Sub-continuum ion transport in air and phonon transport in nanostructures
In this dissertation, atmospheric ion generation were first simulated using a Particle In Cell-Monte Carlo (PIC-MC) method to predict ion generation and breakdown characteristics in microscale gaps. Simulation results were validated and have revealed the significance of each relevant electron-molecule reaction. Self-sustaining discharge and ionization were reproduced numerically under sufficient voltage bias, and the predicted trends of breakdown voltage are similar to prior results. Experimentally, the atmospheric field emission and ionization capabilities of highly graphitic polycrystalline diamond (HGPD) film were characterized. The HGPD sample was activated by applying a moderate voltage bias for an extended period. After activation, a smaller turn-on voltage and a larger, more sustainable current of 10 μA were observed. A hydrogen plasma treatment after long-term operation was shown to restore emission current back to or even exceeding the original level, which suggests an important role of surface termination in electron emission processes. An atomistic Green's function (AGF) method was developed to simulate nanoscale phonon transport in the second part of this dissertation. The theory of the AGF method was established with a full derivation of transmission function and heat flux expressions based on lattice dynamics and Green's function theories. Basic implementations were demonstrated using simple homogeneous and heterogeneous atomic chain examples. The thermal impact of lattice strain was studied in a strained Si/Ge thin film and was shown to be less important than the heterogeneous material effect. The thermal boundary resistance of a single Si/Ge interface was obtained and compared well with the acoustic mismatch model (AMM) at low temperatures. Ballistic behavior of phonons was illustrated through length-dependence and multi-interface case studies. The AGF method was also extended to calculate the thermal conductance of a nanowire-plane junction structure. A method to evaluate self-energy matrices of the abrupt interface was introduced. The dependence of thermal transport on nanowire diameter and length were investigated. Normalized thermal conductance of nanowire-plane junction structure converges with thin-film conductance at large nanowire diameters. Increasing nanowire length reduces thermal conductance due to the weakened coupling between contacts and the device.
Garimella, Purdue University.
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