CHEMICAL DIFFUSION OF OXYGEN IN SINGLE-CRYSTAL AND POLYCRYSTALLINE YBA2CU3O6+X DETERMINED BY ELECTRICAL-RESISTANCE MEASUREMENTS

Isothermal electrical-resistance measurement's were used to monitor oxygen diffusion kinetics in both single-crystal and polycrystalline YBa2Cu3O6+x between 350 and 780-degrees-C. Distinctions are made between intrinsic diffusion behavior and microstructure-dependent properties. An activation energy of 0.4(1) eV was determined for oxygen in-diffusion in dense polycrystalline material from 400 to 780-degrees-C, and 1.10(6) eV for single crystals between 600 and 780-degrees-C. Below 600-degrees-C, the activation energies for out-diffusion in porous and dense polycrystalline specimens were determined to be 0.5(1) eV and 0.6(1) eV, respectively. The lower activation energy in polycrystalline specimens was attributed to (i) shell effects at either grain boundaries or the specimen surface in which the rapid formation of a highly oxygenated skin created short-circuit pathways for current flow, and (ii) high-diffusivity pathways along grain boundaries. Measurements on single crystals reveal that the intrinsic rate of oxygen in-diffusion was comparable to, if not slower, than out-diffusion, which was contrary to measurements on polycrystalline specimens. This type of behavior was attributed to the formation of a highly oxygenated shell during in-diffusion which behaved as both (i) a high-conductivity pathway and (ii) a barrier to bulk oxygen in-diffusion. The use of single crystals enabled these effects to be clearly distinguished.