The effects of Al2O3 addition on both the sintering behavior and microwave dielectric properties of PbO-B2O3-SiO2 glass ceramics were investigated by Fourier transform infrared spectroscope (FTIR), differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that with the increase of Al2O3 content the bands assigned to [SiO4] nearly disappear. Aluminum replaces silicon in the glass network, which is helpful for the formation of boron-oxygen rings. The increase of the transition temperature Tg and softening temperature Tf of PbO-B2O3-SiO2 glass ceramics leads to the increase of liquid phase precipitation temperature and promotes the structure stability in the glasses, and consequently contributes to the decreasing trend of crystallization. Densification and dielectric constants increase with the increase of Al2O3 content, but the dielectric loss is worsened. By contrast, the 3% (mass fraction) Al2O3-doped glass ceramics sintered at 725 °C have better properties of density ρ=2.72 g/cm3, dielectric constant εr=6.78, dielectric loss tan δ=2.6×10-3 (measured at 9.8 GHz), which suggest that the glass ceramics can be applied in multilayer microwave devices requiring low sintering temperatures.
CaO-B2O3-SiO2 (CBS) glass powders are prepared by conventional glass melting method at different melting temperatures, whose properties and microstructures are characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is found that there are SiO2 and some unknown phases in CBS glass melting liquid from 1 300 °C to 1 500 °C and the amount of these phases decreases with the increase of the melting temperature. The CBS glass melted at 1 450 °C could be sintered from 830 °C to 930 °C and the bulk densities of the samples are all higher than 2.4 g/cm3. From the points of the properties and energy conservation, the melting temperature of 1 450 °C is the optimal melting temperature. The glass ceramic sintered at 850 °C exhibits better dielectric properties:εr=6.33, tan δ=2.2×10-3 at 10 GHz, and the major phases of the samples are CaSiO3, CaB2O4 and SiO2.
The effects of alkali oxides(Na2O and K2O) addition on both the sintering behavior and dielectric properties of Ca-Al-B-Si-O glass/Al2O3 composites were investigated by Fourier transform infrared spectroscopy(FTIR),differential scanning calorimeter(DSC),X-ray diffractometry(XRD) and scanning electron microscopy(SEM).The results show that the increasing amount of alkali oxides in the glass causes the decrease of [SiO4],which results in the decrease of the continuity of glass network,and leads to the decrease of the softening temperature Tf of the samples and the increasing trend of crystallization.And that deduces corresponding rise of densification,dielectric constant,dielectric loss of the low temperature co-fired ceramic(LTCC) materials and the decrease of its thermal conductivity.By contrast,the borosilicate glass/Al2O3 composites with 1.5%(mass fraction) alkali oxides sintered at 875 °C for 30 min exhibit better properties of a bulk density of 2.79 g/cm3,a porosity of 0.48%,a λ value of 2.28 W/(m·K),a εr value of 7.82 and a tan δ value of 9.1×10-4(measured at 10 MHz).
