Clathrate materials comprise compounds in which guest atoms or molecules can be encapsulated inside atomic cages formed by host framework polyhedra. The unique relationship that exists between the guest species and its host results in a wide range of physical phenomena, and offers the ability to study the physics of structure-property relationships in crystalline solids. Clathrates are actively being investigated in fields such as thermoelectrics, superconductivity, optoelectronics, and photovoltaics among others. The structural subset known as type II clathrates have been studied far less than other clathrates, and this forms the impetus for the present work. In particular, the known “composition space” of type II clathrates is small, thus the need for a better understanding of possible compositions is evident. A basic research investigation into the synthesis and characterization of silicon and germanium type II clathrates was performed using a range of synthetic, crystallographic, chemical, calorimetric, and transport measurement techniques. A series of framework substituted type II germanium clathrates has been synthesized for the first time, and transport measurements indicate that these compounds show metallic behavior. In the course of the investigation into type II germanium clathrates, a new zeolite-like framework compound with its corresponding novel crystal structure has been discovered and characterized. This compound can be described by the composition Na 1-xGe3 (0 < x < 1), and corresponds to a new binary phase in the Na-Ge system. One of the most interesting aspects of type II clathrates is the ability to create compounds in which the framework cages are partially occupied, as this offers the unique opportunity to study the material properties as a function of guest content. A series of type II sodium-silicon clathrates Na xSi136 (0 < x < 24) has been synthesized in higher purity than previously reported for as-synthesized products. The transport properties of the Na xSi136 clathrates exhibit a clear dependence on the guest content x. In particular, we present for the first time thermal conductivity measurements on Na xSi136 clathrates, and observe evidence that the guest atoms in type II clathrates affect the thermal transport in these materials. Some of the crystalline Na xSi136 compounds studied exhibit very low thermal conductivities, comparable in magnitude to amorphous materials. In addition, for the first time clear evidence from transport measurements was found that resonance phonon scattering may be present in type II clathrates, as is also the case in the type I subset.

Open celled micro and nano foams fabricated from polymers and metals have attracted tremendous attention in the recent past because of their applications in numerous areas such as catalyst carriers, filtration media, ion exchange membranes and tissue engineering scaffolds. In this study open celled polymer micro- and nano foams with controllable pore size and porosity were fabricated via solid state foaming of immiscible blends. The polymer foams were used as templates for fabricating nickel foams using an ethanol based electroless plating process. Thermal conductivity of micro- and nano foams was studied as a function of pore size and porosity using finite element and molecular dynamics based models. The effect of pore size and porosity on performance of phase change material infiltrated metal foams for thermal management was investigated via numerical models. Open celled micro foams were fabricated via solid state foaming of ethylene acrylic acid (EAA) and polystyrene (PS) co-continuous blends. Blending temperature was the main parameters affecting the formation of co-continuous structure. Gas saturation and foaming studies were performed to determine ideal processing conditions for the blend. The results indicated that saturation pressure and foaming temperature were major process parameters determining the porosity of the foamed samples. Open celled polymer templates were obtained by selective extraction of PS phase using dichloromethane (DCM). Foaming resulted in faster extraction of PS and also in a higher porosity. Open celled nano foams were fabricated via solid state foaming of polyetherimide (PEI) and polyethersulfone (PES). The effect of process parameters namely saturation pressure and temperature, desorption time, and foaming temperature and time on porosity and pore size was studied. A high gas concentration and foaming temperature were required to obtain nano pore-sized foams. Throughout the cross section there existed regions with varying pore size and porosity and solid skins at the surface regions of the foam. A solvent surface dissolution process using dimethylformamide (DMF) was employed to access the internal porous structure. Micro- and nano cellular nickel foams were fabricated from EAA and PES templates via electroless plating. The structure of the nickel foams was an inverse of the polymer templates. Ethanol based electroless plating solutions were used to ensure infiltration into the porous structure because of the small pore sizes. Finite element and molecular dynamics based models were developed to predict thermal conductivity of polymer foams as a function of pore size and porosity. Pore sizes ranging from 1 nm to 1 mm were studied. Models were partially validated using experimental data. The results showed that pore size has significant effect on thermal conductivity even for microcellular and conventional foams. When the pore size is reduced to the nanometer scale, the thermal conductivity of the nano foam dramatically reduces and the value could be lower than that of air for certain porosity levels. The extremely low thermal conductivity of polymer nanofoams is possibly due to increased phonon-phonon scattering in the solid phases of the polymer matrix in addition to low thermal conductivity of gas trapped in nano sized pores. Finite element based models were also developed to study the effect of pore size and porosity on performance of phase change material infiltrated metal foams for thermal management applications. The results showed that foams with smaller pore sizes can delay the temperature rise of the heat source for an extended period of time by rapidly dissipating heat in the phase change material. The lower temperatures resulting from the use of a smaller pore size metal foam could significantly increase the lifetime of IC chips. / text

The hydrogen (H2) gas sensing mechanism driving 3C-SiC resistive gas sensors is investigated in this work in which two hypotheses are proposed. One hypothesis involves the surface adsorption of H2 on the sensor surface with the adsorbed molecules influencing the flow of current in a resistive gas sensor, termed the surface adsorption detection mechanism. The second hypothesis includes the transfer of heat from the sensor to the gas, producing a change in the temperature of the device when the heat transfer characteristics of the gas change, termed the thermal detection mechanism. The heat transfer characteristics of the gas are dependent on the thermal conductivity of the gas, a property which is a strong function of gas composition. Thus, the thermal detection mechanism mainly detects changes in the thermal conductivity of a gas or gas mixture.Initial experiments suggested the surface adsorption mechanism as the detection mechanism of resistive 3C-SiC gas sensors. However, these experiments were performed in the absence of device temperature measurements. Recent experiments in which the device temperature was measured with a resistance temperature detector (RTD) in thermal contact with the device strongly support the thermal detection mechanism as being responsible for hydrogen gas detection. Experimental observations show the temperature of the resistive 3C-SiC hydrogen gas sensors changes greatly with changing hydrogen gas composition. For example, a 3C-SiC/SOI resistive sensor biased at 10 Vdc displayed a change in temperature from ~400°C to ~216°C, correlating to a change in current from ~41 mA to ~6mA, upon the introduction of 100% H2. The this 3C-SiC/SOI resistive sensor, this large decrease in temperature caused a large increase in resistance which is detected as a decrease in current. Several different experiments have also been performed to confirm the thermal detection mechanism hypothesis.

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This dissertation is focused on thermal transport at nanometer scale point and line constrictions and in nanowires with sawtooth surface roughness. To better understand thermal transport at a point contact such as that at the tip-sample junction of a scanning probe microscope, a Non Equilibrium Molecular Dynamics (NEMD) method is employed to calculate the temperature distribution and thermal resistance of a nanoscale point constriction formed between two silicon substrates. The simulation reveals surface reconstruction at the two free silicon surfaces and at the constriction. The radius of the heated zone in the cold substrate approaches a limit of about 20 times the average nearest-neighbor distance of boron doping atoms when the constriction radius (a) is reduced below the inter-dopant distance. The phonon mean free path at the constriction is suppressed by diffuse phonon-surface scattering and phonon-impurity scattering. The MD thermal resistance is close to the ballistic resistance when a is larger than 1 nm, suggesting that surface reconstruction does not reduce the phonon transmission coefficient significantly. When a is 0.5 nm and comparable to the dominant phonon wavelength, however, the NEMD result is considerably lower than the calculated ballistic resistance because bulk phonon dispersion and bulk potential are not longer accurate. The MD thermal resistance of the constriction increases slightly with increasing doping concentration due to the increase in the diffusive resistance. The NEMD method is further employed to calculate the temperature distribution and thermal resistance at nanoscale line constrictions formed between two silicone substrates. Similar to the nano point constriction, the thermal resistance at the nano line constriction is dominated by the ballistic resistance for constriction width in the range of 1 nm to 12 nm. An additional question that this dissertation seeks to answer is whether one can engineer the surface roughness on a nanowire to facilitate phonon backscattering so as to reduce the thermal conductivity below the diffuse surface limit. Monte Carlo simulation is used to show that phonon backscattering can occur at sawtooth surfaces of a silicon nanowire, suppressing the thermal conductivity below the diffuse surface limit. Asymmetric sawtooth nanowire surfaces can further cause phonon rectification, making the axial thermal conductance of the nanowire direction dependent. The phonon backscattering and rectification effects can be employed to enhance the thermoelectric figure of merit of nanowires. / text

As silicon-based transistor technology continues to scale ever downward, anticipation of the fundamental limitations of ultimately-scaled devices has driven research into alternative device technologies as well as new materials for interconnects and packaging. Additionally, as power dissipation becomes an increasingly important challenge in highly miniaturized devices, both the implementation and verification of high mobility, high thermal conductivity materials, such as low dimensional carbon nanomaterials, and the experimental investigation of heat transfer in the nanoscale regime are requisite to continued progress. This work furthers the current understanding of structure-property relationships in low dimensional carbon nanomaterials, specifically carbon nanotubes (CNTs) and graphene, through use of combined thermal conductance and transmission electron microscopy (TEM) measurements on the same individual nanomaterials suspended between two micro-resistance thermometers. Through the development of a method to measure thermal contact resistance, the intrinsic thermal conductivity, [kappa], of multi-walled (MW) CNTs is found to correlate with TEM observed defect density, linking phonon-defect scattering to the low [kappa] in these chemical vapor deposition (CVD) synthesized nanomaterials. For single- (S) and double- (D) walled (W) CNTs, the [kappa] is found to be limited by thermal contact resistance for the as-grown samples but still four times higher than that for bulk Si. Additionally, through the use of a combined thermal transport-TEM study, the [kappa] of bi-layer graphene is correlated with both crystal structure and surface conditions. Theoretical modeling of the [kappa] temperature dependence allows for the determination that phonon scattering mechanisms in suspended bi-layer graphene with a thin polymeric coating are similar to those for the case of graphene supported on SiO₂. Furthermore, a method is developed to investigate heat transfer through a nanoscale point contact formed between a sharp silicon tip and a silicon substrate in an ultra high vacuum (UHV) atomic force microscope (AFM). A contact mechanics model of the interface, combined with a heat transport model considering solid-solid conduction and near-field thermal radiation leads to the conclusion that the thermal resistance of the nanoscale point contact is dominated by solid-solid conduction. / text

The ongoing search for energy conservation in built structures and during the construction process prompted this thesis work to explore the use of sustainable technologies for floor heating systems. The thesis work explores the use of thermoplastic material as a sustainable substitute material for future floor heating systems. Concrete materials are presently used extensively for floor heating systems. Thermoplastic materials are seldom used for floor heating and the primary focus of this thesis is to explore the suitability & adaptability of thermoplastics as an innovative energy saving floor heating material. A thorough study of energy demands and the impact on environment due to greenhouse gas emissions has been done. Thermoplastic materials are environmental friendly and light weight. They exhibit high thermal conductivity which is favourable for the floor heating systems. A design technique has been developed for the use of thermoplastic materials as an energy efficient floor heating material. The present technique creates a new modular floor heating system. The design technique uses thermoplastic material of size 2.4m x1.2m with embedded electric heaters. Thermoplastic foam panels act as a single building block. A numerical simulation has been carried out to study the heat transfer characteristics of the proposed material. Limited experiments were conducted to verify the validity of the simulation results. The results from the experiments indicate good agreement with simulation results. The energy savings from the thermoplastic floor heating systems have been compared with that of electrical floor heating systems. The adaptability of the new floor heating system in terms of energy savings and cost benefit analysis is also discussed. / sustainable floor heating system

The A2Mo3O12 family, where A3+ is a large trivalent cation, can show interesting thermal properties such as negative thermal expansion, also known as thermomiotic behavior, where the overall volume of the material contracts with increasing temperature. A selection of compounds in this family, namely HfMgMo3O12, In2Mo3O12, Y2Mo3O12, Al2Mo3O12, In(HfMg)0.5Mo3O12, and In1.5(HfMg)0.25Mo3O12, have been synthesized using solid-state and mechanical activation techniques as well as a simplified sol-gel approach (Al2Mo3O12). Coefficients of thermal expansion were found to range from large-negative to low-positive in the orthorhombic phase, including near-zero in In(HfMg)0.5Mo3O12 and In1.5(HfMg)0.25Mo3O12. This set of materials provided insight into the role of low-frequency phonon modes in open-framework materials. Low-temperature heat capacity and thermal conductivity measurements confirmed that low-frequency modes were active in thermomiotic materials, and also present to some extent in all members of the open-framework A2Mo3O12 family examined. A clear correlation exists between the magnitude and sign of the coefficient of thermal expansion in the orthorhombic phase and the contribution of low-energy modes to the low-temperature heat capacity, with negative thermal expansion materials having a larger contribution. The low-frequency phonon modes result in low thermal conductivity and reduced phonon mean free paths when compared to conventional ceramics and indicate that these low values are characteristic of open-framework materials in NTE families even if the materials in the families are not thermomiotic themselves.

The objective of this research was to design aluminum powder metallurgy (PM) alloys and processing strategies that yielded sintered products with thermal properties that rivaled those of the cast and wrought aluminum alloys traditionally employed in heat sink manufacture. Research has emphasized PM alloys within the Al-Mg-Sn system. In one sub-theme of research the general processing response of each PM alloy was investigated through a combination of sintering trials, sintered density measurements, and microstructural assessments. In a second, the thermal properties of sintered products were studied. Thermal conductivity was first determined using a calculated approach through discrete measurements of specific heat capacity, thermal diffusivity and density and subsequently verified using a transient plane source technique on larger specimens. Experimental PM alloys achieved >99% theoretical density and exhibited thermal conductivity that ranged from 179 Wm-1K-1 to 225 Wm-1K-1. Thermal performance was largely dominated by the amount of magnesium present within the aluminum grains and in turn, bulk alloy chemistry. Data confirmed that the novel PM alloys were highly competitive with even the most advanced heat sink materials such as wrought 6063 and 6061. Two methods of thermal analysis were employed in order to determine the thermal conductivity of each alloy. This first consisted of individual analysis of the specific heat capacity (Cp), thermal diffusivity (?) and density (?) as a function of temperature for each alloy. The thermal conductivity (K) was subsequently determined through the relationship: K=C_p ??. The second means of thermal analysis was a direct thermal conductivity measure using a transient plane source (TPS). The thermal diffusivity and density of samples were both found to decrease with temperature in a linear fashion. Conversely, the specific heat capacity was found to increase with temperature. The only measured thermal property that appeared to be influenced by the alloy chemistry was the thermal diffusivity (and subsequently the calculated thermal conductivity). Both means of thermal analysis showed high thermal conductivity in alloys with low concentrations of magnesium, demonstrating the significance of having alloying elements in solid solution with aluminum. Overall, several alloys were developed using a press and sinter approach that produced higher levels of thermal conductivity than conventional aluminum heat sink materials. The highest thermal conductivity was achieved by alloy Al-0.6Mg-1.5Sn with a calculated value of 225.4 Wm-1K-1. This novel aluminum PM alloy was found to exceed both wrought 6061 and 6063 (195 and 217 Wm-1K-1 respectively). Furthermore, PM alloy Al-0.6Mg-1.5Sn was found to have a significant advantage over die-cast A390 (142 Wm-1K-1).

There is a growing need to better understand the relationship between time, strain rate, and temperature on the load-deformation behaviour of clay soils in engineering applications. These applications may include: infrastructure constructed in northern regions where climate change is a growing concern; disposal of nuclear waste; and, industrial structures, such as furnaces, foundries, and refrigeration plants. Temperature variations may induce changes in internal pressure in the soil, swelling and shrinkage, and affect the mechanical properties of the soil. This thesis presents thermal numerical modeling for two instrumented field sites in northern Manitoba. Thermal conductivity testing on samples from these sites and field data are used to calibrate these thermal numerical models. Various boundary conditions are examined. The capabilities of the models are evaluated to determine if the models adequately simulate and predict changes in temperature in geotechnical structures. A discussion is presented on the strengths and weaknesses in the models and the predictive capabilities of the models. The thesis then shifts into understanding the concepts of thermoplasticity and viscoplasticity and the mathematics relating these concepts. Mathematical models that describe these concepts are examined and compared with traditional soil mechanics approaches. The concepts of thermoplasticity and viscoplasticity are combined in an encompassing elastic thermo-viscoplastic (ETVP) model using a semi-empirical framework. A sensitivity analysis is used to evaluate quantitatively the response of the model. The model is then validated qualitatively against published laboratory data. Applications of the ETVP model are discussed.