Equivalent Model Of Surface Catalytic Properties Of ZrB₂-SiC Ultra High Temperature Ceramic Material

The development of thermal protection materials is one of the key technologies to ensure the safe service of hypersonic vehicles under extreme environments.Many studies carried out in United States,Japan and Russia have shown that catalytic thermal recombination reaction in the surface of heat-resistant materials can increase thermal load by 50%,which has a significant influence on the design of thermal protection system.The catalytic property of suface is an intrinsic property.In order to reduce the thermal load generated by catalytic recombination,it is necessary to explore the reaction mechanism based on the material itself,control catalytic property initiatively and reduce the thermal load of protection system.Although some diagnostic methods such as wind tunnel,Optical Emission Spectroscopy?OES?and Laser Induced Fluorescence?LIF?can be used to test catalytic properties of surface at present,only few typical temperatures and pressures can be obtained under the limit of experimental ability and cost.The discrete data doesn't meet the needs of engineering application.Therefore,it is necessary to predict catalytic heat on the surface of heat-resistant material based on numerical simulation method.In addition,the oxidation reaction accompanying the use of heat-resistant material will affect the characterization and modeling of catalytic effect on the surface greatly.Taking ZrB₂-SiC as an example,a stable oxide layer whose thickness is about¹0^-10-10^-9m would be form on the surface of ZrB₂-SiC after¹0^-1s.The matrix material has been completely covered by oxidation products on the surface,cause the actual space area involved in catalytic recombination reaction is¹0^-10m.Therefore,the actual material is the oxidation product in the reaction.The result also shows that catalytic properties on the surface of material are close to oxide at high temperature.This paper takes ZrB₂-SiC typical ultra-high temperature ceramic material as the research object.Building an equivalent model of the catalytic characteristics and simulating catalytic characteristics of actual oxides by molecular dynamics under the environment of oxygen atoms.The study will be carried out from three aspects below:?1?Measurement and oxidation response analysis on catalytic characteristics of the surface of ZrB₂-SiC ultra-high temperature ceramic materialsTesting catalytic properties of ZrB₂-SiC Using LIF experimental test method,obtained catalytic coefficients of suface under the pressure of 10 Pa and 100 Pa.Studying the molecular and atomic oxygen based on the oxidation reaction of the main components.Thus,getting the main oxidation products on the surface of materials at different temperature intervals under 10 Pa,100 Pa and 1000 Pa,characterizing catalytic properties in this state approximately.?2?Numerical simulation and experimental verification study of catalytic characteristics on the surface of oxide based on ReaxFFConstructing the surface of massive a-SiO₂ based on the principle of molecular dynamics,calculating the surface structure characteristics and correlation functions and comparing with the experimental values.Adding flux boundary conditions to determine the type and concentration of surface sites on the surface of a-SiO₂,establishing a finite-rate catalytic model based on the catalytic mechanism of a-SiO₂.The relationship between surface catalytic properties of a-SiO₂ and temperature and pressure was obtained by simulating single reaction events of ReaxFF.In order to verify the finite rate catalytic model,this paper comparing the experimental and simulation results and calculating the catalytic properties of ZrO₂ and B₂O₃ to obtain the relationship between temperature,pressure and catalytic properties.?3?Equivalent model and key control mechanism of surface catalytic characteristics of ZrB₂-SiCSurface oxidation partition and catalytic coefficient of three oxides are obtained after measuring catalytic coefficient by LIF.Equivalent model of catalytic characteristics of ZrB₂-SiC was constructed by mathematical and physical methods and comparing with the test results for predicting catalytic properties in non-specific environments.Thus,revealing catalytic mechanism by key control mechanism combined reaxff-finite rate catalytic model with the study mention above.In this paper,LIF measurement and ReaxFF simulation were used to establish an equivalent model for predicting catalytic properties of oxidation reaction.The key control mechanism of catalytic recombination reaction on the surface was revealed.The research results can provide boundary conditions for the aerodynamic calculation of the thermal protection system directly,which can improve the design refinement greatly.The method and model can be extended to other heat protection material systems.