Synthesis and doping of fine-crystalline corundum in sub- and supercritical conditions

Abstract

In the report the results of investigations of the mechanism and kinetics of the formation of cobalt doped corundum in water vapor medium are represented. Moreover, it is given the study result of changing of lattice position of cobalt ions and their state during synthesis. Aluminium hydroxide was using as raw material. Doping agent was cobalt nitrate. The doping of synthesis products (boehmite and corundum) occurs during their formation in solvothermal conditions and then in supercritical water fluid. The additives with reducing or oxidizing properties were brought in reaction medium. The products of the synthesis doped with cobalt were explored by various physicochemical methods: the X-ray phase-analysis, DR-spectroscopy, the magnetometry and SEM. It was found that the processes of the lattice rearrangement and doping take place simultaneously. In solvothermal conditions, below water critical point cobalt ions are initially chaotically distributed in the interlaminar space of the boehmite structure. At supercritical conditions the doped boehmite transforms into cobalt doped corundum. Such crystals of corundum exhibit superparamagnetic properties. After heating in vacuum (1900°C) the corundum doped with higher concentrations of cobalt acquire the ferromagnetic properties. The authors guessed that cobalt ions aggregate into the nano-sized particles of cobalt in vacancies complexes of corundum structure.