Cold Sintering of Zinc Oxide (ZnO) and Silica (SiO2)

Cold sintering was investigated as a potential densification method for silica polymorphs. Initial experiments achieved a porosity of 5.1 percent when using amorphous silica powder in 25 wt percent of 5-M sodium hydroxide (NaOH) solution at 250 deg C and 500 MPa for 24 h. A design of experiments study investigated the influence of pressure, silica phase, timing of the heating relative to load, and NaOH concentration on densification. Higher NaOH concentrations and applying pressure before heating helped maximize density. A two-way interaction between either NaOH concentration and powder phase or pressure and heating timing was also identified. The dependence on silica phase and chemistry were explored by comparing the cold sintering of neat and metal ion doped amorphous silica and the cristobalite and quartz phases of silica under the same conditions. Cold sintered amorphous silica had a final mean porosity of 8.0+- 2.7 percent, compared to quartz s 19.1 percent and cristobalites 12.5 percent. X-ray diffraction determined that all sintered specimens comprised primarily of quartz, but cristobalite could be retained with temperatures below 175 C. Cold sintering of silica and its polymorphs to more than 90 percent relative density appears feasible, but phase transformations to quartz appear to be a feature of the densification process with a NaOH solution.