Date

2016

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Oztekin, Alparslan

Other advisers/committee members

Neti, Sudhakar; Rickman, Jeffrey M.; Singh, Dileep

Abstract

Advanced semiconductor materials for thermoelectric applications often comprise of nanostructured grains in order to take advantage of phonon scattering phenomenon at the grain boundaries and thus increase the thermoelectric figure of merit for the material. Opportunities for further improvements in the figure of merit are available via usage of appropriate dopants. In this report, thermal transport across low-angle, symmetric tilt grain boundaries in β-SiC is studied and the influence of dopants, introduced at these grain boundaries, on the phononic transmission across the grain boundary is investigated.Non-equilibrium molecular dynamics (NEMD) simulation are used to gain insights into the impact of grain-boundary segregation on Kapitza resistance of doped β-SiC at high-temperature. In particular, the role of dopant concentration and dopant/matrix interaction strength in determining the resistance is assessed. Dopants that adhere to the matrix material with the same strength as they adhere to other dopant atoms are determined to spread out across the grain boundary cross-section forming a layered structure and resulted in a concomitant gradual increase in resistance with increase in dopant concentration. Whereas, for relatively weak dopant/matrix interaction strengths, dopant clustering predominates, and the Kapitza resistance increases significantly for small changes in dopant concentration. The different interaction strength regimes are investigated by mapping the spatial distribution of temperature at the grain boundary cross-section and calculating the degree of structural disorder. It was found that the dopant clusters lead to a heat flux parallel to the grain boundary plane and a significant increase in boundary disorder, partly explaining the observed increase in Kapitza resistance at the boundary. A comparison of the local vibrational density of states for the weak and strong dopant/matrix interaction strength cases is performed and a subset of modes that are significant for thermal transport in this system are identified. It is determined that for the nano-structures studied, the loss of optical phonon modes that have typically been ignored for thermal transport analyses, resulted in a more significant increase in Kapitza resistance at the grain boundary. This analysis is complemented by calculations of the projected density of states and a corresponding eigenmode analysis of the dynamical matrix that highlight important phonon polarizations and propagation directions. We also examine the dependence of the Kapitza resistance on temperature, dopant mass and dopant/matrix interaction strength, the latter parameter affecting grain-boundary structure and, hence, phonon scattering.The study concludes with an investigation into the effects of grain boundary orientation and the local grain boundary energy on phonon scattering at the boundaries. More specifically, the impact of dopants on the interface resistance is examined for these boundaries. It is observed that for the methodology used to create the grain boundary systems, the interface resistance was independent of the grain boundary orientation irrespective of the dopant concentration. However, grain boundary energy had distinct effects on the interface resistance. Layering dopant caused an increase in disorder at the grain boundaries in higher energy system resulting in an increase in phonon scattering and therefore higher interface resistance.

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