Chemical Diffusion of Oxygen in YBa2Cu3O6+x

Abstract

Chemical diffusion of oxygen has been measured in both single crystal and polycrystalline YBa2Cu3O6+x in order to distinguish between intrinsic diffusion behavior and extrinsic characteristics which depend on microstructure. Isothermal electrical resistance measurements were used to monitor the dynamics of oxygen diffusion from 350-780°C under both oxidizing and reducing conditions. Measured activation energies depended on sample morphology, temperature, and whether indiffusion or out-diffusion of oxygen was occurring. Below 600°C, the activation energies for out-diffusion in porous (~75%) and dense (~95%) polycrystalline material were found to be 0.5(1)eV and 0.6(1) eV, respectively. These low energies suggest a high diffusivity pathway such as a grain boundary. Polycrystalline material exhibited a change in functional form for out-diffusion near 600°C in both dense and porous samples which was attributed to a change from primarily grain boundary diffusion to significant bulk lattice diffusion. Above 600°C, oxygen outdiffusion in dense polycrystalline and single crystal YBa2Cu3O6+x was found to be surface-reaction controlled. The activation energies for outdiffusion in the dense material were 1.93(6)eV and 1.7(1)eV above and below ~700°C; and in the single crystal 1.6(1)eV and 1.00(4)eV above and below ~680°C. Oxygen in-diffusion was found to have activation energies of 0.4(1)eV from 400-780°C in dense polycrystalline material and 1.16(6)eV in single crystal material from 600-780°C. The lower activation energy in the polycrystalline material may be due to percolation effects in which the rapid formation of a highly oxygenated shell masks intrinsic diffusion behavior.