Molecular Dynamics Simulation Of Thermal Energy Transport In Nanocomposites

As the ongoing scientific and technological revolution progress towards microscopic world, nanoscale material engineering becomes a fresh and promising subject. Therefore, the ability to predict and control thermal energy transport in and across these complex micro/nano structures becomes increasing important. Nevertheless, there is limited understanding of the mechanisms (eg. phonon confinement effect, thermal boundary resistance, interface roughness and phonon wave interaction) that affect thermal transport in micro/nano systems. In this thesis, thermal transport in micro/nano structures is explored using computational methods and the results are compared with the experimental ones. Better insight into these mechanisms will offer the opportunity to design customized nanostructures for enhancing the performance of energy conversion device and manipulating the heat flow in complex micro/nano systems.Nonequilibrium molecular dynamics simulations are employed to explore the heat conduction in platinum nanofilm and silicon nanowire embeded in germanium matrix composites. After the simulation of the cross-plane thermal conductivity of platinum nanofilm, it is concluded that with the thickness between 100 nm and 500 nm the cross-plane thermal conductivity of platinum nanofilm is much lower than the bulk value, and also lower than the in-plane thermal conductivity. Obviously, this is "size effect". The temperature response under pulse heating in platinum nanofilm is also investigated. Results indicate that the temperature response time in platinum film with micrometer thickness is in nanosecond magnitude, so it is totally suitable for temperature measurement in explosive boiling experiment in which the microsecond pulse heating is used. It also found that in the initial stage of pulse heating, there is a faster non-fourier heat wave with the speed of approximately 3400 m/s across the film. The thermal conductivity of Si/Ge nanocomposites is influenced by a lot of factors, such as the interface, temperature, heat flow, atomic percentage, characteristic scale and void. The investigation indicates that: "reflecting effect" exists before the interfaces, which means the temperature increases before the interface along the heat flow direction; there is temperature drop at the interface because of the thermal interface resistance; the thermal conductivity is essentially constant over the temperature range from 200 K to 1200 K, and it will decrese in the temperature below 200 K; the influence of heat flow even in 10 GW/m~2 magnitude on thermal conductivity is not prominent; the thermal conductivity increases with the atomic percentage of germanium, increases with the characteristic length of silicon nanowires and decreses with the void density; with the same atomic percentage of silicon and germanium, the thermal conductivity of Si/Ge nanocomposites is much lower than the value of Si/Ge alloy, which predicts it has the strong potential of thermoelectric materials with high performance for subtituting the expensive superlattice which is not suitable for large-scale application.