Synthesis of nano [alpha]-alumina powders using hydrothermal and precipitation routes: a comparative study

Two different synthesis methods, viz. precipitation and hydrothermal treatment, were used to synthesise ultra-fine α-alumina powders from aluminium chloride, ammonia solution and TEAH (tetraethyl ammonium hydroxide). XRD, BET surface area analysis, TEM and FEG-SEM were used to characterise the powders produced. The presence of industrial α-alumina powder as seed particles did not affect the transformation to α-alumina phase during the hydrothermal treatment at 220 C, either in basic or acidic environments. The results obtained from the precipitation route, however, showed that the combined effect of adding α-alumina seeds and surfactants to the precursor solution could lower the transformation temperature of α-alumina from about 1200 C for unseeded samples to about 800 C, as well as reducing the level of agglomeration in the alumina powders. The difference in transformation temperature mainly results from the nucleation process caused by the α-alumina seeds, which enhanced the θ→α transformation kinetics. The lower level of agglomeration present in the final powders could be due to a surface modifying role of the surfactants, preventing the particles from coalescing during the synthesis process.