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Porter, Aaron

Mechanical Engineering

2014

MS

Thermoelectrics materials offer many advantages over traditional mechanical energy conversion devices but their low efficiency is problematic. Recent research progress suggests that materials with nanoscale dimensions and/or made of nano-sized grains could provide a solution due to their unique thermal transport characteristics. However, our fundamental understanding of heat conduction in such nanomaterials is still limited. In particular, the role played by interfaces such as twin boundaries in thermal conductivity of nanowires (NWs) remains an unsettled question, despite the fact that NWs can easily form either coherent twinning superlattices or lengthwise twins in model semiconductors like Si.
This thesis presents a molecular dynamics simulation study of thermal transport in nanotwinned Si nanowires and bulk systems. The results show a 72.5% reduction in thermal transport in <111>-oriented Si NWs with twinning superlattices compared to <112> NWs with lengthwise twins, with twin boundaries running perpendicular or parallel to the wire axis, respectively, due to surface and interface scattering effects. Furthermore, it is found that thermal transport decreases, then increases, as the twin boundary spacing decreases, leading to a minimum in the thermal conductivity at a critical twin boundary density. These findings are critically important to understand thermal conductivity and achieve high thermoelectric figure of merits in nanotwinned systems, given that the growth process directly influences which type of twins will be produced.