Electrical generator or motor structure – Non-dynamoelectric – Nuclear reaction
Reexamination Certificate
2001-12-20
2004-08-10
Dougherty, Thomas M. (Department: 2834)
Electrical generator or motor structure
Non-dynamoelectric
Nuclear reaction
C310S305000, C060S203100, C375S321000, C136S201000
Reexamination Certificate
active
06774532
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to microthermionic self-powered converters having high energy conversion efficiencies and to methods of manufacturing those converters using micromachining manufacturing techniques.
Thermionic generators were first proposed in 1915 by Schlichter, but many of the theoretical problems that existed at the inception of the idea persist today. Thermionic generators convert heat energy to electrical energy by an emission of electrons from a heated emitter electrode. The electrons flow from the emitter electrode, across an interelectrode gap, to a collector electrode, through an external load, and return back to the emitter electrode, thereby converting the heat energy to electrical energy. Historically, voltages produced are low, and the high temperature required to produce adequate current has produced numerous problems in maintaining the devices, including the unintended transfer of heat from the heated emitter electrode to the cold collector electrode. Practical thermionic conversion was demonstrated in 1957 by Hernquist in which efficiencies of 5-10% were reached with power densities of 3-10 W/cm
2
. Generally, such efficiencies and power densities were not sufficient to be financially competitive in the energy market, thus reducing the application of such devices. Furthermore, such devices were too large for use as miniaturized electrical power sources.
Another problem, “space-charge effect,” is described by Edelson (U.S. Pat. No. 5,994,638). A space-charge effect results when the build up of negative charge in the cloud of electrons between the two electrodes deter the movement of other electrons toward the collector electrode. Edelson cites two well-known methods for mitigating the space-charge effect: (1) reducing the spacing between electrodes to the order of microns, or (2) introducing positive ions into the electron cloud in front of the emitter electrode.
Introducing positive ions into the electron cloud in front of the emitter electrode generally consists of filling the interelectrode gap with an ionized gas. Thermionic converters with gas in the interelectrode gap are designed to operate with such ionized species, typically utilizing cesium vapor. Utilization of a cesium vapor results in a space charge neutralization, effectively eliminating the detrimental deterrence of electron flow. Cesium also plays a dual role by decreasing the work function of the device, i.e. the rate of electrons leaving a surface, by absorbing onto the emitter and collector surfaces, thereby allowing greater electron emission. However, too great of a pressure of cesium in the interelectrode gap will cause excess collisions between cesium atoms and electrons leaving the emitter electrode, reducing the efficiency of conversion. Therefore, a careful, complex balance must be maintained in a cesium vapor system. The current apparatus bypasses the complexities and efficiency losses of such a system (and its related expense) by lowering the space-charge effect through reduction of spacing between electrodes to the order of microns (i.e., 1-10 microns).
Reducing the spacing between electrodes to the order of microns has proved impractical with conventional manufacturing techniques. Fitzpatrick (U.S. Pat. No. 4,667,126) teaches “maintenance of such small spacing with high temperatures and heat fluxes is a difficult if not impossible technical challenge.” The present invention overcomes the difficulty of reducing spacing by microengineering. U.S. Pat. No. 6,294,858 to King, et al., “Microminiature Thermionic Converters”, which is hereby incorporated herein by reference, discloses a microminiature thermionic converter having a 1 micron electrode gap manufactured by integrated circuit (IC) semiconductor techniques. U.S. Pat. No. 6,299,083 to Edelson, “Thermionic Converter”, also discloses a microminiature thermionic converter fabricated using MEMS techniques. Both King's device and Edelson's device are powered by an external source of heat; not by an internal, self-contained power source, as in the present invention.
Earlier thermionic converters relied on external heat sources (nuclear power, geothermal energy, solar energy, fossil fuel combustion, wood or waste combustion), which may not be readily available to a user especially if electricity is desired in powering a mobile miniature device.
The present invention, in contrast, with its incorporated thermal source, overcomes the very modern problem of mobility and also provides more choices for operating devices that do not necessarily need to be mobile. For example, devices that are fixed, but may need to be used in a limited space may not be able to harness the thermal energy sources used by earlier devices.
SUMMARY OF THE INVENTION
The apparatus of the present invention is a self-powered microthermionic converter. A preferred embodiment of the converter comprises an emitter electrode and a collector electrode, separated by a micron-scale spaced interelectrode gap, a self-contained (i.e., incorporated, integral) thermal power source in good thermal contact with the emitter electrode, and an electrical circuit connecting the collector electrode and emitter electrode through an external electrical load.
The interelectrode gap of a preferred embodiment is preferably less than about 10 &mgr;m, more preferably, between approximately 1 &mgr;m and 10 &mgr;m, and most preferably, between approximately 1 &mgr;m and 3 &mgr;m. The interelectrode gap preferably comprises a vacuum. Alternate embodiments utilize cesium (or barium) vapor at a low vapor pressure, unlike the more common high vapor pressure cesium systems utilized in prior art inventions. The proposed alternate configuration, using low pressure cesium, differs from a Knudsen diode in that a small quantity of cesium is sealed into the present device during manufacture, whereas the Knudsen diode requires an external source of cesium (i.e., a cesium source apparatus).
A radioactive isotope can be used as the “self-powered” thermal power source, such as alpha-emitting Curium-242, Curium-244, or Polonium-210. Alpha particles emitted from the isotope deposit their energy (heat) within the body of the isotope if the range of the alphas is much smaller than the physical dimensions of the body (e.g., the range of a 6 MeV alpha particle is about 13 microns in copper). If the body of the isotope is very well thermally insulated, then the deposited heat can raise the temperature to very high values, greater than 600 C.
The collector electrode and emitter electrode of the converter are preferably formed by depositing or growing at least one layer of thermionic electron emissive material on a substrate. The thermionic electron emissive material is preferably an alkaline earth oxide in combination with a refractory metal. Thermionic emissive materials can be selected from barium oxide, calcium oxide and strontium oxide; combinations of these oxides; along with additions of aluminum and scandium oxides, as adjunct oxides. The preferable refractory metal to incorporate into the electron emissive oxide is tungsten, but could also include rhenium, osmium, ruthenium, tantalum, and iridium, or any combination of these metals. Tungsten, rhenium, osmium, ruthenium and iridium, or any combination of these metals can also be used as terminating (capping) top layers on the oxide or mixed oxide/metal layer. Alternately, low-pressure alkali or alkaline earth metals, such as cesium and barium, can be used with a high work function metal like tungsten, tantalum, rhenium, osmium, ruthenium, molybdenum, iridium and platinum, or any combination of these metals. The oxides of like tungsten, tantalum, rhenium, osmium, ruthenium, molybdenum, iridium and platinum, or any combination of these metals can also be used with low-pressure alkali or alkaline earth metals, such as cesium and barium.
The emitter electrode length is preferably less than approximately 200 &mgr;m, more preferably, between approximately 50 &mgr;m and approximately 200 &mgr;m, and most preferably, bet
King Donald B.
Kravitz Stanley H.
Marshall Albert C.
Tigges Chris P.
Vawter Gregory A.
Dougherty Thomas M.
Sandia Corporation
Watson Robert D.
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