AC susceptibility study of Bi1.66Pb0.34Sr2Ca2−xMgxCu3Oy (x = 0, 0.2 and 0.4) superconductor systems

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Abstract

A systematic study of the intergranular properties of Bi1.66Pb0.34Sr2Ca2−xMgxCu3Oy (x = 0, 0.2 and 0.4) samples has been done, using the AC susceptibility technique. The samples were prepared by conventional solid state reaction method. It was found that Mg substitution in place of Ca reduces the intergranular coupling of Bi-2223 system. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (Tc) has been done employing Bean's Critical State Model. The observed variation of intergranular critical current densities (Jc) with temperature indicates that the critical current density decreases by increasing the amount of Mg. The higher electronegativity of Mg in the unit cell promotes more intake of oxygen in the material, and the grain boundaries are in more over-doped regime. These over-doped regions reduce the intergranular coupling and increases weak link behavior of Mg doped samples.

Introduction

The high-temperature superconductors are mostly prepared in polycrystalline form for large scale applications such as electric power applications. In this field of applications, it is necessary to fabricate superconducting materials with high critical current densities, Jc. It has been realized that the polycrystalline superconductors can be described as arrays of superconducting grains, coupled weakly by Josephson junctions. These weakly coupled grains are known to limit the Jc values of these materials [1], [2]. Some of the reasons for the formation of weak links are, misorientation of grain boundaries, vacancy, oxygen deficiency, secondary phases and composition variations [3], [4], [5]. Therefore, it is very important to study the intergranular properties of these materials. The (Bi, Pb)2Sr2Ca2Cu3Oy (Bi-2223) is a promising compound from an application point of view due to its application in the fabricating of high critical current superconducting tapes. Since the discovery of Bi-2223 superconductor, great progress has been achieved in enhancing transport properties of this high temperature superconductor. However, the major limitations of the Bi-2223 system applications are the intergrain weak links and weak flux pining capability. For example, the intragrain and intergrain properties of the Bi-2223 system are influenced differently by the partial substitution of (Cu) by 3d elements [6], [7]. Substitution of low-concentration of Sb (in place of Pb) promotes the formation of Bi-2223 phase [8]. Increasing Sm content at Ca-sites results decreasing in the Bi-2223 phase and increasing in the Bi-2212 phase [9]. Further studies revealed that the formation of the Bi-2223 phase is significantly enhanced by partial substitution of Bi cation by Pb cation [10], [11], [12]. In this paper we have studied the intergranular properties of Mg (in place of Ca) substituted Bi-2223 samples.

Section snippets

Experimental

Bi1.66Pb0.34Sr2Ca2−xMgxCu3Oy (x = 0, 0.2 and 0.4) samples were prepared by a conventional solid state reaction method. Extra high purity powders of Bi2O3, PbO, CaCO3, CuO and MgO were well mixed in stoichiometric proportions calcinated at 810 °C for 24 h and sintered at 860 °C in air for 180 h. The calcinations and grinding procedures were repeated three times. The samples will be referred as (x = 0, sample A), (x = 0.2, sample B) and (x = 0.4, sample C). The samples were examined through an X-ray

Results and discussion

The measurement of AC susceptibility is widely used as a nondestructive method for the determination and characterization of the intergrain component in the polycrystalline high-Tc superconductors [14], [15], [16], [17], [18], [19]. In particular, the imaginary component of the AC susceptibility has been widely used to probe the nature of weak links in polycrystalline superconductors. It is also employed to estimate some of the important physical properties like critical current density, Jc and

Conclusion

We have studied the effect of Mg substitution in place of Ca on the intergranular properties of Bi1.66Pb0.34Sr2Ca2−xMgxCu3Oy (x = 0, 0.2 and 0.4) samples using the AC susceptibility technique. The samples were prepared by conventional solid state reaction method. It was found that Mg substitution in place of Ca reduces the intergranular coupling of Bi-2223 system. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (Tc) has been done employing Bean's

Acknowledgement

The authors would like to thank Isfahan University of Technology for supporting this project.

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