Theta-phase stabilization by rare-earth and alkali incorporation in sol-gel derived alumina

Bottom-up
synthesis of alumina suspensions doped with trivalent (Eu3+) or monovalent
(K+) ions is reported as a means to generate crystalline transition-phase
alumina
with increased thermal stability. Dynamic light scattering was used to track the stability
of precursor sols, showing how the dissolved cations contribute to the stabilizing
double layer and particle separation in the liquid phase. The incorporation of alkali or
rare-earth
dopants into the alumina structure further influences the thermal conversion
and crystallization behavior. The Kissinger activation energy of the θ → α phase
transition in Al2O3 indicates an inhibitory effect of Eu3+ and K+ on the formation
of the stable α phase, also confirmed by X-ray
diffraction. The intensity ratio of the
7F2/7F1 characteristic photoemission bands of Eu3+ was used to probe the local symmetry
as a function of calcination temperature to further corroborate this observation.