A miniature, ultra-low power, sensitive, microbridge gas sensor has been developed.The heat loss from the bridge is a function of the thermal conductivity of thegas ambient. Miniature thermal conductivity sensors have been developed for gaschromatography systems [1] and microhotplates have been built with MEMS technologywhich operates within the mW range of power [2]. In this work a lower power microbridgewas built which allowed for the amplification of the effect of gas thermalconductivity on heat loss from the heated microbridge due to the increase inthe surface-to-volume ratio of the sensing element. For the bridge fabrication,CMOS compatible technology, nanolithography, and polysilicon surfacemicromachining were employed. Eight microbridges were fabricated on each die,of varying lengths and widths, and with a thickness of 1 μm. A voltagewas applied to the sensor and the resistance was calculated based upon thecurrent flow. The response has been tested with air, carbon dioxide, helium,and nitrogen. The resistance and temperature change for carbon dioxide was thegreatest, while the corresponding change for helium was the least. Thus the selectivity of the sensor todifferent gases was shown, as well as the robustness of the sensor. Another aspect of the sensor is that it hasvery low power consumption. The measuredpower consumption at 4 Volts is that of 11.5 mJ for Nitrogen, and 16.1 mJ forHelium. Thesensor responds to ambient gas very rapidly. The time constant not only showsthe fast response of the sensor, but it also allows for more accuratedetection, given that each different gas produces a different correspondingtime constant from the sensor. The sensor is able to detect differentconcentrations of the same gas as well. Fromthe slopes that were calculated, the resistance change at 5 Volts operation wasfound to be 2.05mΩ/ppm, 1.14 mΩ/ppm at 4.5 Volts, and 0.7 mΩ/ppm at 4 Volts. Thehigher voltages yielded higher resistance changes for all of the gases thatwere tested. Theversatility of the microbridge has been studied as well. Experiments were donein order to research the ability of a deposited film on the microbridge, inthis case tin oxide, to act as a sensing element for specific gases. In thissetup, the microbridge no longer is the sensing element, but instead acts as aheating element, whose sole purpose is to keep a constant temperature at whichit can then activate the SnO film, making it able to sense methane. In conclusion,the microbridge was designed, fabricated, and tested for use as an electrothermalgas sensor. The sensor responds to ambient gas very rapidly with differentlevels of resistance change for different gases, purely due to the differencein thermal conductivity of each of the gases. Not only does it have a fastresponse, but it also operates at low power levels. Further research has beendone in the microbridge's ability to act as a heating element, in which the useof a SnO film as the sensing element, activated by the microbridge, was studied. REFERENCES: 1. D. Cruz,J.P. Chang, S.K. Showalter, F. Gelbard, R.P. Manginell, M.G. Blain," Microfabricated thermal conductivity detector for themicro-ChemLabTM," Sensors andActuators B, Vol. 121 pp. 414-422, (2007). 2. A. G. Shirke, R. E. Cavicchi, S. Semancik, R. H. Jackson, B.G. Frederick, M. C. Wheeler. "Femtomolar isothermal desorption usingmicrohotplate sensors," J Vac Sci TechnolA, Vol. 25, pp. 514-526 (2007).

As technology has scaled into nanometer regime, manufacturing variations have emerged as a major limiter of performance (timing) in VLSI circuits. Issues related to timing are addressed in the first part of the dissertation. Statistical Static Timing Analysis (SSTA) has been proposed to perform full-chip analysis of timing under uncertainty such as manufacturing variations. In this dissertation, we propose an efficient sparse-matrix framework for a path-based SSTA. In addition to an efficient framework for doing timing analysis, to improve the accuracy of the timing analysis one needs to address the accuracy of: waveform modeling, and gate delay modeling. We propose a technique based on Singular Value Decomposition (SVD) that accurately models the waveform in a timing analyzer. To improve the gate delay modeling, we propose a closed form expression based on the centroid of power dissipation. This new metric is inspired by our key observation that the Sakurai-Newton (SN) delay metric can be viewed as the centroid of current. In addition to accurately analyzing the timing of a chip, improving timing is another major concern. One way to improve timing is to scale down the threshold voltage (Vth). But scaling down increases the subthreshold leakage current exponentially. Sleep transistors have been proposed to reduce leakage current while maintaining performance. We propose a path-based algorithm to size the sleep transistor to reduce leakage while maintaining the required performance. In the second part of dissertation we address power grid and thermal issues that arise due to the scaling of integrated circuits. In the case of power grid simulation, we propose fast and efficient techniques to analyze the power grid with accurate modeling of the transistor network. The transistor is modeled as a switch in series with an RC and the switch itself is modeled behaviorally. This model allows more accurate prediction of voltage drop compared to the current source model. In the case of thermal simulation, we address the issue of ignoring the nonlinearity of thermal conductivity in silicon. We found that ignoring the nonlinearity of thermal conductivity may lead to a temperature profile that is off by 10° C.

Thermal properties of pure methane hydrate, under conditions similar to naturally occurring hydrate-bearing sediments being considered for potential production, have been determined both by a new experimental technique and by advanced molecular dynamics simulation (MDS). A novel single-sided, Transient Plane Source (TPS) technique has been developed and used to measure thermal conductivity and thermal diffusivity values of low-porosity methane hydrate formed in the laboratory. The experimental thermal conductivity data are closely matched by results from an equilibrium MDS method using in-plane polarization of the water molecules. MDS was also performed using a non-equilibrium model with a fully polarizable force field for water. The calculated thermal conductivity values from this latter approach were similar to the experimental data. The impact of thermal conductivity on gas production from a hydrate-bearing reservoir was also evaluated using the Tough+/Hydrate reservoir simulator (Revised version of ICGH paper 5646).

The charge-density-wave (CDW) material orthorhombic tantalum trisulfide (TaS3) is a quasi-one dimensional material that forms long ribbon shaped crystals, and exhibits unique physical behavior. We have measured the dependence of the hysteretic voltage-induced torsional strain (VITS) in TaS3, which was first discovered by Pokrovskii et. al. in 2007, on temperature and applied torque. Our experimental results shows that the application of torque to the crystal could also change the VITS time constant, magnitude, and sign. This suggests that the VITS is a consequence of residual torsional strain originally present in the sample which twists the polarizations of the CDW when voltage is applied. This polarization twist then results in torque on the crystal. Another group of materials that may attract interest is that of small-molecule soluble organic semiconductors. Due to their assumed small phonon thermal conductivities and higher charge carrier mobilities, which will increase their seebeck coefficients with doping as compared to polymers, the small-molecule organic materials are promising for thermoelectric applications. In our experiments, we have measured the interlayer thermal conductivity of rubrene (C42H28), using ac-calorimetry. For rubrene, we find that the interlayer thermal conductivity, ≈ 0.7 mW/cm·K, is several times smaller than the (previously measured) in-plane value. Also, we have measured the interlayer and in-plane thermal conductivities of 6,13-bis((triisopropylsilyl)ethynyl) pentacene (TIPS-Pn). The in-plane value is comparable to that of organic metals with excellent π-orbital overlap. The interlayer (c-axis) thermal diffusivity is at least an order of magnitude larger than the in-plane, and this unusual anisotropy implies very strong dispersion of optical modes in the interlayer direction, presumably due to interactions between the silyl-containing side groups. Similar values for both in-plane and interlayer conductivities have been observed for several other functionalized pentacene semiconductors with related structures.

Rapportens huvudsyfte är att ta fram en isoleringskombination av högpresterande material, som ska bidra till ett förbättrat U-värde och reducerad väggtjocklek. Konstruktionen är baserad på en befintlig sandwichvägg från Strängbetong, där författarna ersatt den ursprungliga isoleringen med det utvecklade isolerskiktet för att slutligen studera väggarna med lika villkor. Den framtagna väggens isolerings- förmåga presenteras genom handberäkningar, där det erhålls U-värde och temperaturfördelningar mellan elementens olika skikt vid stationära förhållanden. Det har även utförts värmesimuleringar för att analysera samma fysikaliska faktorer dock baserat på 3D förhållanden. I samarbete med företag har flera högpresterande isoleringsmaterial valts ut att ingå i väggkonstruktionen. Genom fördjupade studier av materialens fysikaliska egenskaper kunde en komplett isoleringskombination utvecklas och fick namnet InsuFlex. InsuFlex applicerades sedan i en sandwichkonstruktion för vidare analyser och värmesimuleringar. De nya väggresultaten visade mycket goda förbättringar av den ursprungliga sandwichväggen, tack vare det utvecklade skiktet av InsuFlex. Genom utförda beräkningar kunde författarna konstatera att isoleringsförmågan förbättrats med 46,5- samt 29 %, samtidigt som tjockleken reducerats med 5,5- samt 16,5 %, i jämförelse med Strängbetongs befintliga produkt. Den nya väggen erbjuder goda förutsättningar att reducera energiförlusterna och komma närmare framtida energikrav. Isoleringsmetoden förväntas även kunna appliceras i flera olika konstruktionselement. / The main objective of this report is development of an insulation-layer of high performance materials for a sandwich structure, which will contribute to an improved U-value and reduced wall thickness. The design is based on an existing sandwich wall, where the authors replaced the original insulation with the developed insulation-layer, to study the walls with equal conditions. The insulating ability is presented through calculations and thermal simulation to analyze the thermal aspects of the stationary conditions and 3D conditions. A complete insulation combination was developed through extensive studies of material’s physical properties, and named “InsuFlex”. The insulation-layer was then applied in a sandwich construction for further analysis and thermal simulations. The new design showed improvements in several areas.

The High Pressure Test Unit (HPTU) forms part of the Pebble Bed Modular Reactor (PBMR) Heat Transfer Test Facility (HTTF). One of the test sections that forms part of the HPTU is the Braiding Effect Test Section (BETS). This test section allows for the evaluation of the so–called ‘braiding effect’ that occurs in fluid flow through a packed pebble bed. The braiding effect implies an apparent enhancement of the fluid thermal conductivity due to turbulent mixing that occurs as the flow criss–crosses between the pebbles. The level of enhancement of the fluid thermal conductivity is evaluated from the thermal dispersion effect. The so–called thermal dispersion quantity r K is equivalent to an effective Peclet number eff Pe based on the inverse of the effective thermal conductivity eff k . This thesis describes the experiments carried out on three different BETS test sections with pseudo–homogeneous porosities of 0.36, 0.39 and 0.45, respectively. It also provides the values derived for the enhanced fluid thermal conductivity for the range of Reynolds numbers between 1,000 and 40,000. The study includes the following: * Compilation of a literature study and theoretical background. * An uncertainty analysis to estimate the impact of instrument uncertainties on the accuracy of the empirical data. * The use of a Computational Fluid Dynamics (CFD) model to simulate the heat transfer through the BETS packed pebble bed.* Application of the CFD model combined with a numerical search technique to extract the effective fluid thermal conductivity values from the measured results. * The assessment of the results of the experiments by comparing it with the results of other investigations found in the open literature. The primary outputs of the study are the effective fluid thermal conductivity values derived from the measured data on the HPTU plant. The primary variables that were measured are the temperatures at radial positions at different axial depths inside the bed and the total mass flow rate through the test section. The maximum and minimum standard uncertainties for the measured data are 10.80% and 0.06% respectively. The overall effective thermal conductivities that were calculated at the minimum and maximum Reynolds numbers were in the order of 1.166 W/mK and 38.015 W/mK respectively. A sensitivity study was conducted on the experimental data and the CFD data. A maximum uncertainty of 5.92 % was found in the calculated effective thermal conductivities. The results show that relatively high values of thermal dispersion quantities or effective Peclet numbers are obtained for the pseudo–homogeneous packed beds when compared to randomly packed beds. Therefore, the effective thermal conductivity is low and it can be concluded that the radial mixing in the structured packing is low relative to the mixing obtained in randomly packed beds. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.

The High Pressure Test Unit (HPTU) forms part of the Pebble Bed Modular Reactor (PBMR) Heat Transfer Test Facility (HTTF). One of the test sections that forms part of the HPTU is the Braiding Effect Test Section (BETS). This test section allows for the evaluation of the so–called ‘braiding effect’ that occurs in fluid flow through a packed pebble bed. The braiding effect implies an apparent enhancement of the fluid thermal conductivity due to turbulent mixing that occurs as the flow criss–crosses between the pebbles. The level of enhancement of the fluid thermal conductivity is evaluated from the thermal dispersion effect. The so–called thermal dispersion quantity r K is equivalent to an effective Peclet number eff Pe based on the inverse of the effective thermal conductivity eff k . This thesis describes the experiments carried out on three different BETS test sections with pseudo–homogeneous porosities of 0.36, 0.39 and 0.45, respectively. It also provides the values derived for the enhanced fluid thermal conductivity for the range of Reynolds numbers between 1,000 and 40,000. The study includes the following: * Compilation of a literature study and theoretical background. * An uncertainty analysis to estimate the impact of instrument uncertainties on the accuracy of the empirical data. * The use of a Computational Fluid Dynamics (CFD) model to simulate the heat transfer through the BETS packed pebble bed.* Application of the CFD model combined with a numerical search technique to extract the effective fluid thermal conductivity values from the measured results. * The assessment of the results of the experiments by comparing it with the results of other investigations found in the open literature. The primary outputs of the study are the effective fluid thermal conductivity values derived from the measured data on the HPTU plant. The primary variables that were measured are the temperatures at radial positions at different axial depths inside the bed and the total mass flow rate through the test section. The maximum and minimum standard uncertainties for the measured data are 10.80% and 0.06% respectively. The overall effective thermal conductivities that were calculated at the minimum and maximum Reynolds numbers were in the order of 1.166 W/mK and 38.015 W/mK respectively. A sensitivity study was conducted on the experimental data and the CFD data. A maximum uncertainty of 5.92 % was found in the calculated effective thermal conductivities. The results show that relatively high values of thermal dispersion quantities or effective Peclet numbers are obtained for the pseudo–homogeneous packed beds when compared to randomly packed beds. Therefore, the effective thermal conductivity is low and it can be concluded that the radial mixing in the structured packing is low relative to the mixing obtained in randomly packed beds. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.

Understanding thermal transport in nanoscale is important for developing nanostructured thermolelectric materials and for heat management in nanoelectronic devices. This dissertation is devoted to understand thermal transport through SiGe based superlattices. First, we systematically studied the cross-plane thermal conductivity of SiGe superlattices by varying the thickness of Si(Ge) spacers thickness. The observed additive character of thermal resistance of the SiGe nanodot/planar layers allows us to engineer the thermal conductivity by varying the interface distance down to ~1.5 nm. Si-Ge intermixing driven by Ge surface segregation is crucial for achieving highly diffusive phonon scattering at the interfaces. By comparing the thermal conductivity of nanodot Ge/Si superlattices with variable nanodot density and superlattices with only wetting layers, we find that the effect of nanodots is comparable with that produced by planar wetting layers. This is attributed to the shallow morphology and further flattening of SiGe nanodots during overgrowth with Si. Finally, the experiments show that the interface effect on phonon transport can be weakened and even eliminated by reducing the interface distance or by enhancing Si-Ge intermixing around the interfaces by post-growth annealing. The results presented in this dissertation are expected to be relevant to applications requiring optimization of thermal transport for heat management and for the development of thermoelectric materials and devices based on superlattice structures. / Verständnis des thermischen Transport auf Nanoskala ist sowohl grundlegend für die Entwicklung nanostrukturierter Materialien, als auch für Temperaturkontrolle in nanoelektronischen Bauteilen. Diese Dissertation widmet sich der Erforschung des thermischen Transports durch SiGe basierenden Übergittern. Variationen, der Si(Ge) Schichtdicken, wurden zur systematischen Untersuchung der Normalkomponente zur Wachstumsrichtung der Wärmeleitfähigkeit, von SiGe Übergittern, genutzt. Die Beobachtung des additiven Charakters, des thermischen Widerstands, der SiGe Schichten, mit oder ohne Inselwachstum, ermöglicht die Erstellung von Strukturen mit bestimmter Wärmeleitfähigkeiten durch die Variation der Schichtdicken bis zu einer Minimaldistanz zweier Schichtübergänge von ~1.5nm. Die Ge Segregation führt zu einer Vermischung, von Si und Ge, welche eine essentielle Rolle zur diffusen Phononenstreuung spielt. Unsere Untersuchungen, von planaren Übergittern und Übergittern mit variabler Inseldichte, zeigen, dass Inseln und planare Schichten zu einer vergleichbaren Reduktion, der Wärmeleitfähigkeit, führen. Diese Beobachtung lässt sich, sowohl auf die flache Morphologie als auch die Abplattung der SiGe Inseln, aufgrund der Überwachsung mit Si, zurückführen. Die Experimente zeigen außerdem, dass sich der Barriereneffekt, der Schichtgrenzen, durch Reduktion der Schichtabstände und durch verstärkte Vermischung im Bereich der Schichtgrenzen, durch Erhitzung, eliminieren lässt. Die präsentierten Messungen sind sowohl, für die Entwicklung jener Bauteile, die eine Optimierung des thermischen Transports oder Temperaturmanagment erfordern, als auch von thermoelektrischen Matieralien und Bauteilen, basierend auf Übergittern, relevant.

The thermal efficiency of concentrated solar power (CSP) system depends on the maximum operating temperature of the system which is determined by the operating temperature of the TES device. Organic materials (such as synthetic oil, fatty acid, or paraffin wax) are typically used for TES. This limits the operating temperature of CSP units to below 400 degrees C. Increasing the operating temperature to 560 degrees C (i.e., the creeping temperature of stainless steel), can enhance the theoretical thermal efficiency from 54 percent to 63 percent. However, very few thermal storage materials are compatible for these high temperatures. Molten salts are thermally stable up to 600 degrees C and beyond. Using the molten salts as the TES materials confers several benefits, which include: (1) Higher operating temperature can significantly increase the overall cycle efficiency and resulting costs of power production. (2) Low cost of the molten salt materials can drastically reduce the cost. (3) The molten salts, which are environmentally safe, can also reduce the potential environmental impact. However, these materials suffer from poor thermo-physical properties. Impregnating these materials with nanoparticles can enhance these properties. Solvents doped with nanoparticles are termed as nanofluids. Nanofluids have been reported in the literature for the anomalous enhancement of their thermo-physical properties. In this study, the poor thermal properties of the molten salts were enhanced dramatically on mixing with nanoparticles. For example the specific heat capacity of these molten salt eutectics was found to be enhanced by as much as ~ 26 percent on mixing with nanoparticles at a mass fraction of ~ 1 percent. The resultant properties of these nanomaterials were found to be highly sensitive to small variations in the synthesis protocols. Computational models were also developed in this study to explore the fundamental transport mechanisms on the molecular scale for elucidating the anomalous enhancements in the thermo-physical properties that were measured in these experiments. This study is applicable for thermal energy storage systems utilized for other energy conversion technologies – such as geothermal energy, nuclear energy and a combination of energy generation technologies.

The presented thesis focuses on study of transport and thermoelastic properties of materials under conditions of planetary interiors by means of high-pressure experimental tools and finite element modeling, and their role in the dynamics and states of cores of terrestrial planets. Experiments in laser-heated diamond anvil cell (LHDAC) in combination with numerical simulations of heat transfer in DAC are shown to yield information on thermal conductivity of a pressurized sample. The novel technique consists of one-sided laser heating and double-sided temperature measurements and utilizes a precise determination of several parameters in course of the experiment, including the sample geometry, laser beam power distribution, and optical properties of employed materials. The pressure-temperature conditions at the probed portion of the sample are, however, not uniform. To address this problem, thermal pressure in the laser-heated diamond anvil cell and anisotropic thermal conductivity originating from the texture development upon uniaxial compression have been studied by means of numerical simulations. The method for determination of thermal conductivity is applied to iron at pressures up to 70 GPa and temperatures of 2000 K, meeting the Earth’s lower mantle conditions and covering Mercury’s entire core. The obtained results are extrapolated to the conditions of the Earth’s core-mantle boundary using a theoretical model of the density dependence of thermal conductivity of metals and published values on Grüneisen parameter and bulk modulus. After considering the effect of minor core elements, the obtained value at these conditions supports case for the downward revision of the thermal conductivity in the core. From the point of view of core dynamics and energy budget, the lower thermal conductivity implies more favorable conditions to drive the dynamo. Similar scenario applies for Mercury where, for high values of thermal conductivity, heat flux conducted along the iron-core adiabat exceeds the actual heat flux through the core-mantle boundary. This leads to a negative rate of entropy production in the core that makes it impossible to sustain the dynamo process presumably responsible for the observed magnetic field of Mercury.