The ability of uniquely functional thermoelectric materials to convert waste heat directly into electricity is critical considering the global energy economy. Profitable, energy-efficient thermoelectrics possess thermoelectric figures of merit ZT ≥ 1. We examined the effect of metal nanoparticle – oxide film interfaces on the thermal conductivity κ and Seebeck coefficient α in bilayer and multilayer thin film oxide thermoelectrics in an effort to improve the dimensionless figure of merit ZT. Since a thermoelectric's figure of merit ZT is inversely proportional to κ and directly proportional to α, reducing κ and increasing α are key strategies to optimize ZT. We aim to reduce κ by phonon scattering due to the inclusion of metal nanoparticles in the bulk of thermoelectric thin films deposited by Pulsed Laser Deposition. XRD, AFM, XPS, and TEM analyses were carried out for structural and compositional characterization. The electrical conductivities of the samples were measured by a four-point probe apparatus. The Seebeck coefficients were measured in-plane, varying the temperature from 100K to 310K. The thermal conductivities were measured at room temperature using Time Domain Thermoreflectance. / Master of Science

Materials at the nanoscale show properties uniquely different from the bulk scale which when controlled can be utilized for variety of thermal management applications. Different applications require reduction, increase or directional control of thermal conductivity. This thesis focuses on investigating thermal transport in two such application areas, viz., 1) thermoelectric energy conversion and 2) thermal rectification. Using molecular dynamics simulations, several methods for reducing of thermal conductivity in polyaniline and polyacetylene are investigated. The reduction in thermal conductivity leads to improvement in thermoelectric figure of merit. Thermal diodes allow heat transfer in one direction and prevents in the opposite direction. These materials have potential application in phononics, i.e., for performing logic calculations with phonons. Rectification obtained with existing material systems is either too small or too difficult to implement. In this thesis, a more useful scheme is presented that provides higher rectification using a single wall carbon nanotube (SWCNT) that is covalently functionalized near one end with polyacetylene (PA). Although several thermal diodes are discussed in literature, more complex phononic devices like thermal logic gates and thermal transistors have been sparingly investigated. This thesis presents a first design of a thermal AND gate using asymmetric graphene nanoribbon (GNR) and characterizes its performance. / Ph. D.

In this work, a thermoelectric energy harvester system aimed at harvesting energy for locally powering sensor nodes in nuclear power plant coolant loops has been designed, fabricated and tested. Different mathematical modeling methods have been validated by comparing with experimental results. The model developed by this work has the best accuracy in low temperature range and can be adapted and used with any heat sink, heat pipe, or thermoelectric system, and have proven to provide results closely matching experimental data. Using the models, an optimization of the thermoelectric energy harvesting system has been performed which is applicable to any energy harvester of this variety. With experimental validation, the system is capable of generating sufficient energy to power all the sensors and electronical circuits designed for this application. The effect of gamma radiation on this thermoelectric harvester has also been proved to be small enough through radiation experiment. / Master of Science

Energy harvesting is a technology for generating electrical power from ambient or wasted energy. It has been investigated extensively as a means of powering small electronic devices. The recent proliferation of devices with ultra-low power consumption - devices such as RF transmitters, sensors, and integrated chipsets - has created new opportunities for energy harvesters. There is a variety of ambient energies such as vibration, thermal, solar, stray current, etc. Depending on energy sources, different kinds of energy conversion mechanism should be employed. For energy harvesters to become practical, their energy conversion efficiency must improve. This efficiency depends upon advances in two areas: the system or structural design of the energy harvester, and the properties of the materials employed in energy conversion. This dissertation explores developments in both areas. In the first area, the role of nano-, micro-, and bulk structure of the energy conversion materials were investigated. In the second area, piezoelectric energy harvesters and a magneto-thermoelectric generator are treated from the perspective of system design. In the area of materials development, PbTiO3 (PTO) nanostructures consisting of nanofibers and three-dimensional (3-D) nanostructure arrays were hydrothermally synthesized. The growth mechanism of the PTO nanofibers and 3-D nanostructures were investigated experimentally and theoretically. The PTO nanostructures were composed of oriented PTO crystals with high tetragonality; these arrays could be promising candidates for nanogenerators. Different designs for energy harvesters were explored as a means of improving energy conversion efficiency. Piezoelectric energy harvesters were designed and constructed for applications with a low frequency vibrational energy and for applications with a broadband energy spectrum. A spiral MEMS piezoelectric energy harvester design was fabricated using a silicon MEMS process and demonstrated to extract high power density at ultra-low resonance frequencies and low acceleration conditions. For a broadband energy harvester, a magnetically-coupled array of oscillators was designed and built that broadened the harvester's effective resonance frequency with considerably improved output power. A new design concept for thermal energy harvesting that employs a magneto-thermoelectric generator (MTG) design was proposed. The MTG exploits a thermally-induced second order phase transition in a soft magnetic material near the Curie temperature. The MTG harvested electric power from oscillations of the soft magnet between hot and cold sources. For the MTG design, suitable soft magnetic materials were selected and developed using La0.85Sr0.15MnO3-Ni0.6Cu0.2Zn0.2Fe2O4 magnetic composites. The MTG was fabricated from a PVDF cantilever and a gadolinium (Gd) soft magnetic material. The feasibility of the design for harvesting energy from the waste heat was demonstrated by attaching an MTG array to a computer CPU. / PHD

Thermoelectric materials are playing an increasingly significant role in the global effort to develop sustainable energy technologies. Consequently, the demand for materials with greater thermoelectric efficiency has stimulated the development of state-of-the-art interstitially doped skutterudite-based materials. However, since intermetallics are often embrittled by interstitial substitution, optimal skutterudite-based device design, manufacture, and operation require thorough assessment of the fracture toughness of interstitially doped skutterudites. This research determines whether the fracture toughness of skutterudites is sacrificed upon interstitial doping. Both pure and interstitially doped cobalt antimonide skutterudites were synthesized via a solid-state technique in a reducing atmosphere with antimony vapor. Their crystal structures were analyzed by X-ray diffraction, and then sintered by hot uniaxial pressing into dense pellets. The electronic properties of the sintered samples were characterized. Fracture toughness of the pure Co₄Sb₁₂ and interstitially doped In₀.₁Co₄Sb₁₂ samples was evaluated by the Vicker's indentation technique and by loading beam-shaped singe-edge vee-notched bend specimens (SEVNB) in 4-point flexure. The intrinsic crack-tip toughness of both materials was determined by measuring the crack-tip opening displacements (COD's) of radial cracks introduced from Vicker's indentations. The intrinsic crack-tip toughness of both pure Co₄Sb₁₂ and interstitially doped In₀.₁Co₄Sb₁₂ were found to be similar, 0.523 and 0.494 MPa√m, respectively. The fracture toughness of both pure and interstitially doped skutterudites, derived from SEVNB specimens in 4-point flexure were also found to be statistically identical, 0.509 and 0.574 MPa√m , respectively, and are in agreement with the intrinsic crack-tip toughness values. However, the magnitude of the toughness was found to be much lower than previously reported. Moreover, fracture toughness values derived from Vickers's indentations were found to be misleading when compared to the results obtained from fracture toughness tests carried out on the micronotched (SEVNB) specimens loaded in 4-point flexure. / Graduation date: 2012

L'incertitude de l'énergie mondiale avec l'augmentation constante de la demande d'énergie déclenche la recherche de technologies de conversion d'énergie à haut rendement. Les dispositifs thermoélectriques (TE) peuvent jouer un rôle très important dans la collecte et la valorisation de l'énergie car ils peuvent être employés pour récupérer la chaleur résiduelle. Par exemple, la quantité de chaleur émise sous forme de déchets par les différents moteurs thermiques est évaluée en centaines de millions de MWh /an.Cette thèse vise à démontrer la faisabilité de fabrication des systèmes de récupération de la chaleur issue des déchets à l'échelle industrielle en utilisant des générateurs thermoélectriques (TE). Les techniques de fabrication proposées sont basées sur l'utilisation de technologies avancées comme le frittage par spark plasma, le broyage, la fusion laser sélective et la technologie de projection thermique. Ces techniques rendent possible l'élaboration de revêtements de matériau thermoélectrique avec des performances thermoélectriques supérieures et une flexibilité forte liées aux choix multiples de tailles, de formes et de matériaux.Nous nous sommes intéressés à l'étude du matériau semi-conducteur ß-FeSi2 car il présente un coefficient de mérite fort dans une plage de température de 300-800oC qui est la température des gaz en sortie de moteur voiture.Les techniques de SLM (Selective Laser Melting), de broyage, de frittage et de frittage flash (SPS) ont été successivement utilisées pour aboutir à l'élaboration de l'alliage ¿-FeSi2. Les revêtements ont ensuite été obtenus par la technique de projection plasma sous basse pression.Concernant le revêtement formé à partir de l'alliage par procédé LPPS, la transformation de phase de la phase cubique -ferrosilicium et de la phase quadratique ¿-Fe2Si5 en phase orthorhombique ß-FeSi2 se produit en obéissant aux réactions péritectique et eutectique. Après recuit sous température et temps appropriés, les revêtements présentent une phase complète ß-FeSi2 sur le substrat céramique.En outre pour une application à grande échelle, il est nécessaire de déposer ce type de revêtement sur un substrat en acier inoxydable et il convient dans ce cas d'utiliser un masque approprié pour fabriquer le dispositif thermoélectrique. / The uncertainty in the global energy with the constant increase in energy demand triggers the search for energyconversion technologies with high efficiency. The thermoeletrical devices (TE) can play a relevant role in thecollection and recovery of energy because they can be used to recover waste heat. For example, the amount of heatemitted as waste by different ombustion engines is evaluated hundreds of millions of MWh / year.This thesis aims to demonstrate the feasibility of anufacturing heat recovery systems from waste on an industrialscale using thermoelectric generators (TE). The proposed manufacturing techniques are based on the use ofadvanced technologies such as spark plasma sintering, crushing, selective laser melting and thermal spraytechnology. These techniques make possible the development of thermoelectric material coatings with superiorthermoelectric performance and high flexibility related to multiple choices of sizes, shapes and materials.The study of semiconductor ß-FeSi2 material was conducted in this goal because it has a strong merit coefficient(ZT) in the temperature range of 300-800°C which is the temperature of the output gas of the cars.Selective Laser Melting, sintering and spark plasma sintering (SPS) were successively used to lead to themanufacture of ¿-FeSi2 alloy. The coatings were then obtained by low pressure plasma spraying.Concerning the coating formed from the alloy, the phase transformation of the cubic phase ¿-ferro-silicon and thetetragonal phase ¿-Fe2Si5 in the orthorhombic phase ß-FeSi2 is produced by obeying the eritectic and eutecticreactions. After annealing under suitable temperature and time, the coatings sprayed on the ceramic bstratepresent a complete phase ß-FeSi2.In view of a large-scale application, it is necessary to spray this type of coating on a stainless steel substrate and inthis case to use a suitable mask for making the appropriate thermoelectric device.

<p>Energy harvesting is an interesting topic for today since we face running out of energy source, a serious problem in the world. Thermoelectric devices are a good candidate. They can convert heat (i.e. temperature gradient) to electricity. This result leads us to use them to harvest waste heat from engines or in power plants to generate electricity. Moreover, thermoelectric devices also perform cooling by applied voltage to device. This process is clean, which means that no greenhouse gases are emitted during the process. However, the converting efficiency of thermoelectrics are very low compare to a home refrigerator. The thermoelectric figure of merit (ZT_m) is a number which defines the converting efficiency of thermoelectric materials and devices. ZT_m is defined by Seebeck coefficient, electrical conductivity and thermal conductivity. To improve the converting efficiency, nanolaminated materials are good candidate.</p><p> </p><p>This thesis studies TiN/ScN artificial nanolaminates, or superlattices were grown by reactive dc magnetron sputtering from Ti and Sc targets. For TiN/ScN superlattice, X-ray diffraction (XRD) and reciprocal space map (RSM) show that we can obtain single crystal TiN/ScN superlattice. X-ray reflectivity (XRR) shows the superlattice films have a rough surface, supported by transmission electron microscopy (TEM). Also, TiN/ScN superlattices grew by TiN as starting layer has better crystalline quality than ScN as starting layer. The electrical measurement shows that our superlattice films are conductive films.</p><p> </p><p>Ca-Co-O system for inherently nanolaminated materials were grown by reactive rf magnetron sputtering from Ca/Co alloy target. The XRD shows we maybe get the [Ca₂CoO₃]_xCoO₂ phase, so far. The energy dispersive X-ray spectroscopy (EDX) reported that our films have Al conmination. We also discovered unexpected behavior when the film grown at high temperature showed larger thickness instead of thinner, which would have been expected due to possible Ca evaporation. The Ca-Co-O system requires further studies.</p>

Energy harvesting is an interesting topic for today since we face running out of energy source, a serious problem in the world. Thermoelectric devices are a good candidate. They can convert heat (i.e. temperature gradient) to electricity. This result leads us to use them to harvest waste heat from engines or in power plants to generate electricity. Moreover, thermoelectric devices also perform cooling by applied voltage to device. This process is clean, which means that no greenhouse gases are emitted during the process. However, the converting efficiency of thermoelectrics are very low compare to a home refrigerator. The thermoelectric figure of merit (ZTm) is a number which defines the converting efficiency of thermoelectric materials and devices. ZTm is defined by Seebeck coefficient, electrical conductivity and thermal conductivity. To improve the converting efficiency, nanolaminated materials are good candidate.   This thesis studies TiN/ScN artificial nanolaminates, or superlattices were grown by reactive dc magnetron sputtering from Ti and Sc targets. For TiN/ScN superlattice, X-ray diffraction (XRD) and reciprocal space map (RSM) show that we can obtain single crystal TiN/ScN superlattice. X-ray reflectivity (XRR) shows the superlattice films have a rough surface, supported by transmission electron microscopy (TEM). Also, TiN/ScN superlattices grew by TiN as starting layer has better crystalline quality than ScN as starting layer. The electrical measurement shows that our superlattice films are conductive films.   Ca-Co-O system for inherently nanolaminated materials were grown by reactive rf magnetron sputtering from Ca/Co alloy target. The XRD shows we maybe get the [Ca2CoO3]xCoO2 phase, so far. The energy dispersive X-ray spectroscopy (EDX) reported that our films have Al conmination. We also discovered unexpected behavior when the film grown at high temperature showed larger thickness instead of thinner, which would have been expected due to possible Ca evaporation. The Ca-Co-O system requires further studies.

Global warming and developments of alternative energy technologies have become important issues nowadays. Subsequently, the concept of energy harvesting is rising because of its ability of transferring waste energy into usable energy. Thermoelectric devices play a role in this field since there is tremendous waste heat existing in our lives, such as heat from engines, generators, stoves, computers, etc. Thermoelectric devices can extract the waste heat and turn them into electricity. Moreover, the reverse thermoelectric phenomenon has the function of cooling which can be applied to refrigerator or heat dissipation for electronic devices. However, the energy conversion efficiency is still low comparing to other energy technologies. The efficiency is judged by thermoelectric figure of merit (ZT), defined by Seebeck coefficient, electrical conductivity and thermal conductivity. In order to improve ZT, thin film materials are good candidates because of their structural effects on altering ZT.    Ca3Co4O9 thin films grown by reactive radio frequency magnetron sputtering followed by post-annealing process is studied in this thesis. Structural properties of the films with the evolution of elemental ratio (Ca/Co) of calcium and cobalt have been investigated. For the investigations, three samples having elemental ratio 0.82, 0.72, and 0.66 for sample CCO1, CCO2 and COO3, respectively, have been prepared. Structural properties of the films have been investigated by X-ray diffraction (XRD) θ-2θ and pole figure analyses. Surface morphology of the films has been investigated by scanning electron microscopic (SEM) analyses. The highly oriented and phase pure epitaxial Ca3Co4O9 thin films were obtained in the end.   Mixing of ScN and CrN to obtain ScxCr1-xN solid solution thin films by DC magnetron sputtering is the other task in this thesis. Growth of ScN and CrN thin films were studied first in order to get the best mixed growth conditions. The phase shifts between ScN (111) and CrN (111) peaks were observed in mixed growth films by XRD θ-2θ measurements, indicating the formation of ScxCr1-xN. Surface morphology of the films were investigated by SEM. The (111)-oriented ScxCr1-xN thin films with decent surface smoothness grown by DC magnetron sputtering at 600 °C in pure nitrogen with bias were developed.

近幾年來，熱電材料蓬勃發展是許多物理、化學以及材料科學家的熱門研究的方向，然而此一跨領域的基礎研究工作處於萌芽的階段。熱電材料的益處在於可將熱機或是冷凍機之上所產生的廢熱轉化成電能。本研究利用鉍化碲(Bismuth Tellurium)在室溫附近具有一熱電優質係數(ZT)為1.0的熱電表現，其具有非常低的熱傳導率以及適當的載子傳輸性質，因此Bi-Te的合金系列成為大家研究的趨勢，成為另一項重大的焦點引發相當的關注。鉍化碲元素皆是地球殼中豐富的元素，且鉍化碲是對人無毒且對環境無害的化合物，相較於其他高性能熱電材料(一般由稀少元素/貴金屬組成)，具有非常大商業化的潛力。鉍化碲本身為非常穩定的多層層狀結構(Quintuple Layer)，表現出極低的熱傳導率以及良好的導電性。為了未來能製作出微小的熱電模組，本研究利用射頻磁控濺鍍系統(Radio-Frequency Magnetron Sputtering System)調控濺鍍參數的方式，得到最佳熱電性質之薄膜後，再使用半導體製程技術製作微結構的陣列熱電薄膜，利用光微影製程及金屬遮罩兩種分別不同的方式決定所需之電極和薄膜陣列之圖形。本論文使用磁控濺鍍設備，靶材n-type和p-type分別選用Cu0.02Bi2Te2.7Se0.3 和Bi0.5Sb1.5Te3+0.33 wt% Aerogel之熱電材料，經由實驗改變磁控濺鍍的工作壓力、RF power，再透過ZEM-3、EDS對薄膜的研究分析得到(最佳鍍膜參數) 最佳鍍膜品質參數(seebeck、電阻)。決定鍍膜參數後使用本研究開發的兩種方式製作微結構熱電元件，一使用光微影半導體製程，二使用金屬遮罩，針對兩種製程方式所得的n-type和p-type陣列熱電薄膜成長過程做比較與研究探討。 / In recent years, physicists, chemists and material scientists at many major universities and research institutions throughout the world are attempting to create novel materials with high thermoelectric (TE) efficiency. It will be beneficial to harvest waste heat into electrical energy. Especially heating and cooling are other major applications for this class of new TE materials. At present the thermoelectric (TE) material bismuth telluride (Bi2Te3) baesd systems exhibit best figure of merit (ZT). Bismuth and tellurium are earth-abundant elements and Bi2Te3 is non-toxic to human beings and the environment. Therefore, it has great potential in commercial implements. Bismuth telluride is a quintuple layer-structured compound possessing ultralow thermal conductivity and moderate electrical conductivity. In this work, the TE thin film and device are fabricated and optimized by Radio-Frequency Magnetron Sputtering System (RFMSS) and the influence of the preparative parameters such as working pressure and working power of RF sputtering are investigated. In this study, we used the magnetron sputtering equipment and the thermoelectric materials n-type target and p-type target were Cu0.02Bi2Te2.7Se0.3 and Bi0.5Sb1.5Te3+0.33 wt% aerogel, respectively. In this study, the experimental changes the magnetron sputtering working pressure, RF power before the ZEM-3, EDS analysis the thin film thermoelectric properties to get the best thin film quality parameters (Seebeck coefficient, resistivity, power factor). After the thin film parameters were determined, the microstructural thermoelectric 442 pairs device were fabricated by the photolithography semiconductor process, and n-type and p-type arrays used by photolithography to define a pattern and deposit Au electrodes onto the substrate by thermal evaporation.