ALFRED B. GARRETT The Ohio State University, Columbus, Ohio
THEsuccessful conversion of atomic energy into heat energy on a large-scale basis was done in 1945. At once the question was raised whether it would be possible to devise an apparatus to convert atomic energy directly to electrical energy. That conversion can now be done by several types of nuclear batteries.' Even though such batteries produce only a very low current, there are many applications for which they can be used successfully. Nuclear batteries are composed of cells of the following types: (1) the 8-current type, (2) the contact potentialdifference type, (3) the p-n junction type, (4) the thermojunction type, (5) the secondary-emission type, and (6) the photoelectric type. The 8-current Type.= This nuclear cell was first demonstrated by Moseley3in 1913. The scheme of this
is limited by the leakage across the terminals due to the presence of ions in the air which have been produced by the fast-moving electrons. The first cells were prepared with an evacuated space between the emitter and the collector. Now this space is filled with a plastic such as polyethylene which allows the high-energy electrons to penetrate but prevents the reverse flow of low-energy electrons. A considerable amount of research had to be done to find a proper plastic which would not be broken down by nuclear radiation. The source can be almost any radioactive substance that is an emitter of 6 particles and has long enough half-life to be practical. Gamma-ray emitters cannot be used because of shielding problems. One of the convenient substances is strontium-90 which has a half-life of 25 years and is available in adequate quantity for use. A cell of this type built by the Radiation Research Corporation has the following characteristics: voltage10,000 volts (open circuit) ; current-50 micromicro-amperes (no load); capacity-less than 10 micromicroRodiooctive source farads; radioactive isotope-strontium-90; external e m i t t i n g electrons radiation-less than permissible tolerance for a 40-hour lcolled /3-porticlesl week one inch from surface: dimensions-height - 13/8 in. and diameter 1 in.; weight-5 0s.; operating temperature-from 65' to -65°C. This type of cell operates efficiently only a t high voltages; it is the simplest of all the cells in design but has no amplification factor (multiplication factor). I t is sometimes called a constant-current generator. The Crmtact Potentialdifference Type. The operation of this type of cell was first demonstrated by Kramer4 in 1924. He used carbon and zinc electrodes with the space between them filled with air or the radioactive substance itself. The cell operates as follows: The Figure 1. A @-CurrentColl particles from the radioactive gas or other substance in cell is shown Figure 1. 8 particles, emitted from the the cell collide with the gas molecules, knock off elecnuclei of atoms of radioactive substances, are collected trons, and thus produce positively-charged ions or are on a plate (electrode) which then becomes negatively captured by molecules to form negatively-charged ions. charged. These electrons move through the external These charged particles (electrons and ions) serve as a circuit in the conventional manner. They can be conducting medium and so complete the electrical cirmade to do such things as flash a lamp, throw a switch, cuit between the electrodes. Mctals differ from each or send a signal. The voltage across the terminals rises other with respect t o the ease with which they give up to several thousand volts; further increase in voltage electrons. For exam~le.cesium does so verv easily: fact, visible light will'drive the electrons off. ~ o i d ' LINDER,E. G., P. RAPPAPORT,J. J . LIFERSKI, International in on very tightly. The relative Conferenoe on the Peaceful Uses of Atomic E ~ A , / c~~ . holds ~ ~ to ~its electrons . 8/P/169 U. 8. A,, June, 1955. difficulty of removing electrons from two dissimilar COLEMAN, JOHN H., Nucleonics, 11, No. 12, 42 (1953). metals can be measured if the metals are placed in a 3 MOSELWT. I . -P r n ~---*. Row S O PiT.mwhnl A. RR. -.... ...,, . . . , 471 . .. , H G .-., ~
(1913).
\
"RIMER,
3. V., The Electrician, 93, 497 (1924).
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VOLUME 33, NO. 9, SEPTEMBER, 1956
circuit similar to that shown in Figure 2. I n this cell the positive ions not only conduct the current but also tend to pull the electrons out of the metals. This type of cell is called the contact potentialdifference type because the theoretical voltage of the cellis determined by the relative difficulties of removing electrons from the two metals (the arithmetical differences between the terms called the work functions of the metals). Lord Kelvin's work in 1898 on measurements of contact potentials of metals suggested the use of this circuit for a nuclear battery. There are several radioactive substances that can be used in this cell. Tritium, Ha, gives promise of being useful in cells of this type. Its betas are emitted with relatively low energy (0.018 million electron volts), and it has a half-life of 12.5 years and produces no gamma rays. The advantage of this type of cell over the 0-current type is that one high-energy electron produces many ions and electrons (it ionizes the many gas molecules with which it collides before it is finally stopped). Each of these new charged particles is effectivein placing a charge on the electrodes. Hence, rather than have only one charged particle t o produce a charge on the electrodes there is an avalanche of particles that can produce that charge. This increase is called the multiplication factor of the cell. The cell using tritium gas has a multiplication factor of about 200. The voltage of the contact potential-difference type of nuclear cell is about one volt. This is very low compared to the @-currenttype of cell. The current drawn from each of these types also is very low, about 10-lo amps. These cells can be used successfully in circuits where a capacitor is charged from the cell. Then a surge of current from the charged capacitor can be used to trip a circuit or produce a signal. The p-n Junction Type? This type cell derives its power in the same way that the p-n junction type solar cell does (see Figure 3). The two electrodes of the cell are made of wafers of high-purity silicon. Each silicon atom contains four electrons in its outer electron energy level, and is surrounded by four other silicon atoms. Each atom shares a pair of electrons with each of these four neighbors to establish four bonds. Into the surface of one wafer is diffused a tiny amount of antimony whose atoms have five electrons in the outer energy level. If an antimony atom takes the place of a silicon atom in the crystal lattice, eight electrons will be used to form the four bonds with neighboring atoms and an extra electron will remain. I t appears to require only low energy to move that extra electron. Into the other electrode of high-purity silicon is diffused a slight surface impurity of an element such as boron whose atoms have only three electrons in their outer energy level. When a boron atom takes the place of a silicon atom only seven electrons are available
' BRIDGES, HENRYE., C h m . Eng. News, 34, 220 (1956).
,
Electrode f r o m which electrons connot be driven o f f readily
I
Electrode from which electrons con b e driven off r e a d i l y
-?
Rodiooctive 90s or other medium
to form the four bonds between boron and its four silicon neighbors. Since eight electrons are required and there are provided only seven, an electron deficiency or a hole in the electron lattice results. This is called the positive or p wafer and the other one (the one with 9 electrons) the negative or n wafer. When these two wafers are placed with the sides of the wafers containing these impurities next to each other, a p-n junction is formed. If a radioactive substance such as strontium90 is placed near this junction the energy from the radiations (0 particles or y rays) will cause the excess elec/--Silicon
cp - n
with boror
junction
-silicon
Strontium-
with antimony
JOURNAL OF CHEMICAL EDUCATION
trons to move through the circuit to the wafer contain- cell fails in a few weeks because of changes in the crysing the electron holes in its electron lattice but not in the talline stmcture of the silicon electrodes. Work with opposite direction. of cell a lower-energy P emitters may make this type These p-n junction type cells have a very low voltage, more promising one. about 0.2 volt, and a current of several microamperes. The Thennojunction T y ~ e .This ~ type relies on the But they have a huge amplification factor-about heat uroduced bv the nuclear changes in the radioactive 200,000 electrons are produced for every 0 particle substance. he heat is absorbedYby a sensitive theremitted in the electrode from the strontium-90. The mopile placed near the source of the activity (Figure 4). disadvantage of this type of cell, which uses high- To minimize the effects of fluctuations in the voltage due to changes in the cold junction which is a t room Insulator temperature, the hot junction is operated a t a high temperature; this requires a large amount of radioac~ T h e r r n o c o u ~ liunction e tive substance. Hence, the cost is high. Polonium has been used as the source of nuclear radiation for this type of cell. A temperature difference of about 450°F. was maintained between the hot and cold junctions. Polonium is safe to use because it emits only alpha particles. A person working with such a cell can be safely shielded by only a sheet of paper. But polonium decays with the relatively short half-life of 138 days and is quite expensive. The cell built by the Mound Laboratory of the Monsanto Chemical Company contained 150 curies of polonium-210 (today's cost $375,000), delivered 0.75 volt and 25 milliamperes, and weighed 31 grams. Other Types of Nuclear Batteries.' New types of L R o d i o o c t i v e source nuclear batteries are constantly being proposed. A secondary-emission type of cell makes use of a sensitive Figure 4. A Thsrrnojuncti~nCell surface from which electrons can he driven out when the energy P particles, is its short life which results from surface is bombarded with 0 particles (Figure 5). The the radiation damage of the silicon crystals. The secondary electrons are collected on an insensitive surthreshold for radiation damage to the silicon crystals is face. The maximum energy of the secondary electrons about 0.2 Mev. This is much lower than the energy of is about 20 volts, hence, the open-circuit voltage will the p particles from strontium which is 0.61 Mev. be about the same. There is some current multiplicaWith strontium-90 as the radiation source this type of tion. Photoelectric types of nuclear cells have been proy-Rodioactivated emission surface posed (Figure 6). I n these, the radioactive particles strike a phosphor which scintillates (gives off light). The light then liberates electrons from a photoelectric or a photojunction surface. This type of cell could Vacuum probably be used effectively with a p-n junction type of photocell. Limitations in the Development and Use of Nuclear Batteries. All types of nuclear batteries have an efficiency of between 0.1 and 2 per cent. Some estimates would put a limit of two per cent on the ultimate attainable efficiency of any of these types. Except for the ¤t type, the low voltage and low steady current result in low power output. In general, for personal safety reasons, alpha and beta particle emitters are used. It requires too much material to shield personnel from y-emitting substances where the persons may be in contact with these batteries for long periods of time. The beta particles must not be of too high energy or they will produce secondary ~
[
t
" h a . Eng. News, 32, 4183 (1954). THOMAS, A., of Tracerlab, Inc., paper presented a t National Industrial Conference Board's Atomic Energy Course for Management, April, 1955.
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VOLUME 33, NO. 9, SEPTEMBER, 1956
radiations that are difficult to shield. The half-life must be long enough to be practical and the cost must be reasonable. All of these limitations suggest that nuclear batteries, as we now know them, will never be used to meet conventional domestic or industrial power demands or in automobiles. However, their unique properties suggest special uses. They can be used for some types of work a t low and high temperatures where the conventional battery would fail quickly. Uses that demand long shelf life or continued use over a long period of time a t low drain might be found in the recording apparatus of an earth satellite, in the arctic regions, in remote weather stations, or in ocean signal buoys. Other uses may be in survey meters to test for radioactive contamination, pocket dosimeters to test for the amount of radiation a person has received, one-shot burglar alarm systems, periodic operation of timing devices and clocks, and a high-voltage source to trip many types of electrical circuits of civilian or military operations. The expense of this type of cell is governed primarily by the cost of the radioactive substance used. At
A
Figure 6.
A Photoelestris-typeCell
present the lowest cost is about $30 per watt-hour of useful electric energy. This compares with about 4k per watt-hour obtained from an ordinary flashlight cell and indicates that the nuclear battery is noncompetitive for common uses.
