The Fluorescence Scanning Thermal Microscope (FSTM v3.0), was designed to create an inexpensive, and easily manufactured, device for measuring the diffusivity of samples with microscopic locational precision. This was accomplished by using a Blu-ray device known as a PHR-803T, referred to in this work as a PHR. The optics in the PHR are nearly identical in function to conventional devices used in thermoreflectance microscopy, making the PHR extremely useful to integrate into the FSTM design. The focus of this thesis is the application of the FSTM as a confocal microscope using 3D printed components and various low-cost devices to operate with comparable sampling accuracy to existing confocal microscopes. The electronics and optical filters were then adapted to enable the measurement of thermal waves, particularly by detecting a linear relationship between phase delay and the spacing between heating and sensing lasers, as predicted by previous work on the FSTM.

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High energy consumption is a crucial challenge in gas separation processes. With current energy intensive separation methods, there is a real need for more energy-efficient alternative technologies. Membrane technology demonstrates potential uses in industrial separation processes due to its potential energy efficiency, environmental friendliness, and small footprint. The continuous developments in material science contributed directly in enhancing the membrane performance through several engineering modifications such as thermal annealing, which presented visible improvements in gas permeation properties. The objective of this project was to investigate the thermal annealing of three fluorinated polymers (PAE1, PAE2, and TFMPD), aiming for favorable changes in gas permeation properties. In particular, each polymer was annealed for 3 h at various temperature values, targeting the intermediate stage, which is the zone where degradation started but a pure carbon structure stage was not formed yet. Overall, the thermal annealing study revealed that TFMPD had highest pure-gas separation performance among other polymers, in which the Robeson plots displayed that treated sample at 500 ºC surpassed the 2015 H2/CH4 upper bound, whereas the treated sample at 550 ºC surpassed 2019 upper bound of both CO2/CH4 and CO2/N2. Therefore, TFMPD can be a potential candidate polymer for membrane-based gas separation, especially for CO2 and H2 applications. This performance could be attributed to the internal structural changes in the polymer that occurred during thermal annealing. Hence, several characterization techniques were performed to detect these changes. For instance, it was realized that all polymers started crosslinking upon the thermal treatment at 350 ºC. Moreover, FTIR analysis indicated the release of several functional groups from treated polymers at high temperature values. Raman spectroscopy also confirmed that the observed substantial enhancement in gas permeation of annealed TFMPD at 550 ºC was due an early-stage carbon structure formation. Furthermore, several recommendations are proposed to continue the work in this project, which could lead to potential success of the thermally annealed polymers tested in this study in membrane-based gas separations applications.

To assess the long-term performance of a solar thermal system, mathematical models that accurately capture the effects of heat transfer within and interactions between individual components are required. For solar domestic hot water systems, the components can include the solar collectors, storage tanks, heat exchangers, pumps, and associated piping. In addition, weather data and demand profiles are also required. Simplified models for each component are needed to reduce the computational time required to run long-term simulations. The simplified models, however, must also be sufficiently accurate in order to provide meaningful system-level results. Accurate prediction of the temperature profiles in the storage tanks of these systems is important since the temperature within the tank has a large impact on the efficiency of the entire system. TRNSYS, which is a commercial code commonly used for such simulations, contains a variety of different one-dimensional storage tank models. Previous research has indicated that these models have deficiencies in predicting experimental data. Therefore, this thesis is focussed on the analysis of the tank modelling used in TRNSYS. Results of this thesis show that the poor predictions are a result of numerical diffusion due to insufficient grid resolution. The correct theoretical profiles could be obtained by using a large number of nodes. However, this would lead to a significant increase in computational time. Alternative modelling strategies were also developed using analytical techniques to more accurately predict the temperature profiles within a storage tank while keeping a relatively low computational cost. Different models were created which considered the different mixing mechanisms present in a storage tank, such as increasing inlet temperatures with time, heat losses to the surroundings, tank wall heat conduction, and inlet jet mixing. / Thesis / Master of Applied Science (MASc)

Since strain changes the interatomic spacing of matter and alters electron and phonon dispersion, an applied strain ϵ can modify the thermal conductivity κ of a material. This thesis shows how the strain induced by heteroepitaxy is a passive mechanism to change κ in a thin film and how the film thickness is key to the functional form of κ(ϵ). Molecular Dynamics simulations of the physical vapor deposition and epitaxial growth of ZnTe thin films provide insights into the role of interfacial strain on the thermal conductivity of a deposited film. ZnTe films grown on a lattice mismatched CdTe substrate exhibit ~6% in-plane biaxial tension and ~7% out-of-plane uniaxial compression. In the T=700 K to 1100 K temperature range, the conductivities of strained ZnTe layers that are 5 unit cells thick decrease by ~ 35%, a result that is relevant to thermoelectric devices since strain can also enhance charge mobility and increase their overall efficiency. The resulting understanding of dκ/dT shows that strain engineering can also be used to create a thermal rectifier in a material that is partly strained and partly relaxed, like at the junction of an axial nanowire heterostructure. To better isolate the role of strain, the study is extended to free-standing ZnTe films with thicknesses between 116 Å to 1149 Å under the application of both uniform and biaxial strain between -3% to 3% at 300 K. Since the boundaries of the film are diffuse, κ becomes size dependent when the film thickness approaches the order of the mean free path of the phonons. As this thickness is decreased, the magnitude of κ decreases until boundary scattering dominates so that κ(ϵ) depends on v_g (ϵ). This conclusion is important as it can be generalized to other materials and potential functions; it suggests that if a film is thin enough for boundary scattering to dominate, then the behavior of κ(ϵ) can be predicted based on the bulk dispersion curve alone, which should greatly simplify strain-based device design. / Thesis / Doctor of Philosophy (PhD) / Since strain changes the interatomic spacing of matter and alters electron and phonon dispersion, an applied strain ϵ can modify the thermal conductivity κ of a material. This thesis shows how the strain induced by heteroepitaxy is a passive mechanism to change κ in a thin film and how the film thickness is key to the functional form of κ(ϵ). Molecular Dynamics simulations of the physical vapor deposition and epitaxial growth of ZnTe thin films provide insights into the role of interfacial strain on the thermal conductivity of a deposited film. The result is relevant to thermoelectric devices since strain can also enhance charge mobility and increase their overall efficiency. The resulting understanding of dκ/dT shows that strain engineering can also be used to create a thermal rectifier in a material that is partly strained and partly relaxed, like at the junction of an axial nanowire heterostructure.

The thermal performance of the power MOSFET module is subject to change over its lifetime. This is caused by the growth of voids and other defects in the die-attach layer. The goal of this dissertation is to develop measurement techniques and finite element simulations that can measure the changes in thermal performance caused by changes in die-attach voided area. These experimental results and simulations can then be used to create predictions of the thermal performance of a particular power semiconductor module at various stages of die-attach fatigue. In the results and simulations presented, a relationship is developed between thermal impedance and void area coverage. This dissertation starts by presenting an analysis of the thermal and mechanical stresses needed for crack and void growth in the power semiconductor die-attach region. Accelerated life testing is then performed for both commercial and prototype power semiconductor devices to generate the stresses needed to precipitate void growth. Representative groups of lead and lead-free solders are then tested to compare levels of die-attach degradation under accelerated life conditions. Hardware is developed to experimentally measure thermal impedance using temperaturesensitive characteristics of the power MOSFET. The power semiconductor devices that were subjected to accelerated life testing are then measured with this hardware. The results show that die-attach voided area coverage increases thermal impedance. Representative lumped parameter thermal models that use R-C circuits are derived to demonstrate the ability of the thermal impedance analyzer to determine the differences in the die-attach layer. Finite element modeling (FEM) is then used on representative voided devices to support these results, with additional emphasis on peak temperatures caused by hotspots located over the voided areas. Experimental techniques are further applied to measurement of cooling trends that occur due to the existence of voids in the die-attach layer. These measurements are correlated with finite element thermal simulations to develop a relationship between thermal impedance, hotspot temperature, die-attach void size, and total voided area coverage. / Ph. D.

There is growing interest in the use of microwaves to process materials. The main application of microwave processing of materials is in heating. The most important characteristic of microwave heating is {\it volumetric} heating, which is quite different from conventional heating where the heat must diffuse in from the surface of the material. Volumetric heating means that materials can absorb microwave energy directly and internally and convert it to heat. It is this characteristic that leads to advantages such as rapid, controlled, selective, and uniform heating. However, some problems hinder the widespread use of microwave energy. One of these problems is called thermal runaway, which is a type of thermal instability due to the interaction between the electromagnetic waves and materials. As thermal runaway occurs, the temperature of the heated material rises uncontrollably. The normal consequence of thermal runaway is the damage of the processed materials. The origins of thermal runaway are different under different processing conditions. When processing ceramic materials, thermal runaway is mainly due to the positive temperature dependence of dielectric loss of the material. These materials absorb more microwave energy as they are being heated. The most plausible explanation of this phenomenon is the so-called "S-curve" theory. However, prior to this work, no direct experimental evidence has been published to verify this theory. In this dissertation, we report the direct experimental evidence of the so-called "S-curve" by heating thermal runaway materials in a microwave resonant cavity applicator. A complete discussion of how the experimental results were achieved is presented. From the experimental results, we find that by the use of the cavity effects thermal runaway can be controlled. To explain the experimental findings, a theoretical model based on equivalent circuit theory is developed. Also, a coupled heat transfer and electromagnetic field model is developed to simulate the heating process. Both models give reasonably good comparison with our experimental results. Finally, a method to control thermal runaway is described. / Ph. D.

Thermal management has become a major concern in the design of current and future more and all electric aircraft (M/AEA). With ever increasing numbers of on-board heat sources, higher heat loads, limited and even decreasing numbers of heat sinks, integration of advanced intelligence, surveillance and reconnaissance (ISR) and directed energy weapons, requirements for survivability, the use of composite materials, etc., existing thermal management systems and their components have been pushed to the limit. To address this issue, more efficient methods of thermal management must be implemented to ensure that these new M/AEA aircraft do not overheat and prematurely abort their missions. Crucial to this effort is the need to consider advanced heat exchanger concepts, comparing their designs and performance with those of the conventional compact exchangers currently used on-board aircraft thermal management systems. As a step in this direction, the work presented in this thesis identifies two promising advanced heat exchanger concepts, namely, microchannel and phase change heat exchangers. Detailed conceptual design and performance models for these as well as for a conventional plate-fin compact heat exchanger are developed and their design and performance optimized relative to the criterion of minimum dry weight. Results for these optimizations are presented, comparisons made, conclusions drawn, and recommendations made for future research. These results and comparisons show potential performance benefits for aircraft thermal management incorporating microchannel and phase change heat exchangers. / Master of Science