Fabrication And Improved Thermal Insulation Property Of Porous Si₃N₄ Ceramics With Fluorides Additives

With the development of missiles,radomes for high-speed aircraft must have enough mechanical strength,thermal isulation and dielectric properties.Porous Si₃N₄ceramics,with low density,low thermal expansion,high strength and acceptable dielectric properties,gradually show unique advantage on the supersonic and hypersonic radome materials.In this study,different kinds of fluorides additives,polymethylmethacrylate microspheres were added as pore-forming agent and BN were used as second phase to fabricate silicon nitride ceramics.Phase composition,microstructure and mechanical properties of the samples were characterized.The influence of these factors on the porosity and flexure strength of Si₃N₄ ceramics were analyzed.Meanwhile,the thermal insulation property of the porous Si₃N₄ ceramics was improved by composting aerogels.The properties of Si₃N₄ ceramics with different fluorides additives were studied in this research.The weight loss,volume shrinkage,microstructure and mechanical properties of the sintered samples were analyzed.Fluorides additives can react with the silica to form a low temperature liquid and promote the?-Si₃N₄??-Si₃N₄transformation.During the sintering of Si₃N₄ ceramics,the SiF₄ gas and fluorides additives will release and leave behind pores in the Si₃N₄ ceramics.When the CaF₂contents reached 1.0 wt%,the porous Si₃N₄ ceramics with porosity of 35.8%,?-Si₃N₄??-Si₃N₄ transformation of 79.7%and flexure strength of 226.0 MPa was obtained.Effects of the adding pore-forming agent and second phase on the porosity of Si₃N₄ceramics were discussed,respectively.Adding pore-forming agent can easily increase the porosity of the Si₃N₄ ceramics,the transformation and aspect ratio of?-Si₃N₄increased with the increasing of PMMA content,When the PMMA contents reached 60vol%,the porous Si₃N₄ ceramic with porosity of 65.9%,?-Si₃N₄??-Si₃N₄transformation of 98.6%and flexure strength of 38.6 MPa was obtained.The addition of BN can also increase the porosity of Si₃N₄ ceramic,but the transformation and aspect ratio of?-Si₃N₄ will be impeded.When BN contents reached 25 wt%,the porous Si₃N₄ceramic with porosity of 56.8%,?-Si₃N₄??-Si₃N₄ transformation of 76.9%and flexure strength of 56.0 MPa was obtained.The thermal conductivity of the porous Si₃N₄ ceramic was reduced by composting aerogels creatively.Firstly,the porous Si₃N₄ ceramic with a porosity of 64.1%was produced by adding pore-forming agent method.Then the porous Si₃N₄ ceramics was impregnated into ZrO₂-SiO₂ composite sols under vacuum,and the supercritical drying method was used to synthesize the aerogels/porous Si₃N₄ ceramic composite.The gas was eliminated by aerogels in the composite and the thermal conductivity of the ZrO₂-SiO₂ aerogels/porous Si₃N₄ ceramic composite was decreased from 9.8 Wm^-1K^-1 to 7.3Wm^-1K^-1.Meanwhile,the low density and low dielectric constant of the porous Si₃N₄ceramics was retained after introducing ZrO₂-SiO₂ aerogels.