abstract_textpThe transparent B2O3 bulks were prepared by quenching B2O3 melt or cooling it down to room temperature with the rate of 0.1-1 degrees 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 (epsilon(r)) with decreasing the cooling rate. Both the amorphous and cubic phases of B2O3 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 (tau(f)) was insensitive to the cooling rate, and the microwave dielectric properties with epsilon(r) = 3.86-3.97, Qf = 3,200-5,300 GHz and tau(f) = -48.9 to -42.8 ppm/degrees C were obtained in the B2O3 bulks. The reported microwave dielectric properties of B2O3 bulk are helpful to estimate the effects of the residual B2O3 on the properties of the microwave dielectric ceramics with B2O3 and H3BO3 as the sintering aid./p/abstract_text
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