Influence of the compacting pressure on the dependence of the critical current with magnetic field in polycrystalline (Bi–Pb) 2Sr 2Ca 2Cu 3O x superconductors

Abstract

The hysteretic behavior of the superconducting critical current density J c( B a) dependence under low applied magnetic fields ( B a<50 mT) of nearly single phase Bi 1.64Pb 0.36Sr 2Ca 2Cu 3O 10 Bi-2223 samples was investigated. The samples were prepared from the same batch obtained by solid-state reaction method, pressed uniaxially at compacting pressures ranging from 40 to 250 MPa before the last heat treatment. The presence of up to ∼10% of additional phases such as Bi-2212 and Ca 2PbO 4 was estimated from the analysis of the X-ray diffraction patterns. These results also indicate that higher compacting pressures yielded a reduced volume fraction associated with the extra phases. The granular structure of these samples was studied by scanning electron microscopy which revealed grains with platelet-like geometry and typical dimensions of ∼3×1 μm 2 and thickness of ∼0.5 μm. Results of magnetization versus applied magnetic field performed in powder samples indicate that the lower critical field H clg∼10 mT at 77 K is sample independent. In addition, the estimated intragranular current density J cg at 77 K by using the Bean model yields J cg≈6.6×10 8 A/m 2, which is in excellent agreement with the literature. Measurements of electrical resistivity and magnetization as a function of temperature were performed and compared to J c( B a) measurements. We have found that the grain properties are very similar in all samples but that the J c( B a) dependence is very sensitive to the compacting pressure. The experimental data are discussed within the framework of a three-level superconducting system.