A kinetic and microstructural study of the gamma-to-alpha alumina phase transformation: Effect of controlled nucleation and alternative material transport paths.

摘要

The present study focused on enhancing the transformation behavior and tailoring the particle characteristics of a boehmite-derived {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} by controlling nucleation and modifying the growth mechanism during transformation. Controlled nucleation of {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} was effected by the addition of 0.1 {dollar}mu{dollar}m {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} seed particles to a boehmite sol precursor. Seed particle concentrations ranged from 0 to 10{dollar}sp{lcub}14{rcub}{dollar} seeds/cm{dollar}sp3{dollar}. The effect of liquid and vapor phase transport paths on {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} transformation kinetics and particle morphology was studied by adding 10 wt.% V{dollar}sb2{dollar}O{dollar}sb5{dollar} and 5 wt.% AlF{dollar}sb3{dollar}, respectively, to unseeded and seeded {dollar}gamma{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} powders.; During solid state transformation of boehmite-derived {dollar}gamma{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} powders, {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} seed particles function as low energy barrier, heterogeneous nucleation sites for {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar}. A simple model where each seed particle is responsible for a single {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} grain after transformation is in reasonable agreement with microstructural observations. Increasing the seed concentration results in faster transformation rates, a lower apparent activation energy for the {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} transformation, lower transformation temperatures, and improved microstructures. Below approximately 950{dollar}spcirc{dollar}C the {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} transformation in seeded aluminas becomes growth limited due to low solid state diffusivities for material transport.; Alumina transport through liquid V{dollar}sb2{dollar}O{dollar}sb5{dollar} occurs by a solution-precipitation mechanism. The advantages of liquid phase transport are not realized in the alumina system unless the energy barrier to {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} nucleation is lowered and the nucleation frequency increased through {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} seed particle additions. Both seeding and liquid phase transport result in increased transformation kinetics and transformation temperatures as low as 800{dollar}spcirc{dollar}C. Transformation of seeded {dollar}gamma{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} powders in liquid V{dollar}sb2{dollar}O{dollar}sb5{dollar} resulted in fine grained, highly faceted, equiaxed {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} grains containing approximately 1 wt.% vanadium impurity.; Alumina transport through the vapor phase occurs by a chemical transport reaction involving gaseous AlF{dollar}sb3{dollar} and water vapor. The lower energy barrier and higher diffusivities for vapor transport result in greatly reduced transformation temperatures (i.e., 700{dollar}spcirc{dollar}) and increased transformation rates relative to the liquid phase and solid state systems. Transformation of seeded {dollar}gamma{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} in the presence of a fluoride vapor phase results in the formation of 0.5 to 1.0 {dollar}mu{dollar}m single crystal, hexagonal {dollar}alpha{dollar}-Al{dollar}sb2{dollar}O{dollar}sb3{dollar} plates.