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Thermoelectric generators convert heat energy to electricity and can be used for waste heat recovery, enabling sustainable development. Selective Laser Melting (SLM) based additive manufacturing process is a scalable and flexible method that has shown promising results in manufacturing high ZT Bi2T e3 material and is possible to be extended to other material classes such as M g2Si. The physical phenomena of melting and solidification were investi- gated for SLM-based manufacturing of thermoelectric (M g2Si) powders through comprehen- sive numerical models developed in MATLAB. In this study, Computational Fluid Dynamics (CFD)-based techniques were employed to solve conservation equations, enabling a detailed understanding of thermofluid dynamics, including the temperature evolution and the con- vection currents of the liquid melt within the molten pool. This approach was critical for optimizing processing parameters in our investigation, which were also used for printing the M g2Si powders using SLM. Additionally, a phase field-based model was developed to sim- ulate the directional solidification of the M g2Si in MATLAB. Microstructural parameters like the Secondary and Primary Dendritic Arm Spacing were studied to correlate the effects of processing parameters to the microstructure of M g2Si. / Master of Science / Thermoelectric generators are devices that transform heat energy into electricity, offering a way to capture and utilize waste heat for sustainable purposes. A cutting-edge manufacturing method called Selective Laser Melting (SLM) has shown great potential in creating high-performance materials like Bi2T e3 for thermoelectric applications. Researchers are now exploring the extension of this technique to other materials, such as Mg2Si. This study delves into the intricate process of melting and solidifying Mg2Si powders using SLM. Advanced computer models were created in MATLAB, to simulate these processes in detail. By employing Computational Fluid Dynamics (CFD) techniques, heat and fluid flow within the molten material was also closely examined. These simulations were vital for fine-tuning the printing settings used to fabricate Mg2Si powders via SLM. Moreover, a specialized model based on phase field theory was developed to mimic the solidification of Mg2Si. The effects of changing manufacturing parameters on the microstructure of the final product were examined. Understanding these microstructural aspects is crucial for optimizing the manufacturing process and ultimately enhancing the performance of Mg2Si for thermoelectric applications.

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Thermal systems involve multiple components assembled to store or transfer heat for power, cooling, or insulation purpose, and this research focuses on modeling the performance of two novel thermal systems that are capable of functioning in environments subjected to high heat fluxes. The first investigated thermal system is a cascaded thermoelectric generator (TEG) that directly converts heat into electricity and offers a green option for renewable energy generation. The presented cascaded TEG allows harvesting energy in high temperatures ranging from 473K to 973K, and being a solid-state device with no moving parts constitutes an excellent feature for increase device life cycle and minimum maintenance in harsh, remote environments. Two cascaded TEG designs are analyzed in this research: the two-stage and three-stage cascaded TEGs, and based on the findings, the two-stage cascaded TEG produces a power output of 42 W with an efficiency of 8.3% while the three-cascaded TEG produces 51 W with an efficiency of 10.2%. The second investigated novel thermal system is a thermal protection system inspired by the porous internal skeleton of the cuttlefish also known as cuttlebone. The presented bio- inspired thermal protection has excellent features to serve as an integrated thermal protection system for spacecraft vehicles including being lightweight (93% porosity) and possessing high compressive strength. A large amount of heat flux is generated from friction between air and spacecraft vehicle exterior, especially during reentry into the atmosphere, and part of the herein presented research involves a thermomechanical modeling analysis of the cuttlebone bio-inspired integrated thermal protection system along with comparing its performance with three conventional structures such as the wavy, the pyramid, and cylindrical pin structures. The results suggest that the cuttlebone integrated thermal protection system excels the best at resisting deformation caused by thermal expansion when subjected to aerodynamic heat fluxes. / Doctor of Philosophy / Operating engineering systems in extremely hot environments often decreases systems' reliability, life cycle, and creates premature failure. This research investigates two novel thermal systems capable of functioning in high temperatures including a cascaded thermoelectric generator (TEG) and a bio-inspired thermal protection system. The first evaluated novel thermal systems is a cascaded TEG that directly converts waste heat into power, and being a solid-state device with no moving parts forms an excellent feature for device life cycle improvement and minimum maintenance in harsh, remote environments. The research findings show that the designed cascaded TEGs can produce power when subjected to high temperatures ranging from 473K to 973K. The remaining part of the research presented in this dissertation models the thermomechanical performance of a lightweight structure, which is inspired by the internal skeleton of the cuttlefish, also knows as the cuttlebone. The cuttlefish's natural ability to support high-deep sea pressure translates into possessing high compressive strength, and when added the fact of being lightweight (up to 93% porosity), the cuttlebone forms an excellent candidate to serve as integrated thermal protection for spacecraft vehicles. The last part of the presented research discuss the thermomechanical analysis of the cuttlebone when subjected to high aerodynamics heat flux generated from friction between the air and spacecraft vehicle exterior, and it was found that the cuttlebone structure resists deformation associated with the steep temperature gradient experienced by the spacecraft vehicle during travel.

Nanofabrication techniques continue to advance and are rapidly becoming the primary route to enhancement for the electrical, thermal, and optical properties of materials. The work presented in this dissertation details fabrication and characterization techniques of thin films and nanoparticles for these purposes. The four primary areas of research presented here are thermoelectric enhancement through nanostructured thin films, an alternative frequency-domain thermoreflectance method for thin film thermal conductivity measurement, thermal rectification in nanodendritic porous silicon, and plasmonic enhancement in silver nanospheroids as a reverse photolithography technique. Nanostructured thermoelectrics have been proposed to greatly increase thermopower efficiency and to bring thermoelectrics to mainstream power generation and cooling applications. In our work, thermoelectric thin films of SbTe, BiTe, and PbTe grown by atomic layer deposition and electrochemical atomic layer deposition were characterized for enhanced performance over corresponding bulk materials. Seebeck coefficient measurements were performed at temperatures ranging from 77 K to 380 K. Atomic composition was verified by energy-dispersive X-ray spectroscopy and structures were imaged by scanning electron microscopy. All thin films measured were ultimately found to have a comparable or smaller Seebeck coefficient to corresponding materials made by conventional techniques, likely due to issues with the growth process. Frequency-domain thermoreflectance offers a minimally invasive optical pump-probe technique for measuring thermal conductivity. Like time-domain thermoreflectance, the version of frequency-domain thermoreflectance demonstrated here relies on a non-zero thermo-optic coefficient in the sample, but uses moderate cost continuous wave lasers modulated at kHz or MHz frequencies rather than a more expensive ultrafast laser system. The longer timescales of these frequency ranges enables this technique to take measurements of films with thicknesses ranging from 100 nm to 10 um, complimentary to time-domain thermoreflectance. This method differentiates itself from other frequency-domain methods in that it is also capable of simultaneous independent measurements of both the in plane and out of plane values of the thermal conductivity in anisotropic samples through relative reflective magnitude, rather than phase, measurements. We validated this alternate technique by measuring the thermal conductivity of Al2O3 and soda-lime and found agreement both with literature values and with separate measurements obtained with a conventional time-domain thermoreflectance setup. Thermal rectification has the potential to enhance microcircuit performance, improve thermoelectric efficiency, and enable the creation of thermal logic circuits. Passive thermal rectification has been proposed to occur in geometrically asymmetric nanostructures when heat conduction is dominated by ballistic phonons. Here, nanodendritic structures with branch widths of ~ 10 nm and lengths of ~ 20 nm connected to ~ 50 um long trunks were electrochemically etched from <111> silicon wafers. Thermal rectification measurements were performed at temperatures ranging from 80 K to 250 K by symmetric thermal conductivity measurements. No thermal rectification was ultimately found in these samples within the margin of thermal conductivity measurement error 1%. This result is consistent with another study which found thermal rectification with greater conduction in the direction opposite to what ballistic phonon heat conduction theories predicted. Plasmonic resonance concentrates incident photon energy and enables channeling of that energy into sub-wavelength volumes where it can be used for nanoscale applications. We demonstrated that surface plasmon polaritons induced in silver nanosphereoid films by 532 nm light defunctionalize previously photocleaved ligands adsorbed onto the films, to yield a reverse photolithographic technique. In this method, gold nanosphere conjugation were conjugated to a photocleaved ligand, however conjugation could be inhibited by exposing the cleaved ligand to 532 nm light and consequently yield a reversal technique. This defunctionalizion effect did not occur on gold films or nanoparticles conjugated with the ligand in IR spectroscopy, and was observed to have a reduced effect in silver films relative to silver nanospheroid film. As silver nanospheroid films and gold nanospheres of the size used in this study are known to have plasmon resonance in the green wavelengths, while gold and silver continuous films do not, this defunctionalization likely results from plasmonic effects. / Ph. D.

In this project, the time-dependent one-dimensional heat equation with internal heating is solved using eigenfunction expansion, according to the thermoelectric boundary conditions. This derivation of the equation describing time-dependent heat flow in a thermoelectric sample or device yields a framework that scientists can use (by entering their own parameters into the equations) to predict the behavior of a system or to verify numerical calculations. Allowing scientists to predict the behavior of a system can help in decision making over whether a particular experiment is worthy of the time to construct and execute it. For experimentalists, it is valuable as a tool for comparison to validate the results of an experiment. The calculations done in this derivation can be applied to pulsed cooling systems, the analysis of Z-meter measurements, and other transient techniques that have yet to be invented. The vast majority of the calculations in this derivation were done by hand, but the parts that required numerical solutions, plotting, or powerful computation, were done using Mathematica 8. The process of filling in all the steps needed to arrive at a solution to the time-dependent heat equation for thermoelectrics yields many insights to the behavior of the various components of the system and provides a deeper understanding of such systems in general.

The Moon has always been an important milestone in space exploration. After the Apollo landings, it is logical to think that the next step should be a permanent habitation module, which would serve as a testing ground for more ambitious projects to Mars and beyond. For a lunar base to come into realization, it is necessary to assess a number of technological challenges which are due to the harsh conditions that can be found on the Earth's satellite. One of these tasks revolves around energy storage: During the day it is possible to use photovoltaic cells and convert the solar irradiance into electrical energy to power an outpost, however during the lunar night this source is not available. Current investigations establish that the optimal landing site for a permanent mission would be on the rim of the Shackleton crater, near the South Pole. This would reduce the night duration from 14 days to 52 hours of the lunar cycle, which is 29.5 days. While this significantly decreases the exposure to the cold temperatures of the Moon when there is no sunlight, there is still a need for a system to provide energy to the lunar base over this period. Therefore, this study pretends to serve as a possible solution for the aforementioned problem, by developing a system storing energy as thermal energy and then harvesting it as electricity using thermoelectrics. First, a theoretical introduction is presented, where the problem statement is exposed, along with background information regarding the solar illumination and the lunar soil. At the same time, an insight on regolith sintering techniques is given. These techniques are important as a means to providing thermal energy storage during the night cycle. After this, the core of the study is developed: The ideal system for energy storage is broken down into segments, and each of them is explained attending to the possible requirements of a lunar base, while providing supporting simulations when deemed appropriate. These are the solar concentrator, thermal mass, thermoelectric array, cold sink and, if necessary, a pipe network. Following this chapter, a device is proposed. Based on the previously mentioned guidelines, an ideal thermal energy system is simulated and evaluated. Although it is not optimized for efficient energy harvesting, it serves as insight on the design and simulation constraints that appear when one wants to collect electrical energy from thermoelectrics with relatively low efficiency. It was estimated that the prototype would output a mean power of 3.6 Watts over the whole duration of the lunar night. Although in its current state this technology would not present significant benefits over existing energy storage methods such as nickel-hydrogen batteries, this study also proposed several optimization methods which could vastly increase the performance of the device. These include adding more efficient thermoelectric patterns, or modifying the properties of the semiconductors by doping or using nanostructures, and present follow-on opportunities for further research.

Thesis advisor: Zhifeng Ren / Solid-state cooling and power generation based on thermoelectric effects are attractive for a wide range of applications in power generation, waste heat recovery, air-conditioning, and refrigeration. There have been persistent efforts on improving the figure of merit (ZT) since the 1950's; only incremental gains were achieved in increasing ZT, with the (Bi1-xSbx)2(Se1-yTey)3 alloy family remaining the best commercial material with ZT ~ 1. To improve ZT to a higher value, we have been pursuing an approach based on random nanostructures and the idea that the thermal conductivity reduction that is responsible for ZT enhancement in superlattices structures can be realized in such nanostructures. The synthesis and characterization of various nanopowders prepared by wet chemical as well as high energy ball milling methods will be discussed in this dissertation. The solid dense samples from nanopowders were prepared by direct current induced hot press (DC hot press) technique. The thermoelectric properties of the hot pressed samples have been studied in detail. By ball milling ingots of bulk alloy crystals and hot pressing the nanopowders, we had demonstrated a high figure-of-merit in nanostructured bulk bismuth antimony telluride. In this dissertation, we use the same ball milling and hot press technique, but start with elemental chunks of bismuth, antimony, and tellurium to avoid the ingot formation step. We show that a peak ZT of about 1.3 can be achieved. Our material also exhibits a ZT of 0.7 at 250 °C, close to the value reached when ingot was used. This process is more economical and environmentally friendly than starting from bulk alloy crystals. The ZT improvement is caused mostly by the low thermal conductivity, similar to the case using ingot. Transmission electron microscopy observations of the microstructures suggest that the lower thermal conductivity is mainly due to the increased phonon scattering from the high density grain boundaries and defects. The performance of thermoelectric materials is determined by its dimensionless figure-of-merit (ZT) which needs to be optimized within a specific temperature range for a desired device performance. Hence, we show that by varying the Bi/Sb ratio, the peak ZT can be shifted to a higher or lower temperature for power generation applications or a cooling mode operation. A peak ZT of about 1.3 is achieved from a Bi0.4Sb1.6Te3 composition which is highest among the different compositions. These nanostructured bulk samples have a significantly low lattice thermal conductivity compared to the bulk samples due to the increased phonon scattering in the grain boundaries and defects. This study shows that Bi0.5Sb1.5Te3 may potentially perform better for cooling devices, while Bi0.3Sb1.7Te3 should be able to show better power generation efficiency. Several issues related to accurate measurement of thermoelectric properties were identified and many of them were solved during my studies and these are discussed in this thesis. With the data we obtained, it is clear that nanopowder-based thermoelectric materials hold significant promise. Therefore, a review of synthesis of nanostructured materials by solution-based methods, including a hydrothermal process for the Bi2Te3, Bi2Se3, and Bi2Te2.25Se0.75 nanoparticles, a solvothermal route for Sb2Te3 nanostructures, and a polyol process for the preparation of Bi nanostructures is presented in this dissertation. These new nanostructures may find applications in enhancing the thermoelectric performance. Although small sized and well dispersed nanopowders of various thermoelectric materials could be prepared by a solution method in large scale, contamination and partial oxidation are always big challenges in a chemical approach. Hence, a high energy ball milling technique to prepare thermoelectric nanopowders in large scale and without major contamination is still found to be more efficient and preferred. / Thesis (PhD) — Boston College, 2009. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

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Oxide ceramics have been increasingly researched for high temperature thermoelectric (TE) applications. SrTiO3 based materials are promising candidates due to its chemical and thermal stability. In this study, oxide ceramics of composition (1-x)SrTiO3-(x)La1/3NbO3 (0 smaller or equal to x smaller or equal to 0.3) were prepared by single-step solid state sintering in Ar/5%H2 at 1700 K. The density of all the samples prepared was above 90%. All the samples were predominantly single-phase compositions crystallised with a cubic structure in Pm ̅3m space group. The impact of oxygen deficiency, A-site vacancies and mixed oxidation states of Ti3+/Nb4+ on electrical and thermal transport properties was assessed. Optimum TE properties were obtained for x=0.2 (Sr0.8La0.067Ti0.8Nb0.2O(3-delta) = L2), which has 13.4% A-site vacancies. The ZT values improved from 0.2 to 0.27 at 1000 K, with an increase in sintering time from 8 hours to 48 hours, due to increased carrier concentration. Complex interplay of oxygen vacancies and excess donor substitution on A/B-sites of L2 (substituting 5-10% Sr/Ti with La/Nb) exhibited 35% improvement in ZT values, whilst maintaining the A-site vacancies and core-shell structures within grains, which reduced the thermal conductivity by ~50% compared to undoped SrTiO3 samples, due to strong phonon scattering. A facile method to incorporate metallic inclusions (2.5 wt% Fe/Cu) at grain boundaries in L2 ceramics is demonstrated. The modified compositions displayed a maximum ZT of ~0.37 at 1000 K for L2 samples containing metallic inclusions due to increased carrier concentration (5.5 x 10^21 carriers/cm^3) and carrier mobility (2.4 cm^2/(V.s).The addition of graphene/Graphene Oxide (GO) flakes in L2 ceramics has been investigated to improve the electrical conductivity of L2 composites without significantly increasing the thermal conductivity. Spark plasma sintering (SPS) of the composite powders at 1473 K and 50 MPa produced dense samples (>95% relative density) with a homogeneous dispersion of graphene/GO flakes, for loadings smaller or equal to 1.0 wt%. The effect of interaction and distribution of graphene/GO flakes within the ceramics on TE properties is investigated. The composite samples demonstrate anisotropic ZT values, with 20% improvement in the direction normal to the orientation of graphene flakes. A novel sintering method has been proposed which has strong industrial potential. The L2 based composites were sintered in Air at 1700 K (ramp rate: ±300 K/min), whilst samples were covered uniformly. Strong reducing conditions and evolution of secondary phases in the microstructure helped achieve, the very low electrical resistivity of ~3.0 x 10^(-6) ohm.m at room temperature. Secondary phases, sub-micron voids in the grains and A-site vacancies reduced the lattice thermal conductivity (~2.0 W/m.K), comparable to the lowest lattice thermal conductivity achievable (~1.5 W/m.K) at 1000 K and obtain a maximum ZT of 0.4 at 1000 K for L210G-Air/C composites.