Discover of new energy sources and solutions are one of the important global issues nowadays, which has a big impact on economy as well as environment. One of the methods to help to mitigate this issue is to recover wasted heat, which is produced in large quantities by the industry, through vehicle exhausts and in many other situations where we consume energy. One way to do this would be using thermoelectric (TE) materials, which enable direct interconversion between heat and electrical energy. This thesis investigates how the novel material combinations and nanotechnology could be used for fabricating more efficient TE materials and devices. The work presents synthesis, processing, and electrical characterization of group IV materials for TE applications. The starting point is epitaxial growth of alloys of group IV elements, silicon (Si), germanium (Ge) and tin (Sn), with a focus on SiGe and GeSn(Si) alloys. The material development is performed using chemical vapor deposition (CVD) technique. Strained and strain-relaxed Ge1-x Snx (0.01≤x≤0.15) has been successfully grown on Ge buffer and Si substrate, respectively. It is demonstrated that a precise control of temperature, growth rate, Sn flow and buffer layer quality is necessary to overcome Sn segregation and achieve a high quality GeSn layer. The incorporation of Si and n- and p-type dopant atoms is also investigated and it was found that the strain can be compensated in the presence of Si and dopant atoms.  Si1-xGexlayers are grown on Si-on-insulator wafers and condensed by oxidation at 1050 ᵒC to manufacture SiGe-on-insulator (SGOI) wafers. Nanowires (NWs) are processed, either by sidewall transfer lithography (STL), or by using conventional lithography, and subsequently manufactured into nanoscale dimensions by focused ion beam (FIB) technique. The NWs are formed in an array, where one side is heated by a resistive heater made of Ti/Pt. The power factor of NWs is measured and the results are compared for NWs manufactured by different methods. It is found that the electrical properties of NWs fabricated with FIB technique can be influenced due to Ga doping during ion milling. Finally, the carrier transport in SiGe NWs formed on SGOI samples is tailored by applying a back-gate voltage on the Si substrate. In this way, the power factor is improved by a factor of 4. This improvement is related to the presence of defects and/or small fluctuation of nanowire shape and Ge content along the NWs, generated during processing and condensation of SiGe layers. The SiGe results open a new window for operation of SiGe NWs-based TE devices in the new temperature range of 250 to 450 K. / <p>QC 20160907</p>

Approved for public release; distribution is unlimited. / Thermophotovoltaic cells are a good candidate for use in high efficiency radioisotope thermoelectric generator (RTG) power devices for deep space missions. This thesis examines the use of Silvaco Virtual Wafer Fabrication Software as a tool for designing and optimizing TPV cells for different possible spectra. It gives results for GaSb and InGaAs cells optimized to the AM0 spectrum which closely match published data as well as hypothetical cells optimized to the spectrum of a 1300K blackbody. / Ensign, United States Navy

Thesis advisor: Zhifeng Ren / Thermoelectrics, as one promising approach for solid-state energy conversion between heat and electricity, is becoming increasingly important within the last a couple of decades as the availability and negative environmental impact of fossil fuels draw increasing attention. Therefore, various thermoelectric materials in a wide working temperature range from room temperature to 1000 degrees Celsius for power generation or below zero for cooling applications have been intensively studied. In general, the efficiency of thermoelectric devices relies on the dimensionless figure-of-merit (ZT) of the material, defined as ZT=(S<super>2</sup>&sigma;)T/&kappa;, where S is the Seebeck coefficient, [sigma] the electrical conductivity, [kappa] the thermal conductivity (sum of the electronic part, the lattice part, and the bipolar contribution at high temperature region), and T the absolute temperature during operation. Techniques to measure those individual parameters will be discussed in the 2nd chapter while the 1st chapter mainly covers the fundamental theory of thermoelectrics. Recently, the idea of using various nanostructured materials to further improve the ZT of conventional thermoelectric materials has led to a renewed interest. Among these types of nanostructured materials, nanocomposites which mainly denote for the nano-grained bulk materials or materials with nano-sized inclusions are the major focus of our study. For nanocomposites, the enhancement in ZT mainly comes from the low lattice thermal conductivity due to the suppressed phonon transport by those interfaces or structure features in the nanometer scale without deteriorating the electron transport. In the last few years, we have successfully demonstrated in several materials systems (Bismuth Telluride, Skutterudites, Silicon Germanium) that ball milling followed by hot pressing is an effective way for preparing large quantities of those nanocomposite thermoelectric materials with high ZT values in the bulk form. Therefore, in the 3rd part of this thesis, I will talk about how I applied the same technique to the Thalllium (Tl) doped Lead Telluride (PbTe) which was reported for an improved Seebeck coefficient due to the creation of resonant states near the Fermi level, leading to a high ZT of about 1.5 at around 500 degrees Celsius. I showed that comparing with conventional tedious, energy consuming melting method, our fabrication process could produce such material with competing thermoelectric performance, but much simpler and more energy effective. Potential problems and perspectives for the future study are also discussed. The 4th chapter of my thesis deals with the challenge that in addition to those nanostructuring routes that mainly reduce the thermal conductivity to improve the performance, strategies to enhance the power factor (enhancing [sigma] or S or both) are also essential for the next generation of thermoelectric materials. In this part, modulation-doping which has been widely used in thin film semiconductor industry was studied in 3-D bulk thermoelectric nanocomposites to enhance the carrier mobility and therefore the electrical conductivity [sigma]. We proved in our study that by proper materials design, an improved power factor and a reduced thermal conductivity could be simultaneously obtained in the n-type SiGe nanocomposite material, which in turn gives an about 30% enhancement in the final ZT value. In order to further improve the materials performance or even apply this strategy to other materials systems, I also provided discussions at the end of chapter. In the last chapter, the structural and transport properties of a new thermoelectric compound Cu₂Se was studied which was originally regarded as a superionic conductor. The [beta]-phase of such material possesses a natural superlattice-like structure, therefore resulting in a low lattice thermal conductivity of 0.4-0.5 Wm^-1K^-1 and a high peak ZT value of ~1.6 at around 700 degrees Celsius. I also studied the phase transition behavior between the cubic [beta]-phase and the tetragonal [alpha]-phase of such material from the discontinuity of transport property curves and the change in crystal structure. In addition, I also talk about the abnormal decrease in specific heat with increasing temperature that I observed in the as-prepared Cu₂Se samples. I suggest this material is of general interest to a broad range of researchers in Physics, Chemistry, and Materials Science. / Thesis (PhD) — Boston College, 2012. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Neste trabalho foram estudadas deposições químicas de ligas CuNixPy e foram fabricados termopares Cu/CuNixPy sobre superfícies de lâminas de silício. Inicialmente, as superfícies foram pré-ativadas em uma solução diluída de ácido fluorídrico contendo PdCl2. Em seguida, foi empregado um banho químico alcalino diluído em água deionizada contendo 15 g/l NiSO4.6H2O; 0,1 a 0,3 g/l CuSO4.5H2O; 15 g/l Na2HPO2.H2O e 60 g/l Na3C6H5O7.2H2O na temperatura de 80OC sendo que NH4OH foi adicionado até que o pH do banho atingisse o valor de 8,0. Verificamos que a concentração do sal de cobre na solução de deposição afeta substancialmente a quantidade de cobre nos depósitos de CuNixPy. As concentrações planares e as composições dos filmes depositados foram obtidas através da técnica de espectrometria de retroespalhamento de Rutherford (RBS) e a morfologia superficial foi caracterizada através da técnica de microscopia de força atômica (AFM). A solução: 15 g/l NiSO4.6H2O + 0,3 g/l CuSO4.5H2O + 15 g/l Na2HPO2.H2O + 60 g/l Na3C6H5O7.2H2O + NH4OH (pH 8.0) na temperatura de 80OC foi a escolhida na obtenção da liga CuNiP0,5 para a construção de termopares Cu/CuNiP0,5 os quais apresentaram potência termoelétrica de aproximadamente (866) V/oC semelhante aos valores típicos apresentados na literatura para Cu/CuNi. / In this work, it was studied chemical depositions of CuNixPy alloys and it was fabricated Cu/CuNixPy thermocouples onto silicon wafer surfaces. Initially, surfaces were pre-activated in a diluted hydrofluoric acid solution containing PdCl2. Following, it was used a de-ionizedwater- diluted alkaline chemical bath containing 15 g/l NiSO4.6H2O; 0,1-0,3 g/l CuSO4.5H2O; 15 g/l Na2HPO2.H2O and 60 g/l Na3C6H5O7.2H2O at temperature of 80OC where NH4OH was added until ph was 8.0. The concentration of copper salt in the deposition solution greatly affected the Cu content of the CuNixPy deposits. Areal concentration and composition were measured by Rutherford Backscattering Spectrometry (RBS) and surface morphology was characterized by Atomic Force Microscopy (AFM). The solution: 15 g/l NiSO4.6H2O; 0.3 g/l CuSO4.5H2O; 15 g/l Na2HPO2.H2O; 60 g/l Na3C6H5O7.2H2O; NH4OH (pH 8.0) at the temperature of 80OC was chosen to obtain the CuNiP0.5 alloy to fabricate Cu/CuNiP0.5 thermocouples with thermoelectric power of about (866) V/oC, which is similar to the typical values reported in literature for Cu/CuNi.

The Cu-Sb-S compounds (Cu12Sb4S13, CuSbS2, Cu3SbS3 and Cu3SbS4) have the advantages of earth-abundance, low-toxicity and low-cost, compared with conventional thermoelectric materials. This work provides a comprehensive study on the synthesis methods, crystal structures and thermoelectric properties of Cu-Sb-S compounds. All of the samples were prepared by mechanical alloying combined with SPS, which had high density, high purity and very fine microstructure. The lone-pair electrons of Sb and the [CuS3] plane play important roles in realizing very low lattice thermal conductivity of these compounds. Except for Cu12Sb4S13, which is known as a good thermoelectric material, the other three compounds showed very poor thermoelectric performance due to their high electrical resistivities. A phase transition at 398 K was found in Cu3SbS3, which makes it unsuitable for applications and attempts to optimize electrical properties of CuSbS2 failed. Different p-type dopants were studied to improve the electrical properties of Cu3SbS4. Both Ge-doping and Sn-doping on Sb sites increased the carrier concentration of Cu3SbS4 significantly. The electrical transport properties were analyzed using SPB model, and a large effective mass of 3.0 me was found for all of the samples. A maximum zT value of 0.69 was obtained at 623 K in 5 mol. % Sn-doped sample which was about 6 times higher than that of undoped sample. The solid-solutions of Cu3SbS4(1-y)Se4y were studied to further improve the thermoelectric properties. The lattice thermal conductivity was reduced in solid-solution due to the local mass contrast and alloying scattering, but there was no further improvement in zT value due to the decrease in Seebeck coefficient. Another solid solution of Cu3Sb1-xBixS4 was studied, but Bi had very low solubility and a second phase was formed instead of forming the solid solution. Future work should focus on reducing the lattice thermal conductivity of Cu3SbS4 without impacting its electrical properties.

Este estudo teve como finalidade levantar dados para uma avaliação das alternativas tecnológicas (cultivos de microalgas e reflorestamento) para a biofixação de CO2 da atmosfera próxima à usina termelétrica; tendo sido utilizada como referência a Usina Barbosa Lima Sobrinho. Já existe um projeto de avaliação do efeito do reflorestamento na fixação do CO2 nesta usina e, neste trabalho, foi avaliada a alternativa do cultivo de microalgas. Uma pesquisa inicial foi feita na literatura para verificar qual a espécie de microalga seria a mais adequada para ser utilizada no estudo, tendo sido a espécie Chlorella sp. a selecionada. Posteriormente os sistemas de cultivo de microalgas mais comumente empregados no mercado foram levantados e foi selecionado o cultivo em tanques abertos como referência para a modelagem do processo. Utilizando os dados da termelétrica e da literatura foi possível estimar a quantidade de CO2 que será retirada da atmosfera caso um sistema de cultivo seja efetivamente instalado na usina termelétrica. Uma análise econômica foi realizada para determinar a viabilidade do projeto. Os resultados indicam que a utilização deste tipo de tecnologia é promissora / This study aimed to collect data for an evaluation of alternative technologies (microalgae cultivations and reforestation) for biofixation of CO2 from the atmosphere near a power plant, named Barbosa Lima Sobrinho, which uses natural gas and diesel as fuels. There is a project to assess the effect of reforestation for CO2 fixation in this facility and, in this study, we evaluated the alternative of cultivating microalgae. An initial review was done in the literature to determine what species of microalgae would be most suitable to use and Chlorella sp was selected. Thereafter cultivation of microalgae media, commonly employed in the market, were evaluated and the culture selected was in open tanks as a reference for the modeling process. Using data from the thermoelectric and the literature it was possible to estimate the amount of CO2 that will be capture from the atmosphere if a microalgae farming will be installed in thermal power plant area. An economic analysis was performed to determine the feasibility of the project. The results indicate that the use of this technology is promising

Neste trabalho foram estudadas deposições químicas de ligas CuNixPy e foram fabricados termopares Cu/CuNixPy sobre superfícies de lâminas de silício. Inicialmente, as superfícies foram pré-ativadas em uma solução diluída de ácido fluorídrico contendo PdCl2. Em seguida, foi empregado um banho químico alcalino diluído em água deionizada contendo 15 g/l NiSO4.6H2O; 0,1 a 0,3 g/l CuSO4.5H2O; 15 g/l Na2HPO2.H2O e 60 g/l Na3C6H5O7.2H2O na temperatura de 80OC sendo que NH4OH foi adicionado até que o pH do banho atingisse o valor de 8,0. Verificamos que a concentração do sal de cobre na solução de deposição afeta substancialmente a quantidade de cobre nos depósitos de CuNixPy. As concentrações planares e as composições dos filmes depositados foram obtidas através da técnica de espectrometria de retroespalhamento de Rutherford (RBS) e a morfologia superficial foi caracterizada através da técnica de microscopia de força atômica (AFM). A solução: 15 g/l NiSO4.6H2O + 0,3 g/l CuSO4.5H2O + 15 g/l Na2HPO2.H2O + 60 g/l Na3C6H5O7.2H2O + NH4OH (pH 8.0) na temperatura de 80OC foi a escolhida na obtenção da liga CuNiP0,5 para a construção de termopares Cu/CuNiP0,5 os quais apresentaram potência termoelétrica de aproximadamente (866) V/oC semelhante aos valores típicos apresentados na literatura para Cu/CuNi. / In this work, it was studied chemical depositions of CuNixPy alloys and it was fabricated Cu/CuNixPy thermocouples onto silicon wafer surfaces. Initially, surfaces were pre-activated in a diluted hydrofluoric acid solution containing PdCl2. Following, it was used a de-ionizedwater- diluted alkaline chemical bath containing 15 g/l NiSO4.6H2O; 0,1-0,3 g/l CuSO4.5H2O; 15 g/l Na2HPO2.H2O and 60 g/l Na3C6H5O7.2H2O at temperature of 80OC where NH4OH was added until ph was 8.0. The concentration of copper salt in the deposition solution greatly affected the Cu content of the CuNixPy deposits. Areal concentration and composition were measured by Rutherford Backscattering Spectrometry (RBS) and surface morphology was characterized by Atomic Force Microscopy (AFM). The solution: 15 g/l NiSO4.6H2O; 0.3 g/l CuSO4.5H2O; 15 g/l Na2HPO2.H2O; 60 g/l Na3C6H5O7.2H2O; NH4OH (pH 8.0) at the temperature of 80OC was chosen to obtain the CuNiP0.5 alloy to fabricate Cu/CuNiP0.5 thermocouples with thermoelectric power of about (866) V/oC, which is similar to the typical values reported in literature for Cu/CuNi.

Skutterudites have been of great interest for thermoelectric applications over the last ten years. Scientific interest has focused on the unique transport properties Skutterudites possess due to the unique crystal structure. Technical interest has grown since it was discovered that some compounds rival the current best thermoelectric materials. To further the understanding of this material system, and optimize its thermoelectric properties, the synthesis and characterization of polycrystalline n- and ptype CoGe1.5Se1.5 was undertaken. Structural, morphological, chemical, electrical, thermal and magnetic properties were studied. These data are compared to those of the binary Skutterudite CoSb3. The results of this study show a very sensitive dependence of the physical properties on stoichiometry. While the thermoelectric figure of merit is low in these materials, it is apparent that optimization via doping and “void filling” will lead to improved thermoelectric properties.

Carbon nanotubes were dispersed and functionalized with polyethylene imine (PEI) before incorporation in a polyvinyl acetate matrix. The resulting samples exhibit air-stable N-type characteristics with electrical conductivities as great as 1600 S/m and thermopowers as high as 100 microV/K. Thermopowers and electrical conductivities correlate, in a reversal of the trend found in typical materials. This phenomenon is believed to be due to the increase in the number of tubes that are evenly coated in a better dispersed sample. Increasing the amount of PEI relative to the other constituents positively affects thermopower but not conductivity. Air exposure reduces both thermopower and conductivity, but a stable value is reached within seven days following film fabrication. The atmospheric effects on the electrical conductivity prove to be reversible. Oxygen is believed to be the primary contributor to the decay.

Thermoelectric energy (TE) conversion can be used to create electricity from temperature gradients. Hence power can be generated from waste heat using TE materials, e.g. from the exhaust in automotives. This power in turn may lead to a reduction of gas consumption by reducing the alternator load on the engine. Because of the increasing demand and limited availability of energy sources, there is strong and renewed interest in advancing thermoelectric materials. Past research shows that the best TE materials are narrow band gap semiconductors composed of heavy elements, exhibiting a large Seebeck coefficient, S, combined with high electrical conductivity, σ, and low thermal conductivity, κ. Various research projects have been attempted during the past four years of my Ph.D. studies. These include the synthesis, crystal structure studies, electronic structure calculations and thermoelectric properties of transition metal oxides and thallium main group chalcogenides. Because of the good thermal stability, lack of sensitivity to the air, and non-toxicity, transition metal oxides are potential candidates for commercial thermoelectric applications. During the investigation of oxides for thermoelectric application, several interesting features of different transition metal oxides have been discovered: 1. A new quaternary layered transition-metal oxide, Na2Cu2TeO6, has been synthesized under air using stoichiometric mixtures of Na2CO3, CuO and TeO2. Na2Cu2TeO6 crystallizes in a new structure type, monoclinic space group C2/m with a = 5.7059(6) Å, b = 8.6751(9) Å, c = 5.9380(6) Å,  = 113.740(2)°, V = 269.05(5) Å3 and Z = 2, as determined by single crystal X-ray diffraction. The structure is composed of[Cu2TeO6] layers with the Na atoms located in the octahedral voids between the layers. Na2Cu2TeO6 is a green nonmetallic compound, in agreement with the electronic structure calculation and electrical resistance measurement. 2. An n-type narrow band gap semiconductor, LaMo8O14, exhibiting the high Seebeck coefficient of -94 μVK-1 at room temperature has been investigated. 3. Pb0.69Mo4O6 with a new modulated structure and stoichiometry was determined from single-crystal X-ray diffraction data. The compound crystallizes in the tetragonal super space group, P4/mbm(00g)00ss, with a = 9.6112(3) Å, c = 2.8411(1) Å, q = 0.25c*, which is different from the previously reported structure. As for the research of thermoelectric properties of thallium main group chalcogenides, three new ternary thallium selenides, Tl2.35Sb8.65Se14, Tl1.97Sb8.03Se13 and Tl2.04Bi7.96Se13, have been discovered. All three compounds crystallize in the same space group P21/m with different cell parameters, and in part different Wyckoff sites, hence different structure types. The three selenides with similar structures are composed of distorted edge-sharing (Sb,Bi)Se6 octahedra, while the distorted Tl/(Sb, Bi) sites are coordinated by 8 - 9 Se atoms. Electronic structure calculations and physical property measurements reveal they are semiconductors with high Seebeck coefficient but low electrical conductivity, and therefore not good thermoelectrics. On the other hand, our transport property measurements on the unoptimized Tl2SnTe3 sample show interesting thermoelectric properties of this known compound. Advanced thermoelectrics are dominated by antimonides and tellurides so far. The structures of the tellurides are mostly composed of NaCl-related motifs, hence do not contain any Te–Te bonds. All of the antimonide structures containing Sb–Sb bonds of various lengths are much more complex. The Sb atom substructures are Sb24– pairs in β-Zn4Sb3, linear Sb37– units in Yb14MnSb11, planar Sb44– rectangles in the skutterudites, e.g., LaFe3CoSb12, and Sb8 cubes interconnected via short Sb–Sb bonds to a three-dimensional network in Mo3Sb5Te2. The results of electronic structure calculations suggested that these interactions have a significant impact on the band gap size as well as on the effective mass around the Fermi level, which represent vital criteria for advanced thermoelectrics. The crystal structure and electronic structure investigation for the unique T net planar Sb–Sb interactions in Hf5Sb9 will be also presented, although Hf5Sb9 is metallic compound with poor thermoelectric performances.