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Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Two diode designs are being studied that require basically different fabrication techniques. The fabrication of electrodes was begun to test the concepts and the feasibility of these designs. This work was done with polished molybdenum electrode surfaces and vapor-deposited alumina insulator structures. Initial results were obtained in the close spacing of molybdenum electrodes with alumina insulators. Spacings were demonstrated with 1/2-inch diameter discs in the range of 3 to 5 .mu.m, using 1-.mu.m thick insulators. The deposition of thin alumina films on a molybdenum substrate was demonstrated, upon which arrays of thin film electrodes about 1/32-inch in diameter were deposited. A second approach uses a thin film as one of the electrodes. An analysis of the thermal and electrical transport properties of 1-.mu.m thick tungsten film shows that edge-connected films about 1 mm square in area could withstand the stresses generated by differential thermal expansion when the film is attached to the opposite electrode by stand-off insulators.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Two diode designs are being studied that require basically different fabrication techniques. The fabrication of electrodes was begun to test the concepts and the feasibility of these designs. This work was done with polished molybdenum electrode surfaces and vapor-deposited alumina insulator structures. Initial results were obtained in the close spacing of molybdenum electrodes with alumina insulators. Spacings were demonstrated with 1/2-inch diameter discs in the range of 3 to 5 .mu.m, using 1-.mu.m thick insulators. The deposition of thin alumina films on a molybdenum substrate was demonstrated, upon which arrays of thin film electrodes about 1/32-inch in diameter were deposited. A second approach uses a thin film as one of the electrodes. An analysis of the thermal and electrical transport properties of 1-.mu.m thick tungsten film shows that edge-connected films about 1 mm square in area could withstand the stresses generated by differential thermal expansion when the film is attached to the opposite electrode by stand-off insulators.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
In a theoretical and experimental program to evaluate those areas where three-dimensional microfabrication techniques could be important for improving methods of thermionic energy conversion, effort in the first reporting period has been directed toward a theoretical study of microstructures of electrodes for thermionic energy converters. The properties of a cesiated tungsten thermionic energy converter were analyzed with electrode temperatures compatible with a flame-generated heat source (T/sub c/ = 1650°K and T/sub a/ = 700°K), in order to estimate the efficiency, power production, and appropriate electrode spacing for microfabricated devices. The analysis yielded a maximum efficiency of 16 percent and corresponding electrical power of 11 W/cm2, requiring an emission current of 18 A/cm2. The study revealed that to attain these parameters, electrode spacing must be approximately 1 .mu.m, and that such a close-spaced diode with cesiated tungsten electrodes would operate approximately as a vacuum diode. That is, the principal function of the cesium would be to control the work function of the electrode surfaces. Operating at the point of peak efficiency, little space-charge limitation of the emission and little plasma resistance would be produced, because the atom/atom and electron/atom mean free paths would be larger than the interelectrode space.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Two applications of microfabrication technology to thermionic converters have been investigated theoretically. The first is a novel method of maintaining micron or submicron spacings over large areas (>1 cm2), using metals of different expansion coefficients to eliminate the shear stresses on the insulating pillars separating the electrodes. The second uses low-voltage field-emission sources to create ions in a large (approx. 1 mm) interelectrode gap for space charge neutralization. The theoretical results for both these approaches are highly encouraging.
Author: Publisher: ISBN: Category : Languages : en Pages :
Book Description
Considerable effort was directed toward the fabrication of a micron-spaced thermionic converter. In the process, a new technique has evolved which appears more promising and has been investigated theoretically. In both techniques, electrodes of similar differential thermal expansions are utilized to prevent shear stresses from disrupting the spacing pillars. The newer technique has additional advantages that simplify the fabrication of the diode structure, thus making it more practical. Progress is described.
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.
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Author: Jae Hyung Lee
Author: Institute of Electrical and Electronics Engineers