Sintering Magnesium Silicate Ceramics Under High Pressure And Its Microwave Dielectric Properties

The main purpose of this thesis is to explore a new method for preparing microwave dielectric ceramics under high pressure. On the basis of the applications of microwave dielectric ceramics, a project of rapid fabrication for magnesium silicate(MgSiO3) microwave dielectric ceramics is presented. The method for preparing microwave dielectric ceramics is taking magnesium oxide(MgO) and silicon dioxide(SiO2) as raw materials, and the cubic large volume press is the experimental platform.We calibrated the cell pressure and the temperature of cubic large volume press before sintering experiments, then compared our results with the former calibration results. The results confirmed the square ratio relationship between the cell pressure and the diameter of cubic press piston.Seven series of MgSiO3 ceramics were prepared with different sintering conditions. The properties of MgSiO3 ceramics prepared under high pressure were measured including the density, SEM, XRD, bending strength(BS) and dielectric parameters. The results of this thesis as follows:1. We achieved MgSiO3 ceramics within several minutes with appropriate pressure and temperature conditions. The crystal structure of MgSiO3 could convert in the high-pressure sintering process. MgSiO3 prepared at atmospheric pressure was monoclinic, however, its crystal structure transforms to orthorhombic when the sintering pressure was 4GPa and the sintering temperature was higher than 1100 degree centigrade.2. Higher the sintering temperature and longer sintering time can both promote the growth of MgSiO3 crystal, however, higher sintering pressure will decrease the growth of MgSiO3 crystal. With the smaller of the crystal, the density and bending strength of the ceramic will be larger.3. The minimum value of dielectric constant of MgSiO3 ceramic sintered in high pressure was 6.7, which had met requirements of the performance of microwave dielectric ceramic and sintered body with good mechanical strength.4. The dielectric constant was smaller and the quality factor was relatively higher when sintered crystal has larger size and higher density. The abnormal crystal growth led to uneven distribution of crystal and crystal combine circumstances changed, the frequency fluctuation would be more apparent while the quality factor increased.With high-pressure facility, this research had achieved high density MgSiO3 ceramic in a short time. On the one hand, it indicated that the high-pressure as supplementary condition can greatly improve sintering efficiency and provide a new reference method for producing dielectric ceramic. On the other hand the dielectric loss of MgSiO3 ceramic was relatively larger, so we should try to avoid the formation of MgSiO3 in the process of sintering Mg-Si-O system ceramic.