Mechanisms and energetics in the early stages of solvent-assisted low-temperature sintering of ZnO

The Hydro/Solvothermal Sintering (HSS) is a technique enabling the low temperature sintering of ceramics and composites, by using a solvent and uniaxial pressure. This energy-efficient technique also opens new perspectives in designing new composites with tailored functional properties. While the solvent plays a key role in reducing the sintering temperature, the modeling and deep understanding of sintering mechanisms remain an open field of investigation. This study unveils the energetics and mechanisms involved in the first stage of HSS. The strategy highlighted in this paper includes: (i) the building of a hydrothermal sintering device equipped with a dilatometer to monitor the shrinkage in situ, (ii) numerical stress calculations at the contact between particles to show the suitability of the two-particle kinetic equation, and (iii) the use of anisothermal and non-conventional stepwise isothermal methodologies for the investigation of mechanisms and energetics. The model material, ZnO, was densified to high relative densities with acetic acid as a solvent, and a pressure and temperature of 320 MPa and 150 °C, respectively. With these experimental conditions, the kinetic analysis obtained from the two methodologies is consistent and implies a dissolution reaction of the material as the rate controlling mechanism with possible coupling to grain boundary sliding. The activation energy of 90 kJ.mol^—1 is determined with the different analysis methodologies. The interest of the stepwise isothermal methodology in obtaining accurate activation energy is also discussed.