Exploring electron and phonon transport at the nanoscale for thermoelectric energy conversion

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 147-155).Thermoelectric materials are capable of solid-state direct heat to electricity energy conversion and are ideal for waste heat recovery applications due to their simplicity, reliability, and lack of environmentally harmful working fluids. Recently, nanostructured thermoelectrics have demonstrated remarkably enhanced energy conversion efficiencies, primarily due to a reduction in lattice thermal conductivity. Despite these advances, much remains unknown about heat transport in these materials, and further efficiency improvements will require a detailed understanding of how the heat carriers, electrons and phonons, are affected by nanostructures. To elucidate these processes, in this thesis we investigate nanoscale transport using both modeling and experiment. The first portion of the thesis studies how electrons and phonons are affected by grain boundaries in nanocomposite thermoelectric materials, where the grain sizes are smaller than mean free paths (MFPs). We use the Boltzmann transport equation (BTE) and a new grain boundary scattering model to understand how thermoelectric properties are affected in nanocomposites, as well as to identify strategies which could lead to more efficient materials. The second portion of the thesis focuses on determining how to more directly measure heat carrier properties like frequency-dependent MFPs. Knowledge of phonon MFPs is crucial to understanding and engineering nanoscale transport, yet MFPs are largely unknown even for bulk materials and few experimental techniques exist to measure them. We show that performing macroscopic measurements cannot reveal the MFPs; instead, we must study transport at the scales of the MFPs, in the quasi- ballistic transport regime. To investigate transport at these small length scales, we first numerically solve the frequency-dependent phonon BTE, which is valid even in the absence of local thermal equilibrium, unlike heat diffusion theory. Next, we introduce a novel thermal conductivity spectroscopy technique which can measure MFP distributions over a wide range of length scales and materials using observations of quasi-ballistic heat transfer in a pump-probe experiment. By observing the changes in thermal resistance as a heated area size is systematically varied, the thermal conductivity contributions from different MFP phonons can be determined. We present the first experimental measurements of the MFP distribution in silicon at cryogenic temperatures. Finally, we develop a modification of this technique which permits us to study transport at scales much smaller than the diffraction limit of approximately one micron. It is important to access these length scales as many technologically relevant materials like thermoelectrics have MFPs in the deep submicron regime. To beat the diffraction limit, we use electron-beam lithography to pattern metallic nano dot arrays with diameters in the hundreds of nanometers range. Because the effective length scale for heat transfer is the dot diameter rather than the optical beam diameter, we are able to study nanoscale heat transfer while still achieving ultrafast time resolution. We demonstrate the modified technique by measuring the MFP distribution in sapphire. Considering the crucial importance of the knowledge of MFPs to understanding and engineering nanoscale transport, we expect these newly developed techniques to be useful for a variety of energy applications, particularly for thermoelectrics, as well as for gaining a fundamental understanding of nanoscale heat transport.by Austin Jerome Minnich.Ph.D

Energy: A special bibliography with indexes, April 1974

This literature survey of special energy and energy related documents lists 1708 reports, articles, and other documents introduced into the NASA scientific and technical information system between January 1, 1968, and December 31, 1973. Citations from International Aerospace Abstracts (IAA) and Scientific and Technical Aerospace Reports (STAR) are grouped according to the following subject categories: energy systems; solar energy; primary energy sources; secondary energy sources; energy conversion; energy transport, transmission, and distribution; and energy storage. The index section includes the subject, personal author, corporate source, contract, report, and accession indexes

The development of sub-25 nm III-V High Electron Mobility Transistors

High Electron Mobility Transistors (HEMTs) are crucially important devices in microwave circuit applications. As the technology has matured, new applications have arisen, particularly at millimetre-wave and sub-millimetre wave frequencies. There now exists great demand for low-visibility, security and medical imaging in addition to telecommunications applications operating at frequencies well above 100 GHz. These new applications have driven demand for high frequency, low noise device operation; key areas in which HEMTs excel. As a consequence, there is growing incentive to explore the ultimate performance available from such devices. As with all FETs, the key to HEMT performance optimisation is the reduction of gate length, whilst optimally scaling the rest of the device and minimising parasitic extrinsic influences on device performance. Although HEMTs have been under development for many years, key performance metrics have latterly slowed in their evolution, largely due to the difficulty of fabricating devices at increasingly nanometric gate lengths and maintaining satisfactory scaling and device performance. At Glasgow, the world-leading 50 nm HEMT process developed in 2003 had not since been improved in the intervening five years. This work describes the fabrication of sub-25 nm HEMTs in a robust and repeatable manner by the use of advanced processing techniques: in particular, electron beam lithography and reactive ion etching. This thesis describes firstly the development of robust gate lithography for sub-25 nm patterning, and its incorporation into a complete device process flow. Secondly, processes and techniques for the optimisation of the complete device are described. This work has led to the successful fabrication of functional 22 nm HEMTs and the development of 10 nm scale gate pattern transfer: simultaneously some of the shortest gate length devices reported and amongst the smallest scale structures ever lithographically defined on III-V substrates. The first successful fabrication of implant-isolated planar high-indium HEMTs is also reported amongst other novel secondary processes

Étude du comportement d'ADN en solution et aux interfaces et le rôle de la dynamique micellaire et la rhéologie dans la libération contrôlée de médicaments

Nowadays, the target for reaching a greater efficiency in DNA compaction processes, the innovation ofDNA sensors development and the study of changes in the interfacial properties generated between metalsurfaces and DNA molecules has become an area of great interest in bioengineering. This section of thethesis proposes the coupling of rheological, electrochemical and optical techniques to perform a detailedstudy of DNA molecules behavior in the bulk state of the solution and at the interface with two differentmetallic surfaces, as a function of parameters such as temperature, DNA concentration and electricpotential. Firstly, the rheological behavior of DNA/buffer solutions, as well as the evidence of the criticalconcentrations (C★ and Ce) is discussed from simple steady state and oscillatory dynamic shearexperiments. After studying DNA solutions properties, electrochemical and optical techniques are used toidentify structural changes in Au/DNA and Pt/DNA interfaces and to describe the arrangement of DNAchains in the electrochemical double-layer as a function of concentration and within each characteristicregime, i.e. dilute and semi-dilute regimes. The obtained response trough Electrochemical ImpedanceSpectroscospy (EIS), Modulation Interfacial of the Capacitance (MIC) and Surface Plasmon Resonance(SPR) techniques reflects an adsorption process of DNA molecules taking place onto the metal surfaces.Finally, by selecting DNA concentrations in the dilute regime, we studied the formation of chitosan-DNAnanoparticles with defined stoichiometry for gene transfer.The specific delivery of active ingredients, known as vectorization, has actually become a greatchallenge in therapeutic research. This process has been used to control the distribution of activeingredients such as proteins, genes for gene therapy and drugs, to a target by associating it with avector. Molecules for chemotherapy are frequently hydrophobic and require vectorization to betransported to the target cell. In this section of the thesis, we look up to understand the collectiveexchange dynamics (fusion and fission) between amphiphilic block copolymer micelles at the equilibriumand out of the equilibrium, and the exchange dynamics between these micelles (representing vectors)and the simplest model of cells (liposomes). We used a fluorescent technique with hydrophobic pyrenederivative to probe the fusion and fission of micelles at equilibrium. After characterizing amphiphilicblock copolymers structure and studying their dynamics in and out of equilibrium, we proposed a timescan fluorescence technique to quantify the collective vectorization dynamics between amphiphilic blockcopolymer micelles and liposomes. The effect of the variation of several parameters such as liposomeconcentration and a chitosan adsorption were investigated in order to control the vectorizationdynamics between these vectors and cells models.Étude du comportement d'ADN en solution et aux interfaces : actuellement, l'objectif de parvenir à une plus grande efficacité dans les processus de compaction de l'ADN, dans l'innovation des capteurs d'ADN et dans l'étude des changements dans les propriétés interfaciales générés entre les surfaces métalliques et les molécules d'ADN, est devenue un grand domaine d'intérêt en bioingénierie. Cette section de la thèse propose le couplage des techniques rhéométriques, électrochimiques et optiques afin d'effectuer une étude détaillée du comportement de molécules d'ADN en solution et aux interfaces, en fonction de la température, de la concentration en ADN et du potentiel électrique. Tout d'abord, le comportement rhéologique des solutions d'ADN, ainsi que la détection des concentrations critiques (C* et Ce), est discuté à partir d’expériences de rhéométrie en cisaillement permanent et harmonique. Après avoir étudié les propriétés des solutions d'ADN, des techniques électrochimiques et optiques ont été utilisés pour identifier les changements structurels aux interfaces Au/ADN et Pt/ADN, ainsi que pour décrire l'arrangement des chaînes d'ADN dans la double couche électrochimique pour les régimes dilué et semi-dilués. La réponse obtenue par Spectroscopie d'Impédance Électrochimique (EIS), Modulation Interfaciale de la Capacitance (MIC) et Résonance des Plasmons de Surface (SPR) reflète un processus d'adsorption des molécules d'ADN sur les surfaces métalliques. Finalement, en utilisant des concentrations d’ADN dans le régime dilué, on a étudié la formation de nanoparticules de chitosane-ADN avec stœchiométrie définie pour le transfert de gènes.Le rôle de la dynamique micellaire et la rhéologie dans la libération contrôlée de médicaments: le transfert ciblé d'ingrédients actifs (vectorisation) est un grand défi pour la recherche thérapeutique. Ce procédé est utilisé pour contrôler le transfert des protéines, des gènes et des médicaments vers une cellule cible en l'associant à un vecteur. Les molécules pour la chimiothérapie sont souvent hydrophobes et ont besoin d’un vecteur pour être transférées. Dans cette section de la thèse, on cherche à comprendre les dynamiques d'échange collectives (fusion et fission) entre micelles de copolymères triblocs amphiphiles à l'équilibre et hors équilibre. Ensuite, on étudie les dynamiques d'échange collectives entre ces micelles, choisies comme vecteurs, et des liposomes, choisis comme cellules modèles. On utilise une technique de fluorescence avec un dérivé de pyrène hydrophobe pour suivre les processus de fusion et de fission. Après avoir caractérisé la structure des copolymères amphiphile et avoir étudié leur dynamique à l'équilibre et hors l'équilibre, nous proposons une technique de fluorescence qui permet de quantifier les dynamiques collectives de vectorisation entre les micelles et les liposomes. Les effets de la variation de la concentration de liposomes et de l’adsorption du chitosane sur la membrane du liposome et sur les micelles ont été étudiés

Energy: A continuing bibliography with indexes, supplement 16, January 1978

This bibliography lists 1287 reports, articles, and other documents introduced into the NASA scientific and technical information system from October 1, 1977 through December 31, 1977

A cryogenic detector for submillimetre astronomy

xiv, 156 leaves : ill. ; 28 cm.Over the last several years, a submillimetre astronomical polarizing Fourier Transform Spectrometer (FTS) has been developed for use at the James Clerk Maxwell Telescope (JCMT) located atop Mauna Kea, Hawaii. A new liquid3 He cooled dual polarization detector system has been carefully designed for use with this FTS to eliminate noise problems encountered with UKT14, the JCMT facility bolometric detector. The objective of this thesis is to evaluate and optimize the performance of the new detector system. The design of the detector system is discussed, and the noise performance of the system is evaluated. The system performance is determined from photmetric, classical B-I, and spectroscopic measurements. Compared to UKT14, the intrinsic bolometer noise is reduced by a factor of - 2. More importantly, the spectral signal to noise ratio is improved by a factor of - 10 due to the elimination of electrical pickup and microphone noise in the detector signal