Novel water-assisting low firing MoO ₃ microwave dielectric ceramics

MoO ₃ ceramics can not be well densified via conventional solid state method and a low relative density (ρ) was obtained (˜64.5% at 680 °C) with a permittivity (ε _r ) ˜ 7.58, a quality factor (Qf) ˜ 35,000 GHz and a temperature coefficient of resonant frequency (TCF) ˜ − 39 ppm/°C. However, cold sintering at 150 °C using 4 wt. % H ₂ O at 150 MPa enhanced densification and give a relative ρ ˜76.8% and ε _r ˜ 8.31 but with a Qf of only ˜ 900 GHz. The addition of (NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O further improved densification to give a relative ρ ˜ 83.7% after annealing at 700 °C, resulting in a ε _r ˜ 9.91 with a Qf ˜ 11,800 GHz. We conclude therefore that oxides that are difficult to be sintered via a conventional solid state route may benefit from cold sintering but despite the higher density, lower Qf cannot be avoided due to the impurities and grain boundary phases that are introduced.