| Nowadays, low permittivity microwave materials have attracted much attention for their applications as substrate in microwave integrated circuits. The microwave substrate material should have a low permittivity (εr), high Q·f value and near zero temperature coefficient of resonant frequency (τf). Among the microwave substrate materials, SiO2-based composite was used in the field of microwave substrates due to its excellent chemical and thermal stabilities. It’s attractive advantages such as low dielectric constant, low dielectric loss, low τf, high mechanical strength, chemical compatibility with Ag metal electrodes, no toxicity, and abundant raw materials source and low cost constituents make the SiO2ceramic as potential candidates for substrate and LTCC devices. However, the main disadvantages of SiO2ceramics are its high sintering temperature, not near zero τf and microscopic cracks limiting its practical application.SiO2nanospheres were synthesized by wet chemical methods, such as the Sol-Gel, hydrothermal method, and solvent thermal method, and then the powders were used as raw material to fabricate the SiO2based ceramics. The phase-formation and transformation, sintering behavior, microstructures, and dielectric properties of SiO2based ceramics were investigated in detail. The SiO2ceramics sintered at1550℃exhibited excellent microwave properties of εr～3.52, Q·f·92400GHz and τf～-14.5ppm/℃. The optimum sintering temperature of SiO2ceramics was reduced125℃by aqueous Sol-Gel process compared to a conventional solid-state method. The Q·f and εr values of SiO2ceramics are sensitive to the measured temperatures, which was explained by a simple composite dielectric model. TiO2coated SiO2powders were prepared by a SGS and were used as raw material to fabricate the (1-x) SiO2-xTiO2(0.1≤x≤0.3) ceramics. The0.85SiO2-0.15TiO2ceramics sintered at1200℃possess excellent microwave properties of εr～5.4, Q·f～40500GHz and τf near to0ppm/℃. The optimum sintering temperature of ceramics was reduced475℃compared to a conventional solid-state method. The sintering temperature of0.9SiO2-0.1TiO2ceramic is lowered from1200℃/3h to950℃16h by the addition of H3BO3.0.9SiO2-0.1TiO2composite ceramics mixed with10wt%H3BO3sintered at950℃/16h posses a excellent microwave dielectric properties of εr～4.67, Q·f～78000GHz and τf -0.7ppm/℃. From the X-ray diffraction and EDS analysis of cofired ceramics, the H3BO3added0.9SiO2-0.1TiO2powders does not react with Ag at950℃. The aging property, high-low temperature Raman spectra and the improvement for preparation of SiO2ceramics were investigated. The principal expariment results were shown as follows.In the first part, SiO2nanoblocks and nanospheres were synthesized using a Stober method by hydrolysis and condensation of TEOS (tetraethylorthosilicate Si(OC2H5)4) at room temperature under the acidic and alkaline conditions respectively. The powders used as raw materials to fabricate SiO2ceramics. The SiO2ceramics using nanoblock sintered at above1250℃are to be the cristobalite phase, and when the sintering temperature reaches to1600℃, have the density of2.03g/cm3(relative density of85%).The ceramics exhibited microwave dielectric properties of εr～3.13, Q·f～68000GHz and τf～-14.2ppm/℃measured at room temperature. In addition, it was found that the Q·f values of specimens increased significantly with the increase of sintering temperature. The density and microwave dielectric properties of fabricated ceramics using SiO2powder prepared in alkaline environment got improved apparently, and reached up to the optimized values of microwave dielectric properties(εr·3.52, Q·f～92400GHz and τf～-14.5ppm/℃) for specimens sintered at1550℃for3h. The optimum sintering temperature of SiO2ceramics was reduced (125℃) by aqueous sol-gel process compared to a conventional solid-state method. Microwave dielectric properties of SiO2ceramics were firstly measured at varying temperatures from room temperature to80℃.We found that the Q-f values are sensitive to the measurement temperature, due to contribution from silica medium, void volume and adsorbed water. The Q·f values measured at room temperatures after cooling down from80℃are obviously higher than those measured at original room temperatures, but lower than the ones measured at80℃.In the second part, SiO2@TiO2composite microspheres were prepared using electrostatic adsorption deposition method and sol-gel process under solvothermal conditions (SGS). Fabricatinging composites with electrostatic adsorption deposition method exhibited a not good coating effect, and then led a poor uniformity and also relatively lower microwave dielectric properties. TiO2coated SiO2powders with homogeneous coating were prepared by a Sol-Gel process under solvothermal conditions and were used as raw material to fabricate the (1-x) SiO2-xTiO2(0.1≤x≤0.3) ceramics. The0.85SiO2-0.15TiO2ceramics sintered at1200℃possess excellent microwave dielectric properties of εr～5.4, Q·f～40500GHz and τf near to0ppm/℃. The optimum sintering temperature was reduced475℃compared to that by a conventional solid-state method.In the third part, using a conventional solid-state reaction method, SiO2ceramics with addition of Li2TiO3were synthesized and their phase evolution, chemical reactions, microstructure and microwave dielectric properties were studied. The microwave dielectric properties of SiO2ceramics with addition of Li2TiO3strongly depend on the formation and the decomposed of Li2(TiO)(SiO4) intermediate phase. The tridymite phase was formed for all samples sintered above1150℃and its microwave dielectric properties were reported for the first time (εr=2.58,Q·f=258OO GHz, τf=-17.8ppm/℃). The promising ceramics of0.945SiO2-0.055Li2TiO3sintered at1050℃were found to possess good microwave dielectric properties:εr=3.21,Q·f=10180GHz and τf～0.17ppm/℃. The sintering temperature of0.9SiO2-0.1TiO2ceramic is lowered from1200℃/3h to950℃/6h by the addition of H3BO3.0.9SiO2-0.1TiO2ceramics with10wt%H3BO3sintered at950℃/6h possesses excellent microwave dielectric properties of εr～4.67, Q·f～78000GHz and τf～0.7ppm/℃. From the X-ray diffraction and EDS analysis of cofired ceramics, the H3BO3added0.9SiO2-0.1TiO2ceramic does not react with Ag at950℃.In the fourth part, the aging property, high-low temperature Raman spectra, and the improvement for preparation process of SiO2ceramics were investigated. Experimental results indicated that the εr value of SiO2ceramics was almost constant approximately equal to4.2at low frequency, slightly bigger than that of the former. After two years aging, microwave dielectric properties of SiO2ceramic did not decrease, nevertheless improved apparently compared to those as-sintered samples, but the annealing led to a decline in Q·fvalue.In the fifth part, the forsterite powders of pure phase were synthesized with a simple method for the first time, and were used as raw materials to fabricate the Mg2SiO4ceramics.The Mg2SiO4@CaTiO3composite ceramics were prepared with electrostatic adsorption-deposition and solvent thermal methods, and their microwave dielectric properties were also investigated. Although the τfof Mg2SiO4@CaTiO3could reach to near zero, its sintering temperature was very high (1300℃), and the Q·f value was equal to19000GHz. |
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