Dielectric Properties Of SiC Ceramics At Low Frequency

Being a typical covalently bonded material,silicon carbide(SiC)has been known to h ave excellent mechanical strength,good thermal stability and thermal conductivity,and high melting point.These merits make SiC a promising material used for electric device s working at high temperatures.Besides,the appealing dielectric properties of low dielectric loss and temperature-stab le dielectric permittivity of SiC in microwave frequency range make it to be a hot-topic microwave absorbing material researched worldwide.Thus far,numerous research activi ties,such as doping,compositing,and surface modification,have been performed in ord er to improve the microwave dielectric properties of SiC.To better understand the micro wave dielectric properties of SiC,a full knowledge of the dielectric properties over a wi de frequency range from below megahertz up to gigahertz is required.For example,extr insic loss part resulting from interfacial effects,caused by porosities,secondary phases,and grain boundaries,strongly affects the microwave dielectric properties.The dielectric polarization of interfacial effects known as Maxwell-Wagner polarization cannot respon se to the variations of external applied field in microwave frequency range.Hence.it can not be characterized by microwave dielectric investigations.Consequently,investigation on low-frequency(below megahertz)dielectric properties of SiC is of vital importance.However,to the best of our knowledge,little work has been reported on the low-fre quency dielectric properties of SiC.The existing few reports revealed exciting findings.For example,extremely huge dielectric constants were observed in SiC ceramics with Al2O3-MgO-SiO2 glassy insulating grain-boundary phase,SiC added BaTiO3 and PbTiO3 ceramics.Motivated by these results,we performed detailed investigations on the low-f requency(100 Hz–1 MHz)dielectric properties of ?-SiC in a wide temperature range fr om room temperature(RM)to 873 K.SiC ceramic samples are prepared by the solid-state sintering method.The XRD,SEM and XPS were used to characterize the structure,morphology and valence of the elemen t.of ?-SiC ceramic samples was measured over a wide temperature range from room te mperature(RM)to 873 K inhe low frequency(100Hz-1MHz)range.The dielectric prop erties were studied in detail.Through the measurement and analysis of the dielectric tem perature spectrum,frequency spectrum,and impedance spectrum,it was concluded that the parameters of dielectric constant,dielectric loss,and bias voltage change with freque ncy and temperature.The system's dielectric properties were analyzed.SiC ceramics are prepared by solid-phase sintering in an air atmosphere,resulting in t he formation of Oxygen vacancy,due to the non-uniform grain size and the presence of voids,and a SiO2 oxide layer on the surface.Therefore,there are vacancies and electron s in the carrier of the sample.The sample had a dipole-dominant polarization-dominant intrinsic polarization at I te mperature range which is less than 600 K,and it was concluded that the intrinsic dielect ric constant of the sample was 9.285.The dielectric constant in this zone is related to the thickness of the sample.With the thickness decreasing,its dielectric constant decreases significantly.In the II temperature range above 600 K,there are two distinctly different relaxation m echanisms,R1 and R2,respectively.In the high frequency area,the oxygen content in th e interior of the sample is much higher than the surface layer.With the temperature risin g,the oxygen vacancies in the surface layer are activated and exchange positions with th e oxygen atoms,and they jump into the interior of the sample,causing the R2 polarizing-relaxtion.R1 is a low-frequency dielectric response that is caused by Maxwell Wagner relaxation due to surface layer effects.The presence of pores has a significant effect on its dielectric properties and can be se en as insulating particles filled in a SiC matrix.The porosity is 27.64%,and the effective dielectric constant of the sample is 67% of its intrinsic dielectric constant.