Effect of different temperature conditions on mechanical characteristics of Cu diffusion doped Bi-2212 ceramics

The aim of this thesis is to determine the changes of crystal structure, electrical, superconducting, mechanical performance and mechanical characteristics of one surface Cu-layered bulk Bi-2212 superconducting materials with the diffusion annealing temperature in the range of 650 °C-850 °C by means of bulk density, dc resistivity (ρ-T), transport critical current density (Jc), powder X-ray diffraction (XRD) and Vickers hardness measurements. All the experimental findings display that the extracted characteristic properties (crystallanity, crystal plane alignments, volume fractions, phase purity, average crystallite size, lattice constant parameters, normal state resistivity, residual resistivities, residual resistivity ratios, ρnorm, ρ, critical transition temperatures, degree of broadening, pinning nature, metasibility, σ-gap anisotropy, yield strength, fracture toughness, brittleness index, elastic modulus and flexural strength parameters) tend to improve significantly with the increase of diffusion annealing temperature up to 750 °C as a result of regression in the local structural distortions, lattice strains, disorders, defects and grain boundary interaction problems in the Cu-O2 consecutively stacked layers. However, after the critic annealing temperature of 750 °C, the parameters given above degrade rapidly, and in fact they reach to their own global minimum points as the annealing temperature is 850 °C. In more detail, the optimum diffusion annealing temperature of 750 °C resulting in the optimum copper concentrations embedded in the superconducting grains and over grain boundaries transits intrinsically the over-doped feature of Bi-2212 crystal system to optimally doped state as a consequence of the increment in density of electronic states at the Fermi energy level. Thus, the maximum critical transition temperatures (onset and offset) and minimum resistivity parameters are observed for the Bi-2212 material prepared at 750 °C. As for the Jc results, the optimum annealing temperature causes highly dispersed Cu nanoparticles to bind tightly for the formation of the effective nucleation centers along the transgranular and intergranular regions so that the flux pinning of vortices more and more improves in the Bi-2212 crystal structure. Moreover, the powder XRD experimental evidences show that the increase of diffusion annealing temperature up to the temperature value of 750 °C develops the average crystallite size, c-axis length and high phase fraction, after this critical temperature value every value decrease dramatically. Similarly, the optimum annealing temperature value strengthens the mechanical durability and ideal flexural strength as a result of the stabilization of durable tetragonal phase. Thus, the presence of Cu impurities increases the critical stress value of the materials so that the crack-producing flaws and cracks propagation divert or slow down rapidly. Furthermore, we check the mechanical characteristics of every materials by means of available six different theoretical models as regards Meyer's Law (ML), proportional sample resistance (PSR), modified proportional sample resistance (MPSR), Elastic-Plastic Deformation (EPD), Hays-Kendall (HK) and Indentation-Induced Cracking (IIC) approach for the first time. Based on the results of models, while the HK theoretical approach is observed to be the best method to investigate the relations between the load dependent and independent microhardness characterizations for the materials exhibiting the ISE behavior, the IIC approach is noted to be the most suitable method for determination of the true mechanical characteristics for the other material obeying the RISE nature. To sum up, the annealing temperature of 750 °C favors the Bi-2212 superconducting ceramics for usage in the large scale applications. KEYWORDS: Bi-2212 ceramics, Copper diffusion, dc resistivity (ρ-T), transport critical current density (Jc), Vickers hardness , X-ray diffraction (XRD)