Transgranular region preference of crack propagation along Bi-2212 crystal structure due to Au nanoparticle diffusion and modeling of new systems

摘要

This study delves into the changes of mechanical performances and mechanical characteristics of Bi-2212 inorganic materials exposed to the Au nanoparticles diffusion with respect to annealing temperature in the range from 923 K (650 °C) to 1123 K (850 °C) by means of Vickers hardness measurements at different applied indentation test loads (0.245 N ≤ F ≤ 2.940 N) and theoretical approaches. It is found that the mechanical performances belonging to the Bi-2212 superconductors improve considerably with the enhancement of the diffusion annealing temperature of 1073 K (800 °C) as a consequence of the reduced disorders in orientation of adjacent layers, lattice strains, local structural distortions, defects and grain boundary couplings. Existence of Au inclusions in the system, accordingly, makes the critical stress values augment for the diversion of crack-initiating flaws, crack and dislocation propagation. In other word, the optimum annealing temperature of 1073 K (800 °C) leads to develop the mechanical durability, fracture toughness and flexural strength towards the static compression loads as a consequence of the stabilization of durable tetragonal phase. Nevertheless, the excess annealing temperature causing the further Au impurities inserted in the Bi-2212 crystal structure serves as the stress raisers and crack initiation sites. Thus the induced cracks, voids and dislocations reach immediately to the critical propagation speed and the propagation of induced cracks, voids and dislocations is difficult to control. At the same time, the experimental findings enable us to determine the variation of mechanical characteristic properties as regards elastic modulus, yield strength, fracture toughness, elastic stiffness coefficient and brittleness index parameters. Based on the results of elastic modulus and yield strength parameters, the optimum Au foreign additives in the Bi-2212 crystal lattice degrade the limited number of operable slip systems. This is attributed to the fact that the crack propagation more proceeds throughout the transgranular regions. Conversely, the excess annealing temperature results in the increment of intergranular fracture. As for the theoretical examinations, all the materials prepared shows typical indentation size effect (ISE) feature as a result of reversible (elastic) and irreversible (plastic) deformations simultaneously. Moreover, Hays-Kendall (HK) theoretical approach is observed to be the best model for determination of the real mechanical characteristics for the pure and Au surface-layered Bi-2212 superconducting materials.