Aluminum-doped zirconia nanopowders: Chemical vapor synthesis and structural analysis by rietveld refinement of X-ray diffraction data

Abstract

Zirconia nanoparticles doped with up to 50 mol % Al2O3 were prepared by chemical vapor synthesis (CVS). XRD data of these powders were refined by Rietveld analysis. Doping with alumina decreases the fraction of the monoclinic ZrO2 phase and stabilizes the tetragonal or cubic ZrO2 phase, forming different ZrO2/Al2O3 solid solutions. The tetragonal zirconia solid solution is obtained in the as-synthesized nanopowders doped with 3 and 5 mol % Al2O3, whereas the cubic zirconia solid solution is formed in the nanopowder with 15 mol % Al2O3. Zirconia nanoparticles with higher alumina content (30 and 50 mol % Al2O3) are amorphous and crystallize during annealing, forming the single-phase tetragonal ZrO2 at 800 °C and a mixture of tetragonal ZrO2 and γ- or ε-Al2O3 at 850 °C, respectively. During annealing, the stability of the high-temperature zirconia phases decreases considerably with increasing temperature. Thus, the cubic ZrO2 phase transforms to the tetragonal. The tetragonality (c/a ratio) and the fraction of the monoclinic ZrO2 increase with temperature. Zirconia-alumina phase separation is observed in highly doped powders (≥15 mol % Al2O3). The observed maximum solubility of alumina in the cubic/tetragonal zirconia is about 15 mol % Al2O3 (or 26 at. % Al).