA CNRS 1495, UniversitO de Rennes 1, 35042 Rennes Cedex (France) (Received December 23, 1992)

Abstract The effect of excess Ca2PbO 4 on the superconducting properties of Bil.7Pb0.3Sr2Ca2Cu3OvB~(B -- Ca2PbO4) is investigated through X-ray diffraction, resistivity and a.c. magnetic susceptibility measurements. The X-ray diffraction results show that the volume fraction of the low Tc (2212) phase decreases and that of the (2223) phase increases with the addition of Ca2PbO4. For x = 0.6 and 1.5, only the (2223) phase exists. At higher Ca2PbO4 concentrations, i.e. for x = 3.0, the low T~ phase again appears along with the high Tc phase. Diffraction peaks appear at d = 2.02, 2.814 and 2.85, which are characteristic peaks of Ca2PbO4, and their intensities increase with increasing x, indicating that Ca2PbO 4 exists in the lattice in the same form. However, the T~(0) values decreased gradually from 107 to 98 K with increasing Ca2PbO 4 concentration. Compared with the undoped sample, the width of the transition A T~ is broad for impurity-added samples and reaches a maximum value for x = 3.0. Pure and Ca2PbO4-doped samples showed two peaks in Z vs. T curves and two steps in ;~' vs. T curves. The first peak is close to the transition temperature and corresponds to the midpoint of the first step in the g' curve. The second peak appears below 98 K and this peak maximum corresponds to the midpoint of the second step. The position of both sets of peaks remained almost the same up to x = 1.5 and shifted towards lower temperatures thereafter. The role of Ca~PbO 4 in the growth of the (2223) phase and the mechanism of energy loss are discussed,

1. Introduction

In the Bi 2_xPbxSr2Ca2Cu3Oy superconducting system, several workers [1-6] have found a decrease in Tc and also in the volume fraction of the high Tc (2223) phase at higher concentrations of Pb (x-> 0.4). Our results [7, 8] also showed that, for higher concentration of Pb, not only Tc but also the volume fraction of the high Tc (2223) phase and the Meissner signal decreased with a corresponding increase in the low Tc phase. Various mechanisms were suggested for this behaviour. One mechanism suggested was the formation of an impurity phase, such as CazPbO4. Shi et al. [9] and Chen et al. [10] studied the effect of Ca2PbO4 on the growth of the (2223) phase. They suggested that the CaO which decomposed from the C a 2 P b O 4 reacts with CuO to form C a z C u O 3 and that this accelerated the formation of the (2223) phase. According to the Hatano et al. [11], Pb in the mixture lowers the partial melting temperature of the sample and this could effectively enhance the growth rate of the (2223) phase. Uzumaki et al. [12] found that the 0921-5107/93/$6.00

high 7"~ phase can be synthesized by adding CazPbO4 to the B i - P b - S r - C a - C u - O system which has a single CuO layer. According to Liu et al. [13], the formation and decomposition of the high Tc phase occurred through the CazPbO 4 medium. As discussed above, Ca2PbO 4 is found to affect the formation of the high Tc (2223) phase but its role is not clearly understood. Therefore, we have undertaken a systematic study of the formation and growth of the (2223) phase by adding different concentrations of Ca2PbO 4 to the Bil.7Pb0.3Sr2Ca2Cu3Oy system. The results obtained from d.c. resistance, a.c. magnetic susceptibility and X-ray diffraction (XRD) studies are presented and discussed.

2. Experimental details

The samples with a nominal composition of Bil.7Pb0.3Sr2Ca2Cu3Oy-Bx, with B~Ca2PbO 4 and x = 0.0, 0.3, 0.6, 1.5 and 3.0, were prepared by solid state reaction from high purity (> 99%) Bi203, PbO, © 1993 - Elsevier Sequoia. All rights reserved

M. Muralidhar et al.

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Effect of Ca2PbO u on (2223) phase in Bi-Pb-Sr-Ca-Cu-O

SrCO 3, CaO and CuO. Stoichiometric mixtures were calcinated at 820 °C for 72 h with two intermediate grindings. The Ca2PbO4 matrix (B) was prepared from CaO and PbO. The mixtures of Bil.TPb0.3SrzCa2Cu3Oy and Ca2PbO4 were grounded thoroughly and kept at 820 °C for 24 h. The resulting powders were pressed into pellets 1 cm in diameter by applying a load of 5 tonnes. The pellets were then sintered at 850 °C for 100 h, followed by slow cooling to room temperature. The samples with x = 0.0, 0.3, 0.6, 1.5 and 3.0 will be called B1, B2, B3, B4 and B5 respectively. The d.c. resistance was measured with a standard Vander Pauw technique, using a Keithley nanovoltmeter and a constant-current source. A current of 5 mA was sent through the sample. The phase formation was tested by XRD, using a Philips, PW-1140 unit (Cu Ka radiation; Ni filter, 40 kV; 20 mA). The a.c. magnetic susceptibility was measured in the range 70_< T_<120 K using a standard mutual inductance technique operating at 119 Hz. The diamagnetic signal was measured with a home-built a.c. susceptometer.

3. Results and discussion

313

decrease gradually on adding Ca2PbO 4 in concentrations up to x = 1.5, but increased for x = 3.0. Diffraction peaks appear with d=2.02, 2.814 and 2.85 A, which correspond to characteristic peaks of CazPbO4, and their intensities increased with increasing CazPbO4, indicating that Ca2PbO 4 is present within the sample and has not decomposed as suggested by Chen et al. [10]. In Fig. 3, we can see that samples B3 and B4, with x = 0.6 and 1.5, have the maximum percentage of the high Tc (2223) phase. In sample B5, with x = 3.0, low T~ (2212) phase and Ca2PbO4 impurity phase peaks, along with the high Tc (2223) phase peaks can be seen, suggesting that it has a multiphase nature. The relative fractions of the high T c (f2223) and low Tc (f22J2) phases were determined from the peak intensities of some prominent reflections, using the following well-known relationships: f2223 = 12223(0 0 10)/[12223(0 0 10)+ 12212(0 0 8)]

f2212 = 12212(0 0 8)/[I2m(0 0 8)+ I 2 3(0 0 10)] The results obtained are given in Table 2. From the table we can see that, for x = 0.6 and 1.5, the low Tc (2212) phase is totally absent and only the high T~ (2223) phase is present. However, the low T~ phase is present for the samples with x = 0, 0.3 and 3.0.

3.1. Resistivity m e a s u r e m e n t s

Figures l(a) and l(b) show the R T / R m a x vs. temperature plots of pure and Ca2PbO4-added Bil.7Pb0.3Sr2Ca2Cu3Oy superconductors. Tc(0) of the undoped sample was found to be 107 K (sample B1). With the addition of Ca2PbO 4, the Tc(0) value decreased gradually from 105 K (for sample B2) to 98 K (for sample B5). Compared with the undoped sample, the width of the transition is broad for impurity-added samples and is found to increase with Ca2PbO 4 concentration. The To(0), Tc(onset ) and A T~(0) results are shown in Table 1. The results show that To(0) decreased and the transition width A T~(0) increased with the Ca2PbO4 concentration, suggesting that the multiphase nature has increased. 3.2. X-ray diffraction

The XRD pattern of the Ca2PbO 4 sample is shown in Fig. 2. Four prominent peaks at 2 0 = 17.7 °, 31.3 °, 31.8 ° and 44.90 ° can be seen and their corresponding d values are 5.05, 2.85, 2.81 and 2.02 A. Figure 3 shows the XRD patterns of samples B1 to B5. Sample B1 (no CazPbO4) shows that it has both low Tc (2212) and high Tc (2223) phases. The d values and intensity ratios of the prominent peaks, i.e. (0 0 8), (1 1 5) and (1 1 7) of low Tc (2212) phase and (1 1 3), (1 1 5), (0 0 12), (1 1 9),(2 0 0),(1 1 11),(2 0 12),(0 0 20)and(0 0 24) of high T c (2223) phase, are found to be in good agreement with those reported in the literature [14-16]. The prominent low Tc reflections (0 0 8), (1 1 5) and (1 1 7)

3.3. A.c. susceptibility m e a s u r e m e n t s

Figures 4(a)-4(e) show the temperature dependence of the a.c. susceptibility for samples B1 to B5 respectively. From the figures pertaining to samples B1-B4, we can see clearly that the X"-T curves have two peaks and that the ) ( - T curves have two steps. In each figure, the first peak is close to the transition temperature and nearly at the midpoint of the first step in the x ' - T curve. This peak decreased from 107 to 104 K with increasing x. However, the second peak lies between 100 and 95 K for compositions up to x = 1.5 and decreased to 85 K for x = 3.0. This is close to the middle position of the second step of the diamagnetic transition in the z ' - T curve. Values of To(onset ) and the loss peak temperatures are summarized in Table 3. Comparison of Figs. 4(a) and 4(e) shows that the transitions are sharper in the x = 0 sample than in the sample with x = 3.0. These results again confirm the multiphase nature of the sample with x = 3.0. The diamagnetic signal for all the samples was 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. 5. The signal increases sharply up to x = 0.6 and decreases thereafter. These results and those of the XRD confirm that an optimum concentration of Ca2PbO 4 (between x=0.6 and 1.5) helps in the formation and stabilization of the high Tc (2223) phase, although Tc(0) decreases marginally from 107 to 102 K.

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TABLE 1. Resistance results on BiwPb0aSr2Ca2Cu3OyBx (B -= Ca2PbO4) samples

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Our previous results [18] on Ca2CuO3-doped samples showed two loss peaks which were quite intense and of equal magnitude. The lower temperature peak was observed in the 40-60 K temperature range and this was attributed to energy losses at the grain boundaries of the (2212) phase. In Ca2PbO aadded samples, the high temperature loss peaks appeared in the temperature range 107-104 K and these could be due to intrinsic losses within grains. The low temperature loss peaks occurred between 100 and 85 K, and they are probably formed due to losses at grain boundaries which serve as weak links. As the Ca2PbO4 concentration increases, both peaks shift towards lower temperatures, the shift being more pronounced in the low temperature peak pertaining to the x = 3.0 sample. The higher concentration of Ca2PbO4 also resulted in broadening of the coupling peak to a greater extent when compared with those for lower concentrations of Ca2PbO4. The broadening above x = 1.5 and the shift of the low temperature loss peak from 98 to 85 K may be due to an increase in the multiphase nature of the material, which in turn increases the number and size of large-angle grain boundaries.

2 0 (Degrees/ Fig. 3. XRD pattern for Bi17Pb03Sr2Ca2Cu3OyBx superconductor, with B ~-Ca2PbO4 (x = 0.0, 0.3, 0.6, 1.5 and 3.0). The peaks denoted by H and L correspond to the high Tc (2223) and low Tc (2212) phases respectively.

T A B L E 2. Volume fraction of the high Tc and low Tc superconducting phases, calculated from the intensity of XRD reflections Concentration of Ca2PbO4 (x)

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(diamagnetism) within each grain, which they called intrinsic, and the broad peak at lower temperatures was attributed to hysteresis losses at the grain boundaries or to diamagnetism of Josephson-like coupling or "coupled phase". From Figs. 4(a)-4(e), we can also see that the first loss peak, which occurs at the mid-point of the first step in the Z ' - T curve, is always smaller than the second peak, suggesting that losses owing to Josephson-like coupling of grain boundaries are predominant over losses within the grains. This is understandable, since the samples are of bulk polycrystalline nature and contain large-angle grain boundaries.

4. Conclusions

Our XRD results have shown that, for x = 0.6 and 1.5, the low Tc phase is almost absent and the material consists mostly of the high Tc (2223) phase. Shi et al. [9] and Chen et al. [10] added CazPbO4 to the low Tc Bi2Sr2CalCu208 (2212) phase, which accelerated the growth of the (2223) phase. According to them, the addition of Ca2PbO 4 produced a Pb-rich liquid phase and CaO when the samples were sintered above 882 °C. The CaO decomposed from C a z P b O 4 c a n react with CuO to form Ca2CuO3 and this accelerated the formation of the (2223) phase. Our earlier results [18] also showed that the addition of an optimum concentration of Ca2CuO3 not only enhanced the volume fraction of high Tc phase but also the critical current density. In the present studies, we have added CazPbO 4 to the Bi~.vPb0.3Sr2CazCu30 ~, system which initially contained 41.4% (2223) phase and 58.6% (2212) phase. The addition of an optimum concentration (x = 0.6-1.5) of CazPbO4 increased the volume fraction of the (2223) phase. Our XRD results given in Fig. 2 show that Ca2PbO4 is present in the sample and has not decomposed as suggested by Shi et al. [9] and Chen et al. [10]. In our view, a small fraction of Ca2PbO4 might have decomposed initially and helped in the formation of the (2223) phase, the rest remaining unchanged. For the highest concentration, i.e. x = 3.0,

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TABLE 3. A.c. magnetic susceptibility results on Bi~ 7Pb 3Sr2Ca2Cu30,,B, (B -=Ca2PbO4) samples

Acknowledgments

Sample x

The authors thank the Department of Science and Technology, New Delhi for providing financial assistance, and the CSIR-New Delhi for awarding a Senior Research fellowship (M. M. Dhar).

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References

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Fig. 5. Diamagnetic signal (at liquid nitrogen temperature) of samples B1, B2, B3, B4 and B5.

the volume fraction of the high Tc phase again decreased and that of the low Tc phase increased. T h e s e results show that the material must have decomposed into Bil.TPbo.3Sr2CaCu2Os, Ca2CuO3, C u O and Ca2PbO4. T h e decrease in To(0), increase in A Tc(0 ) and the broadening of the low t e m p e r a t u r e loss peaks for the x = 3.0 sample all support the above mechanism.

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