Study On Surface Oxidation And SiO₂ Coating Technology Of Carbon Fiber Porous Material

The thermal protection of aircraft has always been the top priority in the aerospace field,while traditional large-area thermal protection materials for spacecraft are usually made of rigid ceramic fiber insulation tiles.With the rapid development of aerospace science and technology,the speed of aircraft is getting faster and faster,and the service environment is getting worse and worse.Increasing the maximum use temperature and surface heat emissivity of materials has gradually become the focus and focus of current research.Carbon bonded carbon fiber composites(CBCF)have received widespread attention due to their low density,high porosity,and excellent high temperature resistance.However,the oxidation resistance of carbon fiber is low.Under high temperature conditions,the composite material will reduce the overall performance due to low oxidation resistance.Therefore,it is necessary to improve the high temperature oxidation resistance of carbon fiber.At the same time,because the surface of carbon fiber is chemically inert and lacks chemically active functional groups,it is necessary to perform surface pre-oxidation treatment before the oxidation-resistant modification of carbon fiber.By introducing a certain number of polar functional groups on the surface of carbon fiber,Its surface activity,in turn,makes the subsequent anti-oxidation modification effect better.In this project,by combining simulation and experiment,the interface model of silanol molecule and carbon fiber was firstly built by Materials Studio software,and then the binding energy of silanol molecule and carbon fiber when different polar groups were introduced on the surface was calculated by molecular dynamics principle.According to calculation,when the group introduced on the surface of carbon fiber is carboxyl,the adsorption effect between carbon fiber and silanol molecules is the best,and the binding energy is-1654.51kcal/mol.Then the carbon fiber was preoxidized on the surface,and the surface of CBCF was preoxidized mainly by liquid phase oxidation and gas phase oxidation.Among them,the liquid phase oxidation method respectively uses concentrated nitric acid and hydrogen peroxide two kinds of oxidants to carry on the test treatment,the gas phase oxidation method is mainly air gas phase oxidation.Through the characterization of the treated CBCF,it can be seen that the three surface treatment methods can effectively increase the surface polar oxygen-containing group content of carbon bonded carbon fiber composites.In contrast,nitric acid treatment is the best.The content of oxygen element and carboxyl group on the surface of fibers were increased to 11.76% and 6.43% when treated with 68wt% nitric acid solution at60? for 1h.The effect of 30wt% hydrogen peroxide treatment was the next,and the effect of gas phase oxidation treatment was the worst.The mechanical properties of the fibers modified by the three treatment methods were characterized,and it was found that the mechanical properties of the fibers were damaged greatly by nitric acid treatment.Finally,the carbon fiber skeleton material treated by hydrogen peroxide liquid phase oxidation method was selected as the matrix material.Finally,the pre-oxidized CBCF was modified by silane in situ.CBCF was modified in situ by sol-gel method with methyl trimethoxysilane(MTMS),dimethyldiethoxysilane(DMDES)and ethyl orthosilicate(TEOS)as precursors and ethanol and deionized water as solvents.The effects of curing temperature,catalyst type,hydrolysis time and precursor ratio on the in-situ modification of CBCF were investigated.The results show that different curing temperatures have obvious influence on the coating effect.When the curing temperature is 140?,the coating effect is the best,and the silicon content on the fiber surface can reach 4.81%.Different hydrolysis catalysts have different coating effects.When the catalyst is hydrochloric acid,the coating effect on the fiber surface is more uniform than that of glacial acetic acid,and there is no obvious defect on the surface.When the hydrolysis time was 6h,the hydrolysis effect of the precursor was better than that of 3h and 9h,and the coating effect was the best,and the silicon content on the surface of the fiber reached 11.31%.The proportion of the precursor will directly affect the viscosity and gel time of the sol and the effect of the gel coating on carbon fiber.When the volume of ethanol and water is fixed at 50 m L and the ratio of three precursors is 10:10:6,the gel coating effect on the fiber surface is the best.