Design,preparation And Properties Of High Temperature Resistant SiO₂ Aerogel Thermal Insulation Composites

The new type of high-speed aircrafts flying in the atmosphere need to withstand long time strong aerodynamic heating,which puts forward a more stringent demand for the high-efficiency thermal insulation materials in thermal protection structures.As an important part of thermal protection structure,high-efficiency thermal insulation materials need to have the characteristics of light weight,high strength,high temperature resistance and low thermal conductivity.For the above needs,this paper presents a study on sintering behavior of acid-base two step SiO₂ aerogel thermal insulation composites under fast heating condition and proposed strategies for improving the temperature resistance of SiO₂ aerogels and thermal insulation composites.A modified acid-base two step method and monodisperse silica sol are applied to design and prepare SiO₂ aerogel thermal insulation composites with high temperature resistance and low thermal conductivity for the first time.By studying the effects of sol properties and technological conditions on the properties of SiO₂ aerogels and thermal insulation composites,the optimal process parameters for preparation of high temperature resistant SiO₂ aerogel thermal insulation composites were determined,and the high temperature resistant mechanism of high temperature SiO₂ aerogels and thermal insulation composites were studied.The main conclusions are as follows:（1）The sintering process and failure mechanism of acid-base two step SiO₂aerogels and composites at different temperature stages under rapid heating conditions was studied by in situ heating TEM combined with SEM,BET and XRD.At600～1000°C,the shrinkage of the SiO₂ aerogels occurred during the initial time of the heating process,and then the structure of SiO₂ aerogel became stable.And the shrinkage of SiO₂ aerogels was primarily caused by the fusion of aerogel particles.At 1100°C,the aerogel shrinkage can be observed all the time with the heating process continues,and finally closed to the dense state.And the shrinkage of SiO₂ aerogels was caused by the fusion of aerogel particles and the macropore collapse caused by rapid fusion of particles.And at 1200～1300°C,the aerogel will shrink rapidly and reach a dense state in a very short time（within 0.5h）.And the shrinkage of SiO₂ aerogels was caused by the rapid fusion of aerogel particles,rapid pore collapse and shrinkage of fused aerogel particles.The above heating process had little influence on the mullite fibers,and the shrinkage failure of the composites was mainly caused by the aerogel matrix.（2）The research direction of improving the temperature resistance of SiO₂ aerogel thermal insulation composites was clarified,and the strategies of improving the temperature resistance of SiO₂ aerogel thermal insulation composites was put forward.For SiO₂ aerogel matrix,it is necessary to improve the particle size of aerogel,reduce the content of small particle size,and reduce the content of macropore to improve the temperature resistance.For mullite reinforced fibers,it is necessary to select the appropriate fiber bulk density and optimize the cooperation between aerogel and fiber,so that the fiber has sufficient supporting effect in the thickness direction to improve the temperature resistance.（3）The high temperature resistant SiO₂ aerogel and thermal insulation composites were designed and prepared by a modified acid-base two step method.By increasing the R value（H₂O/TEOS molar ratio）to fully hydrolyze the TEOS and then polycondensation to obtain larger particle SiO₂ aerogels,the temperature resistance of SiO₂ aerogel thermal insulation composites was improved.The effects of sol properties and process conditions on the structure and properties of the composite samples were studied,and a better preparation process was determined.The modified acid-base two step SiO₂ aerogel thermal insulation composite had the characteristics of high temperature resistance and low thermal conductivity.In composite samples,the density of the aerogel matrix was 0.241g?cm^-3,the specific surface area was 252.36m²?g^-1,the average pore size was 21.64nm,and the mean particle size was 20.08nm.The density of composites was 0.393g?cm^-3,there was almost no shrinkage in the thickness direction after 0.5h heat treatment at 800～1100°C.Compared with the acid base two step method samples,the maximum temperature of the composite material was increased from800°C to 1100°C.The thermal conductivity at 1000°C and 1100°C was only 0.055W?m^-1?K^-1 and 0.063 W?m^-1?K^-1 respectively,which was obviously lower than the same temperature resistance grade aerogel thermal insulation composites.And the compression strength of 3%,5%and 10%strain were 0.149,0.236 and 0.438MPa respectively.（4）The temperature resistance mechanism of the modified acid-base two step method SiO₂ aerogel thermal insulation composites was studied.The skeleton of modified acid-base two-step SiO₂ aerogels was composed of larger particles.The particles were partly agglomerated and the particle size distribution was large.The large particle structure of aerogels can effectively inhibit the viscous flow between aerogel particles.Since there were many small size particles,the aerogel will appear obvious fusion at the initial heating stage at 1000°C,but it can maintain relatively complete skeleton and pore structure.At 1100°C heat treatment,the small size particles of aerogel will disappear and aerogels can retain relatively complete skeleton and pore structure in0.5h.Combined with reinforced fibers,the shrinkage of the aerogel matrix was inhibited because the fibers good supporting effect,and the composites can maintain a stable shape after 0.5 h heat treatment at 1100°C.（5）The high temperature resistant SiO₂ aerogel and thermal insulation composites were designed and prepared by a self-made monodisperse silica sol.The self-made monodisperse silica sol has the characteristics of adjustable particle size,narrow particle size distribution and small organic residue SiO₂ particles.The effects of sol properties and process conditions on the structure and properties of the composite samples were studied,and a better preparation process was determined.The monodisperse SiO₂aerogel thermal insulation composites had the characteristics of low density,low shrinkage,high temperature resistance,low thermal conductivity and high strength,which had excellent comprehensive properties.In composite samples,the density of the aerogel matrix was 0.206g?cm^-3,the specific surface area was 286.63m²?g^-1,the average pore size was 25.41nm,and the mean particle size was 20.31nm.The density of composites was 0.360g?cm^-3,there was almost no shrinkage in the thickness direction after 0.5h heat treatment at 800～1100°C,and the maximum operating temperature of the prepared SiO₂ aerogel insulation composite was up to 1100°C.The thermal conductivity at 1000°C and 1100°C was only 0.054W?m^-1?K^-1 and 0.062 W?m^-1?K^-1 respectively,which was slightly lower than that of the modified acid-base two step method samples.The compression strength of 3%,5%and 10%strain were 0.291,0.432 and 0.769MPa respectively.And the composites in supercritical drying process had near zero shrinkage（<0.5%）,which can realize the near net forming of complex heterogeneous heat insulation products.（6）The temperature resistance mechanism of the monodisperse SiO₂ aerogels and thermal insulation composites was studied.The monodisperse SiO₂ aerogels had a good three-dimensional porous structure,uniform pore size and particle size,and low particle agglomeration.The large particle structure and narrow particle distribution of aerogels were favorable for inhibiting the viscous flow between aerogel particles.Since there were little small size particles,the aerogels had no obvious structural changes at the initial stage of heat treatment at 1000°C heat treatment.At 1100°C heat treatment,the aerogels can retain relatively complete skeleton and pore structure in 0.5h.Combined with reinforced fibers,the shrinkage of the aerogel matrix was inhibited because the fibers good supporting effect,and the composites can maintain a stable shape after 0.5 h heat treatment at 1100°C.