Microwave absorption in insulating dielectric ionic crystals including the role of point defects

A theoretical model of microwave absorption in linear dielectric (nonferroelectric) ionic crystals that takes into account the presence of point defects has been synthesized and specifically applied to NaCl single crystals by considering all relevant interaction mechanisms between a harmonic electric field and single-crystal ionic crystalline solids, including ionic conduction, dielectric relaxation, and multiphonon processes. The loss factor epsilon '' f has been measured by a cavity resonator insertion technique for nearly pure and Ca2+ doped NaCl single crystals at frequencies from 2 to 16 GHz and at the temperatures from 300 to 700 K. The experimental results are in good agreement with the theoretical model. The theoretical model predicts a transition between low- and high-temperature absorption processes that may partly account for the phenomenon of thermal runaway observed during microwave processing of ceramics.