Nanoscale thermal transport through solid-solid and solid-liquid interfaces

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Virginia Tech

This dissertation presents an experimental investigation of heat transport through solid- solid and solid-liquid interfaces. Heat transport is a process initiated by the presence of a thermal gradient. All interfaces offer resistance to heat flow in the form of temperature drop at the interface. In micro and nano scale devices, the contribution of this resistance often becomes comparable to, or greater than, the intrinsic thermal resistance offered by the device or structure itself. In this dissertation, I report the resistance offered by the interfaces in terms of interface thermal conductance, G, which is the inverse of Kapitza resistance and is quantified by the ratio of heat flux to the temperature drop. For studying thermal transport across interfaces, I adapted a non-contact optical measurement technique called Time-Domain Thermoreflectance (TDTR) that relies on the fact that the reflectivity of a metal has a small, but measurable, dependence on temperature.

The first half of this dissertation is focused on investigating heat transport through thin films and solid-solid interfaces. The samples in this study are thin lead zirconate- titanate (PZT) piezoelectric films used in sensing applications and dielectric films such as SiOC:H used in semiconductor industry. My results on the PZT films indicate that the thermal conductivity of these films was proportional to the packing density of the elements within the films. I have also measured thermal conductivity of dielectric films in different elemental compositions. I also examined thermal conductivity of dielectric films for a variety of different elemental compositions of Si, O, C, and H, and varying degrees of porosity. My measurements showed that the composition and porosity of the films played an important role in determining the thermal conductivity.

The second half of this dissertation is focused on investigating heat transport through solid-liquid interfaces. In this regard, I functionalize uniformly coated gold surfaces with a variety of self-assembled monolayers (SAMs). Heat flows from the gold surface to the sulfur molecule, then through the hydrocarbon chain in the SAM, into the terminal group of the SAM and finally into the liquid. My results showed that by changing the terminal group in a SAM from hydrophobic to hydrophilic, G increased by a factor of three in water. By changing the number of carbon atoms in the SAM, I also report that the chain length does not present a significant thermal resistance. My results also revealed evidence of linear relationship between work of adhesion and interface thermal conductance from experiments with several SAMs on water. By examining a variety of SAM-liquid combination, I find that this linear dependency does not hold as a unified hypothesis. From these experiments, I speculate that heat transport in solid-liquid systems is controlled by a combination of work of adhesion and vibrational coupling between the omega-group in the SAM and the liquid.

Nanoscale heat transport, kapitza resistance, thermoreflectance, solid-liquid interfaces, self-assmebled monolayer