| Along with progress of the microelectronic processing technology, electronic devices move forward to micro-scale, from the three dimensional bulk materials to two-dimensional plane materials such as graphene, grapheye and black phosphorus, from one dimensional carbon nanotube to zero dimensional nanocrystalline materials such as quantum dots. In the research on the heat transport of the materials, we found the transport properties are different from macro materials, such as anomalous thermal conduction, ballistic transport and heat rectification. The reason of these phenomena, which are not clear, collect many attention from researchers. At the same time, these special properties have rich applications, such as materials with high heat conductivity used in microprocessor can effectively solve the heat dissipation problem, which can promote the integration level of microprocessor. Heat-resistant materials with low heat conductivity can be made into effective heat barrier materials, which can protect the machine working in high temperature and high pressure environment to extend service life. This kind of materials can also been used for building materials for effective insulation. Pyroelectric effect can achieve the exchange of heat and electricity. The efficient use of energy with this property can solve the global energy crisis and environmental problems. At the same time, thermal device with controlled heat flow direction has heat rectification, which make quantum computer be possible.We want to know more properties of low dimensional materials from studying phonon relaxation and heat transport in low dimensional system. Molecular dynamics method was employed as traditional tool in the heat transport research, while LAMMPS was adopted as the mature simulation tool for molecular dynamics method.In bulk materials, heat transport obeys Fourier Law. But in low dimensional system, heat conductivity is related to dimension, which called anomalous heat conduction. It is well known that the main interaction in materials is nonlinear interaction. So the influence of thermal conductivity with nonlinear interaction is the first part of our work. We chose the classical Fermi-Pasta-Ulam model. The degree of nonlinear interaction is controlled by changing coefficient in the FPU potential. The system achieves equilibrium with the interaction with Nose-Hoover Chains heat bath. We get the phonon lifetime changed with the degree of nonlinear interaction. It shows that stronger the nonlinear interaction is, shorter the phonon lifetime is. And the phonon frequency offset in the direction of low frequency.Apart from the nonlinear interaction for heat conductivity, we also found heat transport is different in grapheye with different structures. Different from graphene, there are abundant kinds of linkage in grapheye, as well as more kinds of structures. So we conjectured acetylenic linkage will affect the heat conductivity of the systems. We studied heat conductivity of graphene and grapheye with five different structures. We found the heat conductivity damped with the increasing of the acetylenic linkage percentage. It is important that we can control the heat conductivity with the control of the acetylenic linkage percentage. To explain the reason of this phenomena, we calculated the relations of phonon dispersion, the distribution of group velocity, the vibrational density of states (VDOS), and the phonon mode participation by the lattice dynamics approach. Furthermore, the phonon lifetimes are obtained from the analysis of the mode-projected velocity from molecular dynamics simulations. It is demonstrated that the phonon mode is localized with the increasing of the acetylenic linkages percentage. So the rate of the phonon scattering increased and the phonon group velocity reduced. As a result, the phonon lifetime was cut down, and the thermal conductivities of these allotropes reduced. It also provided some theoretical foundation to design new thermoelectric material in experiment. |
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