Impact of phonon scattering and transmission at interfaces on thermal conduction in nanostructured materials [electronic resource]
Keywords: Mechanical Engineering.

Controlling heat transfer is important for many applications, such as in electronic devices that increasingly need to dissipate heat, and energy conversion devices that require selective control of dissimilar energy carriers. Depending on the application, thermal properties may need to be either suppressed or enhanced beyond what is possible with conventional materials. While much previous research has focused on these two aims, interfaces still pose a major obstacle to heat dissipation, and fine control of thermal energy carriers remains challenging. Nanostructuring offers a promising approach towards either reducing or increasing the conductivity: on the one hand, there is increased scattering of energy carriers at the nanoscale, while on the other, one can enable increased surface area to volume ratio and hence, material interaction at small length scales. First we investigate the impact of phonon scattering from boundaries that are oriented normal to the direction of a dominant heat flow. Specifically, we study phonon conduction along tortuous silicon nanobeams resembling S-shaped labyrinths, which feature more than two orders of magnitude variation of the free "line-of-sight" available to phonons between the heat sources and sinks. These features enable a detailed study of the impact of phonon scattering from interfaces that are oriented normal to heat flux. In the tortuous labyrinths studied here, phonons scatter from both the longitudinal and transverse constrictions as they are forced around nanoscale corners and bends. Experimental thermal transport measurements are combined with a theoretical Boltzmann transport description to quantify the impact of forward scattering on phonon conduction. The phonon line-of-sight is proposed as an important metric for characterizing and modulating phonon propagation in nanostructures. We further explore the impact of interface orientations depending on whether scattering characteristics are diffusive or specular. A combination of low-temperature thermal conductivity measurements and simulations using the Monte Carlo - Boltzmann transport formalism shows that ballistic phonons are confined when the line-of-sight is limited and surface scattering is specular. This finding suggests that heat-carrying phonons can be manipulated by controlling the orientation of scattering surfaces relative to heat flow direction below the conventional limit with specular phonon scattering. Second, we explore thermal conduction through nonplanar interfaces with a focus on sidewall interfaces that lie perpendicular to the primary plane of the substrate in microfabrication. We report the first direct measurements of thermal resistance of metal-dielectric sidewall interfaces. Achieving this result required major steps forward in metrology as well as efforts to interpret the physical mechanisms including phonon transport and modified defect spectra. We demonstrate here a novel experimental technique based on a suspended nano-grating platform and electrical thermometry. Using transmission electron microscopy, we investigate the impact of microstructural disorder on energy carrier transport, particularly the connection between etching direction and interface roughness. The results suggest potential nonuniformity in thermal boundary resistance for nonplanar interfaces. Furthermore, we utilize the nonplanar interfaces to enhance thermal conduction across interfacial region. In this thesis, we demonstrate a significant enhancement in thermal conduction at an Al-SiO2 interface using a ~65 nm thick nanostructured fin array. The interface consists of interdigitating Al pillars embedded within SiO2 with characteristic feature size ranging from 100 nm to 800 nm, and the total sidewall surface area is increased by up to 8 times while maintaining the same Al:SiO2 fill fraction. We experimentally show that the effective conductance of a nanostructured interface layer monotonically increases with decreasing pitch and, at the smallest pitch, is more than twice that of a planar layered stack with the same volume ratio. This study suggests that controlled nonplanarity can be useful for enhancing the effective thermal conductance of an interface between any pair of solid materials with sufficient contrast in thermal conductivity, and comparable magnitudes of volumetric and interfacial resistances.