Solar sintering of alumina ceramics: Microstructural development

Alumina powders were lab-synthesized and then sintered on a solar furnace (SF) in order to test the capability of these solar devices to produce dense ceramic bodies. The special configuration of the SF at Plataforma Solar de Almería (PSA-CIEMAT) in Spain, allowed to perform several experiments using high temperatures (up to 1780 °C), fast heating rates (50 and 100 °C min −1) and different atmospheres (air, Ar and 95N 2:5H 2). For comparison, similar alumina samples were sintered in an electric furnace (EF) using standard conditions (5 °C min −1 at 1600 °C during 240 min in air). An exhaustive microstructural characterization by scanning (SEM) and transmission (TEM) electron microscopies were performed on the sintered materials. Results for SF-samples showed a well-sintered alumina matrix of polyhedral grains even using shorter dwell times and higher heat-up rates than the conventional sintering. Obtained microstructures are in agreement with the presence of some impurities (mainly SiO 2, CaO, ZrO 2 and MgO) which are distributed at grain boundaries, triple points and matrix voids. For solar treatments, the variations of sintering parameters produced significant changes on matrix grain size, porosity and distribution of second phases. An important grain growth and density increase was observed after solar sintering on those tests performed at 1780 °C and under N 2:H 2 sintering atmosphere. The gathered data point out once more the convenience of SFs as sintering reactors to obtain ceramic materials with improved grain sizes.