U.S. patent number 3,706,893 [Application Number 04/859,266] was granted by the patent office on 1972-12-19 for nuclear battery.
This patent grant is currently assigned to McDonnel Douglas Corporation. Invention is credited to Charles J. Ambrose, Bobby I. Griffin, Larry C. Olsen, Stephen E. Seeman.
United States Patent |
3,706,893 |
Olsen , et al. |
December 19, 1972 |
NUCLEAR BATTERY
Abstract
Nuclear battery including one or more cells each comprising a
radioactive fuel element or source and a semiconductor element
positioned contiguously to the source and irradiated by it. The
fuel element includes a radioactive material which is preferably
promethium-147 metal or its oxide, promethia, and the semiconductor
element includes a N.sup.+/P or N.sup.+/P/P.sup.+ semiconductor
wafer which is preferably silicon. The semiconductor wafer has an
energy threshold of radiation damage which is compatible with the
maximum energy of the nuclear particles or radiation emitted by the
radioactive material, to provide a long-life (minimal radiation
damage) cell of optimum power output. Other versions include a
nuclear battery utilizing a bi-directional fuel element, and
certain compact and useful embodiments utilizing a multiple section
cell therein.
Inventors: |
Olsen; Larry C. (Richland,
WA), Seeman; Stephen E. (Kennewick, WA), Griffin; Bobby
I. (Richland, WA), Ambrose; Charles J. (Richland,
WA) |
Assignee: |
McDonnel Douglas Corporation
(N/A)
|
Family
ID: |
25330453 |
Appl.
No.: |
04/859,266 |
Filed: |
September 19, 1969 |
Current U.S.
Class: |
310/303; 136/253;
976/DIG.413 |
Current CPC
Class: |
G21H
1/06 (20130101) |
Current International
Class: |
G21H
1/00 (20060101); G21H 1/06 (20060101); G21d
007/00 () |
Field of
Search: |
;310/3,3A,3B
;317/235 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Bennett, Jr.; Rodney D.
Assistant Examiner: Potenza; J. M.
Claims
We claim:
1. In a nuclear battery, a cell comprising:
a fuel element including a radioactive source of nuclear particles
having a known maximum energy, said radioactive source comprising a
source of beta particles; and
a semiconductor element positioned in at least close proximity to
said fuel element and irradiated by the same, said semiconductor
element including a semiconductor wafer and relatively thin,
electrically conductive, distributed ohmic contact members provided
on respective faces of said wafer, and said wafer comprising a
system of at least a high carrier concentration first-type layer
and a lower carrier concentration second-type layer of
semiconductor material having respectively different
electrochemical potentials and having an energy threshold level of
radiation damage compatible with, and at least of the same order
as, the maximum energy of said nuclear particles, said layers
having a junction therebetween and said fuel element being
positioned operatively close to said junction adjacent to said high
carrier concentration first-type layer, at least said ohmic contact
member adjacent to said fuel element providing a relatively large
fractional open area for passage therethrough of said nuclear
particles whereby a long life cell of maximum output voltage and
high power output is obtained.
2. The invention as defined in claim 1 wherein said fuel element
includes a unitary radioactive source member, and said wafer system
further comprises a high carrier concentration layer of said
second-type electrochemical potential material, said latter layer
serving primarily as a reflector barrier whereby a long life cell
of maximum output voltage and high power output is obtained.
3. The invention as defined in claim 2 wherein said source member
includes a backing disc having a layer of radioactive material
provided on a surface thereof and which is a promethium source of
beta particles, and said system includes a silicon system of
N.sup.+ -type, P-type and P.sup.+ -type layers of different
electrochemical potential material and having carrier
concentrations of the order of approximately 10.sup.19, 10.sup.16
and 10.sup.19 atoms/cm.sup.3, respectively, said N.sup.+ -type and
P-type layers having said junction therebetween, said P.sup.+ -type
layer serving primarily as said reflector barrier and said fuel
element being positioned operatively close to said junction
adjacent to said N.sup.+ -type layer.
4. The invention as defined in claim 1 wherein said fuel element
includes a bi-directional source which is a unitary radioactive
disc member comprising promethia and a matrix to hold and support
said promethia, and said semiconductor element is positioned in at
least close proximity to one side of said bi-directional source,
and further comprising another similar semiconductor element
positioned symmetrically to the other side of said bi-directional
source.
5. The invention as defined in claim 2 wherein said fuel element is
of a planar form, and said semiconductor element is of a
correspondingly similar planar form and having multiple sections
which are insulated from each other and operatively connected in at
least a partially series arrangement to provide an output voltage
higher than that available from a commensurate cell having a
unitary semiconductor element.
6. The invention as defined in claim 2 wherein said fuel element is
of a planar form, and said semiconductor element is of a laminar
form having laterally disposed multiple sections with planes
oriented perpendicularly to the plane of said fuel element, said
sections being operatively coupled in series to provide an output
voltage higher than that available from a commensurate cell having
a unitary semiconductor element.
7. The invention as defined in claim 1 wherein said fuel element is
of an elongated and hollow cylindrical form, and said semiconductor
element is of a correspondingly elongated and solid cylindrical
form positioned concentrically within said fuel element and having
axially disposed multiple sections which are insulated from each
other and operatively connected in at least a partially series
arrangement to provide an output voltage higher than that available
from a commensurate cell having a unitary semiconductor
element.
8. The invention as defined in claim 1 wherein said fuel element
includes a unitary radioactive source member comprising a backing
disc having a layer of radio-active material provided on a surface
thereof and which is a promethium source of beta particles, and
said system includes a silicon system of N.sup.+ -type and P-type
layers of different electrochemical potential material and having
carrier concentrations of the order of approximately 10.sup.19 and
10.sup.16 atoms/cm.sup.3, respectively, said N.sup.+ -type and
P-type layers having said junction therebetween and said fuel
element being positioned operatively close to said junction
adjacent to said N.sup.+ -type layer.
9. A nuclear battery comprising:
a plurality of cells arranged in a series stack, each of said cells
including
a fuel element comprising a radioactive source of nuclear particles
having a known maximum energy, said fuel element including a
unitary radioactive source member, and
a semiconductor element positioned in at least close proximity to
said fuel element and irradiated by the same, said semiconductor
element comprising a semiconductor wafer and relatively thin,
electrically conductive, distributed ohmic contact members provided
on respective faces of said wafer, and said wafer including a
system of at least a high carrier concentration first-type layer
and a lower carrier concentration second-type layer of
semiconductor material having respectively different
electrochemical potentials and having an energy threshold level of
radiation damage compatible with, and at least of the same order
as, the maximum energy of said nuclear particles, said layers
having a junction therebetween and said fuel element being
positioned operatively close to said junction adjacent to said high
carrier concentration first-type layer;
a plurality of electrically conductive ring members corresponding
in number to said cells and positioned concentrically about said
fuel elements, respectively;
electrically conductive end members sandwiching said cells of said
stack, said ring members axially contacting said end members and
said ohmic contact members of said wafers and providing a series
connection from one of said end members through said semiconductor
elements of said cells to the other of said end members;
spring means for biasing and maintaining said end members and cells
of said stack in effective series contact throughout the same;
and
a pair of electrode terminals adapted to be connected respectively
to said end members, whereby a long life battery of maximum output
voltage and high power output is obtained.
10. A nuclear battery comprising:
a fuel element including a radioactive source of nuclear particles
having a known maximum energy, said fuel element being of a planar
form; and
a semiconductor element positioned in at least close proximity to
said fuel element and irradiated by the same, said semiconductor
element being of a correspondingly similar planar form to said fuel
element and including multiple sections which are insulated from
each other and electrical leads operatively connecting said
sections in at least a partially series arrangement to provide a
higher output voltage therefrom, each of said sections comprising a
semiconductor wafer and relatively thin, electrically conductive,
distributed ohmic contact members provided on respective faces of
said wafer and connecting with said leads, and said wafer including
a system of at least a high carrier concentration first-type layer
and a lower carrier concentration second-type layer of
semiconductor material having respectively different
electrochemical potentials and having an energy threshold level of
radiation damage compatible with, and at least of the same order
as, the maximum energy of said nuclear particles, said layers
having a junction therebetween and said fuel element being
positioned operatively close to said junction adjacent to said high
carrier concentration first-type layer whereby a long life battery
of maximum output voltage and high power output is obtained.
11. The invention as defined in claim 9 wherein said source member
includes a backing disc having a layer of radioactive material
provided on a surface thereof and which is a promethium source of
beta particles, at least said ohmic contact member adjacent to said
fuel element provides a relatively large fractional open area for
free passage therethrough of said nuclear particles, said system
includes a silicon system of at least N.sup.+ -type and P-type
layers of different electrochemical potential material and having
carrier concentrations of the order of approximately 10.sup.19 and
10.sup.16 atoms/cm.sup.3, respectively, said N.sup.+ -type and
P-type layers having said junction therebetween and said fuel
element being positioned operatively close to said junction
adjacent to said N.sup.+ -type layer, and said spring means
includes an electrically conductive spring ring for axially biasing
uniformly over a wide base against one of said end members of said
stack, and further comprising a shielding container for containing
and primarily providing radiation protection containment of said
stack, and a fire container for containing said shielding container
and primarily providing high temperature protection containment of
said stack, one of said electrode terminals being mounted on and
insulated from said fire container and said spring ring
electrically connecting the one of said end members to said
shielding container which is in electrical contact with said fire
container serving as the other of said electrode terminals.
12. The invention as defined in claim 10 wherein said fuel element
includes a unitary radioactive source member which is a promethium
source of beta particles, at least said ohmic contact member
adjacent to said fuel element provides a relatively large
fractional open area for free passage therethrough of said nuclear
particles, and said system includes a silicon system of at least
N.sup.+ -type and P-type layers of different electrochemical
potential material and having carrier concentrations of the order
of approximately 10.sup.19 and 10.sup.16 atoms/cm.sup.3,
respectively, said N.sup.+ -type and P-type layers having said
junction therebetween and said fuel element being positioned
operatively close to said junction adjacent to said N.sup.+ -type
layer.
Description
BACKGROUND OF THE INVENTION
Our present invention pertains generally to the field of batteries
and more particularly to a nuclear battery wherein nuclear energy
is usefully converted into electrical energy.
Generally, prior radioactive or nuclear batteries which involve the
concept of coupling a radioactive source with one or more
semiconductor elements have utilized a source of high energy
radiation to irradiate the semiconductor elements that each include
a NP junction therein. The radiation damage created by the high
energy particles or radiation in these prior batteries is so great,
however, that their useful lifetimes are so short as to render such
batteries to be of questionable value.
Further, the power density, maximum output power, maximum output
voltage and device efficiencies are relatively quite low for the
prior radioactive or nuclear batteries. The various known concepts
of radioactive or nuclear batteries do not in most instances appear
to be particularly feasible, and all of such concepts are either
clearly impractical or are of little useful value. It is, of
course, well known that a general purpose nuclear battery other
than this invention and of a similar category is not presently and
readily available on the commercial market.
SUMMARY OF THE INVENTION
Briefly, and in general terms, our invention is preferably
accomplished by providing a nuclear battery including one or more
cells each comprising a radioactive fuel element or source and a
semiconductor element positioned in close proximity or contiguously
to the source and irradiated by it.
The semiconductor element is made of a material which has an energy
threshold level of radiation damage that is compatible with the
maximum energy of the nuclear particles or radiation emitted by the
radioactive source. The fuel element includes a radioactive
material which is preferably promethium-147 (Pm.sup.147) metal or
its oxide, promethia (Pm.sub.2 O.sub.3), and the semiconductor
element includes a N.sup.+ /P or N.sup.+ /P/P.sup.+ semiconductor
wafer which is preferably silicon (Si), to provide a long-life
(minimal radiation damage) cell of optimum power output. The
semiconductor wafer is preferably characterized by having at least
a N.sup.+ -type layer which has a very high density of conduction
electrons. In the three-layer semiconductor wafer, the P.sup.+
-type layer has a very high density of "holes," of course. In both
instances (N.sup.+ or P.sup.+ ), the carrier density preferably
approaches 10.sup.20 or 10.sup.21 carriers/cm.sup.3 in order that
maximum voltage be obtained.
The radioactive fuel element and the semiconductor element of a
cell are preferably discs of similar sizes. The fuel element is
also preferably positioned virtually against the N.sup.+ -type
layer surface of the semiconductor wafer. The N.sup.+ /P and
N.sup.+ /P/P.sup.+ (silicon) semiconductor wafers are preferred
because they are more resistant to damage caused by (beta)
radiation. However, P.sup.+ /N and P.sup.+ /N/N.sup.+
semiconductors (N-type semiconductor stock) can, of course, be used
satisfactorily in this invention.
In the N.sup.+ /P semiconductor wafer, the highly doped N.sup.+
-type layer face thereof provides a large number of conduction
electrons which are available to an external circuit or load. In
the N.sup.+ /P/P.sup.+ semiconductor wafer, both faces thereof are
highly doped to have high carrier concentrations such that the
resulting N.sup.+ -type layer and P.sup.+ -type layer faces prevent
any bending of the electron bands in the wrong sense and thus
generate an electromotive force of the incorrect polarity. These
highly doped surfaces essentially "lock" the electron bands in
place whereby the semiconductor surfaces can be treated in any
manner desired without degrading device performance. In such an
arrangement, ohmic contacts can be easily made to the semiconductor
faces and, in fact, any adjacent oxide layers formed thereon will
not alter the electron band structure to affect the performance of
the device. The resulting nuclear batteries can be manufactured
with power levels covering a wide range and have long lifetimes (of
over three years) with high power densities (of one
milliwatt/cm.sup.3 or more).
Other versions of our invention include a two-cell nuclear battery
utilizing a bi-directional fuel element or source, a planar
multiple section cell nuclear battery embodiment, a laminar
multiple section cell nuclear battery configuration and an
elongated, cylindrical multiple section cell nuclear battery
embodiment. The nuclear battery configurations including a multiple
section cell therein are particularly compact and practical forms
of our invention, and can supply electrical power at voltages much
greater than that available from an equivalent single cell which
utilizes only one active area.
BRIEF DESCRIPTION OF THE DRAWINGS
Our invention will be more fully understood, and other features and
advantages thereof will become apparent, from the following
description of certain illustrative embodiments of the invention.
The description is to be taken in conjunction with the accompanying
drawings, in which:
FIG. 1 is a central sectional and elevational view, perspectively
shown, of a nuclear battery constructed in accordance with our
invention;
FIGS. 2A and 2B are enlarged fragmentary and central sectional
views of a fuel element and its concentric ring, and a
semiconductor element, respectively, of the nuclear battery shown
in FIG. 1;
FIG. 3 is an enlarged fragmentary and central sectional view of a
directly fueled nuclear battery cell;
FIG. 4 is an electron energy level diagram for a N.sup.+ /P/P.sup.+
semiconductor device;
FIG. 5 is a circuit diagram showing a resistive load connected to
positive and negative electrodes of a N.sup.+ /P semiconductor
device;
FIG. 6 is a circuit diagram of an equivalent circuit of the
semiconductor device shown in FIG. 5 connected to its resistive
load;
FIG. 7 is a graph illustrating current density and power density
versus output voltage characteristics of a single cell device
constructed according to this invention;
FIGS. 8A, 8B, 8C and 8D are graphs which were plotted from
experimental results and show typical characteristics of
promethia-fueled, silicon semiconductor, betavoltaic single cells
as a function of promethia layer thickness;
FIGS. 9A and 9B are respectively a perspective view of a
bi-directional fuel element or source, and a fragmentary and
enlarged sectional view thereof;
FIGS. 10A and 10B are exploded perspective views of a two-cell
betavoltaic energy converter or battery using the bi-directional
source of FIG. 9A, wherein the cells are connected respectively in
parallel and in series;
FIG. 11 is an elevational view of a N.sup.+ /P semiconductor wafer
having a promethium coating deposited directly onto the N.sup.+
-type layer surface of the wafer;
FIG. 12 is an exploded perspective view of a two-cell betavoltaic
battery having a radioactive material coated directly on at least
one semiconductor wafer of the cells;
FIG. 13 is an exploded perspective view of a highly compact and
practical form of a nuclear battery constructed according to this
invention;
FIG. 14 is a graph illustrating the current and power versus
voltage characteristics of a nuclear battery such as that shown in
FIG. 13;
FIG. 15 is an exploded perspective view of a planar, multiple
section, cell embodiment of a nuclear battery;
FIG. 16 is a partially fragmentary and exploded perspective view of
a laminar, multiple section, cell configuration of a nuclear
battery;
FIG. 17 is an electron energy level diagram for about two sections
of the N.sup.+ /P.sup.+ /P semiconductor device shown in FIG. 15;
and
FIG. 18 is an exploded and partly fragmentary perspective view of
an elongated cylindrical, multiple section, cell embodiment of a
nuclear battery.
DESCRIPTION OF THE PRESENT EMBODIMENTS
FIG. 1 is a central sectional and elevational view, perspectively
shown, of a nuclear battery 30 constructed in accordance with our
invention. The battery 30 broadly includes a radioactive
fuel-semiconductor stack 32, and inner and outer containers 34 and
36 for suitably enclosing and housing the stack. The stack 32
includes lower and upper metallic discs 38 and 40 which sandwich a
number of radioactive fuel elements 42 and semiconductor elements
44 that are contiguously positioned in alternate layers. The
diameter of the fuel elements 42 is smaller than that of the
semiconductor elements 44, such that metallic rings 46 having an
inner diameter larger than that of the fuel elements and an outer
diameter equal to that of the semiconductor elements, can be
positioned concentrically around and radially spaced from the
semiconductor elements in the same corresponding layers
thereof.
The lower metallic disc 38 is in direct contact with the lower fuel
element 42 and its concentric metallic ring 46. The upper metallic
disc 40 is in direct contact with the upper semiconductor element
44. An insulating liner 48 made of radiation resistant rubber or an
oxide, for example, is provided around the sides or circumferential
surface of the stack 32 and over the upper surface of the metallic
disc 40 except for a central area 40a thereof, as shown in FIG. 1.
The liner 48 insulates the stack 32 from the inner sleeve 34a of
inner container 34 which can be fabricated of tantalum (Ta), for
example. The inner container 34 includes an outer sleeve 34b which
is closed at its lower end to support the stack 32 through a
metallic spring ring 50. The spring ring 50 also provides an
electrical connection between the lower disc 38 and the inner
container 34.
The inner container 34 is enclosed by and in direct contact with
the outer container 36 which is preferably a closed stainless steel
fire can. The outer container 36 includes a lower can portion 36a
and an upper cover portion 36b which can be electron beam welded to
the can portion at the juncture 52 thereof. The cover portion 36b
has a central and countersunk hole 54 therein. A small insulating
sleeve 56 of ceramic or glass, for example, is positioned within
the countersunk portion of the hole 54 and supports a closing
metallic disc 58 which is electrically connected by lead 60 to the
central area 40a of the upper metallic disc 40. Glass-to-metal
seals are, of course, suitably provided at the lower and upper ends
of the insulating sleeve 56. Thus, the disc 58 is one (positive)
electrode and the outer container 36 is the other (negative)
electrode of the nuclear battery 30. The battery 30 as shown
illustratively in FIG. 1 is cylindrical in configuration; however,
it obviously can be made in rectangular or other configurations.
The battery 30 has outside or overall dimensions of diameter D and
length L.
FIGS. 2A and 2B are enlarged fragmentary and central sectional
views of a fuel element 42 and its concentric ring 46, and a
semiconductor element 44, respectively. In FIG. 2A, the ring 46 can
be made of aluminum (Al) and can have an outer diameter of
approximately 1 cm, for example. The fuel element 42 includes an
aluminum disc 42a and a thin layer 42b of promethia (Pm.sub.2
O.sub.3) thereon. The ring 46 can have an inner diameter of
approximately 0.95 cm, and the disc 42a can have a diameter of
approximately 0.93 cm and a thickness typically of from 3 to 10
mils. This thickness is primarily for strength and handling. Nickel
(Ni) or tantalum, for example, can be used instead of aluminum. The
disc 42a is preferably sand-blasted on a face first and then the
promethia layer 42b can be provided tenaciously thereon by vapor
deposition. The promethia layer 42b can be provided on the face
with a layer thickness of approximately 5.3 milligram/cm.sup.2, for
example. Promethia is preferably used since it is a good and stable
source of beta particles or radiation. It is, of course, to be
understood that the various types of materials and different
dimensions noted herein are given by way of example only and are
not intended to be restrictive or limiting on the scope of our
invention in any manner.
In FIG. 2B, the semiconductor element 44 has an outer diameter
which is preferably equal to the outer diameter of the ring 46
(FIG. 2A). The semiconductor element 44 includes a N.sup.+ /P
silicon (Si) wafer 44a, a thin metallic upper layer 44b and a thin
metallic lower layer 44c. The wafer 44a can have a thickness of
from 3 to 15 mils, for example. This wafer 44a can be, if desired,
a N.sup.+ /P/P.sup.+ silicon wafer in accordance with this
invention. The upper and lower layers 44b and 44c can be of
aluminum, and the upper layer 44b provides an electrical ohmic
contact from the N.sup.+ -type surface of wafer 44a to the lower
surface of (an upper) ring 46 while the lower layer 44c provides an
electrical ohmic contact from the P-type surface of wafer 44a to
the upper surface of (a lower) ring 46 and, incidentally, to the
upper surface of (a lower) disc 42a. The upper layer 44b is, of
course, sufficiently thin (less than 2 microns thick, for example)
to permit adequate penetration thereof by the beta radiation from
the contiguous promethia layer 42b above it. In one particular
embodiment, the upper layer 44b is actually in the form of a grid
(generally similar to a wire screen) wherein each mesh or enclosed
space allows free passage of the beta particles or radiation
therethrough. The upper and lower layers 44b and 44c can be of
silver (Ag) instead of aluminum, and are preferably vapor deposited
or plated on the wafer 44a. The lower layer 44c can be 1 mil thick
although 2 microns would be adequate.
The thin metallic layer and grid are both distributed or extended
electrode forms which provide good ohmic contact with the full
lower and upper surfaces of the semiconductor wafer 44a. A grid
providing an ohmic contact to a given semiconductor surface (unit
reference) area has a certain fractional ohmic contact area and a
remaining fractional open area for passage of beta particles or
other radiation. A thin metallic layer obviously provides maximum
(lowest resistance) contact over the full semiconductor surface
with, however, a zero fractional open area for free passage of the
beta particles or radiation. Of course, the distributed ohmic
contact area of a grid or any other such extended electrode form
with a given semiconductor surface area must be adequate to achieve
a suitable balance between the fractional open area obtainable for
free passage of the beta particles or radiation commensurate with
maintaining a sufficiently extensive and good (low resistance)
ohmic contact for the entire semiconductor surface area. In this
respect, a minimum distributed ohmic contact area is required with
a highly conductive (high carrier concentration) semiconductor
surface.
In our invention, beta radiation is preferably coupled with either
a N.sup.+ /P or N.sup.+ /P/P.sup.+ silicon wafer. The N.sup.+ /P
silicon wafer is a two-layer silicon system including a N.sup.+
-type layer characterized by a phosphorus (P) concentration of
approximately 10.sup.19 to 10.sup.20 atoms/cm.sup.3 and a P-type
layer characterized by a boron (B) concentration of approximately
10.sup.16 atoms/cm.sup.3 (or a P-type layer having a resistivity of
approximately 1 ohm-cm). The N.sup.+ /P/P.sup.+ silicon wafer can
be produced from a N.sup.+ /P silicon wafer by diffusing a suitable
amount of boron into the lower surface of the P-type layer or
region of the silicon wafer to produce a P.sup.+ -type layer
therein. The N.sup.+ /P/P.sup.+ silicon wafer is a three-layer
silicon system including N.sup.+ -type and P-type layers as in the
above N.sup.+ /P silicon wafer, and a P.sup.+ -type layer
characterized by a boron concentration of approximately 10.sup.19
to 10.sup.20 atoms/cm.sup.3. In both of these types of silicon
wafers, arsenic (As), antimony (Sb) or bismuth (Bi) can be
substituted for phosphorus, and aluminum, gallium (Ga), indium (In)
or thallium (Tl) can be substituted for boron. Germanium (Ge),
cadmium sulfide (CdS) and gallium arsenide (GaAs) semiconductors,
among others, can be used instead of silicon. However, more power
can be obtained from silicon cells than from the others. For
example, roughly five times more power can be usually derived from
a silicon cell than from a generally similar germanium cell.
FIG. 3 is an enlarged fragmentary and central sectional view of a
directly fueled nuclear battery cell 62. A N.sup.+ /P (or N.sup.+
/P/P.sup.+ ) semiconductor wafer 64 of silicon, for example, has
both upper and lower surfaces coated with thin metallic layers 66
and 68, respectively, which can be of aluminum, nickel or silver.
Suitably deposited thereon are respective layers 70 and 72 of a
source of beta radiation material such as promethia or
promethium-147 (Pm.sup.147) metal. The radioactive layers 70 and 72
are smaller in diameter than that of the semiconductor wafer 64, as
illustrated. The radioactive layers 70 and 72 are then fully
covered by outer metallic layers 74 and 76, respectively, which can
be of aluminum or silver and are for electrical contact.
The radially outer circumferential (lower) surface portion of the
upper cover layer 74 contacts the corresponding portion of the thin
metallic layer 66 which is in contact with the surface of the
P-type layer of the semiconductor wafer 64. Similarly, the radially
outer circumferential (upper) surface portion of the lower cover
layer 76 contacts the corresponding portion of the thin metallic
layer 68 which is in contact with the surface of the N.sup.+ -type
layer of the wafer 64. Thus, the upper cover layer 74 is of
positive polarity and the lower cover layer 76 is of negative
polarity for the battery cell element 62. The cell elements 62 can
be readily stacked in series for a higher output voltage and, of
course, the stacks can also be suitably connected in parallel as
may be desired or required.
FIG. 4 is an electron energy level diagram for a N.sup.+ /P/P.sup.+
semiconductor wafer 78. Nuclear-voltaic effects are initiated when
radiation 80 enters the semiconductor 78 and creates electron 82
and hole 84 pairs. A large number of these carriers (electrons and
holes) diffuse to the vicinity of the abrupt (N.sup.+ P) junction
86 where the junction electric field accelerates them to device
terminals 88 and 90. By suitably adjusting the device parameters,
most of the carriers produced are collected under short circuit
conditions. The electrochemical potential or Fermi level of the
device under equibrium conditions is indicated by full line 92, and
broken lines 94 and 96 represent the electrochemical potentials for
electrons and holes, respectively, resulting from electron and hole
pair production of the device under radiation excitation. Brackets
98 shown near the line 92 represent donor impurity atoms which have
donated their excess valence electrons to the conduction band above
the energy gap of the forbidden region. Similarly, brackets 100
shown near the line 92 represent acceptor impurity atoms which have
captured extra electrons from the valence band below the forbidden
energy gap. It can also be seen that the P.sup.+ -type layer acts
as a reflector of the free electrons.
Nuclear-voltaic energy conversion involves collecting electrons and
holes at the terminals of an inhomogeneous semiconductor or
metallic wafer. The electrons and holes result from electron-hole
pair creation by the absorption of nuclear particles within the
material. These charge carriers are accelerated to the device
terminals as a result of an internal electric field which exists
because of the inhomogeneous nature of the medium. Electrons and
holes live longer in semiconductors than in metals. Thus, the most
practical approach to nuclear-voltaic energy converters involves
the use of a semiconductive material for at least part of the
system. For example, a two-electrode system can contain one layer
of semiconductive material and another layer characterized as (1)
the same kind of semiconducting material but with a different
electrochemical potential as can be obtained by doping the layer
differently than the first layer, (2) a different kind of
semiconductor, or (3) a metal. The first approach is, of course,
followed in accomplishing this invention.
Radiation employed in the various embodiments of this invention is
preferably beta nuclear particles supplied by a source of promethia
or promethium-147 metal. A flux of approximately 5 .sup.. 10.sup.10
betas/cm.sup.2 /sec can be obtained from a layer of promethia of
about 0.001 cm thickness and approximately 10.sup.11 betas/cm.sup.2
/sec in the case of a layer of promethium-147 metal of about the
same thickness. The energy spectrum of beta particles emitted from
a radioactive promethium source (the metal or its oxide) is such
that very little radiation damage is caused in silicon
semiconductors. The thickness of the semiconductor wafer is
selected so that most of the radiation is absorbed by the
semiconducting medium. Where promethium-147 metal or its oxide is
used in conjunction with a N.sup.+ /P or N.sup.+ /P/P.sup.+ silicon
semiconductor wafer, the thickness of the wafer need only be
approximately 0.01 cm, for example.
Beta sources other than promethium include the isotopes of tritium
(H.sup.3), nickel-63 (Ni.sup.63), and strontium-90 and yttrium-90
(Sr.sup.90 -Y.sup.90). Such other beta sources as these, however,
are used in this invention largely for special applications. More
power can be obtained from promethium-147 metal or its oxide than
with H.sup.3 and Ni.sup.63. For example, tritium may be preferred
under certain circumstances because of its longer half-life.
Similarly, the available energy in strontium-90 and yttrium-90
sources greatly exceeds that of Pm.sup.147 sources, and this is
useful for certain applications even though such high energy beta
particles would cause greater radiation damage in the semiconductor
than the lower energy beta particles emitted from the other
sources.
Beta particles emitted by a promethium source have a maximum energy
of approximately 0.230 million electron volts (mev), approximately
0.0186 mev from tritium, and approximately 0.54 and, 2.26 mev from
strontium-90 and yttrium-90 (Sr.sup.90 and Y.sup.90 are in secular
equilibrium and cannot be separated due to the short half-life of
Y.sup.90, as is well known). Radiation damage in P-type silicon
starts at about 0.20 mev so that a promethium source coupled with a
silicon N.sup.+ /P or N.sup.+ /P/P.sup.+ cell is almost ideal and
provides optimum power output. Germanium has a higher threshold
level (of about 0.4 mev) for the start of radiation damage than
silicon but the power obtainable from silicon is about five times
better. Other nuclear particles or radiation such as alpha,
neutrons or gamma can be used besides beta in this invention.
However, use of such other particles or radiation is limited to
other specific purposes and is not particularly practical for the
purpose of a nuclear battery. For example, the most energetic alpha
particles from radionuclides lose all their energy within several
microns in silicon or other semiconducting materials.
FIG. 5 is a circuit diagram showing a resistive load R.sub.L
connected to positive and negative terminals 102 and 104 of a
N.sup.+ /P (or N.sup.+ /P/P.sup.+ ) semiconductor wafer 106. The
terminals 102 and 104 connect respectively with thin metallic
coatings on the faces of the wafer 106. The thin, low resistance,
metallic coatings are only needed to cover large area faces, of
course. An ammeter 108 is connected in series with the load R.sub.L
and a voltmeter 110 is connected across the terminals 102 and 104.
The wafer 106 is operatively associated with fuel element 112 which
is a radioactive source providing a nuclear particle flux 114 (of
beta radiation) to the wafer. The fuel element 112 is, of course,
normally positioned substantially against the wafer 106 to form a
battery cell 116. A current I will flow through the load R.sub.L
and can be measured by the ammeter 108. Similarly, the voltage V
across the load R.sub.L can be measured by the voltmeter 110.
FIG. 6 is a circuit diagram showing an equivalent circuit for the
single cell 116 (FIG. 5) having its terminals 102 and 104 connected
across the resistive load R.sub.L. The voltage-current
characteristics of the cell 116 are well described by the following
relationship:
I = I.sub.g - (V/R.sub. sh) - I.sub.o [exp (V-R.sub.s I/AkT ) -1]
[Eq. 1]
[= I.sub.g - I.sub.sh - I.sub.d ]
where I = Current through resistive load R.sub.L
v = voltage across R.sub.L
r.sub.s = Series resistance of nuclear-voltaic cell resulting from
imperfect terminal contacts to cell surfaces
R.sub.sh = Effective shunt resistance of cell
I.sub.o = Reverse current of semiconductor wafer
A = Parameter characterizing N.sup.+ P junction
I.sub.g = Generation current
k = Boltzmann's constant
T = Absolute temperature
The diode d in FIG. 6 represents the N.sup.+ P junction, and the
series contact resistance R.sub.s is usually negligible and it can
be henceforth considered that R.sub.s = 0. In that case, the short
circuit current I.sub.sc (I measured when V = 0) is equal to
I.sub.g. On a per-unit-area basis, the current density of a single
cell is given by the following equation:
J = J.sub.sc - J.sub.o [exp (V/AkT ) -1] - G.sub.sh V [Eq. 2]
where I.sub.sc = I.sub.g
J = Current (I) per unit area of single cell
J.sub.sc = Short circuit current (I.sub.sc) per unit area of single
cell
G.sub.sh = Effective shunt conductance per unit area of single cell
[= 1/(R.sub.sh .sup.. area)]
The open circuit voltage V.sub.oc of a single cell is given by the
following equation:
V.sub.oc = AkT .sup.. log [J.sub.sc + J.sub.o - G.sub.sh V.sub.oc
/J.sub.o
The reverse current density J.sub.o can be appropriately denoted as
a "leakage current parameter" and the proper value of J.sub.o to
use when describing betavoltaic results is that value obtained when
the N.sup.+ P junction is subjected to a forward bias voltage of
about 0.0 to 0.4 volt. When reverse bias conditions are used to
determine J.sub.o, an improper result is generally obtained.
The short circuit current is determined primarily by the diffusion
length (which is related to lifetime) of the minority carriers and
the generation rate of those carriers. The latter depends on how
effectively beta particles are absorbed in the semiconducting
medium and upon the average energy of the beta particles. The
generation rate is directly proportional to the beta particle
average energy; however, radiation damage effects to the
semiconductor must also be considered in selecting a suitable beta
source. The parameters J.sub.o and A are tightly coupled, and these
parameters reflect the quality of the semiconductor element or
device. In the case of an ideal silicon NP junction, J.sub.o is
about 10.sup..sup.-11 amp/cm.sup.2 and A = 1.0. In real devices,
however, defects on the surface and near the junction change these
values considerably. For example, in the voltage range of interest,
typical commercial silicon solar cells are characterized by values
of J.sub.o of the order of 10.sup..sup.-6 amp/cm.sup.2 and A of
approximately 2.5.
The net effect of the change in values from the ideal case is that
V.sub.oc of the device is considerably lowered, and the maximum
power is also lowered. The existence of defects in the junction
region causes electron-hole recombination currents -- or leakage
currents-- that subtract from the available output current. Surface
defects also account for losses, but the junction recombination
currents are the primary reason for solar cells not behaving as
ideal cells under low bias voltage conditions. The parameter
G.sub.sh takes into account current shunting paths which are, of
course, sources of loss in a betavoltaic energy converter. The
paths can arise from inhomogeneities in the planar junction, or
defective regions near the edge of the device. Commercial silicon
solar cells have values of G.sub.sh of about 10.sup.-.sup.5
ohm.sup.-.sup.1 cm.sup.-.sup.2. Thus, if the effective area of such
a cell is 1 cm.sup.2, the shunt resistance, R.sub.sh = 1/(G.sub.sh
.sup., area), would be about 10.sup.5 ohms.
FIG. 7 is a graph illustrating the J-V and Pw-V characteristics of
a single cell constructed according to our invention. The cell had
a planar junction area of 2.85 cm.sup.2 and was fueled with a
promethium-147 radioisotope, by utilizing a promethia source having
an area of 2.38 cm.sup.2 and a layer thickness of approximately 5.3
mg/cm.sup.2. Promethia activity was approximately 678 curies/gm
such that a total source strength of 1288 microwatts/cm.sup.2 was
available. Such cells or devices are much more suited to
betavoltaic energy conversion than the solar cells mentioned above.
Devices of the nature as that having the typical betavoltaic data
of FIG. 7 are characterized by the following parameters: J.sub.o
.congruent.10.sup.-.sup.9 amp/cm.sup.2, A.congruent.1.5 and
G.sub.sh .congruent.(0.5 .sup.. 10.sup.-.sup.6) ohm.sup.-.sup.1
cm.sup.-.sup.2. With the promethium-147 radioisotope fuel, the
energy spectrum of beta particles emitted therefrom is such that
very little radiation damage occurs in silicon devices.
The J-V characteristic of the single cell fueled with
promethium-147 is shown at the "beginning of life" by a full line
118 in FIG. 7, and is shown by broken lines 120, 122 and 124 after
1, 2 and 3 years, respectively. The beginning of life output power
density (Pw) available from a single cell (output power per unit
area of its active area) is shown by a full line 126. The short
circuit current density J.sub.sc generally decays as does the
promethium-147 radioisotope (of 2.6 years half-life). Further, the
single cell open circuit voltage V.sub.oc decays as follows:
V.sub.oc = (V.sub.oc).sub.t .sub.0 - (0.0067)At [Eq. 4]
where t = Elapsed time (years)
The net effect on the value of maximum power (P.sub.max) is that it
also decays, with a half-life of (t.sub.1/2).sub.P .congruent.2.13
years. At the maximum power point in FIG. 7, efficiency of the
exemplary cell is 1.1 percent. The efficiency is based on the total
amount of power available from the promethia source.
FIGS. 8A, 8B, 8C and 8D are graphs which were plotted from
experimental results and show typical characteristics of a
promethia-fueled, silicon, betavoltaic single cell as a function of
promethia layer thickness. The cell included a N.sup.+ /P/P.sup.+
silicon wafer with a resistivity .rho..sub.P .congruent. 0.3
ohm-cm, A = 1.5, J.sub.o = 8.8 .sup.. 10.sup.-.sup.10 amp/cm.sup.2
and the promethia activity was 678 curies/gm. In FIG. 8A, the short
circuit current density J.sub.sc (microamps/cm.sup.2) is plotted as
a function of promethia layer thickness (milligrams/cm.sup.2).
Similarly, in FIGS. 8B, 8C and 8D, the open circuit output voltage
V.sub.oc (volts), the maximum output power density Pw.sub.max
(microwatts/cm.sup.2) and device or cell efficiency (%) are
respectively plotted as a function of promethia layer thickness
(mg/cm.sup.2). This data is used to determine the optimum layer
thickness promethia source for a given application. For example, if
device efficiency is the dominating factor in any consideration, it
is apparent from FIG. 8D that promethia sources having a layer
thickness of about 1 or 2 mg/cm.sup.2 should be used; however,
where power output is the dominating factor, it can be seen from
FIG. 8C that promethia sources having a layer thickness of the
order of 10 mg/cm.sup.2 should be used.
FIGS. 9A and 9B are respectively a perspective view of a
bi-directional fuel element or source 128, and a fragmentary and
enlarged sectional view thereof. The fuel element 128 is preferably
a thin disc 130 as shown in FIG. 9A, and emits nuclear particles in
two directions as indicated by arrows 132. Radioisotopic fuel
material is, for example, held and supported in a suitable matrix.
In this instance, small grains 134 of promethia are used to form a
disc and then these grains are thinly coated with approximately 1
micron of metal 136 to hold them together as indicated in FIG. 9B.
The metal 136 is preferably aluminum; however, nickel, copper (Cu)
or tantalum can also be satisfactorily used. The metal 136 can be
deposited on the grains 134 by vapor deposition or sputtering. The
disc 130 can alternatively be made of promethium-147 metal.
However, such a metal disc cannot be much greater than 10 microns
in thickness if the device efficiencies of the improved later
models are to be maintained. It appears that promethium-147 metal
cannot presently be rolled-out much thinner than 50 to 75 microns,
and direct depositing of the metal is required.
FIGS. 10A and 10B are exploded perspective views of a two-cell
betavoltaic energy converter 138 using the bi-directional source
128 (FIG. 9A), wherein the cells are connected respectively in
parallel and in series. A cell is, of course, a unit including a
fuel element or source and an associated N.sup.+ /P or N.sup.+
/P/P.sup.+ semiconductor element or wafer. The source and wafer are
preferably promethium (metal or its oxide) and silicon,
respectively. Thus, the bi-directional source 128 in FIGS. 10A and
10B is a mutual source for the wafers 140 and 142. The source 128
is normally positioned contiguously to the N.sup.+ -type layers
140a and 142a of the wafers 140 and 142. As shown in FIG. 10A, the
P-type layers 140b and 142b (with thin metallic coatings thereon)
are both connected to positive terminal 144, and the N.sup.+ -type
layers 140a and 142a (with thin metallic coatings or grids thereon)
are both connected to negative terminal 146. In FIG. 10B, however,
the P-type layer 142b is connected to positive terminal 148, the
N.sup.+ -type layer 142a is connected to the P-type layer 140b and
the N.sup.+ -type layer 140a is connected to negative terminal
150.
FIG. 11 is an elevational view of an N.sup.+ /P (or N.sup.+
/P/P.sup.+ ) semiconductor wafer 152 having a coating 154 of
promethium-147 metal or promethia deposited directly onto the
N.sup.+ -type layer 156 of the wafer 152. The promethium coating
154 can be deposited directly onto the surface of the N.sup.+ -type
layer 156 either by vapor deposition, sputtering or electroplating
techniques. Negative electrode strip 158 is bonded to the N.sup.+
-type layer 156 at its peripheral margin (which has a thin metallic
coating thereon), and positive electrode strip 160 is bonded to the
(thinly metal coated) P-type (or P.sup.+ -type) layer 162 at its
peripheral margin as illustrated. The surface of the N.sup.+ -type
layer 156 provides direct support for the promethium coating 154,
and an intervening conductive metal coating or grid is not used.
The advantages include greater simplicity of construction and
significantly increased efficiency.
FIG. 12 is an exploded perspective view of a two-cell betavoltaic
energy converter 164 having a radioactive material coated on at
least one semiconductor wafer. The converter 164 includes N.sup.+
/P of an enclosing pair, (or N.sup.+ /P/P.sup.+ ) semiconductor
wafers 166 and 168. The N.sup.+ -type layer 168a of the wafer 168
has a coating 170 thereon of promethium-147 metal or promethia, for
example, just as the wafer 152 (FIG. 11). The wafer 166 need not
have a radioactive coating thereon and is normally positioned with
its N.sup.+ -type layer 166a contiguous to the coating 170 to take
advantage of its emitted nuclear particles. Of course, the N.sup.+
-type layer 166a can have a coating thereon similar to the coating
170. In this instance, each of such coatings can be formed with a
significantly lower layer thickness. The two cells of the converter
164 are shown connected in parallel to the positive and negative
terminals 172 and 174; however, it should be clear that the cells
can be readily connected in series to the terminals 172 and 174. Of
course, the faces of the P-type (or P.sup.+ -type) layers 166b and
168b have thin metallic coatings thereon, and the marginal ring
areas of the N.sup.+ -type layers 166a and 168a also have such
coatings thereon.
FIG. 13 is an exploded perspective view of a highly compact and
practical form of a nuclear battery 176. Most applications require
several cells in a nuclear battery. The voltage requirements
determine the number of cells to be connected in series, while the
current requirements determine the cell areas and/or the number of
cells to be connected in parallel. The final design, therefore,
includes one or more cells combined in a series and/or parallel
network to meet required specifications. For example, the nuclear
battery 30 embodiment shown in FIG. 1 is a series connection of
cells, and certain models (DL-10A-2 and DL-100A-1) thereof meet the
following specifications.
Model DL-10A- 2 DL- 100A- 1
__________________________________________________________________________
Maximum power, P.sub.max (microwatts) 43 212 Voltage at P.sub.max
(volts) 1.35 3.35 Open circuit voltage, V.sub.oc (volts) 1.79 4.75
Short circuit current, I.sub.sc (microamps) 44.0 77.0 Length of
cylindrical envelope, L (cm) 0.57 1.22 Diameter of cylindrical
envelope, D (cm) 1.18 1.55 Device efficiency (%) 1.04 0.84
__________________________________________________________________________
The planar, multiple section, cell embodiment of the nuclear
battery 176 can meet the above specifications (taken on a per-cell
basis) with relatively greater compactness and, certainly, with a
simpler construction than that of the battery 30. The battery 176
includes a fuel element 178 and a multiple section N.sup.+
/P/P.sup.+ semiconductor element 180. The fuel element 178
comprises a disc 182 and a thin coating 184 of radioactive material
thereon. The disc 182 is made of aluminum and the coating 184 is of
promethia, for example. The semiconductor element 180 can be made
by first diffusing phosphorus into the upper surface active areas
of a P-type silicon wafer 186 using a suitable mask (not shown) to
produce a N.sup.+ -type layer 188 thereon. Boron is then similarly
diffused into the lower surface active areas of the wafer 186 to
produce a P.sup.+ -type layer 190 thereon. The wafer 186 is next
cut into appropriate sectors 192 which are suitably cemented back
together in disc form to an electrically insulating cross 192a made
of material such as ceramic. In one construction, the sectors 192
were cemented directly on the surface of a ceramic substrate plate
so that the cross 192a was of air. The sectors 192 were connected
in series by (three) leads 194 which connect a N.sup.+ -type layer
188 of one sector to the P.sup.+ -type layer 190 of a succeeding
sector in a clockwise direction. Positive output lead 196 is
connected to the P.sup.+ -type layer of the first (upper left)
sector, and negative output lead 198 is connected to the N.sup.+
-type layer of the last (lower left) sector of the semiconductor
element 180. As in the other versions of this invention, the
surfaces of the N.sup.+ -type and P.sup.+ -type layers 188 and 190
of each sector 192 have a thin metallic coating provided
thereon.
FIG. 14 is a graph of the current (I) and power (Pwr) versus
voltage (V) characteristics of a planar, four section cell, nuclear
battery embodiment similar to that shown in FIG. 13. The
semiconductor element was coupled to a promethia fuel element or
source wherein the fuel disc is coated with promethia at a layer
thickness of approximately 8 mg/cm.sup.2 and having an activity of
about 660 curies/gm. It can be noted that maximum power occurs at a
relatively high voltage, for a single cell configuration.
FIG. 15 is an exploded perspective view of a planar, multiple
section, cell embodiment of a nuclear battery 200 which is superior
from a fabrication point of view than the battery 176 of FIG. 13.
The battery 200 includes fuel element 202 and a multiple section
N.sup.+ /P/P.sup.+ semiconductor element 204. In this
configuration, P-type (silicon) wafer 206 need not be cut into
sectors since P.sup.+ -type layers 208 and N.sup.+ -type layer 210
are formed by deep diffusion or ion implantation (ion beam directed
into material) techniques in the wafer as axial guard and isolating
walls, respectively, and completely separate and isolate each cell
section. The P.sup.+ -type wall layers 208 are located between the
planar N.sup.+ -type layers 210a and the N.sup.+ -type wall layer
210. The wall layer 210 can be ceramic but this would require
cutting of the wafer 206. The cell sections can be connected in
series by (three) leads 212, and positive and negative output leads
214 and 216 are connected to the first and last sections as
illustrated. The exposed surfaces of the guard layers 208 and of
the planar layers 210a are, of course, plated with a thin metallic
low resistance coating. It may be noted that a single cell
including a promethia source and a silicon N.sup.+ P or N.sup.+
/P/P.sup.+ wafer, and which utilizes only one active area or
section, supplies electrical power characteristically at voltages
of about 0.3 volt. Thus, multiple section cell embodiments of
nuclear batteries are highly desirable to obtain much higher output
voltages from a relatively compact device.
FIG. 16 is a partially fragmentary and exploded perspective view of
a laminar, multiple section, cell configuration of a nuclear
battery 218. The battery 218 includes a fuel element 220 and a
multiple section N.sup.+/P/P.sup.+ semiconductor element 222. The
illustrated structure is believed to be self-explanatory in view of
the preceding description on multiple section cell embodiments. The
laminar, multiple section battery 218 may at first sight appear to
be equivalent to the planar devices of FIGS. 13 and 15. It is not,
however. The nuclear particle flux 224 is parallel to the main
N.sup.+ P.sup.+ junctions in this embodiment. The silicon
semiconductor element 222 preferably has, for example, a height h
greater than approximately 100 microns and a width w of the order
of 1 cm. These dimensions can be varied by using another fuel
element similar to the fuel element 220 on the opposite (lower)
side of the semiconductor element 222, and also on the front and
rear sides thereof. One of the advantages of this battery (218)
configuration is that very high voltage devices can be easily made
by having any desired number of lateral cell sections. The lateral
end surfaces have, of course, metallic coatings deposited thereon
to which are soldered the positive and negative electrode strips
226 and 228.
FIG. 17 is an electron energy level diagram for about two sections
of the N.sup.+ /P/P.sup.+ semiconductor element 222 shown in FIG.
16. Its operation under exposure to radiation is indicated in the
diagram. It can be seen from the diagram that the main junctions
are those between the N.sup.+ -type and P.sup.+ -type layers.
Current flows from the electrode strip 226 (FIG. 16) attached to
the coated left end P.sup.+ -type layer and to the electrode strip
228 attached to the coated right end N.sup.+ -type layer when a
load is connected between the electrode strips.
FIG. 18 is an exploded and partly fragmentary perspective view of a
"needle-like" or elongated cylindrical, multiple section, cell
embodiment of a nuclear battery 230. The battery 230 is fabricated
in a manner generally similar to that of the planar battery 176 of
FIG. 13. The battery 230 includes a fuel element 232 and a multiple
section N.sup.+ /P (or N.sup.+ /P/P.sup.+ ) semiconductor element
234. In this instance, however, the fuel element 232 comprises a
thin cylindrical shell 232a of, for example, aluminum foil having a
promethia layer 232b deposited on its inner surface. The
semiconductor element 234 is fabricated by diffusing phosphorus
circumferentially into a P-type cylinder of semiconductor material.
The cylinder is then cut into uniform sections 236 having a P-type
core 236a and a surrounding N.sup.+ -type outer layer 236b. The
normally upper surface of the P-type core 236a is covered with a
metallic coating or sheet 238 which can be aluminum, and the
circumferential surface of the N.sup.+ -type layer 236b is covered
with a metallic grid or thin metallic coating 240 which can also be
of aluminum. The periphery of the coating or sheet 238 covering the
normally upper surface of the P-type core 236a is, of course,
spaced from the N.sup.+ P junction so that there is no interaction
therewith. For a N.sup.+ /P/P.sup.+ semiconductor, the coating 238
would be deposited on top of a central P.sup.+ -type cylinder
formed axially in the P-type core 236a.
The sections 236 are separated by insulator discs 242 which are
preferably made of ceramic. Each disc 242 has a hole 242a through
which passes a lead 244 connecting a P-type core 236a of one
section 236 to the N.sup.+ -type layer 236b of an adjacent upper
section. The sections 236 are thus connected in series with a
positive output lead 246 and a negative output lead 248. The
insulator discs 242 can be cemented or suitably secured to adjacent
sections 236, and then inserted and positioned in the cylindrical
fuel element 232 shell. While a cylindrical junction surface is
utilized in obtaining the elongated cylindrical battery 230, it is
apparent that other configurations of nuclear batteries can be
obtained so long as a suitable junction can be formed therein.
It is to be understood that the exemplary embodiments of this
invention are merely illustrative of, and not restrictive on, our
broad invention and that various changes in design, structure and
arrangement may be made therein without departing from the true
spirit and scope of the invention.
* * * * *