UC San Diego

Calorimetry is a heat measuring process that quantifies heat generation, loss, and transport. Calorimetry holds significant values to a multitude of scientific disciplines such as nanoscale heat transfer, bolometer infrared detection, and drug discovery. This dissertation focuses on two aspects of calorimetry: 1) application of a calorimetry technique to investigate fundamentals of thermal transport at micro/nanoscale level, and 2) development of high-resolution microfluidic calorimeters for biological applications.

A modulated calorimetry technique, namely, the 3ω method, was used to study thermal transport across highly mismatched interfaces, where Au/Si multilayers (MLs), were used as the model material. By leveraging thermal resistance at interfaces of Au and Si, which have highly dissimilar acoustic impedance, we experimentally demonstrated an ultralow thermal conductivity, κ, of 0.33 ± 0.04 W m-1K-1 at room temperature with a high interfacial density of ~0.2 interface nm-1. The measured κ was the lowest amongst inorganic MLs with similar interfacial density. The 3ω method was also applied to measure thermal conductivity of the near-surface regime of tungsten damaged by ion irradiation used in plasma facing components (PFCs) in fusion reactors. The measurement showed a nearly 60\% reduction in κ in comparison to pristine tungsten, which can be detrimental to the mechanical integrity of the PFCs under heat fluxes

The second thrust of the dissertation is focused on the development of high-resolution calorimeters. A high performance resistive thermometer material, NbNx, was developed and integrated into a suspended bridge calorimeter. When used with a modulated heating current and a differential instrumentation schemes, the NbNx-based calorimeter demonstrated an exceptional high power resolution of 0.26 pW at room temperature. Furthermore, the high-resolution calorimetry was extended to microfluidic platform for biological applications. Owing to the low parasitic heat loss and a long-term temperature stability platform, the biocalorimeters developed from this study possess a significant enhancement over the state-of-the-art long-term power detection limit, which enable the detection of metabolic rate of individual living cells in the microfluidic channels. Such a biocalorimetry technique will open up new opportunities in a biomedical field such as in cell and tissue metabolism, drug discovery and screening, and antibody-antigen detection.