ИСТИНА

Analysis of scanning electron microscopy images was used to study the changes in the crystal size distribution of zinc oxide which occurred during its processing in an aqueous medium at 220-296°C and an equilibrium vapor pressure in an autoclave. The results were compared with those of zinc oxide placed in a die for the treatment under similar conditions supplemented with mechanical pressure application. In both cases, ZnO was treated in the presence of an activating additive: either zinc acetate or ammonium chloride. During autoclaving, a powder consisting of fine ZnO monocrystals was obtained, while under mechanical pressure, a process known as cold sintering led to ceramics formation. Under vapor pressure as well as under mechanical pressure, the aqueous medium affected zinc oxide transformation by the same mechanism of solid-phase mobility activation due to the influence of the additives. The higher the content of additives in the medium as well as the higher the mechanical pressure the more pronounced activating effect was observed. Mass transfer during the cold sintering occurred mainly by the coalescence of crystals, while without mechanical pressure, the predominance of surface spreading was revealed. In the initial ZnO powder, average crystal size was of 0.2 μm. It grew up to 0.3 - 0.4 μm in a fine-crystalline powder formed in autoclave and to an average grain size of 0.26-0.8 μm in the ceramics, which relative density reached 0.83-0.98. A scheme explaining the influence of aqueous medium on the solid-phase mobility of ZnO structure was proposed.