The push toward the miniaturization of electromechanical devices and the resulting need for micro-power generation (milliwatts to watts) with low-weight, long-life devices has led to the recent development of the field of micro-scale combustion. Since batteries have low specific energy (~200 kJ/kg) and liquid hydrocarbon fuels have a very high specific energy (~50000 kJ/kg), a miniaturized power-generating device, even with a relatively inefficient conversion of hydrocarbon fuels to power, would result in increased lifetime and/or reduced weight of an electronic or mechanical system that currently requires batteries for power. Energy conversion from chemical energy to electrical energy without any moving parts can be achieved by a thermophotovoltaic (TPV) system. The TPV system requires a radiation source which is provided by a micro-combustor. Because of the high surface area to volume ratio for micro-combustor, there is high heat loss (proportional to area) compared to heat generation (proportional to volume). Thus the quenching and flammability problems are more critical in a micro-scale combustor. Hence innovative schemes are required to improve the performance of micro-combustion. In the current study, a micro-scale counter flow combustor with heat recirculation is adapted to improve the flame stability in combustion modeled for possible application to a TPV system. The micro-combustor consists of two annular tubes with an inner tube of diameter 3 mm and 30 mm long and an outer tube of 4.2 mm diameter and 30 mm long. The inner tube is supplied with a cold premixed combustible mixture, ignited and burnt. The hot produced gases are then allowed to flow through outer tube which supplies heat to inner tube via convection and conduction. The hot outer tube radiates heat to the TPV system. Methane is selected as the fuel. The model parameters include the following: diameter d , inlet velocity u , equivalence ratio φ and heat recirculation efficiency η between the hot outer flow and cold inner flow. The predicted performance results are as followings: the lean flammability limit increased from 7.69% to 7.86% and the quenching diameter decreased from 1.3 mm to 0.9 mm when heat recirculation was employed. The overall energy conversion efficiency of current configuration is about 2.56.

MBE ( Molecular Beam Epitaxy ) technique can obtain high quality of GaSb/GaAs hetero-junction structure and control epilayers precisely.It has 7¢Mlattice mismatch between GaAs ( substrate ) and GaSb ( thin film ), but if we control beam flux ratio (V/III) and substrate temperature exactly, we can obtain high quality of epilayer. The growth mechanisms related to the major factors of (1) Beam flux ratio (V/III)¡B(2) Substrate temperature. The properties of GaSb epilayers are characterized by different methods such as the X-ray diffraction. The optimum growth conditions 500¢J of substrate temperature and the V/III flux ratio about 2.5 have been obtained. On the basis of this condition, We use simulation program for solar cell ( Scaps ), to simulate GaSb/GaAs hetero-junction solar cell structure, to try the possibility of the GaSb/GaAs hetero-junction structure in solar cells. From the simulation result, we know if the doped concentration in the two semiconductor materials and thickness are suitability, and if we can control the concentration of interface states under a suitable value, the efficiency of the solar cell can well. On the basis of this result, the same for thermo-photovoltaic (TPV), its' efficiency will also well.

No description available.

Optoelectronic devices operating in the mid-infrared spectral region are attracting increasing attention due to potential applications in a wide range of disciplines. For example, mid-infrared photodetectors play a key role in thermophotovoltaic (TPV) energy conversion, whereby a photovoltaic device is used to extract electrical power from heat radiation. This technology is attractive for waste heat harvesting and clean energy production in several different environments. Similarly, mid-infrared light sources are particularly useful for biochemical sensing and spectroscopy, where the distinctive absorption features of many molecular species of interest can be exploited for their sensitive identification and detection. Both devices are investigated in this thesis work. In the area of TPV energy conversion, I have studied the use of intersubband transitions in semiconductor quantum cascade structures as a means to overcome the fundamental limitations of existing TPV devices using mature InP-based technology. Very efficient coverage of the incident radiation spectrum and optimal current matching can be achieved using multiple quantum-cascade structures monolithically integrated with a p-n junction, by taking advantage of their intrinsic cascading scheme, spectral agility, and design flexibility. Numerical simulations indicate that this approach can effectively double the present state-of-the-art in TPV output electrical power. In the area of mid-infrared light sources, my work has focused on developing efficient light emitters based on tensilely strained Germanium nanomembranes (Ge NMs). These ultrathin (a few ten nanometers) single-crystal membranes are good candidates for the development of CMOS-compatible Group-IV light sources, by virtue of their ability to sustain large strain levels and in the process become direct-bandgap materials. My research efforts have concentrated on the development of optical cavities based on Ge NMs that can satisfy the mechanical flexibility requirement of this materials platform. In particular, photonic-crystal (PhC) cavities in the form of disconnected dielectric-column arrays have been designed and fabricated based on a novel membrane assembly method, producing clear cavity-mode features in NM photoluminescence spectra. / 2016-08-17T00:00:00Z

Approved for public release, distribution is unlimited / Thermophotovoltaic energy conversion offers a means of efficiently converting heat into electrical power. This has potential benefits for space nuclear reactor power systems currently in development. The primary obstacle to space operation of thermophotovoltaic devices appears to be the low heat rejection temperatures which necessitate large radiator areas. A study of the tradespace between efficiency and radiator size indicates that feasible multi-junction TPV efficiencies result in substantial overall system mass reduction with manageable radiator area. The appendices introduce the endothermodynamic model of a TPV cell and briefly assess the utility of advanced carbon-carbon heat pipe radiator concepts. / Lieutenant, United States Navy

No description available.

Two classes of carbon nanomaterials—carbon nanotubes and graphene—have promoted the advancement of nanoelectronics, quantum computing, chemical sensing and storage, thermal management, and optoelectronic components. Studies of the thermal radiative properties of carbon nanotube thin film arrays and simple graphene hybrid structures reveal some of the most exciting characteristic electromagnetic interactions of an unusual sort of material, called hyperbolic metamaterials. The features and results on these materials in the context of both far-field and near-field radiation are presented in this dissertation. Due to the optically dark nature of pyrolytic carbon in the wavelength range from visible to infrared, it has been suggested vertically aligned carbon nanotube (VACNT) coatings may serve as effective radiative absorbers. The spectral optical constants of VACNT are modeled using the effective medium theory (EMT), which is based on the anisotropic permittivity components of graphite. The effects of other EMT parameters such as volume filling ratio and local filament alignment factor are explored. Low reflectance and high absorptance are observed up to the far-infrared and wide range of oblique incidence angles. The radiative properties of tilt-aligned carbon nanotube (TACNT) thin films are illustrated. Energy streamlines by tracing the Poynting vectors are used to show a self-collimation effect within the TACNT thin films, meaning infrared light can be transmitted along the axes of CNT filaments. Graphene, a single layer sheet of carbon atoms, produces variable conductance in the terahertz frequency regime by tailoring the applied voltage gating or doping. Periodically embedding between dielectric spacers, the substitution of graphene provides low radiative attenuation compared to traditional metal-dielectric multilayers. The hyperbolic nature, namely negative angle of refraction, is tested on the graphene-dielectric multilayers imposed with varying levels of doping. EMT should be valid for graphene-dielectric multilayers due to the nanometers-thick layers compared to the characteristic wavelength of infrared light. For metal- or semiconductor-dielectric multilayers with thicker or lossier layers, EMT may not hold. The validity of EMT for these multilayers is better understood by comparing against the radiative properties determined by layered medium optics. When bodies of different temperatures are separated by a nanometers-size vacuum gap, thermal radiation is enhanced several-fold over that of blackbodies. This phenomenon can be used to develop more efficient thermophotovoltaic devices. Due to their hyperbolic nature, VACNT and graphite are demonstrated to further increase evanescent wave tunneling. The heat flux between these materials separated by vacuum gaps smaller than a micron is vastly improved over traditional semiconductor materials. A hybrid structure composed of VACNT substrates covered by doped graphene is analyzed and is shown to further improve the heat flux, due to the surface plasmon polariton coupling between the graphene sheets.

Ce manuscrit traite du rayonnement thermique à l’échelle nanométrique et du contrôle de l’échange d’énergie radiative entre deux corps, afin d’augmenter les performances de conversion énergétique des systèmes thermophotovoltaïques (TPV). Les bases du rayonnement thermique et de la conversion photovoltaïque sont tout d’abord rappelées. Les flux rayonnés par des émetteurs multicouches supportant des phénomènes d’interférence sont ensuite calculés numériquement. Ces phénomènes permettent de contrôler le spectre d’émission et donc l’optimisation d’un émetteur sélectif pour des applications TPV. Il s’avère important de prendre en compte l’évolution en température des propriétés optiques des matériaux constituant l’émetteur. Il est démontré que le contrôle des phénomènes d’interférences au sein des structures multicouches sur substrat métallique permet d’obtenir des émissivités spectrale et totale hémisphérique 20 fois supérieures à celles du substrat seul. Le chapitre suivant est dédié au rayonnement thermique en champ proche entre un émetteur semi-infini et une couche mince. Cette configuration est proche d’un système TPV, où l’émetteur semi-infini peut être assimilé au corps rayonnant, et le film à une cellule PV. Différent phénomènes sont analysés : le comportement des résonances de polaritons de surface, l’absorption spatiale de la puissance radiative en champ proche et les phénomènes d’interférences dans le régime de transition champ proche-champ lointain. Ces phénomènes peuvent être mis à profit pour la conception de spectres optimisés. Dans le dernier chapitre, les performances de systèmes TPV en champ proche (TPV-CP) sont simulées numériquement à l’aide d’un code couplé transport des charges-rayonnement. Les modèles basés sur l’hypothèse de faible injection utilisés généralement pour simplifier le problème du transport des charges électriques dans la cellule PV sont évalués en détails. Différentes architectures de cellules permettant d’optimiser les performances du système sont présentées en conclusion. Ces travaux offrent un nouvel éclairage sur le rayonnement des structures multicouches et leur application à la conversion thermophotovoltaïque. / This thesis deals with thermal radiation at nanoscale in order to increase the energy conversion performances of thermophotovoltaic systems (TPV) The basics of thermal radiation and of photovoltaic energy conversion are recalled first. The flux radiated by few-layers emitters supporting interference phenomena are then calculated numerically. These phenomena allows controlling the emission spectrum, and thus the optimization of a selective emitter for TPV application. The next chapter is dedicated to near-field thermal radiation between a semi-infinite emitter and a flat film. This configuration is close to a TPV system, where the semi-infinite emitter can be related to the radiating body, and the film to the photovoltaic device. Different phenomena are analyzed: the behavior of the surface polariton resonances, the spatiale absorption of the radiative power and the interference phenomena in the near-to-far field transition regime. These phenomena can be used to design optimal spectra. In the last chapter, the performances of TPV system under near-field regime (NFR-TPV) are numerically simulated with a coupled charge transport/thermal radiation code. The models based on the low-injection approximation commonly used to simplify the charge transport problem inside the PV device are evaluated in details. Several cell architectures optimizing the performances of the system are then presented. All these results shed new light on thermal radiation of multilayers and their application to thermophotovoltaic conversion.

This semestral thesis focuses on the description of specific renewable resources in the form of thermophotovoltaic cells using selective radiators with micro/nano structures. This work deals with an introduction of renewable resources and specifically focuses on thermophotovoltaic. Thesis describes basic principles, but also influences affecting the proper functioning of these systems. It also focuses on selective radiators, which are created by mikro/nano structures, and factors that can affect their implementation or simulation. Part of the work are also examples of calculations of basic parameters of the structures, which will be used in the simulations. Next chapters are dealing with simulations which are analyzing thermal effects on termophotovoltaic system. Except the analysis itself there is also partial optimalization solving some of the negative thermal effects.

Recently, remarkable advances have been made in the understanding of micro/nanoscale energy transport, opening new opportunities in various areas such as thermal management, data storage, and energy conversion. This dissertation focuses on thermally-sensed nanotopography using a heated silicon microcantilever and near-field thermophotovoltaic (TPV) energy conversion system. A heated microcantilever is a functionalized atomic force microscope (AFM) cantilever that has a small resistive heater integrated at the free end. Besides its capability of increasing the heater temperature over 1,000 K, the resistance of a heated cantilever is a very sensitive function of temperature, suggesting that the heated cantilever can be used as a highly sensitive thermal metrology tool. The first part of the dissertation discusses the thermal characterization of the heated microcantilever for its usage as a thermal sensor in various conditions. Particularly, the use of heated cantilevers for tapping-mode topography imaging will be presented, along with the recent experimental results on the thermal interaction between the cantilever and substrate. In the second part of the dissertation, the so-called near-field TPV device is introduced. This new type of energy conversion system utilizes the significant enhancement of radiative energy transport due to photon tunneling and surface polaritons. Investigation of surface and bulk polaritons in a multilayered structure reveals that radiative properties are significantly affected by polariton excitations. The dissertation then addresses the rigorous performance analysis of the near-field TPV system and a novel design of a near-field TPV device.