Study On Fabrication And Properties Of Wave-Transparent Materials With Porous Si₃N₄

With the development of aerospace technology,aircraft require wave-transparent with higher performance.Ceramic and ceramic matrix composite materials have become focus of the study in many countries for their superior properties.The strength,hardness,wear resistance,chemical resistance,thermal shock resistance and electric insulation of silicon nitride are very excellent,so it is regarded as the most promising new wave-transparent materials.The porous ceramic of silicon nitride in this present study was prepared by GPS with La₂O₃ and Sm₂O₃ as sintering aids,which was based on the superior dielectric properties and mechanical performance of Si₃N₄ and BN.The effects of sintering additives and BN on the mechanical properties,themal properties and microstructure dielectric properties are analyzed in detail.The results show that the porous silicon nitride was fabricated successfully with single rare earth oxides(La₂O₃ or Sm₂O₃) as sintering aids.The density,bending strength and fracture toughness of ceramic composites with La₂O₃(5%) as aids is higher,density is 2.43g/cm³,bending strength is 386.61Mpa,fracture toughness is 5.33MPa·m^1/2;but ceramic composites with Sm₂O₃ as aids have high porosity and low permittivity,porosity is 29.86%,permittivity is 3.97.It is discovered that the porosities are structured by the pillarβ-Si3N4 grain size through SEM analysis.The properties of high strength and less defects of and its bridging structure resistant to cracks are the main reasons for high mechanical performance.The dielectric constant and dielectric loss have close relationships with porosities,they tend to go down with the increase of porosities.For improving dielectric and thermal properties of Si₃N₄ wave-transparent materials, Si₃N₄/BN ceramic composite materials was prepared by adding BN to formula based on front test.BN owned better thermal stability,lower dielectric constant and lower dielectric loss than Si₃N₄,its decomposition temperature is 2460℃.Boron nitride can improve overall performance of materials.Study shows that,the addition of the power material of BN can reduce the dielectric constant and improve the thermal shock resistance of the materials effectively,but it will also block the GPS process by its layered structure.When the amount of BN is 25%(5%La2O3 for additives),the parameters of properties are as follows:bending strength is 101.83Mpa,fracture toughness is 1.84MPa·m^1/2,the dielectric constant is 3.81,thermal cycles(at room temperature to 900℃) up to more than 9.