Change of mechanical performance and characterization with replacement of Ca by Gd nanoparticles in Bi-2212 system and suppression of durable tetragonal phase by Gd

Abstract

In this study, we concentrate on crucial evaluations of mechanical performances belonging to Bi2.1Sr2.0Ca1.1−xGdxCu2.0Oy superconducting ceramics with the partial replacement within the molar ratio change 0 ≤ x ≤ 0.30 of aliovalent Gd-sites on the Ca-sites in the system by means of microhardness measurements at different static compression test loads in the range of 0.245–2.940 N. The experimental measurement findings observed display that the mechanical performance tends to regress regularly with the enhancement of the Gd content level due to the increment in the local structural distortions, boundary weak connection between the grains and lattice disorders/strains/defects in the orientation of superconducting adjacent layers. Namely, the presence of Gd foreign impurities in the Bi-2212 crystal lattice leads to induce the crack-producing omnipresent flaws acting as the stress raisers and crack initiation sites. The long and short of it is that the Ca/Gd substitution in the adjacent layers damages seriously the mechanical strength (durability and stiffness), fracture and flexural strengths of the Bi-2212 material due to the increased operable slip systems so that the intergranular fracture becomes more dominant in the Bi-2212 matrix. In other words, the increment in Gd substitution level at the Ca site leads to accelerate the crack propagation and dislocation movement as a result of the decrement in Griffith critical crack length. Thus, the critical stress and durable tetragonal phase values degrade considerably in parallel to the enhancement of Gd content level, and the augmented cracks and dislocations with the existence of foreign impurities reach instantly to the critical propagation speed. The propagation of cracks and dislocation movements is hard to control along with the superconducting samples. At the same time, the Gd inclusions lead to change in mechanical characterization of the Bi-2212 ceramics. The pure sample presents Indentation Size Effect (ISE) behavior while the others exhibit Reverse Indentation Size Effect (RISE) feature, pointing out the increment of induced structural distortions and grain boundary couplings. Besides, the Vickers hardness experimental results are analyzed by the offered theoretical model of Hays–Kendall approach for the first time.