Designing high-temperature thermal insulators based on densification-resistant in situ porous spinel

In situ formation of magnesium aluminate spinel (MgAl2O4) based on Al2O3 and MgO sources is an expansive reaction that hinders its densification. Although its side effects have been extensively investigated for dense materials, its potentially useful role to produce refractory porous structures remains unexplored. In this study, spherical clusters of brucite (Mg(OH)2) nanoparticles were synthesized and combined with Al2O3 in different ratios for producing porous alumina-spinel-MgO structures. The evolution of their microstructure and physical properties was followed in the 300−1500 °C range. Different porogenic mechanisms related to particles' packing, dehydroxylation of brucite, MgO consumption in the reaction with alumina (Wagner's mechanism leading to the formation of Kirkendall voids) and expansive spinel formation were identified during initial heating. Variations in Mg(OH)2-Al2O3 ratio and sintering temperature resulted in structures of tailored porosity, compression strength, thermal conductivity and pore sizes in the 1−5 μm range, suitable for materials to be used at high temperatures.