From nanoporous to transparent MgAl2O4 spinel-Nanostructural flexibility by reaction densification of metastable powders

Polycrystalline magnesium aluminate spinel (MgAl2O4) is one of the most important structural ceramics with proven applications as a hard, temperature resistant material and as a high strength optical window. Here, we present a study of the reaction densification of nano-sized MgO (nMgO) and metastable gamma-Al2O3 to form MgAl2O4 instead of the typical stable alpha-Al2O3 and micrometer-sized MgO (mu MgO). We show that the reaction kinetics of gamma-Al2O3 - mu MgO are substantially faster and occur at significantly lower temperatures compared to previous kinetic studies of alpha-Al2O3-mu MgO, revealing that they are not controlled by the same mechanism. We propose that the enhanced reaction rates are caused by short diffusion distances, faster diffusion through defected gamma-Al2O3 as well as similarities in crystal structure between products and reactants. Lastly, we show that understanding the reaction and densification kinetics of this reactant combination can be leveraged to produce a wide range of microstructures leading to nanoporous high temperature energy absorbing materials and highly dense optically transparent spinel. (C) 2021 The Authors. Published by Elsevier Ltd.