Molecular Dynamics Simulations Of Energy Transport In One-Dimensional Nanocomposites

With rapid development of the global economy the world’s energy crisis is increasing severe, the efficient utilization of energy has become one of the greatest challenges in modern society. Waste heat recovery and heat dissipation in electronics are the two very urgent energy issues. Thermoelectric materials can directly convert waste heat to electricity for improving the efficiency of energy utilization, and thermal interface materials can improve the efficiency of heat dissipation across the interfaces of electronics. Nanomaterials have demonstrated their unique potential in effective utilization and sustainable development of energy, and especially one-dimensional materials have broad prospects for applications of thermoelectric conversion and heat dissipation in electronics. Si nanowires have extremely low thermal conductivity due to increased phonon boundary scattering, making them promising thermoelectric materials; and due to the unique structure, phonon boundary scattering is nearly absent in carbon nanotubes, making them possessing ultra high thermal conductivity, expected to be promising thermal interface materials. Despite of the advantages above, the thermoelectric conversion efficiency of Si nanowire thermoelectric materials is not enough for widespread applications and the efficiency of heat dissipation in carbon nanotube thermal interface materials is also not high enough due to existence of interfacial thermal resistance. Due to increased phonon scattering, the Si nanowire nanocomposites with modification are expected to achieve lower thermal conductivity and larger thermoelectric conversion efficiency. So, it’s necessary to furtherly investigate the energy transport in the two one-dimensional nanocomposites.Using non-equilibrium molecular dynamics, the thesis investigated thermal transport in modified Si nanowires, interfacial thermal conductance between carbon nanotubes and Si substrate, and thermal transport in a single-walled carbon nanotube bridging two Si slabs under constant high heat flux, respectively.To improve the energy conversion efficiency of Si nanowire thermoelectric materials, the thesis investigated the thermal conductivity of three types of modified Si nanowires, including:(1) Si nanowire decorated with nanoparticles,(2) Si nanowire with nanoparticle inclusions, and (3) Si-based core-shell nanowire. The enhancement in energy conversion efficiency was inferred from the reduction in thermal conductivity of the modified Si nanowires. The effects of the surface coverage and configuration of external particles, the different types of internal inclusions, and the different shell coatings on the thermal conductivity of Si nanowire are investigated. Compared to pristine Si nanowires, it was found that the modified nanocomposite structures have considerably lower thermal conductivity (up to82%reduction), implying～5X enhancement in the ZT coefficient. This significant effect appears to have two origins:(1) increase in phonon-boundary scattering and (2) onset of interfacial interference. The results suggest new fundamental-yet realizable ways to improve markedly the energy conversion efficiency of nanostructured thermoelectrics.The thesis investigated the interfacial thermal conductance between vertically and horizontally aligned single-walled (10,10) carbon nanotubes and Si substrate. Effects of interfacial energy, the direction of heat flux, length of carbon nanotube, temperature of model system on the interfacial thermal conductance, and relative contributions of lattice vibrations in different directions to total interfacial heat flux were investigated. Compared with the results with heat flowing from the carbon nanotube to Si substrate and vice versa, the thesis found that there exists a thermal rectification for the interfacial thermal conductance, especially for the horizontally aligned carbon nanotube, the maximum thermal rectification is up to184%with a critical heat flux of60W/m, which is promising for thermal rectifier applications. By phonon-related analysis, the thesis found that for heat flowing from Si to carbon nanotube the increase of the interfacial thermal conductance with heat flux is due to the better match of phonon density of states between carbon nanotube and Si substrate, while for heat flowing from carbon nanotube to Si the low-frequency phonon modes excited at large heat fluxes dominate the interfacial heat transfer and such low-frequency phonon mode mechanism is responsible for the thermal rectification effect. Moreover, the thesis proposed a simple yet very useful method to quantify the directional contributions of lattice vibrations to the total interfacial heat flux and demonstrated that the out-of-plane lattice vibrations at the interface dominate the heat transfer across the silicon/horizontally aligned carbon nanotube interfaces. This method could be helpful in identifying mechanism in nanoscale heat transfer by analyzing the relative contributions from different phonon branches.The thesis investigated thermal transport in a single-walled carbon nanotube bridging two Si slabs under constant high heat flux. An anomalous wave-like kinetic energy profile was observed and a previously unexplored, wave-dominated energy transport mechanism is identified for high heat fluxes in carbon nanotubes, originated from excited low frequency transverse acoustic waves. The transported energy, in terms of a one-dimensional low frequency mechanical wave, is quantified as a function of the total heat flux applied and is compared to the energy transported by traditional Fourier heat conduction. The results suggest that at low heat fluxes Fourier heat conduction is dominant and the contribution of low frequency waves is negligible; however, as the heat flux exceeds a critical value, low frequency waves are excited and the wave transport energy mechanism overtakes the traditional Fourier conduction, rendering the carbon nanotube significantly more energy conductive. The results reveal an important new mechanism for high heat flux energy transport in low-dimensional nanostructures, such as1-D nanotubes and nanowires, which could be very relevant to high heat flux dissipation such as in micro/nanoelectronics applications.