Fresh water collection techniques have gained significant attention due to global dwindling of fresh water resources and recent scares such as the 2011-2017 California drought. This project explores the economic viability of actively harvesting water from fog, and techniques to maximize water collection. Vapor compression and thermoelectric cooling based dehumidifier prototypes are tested in a series of experiments to assess water collection capability in foggy environments, and what parameters can increase that capability. This testing shows an approximate maximum collection rate of 1.25 L/kWh for the vapor compression prototype, and 0.32 L/kWh for the thermoelectric cooling prototype; compared to 315 L/kWh for desalination or 12 L/m2/day for passive meshes. Exploration of parameters on the thermoelectric cooling prototype show a potential increase in water collection rate of 29% with the addition of a Teflon coating to the collection surface, 15% by clearing the collection surface, and 89% by tilting certain collection surfaces by 60-75°. In combination, these parameters could push active atmospheric water harvesting into economic viability where significant infrastructure investment is not feasible.

The thesis deals with the theoretical investigation of the thermoelectric tensions in lead-free solder. The theory introduces the reader to the formation and composition of solder joints, thermoelectric properties and behavior in circuits. Part of the thesis is the design and optimization of the measuring tool for lead-free solder and product samples with measuring.

Thermoelectric (TE) devices can undergo degradation from reactions in corrosive environments and at higher operating temperatures by sublimation and oxidation. To prevent the degradation, we have applied two high temperature polymers (HTPs) as coatings for TE materials. Sintering temperatures were from 250°C to 400°C. We explain why dip coating is better technique in our study and had two potential HTPs for tests. By applying TGA (thermogravimetric analysis), we were able to figure out which HTPs have better thermal resistivity. Besides, TGA also help us to find proper curing cycles for HTPs. EDS and SEM results show that the coatings prevent oxidation and sublimation of TE materials. We also shorten HTP curing cycle time and lower the energy costs.

This dissertation describes the process of designing, process of optimalization and the following implementation of the changes of the thermoelectric linear actuator which operates the opening and the closing of the valves used in the most modern buildings for example for floor heating. It is a product produced in large series with the annual production of more than half of a million items. The reason for the optimalization is the instability during the assembly in the production and the reduction of costs, whereas the final assembly takes place in Germany. The dissertation deals with designing of the conceptions for solving of the trouble parts of the current version of the actuator. The chosen concept is consequently optimalized, tested and put into production.

Ce travail de thèse porte sur une série d’échantillons de composition nominale AgBiSe2-xSx (avec x= 0 à 2), appartenant à la famille des chalcogénures ternaires de type I-V-VI₂. Les analyses structurales et thermiques ont mis en évidence une solution solide complète sans gap de miscibilité, et des transitions de phase pour toutes les compositions. Nous avons pu obtenir des composés monophasés à la fois des phases hexagonale et cubique, et notre étude de DRX en température à mis en évidence une phase rhomboédrique pour certaines compositions (x=1 à 2 dans AgBiSexS2-x). Les résultats de DSC ont confirmé la présence de transitions de phase pour toutes les compositions, avec un déplacement des températures de transition en fonction de la fraction de soufre/sélénium. Notre étude de DRX sous pression de l’échantillon AgBiSe₂ a montré une transition de phase induite par la pression d’une phase hexagonale à rhomboédrique puis cubique. Suite à cette observation, l’application d’une pression chimique, par la substitution de 30% du Bi par du Sb a été utilisée avec succès pour stabiliser la phase cubique pour toutes les compositions. Le dopage par Nb des échantillons substitués par l’antimoine l’a pas eu d’influence sur la nature des phases stables à l’ambiante en comparaison aux échantillons non dopés. Nous avons ensuite étudié l’influence du dopage sur les propriétés de transport. Les valeurs négatives de S pour toutes les compositions indiquent un comportement de semi-conducteur de type n dans la gamme (50-300K). Par ailleurs, nos mesures ont montré à a fois de très faibles valeurs de κ mais aussi une décroissance de ∣S∣ et ρ avec l’augmentation de la fraction de Nb. Ces résultats devraient permettre d’optimiser le facteur de puissance pour améliorer les valeurs de ZT. Enfin, une étude en collaboration avec une équipe chinoise a permis d’obtenir une valeur de ZT de 1.3 à 890K dans un composé AgPbmSnSe₂. / Here, we report on a series of samples with nominal compositions AgBiSe2-xSx (with x= 0 to 2) belonging to the class of ternary chalcogenides of type I-V-VI₂. The structural and thermal analysis result shows a complete solid solution without miscibility gap and phase transitions for all compositions. We have succeeded in obtaining single phase compounds, of both hexagonal and cubic phase, and the high temperature XRD study showed the rhombohedral phase too for selected compositions (x=1 to 2 in AgBiSexS2-x). The DSC results confirmed the presence of the phase transitions for all compositions, with a shift of the temperature of transition as a function of the sulfur/selenium fraction. The high pressure XRD investigation of the compound AgBiSe₂ showed a pressure induced phase transition from hexagonal-to-rhombohedral-to-cubic phase. In this respect, chemical pressure with 30% Sb on the Bi site has been successfully applied to stabilize the cubic phase for all compositions. Nb doping in the Sb-substituted samples does not show any change in the phase behavior at RT in comparison with the undoped samples. The influence of doping on transport properties was analyzed. The negative value of S for all compositions indicates n-type semiconducting behavior over the range (50-300K). Further, the results not only shows very low value of κ but the ∣S∣ and ρ value also decreases for each composition from Nb fraction 0.02 to 0.04. This gives us the opportunity to optimize the power factor in order to improve the ZT value. At last, collaborative study with Chinese team showed that ZT of 1.3 at 890 K can be achieved for AgPbmSnSe2+m (m = 50).

In the past decade, the use of nanotechnology as a tool to develop and fabricate new structures and devices for biological sensing and energy applications has become increasingly widespread. In this work, a systematic study has been performed on one-dimensional nanomaterials, with a focus on the development of miniaturized devices with a "bottom up" approach. First, members of the nano - carbon family are utilized for biosensing applications; in particular, carbon nanotubes as well as nitrogen - doped and boron - doped nanocrystalline diamond (NCD) films. These carbon - based materials possess several unique electrochemical properties over other conductive materials which make them suitable for biosensing applications. Single walled carbon nanotubes were deposited on a glass carbon electrode and modified for the detection of Salmonella DNA hybridization. Electrochemical impedance spectroscopy (EIS) was used as the method of detection and a detection limit of 10-9 M was achieved. Nanocrystalline diamond was grown using a microwave enhanced plasma chemical vapor deposition method. The diamond electrodes were doped with either boron or nitrogen to provide substrates and characterization was performed using scanning electron microscopy, atomic force microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, UV-vis spectroscopy, as well as by electrochemical methods. Modified boron - doped NCD was able to detect Salmonella DNA hybridization via EIS and fluorescent microscopy. The detection limit for these genosensors was found to be 0.4 micrometer complementary DNA. Boron - doped and nitrogen - incorporated nanocrystalline diamond also served as functionalized electrodes for lactic acid detection. It was found that the boron - doped electrodes could detect 0.5 mM lactic acid in a phosphate buffer solution. Second, bismuth antimony nanowires were grown in an anodized alumina template for the fabrication of a thermoelectric cooling device. Bismuth antimony nanowires were chosen due to their high thermoelectric efficiency compared to their bulk material counterpart. The development of a successful anodized template was achieved and EIS was used to diagnose the optimal etch parameters of the barrier oxide layer for nanowire growth. Bismuth antimony nanowires were grown directly on a silicon substrate and a thermoelectric cooling device was fabricated. The nanowires exhibited a thermoelectric efficiency of 0.18 at room temperature.

Thermoelectric generators (TEGs) are currently a topic of interest in the field of energy harvesting for automobiles. In applying TEGs to the outside of the exhaust tailpipe of a vehicle, the difference in temperature between the hot exhaust gases and the automobile coolant can be used to generate a small amount of electrical power to be used in the vehicle. The amount of power is anticipated to be a few hundred watts based on the temperatures expected and the properties of the materials for the TEG. This study focuses on developing efficient heat exchanger modules for the cold side of the TEG through the analysis of experimental data. The experimental set up mimics conditions that were previously used in a computational fluid dynamics (CFD) model. This model tested several different geometries of cold side sections for the heat exchanger at standard coolant and exhaust temperatures for a typical car. The test section uses the same temperatures as the CFD model, but the geometry is a 1/5th scaled down model compared to an full-size engine and was fabricated using a metal-based rapid prototyping process. The temperatures from the CFD model are validated through thermocouple measurements, which provide the distribution of the temperatures across the TEG. All of these measurements are compared to the CFD model for trends and temperatures to ensure that the model is accurate. Two cold side geometries, a baseline geometry and an impingement geometry, are compared to determine which will produce the greater temperature gradient across the TEG. / Master of Science

The increasing price of oil, global warming and rapid industrial growth has drawn much attention to renewable energy technologies over the last few decades. The total energy consumption is estimated to increase 1.4% per year globally. About 90% of this energy supply is generated through fossil fuel combustion with a typical efficiency of 30-40%. The remaining 60-70% of the energy is lost to the environment via automotive exhaust or industrial processes. It is highly desired to retrieve wasted heat to improve the overall efficiency of the energy conversion. Developing thermoelectric materials and devices is a potential solution to utilize waste heat as an energy source. Skutterudites are known to be promising thermoelectric materials in the temperature range 600K to 900K. Novel nanoengineering approaches and filling of skutterudites structure can further improve the transport properties of the material. In this work, Cobalt Antimonide (Co4Sb12) based skutterudites were fabricated via mechanical milling and alloying. Rear earth material Ytterbium and Cerium are used as fillers to substitute the cages in the crystal lattice of these materials. Base material is synthesized via thermochemical reduction of the precursors under hydrogen. Further processing of the material is performed with ball milling and Spark Plasma Sintering (SPS). Ball milling parameters were optimized for nanostructuring of Co4Sb12. Grain size was significantly reduced after SPS compaction. Finally, Thermoelectric transport properties of the material is evaluated over the temperature range 300K to 900K for five different composition of the skutterudites materials. Significant reduction in materials thermal conductivity was achieved through nanostructuring.

Die vorliegende Arbeit forciert die Steigerung der Leistungsfähigkeit netzautarker thermoelektrischer Systeme. Hierunter werden im Folgenden Apparate verstanden, die mittels thermoelektrischer Generatoren (TEG) thermische Energie in Elektroenergie umwandeln, um damit netzautark und dezentral elektrische Kleinstverbraucher an großtechnischen Anlagen und Maschinen zu speisen. Bei den elektrischen Verbrauchern kann es sich beispielsweise um Sensoren zum Vermessen unterschiedlicher Prozessgrößen handeln. Aber auch eine Energieversorgung von Geräten zur drahtlosen Datenübertragung und Aktoren mit entsprechend geringer Leistungsaufnahme ist mittels netzautarker thermoelektrischer Systeme technisch möglich. Zum Erreichen der Zielstellung werden die TEG zur Energieumwandlung nicht isoliert betrachtet, sondern der optimierte Systemaufbau ganzheitlich forciert. Denn zur Steigerung der Leistungsfähigkeit netzautarker thermoelektrischer Systeme müssen alle Komponenten betrachtet: Angefangen von den TEG über die notwendige Wärmekopplung bis hin zu weiteren Peripheriegeräten. Im Konkreten basiert die in dieser Dissertation dargelegte Entwicklungsarbeit zum einen auf einem mathematischen Modell zur Berechnung verlustbehafteter TEG-Wärmeübertrager-Systeme, zum anderen auf der Entwicklung eines effizienten Wärmeübertragers zur passiven Kühlung thermoelektrischer Module sowie der Darstellung und Diskussion eines Gleichstromwandler-Schaltkreises für die Regelung des Betriebszustands eines angekoppelten TEG.

Many modern technologies are enabled by the use of thin films and/or nanostructured composite materials. For example, many thermoelectric devices, solar cells, power electronics, thermal barrier coatings, and hard disk drives contain nanostructured materials where the thermal conductivity of the material is a critical parameter for the device performance. At the nanoscale, the mean free path and wavelength of heat carriers may become comparable to or smaller than the size of a nanostructured material and/or device. For nanostructured materials made from semiconductors and insulators, the additional phonon scattering mechanisms associated with the high density of interfaces and boundaries introduces additional resistances that can significantly change the thermal conductivity of the material as compared to a macroscale counterpart. Thus, better understanding and control of nanoscale heat conduction in solids is important scientifically and for the engineering applications mentioned above. In this dissertation, I discuss my work in two areas dealing with nanoscale thermal transport: (1) I describe my development and advancement of important thermal characterization tools for measurements of thermal and thermoelectric properties of a variety of materials from thin films to nanostructured bulk systems, and (2) I discuss my measurements on several materials systems done with these characterization tools. First, I describe the development, assembly, and modification of a time-domain thermoreflectance (TDTR) system that we use to measure the thermal conductivity and the interface thermal conductance of a variety of samples including nanocrystalline alloys of Ni-Fe and Co-P, bulk metallic glasses, and other thin films. Next, a unique thermoelectric measurement system was designed and assembled for measurements of electrical resistivity and thermopower of thermoelectric materials in the temperature range of 20 to 350 °C. Finally, a commercial Anter Flashline 3000 thermal diffusivity measurement system is used to measure the thermal diffusivitiy and heat capacity of bulk materials at high temperatures. With regards to the specific experiments, I examine the thermal conductivity and interface thermal conductance of two different types of nanocrystalline metallic alloys of nickel-iron and cobalt-phosphorus. I find that the thermal conductivity of the nanocrystalline alloys is reduced by a factor of approximately two from the thermal conductivity measured on metallic alloys with larger grain sizes. With subsequent molecular dynamics simulations performed by a collaborator, and my own electrical conductivity measurements, we determine that this strong reduction in thermal conductivity is the result of increased electron scattering at the grain boundaries, and that the phonon component of the thermal conductivity is largely unchanged by the grain boundaries. We also examine four complex bulk metallic glass (BMG) materials with compositions of Zr₅₀Cu₄₀Al₁₀, Cu_46.25Zr_44.25Al_7.5Er₂, Fe₄₈Cr₁₅Mo₁₄C₁₅B₆Er₂, and Ti_41.5Zr_2.5Hf₅Cu_42.5Ni_7.5Si₁. From these measurements, I find that the addition of even a small percentage of heavy atoms (i.e. Hf and Er) into complex disordered BMG structures can create a significant reduction in the phonon thermal conductivity of these materials. This work also indicates that the addition of these heavy atoms does not disrupt electron transport to the degree with which thermal transport is reduced. / Ph. D.