Preparation And Investigation Of Porous Gradient Structure And Highly Wave-transparent Materials For Microwave Sintering

Microwave sintering is an advanced material preparation technology because of its excellent properties,such as rapid heating rate,high energy efficiency,high heating efficiency,safety and chean.Therefore,it has become new research hotspots in the field of material sintering.At present,microwave kiln has been widely used below 800?,and the use of high temperature microwave sintering kiln above 1300? is very rare,the main reason is the lack of the refractory materials with thermal shock,thermal insulation and high wave-transparent.Porous Si2N2O ceramics not only have low dielectric constant and dielectric loss,but also have excellent oxidation resistance and thermal shock resistance.Therefore,the study on the preparation and properties of porous Si2N2O ceramics can be extended in microwave kiln refractory materials as well as the application of high temperature heat-resistant components.In this paper,amorphous silicon nitride(Si3N4)was used as raw material,lithium carbonate(Li2CO3)was used as sintering aid because its low-temperature melting and high-temperature strong volatilization,which used to reduce the interphase content and facilitate intergranular phase purification.At the same time,porous Si2N2O ceramics with excellent comprehensive properties was prepared because of high-temperature volatilization of Li2CO3.The crystallization temperature of amorphous Si3N4 was 1450? under N2 atmosphere,and the crystallization products was mainly a-Si3N4.But the amorphous Si3N4 was partially crystallized into Si2N2O at 1450? while it was pre-oxidized at 1000?.After adding 2 wt.%sintering aid of Li2CO3,amorphous Si3N4 crystallize a large amount of Si2N2O phase since oxygen and sintering liquid play an important influence on the crystallized product of amorphous Si3N4.Pure phase Si2N2O ceramics was fabricated with Li2CO3(3 wt.%)at 1550? in N2 atmosphere.However,the crystal phase was single Si2N2O phase while Li2CO3 content was 2 wt.%at 1650?.On the other hand,Si2N2O was decomposed to form Si3N4 phase and SiO2 phase when the content of Li2CO3 was 5 wt.%.The porosity of Si2N2O ceramics decreased with the increase of the sintering aid content,the maximum porosity was 56.26%,and the minimum porosity was 42.39%.The mass loss rate of Si2N2O ceramics increased with the increase of the sintering aid content,and the maximum mass loss rate was 15.27%,indicating that the sintering aid were melted at low temperature and volatiled at high temperature,which not only promoted the formation of Si2N2O,but also facilitated the formation of pores.The study on the properties of the Si2N2O ceramics showed that the flexural strength,fracture toughness and elastic modulus of Si2N2O ceramics increase with the increase of sintering aid content.At the porosity of 49.78%,the flexural strength was 50 MPa,the fracture toughness was 1.34 MPa-m1/2,and the elastic modulus was 22.6 GPa.The oxidation resistance of Si2N2O ceramics was perfect,tending to be stable after oxidation for 15 h at 1400?.When Li2CO3 content was 2 wt.%,the residual strength rate of the porous Si2N2O ceramics was more than 70%at 1300?,and its thermal diffusion coefficient was 0.03 cm2/s at 980?,showing excellent thermal shock resistance and thermal insulation properties.The dielectric constant and dielectric losses of porous Si2N2O ceramics(52.64%)were 3.54 and 0.0029,respectively,which can meet the requirements of thermal insulation materials for microwave kiln.Si2N2O ceramics with controllable density gradient structure was fabricated through the control of pressure.Si2N2O/Si3N4 composites with different component gradient structure were prepared using Si3N4 buried powder.Because the N2 pressures distributed on the surface and inner areas of the samples were different.On the surface of the samples buried Si3N4 powder provided higher N2 pressure at high temperatures.The flow of the gas will also reduced oxygen content on the surface of the sample.In the inner region of the sample,the pressure of N2 was lower,and the large amount of oxygen carried by the amorphous Si3N4 allows the inner to have a higher partial pressure of O2,therefore,Si2N2O was generated in the interior of the samples.