Patent Grant
6949865
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).
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.
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.
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.
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.
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.
As seen in prior art
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.
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.
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.
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.
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.
Referring now to
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.
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
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.
Referring now to an example embodiment shown in
Referring now to an example embodiment shown in
While
Referring still to an example embodiment shown in
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.
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.
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.
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).
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.
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.
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).
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.
According to a further example embodiment, preparation of appropriate silicon wafers 6a and 6b (see
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.
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.
Referring again to an example embodiment shown in
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.
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.
Turning now to an even more specific example embodiment,
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.
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.
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.
| Number | Name | Date | Kind |
|---|---|---|---|
| 2517120 | Linder | Aug 1950 | A |
| 2666814 | Shockley | Jan 1954 | A |
| 2672528 | Shockley | Mar 1954 | A |
| 2745973 | Rappaport | May 1956 | A |
| 2758225 | Annis et al. | Aug 1956 | A |
| 2819414 | Sherwood et al. | Jan 1958 | A |
| 2847585 | Christian | Aug 1958 | A |
| 2864012 | Thomas et al. | Dec 1958 | A |
| 2900535 | Thomas | Aug 1959 | A |
| 2976433 | Rappaport et al. | Mar 1961 | A |
| 3094634 | Rappaport | Jun 1963 | A |
| 3257570 | Dehmelt et al. | Jun 1966 | A |
| 3714474 | Hoff, Jr. | Jan 1973 | A |
| 3934162 | Adler et al. | Jan 1976 | A |
| 4024420 | Anthony et al. | May 1977 | A |
| 4026726 | Carney | May 1977 | A |
| 5082505 | Cota et al. | Jan 1992 | A |
| 5396141 | Jantz et al. | Mar 1995 | A |
| 5529950 | Hoenlein et al. | Jun 1996 | A |
| 5605171 | Tam | Feb 1997 | A |
| 5606213 | Kherani et al. | Feb 1997 | A |
| 5607519 | Krepak | Mar 1997 | A |
| 5642014 | Hillenius | Jun 1997 | A |
| 5721462 | Shanks | Feb 1998 | A |
| 5859484 | Mannik et al. | Jan 1999 | A |
| 5997713 | Beetz, Jr. et al. | Dec 1999 | A |
| 6118204 | Brown | Sep 2000 | A |
| 6204087 | Parker et al. | Mar 2001 | B1 |
| 6238812 | Brown et al. | May 2001 | B1 |
| 6479920 | Lal et al. | Nov 2002 | B1 |
| 6774531 | Gadeken | Aug 2004 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20040150229 A1 | Aug 2004 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10356411 | Jan 2003 | US |
| Child | 10373914 | US |
