The Direct Cold Sintering of α-Al2O3 Ceramics in a Pure Water Medium

Porous alpha-Al2O3 ceramics are a highly sought-after material with a multitude of applications; for example, they are used as filters, substrates, biomedicine materials, etc. Despite the availability of raw materials, a challenge associated with this technology is the high energy budget caused by sintering above 1500 degrees C. For the cold sintering processing (CSP) of ceramics, lowering the alpha-Al2O3 sintering temperature is one of the most urgent challenges in the background of its rapid development. This paper is the first to demonstrate a solution to this problem using the CSP of alpha-alumina ceramics in the presence of pure water as a transient liquid. The manufactured materials were examined using XRD analysis; the evolution of their microstructures during CSP was revealed by SEM; and the porosity was evaluated using the Archimedes method. Ceramics with an open porosity up to 36% were produced at 380-450 degrees C and 220 MPa in 30 min. An increase in the pressure was found to impede alpha-Al2O3 formation from gamma-AlOOH. The development of the microstructure was discussed within the framework of the dissolution-precipitation model and homogenous nucleation. The results of the SEM study pointed to the coalescence of gamma-AlOOH grains during CSP.