Composition-Process-Structure-Property Correlation Of Fluoromica Glass-Ceramics And Ceramic Insulating Materials

Mica glass-ceramics and ceramic insulating materials have excellent machinability,insulation and chemical stability,which can be used in electronics,chemical engineering,machinery,aviation and medical fields.This work investigated the effects of different types and quantities of nucleating agents,B₂O₃ substituting Al₂O₃,sintering temperature and composition on the structure,crystallization,and properties of K₂O-SiO₂-Al₂O₃-MgO-MgF₂ mica glass-ceramics and ceramic insulating materials through X-ray diffraction analysis（XRD）,differential thermal analysis（DSC）,scanning electron microscope analysis（SEM）,infrared spectroscopy analysis（IR）,transmittance test and other analysis methods,and the main results were as follows:（1）With the substitution of MgF₂ by nucleating agent ZrO₂ or TiO₂,the transition temperature and the first crystallization peak temperature of K₂O-SiO₂-MgO-MgF₂ system glasses gradually increase.Two kinds of microcrystals,tetrasilicic fluormica(K_0.88Mg_2.5Si₄O₁₀F₂)and enstatite（MgSiO₃）,are precipitated in the glasses at different crystallization temperatures.The addition of nucleating agents has no obvious effect on the type of crystal phase,and the addition of a small amount of ZrO₂or TiO₂ can promote glass crystallization.（2）The mechanical properties of K₂O-SiO₂-MgO-MgF₂ glass-ceramics are closely related to mica crystal content and the degree of connection between crystals.The hardness of Z4T sample is the highest（5.85 GPa）,and the machinability is the lowest.The dielectric constant of glass-ceramics is higher than that of ordinary glass-ceramics at 25℃ and1MHz due to the precipitation of the non-stoichiometric compound tetrasilicic fluormica.The volume resistivity of glass-ceramics at room temperature is greater than 2×10¹¹Ω·cm,and the dielectric loss reaches10^-4 magnitude（25℃,1MHz）.The higher the content of mica crystal is,the higher the resistivity is,and the resistivity decreases obviously with the increase of temperature.（3）The introduction of proper amount of B₂O₃ in K₂O-SiO₂-MgO-MgF₂-Al₂O₃ system is beneficial to the reduction of glass transition temperature and crystallization temperature,which promotes the migration and diffusion of ions,and accelerates glass crystallization.While excessive amount of B₂O₃ has an inhibiting effect on glass crystallization.As B₂O₃gradually replaces Al₂O₃,the density and coefficient of thermal expansion（CTE）values of glasses decrease gradually.The substitution of B₂O₃ for Al₂O₃ has no effect on the type of crystal phase,and the crystallinity of glass-ceramics increases first and then decreases with the increase of B₂O₃.（4）Under the same heat treatment conditions,K₂O-SiO₂-MgO-MgF₂-Al₂O₃ glass-ceramic without B₂O₃（B0）has the smallest crystal size and the highest transmittance.At 1MHz,the volume resistivity of glass-ceramics is greater than 1×10¹¹Ω·cm,the dielectric constant is in the range of 15.76～17.38,and the dielectric loss is low(8.29×10^-4～1.162×10^-3).The variation law of electrical properties is highly consistent with that of density and crystallinity.The hardness and bending strength of glass-ceramics increase with the increase of grain size because the precipitated crystals have nanometer grain size.（5）Two kinds of glass powders（M powder and G powder）with different proportions and different chemical compositions were pressed and sintered to produce mica glass-ceramics containing fluoromica and forsterite microcrystals.With the increase of M glass powder,the content of precipitated crystal increases.The increase of sintering temperature is beneficial to the precipitation of mica crystals.Sintering temperature and compositions have no effect on the type of crystal phase,but have a certain effect on the degree of crystallinity.At the same sintering temperature,the crystallinity of ceramics increases first and then decreases with the increase of M glass powder content.For the sample with the same composition,the crystallinity increases obviously with the increase of sintering temperature.（6）The increase of sintering temperature is beneficial to decrease the porosity of the ceramics,resulting in the obvious growth of grains and the increase of the degree of connection between crystals.The density of ceramics is mainly concerned with crystal content,and the density of ceramics with high crystallinity is also higher.The dielectric constant of ceramics at 1MHz is in the range of 6.43～7.27.Higher crystallinity,grain size and lower porosity all increase the dielectric constant.With the increase of sintering temperature,dielectric constant decreases first and then increases.Due to the relaxed polarization,the dielectric loss of ceramics increases with the increase of frequency,and the resistivity of ceramics is greater than 2×10¹¹Ω·cm at room temperature.