U.S. patent application number 10/356411 was filed with the patent office on 2004-08-05 for apparatus and method for generating electrical current from the nuclear decay process of a radioactive material.
Invention is credited to Gadeken, Larry.
Application Number | 20040150290 10/356411 |
Document ID | / |
Family ID | 32770798 |
Filed Date | 2004-08-05 |
United States Patent
Application |
20040150290 |
Kind Code |
A1 |
Gadeken, Larry |
August 5, 2004 |
APPARATUS AND METHOD FOR GENERATING ELECTRICAL CURRENT FROM THE
NUCLEAR DECAY PROCESS OF A RADIOACTIVE MATERIAL
Abstract
An apparatus and method for generating electrical power from the
decay process of a radioactive material is disclosed, wherein a
volume of radioactive material and a junction region are enclosed
in a cell. The junction region is formed by appropriate
construction of a number of p-type and n-type dopant sites. At
least a portion of one of the junction regions is disposed within a
porous region having an aspect ratio of greater than about 20:1,
and disposed at an angle of greater than about 55.degree. measured
relative to the surface area in which it is formed. The dimensions
and shapes of the macroporous regions and the improved junction
region surface area available for collecting charged particles
emitted during a radioactive decay series permit an improved
current to be derived from the apparatus than would otherwise be
expected given its external dimensions.
Inventors: |
Gadeken, Larry; (Houston,
TX) |
Correspondence
Address: |
RAYMOND R. FERRERA
INTELLECTUAL PROPERTY SERVICES
2502 LIVELY LANE
SUGAR LAND
TX
77479
US
|
Family ID: |
32770798 |
Appl. No.: |
10/356411 |
Filed: |
January 31, 2003 |
Current U.S.
Class: |
310/303 ;
429/5 |
Current CPC
Class: |
G21H 1/06 20130101 |
Class at
Publication: |
310/303 ;
429/005 |
International
Class: |
H01M 014/00; G21H
001/00 |
Claims
I claim:
1. An apparatus for generating electrical current from a nuclear
decay process of a radioactive material, 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.degree. relative to a second portion of said junction
region.
2. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 1, wherein said
enclosed volume of radioactive material further comprises beta
particles emitted during said nuclear decay process.
3. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 1, wherein said
enclosed volume of radioactive material further comprises alpha
particles emitted during said nuclear decay process.
4. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 1, wherein said
enclosed volume of radioactive material further comprises gamma
particles emitted during said nuclear decay process.
5. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 1, wherein said
enclosed volume of radioactive material further comprises a gaseous
material.
6. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 5, wherein said
gaseous material further comprises a tritium gas.
7. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 1, wherein said
enclosed volume of radioactive material further comprises a liquid
material.
8. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 7, wherein said
liquid material further comprises a .sup.63Ni solution.
9. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 1, wherein said
enclosed volume of radioactive material further comprises a solid
material.
10. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 1, wherein said
first portion of said junction region is formed within at least one
pore formed within a macroporous region of a semiconductor.
11. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 10, wherein said
at least one pore formed within said macroporous region has a
curved shape.
12. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 11, wherein a
throat opening of said at least one pore has a diameter of less
than about a mean free path length of a beta particle emitted from
said radioactive material.
13. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 11, wherein a
throat opening of said at least one pore has a diameter of greater
than about 1 .mu.m and less than about 500 .mu.m.
14. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 11, wherein a
throat opening of said at least one pore has a diameter of greater
than about 10 .mu.m and less than about 100 .mu.m.
15. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 11, wherein a
throat opening of said at least one pore has a diameter of about 70
.mu.m.
16. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 10, wherein said
at least one pore formed within said macroporous region has a
multifaceted shape.
17. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 16, wherein a
throat opening of said at least one pore has a diameter of less
than about a mean free path length of a beta particle emitted from
said radioactive material.
18. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 16, wherein a
throat opening of said at least one pore has a diameter of greater
than about 1 .mu.m and less than about 500 .mu.m.
19. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 10, wherein a
length of said at least one pore terminates within a body portion
of said semiconductor.
20. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 10, wherein a
length of said at least one pore extends entirely through a body
portion of said semiconductor.
21. An apparatus for generating electrical current from a nuclear
decay process of a radioactive material, the apparatus comprising:
a volume of radioactive material enclosed in a bulk silicon
material; and a junction region disposed within at least one pore
formed within a body portion of said bulk silicon material, wherein
said at least one pore has an aspect ratio of greater than about
20:1.
22. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 21, wherein said
at least one pore has an aspect ratio of greater than about
30:1.
23. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 21, wherein said
enclosed volume of radioactive material further comprises beta
particles emitted during said nuclear decay process.
24. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 21, wherein said
enclosed volume of radioactive material further comprises alpha
particles emitted during said nuclear decay process.
25. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 21, wherein said
enclosed volume of radioactive material further comprises gamma
particles emitted during said nuclear decay process.
26. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 21, wherein said
enclosed volume of radioactive material further comprises a gaseous
material.
27. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 26, wherein said
gaseous material further comprises a tritium gas.
28. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 21, wherein said
enclosed volume of radioactive material further comprises a liquid
material.
29. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 28, wherein said
liquid material further comprises a .sup.63Ni solution.
30. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 21, wherein said
enclosed volume of radioactive material further comprises a solid
material.
31. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 21, wherein said
at least one pore formed within a body portion of said bulk silicon
material has a curved shape.
32. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 31, wherein a
throat opening of said at least one pore has a diameter of less
than about a mean free path length of a beta particle emitted from
said radioactive material.
33. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 31, wherein a
throat opening of said at least one pore has a diameter of greater
than about 1 .mu.m and less than about 500 .mu.m.
34. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 31, wherein a
throat opening of said at least one pore has a diameter of greater
than about 10 .mu.m and less than about 100 .mu.m.
35. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 31, wherein a
throat opening of said at least one pore has a diameter of about 70
.mu.m.
36. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 21, wherein said
at least one pore formed within the body of said bulk silicon
material has a multifaceted shape.
37. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 36, wherein a
throat opening of said at least one pore has a diameter of less
than about a mean free path length of a beta particle emitted from
said radioactive material.
38. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 36, wherein a
throat opening of said at least one pore has a diameter of greater
than about 1 .mu.m and less than about 500 .mu.m.
39. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 21, wherein a
length of said at least one pore terminates within said body
portion of said bulk silicon material.
40. The apparatus for generating electrical current from a nuclear
decay process of a radioactive material of claim 21, wherein a
length of said at least one pore extends entirely through said body
portion of said bulk silicon material.
41. A method for generating electrical current from a nuclear decay
process of a radioactive material, the method comprising: enclosing
a volume of radioactive material; 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.degree. relative to a second portion of said junction
region.
42. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 41, the method
further comprising: enclosing a volume of radioactive material that
emits beta particles during said nuclear decay process.
43. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 41, the method
further comprising: enclosing a volume of radioactive material that
emits alpha particles during said nuclear decay process.
44. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 41, the method
further comprising: enclosing a volume of radioactive material that
emits gamma particles during said nuclear decay process.
45. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 41, the method
further comprising: enclosing a volume of gaseous radioactive
material.
46. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 45, the method
further comprising: enclosing a volume of tritium gas.
47. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 41, the method
further comprising: enclosing a volume of liquid radioactive
material.
48. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 47, the method
further comprising: enclosing a volume of liquid .sup.63Ni
solution.
49. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 41, the method
further comprising: enclosing a volume of solid radioactive
material.
50. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 41, the method
further comprising: forming at least one pore in a macroporous
region of a semiconductor; and disposing said first junction region
within said at least one pore.
51. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 50, the method
further comprising: forming said at least one pore into a curved
shape.
52. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 51, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of less than about a mean free path
length of a beta particle emitted from said radioactive material is
obtained.
53. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 51, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of greater than about 1 .mu.m and
less than about 500 .mu.m is obtained.
54. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 51, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of greater than about 10 .mu.m and
less than about 100 .mu.m is obtained.
55. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 51, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of about 70 .mu.m is obtained.
56. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 50, the method
further comprising: forming said at least one pore into a
multifaceted shape.
57. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 56, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of less than a mean free path length
of a beta particle emitted from said radioactive material is
obtained.
58. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 56, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of about greater than about 1 .mu.m
and less than about 500 .mu.m is obtained.
59. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 50, the method
further comprising: forming a length of said at least one pore so
that said length terminates within a body portion of said
semiconductor.
60. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 50, the method
further comprising: forming a length of said at least one pore so
that said length extends entirely through a body portion of said
semiconductor.
61. 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 bulk silicon material;
forming at least one pore within a body portion 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.
62. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 61, the method
further comprising: forming said at least one pore so that said at
least one pore has an aspect ratio of greater than about 30:1.
63. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 61, the method
further comprising: enclosing a volume of radioactive material that
emits beta particles during said nuclear decay process.
64. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 61, the method
further comprising: enclosing a volume of radioactive material that
emits alpha particles during said nuclear decay process.
65. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 61, the method
further comprising: enclosing a volume of radioactive material that
emits gamma particles during said nuclear decay process.
66. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 61, the method
further comprising: enclosing a volume of gaseous radioactive
material.
67. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 66, the method
further comprising: enclosing a volume of tritium gas.
68. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 61, the method
further comprising: enclosing a volume of liquid radioactive
material.
69. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 68, the method
further comprising: enclosing a volume of liquid .sup.63Ni
solution.
70. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 61, the method
further comprising: enclosing a volume of solid radioactive
material.
71. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 61, the method
further comprising: forming said at least one pore into a curved
shape.
72. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 71, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of less than about a mean free path
length of a beta particle emitted from said radioactive material is
obtained.
73. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 71, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of greater than about 1 .mu.m and
less than about 500 .mu.m is obtained.
74. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 71, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of greater than about 10 .mu.m and
less than about 100 .mu.m is obtained.
75. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 71, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of about 70 .mu.m is obtained.
76. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 61, the method
further comprising: forming said at least one pore into a
multifaceted shape.
77. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 76, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of less than about a mean free path
length of a beta particle emitted from said radioactive material is
obtained.
78. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 76, the method
further comprising: forming a throat opening of said at least one
pore so that a throat diameter of greater than about 1 .mu.m and
less than about 500 .mu.m is obtained.
79. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 61, the method
further comprising: forming a length of said at least one pore so
that said length terminates within a body portion of said bulk
silicon material.
80. The method for generating electrical current from a nuclear
decay process of a radioactive material of claim 61, the method
further comprising: forming a length of said at least one pore so
that said length extends entirely through a body portion of said
bulk silicon material.
Description
BACKGROUND
[0001] 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).
[0002] Radioactive materials randomly emit charged particles from
their atomic nuclei. Examples are alpha particles (i.e., .sup.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.
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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
.OMEGA. completes the circuit such that positive charges 6 and
negative charges 7 recombine, power is generated by the induced
current flow.
[0008] 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.
[0009] 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.
[0010] 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., all of which are incorporated herein by reference.
None of these nuclear batteries have been developed commercially
for practical applications.
BRIEF SUMMARY OF THE INVENTION
[0011] 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.degree. 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.
[0012] 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.degree. 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.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic representation of the electrical
current generation process achieved by emission of a charged
particle from a nucleus of a confined mass of radioactive material
as is known in the prior art.
[0014] FIG. 2A is a schematic representation of an example
embodiment of the present invention.
[0015] FIG. 2B is a sectional view of an example embodiment of the
present invention.
[0016] FIG. 2C is a sectional view of an example embodiment of the
present invention.
[0017] FIG. 3 is a schematic representation of an example
embodiment of the present invention.
[0018] FIG. 4 is a sectional view of an example embodiment of the
present invention.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION
[0019] 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 .OMEGA. via a
low-resistivity contact surface region 30 (e.g., a metal, for
example, aluminum). A second low-resistivity contact surface region
disposed between surface plane 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 .OMEGA..
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 30 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 and migrate towards junction region 20 and contact surface
region 30, thereby inducing current flow through load .OMEGA..
[0020] The maximum travel distance of the most energetic tritium
beta particle in silicon is about 4.33 .mu.m; 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 .mu.m. 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.
[0021] 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 .theta. of greater
than about 55.degree. (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 .theta. of about 90.degree. 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.
[0022] 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.
[0023] 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.
[0024] 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).
[0025] 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.
[0026] 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.).
[0027] 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, .sup.63Ni), which
thereafter decays when irradiated or otherwise excited by
appropriate means.
[0028] 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.
[0029] 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.
[0030] 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).
[0031] 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.sup.-9-meters) range.
[0032] In one specific example embodiment of the invention, a
macroporous silicon formation is used in which an individual pore
throat diameter is on the order of a micron (10.sup.-6-meters), for
example, greater than about 1 .mu.m and less than about 500 .mu.m.
In a more specific example embodiment, a pore throat having a
diameter of greater than about 10 .mu.m and less than about 100
.mu.m is formed. In a still more specific example embodiment, a
pore having a throat diameter of about 70 .mu.m is formed.
[0033] 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).
[0034] 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 .mu.m and less than
about 500 .mu.m is formed. In a more specific example embodiment, a
pore throat diameter of greater than about 10 .mu.m and less than
about 100 .mu.m is formed. In a still more specific example
embodiment, a channel having a throat width of about 70 .mu.m is
formed.
[0035] 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.
[0036] 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., N.sub.2 or argon) prior
to introduction of radioactive material 10.
[0037] 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.
[0038] 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.
[0039] 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.
[0040] In other example embodiments, other fluid or solid
radioactive materials that emit alpha and/or gamma particles are
deposited within the cell, for example, .sup.63Ni or .sup.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.
[0041] 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.times.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 .mu.m in both the horizontal and vertical
directions. The diameter of the pore throats is about 70 .mu.m. The
annular shading (extending to about an 80 .mu.m 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.times.10.sup.10 beta decay events per second in a 1 cm.sup.3
volume of tritium gas in atmospheric pressure at 20.degree.
Celsius, and it takes approximately 3.2 eV to create an
electron/hole pair in silicon, a current of about
19.7.times.10.sup.-6 amperes is generated per cubic centimeter of
silicon wafer, thereby assuring a conversion efficiency of about
100%.
[0042] In still further embodiments of the invention, further
radioactive materials (e.g., a liquid .sup.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.
[0043] 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.
* * * * *