The effect of mechanical pressure on the change in the crystal size distribution (CSD) during the treatment of zinc oxide in a water medium in the presence of an activating additive, such as zinc acetate or ammonium chloride, is studied by analyzing the images taken on a scanning electron microscope (SEM). The results of ZnO treatment at temperatures of 220–296°C in ceramic cold sintering processes and under similar conditions without mechanical pressure in an autoclave are compared. It is concluded that the additives to the reaction medium have an effect of the rate of exchange by water molecules between the water medium and the water bonded in the ZnO structure and activate the solid-phase mobility and redistribution of mass between crystals. Mechanical pressure intensifies the solid-phase mobility activation effect and leads to an increase in the probability of crystal coalescence and the size of a ceramic grain under cold sintering. Surface spreading is predominant in the process of treatment in water fluid medium without mechanical pressure. A mechanism is proposed for the effect of a water medium on mass transfer and the change of dispersion composition during the recrystallization of ZnO powder with the formation of new finely dispersed crystals, which differ from initial powder crystals in their properties.

—Electron microscopy was used to study the formation of a dispersed composition of a fine-crystalline powder during the recrystallization of zinc oxide in a water medium at 100–400°C. It was shown that recrystallization in condensed water is difficult but proceeds in a water medium in a vapor or supercritical state in the presence of an activator (ammonium chloride). It occurs under the influence of an introduced activator at temperatures up to 300°C and under the combined influence of the activator and thermal activation of the surface layer of crystals in the range of 300–400°C. In the recrystallization process, monodisperse ZnO powder with an average crystal size of 0.184 μm is converted into a polydisperse powder with several size-distribution components and average sizes from 0.08 to a few μm.

Analysis of scanning electron microscopy images was used to study the changes in the crystal size distribution of ZnO, which occurred during its processing in an aqueous medium at 220–255 °C and an equilibrium vapor pressure in an autoclave. The results were compared with those of ZnO placed in a die for treatment under similar conditions supplemented with mechanical pressure application in the cold sintering process. 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 the cold sintering process led to ceramics formation. Under vapor pressure and mechanical pressure, the aqueous medium affected ZnO transformation by the same mechanism of solid-phase mobility activation due to the additives’ influence. The higher the content of additives in the medium, and the higher the mechanical pressure, the more pronounced activating effect was observed. Mass transfer during the cold sintering process occurred mainly by the coalescence of crystals, while without mechanical pressure, the predominance of surface spreading was revealed. In the initial ZnO powder, the average crystal size was 0.193 μm. It grew up to 0.316–0.386 μm in a fine-crystalline powder formed in the autoclave and to an average grain size of 0.244–0.799 μm in the ceramics, which relative density reached 0.82–0.96. A scheme explaining the influence of an aqueous medium on the solid-phase mobility of ZnO structure was proposed. It was found that the addition of 7.6 mol% ammonium chloride to the reaction medium causes the processes of compaction and grain growth similar to those observed in ZnO Cold Sintering Process with the addition of 0.925 mol% zinc acetate.

The mechanism of synthesis of fine-crystalline corundum from aluminum hydroxide under induced nucleation conditions in a supercritical water fluid at 400°C and initial pressure of 26.4 MPa followed...