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.