Electrochemical Impedance Spectroscopy of Transparent Polycrystalline Magnesium Aluminate (MgAl2O4) Spinel

Transparent polycrystalline magnesium aluminate spinel is one of few materials capable of fulfilling certain demanding optical applications. However, LiF additive typically required to enhance sintering and impart transparency also degrades mechanical properties and causes scatter, precluding wider application. To shed light on the cause of altered properties, electrochemical impedance spectroscopy was performed between 500 degrees C and 900 degrees C, in oxidizing and reducing atmosphere, on fully dense, polycrystalline spinel compacts hot-pressed with and without LiF. In combination with electron microscopy, chemical analysis, and secondary-ion mass spectroscopy, bulk and grain-boundary dielectric behavior were related to microstructure and chemistry. Decades higher conductivity than for comparable single crystals correlated with Al2O3-rich stoichiometry, impurities, and small grain size, and suggested increased conduction and current-line detouring along Mg-depleted, impurity-rich, field-parallel grain boundaries. LiF addition reduced conductivity by one decade and increased the activation energy for conductivity, attributed to impurity removal, larger grain size, and point defects caused by lithium incorporation.