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With the increasing speed of the microprocessor and its rapidly increasing demand for power, determining how to power the microprocessors for our computers becomes an important issue. So far, industry has been struggling to operate the VR/VRM at higher and higher switching frequencies while maintaining acceptable power conversion efficiency. As a consequence, the power switches used in the VR/VRM must be able to work efficiently at a higher switching frequency and with a higher current density. To evaluate the performance of the MOSFET for this low-output-voltage, high-current and high-switching-frequency application, a prevalent criterion, the Figure-of-Merit (FOM), is being widely adopted for determining the top switch of the buck converter in the VR/VRM. By comparing the FOMs of different devices, the device with the lowest FOM value should have the best performance and lead to the lowest loss for this device in the circuit. Qgd*Rdson is a widely accepted and widely used FOM for power devices. Due to the lack of accuracy of the power loss model, this FOM is no longer suitable for VRM applications. Furthermore, the question of how to use this FOM to select the right device for different application is another important issue. This work presents an investigation of a new Figure-of-Merit based on a more accurate loss model, which includes the factor of Qgs2, the gate-driving voltage and the packaging parasitics. Furthermore, a simple method is proposed to select the right device and gate-driving voltage for different circuit conditions. A new simple and accurate closeform model for device loss with packing parasitics is derived. This model can provide more physical information for each of the device parameters. The loss influence of the different packaging method is analyzed and discussed at the end of the work. / Master of Science

International Telemetering Conference Proceedings / October 23-26, 2000 / Town & Country Hotel and Conference Center, San Diego, California / This paper presents the results of installing a distributed feedback (DFB) laser transmitter and the appropriate optical receiver in an operational site. Frequencies from 1435 to 2400 megahertz are transported intact from a remote site to a local site. From the theoretical calculations, 10 dB of dynamic range may need to be recovered by the use of an automatic gain circuit. The actual device is a delight, needing no additional circuitry to meet specifications. Predictions of performance were made from calculations. The installed system was measured for 1 dB compression point and for figure of merit.

Thesis advisor: Zhifeng Ren / Solid state cooling and power generation based on thermoelectric principles are regarded as one of the technologies with the potential of solving the current energy crisis. Thermoelectric devices could be widely used in waste heat recovery, small scale power generation and refrigeration. It has no moving parts and is environmental friendly. The limitation to its application is due to its low efficiency. Most of the current commercialized thermoelectric materials have figure of merit (ZT) around 1. To be comparable with kitchen refrigerator, ZT is required at room temperature. Skutterudites have emerged as member of the novel materials, which potentially have a higher ZT. In the dissertation, my investigation will be focused on the optimization of CoSb₃ &ndash based skutterudites. Starting with Co and Sb elements, CoSb₃ will form through a high energy ball mill. Unfortunately, even after 20 hours, only a small percentage of the powders have transformed in into CoSb₃. Then the powders will be compacted into bulk samples by DC-controlled hot press. CoSb₃ single phase will form after press. Characterization of the structure and thermoelectric properties will be presented with details. The effects of synthesis conditions on thermoelectric properties of skutterudites were studied and discussed. Several possible methods of improving the ZT of N type skutterudites were applied. The highest obtained ZT thus far is about 1.2 from Yb doped CoSb₃. For a group of samples with nominal composition Yb_xCo₄Sb₁₂, the increased Yb concentration in our samples not only enhanced the power factor due to electron doping effect but also decreased the thermal conductivity due to a stronger rattling effect. In addition, the increased grain boundary density per unit volume due to the small grains in our bulk skutterudite materials may have also helped to enhance the phonon scattering and thus to reduce the thermal conductivity. Single and double doping methods with different combinations were also tried. So far, none of them have surpassed ZT of 1.2. Mixing different materials with Yb_0.35Co₄Sb₁₂ so far to increase the phonon scattering was also performed. No dramatic thermal conductivity reduction was observed. Small amounts of Fe/Mn substitution on Co sites will decrease the power factor to undesired degrees. Some results with Nd filled P type sample will be briefly introduced. P type samples are also obtained through substitution on Sb site. Preliminary work on preparing the electrode for CoSb₃ will be presented in the dissertation. CoSi₂ has low resistivity, and a similar coefficient of thermal expansion (CTE) as of doped CoSb₃. It is good electrode candidate. DC controlled hot press is used to make the contact. Thermal stability of the contact was tested. Small cracks will form in the contact area, further improvement is necessary. Finally, my previous work on ZnO nanowire growth is briefly introduced. Large throughput of ZnO nanowire could be obtained with NaCl as the support to promote the conversion of Zn powder to ZnO. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Sustainable and renewable energy is an incredibly important area in today’s society and investigation into avenues to improve this wide ranging technology are underway in many different fields. Thermoelectric devices possess the ability for the direct solid-state interconversion of heat and electrical power, which not only allows for sustainable refrigeration but also waste heat recovery. One current restriction on the efficiency of thermoelectric devices is the disparity in thermoelectric performance of p-type and n-type materials. Furthermore, a key physical attribute shared by the majority of high performing thermoelectric materials is low thermal conductivity. Thus in this thesis three separate p-type material systems exhibiting low thermal conductivity will be discussed. The Cu-excessed quaternary chalcogenides, CuM2InTe4 (M = Zn, Cd), and ternary chalcogenide, CuSbS2, were investigated due to their intrinsically low thermal conductivity. Whereas, skutterudites typically have good electrical properties but do not exhibit an intrinsically low thermal conductivity. Nevertheless low thermal conductivity can be achieved by taking advantage of their unique crystal structure by filling large voids with loosely bound atoms that act as phonon scattering centers. Therefore double-filled Fe substituted skutterudites with nominal compositions Yb0.4In0.02Co3FeSb12 and Yb0.8In0.02Co2.5Fe1.5Sb12 were also investigated. The CuM2InTe4 (M = Zn, Cd) and skutterudite specimens were synthesized by direct reactions, whereas the CuSbS2 specimens were synthesized by mechanical alloying. Structural and stoichiometric compositions were analyzed by a combination of X-ray diffraction, Rietveld refinement and energy dispersive spectroscopy. High-temperature transport properties were measured for all specimens and will be discussed in detail. The Cu-excessed quaternary chalcogenides display intrinsically low thermal conductivity that appears to be unaffected by the change in electrical properties that is a result of differing stoichiometries. This may provide a possible route to furthering the enhancement of the thermoelectric properties of these materials. Similarly the CuSbS2 ternary chalcogenides display a very low thermal conductivity due to stereochemically active lone-pair electrons and would potentially allow an optimization of the power factor without a significant increase of the very low thermal conductivity, thus improving the figure of merit. For the case of p-type skutterudites, (Yb, In) double-filled skutterudites have a maximum ZT of 0.6, which is promising in the hunt for improved p-type materials. This fundamental investigation provides insight that can lead to a deeper understanding of all three material systems outlined in this thesis and provides a platform for new research in the quest for materials suitable for thermoelectric applications.

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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.

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.