Impact of concentration and Si doping on the properties and phase transformation behavior of nanocrystalline alumina prepared via solvothermal synthesis

Solvothermal reaction of 20 g aluminum isopropoxide (AIP) in mineral oil at 300 °C for 2 h gave χ-alumina showing high thermal stability while the reaction with higher amounts of starting AIP (30 and 40 g) contributed contamination of pseudoboehmite. The χ-alumina thus obtained directly transformed into α-alumina completely at approximately 1400 °C bypassing the other transition alumina phases whereas some part of the contaminated product transformed to γ-alumina through θ-alumina and finally α-alumina. When silicon was doped in the alumina matrix (5, 10, 20 and 50 at.%) using tetraethylorthosilicate as the silicon (Si) precursor, χ-alumina was still observed without any contaminations at low concentration doping (5–20 at.%). Amorphous structure was obtained by doping 50 at.% Si. The phase transformation temperature was shifted to the high temperature after loading the Si. The α-phase transformation did not go to completion even after calcinations at 1500 °C. This could be due to the incorporation of Si atom in alumina lattice forming SiO2–Al2O3 solid solution.