Study On The Multiple Functionality Of SiCN Ceramic Aerogel/mullite Thermal Insulation Tile Composites

With the rapid development of wireless communication technology and radar detection technology,electromagnetic interference,electromagnetic pollution and electromagnetic information leakage have become increasingly prominent.Therefore,the development of a high-performance electromagnetic wave absorbing material is of great significance to both civil and military fields.SiCN precursor ceramic aerogel not only has the advantages of designable chemical composition and easily adjustable electrical/dielectric properties of SiCN precursor ceramics,but also has the advantages of low density,high specific surface area and high porosity of aerogel,which makes SiCN aerogel possessing excellent wave absorption properties.However,the application of SiCN precursor ceramic aerogel is greatly limited because of its loose structure,high brittleness,poor mechanical properties,poor oxidation resistance and single function.In this thesis,we creatively impregnated mullite（Mullite）thermal insulation tile with SiCN precursor ceramic aerogel and used the excellent mechanical properties of mullite thermal insulation tile to solve the brittleness problem of SiCN precursor ceramic aerogel;meanwhile,on this basis,SiO₂ aerogel with good thermal stability and oxidation resistance was introduced to form nested aerogel composites to solve the problem of poor thermal stability and oxidation resistance of SiCN precursor ceramic aerogel.The main research contents include the following three aspects:（1）Preparation and microstructure characterization of SiCN_a/Mullite thermal insulation tile composites:SiCN_a/Mullite thermal insulation tile composites were successfully prepared by high pressure impregnation,sol-gel method,polymer precursor transformation method and freeze-drying technology.The composition,and microstructure of SiCN_a/Mullite thermal insulation tile composites were tuned by changing the impregnation times,molar ratio of C=C to Si-H in DVB and PSN and pyrolysis temperatures.With the increase of impregnation times,the disorder of free carbon increased,and the average pore size of the composites decreased.With the increase of the molar ratio of C=C/Si-H,the degree of order of free carbon,graphitization and pore size of SiCN_a/Mullite thermal insulation tile composites decreased.With the increase of pyrolysis temperatures,the degree of order of free carbon and graphitization of SiCN_a/Mullite thermal insulation tile composites increased.（2）The multiple functionality of SiCNa/Mullite thermal insulation tile composites:The effects of impregnation times,molar ratio of C=C to Si-H in DVB and PSN and pyrolysis temperatures on the microwave absorption,thermal insulation and mechanical properties of SiCN_a/Mullite thermal insulation tile composites were investigated.With the increase of impregnation times,the microwave absorbing properties of SiCN_a/Mullite thermal insulation tile composites decreased gradually,the thermal conductivity increased gradually,but still remained in a low range(from 0.077W·m^-1·K^-1 to 0.115 W·m^-1·K^-1),and the compressive strength increased gradually（from 2.17 MPa to 5.17 MPa）.For 1 time impregnation,the minimum reflection loss（RL）of the composites is-47.73 d B and the effective absorption bandwidth is 9.6 GHz.When the molar ratio of C=C/Si-H is 1.25:1 and 1.5:1,the minimum RL and effective absorption bandwidth of the composites are-47.73 d B and-49.66 d B,9.6 GHz and7.52 GHz,respectively.At the same time,the composites show excellent thermal insulation and mechanical properties.When the pyrolysis temperature is 1000°C and1100°C,the minimum RL of the composites is-47.73 d B and-35.97 d B,and the effective absorption bandwidth is 9.6 GHz and 8.08 GHz,respectively.The composites possess excellent microwave absorbing,thermal insulation and mechanical properties.（3）Preparation and multiple functionality of SiCNa/Mullite/SiO2 nested aerogels:Ethyl orthosilicate（TEOS）was used as precursor and SiO₂ sol was introduced into the SiCN_a/Mullite thermal insulation tile composites by high pressure impregnation.And then SiCN_a/Mullite/SiO₂ nested aerogels were prepared by sol-gel and atmospheric pressure drying.Comparing the microstructure,microwave absorbing,thermal and mechanical properties of SiCN_a/Mullite-1（as prepared in（1））and SiCN_a/Mullite/SiO₂,it is found that the minimum RL and effective absorption bandwidth of these two composites are-47.73 d B and-26.90 d B,9.6 GHz and 9.68GHz,respectively.Compared with SiCN_a/Mullite-1,SiCN_a/Mullite/SiO₂ shows better thermal stability,oxidation resistance and mechanical properties（compressive strength from 2.36 MPa to 3.16 MPa）.It shows that the introduction of SiO₂ aerogel can effectively protect and enhance SiCN_a/Mullite-1,realized the integration of multiple functionality of microwave absorbing,thermal insulation and mechanical loading.