Li2O‐doped MgAl2O4 nanopowders: Energetics of interface segregation

Abstract

AbstractManufacturing nanoceramics is challenging owing to the instability of the grain size resulting from the high driving force toward growth associated with the interfaces. Nanometric ceramics of some oxides have exceptional mechanical and optical properties, eg, magnesium aluminate spinel (MgAl2O4). The production of these fully conformed ceramics requires a precursor powder, which generally contains sintering‐promoting additives. Li salts are typically used as sintering promoters for MgAl2O4, but the interface stability associated with the segregation of the additive is poorly understood. In this study, MgAl2O4 samples containing 0‐2.86 mol% Li ions were synthesized via a simultaneous‐precipitation method in an ethylic medium and subsequently calcined at 800°C in air. The nanopowders exhibited only the MgAl2O4 phase, and the crystallite size was determined by the Li2O concentration. The crystallite size was changed via the chemical modification of the interfaces by the segregation of Li ions. The solubility in the bulk material was very low at the fabrication temperature, and small amounts of Li ions saturated the bulk material and segregated to the grain boundaries (GBs), significantly stabilizing the grain–grain interface compared with the surface. The resulting powder was then aggregated further owing to the initial stage of sintering. The surface excess obtained via the selective lixiviation method confirmed that the segregation to the GBs was greater than that to the surface. Energetics calculations confirmed these results, indicating a high enthalpy of segregation at the GBs () compared with that at the surfaces (). The enthalpy of segregation together with the interface excess allowed us to estimate the reduction in the interface energy with Li+ segregation of 0.8% to the surface and 11.2% to the GBs. The Li+ segregation to the surfaces started by Al3+ substitution, and for powders with ≥1.8 mol% Li ions, Mg+2 was preferentially substituted at the surfaces.