Patent Number: 06294858&
Section: description

DETAILED DESCRIPTION OF THE INVENTION As suggested above, planar thermionic diodes can be manufactured using IC fabrication techniques slightly modified as disclosed herein to accomplish the objectives of the invention. All elements of the diode (emitter, collector, and insulating spacer between the electrodes) can be made using standard chemical vapor deposition (CVD) techniques and etch techniques used by the semiconductor industry. The CVD techniques allow for reliable, reproducible and accurate growth of extremely thin layers of metals (for the electrodes) and oxides (for some electrodes and for the spacers). MTCs can be fabricated with gap spaces ranging from 0.1 to 10 microns. With IEGs of this size, gases such as Cs vapor need not be introduced into the gap to reduce the space charge effects resulting from the large current flow from the emitter to the collector. The small gap size itself reduces the density of electrons in the gap. Existing thermionic converter technology employs use of refractory metals such as tungsten or molybdenum to fabricate the emitter and collector electrodes. These materials have high work functions that, in turn, require higher emitter temperatures. The MTCs of the present invention, conversely, use low work function materials that can be selected on the basis of performance criteria, and desired temperature of operation. Examples of such low work function materials that are suitable for MTC electrodes and compatible with the IC-style fabrication techniques used in the present invention include BaO, SrO, CaO, and Sc.sub.2 O.sub.3. In all cases, for thermionic conversion to occur, the work function of the collector electrode must not exceed that of the emitter electrode. Additionally, as noted above, one example of a class of suitable low work function materials, is disclosed in US Patent Application Ser. No. 09/257,336 which is herein incorporated by reference. This class of materials includes a mixture of BaSrCaO, Sc.sub.2 O.sub.3 and metal such as W. Various dielectric materials for separation of the electrodes are likewise suited both to the IC fabrication techniques and to application as spacers in MTCs. Among these are included SiO.sub.2 and Si.sub.3 N.sub.4. As shown below, in certain embodiments, the insulator material itself may serve as an appropriate substrate onto which the electrodes can be deposited using CVD. FIG. 2 illustrates the general concept by which an MTC could be fabricated according to one embodiment of the invention. In this embodiment, CVD techniques are used to deposit various layers of material of which the elements of the thermionic converter are comprised. FIG. 2a shows a deposited substrate or first electrode layer 70, which could form either the emitter or the collector in a finished MTC. This could be any low work function material appropriate for the desired application. As indicated above, materials such as BaO, SrO, CaO, Sc.sub.2 O.sub.3 or a mixture of BaSrCaO, Sc.sub.2 O.sub.3 and metal such as W for example, may be suitable. Likewise, a combination of these materials may be appropriate for given applications. It is also noted that the first electrode layer 70 could represent some combination of metal electrode and low work function material, or even some combination of a thermally and/or electrically insulating substrate with metal and low work function material on its surface. Variations of this sort will be known to those skilled in the art and are considered to be within the scope of the appended claims. In FIG. 2b, an oxide spacer 80 is then deposited on the first electrode layer 70. The depth of the spacer 80 serves to define the distance between the collector and emitter (the interelectrode gap) in the completed MTC. The next step in this embodiment, FIG. 2c, is to deposit another electrode layer 90 on top of the oxide spacer 80 layer. This second electrode layer 90 must be of a material having a work function that is different from that of the first electrode layer. (As with the first electrode layer 70, the second electrode layer 90 could include a combination of metal electrode and low work function material, or some combination of a thermally and/or electrically insulating substrate with metal and low work function material on its surface. Again, variations of this sort will be known to those skilled in the art and are considered to be within the scope of the appended claims.) Again, in the completed MTC, the electrode layer having the higher work function will serve as the emitter and the electrode layer having the lower work function will be the collector. FIG. 2d and 2e illustrate the creation of the interelectrode gap, or IEG 100. This can be accomplished by various means known to those skilled in the arts of chemical vapor deposition and integrated circuit fabrication. Those means may include, but are not limited to, masking the electrodes and spacers and then etching out an IEG region 100 of desired dimensions between the two electrode layers using suitable etchants, or sputtering particles to disrupt the crystal structure in the spacer layer 80 thereby creating a hole to serve as the IEG 100. The size of the IEG 100 is in the range of 0.1 to 10 microns between the first electrode layer 70 and the second electrode layer 90. FIG. 2d shows how one or more etching vias 110 might serve to assist in making the IEG 100. FIGS. 3a through 3d show an alternative embodiment wherein the MTC is manufactured using at least two separate substrate elements which can be subsequently assembled resulting in the completed MTC. Due to the precision of the IC fabrication methods used in making the various components of MTCs, and because only a small number of separate elements are required, the problems alluded to in the background section of this disclosure with regard to assembly of prior art macro-sized close-space thermionic converters are averted when manufacturing MTCs. Benefits of using the design of this embodiment of the invention include easy customization in terms of size, shape and electrical characteristics for use in building banks of MTC to accommodate different power requirements. This embodiment also incorporates use of metal conductors deposited separately from the emitter and collector electrode materials, likewise offering flexibility in design. Referring to FIG. 3a, a first substrate 130 comprising a dielectric and having a substantially flat surface 135 is deposited or otherwise provided. A second substrate 150 is deposited or otherwise provided separately from the first substrate. This second substrate 150 may be comprised of a dielectric or semiconductor, depending on the design requirements of the MTC to be constructed. FIG. 3b shows where a recess or opening 160 is created in the second substrate 150 using any of any of a variety of techniques such as etching or sputtering as previously described for creating the IEG 100 illustrated in FIG. 2(e). The opening 160 has a substantially planar boundary 165 along one dimension which will lie substantially parallel to the substantially flat surface 135 of the first substrate 130 in the completed MTC. The opening also includes at least one wall 163. The reason this element is described as at least one wall is that functional embodiments could include various instances including the following: 1) use of separate and distinct walls (such as in the case where multiple walls define a geometrically angular opening), or 2) use of a single curved wall (such as in the case of a circle or oval). These and other modifications in the wall configuration are considered to be a matter of choice and within the understanding of those skilled in the art. FIG. 3c illustrates where a first conductor 120 has been deposited in the first substrate 130. This conductor is comprised of metal or another electrically conducting material suited to deposition using semiconductor manufacturing techniques known to those skilled in the art. The first conductor 120 includes a surface 125 disposed adjacent to, and in a plane substantially parallel to, the substantially flat surface 135 of the first substrate 130. Also shown in FIG. 3c is a second conductor 140, which is deposited within the second substrate 150. As with the first conductor 120, the second conductor 140 is comprised of metal or another electrically conducting material suited to deposition using semiconductor manufacturing techniques known to those skilled in the art. The second conductor 140 likewise includes a surface 145, however, in this case the surface 145 is disposed adjacent to, and in a plane substantially parallel to, the substantially planar boundary 165 of the opening 160 in the second substrate 150. FIG. 3d shows a completed MTC wherein the first substrate 130 is assembled to the second substrate 150 so that the surface 125 of the first conductor 120 is aligned substantially parallel to the surface 145 of the second conductor 140. Deposited on the surface 125 of the first conductor is a first electrode material 128 having a given work function. Deposited on the surface 145 of the second conductor is a second electrode material 148 having a given work function which is different from that of the first electrode material 128. An interelectrode gap (IEG) 175 is disposed therebetween. As with the earlier described embodiment, the size of the IEG 175 should be in the range of 0.1 to 10 microns between the first electrode material 128 and the second electrode material 148. Choice of the exact size of the IEG as well as what specific low work function materials to use for electrodes will depend on the requirements for any particular MTC. Potentially suitable electrode materials, for the reasons stated above, include BaO, SrO, CaO, and Sc.sub.2 O.sub.3, however, in all cases, the electrode material which serves to collect electrons in the MTC cannot have a work function greater than the electrode material of the electron emitter in the MTC diode. Given the specific requirements of a given MTC, it may be desirable for the anode and cathode to be treated with the same electrode material. It should be noted that the embodiment illustrated in FIGS. 3a though 3d can be modified as needed to accommodate specifications or manufacturing constraints. For example, the boundary 165 of the gap 160 etched in the second substrate 150 and the surface 135 of the first substrate need not necessarily be flat and disposed parallel to one another so long as the coated surfaces 128, 148 of the first and second conductors 120, 140 are substantially flat and disposed parallel to each other. Maximum efficiency of an MTC depends on the anode and cathode in the diode being the same distance apart at all points along the emitting and collecting surfaces Operation of the completed MTC in all cases contemplated by this disclosure requires a temperature difference to exist between the emitter and the collector at the time the MTC is operated. In the best mode known to the inventors, satisfactory electric power generation with MTCs can be accomplished where the emitter temperature is approximately 300.degree. C. higher than the collector temperature. This can be accomplished using any of a variety of methods of temperature regulation known to those skilled in the arts of thermionic conversion and integrated circuit manufacture, and includes use of such means as radiant heat sources for heating the emitter and heat sinks for cooling the collector. FIG. 4a shows how multiple MTCs can be arranged in a bank in series. In the figure, two MTCs 250 are mounted atop a cold plate 260, and secured by collars 270. The cold plate serves to cool the collector electrodes of the MTCs 250. A radiator 280 supported by a radiator support 290 serves to heat the emitter electrodes of the MTCs 250. Electrical interconnects 300 between adjacent MTCs are shown in the figure as bold lines. FIG. 4a illustrates an electrical connection between the heated emitter of one MTC to the cooled collector of the adjacent MTC, thereby creating a series connection. FIG. 4b is similar except that it illustrates a first pair of MTCs 250 in parallel configuration 310 which, in turn, is joined by a series connection 320 to a second pair of MTCs 250 in parallel configuration 310. Thus, the MTCs of the present invention are scalable to a wide range of power levels though series and parallel connections. The design and fabrication of MTCs is guided by modeling of the converter structures and materials as well as the physical processes. FIG. 5 illustrates the dependence of converter efficiency on gap size of the converter. Two emitter work functions (wfe) were selected: 1.6 and 2.2 eV. The upper curve 180 on the graph plots data for wfe=2.2 eV. The lower curve 170 on the graph plots data for wfe=1.6. For the 2.2 eV emitter, the emitter temperature, collector temperature, and collector work function were 1500 K, 673 K, and 1.5 eV, respectively. For the 1.6 eV emitter, the emitter temperature, collector temperature, and collector work function were 1100 K, 573 K, and 1 eV, respectively. For these two cases, efficiencies in the high 20% to low 30% were obtained. Maximum efficiencies occur in the 1-micron gap space range. FIG. 6 illustrates the power and current densities achieved by the cases shown in FIG. 5. Plot 190 shows power (W/cm.sup.2), wfe=2.2 eV; plot 200 shows current (A/cm.sup.2), wfe=2.2 eV; plot 210 shows power (W/cm.sup.2), wfe=1.6 eV; and plot 220 shows current (A/cm.sup.2), wfe=2.2 eV. Current densities in the 1 to 10 A/cm.sup.2 range are readily attainable. Raising the emitter temperature or decreasing the gap size can increase current densities. FIG. 7 illustrates the output voltage that can be achieved versus gap size. Plot 230 shows data for wfe=2.2 eV and plot 240 shows data for wfe=1.6 eV. Output voltage increases as gap size is increased; however, current densities decrease as gap size increases. Larger output voltages can also be achieved by fabricating the miniature converters in series. As has been discussed, the high conversion efficiency (about 30%) of MTCs and their inherent small size makes them suitable for radioisotope thermoelectric generators (RTGs). RTGs have been extensively used for space power systems such as that found on the Gallileo and Ulysses satellites. Currently, these RTGs can deliver at least 285 W of electrical power at an efficiency of about 6.5%. It is believed that MTCs could increase the output of RTGs to&gt;1000 W of electrical power without modifying the design of the radioisotope module and without increasing the mass of the RTG. Terrestrially, it is believed that MTCs could be used as portable power systems. Since energy conversion from these systems can be accomplished at relatively low temperatures (&lt;1000 K), heat sources such as that found from burning kerosene, alcohol, wood, and similar fuels could be used. Therefore, a portable power generator that could be used for emergency power or camping, for example, could be made to fit in the trunk of a car. The preliminary Heat Pipe Power System (HPS) Space Reactor is designed to provide 5 kWe power using 5% efficient unicouple thermoelectrics. Heat pipes provide heat to the thermoelectrics at 1275 K. The excess heat from the thermoelectrics is rejected at 775 K. MTC characteristics could be matched to the thermal operating conditions of the HTS to achieve higher conversion efficiencies. When operating at the temperature range mentioned above and with emitter and collector work functions of 1.6 eV and 1.0 eV, respectively, MTCs could provide energy conversion efficiencies of 25 to 34% for interelectrode gap sizes ranging from 1 to 3 microns. Output currents would range from 3 to 19 A/cm.sup.2, and output power densities would range from 2.7 to 12.8 W/cm.sup.2. Increasing efficiencies would also result in a less massive HPS by decreasing the size of the heat rejection radiator. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the appended claims.