Transparent MgAl2O4 spinel by double sintering process

Transparent polycrystalline ceramic with magnesium aluminate (MgAl2O4) spinel composition is an attractive material for several optical applications [1,2]. The preferred fabrication approach for large size parts, is a two stage, pressure assisted, sintering process, where a linear pressing step performed up to a level of densification at which open porosity is eliminated, is followed by a hot isostatic stage. Both in this and other fabrication procedures spinel powders are generally used. Starting with oxide mixtures - a procedure able to significantly simplify the fabrication process and reduce its cost - is avoided because MgAl2O4 formation is associated with a noticeable volume change that leads to a consistent reduction of its sinterability. Our preliminary tests showed, however, that when pressure assistance is employed along all the sintering process, the volume change is negligible and does not affect the densification. This work presents the results obtained by double sintering spinel starting from a properly balanced mixture of commercial oxide powders (as opposed to the usual already formed spinel). During the hot pressing cycle at moderate temperature and time the material densify up to the closure of the open porosity. During the hot isostatic pressing cycle at moderate pressure the full closure of the porosity is obtained, leading to a transmittance higher than 75 % for thicknesses larger than 5 mm. The advantage of this process is twofold. It allows to simplify and reduce the cost of the press-feed material preparation and to obtain highly transparent components without the use of extremely high pressures; it also limits the length of polishing operations because the use of hot pressing leads to flat surfaces. 1.A. Goldstein, Correlation between MgAl2O4-spinel structure, processing factors and functional properties of transparent parts (progress review)",J. Eur. Ceram. Soc., 32 [11] 2869-2886 (2012). 2.L. Esposito, A. Piancastelli and S. Martelli, "Production and characterization of transparent MgAl2O4 prepared by hot pressing", J. Eur. Ceram. Soc., 33 (2013) 737-747.