Microwave dielectric high-entropy ceramic Li(Gd0.2Ho0.2Er0.2Yb0.2Lu0.2)GeO4 with stable temperature coefficient for low-temperature cofired ceramic technologies

High-entropy ceramics are new single-phase materials with at least four cation or anion types. Their large configurational entropy is believed to enhance the simultaneous solubility of many components, which can be used to optimize certain properties. In this work, a high-entropy oxide, Li(Gd0.2Ho0.2Er0.2Yb0.2Lu0.2)GeO4 (LRG) was explored as a microwave dielectric ceramic for low-temperature cofired ceramic technologies. The LRG high-entropy ceramic with an olivine structure formed in the sintering temperature range of 1020–1100 °C. The minimal distortion (5 × 10-4) of the [RO6] octahedron led to a stable temperature coefficient of resonant frequency (τf) of –5.3 to –2.9 ppm/°C. Optimal microwave dielectric properties were achieved in the high-entropy ceramics at 1080 °C for 4 h with a relative density of 94.9%, a relative permittivity (εr) of 7.2, and a quality factor (Q×f) of 29000 GHz (at 15.3 GHz). For low-temperature cofired ceramic technology applications, the sintering temperature of the LRG high-entropy ceramic was reduced to 900 °C by the addition of 3 wt% H3BO3, which exhibited outstanding microwave dielectric properties (εr = 7.6, Q×f = 11700 GHz, and τf = –7.4 ppm/°C) and a good chemical compatibility with silver.