Microwave dielectric properties of MgAl2O4–CoAl2O4 spinel compounds prepared by reaction-sintering process

MgAl2O4 spinel exhibits fascinating microwave dielectric properties, but the synthesis of dense MgAl2O4 ceramics requires high firing temperatures. In this study, Co is introduced into MgAl2O4 ceramics to improve their sinterability and microwave dielectric properties. An Mg1−xCoxAl2O4 solid solution of a spinel structure was observed in the MgAl2O4–CoAl2O4 system, and dense Mg1−xCoxAl2O4 ceramics were obtained by sintering at 1475–1500 °C in air for 2–6 h. Co addition is effective in lowering the sintering temperature to 1475 °C. Q × f of Mg1−xCoxAl2O4 ceramics was increased to 49,300 GHz with an increase in Co content to 0.2, but degraded with a further increase in Co content. The temperature coefficient of resonant frequency of Mg1−xCoxAl2O4 ceramics was sustained at between −73 and −23 ppm/°C to the variation of Co content.