Preparation and microwave dielectric properties of B ₂ O ₃ bulk

Abstract The transparent B ₂ O ₃ bulks were prepared by quenching B ₂ O ₃ melt or cooling it down to room temperature with the rate of 0.1‐1°C/min. The quenched sample exhibited the dense microstructure, while the pores were observed for low cooling rates, and this is consistent with the slight decrease in density and permittivity ( ε _r ) with decreasing the cooling rate. Both the amorphous and cubic phases of B ₂ O ₃ were indicated by XRD, and the content of the cubic phase increased with the decrease in cooling rate, which was responsible for the decreasing Qf value with the lower cooling rate. The temperature coefficient of resonant frequency ( τ _f ) was insensitive to the cooling rate, and the microwave dielectric properties with ε _r = 3.86‐3.97, Qf = 3,200‐5,300 GHz and τ _f = −48.9 to −42.8 ppm/°C were obtained in the B ₂ O ₃ bulks. The reported microwave dielectric properties of B ₂ O ₃ bulk are helpful to estimate the effects of the residual B ₂ O ₃ on the properties of the microwave dielectric ceramics with B ₂ O ₃ and H ₃ BO ₃ as the sintering aid.