BINARY KINETICS IN THE Y-BA-CU SYSTEM .2. NANOSIZED PARTICLES

The kinetics of binary mixtures of Y2O3-BaCO3(Y-Ba), BaCO3-CuO(Ba-Cu) and Y2O3-CuO(Y-Cu) prepared by citrate sol-gel techniques were studied in both air and helium. Particle sizes resulting from the sol-gel pre paration were between 5 and 100 nm, and the prevailing kinetics differ ed significantly from those observed in the companion study of micron- sized particles. That is, nucleation-growth kinetics of the Avrami-Ero fe'ev type adequately model the kinetics over the entire conversion ra nge as opposed to the diffusion mechanisms that describe the kinetics in larger sized particles. An alternate nucleation-growth model, which neglects overlapping volumes during reaction (Austin-Rickett model), was also adequate in the Ba-Cu system where particle size/spacing was larger, but was only applicable in other reaction systems at lower con versions. Activation energies obtained from the Avrami-Erofe'ev were c onsistently lower than those from the Austin-Rickett model, because th e former model contains two parameters that are temperature-sensitive.