Study On Thermal Properties Of Graphene And Its Composites

Graphene and its composite materials have increasingly become the focus of research in the field of micro/nano-scale heat dissipation due to their superior thermal conductivity.However,the phonon heat conduction theory of graphene is still in development,and the coupling mechanism of graphene and metal materials needs further research.These shortcomings in basic theory limit the optimization and application of graphene and its composites.In this paper,molecular dynamics(MD)is used to simulate graphene and graphene-copper composites,and explored the influencing factors and do qualitative analysis of the effects on thermal conductivity.At first,it is to explore the influencing factors of the thermal conductivity of graphene materials.The effects of temperature,size,layer number and other factors on the thermal conductivity of graphene materials are investigated by using two methods,including Equilibrium Molecular Dynamics(EMD)and Non-Equilibrium Molecular Dynamics(NEMD),and the results are compared.Firstly,when the temperature is between 100 K and 500 K,the thermal conductivity of the graphene is negatively correlated with temperature,and from 100 K to 500 K,the thermal conductivity of graphene is reduced by more than 45%.This is mainly due to an increase in temperature,which leads to an increase in molecular motion,resulting in increased phonon scattering and a decrease in thermal conductivity.Secondly,the thermal conductivity of the graphene model is positively correlated with the material size.Between 10 nm and 800 nm,the thermal conductivity becomes larger as the size of the model becomes larger.When using EMD,the thermal conductivity of graphene with a length of 738 ? is 1.20 times of that of graphene with a length of 147.6 ?.While using NEMD,the ratio is 2.7.This is mainly caused by the increase in size,which causes the boundary scattering effect to be weakened and the phonon heat conduction to be enhanced.For the effect of the number of graphene layers on the thermal conductivity,most of them are affected by interface scattering,causing the thermal conductivity to become smaller as the number of layers becomes larger.Using the EMD method,the thermal conductivity of the twolayer graphene is about 90.20% of the thermal conductivity of the single-layer graphene(k=168.6 W/(m·K)),and the result is 61.4% when using NEMD method.The gaseous environment also has a large effect on the thermal conductivity of graphene.At about 2 atmospheres,the thermal conductivity of graphene is only 73.62 W /(m · K),which is 54.29% of the thermal conductivity(135.6 W /(m · K))of grephene under vacuum.This is because the collision of graphene with gas molecules causes interfacial scattering of phonons,so that the mean free path of phonons is reduced,resulting in a decrease in thermal conductivity.In addition,using the NEMD method,it is found that the thermal conductivity of graphene decreases as the pressure of the gas environment becomes larger.This is because the frequency of collision of gas molecules with graphene varies under different pressure conditions.When the pressure is high,the frequency increases,resulting in enhanced phonon interface scattering,smaller mean phonon free path,and lower graphene thermal conductivity.The thermal conductivity of the graphene-metal copper film material was simulated using the NEMD method.This simulation is mainly to investigate the effect of temperature,size and the number of layers of copper on the thermal conductivity of composite materials.The results show that the thermal conductivity of graphene-copper film material increases first and then decreases with increasing temperature.Here we propose that the thermal conductivity of graphene and metal copper film material consists of three parts: k = kp+ke+kc,the sensitivity of these three parts to temperature is different,the phonon thermal conductivity kp decreases with increasing temperature,and the electronic thermal conductivity ke increases as the temperature rises,the change of the coupling thermal conductivity kc of the two materials is uncertain,but the combination of the three causes the above phenomenon.The thermal conductivity of graphene-copper composites increases with increasing size,and generally increases linearly within the simulated scale.For example,the thermal conductivity of a graphenecopper composite with a length of 738 ? is 1.27 times the thermal conductivity of graphene-copper with a length of 147.6 ?.This is mainly due to the effect of increased size on phonon scattering.At the same time,the thermal conductivity of the graphene-copper film material is significantly better than that of copper.However,as the number of layers of copper increases,the thermal conductivity of the graphene-copper film material and copper decreases and the thermal conductivity of copper is always less than that of the graphene-copper film material.However,the difference in thermal conductivity between the two materials is gradually reduced with layers of copper increasing.For example,when the number of layers of copper is 2,the thermal conductivity of the graphene-copper film material is 1.32 times of that of copper,and when the number of layers is increased to 10,this ratio is reduced to 1.17.