Cold sintering with dimethyl sulfoxide solutions for metal oxides

Cold sintering of ZnO and MnO with dimethyl sulfoxide (DMSO)-based solutions is demonstrated. For ZnO ceramics, density values approach 99% theoretical when cold-sintered at 180 °C with DMSO-HOAc and DMSO-Zn(OAc) ₂ solutions. MnO densified with aqueous HOAc solutions produces ceramics of 84% theoretical density that contain significant amounts of Mn(OH) ₂ secondary phases. In comparison, using DMSO-HOAc solutions produces density values of 94% theoretical at 250 °C with trace quantities of Mn ₃ O ₄ , verified via X-ray diffraction. Scanning electron microscope analysis of sample fracture surfaces containing Mn ₃ O ₄ reveals numerous crystallites smaller than 100 nm that nucleate on or between the considerably larger starting MnO grains. With increasing temperature, these precipitates appear to coalesce and fill the porosity that remains after initial compaction. These results identify an avenue to cold sinter metal oxides that, in the presence of aqueous media, favor hydroxide formation which inhibits further densification.