Department of Applied Physics
Ph.D. Dissertation Defense
Time- and Momentum-Resolved Nanoscale Thermal Transport
Michael Edmund Kozina, PhD Candidate
Research Advisor: Professor David Reis
There is a growing need for materials with non-traditional transport properties. For example in microcircuits, materials that can dissipate heat rapidly while acting as electrical insulators are sought for efficient heat sinks. Thermoelectric materials, useful for converting waste heat into electricity and for providing all solid-state (coolant-free) refrigeration, demand the exact opposite: thermal insulators with large electrical conductivities. Thus there is interest in ways to tailor the thermal transport properties independent of electrical properties.
Currently ways to probe the microscopic physics underlying thermal transport in solids, however, are restricted to external mechanisms, for example contact-based transport measurements or non-contact thermoreflectance imaging. X rays on the other hand, because of their penetrating power, are able to explore the details of thermal transport below the surface. Along these lines, I have developed means of using time-resolved x-ray diffraction to probe microscopic thermal transport in materials with embedded nanoparticles, across thin-film interfaces, and in working miniaturized thermoelectric devices.
In this talk I will focus on one particular experiment exploring embedded ErAs nanoparticles in GaAs. It has been known that ErAs nanoparticles embedded in InGaAs will diminish the thermal conductivity while maintaining a sufficiently large electrical conductivity, thus enhancing the material’s thermoelectric capabilities. It was proposed that the nanoparticles provide additional sites to scatter phonons of nm-scale wavelength while not disrupting the flow of electrical current. To explore this hypothesis, we used the recently-developed Fourier-Transform Inelastic X-ray Scattering (FT-IXS) technique to measure the lifetimes of phonons in the time domain in a model system, GaAs doped with ErAs nanoparticles. We found that phonons in ErAs:GaAs with wavelengths similar to the nanoparticle size do not show significantly reduced lifetimes compared to pure GaAs. In addition, we discovered that the presence of nanoparticles in the GaAs enhances the FT-IXS signal and so opens the possibility of extending the technique to a larger spectral range.