Molecular Dynamics Study Of Interfacial Cascade Damage And Interfacial Thermal Conductance On SiC/C Interface

Continuous silicon carbide(SiC)fiber-reinforced SiC-matrix(SiCf/SiC)composites have been considered as one of the most important candidate structural materials for advanced fission reactors and future fusion reactors,for their high-temperature strength,high creep resistance,high corrosion resistance,high thermal shock resistance,low induced-activation and low after-heat properties.The mechanical properties of SiC/SiC composite materials have been greatly improved during the last decade.But irradiation damage at the SiC and pyrolytic carbon interface would degrade the mechanical integrity of the composite with the mechanisms remaining unknown in the present.Meanwhile,the interfacial thermal conductivity degrade after irradiation.Firstly,the thermal conductivity of liquid argon at 85 K was calculated by means of Equilibrium Molecular Dynamics and reverse-Nonequilibrium Molecular Dynamics method,respectively.The calculated result is 0.129 and 0.122 W/(m · K),respectively,and both of them are in a good agreement with experimental values,which is 0.1322 W/(m · K).This validates the flexibility of LAMWPS program which was used in this paper.Secondly,the finite size effect in the SiC model was studied with rNEMD method,the results show that there is the effect only along the direction of heat flux but not the direction perpendicular to the heat flux.Finally,the relationship that between temperature and the thermal conductivity of bulk SiC was studied,which is consistent well with Taylor experimental values.By extrapolating to infinite length,an approach based on the finite size effect,the thermal conductivity of bulk SiC and graphite(heat flux along the basal plane)are obtained to be 163.93 W/(m ·K)and 322.58 W/(m·K)at 300 K and 325 K,respectively,both are comparable with their experimental values,which is 177.46 W/(m · K)and 241.66 W/(m · K)for single crystal ?-SiC and pyrolytic graphite,respectively.On the basis of the above work,referring to the practical structure of fiber/matrix interfaces,five SiC/graphite composite models were created with different interface structures.The angles between the graphite plane and SiC are 0.00°,28.56°,56.71°,77.32° and 90.00°(denoted as M0,M28,M56,M77and M90),respectively.On the one hand,molecular dynamics simulations have been used to model the irradiation cascade of the 5 composite systems.In each run a collision cascade is initiated by giving one atom 1.5 keV of kinetic energy.The relationship between the distribution of defects and simulation time and PKA position were systematically studied and compared with bulk SiC,the distance between PKA and interface is la0,2a0,3a0 and 4a0(a0 is the lattice constant of SiC),respectively.The results show that the irradiation damage resistance of SiC/C interface is significantly lower than that of bulk SiC,and the interface structure have an impact on the number of defects.Radial distribution function was employed to examine the coordination of interfacial atoms.The results show that the higher the density of interfacial graphite atoms,the larger impact that irradiation exert on the coordination.On the other hand,in order to study the effects of interface structures and irradiation on the interfacial thermal conductance,the thermal conductance of irradiated models,which were irradiated by 1 keV Si atom,was calculated by means of reverse-NEMD method and compared with unirradiated models.Meanwhile,MO was given a series of PKAs with kinetic energy of 0.25 keV,0.5 keV,0.75 keV and 1 keV for studying the relationship between irradiation dose and interfacial thermal conductance.Results show that there is positive correlation between the interfacial energy and interfacial C-Si bond quantity,and irradiated models showed higher interfacial energy compared with their unirradiated counterparts.The interfacial thermal conductance of unirradiated and irradiated(1000 eV)M77,M56,M28 and M0,increases as the increase of interfacial energy,respectively.For irradiated models,lattice defects played an important role in impacting the interfacial thermal conductance.Except M90,the other models showed decreased interfacial thermal conductance after irradiation.With the special structure,the interfacial thermal conductance of both irradiated and unirradiated M90 is an order of magnitude lower than the others.And different from the rest,the interfacial thermal conductance of M90 increased after irradiation.To explain these phenomenons,the vibrational density of states of atoms in the 10 atom layers of interfacial region was calculated to analyze the phonon mismatch at the interface.Results show that there is obvious VDOS mismatch between the interfacial region and bulk SiC or graphite,and this is the reason how interfacial thermal conductance appeared.