ZnO was first cold sintered by Funa

ZnO was first cold sintered by Funahashi et al. [32]. Among solvents tested in this study, acetic acid provided the best results as it let to relative densities of 98 %, electrical conductivities equivalent to conventionally sintered materials and an activation energy for grain growth of 43 KJ. mol−1, which is at least five times lower than the one in conventional sintering [33,34]. Guillon et al. also sintered ZnO in the presence of a small amount of water (1.7 wt. %) using FAST/SPS apparatus. High relative densities were obtained, and authors proposed that the main effects of adsorbed water were a better initial packing due to reduction of interparticle friction,dissolution of Zn2+ and O2− ions on the surface particle, defect formation due to diffusion of H+ and OH− ions into the ZnO crystals, formation of a highly defective diffusion path between grains and carbonate elimination. All these effects would then be amplified by acetates [35].Recently some attempts to experimentally become closer to the conditions of a closed system were made by Kang et al. [36].