Molecular dynamics simulation of thermal conduction in solid and nanoporous thin films.

摘要

Thin solid films and porous materials with nanometer-scale characteristic dimensions possess physical properties fundamentally different from those observed at larger length scales. These novel materials form the building blocks of a small but growing number of innovative new structures and devices in applications such as optical communications, electronics, and chemical sensing. The ultrasmall dimensions of these structures can strongly impede the conduction of heat away from high-temperature regions, possibly causing severe performance degradation or even failure. A basic understanding of thermal conduction phenomena in these structures thus becomes critical to optimize performance in the above applications.; The molecular dynamics computational technique is used in this work to investigate thermal conduction in solid and nanoporous thin films. The simulations are based on the widely used Lennard-Jones argon model and are performed in a full three dimensional domain. For nanoporous thin films, parameters such as average temperature, pore placement, and pore shape, size, and orientation are examined in order to better understand their influence on the temperature profiles and overall thermal conductivity. For solid thin films, the effects of film thickness, lateral extent, temperature, boundary conditions, and applied flux are studied. The results for both cases are compared to experimental data and theoretical calculations, and practical guidelines for effective simulations are outlined.; To complement the molecular dynamics simulations and to pinpoint areas where they can be improved, this work also performs laser-based thermal conductivity measurements on films of nanoporous silicon, a technologically relevant engineering material. The relationship between processing, structure, and thermal conductivity is examined systematically in an attempt to address the inconsistent values for porous silicon thermal conductivity reported in the literature.; Key findings of this work are that the calculated values show reasonable agreement with experimental data, that thermal conductivity increases with film thickness, that thermal conductivity shows little temperature dependence for films with pores larger than five atomic vacancies, and that the proximity of a pore to the hot or cold boundaries of a film influences its overall thermal conductivity. The tuning of material properties by pore placement creates exciting possibilities for the engineering of thermal transport at nanometer scales.