Molecular Dynamics Investigation On Heat Transfer Properties Of Nano-scale Graphite Crystallite In Carbon/Carbon Composite

Carbon/carbon(C/C)composites have been widely used in the heat protective components of aerospace and military defense,such as the missile end cap,shuttle nose cone/wings,rocket engine spray pipe line,and so on.The excellent thermal conductivity and the low density make the C/C composites indispensable in the above fields.The internal structure of C/C composite is mainly composed of carbon fiber/carbon fiber woven fabric and carbon matrix filled with carbon fibers.In nano-scale,the basic structure of carbon fiber and carbon matrix is the graphite crystallite.The thermal performance of graphite crystallite is directly related to the overall performance of C/C composites.However,graphite crystallite has the anisotropy thermal performance with complex structure,which is stacked by the nano-scale graphene layers,so that it is difficult to utilize the traditional theories of solid state physics to study the thermal properties of graphite crystallite.Moreover,the phonon properties of pratical graphite crystallite with complex geometry structure still remain to be explored.In this thesis,molecular dynamics simulation as well as the theory of solid state physics are applied to investigate the following four contents.(1)The applicability of different potential models when predicting thermal conductivity of graphite crystallite.(2)Modifying the normal mode analysis method based on the practical lattice vibrational position.(3)The correlation between the different types of defects and the thermal conductivity of the graphite crystallite.(4)The torsion effect on the thermal conductivity of graphite crystallite.The purpose of research in this thesis is to develop and improve the analysis method of the thermal conductivity of nano-scale graphite crystallite,to reveal the factors of the thermal conductivity,and to provide the theoretical basis to the material design and performance optimization of the C/C composites.Firstly,to make sure the molecular dynamics method can correctly predict the thermal conductivity of graphite crystallite,the applicabilities of REBO,AIREBO,Tersoff and opt-Tersoff potentials are compared.For the single-layer graphene,the Tersoff potential is failed when calculating the phonon dispersion relation.The other three potentials can successfully predicted the G-band in the phonon density of state.Among them,the G-band predicted by the opt-Tersoff potential has larger frequency range and wider amplitude,while the ZO mode is failed in the phonon dispersion relations of REBO and AIREBO.Hence,opt-Tersoff potential can predict the largest and the most accurate thermal conductivity for the single-layer graphene.For the multi-layer graphene,the opt-Tersoff potential can accurately capture the decreasing of the amplitude of ZA+ZO mode caused by the scattering,thus it can qualitatively reproduce the experimental observation.However,the AIREBO potential underestimates the scattering,so that it is not recommended study the thermal conductivity of multi-layer graphene and graphite crystallite.Once the choosing of the potential model is clear,the lattice vibration can obtained by the molecular dynamics method.To deeply analysis the lattice vibration near defects,the lattice asymmetry and excursion are investigated by theoretical analysis and numerical simulation.The deviation caused by the lattice asymmetry is revealed.The modified normal mode analysis method is raised.It is found that there exists distinct lattice asymmetry near the vacancy,and the asymmetry in the normal direction is larger.While the lattice asymmetry near N-doping and-CH3 funtion is negligible.The lattice asymmetry near the vacancy can affect the prediction of normal mode analysis.This work modified the effect of lattice asymmetry by moving the vibration position from the ideal position to the practical position.The modified scattering rate ?SNMA=2.39×10-5 A-1 is raised.After perfecting the investigation method,the effects of different types of defects on the thermal conductivity of graphite crystallite,such as N-doping,-CH3 group,and vacancy,are studied.For the graphite crystallite which contains one type of defect,vacancy is the most remarkable factors of the thermal conductivity.For the graphite crystallite which contains the three types of the defects,the Taguchi orthogonal algorithm is utilized to reveal the combined effects of defects.It is found that the vacancy is the most remarkable type of defect at 300 K and 700 K.While at 1500 K,the-CH3 group becomes the most important type.Torsion is one of the common forces which can remarkably change the lattice asymmetry of the graphite crystallite.Unfortunately,there not exist any kind of method to describe the phonon dispersion relation of twisted graphite crystallite.Hence,this thesis raised the new equivalent method to reflect the effects of torsion on the thermal conductivity and the phonon dispersion relation.Torsion effect is broke up into the translation effect and the compression effect.It is found that the translation effect can be ignored due to it almost do not affect the phonon dispersion relation.On the other hand,the compression effect decreases the phonon lifetime of ZA branches,increases the group velocities of ZA branches as well as the phonon capacity heat.In this work,molecular dynamics method as well as theoretical analysis are utilized to investigate the thermal conductivity of graphite crystallites,which are the basic nano-structure of carbon/carbon composites.The new analysis methods are raised to study the graphite crystallites in reality.Besides,the effects of complex factors on the thermal conductivity of graphite crystallites are revealed,and the physical mechanism is analyzed.