Evolution of porous structure and free-volume entities in magnesium aluminate spinel ceramics

Porous structure evolution in functional humidity-sensitive MgAl2O4 spinel ceramics prepared at different sintering temperatures is studied. It is shown that the increase in sintering temperature from 1100 to 1400 °C results in the transformation of the pore size distribution in ceramics. It is established that porous ceramics with tri-modal pore size distribution including open macro-, meso- and micropores up to a few nanometer is humidity sensitive in the 25–95% relative humidity range. It is demonstrated that positron annihilation lifetime spectroscopy supported by four-component fitting procedure is a promising tool for nanostructural characterization of MgAl2O4 spinel-type ceramics. Microstructure of these materials is improved with the increase in sintering temperature; the effect mainly results in the decrease of the additional near grain boundaries phases amount. These phase extractions and free-volume entities in ceramic structure serve as specific trapping centers for positrons penetrating ceramics. It is demonstrated that Tao–Eldrup model can be adequately used for the calculation nanopores size in MgAl2O4 ceramics using o-Ps lifetimes of the third and the fourth components.