Molecular Dynamics Study On Oxidation Mechanism Of Carbon-based Ablative Materials At High Temperature

Ablative materials are key materials in aerospace thermal protection systems and play an important role in their design and development.They are usually used in hypersonic reentry vehicles.Their performance is critical to the progressiveness and reliability of aircraft.Among many ablative materials,carbon based ablative materials have become one of the most popular heat resistant materials in the research and development of thermal protection systems due to their excellent ablation resistance.However,the properties of carbon based materials themselves are relatively complex,making it difficult to accurately regulate them,making it difficult for researchers to obtain correct and reliable research conclusions.Therefore,it is crucial to understand the ablation and thermal response behavior of carbon based ablative materials under high temperature conditions.In particular,accurately understanding their ablation mechanism and ablation rate during reaction provides an effective reference and basis for optimizing and designing thermal protection systems and ablative materials.In this paper,using the DFTB method and XTB method in CP2K software,a numerical simulation study was conducted on the reaction mechanism and reaction rate of ablative materials at different temperatures,using single and double layer graphite materials and SiBCN structural materials as research objects.The main work includes the following aspects:Firstly,the oxidation reaction process of monolayer graphite was analyzed,and a model of the oxidation reaction of monolayer graphite was established on this premise.Using the DFTB method in CP2K software,the model calculates the ablation rate and counts the small molecule products during the reaction process at different temperatures,and obtains the reaction products,main control factors,and reaction activation energy during the reaction process.Secondly,based on the single layer model,a double layer graphite model was established.The reaction process was simulated for different defect types of model structures at different temperatures,and the main products,ablation rate,activation energy,etc.during the reaction process were obtained.The similarities and differences between the single layer and double layer models were compared.Finally,an ablation model of the SiBCN structure was established,and the reaction process of the structure at different temperatures was simulated using the XTB method in the CP2K software.The reaction mechanism,ablation rate,activation energy,etc.of the structure were obtained,and the similarities and differences between the structure model and the graphite model were compared.The calculation results show that:（1）The main products in the monolayer graphite model reaction process are CO and CO₂,which are mainly formed by the combination of carbon atoms and oxygen atoms after the fracture of the C-C bond on the epoxy group,while CO₂ is mainly formed by the cracking of small molecular clusters（C₂O₂,C₃O₁,C₄O₁）.By analyzing the oxidation reaction rate,we found that C-C bond breakage is the main pathway of graphite ablation reaction.The reaction activation energies calculated by the three models are 7.56,2.4,and 1.6 kcal/mol,respectively,indicating that the graphite model without defects has a high activation energy and exhibits the lowest ablation reaction rate,while atomic defects and hole defects will accelerate the ablation reaction rate.（2）The process of CO and CO₂ production in the oxidation reaction of the double layer graphite model is basically the same as that of the single layer graphite.Compared to the single layer model,the reaction rate of the double layer model is slower,indicating that the effect of layer thickness on the reaction rate is more obvious.The slowest reaction rate of the double layer graphite model is the model with holes,indicating that hole defects will increase the activation energy and reduce the reaction rate.（3）The oxidation reaction of SiBCN precursors begins with the cracking of the Si-N bond.As the temperature increases,the reaction rate increases,so a large amount of Si CN will be produced at high temperatures.Compared to the graphite model,the activation energy of the SiBCN precursor DMTA is lower,so the reaction rate is faster than that of graphite.