Author: Sharon Hsiao-Wei Chou Publisher: ISBN: Category : Languages : en Pages :
Book Description
The work function is the interfacial parameter of a surface that determines how easily electrons can escape into a vacuum or gas environment, with lower work functions generally facilitating electron emission. This thesis employs density function theory methods to a systematic approach in the discovery of new nanostructured multilayer materials with low work functions. Techniques in density functional theory can enable many promising film coating combinations to be efficiently investigated for the first time. Two main sets of screening studies are described: (1) cesiated transition metal surfaces and (2) alloyed alkali-earth oxide films on tungsten. A model is introduced for the effect of cesium adsorbates on the work function of transition metal surfaces. This model builds on the classic point-dipole equation by adding exponential terms that characterize the degree of orbital overlap between the 6s states of neighboring cesium adsorbates. In addition, the model analyzes the effect of orbital overlap on the strength and orientation of electric dipoles along the adsorbate-substrate interface. This new framework improves upon earlier models in terms of agreement with the work function-coverage curves obtained via first-principles calculations based on density functional theory. All the cesiated metal surfaces have optimal coverages between 0.6 and 0.8 monolayers, in accordance with experimental data. Of all the cesiated metal surfaces considered, tungsten has the lowest minimum work function, also in accordance with experiments. The work function and stability of 570 alloyed alkali-earth oxide films on the (100) surface of tungsten have been calculated within density functional theory. Computational screening of this large phase space is enabled by implementing the virtual crystal approximation, where the degree of freedom in the chemical composition is modeled with virtual atoms of mixed calcium, strontium, and barium character. Low work functions are achieved by doping the films with scandium (1.16 eV) or lithium (~1.2 eV) and alloys containing ~15% to ~20% of calcium. In particular, lithium-doped systems with ~15% calcium also show favorable stability indicated via formation energy calculations. Identification of such film alloys outperforming any of the constituents relies on careful sampling of the chemical composition. Covalent interactions within the film limit the reduction in work function from the dipole normal to the surface. Controlling the electronic screening of these intra-film interactions by oxygen atoms is essential for the design of new low-work function materials.
Author: Daniel C. Riley Publisher: ISBN: Category : Languages : en Pages :
Book Description
Thermionic energy conversion (TEC) is a direct heat-to-electricity conversion technology with the potential to leapfrog state-of-the-art solid-state conversion in efficiency and power density. In a thermionic energy converter, electrons evaporate from a hot electrode, the cathode, into a vacuum gap and are collected by a cooler electrode, the anode, to generate electric current. In the 1960s-1970s numerous groups reported thermionic converters with power densities above 10 W/cm^2 and conversion efficiencies of ~15%. However most of this work was tied to the US space-nuclear program which ended in 1973, and thermionics research has never fully recovered. As a result two central challenges yet remain in thermionics: (1) High operating temperatures necessary to produce electric current result in difficult materials challenges, and (2) low operating voltages due to losses associated with space charge and high anode work functions. However, new opportunities to tackle these challenges are available as a result of the breathtaking rise of semiconductor fabrication technology. In this work I present a new physical mechanism called photon enhanced thermionic emission (PETE). This concept is an improvement on thermionic emission by using light to boost the average energy of carriers in a hot p-type semiconductor cathode. Additionally, unlike in a photovoltaic cell, the waste heat from recombination losses and sub-bandgap light absorption is utilized to heat the cathode. Thus a PETE cathode can produce efficient electron emission at lower temperatures than a thermionic cathode. I will describe theoretical calculations showing that a PETE device may exceed 40% solar power conversion efficiency, and the conversion efficiency may exceed 50% if a PETE device is used in tandem with a solar thermal backing cycle. I will also describe an experimental demonstration of the PETE effect in an ultra-high vacuum photoemission measurement. In the cathode of an energy converter based on photon-enhanced thermionic emission (PETE) photoexcited carriers may need to encounter the emissive surface numerous times before having sufficient thermal energy to escape into vacuum and therefore should be confined close to the surface. However, in a traditional planar geometry, a thin cathode results in incomplete light absorption. Nanostructuring has the potential to increase light capture and boost emission by decoupling the lengths associated with photon absorption and electron emission. Nanostructures may complicate the properties of the emissive surface; therefore, the effect of nanostructuring on emission efficiency needs to be studied. In this work I describe results from a suite of simulation tools we have developed to capture the full photoemission process: photon absorption, carrier transport within the active material, and electron ballistics following emission. I show that the theoretical efficiency of a negative electron affinity emitter may be increased with nanostructures if light absorption and electron escape ballistics are considered. I then describe measurements of the photoemission efficiency of fabricated nanostructures that were designed based on the results of the simulation suite. I will also present a fundamentally new method to increase the operating voltage of a TEC by lowering the anode work function using the surface photovoltage effect. When a semiconductor surface is illuminated, photo-excited carriers form an internal dipole, or surface photovoltage (SPV), in the band-bending region and begin to flatten the bands near the surface. This SPV is analogous to the photovoltage in a photovoltaic cell and can reduce the effective work function of the material. I will describe an experimental demonstration using the SPV effect to produce a low work function surface. I will also describe a proof-of-concept demonstration of the SPV effect applied to improve the I-V characteristics of thermionic device. This generic physical process extends across materials systems and forms a realistic path to ultra-low work functions in devices to enable efficient thermionic energy conversion.
Author: Santoshrupa Dumpala Publisher: ISBN: Category : Heat recovery Languages : en Pages : 0
Book Description
More than 50% of the total energy produced is typically rejected in the form of waste heat from various processes. The grand challenge is that most of this waste heat is released at temperatures much lower than 1000 C, which makes it difficult to recover it using traditional methods which require higher operating temperatures for thermal energy conversion. In addition, these traditional methods involve different intermediate processes and do not offer direct conversion into electricity. In this regard, thermal energy conversion through thermionic emission can offer direct conversion of waste heat into electricity with highest theoretical efficiency. So, waste heat recovery through thermionic emission energy conversion is of great interest and is the motivation for the present work. However the greatest challenge involves the discovery or availability of the material with appropriate work functions and stability criteria. To address the need for developing suitable materials toward thermionic energy conversion, we investigated phosphorus doping in individual diamond nanocrystals, conical carbon nanostructures (CCNTs) and diamond nanocrystals supported on conical carbon nanostructures. Hybrid architectures, diamond nanocrystals supported on high aspect ratio structures will allow the study of true performance of nanocrystals free from grain boundaries and also offer field enhancements. First, the synthesis of CCNTs over large area and flat substrates is investigated. From the experimental results, we successfully synthesized CCNTs on planar graphite and tungsten foil substrates with areas as large as (>0.5 cm2). A detailed underlying nucleation and growth mechanism was also demonstrated supported with regrowth experiments and kinetic growth model. Secondly, selective nucleation of the diamond crystals on the tips or complete coating on CCNTs was demonstrated and a likely mechanism for the nucleation and growth of diamond crystals is also presented. Thirdly, the field and thermionic emission characteristics from the as synthesized CCNTs have shown to exhibit enhanced emission characteristics such as low turn-on voltages, large field enhancement factor and lower work function values owing to their higher aspect ratios and optimum density overcoming field screening effects. Finally, phosphorus doping into these individual diamond crystals and diamond films was performed and thermionic emission characteristics were studied. Work function values as low as 1.8 eV from diamond films and 2.2 eV from diamond crystals was obtained. In summary, the main outcomes of this work include growth of large area CCNTs on flat substrates, discovery of the enhanced field and thermionic emission characteristics of CCNTs, selective nucleation and phosphorus doping of individual diamond nanocrystals on CCNTs free from grain boundaries and work function value as low as 2.2 eV from thermionic emission from these crystals.
Author: George N. Hatsopoulos Publisher: MIT Press (MA) ISBN: Category : Science Languages : en Pages : 288
Book Description
Good,No Highlights,No Markup,all pages are intact, Slight Shelfwear,may have the corners slightly dented, may have slight color changes/slightly damaged spine.
Author: Tianyu Ma Publisher: ISBN: Category : Languages : en Pages : 111
Book Description
The work function is a fundamental materials property which governs electron emission and has thus become a key design parameter for materials used in thermionic energy converters and vacuum electronic devices. In this dissertation, we use Density Functional Theory (DFT) calculations to search for low work function materials which is needed for efficient thermionic energy converters and compact high power terahertz vacuum electronic devices.The first part of this dissertation establishes a systematic work function data set for rare-earth hexaborides, rare-earth tetraborides, and transition metal nitrides. The work function trends in these materials can be understood based on the atomic properties of their parent metals, i.e., electronegativity and atomic radius. We find Ba alloyed LaB6 (LaxBa1-xB6) has lower work function than both endmembers, due to the positive surface dipole created by surface rumpling. We also propose HfN as a promising electron emitter material as it has a low work function comparable to pure LaB6 according to our calculations. In the second part of our work, we benchmark the accuracy of DFT calculated work functions for complex oxide surfaces. The results demonstrate a mean absolute error of ~0.2 eV between Heyd-Scuseria-Ernzerhof (HSE) calculated work functions and experimental values for select SrTiO3 ideal and reconstructed surfaces. This accuracy is at the same level as that for elemental metals. By comparing work functions obtained from HSE functional and Perdew-Burke-Ernzerhof (PBE) functional, we find HSE functional yields more accurate work function values, while PBE functional seems to correctly capture the changes in work functions. We also explore the effects of common surface features, including vacancies, adatoms, reconstructions, and surface steps, on the work function. The third part of our work is a high-throughput screening in search of low work function perovskite oxides. We use the linear correlation between the O p-band center and the work function to quickly predict work functions of 2913 perovskites. After further screening based on the stability under typical cathode operating conditions and electrical conductivity, we finally identify seven perovskites as promising thermionic cathode materials which are predicted to have low AO-terminated (001) surface work function, high stability, and high conductivity. We also discover a qualitative relationship between the work function and the number of d electrons, where perovskites with barely filled d bands have the lowest work functions while perovskites with empty d bands or nearly filled d bands have the highest work functions.
Author: Jae Hyung Lee Publisher: ISBN: Category : Languages : en Pages :
Book Description
Thermionic energy converters (TECs) are heat engines that convert heat directly to electricity at very high temperatures. This energy conversion process is based on thermionic emission--the evaporation of electrons from conductors at high temperatures. In its simplest form, the converter consists of two electrodes in the parallel plate capacitor geometry, and it uses the thermionically emitted current to drive an electrical load. This dissertation presents research on five key areas of microfabricated thermionic energy converters ([mu]-TECs). First, the numerical calculation of the emitter-collector gap that maximizes the power conversion efficiency of thermionic energy converters (TECs) is discussed. Thermionic energy converters require emitter and collector work-functions that are relatively low, to reach useful efficiencies at typical operating temperatures of 1000 - 1500 oC. The optimum arises because efficiency drops both at very large gaps, due to space-charge limitations on the TEC current, and at very small gaps, due to the increased heat loss via near-field radiative heat transfer. The numerical calculation results show that, for typical TECs made with cesiated tungsten electrodes, the optimal gaps range from 900 nm to 3 [micrometers]. I then discuss several prototypes of mechanically and thermally robust [mu]-TECs, including the stress-relieved emitter design, emitter-collector structural design, as well as a recent approach for the stand-alone (encapsulated) [mu]-TECs. Thermionic emission from the SiC emitter was demonstrated for the first time. The stress-relieved design emitters were analyzed, and the work-function of the SiC emitter was estimated at temperatures of up to 2900K. Also described are both the planar and the U-shaped suspension for microfabricated TECs ([mu]-TECs). Our initial planar [mu]-TECs achieved emitter temperatures of over 2000 K with incident optical intensity of approximately 1 W/mm2 (equivalent to 1000 Suns), remained structurally stable under thermal cycling, and maintained a temperature difference between the emitter and the collector of over 1000 K. Conformal sidewall deposition of poly-SiC on a sacrificial mold is used to fabricate stiff suspension legs with U-shaped cross sections, which increases the out-of-plane rigidity and prevents contact with the substrate during the heating of the suspended emitter. By extending the conventional technique of cesium coatings to SiC, we reduce the work-function from 4 eV to 1.65 eV at room temperature. Subsequently, we tested [mu]-TECs with both barium and barium oxide coatings. The coatings reduced the work-function of the SiC emitter to as low as ~2.14 eV and increased the thermionic current by 5-6 orders of magnitude, which is a key step toward realizing a efficient thermionic energy converter. Encapsulation of [mu]-TEC was achieved by an anodic bond between pyrex and the silicon substrate with via feedthroughs. Last, I introduce the photon-enhanced thermionic emission (PETE) concept, and show why a single crystal photo-emitter is needed. I cover my recent fabrication development of smart-cut layer transfer using Spin-on-Glass (SoG). In addition, a novel layer transfer technology that can transfer any device materials onto the glass substrate, which I call "Anything on Glass, " is briefly described. I, then, describe how the first demonstration of the photon-enhanced thermionic emission (PETE) from the microfabricated emitter was achieved. The p-type SiC emitter was used to demonstrate PETE in an uncesiated and microfabricated sample, bringing this energy conversion approach closer to practical applications.
Author: Geoffrey A. Ozin Publisher: Royal Society of Chemistry ISBN: 183916316X Category : Science Languages : en Pages : 465
Book Description
Materials have the potential to be the centrepiece for the transition to viable renewable energy technologies and this book provides a perspective on the application of new technologies to this field as well as the broader techno-economic and social context.
Author: NED S. RASOR Publisher: ISBN: Category : Languages : en Pages : 1
Book Description
Contents: Processes Emission limitation on thermionic energy conver sion Correlation of electron, ion, and atom emission energies Potential of an ion in a discrete dipole layer Statistical mechanical treatment of surface ioni zation An experimental search for an elemental surface exhibiting low electron work function Cs-O-H-Mo system interactions and transfer of molybdenum within a thermionic diode Analysis of the space charge mode in the cesium thermionic diode Plasma mode in a thermionic diode with detailed energy balance Plasma resistivity and collective interactions Instabilities in high current discharges in cesium vapor.
Author: Sharon Hsiao-Wei Chou
Author: Daniel C. Riley
Author: Santoshrupa Dumpala
Author: James F. Morris
Author: George N. Hatsopoulos
Author: Tianyu Ma
Author: Jae Hyung Lee
Author: Geoffrey A. Ozin
Author: NED S. RASOR
Author: