Thermal Interface Properties Of Pillared-Graphene By Molecular Dynamics Simulation

In recent years, with the increasing energy density of electron devices, the heat dissipation has become one of the most important topics. As an important part of heat dissipation system, thermal interface material has gotten great attentions from industry and researchers. In order to improve the problem of the traditional thermal interface, such as easy aging, low resistant to fatigue, corrosion and so on, the related thermal interface material of Carbon Nanotube and Graphene has been proposed. However, the high thermal boundary resistance between Carbon Nanotubes and heat sink, the low normal thermal conductivity of Graphene, which made the thermal properties of Carbon Nanotubes and Graphene based thermal interface materials need further enhancement.In order to make full use of the high thermal conductivity in vertical direction of Carbon Nanotubes and the smoothness of Graphene, Pillared-Graphene, a new thermal interface material, is proposed in this paper. The Pillared-Graphene is based on Multiple Layer Graphene and Carbon Nanotubes are used to link the Graphene layers. Thus, the flat plane of Graphene is used to contact with the heat sink and the high conductive Carbon Nanotubes is used to transport the thermal out of plane, which could make the structure behave low thermal boundary resistance and high thermal conductivity out of plane in the same time.Through the Molecular Dynamics Method, the thermal conductance of the structure with different heat sink changing with temperatures, layer number of Graphene and the density of Carbon Nanotubes has been studied. Two kinds of basic Pillared-Graphene were built in this paper:the Pillared-Graphene I, and the Pillared-Graphene II. For the Pillared-Graphene I, the top and the bottom of which are Graphene layers and Carbon Nanotubes inside connect directly with graphene layers. For the Pillared-Graphene II, besides the top and the bottom graphene layers, another graphene layer is used to separate Carbon Nanotubes.Firstly, thermal conductance of Pillared-Graphene itself varied with the number of Carbon Nanotubes and system temperature was studied. The results show that, with the inserted Carbon Nanotubes, the thermal conductance of two types Pillared-Graphene are all linearly increasing with the number of Carbon Nanotubes, the most of which can be 2 magnitude greater than that the same size and same thickness of the 8-layer-Graphene. In addition, the thermal conductance of the Pillared-Graphene increasing with temperatures shows the phonon scattering between Carbon Nanotubes and Graphene decreased with the increasing temperature.Secondly, the thermal conductance of Pillared-Graphene between two copper heat sinks was studied. In the copper/Pillared-Graphene I system, the thermal conductance of the system increased with the increasing Carbon Nanotubes number at first. The thermal conductance reaches the maximum when Carbon Nanotubes number is 10. From the further analysis, the thermal coupling between the Carbon Nanotubes and the increasing porosity of the Graphene made the temperature jump at the interface larger and the thermal conductance will not increase any more. For the Copper/Pillared-Graphene II composite system, as the middle Graphene layer will cause phonons scatter along the vertical distance, the thermal conductance always lower than the former structure. In addition, when the system temperature is more than 450K, the thermal conductance of both two structures arrive the maximums. The result that thermal conductance does not increase in high temperature might be caused by the severe vibration of copper atoms.Fatherly, the thermal conductance of Pillared-Graphene between copper and silicon was studied. The result shows that, the thermal conductance of the system increase with the increasing the carbon nanotube numbers and arrives its maximum when CNT number is 12. In addition, when CNT number in the system of silicon/Pillared-Graphene Ⅱ/copper is more than 6, the thermal conductance of the system also will not increase any more. It shows that, the silicon change the characteristic of the thermal transport. When the Carbon Nanotube close enough, the thermal coupling can be obvious, which hinder the effective thermal transport of Carbon Nanotubes. Besides, when the system temperature is higher than 450K, the thermal conductances of both structures do not increase any more. It shows the little influence that silicon layer on the temperature dependent thermal properties.In summary, thermal conductance of the Pillared-Graphene and its composite structures were simulated by molecular dynamics simulation. The results provide valuable data for the applications of Pillared-Graphene based thermal interface materials.