B₂O₃-Modified SiO₂ Microwave Dielectric Materials

SiO2 is an important insulate material. The amorphous, quartz and cristobalite phases of SiO2 have excellent microwave dielectric properties, while the preparation difficulty is a serious problem. In this thesis, B2O3 was used as the modifier due to the low melting temperature, and its effects on the sintering characteristics, phase constitution, microstructure and microwave dielectric properties of amorphous and quartz SiO2 were investigated systematically.B2O3-modified fused silica bulks with formula of xB2O3-(1-x)SiO2 (x=0.05 0.35) were prepared by the standard solid state sintering method. The content of cristobalite decreases to zero gradually with increasing x for the samples sintered at 1000 ℃. High content of cristobalite in the sintered samples with x=0.05-0.15 results in cracking due to the phase transition from high-cristobalite to low-cristobalite occuring at about 265 ℃ during the cooling process after sintering, which is accompanied by a large volume decrease of about 3.2%, and no crack is observed for x=0.20-0.35. The densification temperature decreases as well as the highest relative density with increasing x from 0.20 to 0.35. The highest Qf value was obtained at the sintering temperature significantly higher than that for the highest density, and this may be due to the more homogenous distribution of B2O3 at higher sintering temperature. The optimal properties with εr= 3.56, Qf=70,600 GHz, τf=-11.4 ppm/℃ were achieved for x=0.20 sintered at 1100 ℃ with the relative density of 95.3%.B2O3-modified quartz bulks with formula of xB203-(1-x)SiO2(x=0.05-0.35) were prepared by the standard solid state sintering. When the sintering temperature is 1000℃, high content of cristobalite in the sintered samples with x=0.05-0.15 results in cracking. Only the diffraction peaks of quartz phase are observed for x 0.20-0.35, and the intensity of diffraction peaks decreases with the increase of sintering temperature, which indicates the transition from quartz to amorphous phase during the sintering process. The densification temperature decreases with increasing x from 0.20 to 0.35, and the highest Qf value was obtained at the sintering temperature significantly higher than that for the highest density, which may be because that higher sintering temperature promotes the transition from quartz to amorphous phase and more homogenous distribution of B2O3. The optimal properties with ultra-low εr =3.76, relatively high Qf=11,000 GHz, and low τf=-13.3 ppm/℃ were achieved for x=0.30 sintered at 750 ℃ with the relative density of 98.2%. The very low sintering temperature indicates the broad possible applications in low-temperature co-firing technology.