| Summary: | 碩士 === 國立交通大學 === 機械工程系所 === 101 === This study aims to investigate the heat transfer on polyethylene (PE) chains. Previous studies have shown that an ultra-drawn PE nanofiber has a thermal conductivity much larger than its bulk counterpart. The enhancement in the nanofiber was speculated as a result of lattice-reconstructing during the ultra-drawing process; however, the exact mechanism causing such enhancement has not been fully understood yet.
In this work, thermal conductivity was determined using the Boltzmann transport equation (BTE), where the phonon group velocity, heat capacity and relaxation time were obtained from molecular dynamics (MD) simulations. The phonon group velocity and heat capacity in the chain were determined from the phonon dispersion relation derived from Lattice dynamics using trajectories from MD simulations. Meanwhile, the phonon relaxation time was found from the phonon normal mode autocorrelation function. These properties were adopted as inputs in BTE to determine the thermal conductivities of single PE chains.
The thermal conductivity of PE chains increases with increasing chain length and saturates to a constant value. The constant value was taken as the thermal conductivity of an infinite PE chain. For longer chains, the thermal conductivity increases with increasing temperatures, reaches a maximum at a certain temperature and decreases with temperature afterwards. However, for shorter chains, the temperature dependence is not significant. The thermal conductivity for a chain with an infinite length from the BTE agrees well with the experimental result of a near ideal PE fibril. It is also found that the non-equilibrium molecular dynamics method over-predicts the thermal conductivity at low temperature as a result of the classical statistics in MD simulation.
At room temperature, the thermal conductivity of the PE chain could be as high as 45 W/m-K, about 100 times that of the bulk PE (0.3~0.5 W/m-K). The enhancement is due to the higher group velocity and longer mean free path in the chain.
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