Amir Farajian (Advisor), Hong Huang (Committee Member), James Menart (Committee Member)
Master of Science in Renewable and Clean Energy Engineering (MSRCE)
Nanomaterials hold great promise for applications in thermal management and thermoelectric power generation. Defects in these are important as they are generally inevitably introduced during fabrication or intentionally engineered to control the properties of the nanomaterials. Here, we investigate how phonon-contributed thermal conductance in narrow graphene, boron nitride (BN), and silicene nanoribbons (NRs), responds to the presence of a vacancy defect and the corresponding geometric distortion, from first principles using the non-equilibrium Green's function method. Analyses are made of the geometries, phonon conductance coefficients, and local densities of states (LDOS) of pristine and defected nanoribbons. It is found that hydrogen absences produce similar reductions in thermal conductance in planar graphene and BN NRs with greater reductions in buckled silicene NRs. Vacancies of larger atoms affect all systems similarly, causing greater reductions than hydrogen absences. Emerging flexible and stiff scattering centers, depending on bond strengths, are shown to cause thermal conductance reduction by changing nearby LDOSs in defected structures relative to pristine ones. This knowledge suggests that inferences on unknown thermal properties of novel defected materials can be made based on understanding how thermal transport behaves in their analogues and how bond characteristics differ between systems under consideration. The thermal conductance contributed by phonons is often a limiting factor to the overall suitability of a material for use in thermoelectric power generation, wherein a voltage is generated in a material by a temperature gradient. The thermoelectric figure of merit (ZT) assesses this suitability, in part based on a ratio of electrical conductance to thermal conductance. These two properties can be decoupled in low-dimensional structures like NRs, with lower thermal conductances generally found in narrower materials. Here, ZT is analyzed in graphene, BN, and silicene nanoribbons of two different widths with engineered edges that are designed to increase the ratio of edge length to NR length. This could conceivably be synthesized by either top-down or bottom-up methods. Analyses are made of how width and material change the maximum ZT attainable by controlling the chemical potential of each system, how these maximum ZTs differ in each system as a result of p- or n- type change to chemical potential, how full-width half-maximum values of ZT peaks behave, and how the different factors of ZT affect its final value in these systems. A very high ZT of 6.26 is reported near the bandgap in the narrow chevron silicene NR at room temperature, and a room temperature ZT greater than 3 is also found in the narrow BN NR, suggesting that edge-engineered NRs offer high promise for thermoelectric applications and may be suitable for emissions-free electricity generation from waste heat sources.
Department or Program
Department of Mechanical and Materials Engineering
Year Degree Awarded
Copyright 2016, all rights reserved. My ETD will be available under the "Fair Use" terms of copyright law.