Thesis (M.S.)--Rochester Institute of Technology, 2009. / Typescript. Includes bibliographical references (leaves 96-98).

Thermoelectric (TE) materials can be ranked by their figure-of-merit, ZT, which is defined as ZT = (S2σT)/κ. Herein, T is the average temperature between the cold (TC) and hot sides (TH) of the thermoelectric device, S is the Seebeck coefficient, σ and κ are electrical and thermal conductivity, respectively. Presently, the applications of TE devices are limited by their low energy conversion efficiencies. The energy conversion efficiency includes the Carnot term [(TH-TC)/TC], an unattainable upper limit of efficiency for any real system, and a loss term, which depends on the ZT value. A larger temperature difference will produce a larger Carnot efficiency, and a higher ZT value will improve the loss efficiency. Thus, the combination of a larger temperature difference and higher ZT will lead to higher energy conversion efficiency, e.g. with ZT = 1, TH = 1000 K, TC = 300K, the efficiency can reach 17%. It is therefore important to explore TE materials that can be used at high temperatures (around 1000 K). Unfortunately there are very few materials that are stable and efficient under such strict conditions. Our group commenced an investigation of the Ir3Ge7 family materials in the late 1990s. My thesis work had been focused on the synthesis and optimization of Mo3Sb7-xTex and Re3(Si, Ge, Sn, Pb)xAs7-x, the ternary substitution variants of Mo3Sb7 and Re3As7. They exhibited the semiconductivity by partial Sb/Te or (Si, Ge, Sn, Pb)/As exchange, as predicted by electronic structure calculations (linear muffin-tin orbital method) and confirmed by physical property measurements. Furthermore, some transition metal atoms with the proper size, such as Fe, Co, Ni, and Cu, were intercalated into the existing cubic voids of the crystal structure in an attempt to reduce the phonon conductivity. Rietveld refinements using GSAS software were employed to analyze the (Si, Sn, Pb)/As ordering and distribution preference. Partial characterizations were conducted at University of Waterloo on cold-pressed pellets. The high temperature measurements were performed on hot-pressed pellets at Clemson University. Another project I worked on was about how to open a band gap by changing the bonding interactions. Some compounds containing the layered square nets structure may behave like a metal or like a semimetal. A band gap can be opened by distorting the square net into less symmetric layers, containing cis–trans or zig–zag chains, resulting in changes of the electronic structure around the Fermi level, therefore widening or opening a band gap. In some pnictide compounds, regular planar square nets of pnictogen atoms with half (“hypervalent” one-electron) bonding are prone to undergo Peierls distortions. Such materials are of great interest for thermoelectric energy conversion because the narrow band gap is advantageous for high-performance thermoelectric properties. The ternary and quaternary chalcogenides, LaAsSe and LaCuxAsSe, were prepared and investigated in this thesis. Seebeck coefficient measurements proved they are semiconductors. Rietveld refinements indicated the As atoms adopt a cis-trans distortion, which changes the structure type from ZrSiS (space group P4/nmm) to GdPS (space group Pnma). Consequently, a band gap is opened, in accordance with the electronic structure calculation. Single crystal structure determination showed LaCu0.12(1)AsSe adopts a derivative structure of the ZrSiS type, with deficient Cu sites inserted between La and As layers. The refinement based on As split sites is preferred.

My main subject is to set up a measuring system of TEP(Thermoelectric Power) which the theory was based on Thomas Seebeck effect, mechanism of the translation between thermal energy and electricity. There are two applications of thermoelectric power. One is temperature dependent measurement which is applied on thermal couple sensor; another one is to study the thermoelectric materials. The Seebeck coeifficient is approached to zero when superconductivity was occurred, also the calibration of the obtained value and offers essential information of carriers. The hardware of thermoelectric power measuring system consists of Digital NanoVoltmeter¡BProgrammable current Source¡BTemperature controller and Cryogenic compressor system. In the software, we use LabView to control the integral system and analyze the obtained data. For the whole system, it is vital to find a suitable compositions of sample holder¡Bheater and pasted way of sample.

Thermoelectric (TE) materials can be ranked by their figure-of-merit, ZT, which is defined as ZT = (S2σT)/κ. Herein, T is the average temperature between the cold (TC) and hot sides (TH) of the thermoelectric device, S is the Seebeck coefficient, σ and κ are electrical and thermal conductivity, respectively. Presently, the applications of TE devices are limited by their low energy conversion efficiencies. The energy conversion efficiency includes the Carnot term [(TH-TC)/TC], an unattainable upper limit of efficiency for any real system, and a loss term, which depends on the ZT value. A larger temperature difference will produce a larger Carnot efficiency, and a higher ZT value will improve the loss efficiency. Thus, the combination of a larger temperature difference and higher ZT will lead to higher energy conversion efficiency, e.g. with ZT = 1, TH = 1000 K, TC = 300K, the efficiency can reach 17%. It is therefore important to explore TE materials that can be used at high temperatures (around 1000 K). Unfortunately there are very few materials that are stable and efficient under such strict conditions. Our group commenced an investigation of the Ir3Ge7 family materials in the late 1990s. My thesis work had been focused on the synthesis and optimization of Mo3Sb7-xTex and Re3(Si, Ge, Sn, Pb)xAs7-x, the ternary substitution variants of Mo3Sb7 and Re3As7. They exhibited the semiconductivity by partial Sb/Te or (Si, Ge, Sn, Pb)/As exchange, as predicted by electronic structure calculations (linear muffin-tin orbital method) and confirmed by physical property measurements. Furthermore, some transition metal atoms with the proper size, such as Fe, Co, Ni, and Cu, were intercalated into the existing cubic voids of the crystal structure in an attempt to reduce the phonon conductivity. Rietveld refinements using GSAS software were employed to analyze the (Si, Sn, Pb)/As ordering and distribution preference. Partial characterizations were conducted at University of Waterloo on cold-pressed pellets. The high temperature measurements were performed on hot-pressed pellets at Clemson University. Another project I worked on was about how to open a band gap by changing the bonding interactions. Some compounds containing the layered square nets structure may behave like a metal or like a semimetal. A band gap can be opened by distorting the square net into less symmetric layers, containing cis–trans or zig–zag chains, resulting in changes of the electronic structure around the Fermi level, therefore widening or opening a band gap. In some pnictide compounds, regular planar square nets of pnictogen atoms with half (“hypervalent” one-electron) bonding are prone to undergo Peierls distortions. Such materials are of great interest for thermoelectric energy conversion because the narrow band gap is advantageous for high-performance thermoelectric properties. The ternary and quaternary chalcogenides, LaAsSe and LaCuxAsSe, were prepared and investigated in this thesis. Seebeck coefficient measurements proved they are semiconductors. Rietveld refinements indicated the As atoms adopt a cis-trans distortion, which changes the structure type from ZrSiS (space group P4/nmm) to GdPS (space group Pnma). Consequently, a band gap is opened, in accordance with the electronic structure calculation. Single crystal structure determination showed LaCu0.12(1)AsSe adopts a derivative structure of the ZrSiS type, with deficient Cu sites inserted between La and As layers. The refinement based on As split sites is preferred.

This dissertation presents experimental and computational study of thermal and electrical transport in one-dimensional nanostructures. Synthesizing materials into one-dimensional nanowire has been proved very effective for suppressing the phonon contribution due to scattering at the wire boundaries. Three one-dimensional nanostructured thermoelectric candidates - SiGe nanowires, SrTiO3 nanowires, and ZnO nanowires – were presented and discussed in this dissertation. SiGe nanowires are successfully synthesized on a cleaned n-type (100) Si substrate coated with gold thin film as a catalytic metal, via the vapor-liquid-solid (VLS) growth method. The thermoelectric properties of SiGe nanowires with different diameter, Ge concentration, and phosphorus doping concentration were measured using a MEMS micro-device consisting of two suspended silicon nitride membranes in the temperature range of 60 K ~450 K. The experimental results were obtained by “simultaneously” measuring thermal conductivity, electrical conductivity, and thermopower. The ZT improvement is attributed to remarkable thermal conductivity reductions, which are thought to derive from the effective scattering of a broad range of phonons by alloying Si with Ge as well as by limiting phonon transport within the nanowire diameters. An improved model based on Boltzmann transport equation with relaxation time approximation was introduced for estimating thermoelectric properties of phosphorus heavily doped SiGe nanowires from 300 to 1200 K. All the electron and phonon scatterings were comprehensively discussed and utilized to develop the new model for estimating electrical conductivity, thermopower, and thermal conductivity of SiGe nanowires. As thermoelectric materials, oxide nanowires have great advantages comparing to other semiconductors. Two nanostructured materials, SrTiO3 nanotubes and ZnO nanowires, are introduced and successfully synthesized by simple methods. Thermal conductivity of ZnO nanowires with different diameter were characterized from 60 K to 450 K. In order to measure thermoelectric properties of one-dimensional nanostructures at temperature up to 800 K, a new temperature vacuum system was carefully designed and built from scratch. The thermal conductivity of ZnO nanowires with different diameters at high temperature were measured from 300 K to 800 K.

Thesis (Ph. D.)--Michigan State University. Dept. of Physics and Astronomy, 2006. / Title from PDF t.p. (viewed on June 19, 2009) Includes bibliographical references (p. 90-98). Also issued in print.

Thesis (M.S.)--Rochester Institute of Technology, 2009. / Typescript. Includes bibliographical references (leaves 80-82).

Thesis (Ph. D.) -- California Institute of Technology, 2010. / Title from home page (viewed 05/26/10). Advisor and committee chair names found in the thesis' metadata record in the digital repository. Includes bibliographical references.

Les matériaux composites – de par leurs propriétés spécifiques élevées – sont l’une des classes de matériauxpermettant de fabriquer des structures aéronautiques plus légères pour une meilleure performance énergétique des aéronefs etune réduction conséquente des émissions de CO2. Les composites à matrice polymère renforcée par des fibres de carbone(CFRP) sont de plus en plus utilisés dans l’industrie aéronautique civile pour la réalisation de parties structurales, dontrécemment les panneaux de fuselage.L’intégration des CFRPs dans ces structures ne se fait pas sans difficultés : le fuselage – normalement soumis aux charges de lapressurisation – peut être soumis à des chocs/impacts, particulièrement dommageables pour les composites, et exposé àl’agression de l’environnement (température, humidité, liquides de refroidissement, …).Le fuselage est aussi le siège de sollicitations de nature électrique, telles que celles liées au retour de courant à la masse ou aufoudroiement : bien que des réseaux secondaires de câbles électriques et de grillages métalliques soient prévus afin de supporterces charges, l’action de courants de fuite, transitant à travers les jonctions boulonnées et rivetées, et les fortes différences depotentiel électrique dans l’épaisseur du fuselage, conséquences du foudroiement, peuvent activer le couplage thermoélectriqueet conduire à des échauffements localisés de ces structures. Les phénomènes liés à ce type de couplage peuvent êtreparticulièrement marqués dans les CFRPs – les matrices polymères étant électriquement et thermiquement quasi-isolantes, lecomportement thermoélectrique global du composite étant fortement anisotrope – et méritent d’être approfondis. L’intégrationde micro et nano charges – en particulier les nanotubes de carbone (NTCs) – dans les résines polymères ou à l’interfacefibre/matrice peut globalement améliorer ce comportement mais il s’agit d’une solution technologique qui nécessite encored’être explorée en détail.Ce travail est consacré à la caractérisation du comportement thermoélectrique anisotrope de matériaux CFRPs - chargés ou nonchargés en NTCs. Cette caractérisation comprend :· la mesure des valeurs de résistivité/conductivité électrique et de leur évolution avec la température,· la caractérisation des champs de température induits par le passage de courants électriques,· l’identification des paramètres pour la modélisation du comportement thermoélectrique de ces matériaux etpour l’interprétation des essais,· la caractérisation de l’effet d’un vieillissement humide sur les valeurs de résistivité/conductivité électrique ducomposite. / Composite materials – since their high specific properties – are used recently to reduce the weight of aircraftstructures and to improve the performance engine in order to reducing the emissions of CO2. Composites of Carbon FibberReinforced Polymer matrix (CFRPs) are increasingly used in civil aviation industry for the production of structural parts:fuselage panels.Integrating CFRPs in the aeronautical structures has also some difficulties: the fuselage – generally subject to pressure of airflux – would be subjected also to shocks/impacts, particularly will be damaging for exposed to the aggressive environment(temperature, humidity, liquid cooling, ...).The fuselage is the principal structure to support the charge of electric current, such as those related to the electric current returnto ground or lightning : although the secondary networks of electrical cables and wires fences are provided to support thesecharges, the action of leakage currents, probably passer through bolted and riveted joints and because of the high differences ofelectrical potential between the surface of panels thickness, can activate the thermoelectric coupling and lead to localizedheating of these structures. Phenomena related to this type of coupling may be particularly remarkable in CFRPs – since thepolymer matrices is electrically and thermally quasi-insulates, for composite the thermoelectric behaviour is highly anisotropic– and need further research and investigation. The integration of micro and nano fillers – especially carbon nanotubes (CNTs) –in the polymer resin or in interface of fibber/matrix can improve the overall behaviour, but it is a technological solution that stillrequires be explored in detail.The aim of this work is to characterize the anisotropic properties of thermoelectric behaviour of materials CFRPs – charged oruncharged in CNTs. This characterization includes:· measurement of electrical resistivity/conductivity and their evolution with temperature,· characterization of the temperature fields induced by the passage of electric current,· identification of parameters for modeling the behavior of these thermoelectric materials and interpretation oftests,· characterization of the effect of moisture aging on the values of resistivity/conductivity of the composite.

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