Thermal Transport in Low Dimensional Graded Structures and Silicon Nanowires

Abstract

Very recently, phononic (thermal) devices have been brought forward theoretically, in which the phonon is used as information carrier. It drives us to search materials fit for thermal devices, such as thermal diodes and thermal transistors. On the first part of this thesis, it is proposed that low dimensional graded materials are good candidates for thermal rectifier. The heat flux in the one dimensional harmonic/anharmonic chain with a mass gradient and the carbon nanocone were studied by using classical non-equilibrium molecular dynamics simulation. It was found that the heat flowed with asymmetric in anharmonic lattices with a mass gradient. Moreover, in a certain temperature region, negative differential thermal resistance was observed. It was also demonstrated that the structural asymmetry in carbon nanocone benefited the rectification ratio remarkably. It was found that there was a larger heat flux in the direction of decreasing diameter and the rectification in carbon nanocone was size independent. Possible applications in constructing thermal rectifiers and thermal transistors by using the graded material were discussed.

The silicon nanowire (SiNW) has been shown to be an efficient thermoelectric material. The thermal conductivity of SiNW is crucial in thermoelectric applications. On the second part of this thesis, using classical nonequilibrium molecular dynamics simulation, it was studied that the reduction of the thermal conductivity of SiNWs with two isotope-doping methods: doping nanowires with isotope impurities randomly and isotopic-superlattice nanowires. It was shown that these two methods led to a large scale decrease of thermal conductivity of SiNWs. The thermal conductivity of isotopic superlattice structured SiNWs depended clearly on the period length of super lattice. The mass effect on thermal conductivity was obvious. The heavier isotope atoms (42Si) could decrease the conductivity much more than the lighter ones (29Si). The remarkable isotopic effect observed in this work provides an efficient approach to decrease thermal conductivity of SiNW, which could be of great benefit to improve the thermoelectric performance. These improvements have raised the exciting prospect that SiNWs can be applied as novel nano-scale thermoelectric materials. It was also studied that the size effect on the thermal conductivity of nanowire structures. It was demonstrated that the thermal conductivity of SiNWs diverged with the longitudinal length, even when the sample length was much longer than the phonon mean free path at the room temperature, which meant Fourier¿s empirical law was broken. The effect of fixed boundary on heat transport in SiNW was researched. It was reported that there was obvious difference between the heat flux of atoms close to boundary and the flux of atoms at the center of cross section.