Apparent viscosity reduction during microwave sintering of amorphous silica

The sintering of arrays of spherical particles of amorphous SiO2 was investigated experimentally under microwave (24 GHz) and conventional heating. The materials under study were compacts formed by gravitational sedimentation of monodisperse silica microspheres (1 µm in diameter). The kinetics of neck growth between individual particles was investigated by analyzing SEM images. It was found that the rates of viscous mass transport under microwave heating were significantly higher than those under conventional heating. The values of the viscosity obtained under microwave heating were significantly (by more than an order of magnitude) lower compared to conventional heating. Possible reasons for the viscosity decrease observed under microwave heating may be associated with the influence of water vapor and the action of the electromagnetic field on impurity ions. The interrelation of the observed effect with flash sintering and microwave-enhanced mass transport is discussed.