Preparation And Properties Of Si₂N₂O Based Wave Transmissive Ceramic Materials

The silicon oxynitride(Si2N20)material is a high-temperature structural ceramic material,which combines the excellent high-temperature resistance of traditional ceramic materials with the excellent wave-transparing properties of dielectric materials.In mechanical properties,high temperature oxidation resistance,thermal shock resistance,dielectric properties,chemical stability and other aspects,it shows excellent comprehensive performance.And it has been extensively studied in the field of high-speed aircraft.So it is a promising high-temperature structure-functional integrated material.At the same time,the silicon oxynitride material is also a special refractory material that can be applied to the advanced manufacturing technology-microwave sintering field.It has great significance to study the preparation process and performance of the silicon oxynitride material for the development of advanced manufacturing technology in China.In this paper,we used amorphous silicon nitride(Si3N4)as the base material,nano-boron nitride(h-BN)as the additive,sodium carbonate(Na2CO3)as the sintering aided to prepare silicon oxynitride ceramic in a pressureless sintering method after they were milled to powders.The effects of densification behavior of the oxynitride ceramics on microstructure and performance were investigated.We focus on the following aspects:(1)After the raw materials were mixed to powders by roller milling and high-energy ball milling,the ceramic materials with different Si2N2O phase content were prepared by pressureless sintering at 1550? and 1650? respectively.XRD,SEM and adiabatic methods were used to study the effects of different milling methods on the phase composition,distribution ratio and microstructure.The results shows when the Na2CO3 content is 6 wt.%and 9 wt.%,the content of Si2N2O phase in high energy ball milling group sintered in 1550? is 77.37 wt.%and 88.82 wt.%,respectively,But the contents in roller milling group were 53.57 wt.%and 58.64 wt.%,respectively.So it is known that high content of Na2CO3 and high energy ball milling are all beneficial to the formation of Si2N2O.The near-pure phase Si2N2O was found in the high-energy ball mill group sample sintered in 1650?,so the elevated temperature is more favorable for the formation of pure phase Si2N2O.(2)High-energy ball milling method was used to prepare ceramic materials with high Si2N2O content at 1650? by adding different contents of Na2CO3.The mechanical properties,high temperature oxidation resistance,thermal shock resistance,thermal expansion coefficient and dielectric properties tests were conducted to explore the effects of densification behavior on related mechanics and dielectric properties.It is found that at the high temperature of 1650?,the density of the material increases with the increase of Na2CO3 content,and the mechanical properties increase continuously.However,the high content of Na2CO3(more than 6 wt.%)will decompose the Si2N2O phase to form the Si2N2O/Si3N4 composite.The results of oxidation resistance showed that although the sample with 3 wt.%Na2CO3 content led to the higher oxidation weight gain rate(4.1%),the phase composition remained better.The thermal shock resistance test and dielectric property test showed that the sample with 3 wt.%Na2CO3 content has a higher thermal shock strength retention rate and a lower dielectric constant.So for superior refractory materials,the content of Na2CO3 should be controlled at 3 wt.%.(3)The porous Si2N2O/BN composite with high Si2N2O content was prepared by adding different contents of h-BN using a high energy ball milling method with 3 wt.%sodium carbonate as sintering aid in 1650?.The addition of h-BN can significantly increase the porosity,reduce the hardness,and improve the processability of the materials.At the same time,it can reduce the dielectric constant and dielectric loss of the material,and stabilize the thermal diffusivity and thermal expansion coefficient of the material.So the addition of h-BN is obviously helpful for preparing high-temperature wave-transparent materials for high-performance microwave kiln.