ConclusionsMoO3 can not be well densified by conventional sintering with a relative ρ ˜ 64.5% obtained at 680℃ giving a εr ˜ 7.58, a Qf ˜ 35,000 GHz and a TCF ˜ − 39 ppm/°C. Water-assisted cold sintering method increased the relative density of MoO3 ceramic to ˜ 78%, which was further improved to ˜ 83% by the addition of (NH4)Mo7O24•4H2O. However, higher temperature annealing was still required to eradicate secondary phases and / or and increase Qf. Calculated εr of MoO3 using Shannon’s additive rule is ˜ 10.96, which is 10% larger than that of the optimum cold sintered samples. Although relative ρ of MoO3 did not achieve > 95%, the methodology of introducing a cold sintering step for materials that are difficult to be sintered is adequately demonstrated.