Ultralow Loss CaMgGeO₄ Microwave Dielectric Ceramic and Its Chemical Compatibility with Silver Electrodes for Low-Temperature Cofired Ceramic Applications

A new ultralow dielectric loss cofired CaMgGeO4 dielectric material with olivine structure was fabricated by the solid-state route. The X-ray patterns, Rietveld refinement, and microstructure revealed the characteristics of the synthesized material. CaMgGeO4 ceramic belongs to the orthorhombic system with a Pbmn space group. Sintered at 1300 °C for 6 h, the ceramic exhibited a densification of 96.5%, an ultrahigh quality factor (Q × f) of 124 900 GHz (tan δ = 1.24 × 10–4) at a frequency of 15.5 GHz, a permittivity (εr) of 6.71, and a temperature coefficient of resonant frequency (τf) of −73.7 ppm/°C, and the average coefficient of thermal expansion of CaMgGeO4 was 12.4 ppm/°C. The sintering temperature of the CaMgGeO4 ceramic was reduced from 1300 to 940 °C with the addition of 5 wt % B2O3. The CaMgGeO4 + 5 wt % B2O3 ceramics exhibited favorable microwave dielectric performances: Q × f = 102 000 GHz (at 16.4 GHz), εr = 5.80, and τf = −64.7 ppm/°C, respectively. In addition, the CaMgGeO4 ceramic did not react with Ag electrodes, which could be advantageous in low-temperature cofired ceramic multilayer microwave devices.