Preparation,Microstructure And Properties Of Nano-SiC Thermal Insulation Materials

Aerogels are excellent thermal insulation materials with extremely low thermal conductivity.However,low thermal stability limited the practical applications of traditional oxide aerogels at high temperature.SiC has excellent thermal stability and sintering resistance,but its intrinsically high thermal conductivity frustrates its application as good thermal insulator.Increasing porosity and reducing particle size to nano scale are effective methods to reduce the thermal conductivity of porous SiC.There remains some difficulties on the advancements of nano-SiC thermal insulation materials,such as high-cost/complex preparation process,low synthesis efficiency,and trade-off between low thermal conductivity and high mechanical properties.In this thesis,from the perspective of pore structural design,nano-SiC raw materials with different microstructures were used to prepare highly porous nano-SiC with high thermal resistance,as well as low thermal conductivity,to balance the mechanical properties and thermal insulation property.The main results are as follows:Bamboo-like SiC nanowires with a diameter of 98 nm and a large aspect ratio were prepared.The SiC nanowires show excellent thermal stability up to 1700℃ in Ar.Flexible SiC nanowire aerogels with ultra-high porosity were prepared by freeze-drying technique.With the increase of temperature(1500℃～1700℃),the sintering between SiC nanowires becomes more serious,and the thermal conductivity of the aerogel increased slightly(0.034 W/(m·K)～0.037 W/(m·K));at high testing temperature(1000℃),the SiC nanowire aerogel also exhibits low thermal conductivity(0.156 W/(m·K)).In addition,one-dimensional SiC nanowires and the ultra-high porosity led to the excellent cyclic compression elasticity of the developed SiC nanowire aerogel.SiC nanowire film was prepared by suction filtration-induced self-assembly method and also combined with PEG to fabricate SiC/PEG phase change material.The SiC nanowire film exhibits excellent flexibility and low thermal conductivity(0.113 W/(m·K)一 0.214 W/(m·K),from room temperature～1000 ℃).By combined with PEG,the SiC/PEG composite has a low thermal conductivity of 0.38 W/(m·K),but this material exhibits the thermal management capability.The phase transition temperatures during the heating and cooling process are 58℃ and 43℃,respectively;and the phase change latent heats are 83.19 J/g and 78.19 J/g,respectively.Isotropic porous nano-SiC ceramics with porosities up to 96%were fabricated by foam-gelcasting-freeze drying method.The thermal conductivity(0.064 W/(m·K)～0.112 W/(m·K))of porous nano-SiC increases with the enhancing of sintering temperature.At high temperature(1000℃),the processed porous nano-SiC maintains the low thermal conductivity(0.15 W/(m·K)).The highest strength of porous nano-SiC occurs when the sample was sintered at 1400℃,which achieving a balance between low thermal conductivity and good mechanical properties.SiC nanowires was used to enhance the mechanical properties of the porous nano-SiC.With the increase of SiC nanowire content,the thermal conductivity of porous nano-SiC increases slightly(0.065W/(m·K)～0.067 W/(m·K)),and the mechanical properties are more obviously improved(6%～17%).Porous nano-SiC with anisotropic pore structure was fabricated by directional freezing method and has ultra-high porosity of 95%.The anisotropic structure results in the anisotropy of the properties:in radial direction,the porous nanoSiC exhibits low thermal conductivity and compressive strength(0.045 W/(m·K)and 0.83 MPa);and along axial direction,the sample shows high thermal conductivity and compressive strength(0.062 W/(m·K)and 1.39 MPa).Dispersible SiC hollow spheres with diameter of 400 nm and wall thickness of 40 nm were prepared by Mg thermal reduction method.The Mg and F impurities lead to the decreased thermal stability(T<1400℃).SiC hollow spheres with better thermal stability(～1600℃)were prepared by silicon-carbon reaction at high temperature.The wall thickness of SiC hollow spheres can be controlled(20 nm～40 nm)by adjusting the C template.The effects of sintering temperature,porosity and size of SiC hollow spheres on the mechanical properties and thermal conductivity of SiC hollow spheres thermal insulation material were studied:the thermal conductivity(0.163W/(m·K)～0.216W/(m·K))and compressive strength(2.87 MPa～3.76 MPa)were increased with the increase of sintering temperature(1400℃～1600℃);with the enhanced porosity(82%～92%),thermal conductivity(0.046 W/(m·K)～0.163W/(m·K))and compressive strength(1.73 MPa～2.87 MPa)were decreased.Reducing the wall thickness(20 nm～40 nm)of SiC hollow spheres can increase the compressive strength(1.73 MPa～1.77 MPa)and decrease the thermal conductivity(0.044 W/(m·K)～0.046 W/(m·K)).