Comparative study on indentation size effect, indentation cracks and superconducting properties of undoped and MgB2 doped Bi-2223 ceramics

This study examines the evaluation mechanism of MgB₂ doped Bi_1.8Pb_0.4Sr₂(MgB₂)_xCa_2.2Cu_3.0O_y (0 ≤ x ≤ 1.0) superconducting ceramics prepared by conventional solid-state reaction method via dc resistivity, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and Vickers micro hardness (H_v) measurements. Variation of room temperature resistivity, critical transition temperatures (onset and offset), phase purity, cell parameter, texturing, grain connectivity, surface morphology, crystallinity and H _v values of the materials are deduced and compared with each other for the determination of the optimum doping level in the Bi-2223 system. It is found that all the properties given above depend strongly on the MgB₂ concentration. From dc resistivity investigations, each sample studied exhibits the superconducting behavior below their variable offset critical temperature values. The maximum onset (T _c ^onset ) and offset (T _c ^offset ) temperatures are found to be about 121.3 and 114.1 K, respectively, for the sample doped with x = 0.05. The minimum T _c ^onset of 118.6 K and T _c ^offset of 109.4 K are observed for the sample doped with x = 1.0. Similarly, XRD and SEM examinations indicate that there is an improvement in the crystal structures and surface morphologies of the superconducting materials with the increment of the MgB₂ inclusions in the Bi-2223 system up to x = 0.05 beyond which the crystallinity, grain connectivity and surface morphology start to degrade regularly and in fact reach to the worst structure appearance for the doping level of x = 1.0. Furthermore, the H_v measurement results being analyzed by Meyer’s law, proportional sample resistance (PSR), modified PSR, elastic–plastic deformation model, Hays–Kendall (HK) approach, Indentation-induced cracking model (IIC) allow us to derive the mechanical properties of the superconducting samples for the potential technological and industrial applications. According to the results obtained, HK approach, among the mechanical analysis methods, is determined as the most successful model for the samples (doped with x = 0, 0.1, 0.3, 0.5 and 1.0) exhibiting indentation size effect behavior whereas the IIC model is noted to be superior to other models for the other samples (doped with x = 0.01, 0.03 and 0.005) presenting reverse indentation size effect feature.