Evolution of the microstructure of disperse ZnO powders obtained by the freeze-drying method

Abstract

Freeze-drying, as a cryochemical powder processing method is applied in the synthesis of ZnO submicrometer to narrosized powders. The process involves rapid freezing of the sprayed precursor solution, drying under vacuum by sublimation of the Solvent and salt decomposition to oxide by thermal treatment. Calcination of dehydrated Zn(NO3)(2) was performed through destruction of the primary crystal structure, i.e. through accumulation of different defect complexes and based on this, the formation of a new state of the ZnO crystal lattice. An analysis of the microstructure evolution of zinc oxide particles in the temperature range from 548 to 898 K is described. The research was performed using differential scanning calorimetry (DSC), scanning electron microscopy (SEM), X-ray powder diffraction (XRPD), electronic paramagnetic resonance (EPR) and infrared spectroscopy (IR). It was shown that the least ("primary") coherent scattering region (580 Angstrom) and the most defective state were ZnO obtained at the lowest calcination temperature (T-c = 548 K). Increase of the calcination temperature (T-c > 548 K) favors uniting of eight closest "primary" domains into "secondary" ones. Unification is accompanied by a pronounced increase in microstrainin "secondary" domains, whose size practically does not depend on the calcination temperature in the region 573-898 K.