Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy süperiletkeninde Au difüzyonunun yapısal, üstüniletkenlik ve mekanik özellikleri üzerine etkisi

In this study, we have investigated the effect of the gold diffusion-doping and diffusion-annealing duration on structural, superconducting and mechanical properties of the Bi1.8Pb0.35Sr1.9Ca2.1Cu3Oy superconductors annealed 830°C for 10, 20, 50 hours. The samples were prepared by the conventional solid-state reaction method in the polycrystalline bulk form. Doping of Bi-2223 was carried out by means of Au diffusion during sintering from an evaporated Au films on pellets. The experimental works in this study consist of DC electrical resistivity, critical current density and AC susceptibility measurements for electrical, magnetic and superconducting properties, microhardness measurements for mechanical properties (such as Vickers microhardness, Young?s modulus, fracture toughness and yield strength). The mechanical properties of the samples were found to be diffusion annealing time and load dependent. Powder X-ray diffraction for phase analyses and lattice parameters, and SEM for microstructure examinations were carried out. These measurements showed that Au doping, in comparison with the undoped samples, increased the lattice parameter c, grain size, critical current density, and critical transition temperature, improved the mechanical properties and enhanced formation of Bi-2223 phase. Moreover, as the diffusion-annealing time decreased the number and size of voids increased, transition temperature, critical current density, lattice parameter c decreased. Additionally, we have calculated the activation energies and diffusion coefficients of Au in BSCCO. For this reason, the undoped samples were coated with an Au layer and diffusion annealing was carried out 500, 600, 700, 750 and 800°C for 10 hours. There are several methods to estimate the diffusion parameter in polycrystalline high-Tc superconductors. The most common four methods are: successive removal of thin layers and (1) the measurement of the sample?s resistivity, (2) the measurements of lattice parameters from XRD patterns, (3) the energy dispersive X-ray fluorescence, and (4) the radio-tracer methods. In this study, we used first two techniques to calculate the diffusion coefficient of Au in Bi(Pb)-Sr-Ca-Cu-O system. The temperature dependence of the Au diffusion coefficients in the range of 500-800°C was described by the relation D = D0 exp(-Ea/kBT). It was obtained that for diffusion of Au atoms in Bi-2223 in the temperature range 500-800°C, the coefficients of diffusion increases, the corresponding activation energy being 1.08 eV. Diffusion at lower annealing temperatures is much less significant. The possible reasons for the observed improvements in structural, superconducting and mechanical properties due to Au diffusion in the superconductor BSCCO samples were discussed. Keywords: Bi-2223, Bi-2212, High temperature superconductor, Diffusion coefficient, Transition temperature, Transport critical current density, Intergrain and Intragrain critical current density, Mechanical properties,