Glass formation and phase transformations in plasma prepared Al2O3-SiO2 powders

Abstract

The structure of Al2O3-SiO2 sub-micron powders prepared by oxidation of mixed aluminium-silicon halides in an oxygen-argon high frequency plasma flame has been studied. The powders were completely amorphous up to at least 52 wt % Al2O3 and partially amorphous in the range 52 to 88 wt % Al2O3. The crystalline phase was mullite up to 75 wt % Al2O3 but at higher Al2O3 contents a metastable solid solution of SiO2 in γ-Al2O3 was observed in addition to mullite. Amorphous particles crystallized to mullite on heating to 1000°C, independently of composition. Extension of glass formation towards the high Al2O3 end of the Al2O3-SiO2 system as the cooling rate is increased and particle size decreased, may be explained by the effect of viscosity on the nucleation rate of mullite from liquid, for Al2O3 contents up to 60 wt %. The viscosity change is relatively small as the Al2O3 content is increased beyond 60% and it is suggested that the change in nucleus-liquid interfacial energy with composition is the predominant factor controlling nucleation rate in this range. At Al2O3 concentrations greater than approximately 80 wt %, γ-Al2O3 is the phase which nucleates from the melt. A double DTA peak was observed for powders containing more than 80 wt % Al2O3. The lower temperature peak is believed to arise from the formation of mullite from a metastable solution of SiO2 in γ-Al2O3, and the higher temperature peak from crystallization of mullite from the amorphous phase. The presence of SiO2 in solution in metastable Al2O3 increases the temperature of transformation to α-Al2O3 to greater than 1500° C compared with 1230° C for pure Al2O3.