Sintering behavior and microwave dielectric properties of a new low-permittivity ceramic system Ca(Mg1−xAlx)(Si1−x/2Alx/2)2O6

The diopside ceramics with a formula of Ca(Mg_1−xAl_x)(Si_1−x/2Al_x/2)₂O₆ (x=0.01–0.3) were synthesized via a traditional solid-state reaction method, and their solid solubility, sintering behavior and microwave dielectric properties were investigated. The results revealed that the solubility limit of Al₂O₃ in Ca(Mg_1−xAl_x)(Si_1−x/2Al_x/2)₂O₆, which is defined as x, was between 0.15 and 0.2, and a second phase of CaAl₂SiO₆ presented when the x value reached 0.2. Appropriate Al³⁺ substitution for Mg²⁺ and Si⁴⁺ could promote the sintering process and lower the densification temperature, and a broadened densification temperature range of 1250–1300 °C was obtained for the compositions of x=0.08–0.15. With the increase of the x value, the dielectric constant (ε_r) increased roughly linearly, and the temperature coefficient of frequency (τ_f) showed a rising trend. The Q×f values increased from 57,322 GHz to 59,772 GHz as the x value increased from 0.01 to 0.08, and then they were saturated in the range of x=0.08–0.2. Further increase of the x value (x≥0.25) deteriorated the microwave dielectric properties. Good microwave dielectric properties of ε_r=7.89, Q×f=59,772 GHz and τ_f=−42.12 ppm/°C were obtained for the ceramics with the composition of x=0.08 sintered at 1275 °C.