The Preparation Of Low Dielectric Constant Fused Silica Microwave Dielectric Materials

The fused silica microwave dielectric materials have a broad application prospect in the fields of high frequency microwave components and microwave substrates due to their very low dielectric constant(εr), high quality factor(Q×f) and a near-zero resonance frequency temperature coefficient(τf). In this paper, the fused silica microwave dielectric materials were prepared by the conventional solid-state ceramic method and Sol-Gel method, respectively. The effects of preparation processes, sintering atmospheres and preparing methods on the phase compositions, crystal content, microstructures and microwave dielectric properties of the materials were systematically investigated by XRD, SEM, FTIR, network analyzer etc.At first, the effects of preparation processes and annealing temperature on the phase compositions, micromophologies and microwave dielectric properties of the fused silica were investigated. The results reveal that although there have little or no cristobalite precipitate when the sintering temperature is low or the holding time is short, it is difficult to obtain high-density materials, so the performance of materials is poor. The volume density increases with increasing sintering temperature or holding time, but the exorbitant temperature or time would result in the amount of cristobalite increaseing noticeably, which leads to the internal stress increasing, surface cracks deepening or even peeling off, finally aggravate the degradation of microwave dielectric properties. In addition, the advisable annealing temperature could reduce the internal stress and heal surface cracks, thus improving microwave dielectric properties, however, an inappropriate annealing temperature might cause the formation of tridymite, which worsen rather than improve the performance. When sintered at 1300 ℃for 3h, then annealed at 1200 ℃for 3h, the fused silica shows good microwave dielectric properties: εr=3.77、Q×f =66 758GHz、τf =-6.60ppm/℃。Phase compositions, cristobalite content, micromophologies, microwave dielectric properties and crystallization kinetics of the fused silica materials in different sintering atmospheres were investigated. The results reveal that sintering atmospheres have no apparent effect on phase composition of fused silica. The vacuum, N2 and buried powder sintering have varying degree inhibition action on crystallization of fused silica and the microwave dielectric properties have improved accordingly compare with air sintering. The rank of crystallization activation energy in different sintering atmospheres from big to small is: vacuum, buried powder, N2 and air. The value of k increases as the temperature rise from 1300℃ to 1375℃, indicating that nucleate cristobalire crystallization rate increases as the temperature rise. When sintered at 1350℃ for 3h in vacuum, the fused silica have best properties: εr=3.83、Q×f =74 182GHz、τf =-6.88ppm/℃.In contrast to the solid-state reaction, smaller size, more homogenous and higher activity powder can be synthesized by Sol-Gel method, which promotes the materials sintering and lowers the sintering temperature. The sintering characteristic, phase compositions, micromophologies and microwave dielectric properties of fused silica synthesized by Sol-Gel method had been explored. There is much Si-hydroxyl and other similar imperfections on the surface of powder, which causes the bulk crystallization of fused silica but not surface crystallization. The samples prepared from finer powder have better density and properties at the same sintering temperature. When sintered at 1150℃ for 3h, the samples prepared from powder whose average particle size is about 120 nm show good properties: εr=3.59、Q×f =61 272GHz、τf =-10.0ppm/℃。...