Apparatus and method for generating electrical current from the nuclear decay process of a radioactive material

[0001] The instant application is a continuation-in-part of prior U.S. application Ser. No. 10/356,411, filed Jan. 31, 2003, now issued as U.S. Pat. No. 6,774,531.

[0002] The present invention relates generally to an apparatus for generating electrical current from the nuclear decay process of a radioactive material. In a specific, non-limiting example, the invention relates to an energy cell (e.g., a battery) for generating electrical current derived from particle emissions occurring within a confined volume of radioactive material (e.g., tritium gas).

[0003] Radioactive materials randomly emit charged particles from their atomic nuclei. Examples are alpha particles (i.e., ⁴He nuclei) and beta particles (i.e., either electrons or positrons). This decay process alters the total atomic mass of the parent nucleus, and produces a daughter nucleus, having a reduced mass, that may also be unstable and continue to decay. In such a nuclear decay series, a fraction of the original material is consumed as energy, and eventually, a stable nucleus is formed as a result of successive particle emissions.

[0004] The principal use of controlled nuclear decay processes relates to generation of energy producing heat sources. Two of the best-known examples are nuclear reactors for producing electric power, and radioisotope thermal generators (RTGs) used in connection with various terrestrial and space applications.

[0005] Nuclear reactors have a heat-generating core that contains a controlled radioactive decay series. Heat generated within the core during the decay series is transferred to an associated working fluid, for example, water. The introduction of heat into the working fluid creates a vapor, which is in turn used to power turbines connected to electric generators. The resulting electricity is then wired to a distribution grid for transmission to users.

[0006] RTGs are also heat-generating devices, wherein electricity is produced by one or more thermocouples. The principle of operation of a thermocouple is the Seebeck effect, wherein an electromotive force is generated when the junctions of two dissimilar materials, typically metals, are held at different temperatures. RTGs are typically used for space applications due to their reasonably high power-to-weight ratio, few (if any) moving parts, and structural durability. RTGs also supply power in space applications where solar panels are incapable of providing sufficient electricity, for example, deep space missions beyond the orbit of Mars.

[0007] Previously, a major drawback when attempting to use energy derived from a nuclear decay series to power devices in remote locations has been an inefficiency of the energy conversion process. For example, it has proven difficult to achieve much greater than a ten percent energy conversion rate, especially when the energy is transferred via a thermodynamic cycle as described above.

[0008] As seen in prior art FIG. 1, a schematic representation of an energy generation process achieved by emission of a charged particle from the nucleus 1 of a radioactive material 2 is shown. Provided that an electric field is maintained between positive electrode 3 and negative electrode 4 by a potential difference 5, a charged decay particle creates electron/hole pairs that migrate toward naturally attractive electrodes 3 and 4. If a resistive load Ω completes the circuit such that positive charges 6 and negative charges 7 recombine, power is generated by the induced current flow.

[0009] Electrical current directly derived from a nuclear decay process is frequently referred to as an “alpha-voltaic” or “beta-voltaic” effect, depending on whether the charged particle emitted by a particular nucleus is an alpha particle or a beta particle, respectively.

[0010] A description of efforts to exploit the nuclear decay process of a radioactive material is found in A Nuclear Microbattery for MEMS Devices, published by James Blanchard et al. of the University of Wisconsin-Madison in August, 2001, and incorporated herein by reference. Blanchard et al. sought to develop a micro-battery suitable for powering a variety of microelectromechanical systems (“MEMS”). Advantages of using such devices to power MEMS include a remote deployment capability, high power-density as compared to other conventional micro-energy sources, and long-term structural durability.

[0011] Other references to nuclear batteries include U.S. Pat. No. 6,479,920 to Lal et al.; U.S. Pat. No. 6,118,204 to Brown; U.S. Pat. No. 5,859,484 to Mannik et al.; and U.S. Pat. No. 5,606,213 to Kherani et al of which are incorporated herein by reference. None of these nuclear batteries have been developed commercially for practical applications.

[0012] An apparatus for generating electrical current from a nuclear decay process of a radioactive material is disclosed, the apparatus comprising: an enclosed volume of radioactive material; and a junction region disposed within said enclosed volume, wherein a first portion of said junction region is disposed at a declination angle of greater than about 55° relative to a second portion of said junction region. Also disclosed is an apparatus for generating electrical current from a nuclear decay process of a radioactive material, wherein the apparatus comprises: an enclosed volume of radioactive material; and a junction region, disposed within said enclosed volume, formed on one or more surfaces of a porous region having an aspect ratio of greater than about 20:1.

[0013] Also disclosed is a method for generating electrical current from a nuclear decay process of a radioactive material, the method comprising: enclosing a volume of radioactive material in a cell; and disposing a junction region within said enclosed volume, so that a first portion of said junction region is disposed at a declination angle of greater than about 55° relative to a second portion of said junction region. Also disclosed is a method for generating electrical current from a nuclear decay process of a radioactive material, wherein the method comprises: enclosing a volume of radioactive material in a bulk silicon material; forming at least one pore within the body of said bulk silicon material so that said at least one pore has an aspect ratio of greater than about 20:1, and disposing a junction region within said at least one pore.

[0020] Referring now to FIG. 2A, an example embodiment is seen in which a silicon wafer 21 has been doped to provide a p-type region 22, an n-type region 24 and a junction region 20. Contact 28 connects p-type region 22 to a first side of a load Ω via a low-resistivity contact region 30 (e.g., a metal, for example, aluminum). A second low-resistivity contact surface region disposed between contact surface region 27 and contact 26 (e.g., a metal deposit, for example, gold) permits a current transport means for charges liberated by energetic decay electron energy absorption in n-type region 24 to reach contact 26 such that n-type region 24 is in electrical communication with another side of load Ω. Tritium gas (not shown), which is disposed in deep pores 23, decays. Each decay event generates an energetic beta particle (not shown) that enters n-type region 24, where an electric field exists relative to junction region 20 and contact surface region 27 caused by the contact potential between p-type region 22 and n-type region 24. In this particular example embodiment, the emitted beta particle enters n-type region 24 and creates, via ionization, positive and negative charges within n-type region 24, so that electrons and holes separate under the influence of the electric field. One charged species migrates towards junction region 20 and thence to contact region 30, while an oppositely charged species migrates toward contact surface region 27, thereby inducing current flow through load Ω via contacts 28 and 26, respectively.

[0021] The maximum travel distance of the most energetic tritium beta particle in silicon is about 4.33 μm; and, in at least one example embodiment employing a silicon wafer and tritium gas, a junction region 20 is created near a boundary of p-type region 22 and n-type region 24 at a depth just past 4.33 μm. Disposition of the junction region at a depth just greater than the maximum travel distance of the beta particle provides a nearly 100% chance that all of the charge generated when a beta particle travels through n-type region 24 will be collected, and therefore contribute to the total generated current.

[0022] The deep pores 23, in various embodiments, have a throat diameter of significantly less than the “mean free path” of the decay particle of the radioactive material disposed in the pore (in the above-described example, tritium) for the purpose of increasing the probability that a decay event will cause current to be generated. In further embodiments, the pores 23 have a length-to-diameter aspect ratio of greater than about 20:1; in a still further embodiment, the pores 23 have an aspect ratio of greater than about 30:1, again for the purpose of increasing the probability that a decay event will result in a particle entering the silicon and generating current. In still further embodiments (for example, see FIG. 2A), the walls of deep pores 23, and consequently the junction region 20 formed between p-type region 22 and n-type region 24, have a declination angle θ of greater than about 55° (measured relative to a surface plane 27 of the semiconductor surface in which they are formed). In the embodiment shown in FIG. 2A, for example, the walls of deep pores 23, and thus the associated longitudinal junction regions 20, have a declination angle θ of about 90° measured relative to the surface plane 27 of the semiconductor in which they are formed. When the radioactive material is disposed in a long, narrow volume in a semiconductor, there is a much greater probability that a beta particle produced by a decay event will enter the junction region 20 and induce a current flow. Disposing the radioactive material in a manner such that a decay particle is produced a significant fraction of a mean free path or further from the nearest energy conversion function causes a much lower current density to result from any particular volume of semiconductor 21.

[0023] It should be noted that the current of a particular device is related, at least in part, to the surface area of the junction region available to collect electrons quickly after the decay event. The greater the area of junction region 20 provided in a particular volume of radioactive material, the greater the induced current. The voltage of a particular device depends, at least in part, on the voltage of the junction region. For silicon-material junction regions, that voltage is about 0.7 volts. For other junction regions, whether derived from different semiconductor materials (e.g., germanium, gallium-arsenide, etc.) and/or other structural configurations (e.g., plated metal disposed over selected portions of a semiconductor material), the voltage is different.

[0024] Referring now to an example embodiment shown in FIG. 2B, voltage is increased by attaching multiple junction regions in series (e.g., by connecting the p-type region 22 of one junction region to the n-type region 24 of another junction region using an appropriate connector 25, for example, a metalization deposit). As seen in the example embodiment shown in FIG. 2C, total current is increased by attaching multiple junction regions in parallel (e.g., by connecting the p-type region 22 of one junction region to the p-type region 22 of another junction region using an appropriate connector 25, for example, a metalization deposit, and connecting the n-type region 24 of a first junction region to the n-type region 24 of another junction region using an appropriate connector, for example, a portion of conductive contact material 29). In this manner, according to various embodiments of the invention, distinct voltage and current characteristics are achieved for each particular application.

[0025] Referring now to an example embodiment shown in FIG. 3, an apparatus for generating electrical current from the decay process of a radioactive material is shown, wherein the apparatus comprises: a metal housing 1 (e.g., a metal canister); an insulated feed-through 2 (which in some embodiments has an evacuation port 3, a fill pipe 4 and an electrical connector 5a; although, in other embodiments, feed-through 2 is a single hollow member, e.g., a metal tube that is crimped after introduction of a radioactive material); an enclosed cell 11 comprising a semiconductor portion 6a and a semiconductor portion 6b, each of which are affixed to opposing sides of a thin conductive ring 6c (e.g., a metal, a doped semiconductor, or another appropriate conductor).

[0026] While FIG. 3 shows an embodiment of the invention having at least two semiconductor portions 6a and 6b connected by a conductive ring 6c, the present invention is practiced in some alternative embodiments using only a single semiconductor wafer. In still further embodiments, multiple layers of semiconductor material are used, thereby increasing the total available voltage. In still further embodiments, wafers suitable for practicing the invention are formed by plating layers of metal (e.g., platinum, silver, nickel, gold, etc.) to selected surfaces of a semiconductor.

[0027] Referring still to an example embodiment shown in FIG. 3, an electricity-generating cell 11 is disposed within housing 1 and adhered to an inner surface 13 of said housing by an adhesive 7 (for example, glue, tape, paint, etc.). In various other examples, adhesive 7 is conductive (e.g., conductive paint, deposited metal film, metal foil, etc.).

[0028] Cell 11 further comprises a plurality of etched pores or channels 8 having doped junction regions 9 formed on the inner surfaces of said pores or channels, and a volume of confined radioactive material 10 (e.g., a tritium gas) confined within the cell. In a further embodiment, radioactive material 10 comprises a non-radioactive material (e.g., nickel), which is converted into an appropriate radioactive species (for example, ⁶³Ni), which thereafter decays when irradiated or otherwise excited by appropriate means.

[0029] In at least one embodiment, existing semiconductor fabrication methods are used to form porous silicon wafers having a plurality of etched pores or channels. See, for example, U.S. Pat. No. 6,204,087 B1 to Parker et al., U.S. Pat. No. 5,529,950 to Hoenlein et al.; and U.S. Pat. No. 5,997,713 to Beetz, Jr. et al., all of which are incorporated herein by reference. Generally, a pore or channel pattern is deposited onto the wafer. Masking is performed using, for example, photolithography and/or photo-masking techniques. Exposed portions of the wafer are etched (for example, by exposure to a chemical solution, or gas plasma discharge), which removes areas of the wafer that were not protected during the masking stage.

[0030] In at least one embodiment, inner surfaces of the etched pores are substantially curved in shape, for example, cylindrical or conic. In an alternative embodiment, however, a series of very narrow channels are etched. In a still further embodiment, the etched pores and/or channels are formed in the wafer in positions that are substantially equidistant from one another. In further examples, pores and/or channels etched into the wafer are substantially the same shape, although, in other examples, some of the pores and/or channels have differing shapes.

[0031] The electrical properties of the etched area are then altered by the addition of doping materials. In at least one embodiment, known doping methods are used to alter the electrical properties of the etched pores or channels. See, for example, Deep Diffusion Doping of Macroporous Silicon, published by E. V. Astrova et al. of the A.F. Ioffe Physico-Technical Institute, Russian Academy of Sciences—St. Petersburg in December 1999 and March 2000, each of which is incorporated herein by reference. In one process, the wafer is doped by applying atoms of other elements to the etched areas. In some embodiments, the added elements have at least one electron more than silicon and are called p-type (e.g., boron). In further embodiments, the added elements have at least one electron less than silicon and are called n-type (e.g., phosphorous).

[0032] An existing classification scheme divides relative silicon pore sizes in semiconductors into three basic classes, viz., nanoporous, mesoporous and macroporous. Nanoporous silicon contains pore sizes in the nanometer (10⁻⁹-meters) range. According to one example embodiment, the invention is practiced using appropriate materials having pore sizes within any of the aforementioned size ranges, (e.g., nanometer-sized structures such as carbon nanotubes), or using a quantum wire of radioactive atoms strung in a polymer chain inserted into a pore slightly larger than the chain.

[0033] In one specific example embodiment of the invention, a silicon formation is used in which an individual pore throat diameter is greater than about 1 nm and less than about 500 μm. In a more specific example embodiment, a pore throat having a diameter of greater than about 1 nm and less than about 100 μm is formed. In a still more specific example embodiment, a pore having a throat diameter of between about 1 nm and about 70 μm is formed.

[0034] In some examples, the pore depth extends through the entire thickness of a semiconductor wafer. In such examples, the junction regions of the pores are interconnected by a variety of means that will occur to those of skill in the art (e.g., exterior wire-bond connection, metalization deposits on the wafer, and/or conductive layers within the wafer itself).

[0035] In a further embodiment, a series of channels are formed in the wafer wherein a width of the channels is on the order of a micron. For example, in one embodiment of the invention, a channel having a throat width of greater than about 1 nm and less than about 500 μm is formed. In a more specific example embodiment, a pore throat diameter of greater than about 1 μm and less than about 100 μm is formed. In a still more specific example embodiment, a channel having a throat width of about 70 μm is formed.

[0036] According to a further example embodiment, preparation of appropriate silicon wafers 6a and 6b (see FIG. 3) is performed using known doping techniques. In one example, pore or channel array 8 is etched into the bodies of wafers 6a and 6b, and then doped to form a plurality of junction regions 9 on the inner wall surfaces of etched pores or channels 8. The porous wafers 6a and 6b are assembled into an enclosed cell 11, in one example, by adhering the two wafer portions onto opposite sides of a conductive ring 6c. A volume of radioactive material 10 (e.g., tritium gas) is introduced into enclosed cell 11.

[0037] In a further embodiment, the risk of a chemical reaction between oxygen and tritium is reduced by removal of oxygen from the cell prior to the insertion of tritium. In at least one example, the interior contents of the cell are evacuated through evacuation port 3, which is then sealed. A radioactive material 10 is then fed into cell 11 through a fill pipe 4; thereafter, fill pipe 4 is sealed. In further example embodiments, cell 11 is purged via evacuation port 3 using an inert gas (e.g., N₂ or argon) prior to introduction of radioactive material 10.

[0038] In some embodiments, enclosed cell 11 is disposed within a housing 1 that prevents radioactive emissions from escaping from the package. For example, certain embodiments of housing 1 comprise a metal, or a ceramic, or another suitable material constructed so as to provide rigorous containment.

[0039] Referring again to an example embodiment shown in FIG. 3, a metal canister 1 is pierced on one side by an insulated feed-through 2, which includes a first electrode 5a disposed in conductive contact with n-type material 14. Metallic outer surfaces of canister 1 serve as a second electrode 5b disposed in conductive communication with p-type material 6a and 6b. Connections 5a and 5b permit current generated within the cell to be transmitted to an external device (not shown) via electrode 5a. In a more specific embodiment, cell canister 1 is enclosed within the body of a durable outer container 12 in a manner similar to existing chemical batteries. In one example embodiment, cell canister 1 is disposed within a thermoplastic shell 12a such that only electrode 5a is exposed; thermoplastic shell 12a is then snugly fitted into metallic outer canister 12b such that only electrode 5a protrudes through the body of metallic outer canister 12b to permit electrical connection with an external device (not shown). In still further embodiments, two or more unit cells 11 are connected either in series or in parallel, again to achieve desired current and voltage characteristics, and then packaged in a single housing as described above; in still further embodiments, two or more individual unit cells 11 are packaged in individual housings, and electrically connected either in series or in parallel to obtain desired voltage and current characteristics.

[0040] As mentioned above, in at least some examples in which tritium gas 10 is deposited within the cell 11, the emitted charged particles are beta electrons. Beta electrons have a relatively low penetrating power. Accordingly, in at least one example, outer canister 1 is formed from a thin sheet of metallic foil, which prevents penetration of energetic particles emitted during the decay process. Thus, the possibility of radioactive energy escaping from the package is reduced. Moreover, tritium is a form of hydrogen, and the uptake of hydrogen gas by the human body is naturally very limited, even in lung tissue, since gaseous hydrogen cannot be directly metabolized. Therefore, fabrication precautions relate primarily to ventilation and dilution in the event of an inadvertent release of the tritium into the external environment.

[0041] In other example embodiments, other fluid or solid radioactive materials that emit alpha and/or gamma particles are deposited within the cell, for example, ⁶³Ni or ²⁴¹Am. In such embodiments, other containment materials and fabrication precautions are employed, and vary depending upon the precise characteristics of the radioactive material used in a particular application.

[0042] Turning now to an even more specific example embodiment, FIG. 4 shows a pore array formed within a macroporous silicon cell for generating electrical current from the decay process of tritium gas is shown. As seen, a 3×3 array of circles represents a sectional view of a few cylindrical pores 8 etched into the silicon wafers 6a or 6b (as shown in FIG. 3). Cylindrical pores 8 (which, in further examples of the invention, are instead formed into multifaceted shapes, e.g., octagonal and/or hexagonal) are separated by about 100 μm in both the horizontal and vertical directions. The diameter of the pore throats is about 70 μm. The annular shading (extending to about an 80 μm diameter) indicates a junction region 9 formed by a p-n junction. Therefore, the volume fraction occupied by the pore channels in this particular example embodiment is about 0.385. Since there are approximately 8.98×10¹⁰ beta decay events per second in a 1 cm³ volume of tritium gas in atmospheric pressure at 20° Celsius, and it takes approximately 3.2 eV to create an electron/hole pair in silicon, a current of about 19.7×10⁻⁶ amperes is generated per cubic centimeter of silicon wafer, thereby assuring a conversion efficiency of about 100%.

[0043] In still further embodiments of the invention, further radioactive materials (e.g., a liquid ⁶³Ni solution) and/or further semiconductors (e.g., germanium, silicon-germanium composite, or gallium arsenide) and/or other materials capable of forming appropriate junction regions are employed. Other methods of forming pores and channels, and other pore and channel shapes and patterns, are used in still further example embodiments. Actual dopants of the semiconductor, and related methods of doping, also vary in other example embodiments, and are not limited to those recited above.

[0044] The foregoing is provided for illustrative purposes only, and is not intended to describe all possible aspects of the present invention. Moreover, while the invention has been shown and described in detail with respect to several exemplary embodiments, those of ordinary skill in the pertinent arts will appreciate that minor changes to the description, and various other modifications, omissions and additions may also be made without departing from either the spirit or scope thereof.