Comparative Study of Cold Sintering Process and Autoclave Thermo-Vapor Treatment on a ZnO Sample

Abstract

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 Role of the Activator Additives Introduction Method in the Cold Sintering Process of ZnO Ceramics: CSP/SPS Approach

“…In addition, in another previous work [21] it was shown that there is a similarity in the conditions of the crystal structure transformation of ZnO in both the water vapor medium at CSP and the autoclave thermovapor treatment (TVT).…”

“…The most detailed data on the effect of the type and amount of the activating additive, temperature, pressure, and powder characteristics on the density and grain size of cold sintered ceramics are available for ZnO [14][15][16][17][18][19][20][21][22]. However, the degree of influence of the activator's introduction method, when using water as a transport phase, remains poorly studied.…”

“…This additive facilitates the interaction of crystals with the medium. The solid-phase mobility appears due to exchange processes between crystals and the medium [21,23]. At the same time, mass transfer and crystal growth begin.…”

“…At the same time, mass transfer and crystal growth begin. The interaction of the activating additive with the initial powder during the TVT process leads to the surface and volume changes in the powder particles crystal structure [21,23,24]. The use of a raw material previously subjected to TVT can drastically change the results obtained after CSP compared to other traditional methods of activator introduction that are applied just before starting the CSP, such as direct powder mixing with water solution [22]; mixing ceramic and activator powders with subsequent water addition [25], and direct solution injecting into the die [14].…”

“…However, subsequent studies have shown that more complex mechanisms are involved in the process of cold sintering, which is not yet fully understood [14,26]. Afterwards, it was noted that there is a problem in the re-deposition model: dissolution and precipitation are exclusively surface events and can only lead to particle enlargement but cannot contribute to their compaction [14,21]. Guo et al [12] established that the liquid volume necessary for the formation of ceramics in CSP is only approximately 5 vol.% of powder, and the liquid was called "The transport phase", since the analogy of liquid-phase sintering.…”