The main objectives of this dissertation is to investigate the prospects of embedded thermoelectric devices integrated in a chip package and to develop a design methodology aimed at taking advantage of the on-chip on-demand cooling capabilities of the thermoelectric devices. First a simulation framework is established and validated against experimental results, which helps to study the cooling capabilities of embedded thermoelectric coolers (TEC) in both a transient and steady state. The potential for up to 15°C of total cooling has been shown. The thermal simulation framework allows for rapid assessment of TEC and system level thermal performance. Next, the thesis develops a co-simulation environment that is capable of simulating the thermal and electrical domain and couples them to design intelligent TEC controllers. These controllers are implemented on chip and can leverage the transient cooling capability of the device. The controllers are simulated within the co-simulation environment and their potential to control high power chip events are thoroughly investigated. The system level overheads are considered and discussions on implementation techniques are presented. The co-simulation framework is also extended to allow for simulation of real predictive technology microprocessor cores and their workloads. Finally the thesis implements a fully on-chip autonomous energy system that takes advantage of the TEC in its reverse energy harvesting mode and uses the same device to harvest energy and use the energy to power the on-chip cooling circuit. This increases the overall energy efficiency of the cooler and verifies the TEC control methods.

<p>In this project, nano powder of CoSb₃ thermoelectric material was synthesized using chemical alloying novel co-precipitation method. This method involved co-precipitation of TE precursor compounds in controlled pH aqueous solutions followed by thermo-chemical treatments including calcination and reduction to produce nano-particulates of CoSb₃. The nano powder was consolidated using rapid solid state spark plasma sintering (SPS) and the processing time was of the order of few minutes. On a result very high densities were achieved and grain growth was almost negligible.</p><p>Various batches of the CoSb₃ nano powder were produced to achieve high purity, minimum particle size and compensate Sb evaporation during thermo-chemical reduction. For de-agglomeration, powder was grinded before and after calcination. Samples were characterized at each stage during synthesis using XRD and SEM (with EDX). Thermal gravimetric analysis (TGA) was done before thermochemical treatments to observe weight losses with heating the powder at high temperatures and other physiochemical changes. Thermal diffusivity of the samples was measured at room temperature using Laser Flash Apparatus (LFA) and heat capacity was measured using Differential Scanning Calorimetry (DSC).   Thermal conductivities are calculated using these thermal diffusivities, heat capacities and densities of the sintered pellets. Average grain size is measure using image size J software.</p><p>It was observed that powder purity and size is affected by batch size, reduction conditions like holding temperature and time.  During sintering with SPS; heating and cooling rates, sintering temperature, holding pressure and time were the main variables. Grain size and morphology was analyzed using SEM.</p><p>It was observed that larger the grain size higher will be the thermal diffusivity, which leads to increase in thermal conductivity. Hence, grain size has affected on thermal conductivity and also on TE performance.</p> / QC 20100708

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The synthesis of NaxCoO2 by Pechini method (with initial sodium stoichiometry x = 1.0, 0.9, 0.8, and 0.7) was studied for the samples heat-treated in the temperature range from 550 řC to 750 řC. The structure characterization and phase composition was carried out by the powder X-ray diffraction (XRD) analysis. The elementary analysis was done using atomic absorption spectroscopy. From the thermoelectric properties, the Seebeck coefficient was measured for both α- and γ- phase. The Weiss constant was determined for α- and γ- phase by linear regression of 1/χ. The real sodium content in sodium cobaltate phase was found to be independent on initial sodium precursor weight and measurement by PXRD and Seebeck coefficient showed that the sodium content in NaxCoO2 falls in the range from x = 0.65 to 0.75. The magnetization measurements showed presence of spinel Co3O4 impurities in the samples and the analysis of magnetic susceptibility showed unusual high values of the Weiss constant for α- phase in comparison with structurally similar γ- NaxCoO2. Thin film of NaxCoO2 with initial sodium stoichiometry x = 1.0, 0.52, 0.32, and 0.175 was deposited by chemical solution deposition method using spin-coating deposition on a 00l-oriented α-Al2O3 single crystal substrate. The formation and structure characterization of...

Master of Science / Department of Electrical and Computer Engineering / William B. Kuhn / This work was a part of NASA EPSCoR Project NNX11AM05A: Biosensor Networks and Telecommunication Subsystems for Long Duration Missions, EVA Suits, and Robotic Precursor Scout Missions. The project’s main goal is the development of a wireless sensor network inside an astronaut’s spacesuit. Antennas are essential components in a wireless network. Since this antenna will be used inside the spacesuit it is important to consider both the physical size limitations and the desired antenna polarization. After exploring the WWVB radio station antenna which provides the preferred vertical polarization and has a suitable aspect ratio, the top hat antenna seemed promising for intrasuit communication. The design of a top hat antenna is outlined. Then, the antennas were tested using 433 MHz radios in a full scale model spacesuit. This spacesuit was designed specifically to model the behavior of aluminized mylar in the real suit. Test results support the feasibility of an intrasuit wireless network. If a gateway radio is placed on the chest or back, a sensor could be placed anywhere on the body and provide an adequate signal. These initial tests did not include a matching network, but the additional link-margin afforded by a matching network, even an imperfect match, is considered. Energy harvesting is explored as an alternative to batteries powering the intrasuit radio. In the oxygen rich environment of a spacesuit, even the smallest spark can be catastrophic. A variety of energy harvesting options are explored with a focus on thermal energy harvesting. The temperature difference between the human skin and the astronaut’s Liquid Cooling and Ventilation Garment can be used to produce a small voltage. To increase the voltage a step-up converter is implemented. Final integration of the two systems with a biosensor is left for on-going work in the three year NASA project.

Anisotropic thermoelectric effects can be measured in certain materials. Anisotropy can also be simulated using a repeated, layered structure of two materials cut at an angle. Various aspect ratios and angles of inclination are investigated in device geometry in order to maximize the thermopower. Eddy currents have been shown to occur in thermoelectric devices, and evidence of these currents are revealed in finite element analysis of the artificially synthesized anisotropic Peltier effect.

Um condicionador de ar para biotérios foi montado com módulos termoelétricos de efeito Peltier. Para troca térmica, foram testados: 1. dissipação externa a ar, com &delta;t de 14°C, rendimento de 16,46%, consumo de 1212 W/h e, 2. dissipação externa água, com &delta;t de 21°C, rendimento de 46,02%, consumo de 524 W/h. A simulação matemática de operação, com mistura de ar não condicionado, mostrou que o sistema pode servir, na dissipação a ar, a aproximadamente 91 microisoladores padrão rato e a aproximadamente 137, na dissipação a água. Quando comparado com um sistema de compressão de freon, o termoelétrico mostrou economia de 26% na implantação e 38% no consumo elétrico por BTU gerado. O sistema termoelétrico mostrou ainda, precisão de &plusmn; 0,1°C, nas temperaturas experimentais, o que é impossível num sistema de freon. Para os testes em animais foram empregados Ratos wistar, mantidos individualmente, em gaiolas metabólicas de arame, sem abrigo, em sistema microambiental, sob fluxo direto de ar a 0,6 m/s, nas temperaturas de 22°, 24°, 26°, 28° e 30°C (E I, E II, E III, E IV e E V). A ingestão de ração e o ganho de peso foram comparados ao final de 5 dias (ANOVA; Tukey-Kramer). No total, 7 grupos de 15 animais cada foram comparados. Para a faixa de 22°C foram utilizados 3 grupos, sendo um grupo experimental e dois grupos controle (CI e C II). Um deles foi mantido em condições ambientais semelhantes a biotérios convencionais sob ventilação geral diluidora (VGD) - C I. O outro grupo controle (C II) foi mantido no interior do equipamento de ventilação microambiental, porém, sem o direcionamento de ar, simulando a VGD. Os resultados obtidos demonstraram claramente que animais mantidos sob ventilação microambiental direta a 26°, 28° e 30° (E III, E IV e E V) apresentaram o mesmo ganho de massa corpórea que animais do grupo C I (22 &plusmn; 2°C). Os grupos E I e E II apresentaram menor ganho de massa corpórea quando comparados a C I (p<0,001 em ambas comparações). O ganho de peso de todos os grupos experimentais apresentou diferença estatística, quando comparado ao C II, exceto o grupo E V que obteve índice de ganho de peso equivalente a C II. A ingestão de ração de todos os grupos se manteve praticamente constante. O grupo E V apresentou uma redução na ingestão de ração quando comparado aos grupos C I, E I e E II (p<0,01; p<0,01; p<0,001 respectivamente). O grupo E III ingeriu menos ração que os grupos C I (p<0,05) e E II (p<0,05). / An air-conditioner for animal facility was assembled with Peltier effect thermoelectric modules. For external exchanger, had been tested: 1. external air dissipation: &delta;t = 14°C; 16,46% of efficiency; 1212 W/h of power consumption and, 2. external water dissipation: &delta;t = 21°C; 46,02% of efficiency; 524 W/h of power consumption. A mathematical simulation of operation, with not conditional air mixture, showed that the system can supply, with air dissipation, to &asymp; 91 microisolator rat cages and to &asymp; 137, with water dissipation. When compared with a freon system, the thermoelectric system shows economy of 26% in implantation and 38% in the electric consumption by generated BTU. The thermoelectric system showed too, a precision of ± 0,1°C, at experimental temperatures, what is impossible in a freon system. For animal tests, Wistar rats had been kept individually, in metabolic wire cages, without shelter, in microenvironmental system, under direct air flow at 0,6 m/s, under temperatures of 22°, 24°, 26°, 28° and 30° C (E I, E II, E III, E IV and E V). The food ingestion and the weight gain had been compared in the end of 5 days (ANOVA; Tukey-Kramer). In the total, 7 groups, 15 animals each, had been compared. For the 22°C temperature, had been used 3 groups, one experimental and two controls (C I e C II). One of them was kept in similar ambient of conventional laboratory animal rooms conditions (general diluitory ventilation, GDV) - C I. The other control group (C II) was kept in the interior of the equipment of microenvironmental ventilation, however, without the direct air flow, simulating the GDV. The gotten results demonstrate clearly that animal kept under direct microenvironmental ventilation at 26°, 28° and 30°C (E III, E IV and E V) have the same gain of corporal mass that C I group (22 &plusmn; 2°C). The E I and E II had less corporal mass gain when compared to C I (p<0,001 for the two comparisons). The weight gain for all the experimental groups, when compared to C II, presents statistical differences, except E V group, that was equal to C II. The food ingestion of all the groups was constant. The E V group presented a reduction in the food ingestion when compared with the groups C I , E I and E II (p<0,01; p<0,01; p<0,001 respectively). The E III group ingested less ration than C I (p<0,05) and E II (p<0,05) groups.

Le déploiement constant de nouveaux réseaux de capteurs dans des endroits confinés des turbomachines ou difficilement accessibles nécessite l'apport d'énergie pour alimenter ces capteurs. De nombreuses recherches s'orientent donc sur une alimentation à demeure des capteurs afin de monitorer de nouveaux composants. Une voie possible d'alimentation (à demeure) est l'utilisation de modules thermoélectriques afin de convertir l'énergie thermique issue de la combustion dans les turbines. Parmi les matériaux thermoélectriques utilisables dans ce genre d'applications (au-delà de 700 °C), les siliciures semblent constituer la famille la plus prometteuse. Le sujet de cette thèse est l'étude de siliciures connus pour leurs résistances à l'environnement en température afin de réaliser des modules thermoélectriques pouvant fonctionner à des températures de l'ordre de 700 °C. À cette fin, différents représentants de la famille des siliciures ont été étudiés par calcul ab initio à l'aide de la théorie de la fonctionnelle de la densité. Cet outil a permis de calculer leurs propriétés thermoélectriques potentielles et a montré que les deux meilleurs candidats à des applications bon marché étaient les disiliciures de manganèse et de fer. Le calcul a également permis de montrer le caractère métallique de nombreux siliciures ternaires. Les siliciures prometteurs ont été élaborés par frittage et leur vieillissement ainsi que les coefficients de dilatation thermique ont été étudiés. Ces connaissances ont permis de mettre au point des modules thermoélectriques à base de siliciures résistants à 900 °C sous air. Bien que possédant des propriétés thermoélectriques modestes, leur résistance à l'oxydation à haute température permet d'envisager la fabrication de modules thermoélectriques / The constant deployment of new sensors networks in confined areas of turbomachines or difficult to access, requires the input of energy to power these sensors. Many researches are thus focused on a permanent power supply of sensors to monitor new components. One possible way of permanent power supply is the use of thermoelectric modules to convert the thermal energy from combustion into the turbines. Among the thermoelectric materials, silicides seem to be the most promising family for high temperature applications (above 700 °C). The subject of this thesis is the study of silicides known for their high temperature oxidation resistance in order to produce thermoelectric modules that can operate at optimal temperatures around 700 °C. To this end, different representatives of the silicide family have been studied by ab initio calculation using the density functional theory. This tool allowed to calculate their potential thermoelectric properties and showed that the two best candidates for cheap applications were the disilicides of manganese and iron. The calculation also showed the metallic character of many ternary silicides. The promising silicides were developed by sintering method and their aging as well as their coefficients of thermal expansion were studied. This knowledge has made it possible to develop thermoelectric modules based on silicides resistant to 900 °C in air. Although possessing modest thermoelectric properties, their resistance to high temperature oxidation makes it possible to envisage the manufacture of thermoelectric modules

Efeitos termoelétricos descrevem o surgimento de campos elétricos em função de gradientes de temperatura e vice-versa. Neste trabalho investigamos as propriedades termoelétricas de materiais de baixa dimensionalidade e nanoestruturas através de cálculos de primeiros princípios das propriedades de transporte destes sistemas, usando o código TRANSAMPA, que é baseado em funções de Green fora do equilíbrio e do código SIESTA, baseado em teoria do funcional da densidade. Inicialmente estudamos nanofitas de grafeno e como estas são alteradas pela presença de impurezas substitucionais de Boro e Nitrogênio. Entre os principais resultados, mostramos que fitas na configuração ferromagnética apresentam efeito Seebeck dependente do spin, que pode ser ajustado por efeito de campo. A seguir, vemos que o coeficiente Seebeck (S) em bicamadas de grafeno pode ser ajustado por potenciais de gate, de forma a escolher os portadores de carga, atingindo S =_250 _V/K. Também estudamos a dependência de S com a temperatura (T) e o tamanho do gate, calculamos a condutividade térmica por dinâmica molecular e a eficiência termoelétrica (ZT). Na seqüencia, mostramos que grafeno dopado com Mn mostra caloritrônica de spin ajustável via gate e como a termocorrente varia com T e _T. Finalmente, calculamos as propriedades termoelétrica de uma junção molecular Au-BDT-Au e como elas variam em função do alongamento da junção. Também propomos um esquema geral para maximizar ZT de junções moleculares em geral. / Thermoelectric effects describe how electric fields arise in response to temperature gradients and vice versa. In this thesis we investigate the thermoelectric properties of low-dimensional materials and nanostructures theoretically. We perform ab initio calculations of the electronic transport properties using the TRANSAMPA code, based in nonequilibrium Greens functions, and the SIESTA code, based in density functional theory. First, we study graphene nanoribbons and how their properties are altered by substitutional impurities. Among our main results for this system, we show that ribbons in the ferromagnetic configuration present spin-dependent Seebeck effect, which can be tuned by a field effect. We show that the Seebeck coefficient (S) of bilayer graphene is highly tunable by a gate potential, with ambipolar behavior, reaching S = _250 _V/K. We also study how S varies with temperature (T) and gate length. We calculate its thermal conductivity by molecular dynamics, and its thermoelectric efficiency (ZT ). Then, we show that Mn doped graphene features a gate-tunable spin-dependent S, which is robust under changes in T and _T, rendering this material suitable for spin caloritronics. Finally, we calculate how the thermoelectric properties of an Au-BDT-Au molecular junction vary with mechanical stretching, and propose a general recipe to improve ZT in molecular junctions in general.

The aim of this work has been to use atomistic computer simulation methods to calculate the thermal conductivity and investigate factors that will modify the behaviour when applied to three different oxide materials: MgO, SiO2 and SrTiO3. These were chosen as they represent distinct classes of materials and are substrates for thermoelectric devices, where one of the primary goals is to tailor the system to reduce the thermal conductivity. Chapter 1 introduces thermoelectric concepts, gives a background of the theory and a review of various important thermoelectric materials. In Chapter 2 an overview of the interatomic interactions is presented along with details on the implementation of these interactions in a simulation of a 3D periodic crystal. Chapter 3 outlines the importance of phonon processes in crystals and several approaches to the calculation of thermal conductivity are presented. MgO results are given in Chapter 4. Both the Green-Kubo and Boltzmann transport equation (BTE) methods of calculating thermal conductivity were used. The effect on thermal conductivity of two different grain boundary systems are then compared and finally extended to MgO nanostructures, thus identifying the role of surfaces and complex nanostructure architectures on thermal conductivity. In Chapter 5 two different materials with the formula unit SiO2 are considered. The two materials are quartz and silicalite which show interesting negative thermal expansion behaviour which may impact upon the thermal transport within the material. Chapter 6 presents results on the promising thermoelectric material STO. Once again the results from both Green-Kubo and BTE calculations are compared. Grain boundaries are also studied and the effect of inter-boundary distance and boundary type on the thermal conductivity is explored. Finally, a nanostructured STO system (assembled nanocubes) with promising thermoelectric applications is studied. Chapter 7 outlines the conclusions made from this work and suggests areas for future study.