Impact of convection and radiation on direct/hybrid heating stability of field assisted sintering

Abstract

Field assisted sintering (FAST) processes allow a direct transmission of the heating energy to the specimen (through the electric, magnetic fields or the electrical current). FAST allows higher heating rates, faster sintering response and a better control of the sintered microstructures. However, FAST suffers high heating instability in direct heating configurations which generally takes the form of a hot spot. The origin of these hot spots is well known and is correlated to the convective/radiative cooling at the specimen surfaces and the thermal dissipation in the specimen. Nevertheless, the impact of these cooling fluxes evolves with the sample dimensions, thermal insulation, heating rate and hybrid heating conditions and there is not clear quantification of the relative importance of these fluxes in regards to the previous cited heating conditions. In this work we develop a finite element (FE) tool which can easily explore the heating stability of an “Equivalent Thermal Cavity” (ETC). We illustrate the ETC concept by the case study of the microwave sintering of zirconia. We show that the dominant heat transfer is radiative, but the convective fluxes have a high importance for the temperatures homogenization, in particular in the case of a hybrid heating configuration.