Relative ρ, εr and Qf of MoO3 ceram

Relative ρ, εr and Qf of MoO3 ceramics prepared via solid state reaction method as a function of sintering temperature (640 ˜ 740℃) are shown in Fig. 1a. εr increased from 6.75 to 7.58 as sintering temperature increased from 640℃ to 680℃ and then decreased a little bit with further increase in sintering temperature. Qf reached a maximum value of 35,000 GHz at 660℃ and remained almost constant for higher sintering temperatures. TCF was ˜− 39 ppm/℃ and therefore the overall MW properties were similar to those reported [13]. Although the properties are attractive for the development of MW ceramics, MoO3 pellets could not be well densified even at tempertaures close to its melting point, only with a maximum relative ρ ˜ 64.5% at 680 °C. An optical image of an MoO3 pellet sintered at 680 °C and the schematic of crystal structure are shown in Fig. 1b. The pellet has a green-yellow color, similar to the starting powders. Conversely, some grains are observed which are > 100 μm in diameter which indicates that grain growth is possible but not uniform throughout the ceramic, resulting in poor mechamical strength and low fracture toughness. All the results suggest MoO3 can not be well sintered using a conventioanl solid-state method even though high Qf is observed. Moreover, its true εr is likely to be far higher than the measured value (˜ 7.58).