Oxygen diffusion, electrical conduction mechanism and supercondictivity in $YBa_2Cu_3O_{7-y}$ high-Tc superconductor

Abstract

I studied oxygen diffusion, electrical conduction mechanism and superconductivity in $YBa_2Cu_3O_{7-y}$ high Tc superconductor. Oxygen diffusion coefficient was measured by electrical conductivity method. The rate of equilibration is faster than during oxygen uptake than during its loss. This was caused by the difference in diffusion coefficient between oxygen uptake and its loss. The change in Tc and $\Delta$ Tc with increasing annealing time can be explained by the disintercalation and stoichiometry gradient of oxygen. The conduction mechanism of semiconducting 1-2-3 phase was variable range hopping. The 3-point bending strength of $YBa_2Cu_3O_{7-x}$ ceramics was higher in tetragonal than in orthorhombic phase, decreased with increasing sintering time. The measured difference in strength between tetragonal and orthorhombic phase was attributed to the residual stress resulted form volume change during tetragonal-orthorhombic phase transformation. With increasing sintering time, Tc increased and $\delta$ Tc decreased. The variation in Tc and $\Delta$Tc for the various sintering time was rather due to the development of phase than due to residual stress. Cr was substituted into Cu(2). The substitution of Cu with Cr in $YBa_2Cu_3O_{7-y}$ high Tc superconductor depressed critical temperature, Tc, increased transition width $\Delta$Tc and decreased orthorhombicity, (b-a)/a. And with Cr concentration, the resitivity increased at 290K. The decrease in the number of Cu-O(plane) lowered number of mobile holes in the Cu-O(plane) that determined the superconductive properties. This resulted in the increase of room temperature resitivity and the increase of Tc with increasing Cr concentration.