Recent developments in materials technology now make it possible to fabricate nonthermal thin-film radioisotopic energy converters (REC) with a specific power of 24 W/kg and a 10 year working life at 5 to 10 watts. This creates applications never before possible, such as placing the power supply directly on integrated circuit chips. The efficiency of the REC is about 25 percent which is two to three times greater than the 6 to 8 percent capabilities of current thermoelectric systems. Radio isotopic energy converters have the potential to meet many future space power requirements for a wide variety of applications with less mass, better efficiency, and less total area than other power conversion options. These benefits result in significant dollar savings over the projected mission lifetime

Brown, Paul M.

English

NASA Technical Reports Server

SOLID-STATE ISOTOPIC POWER SOURCE 
R MEMORY CHIPS 
PAUL M. BROWN, Ph.D. 
IsoGen Radioisotopic Research Laboratory 
315 S. McLoughlin Blvd. 
Oregon City, OR 97045 
(503) 656-4419 
ABSTRACT 
Recent developments in materials technology now make it possible to fabricate nonthermal thh-film 
radioisotopic energy converters (REC) with a specific power of 24 W/kg and a 10 year working life at 5 to 10 
watts. This creates applications never before possible, such as placing the power supply directly on iwtegated 
circuit chips. The efficiency of the REC is about 25% which is two to three times greater than the 6 to 8% 
capabilities of current thermoelectric systems. Radioisotopic energy converters have the potentid to meet mmy 
future space power requirements for a wide variety of applications with less mass, better efficiency, and less total 
area than other power conversion options. These benefits result in significant dollar savings over the projected 
mission lifetime. 
INTRODUCTION 
Placing miniature power supplies right where they're needed on integrated circuit chips is a quick and efficient 
way of getting electrical power to a circuit's microscopic components. This goal now seems within reach with 
the construction of the tiny, thin-film radioisotopic energy converter or REC that generates elect rid^. 
Traditional lead-acid batteries are made from plates of lead and lead peroxide immersed in a sulfuric acid 
solution, called an electrolyte. Charged atoms, called ions, flow through the acid within the bateev from one 
plate to another, setting up an electrical imbalance that causes electrons to flow through wires attached to the 
battery terminals and completing the circuit. These batteries are heavy and corrosive. 
-% 
Potting Compound 7 1 
InrruLator Cartridge 
Insulator Ring 
Sealed Can 
Wigh Work Function 
Electrode 
Low Work Function 
Electrode 
Fill Gas 
ztikHydJ-o@n 
Electrical Contact I 
+ ?$rminal 
FIGURE 1. Contact Potential Gas Ionization Type. 
https://ntrs.nasa.gov/search.jsp?R=19930016407 2020-03-24T07:27:43+00:00Z
Interest in non-chemical space power systems has been rekindled with the recent ambitious manned space 
missions plamed by NASA's Space Exploration Initiative. Deep space missions will require more energy than 
is practicdly available from chemical systems. 
The operation of a contact potential cell was first demonstrated by Gamer in 1924 (Brown 1992a) while new 
developments have been the subject of much research (Brown 1992b). When a gas is irradiated by beta particles, 
i o ~ t i o n  results. If this occurs in an electric field, such as produced by the contact potential difference of two 
disshi la  i a e t a c  electrodes (Figure I), which are electrically c o ~ e c t e d  through an external conductor, the ions 
naigaite to the electrode of opposite charge and conduction through the gas takes place. The electrical 
eneru  debvered to any external load is a result of the io g energy of the beta particles. The contact potential 
&ffcrence enables the neutralization of ions and the transfer of their charge to the circuit to take place. 
Efficiency is Birnited to 1% by charge recombination, leakage current, and space-charge effects but long life of 
20 years is easily achieved. 
Beta ElmiLLer 
FIGURE 2. Basic Approach to Betavoltaic Energy Conversion Consisting 
of a Source Coupled to a Semiconductive Junction Device. 
The first work on p-n Junction converters was reported by Rappaport in 1954 (Linder 1956), since then 
betavoltaic p-n junction batteries have been used only as a source of power in the heart pacemaker, where power 
requirements arte on the order of micro-watts (Olsen 1972). Figure 2 shows a straightforward approach to a 
betavoltaic &a-n junction energy converter; a planar source of beta emitting material is coupled to a planar p-n 
junction device. The n-face and p-face are respectively the negative and positive terminals of the single cell 
power source. Ut g the internal electric field of the junction, the radiation energy is converted directly into 
electricd eneru, in much the same manner as a solar cell converts photons of light into electricity (Olsen 1974). 
Efficiency is good on the order of 25% and fairly reliable using low power isotopes in the microwatt range but 
radiation damage to the junction seriously limits the working life of this type of device, when built with any power 
above one milliwatt, to only a few days. 
I should also mention the most widely used type of nuclear battery which is called the radioisotopic 
thermoelectric generator (RTG). This type of device utilizes the heat produced by large amounts of radioactive 
materid with a thermo-couple to generate electricity. This type of device is limited to only 6 to 8% efficiency 
and requires .high operating temperatures in addition to the fact that much shielding is necessary because of the 
type and mounts of radioactive material used. However, these devices are reliable and have been used 
repeatedly by NASA as well as other agencies. 
The nonthermal thin-film isotopic energy converter is a hybrid contact-potential/betavoItaic deGce. lit is made 
up of alternating thin-film layers of low work-function metal (Figure 3), isotope laced semiconductor media, and 
a high work function metal. Beta particles emitted from the source traverse the media losing eneag and creathg 
electron-hole pairs. Those carriers within a diffusion length of the junction will be swept across the gap 
contributing a current. A United States Patent was granted on this innovative approach to ra&oisotogic decay 
energy conversion February 11, 1992, USP #5,087,533. 
The embodiment shown here in figure 3 consists of a thin film layer of cesium metal on w%aich a second layer 
of selenium is deposited. Other materials will also work in place of the selenium. This seledarnzm layer is lioaded 
with tritium for this configuration although any suitable isotope will work. The t b d  or final layer is of platinum 
metal which yields a composite foil 0.001 mm thick and will deliver 4.4 volts continuously. 
// (Posirtive Electrode) 
ron niow beLween layers 
mtiaLed S e l e ~ m :  
siom from Ha generates 4.4 volh 
in Selenium WE& 
on of 
FIGURE 3. Radioisotope Electric Converter (Paper-Thin Layers 
are Shown Peeled Back for Illustration). 
Just as a solar cell converts light energy directly into electrical energy, the REC converts radioactive decay 
energy directly into electricity. The new technology of the REC works on the same principle as traiblioeaal 
batteries only the ions are not generated by chemical reactions. Rather, the acid solution or electaolj%e is 
replaced by a semiconductor medium that is ionized by the absorption of radioactive decay pztides emitted from 
a radioisotope homogeneously dispersed throughout the semiconductor itself. That is to say, the semiconductor 
is doped with the radioisotope. The semiconductor medium is a thin-film and conducts ions between metd 
electrodes typically made of cesium and platinum. Versions of the REC develop a potentid difference of 1 to 
4.5 volts DC between the electrodes (depending on the type of electrodes used) from a s b d e  cell and a few 
millhatts of power per square centimeter. 
One embodiment of the REC yields 4.4 volts DC from a foil only 0.001 mm thick wMe anoalber embglr&ment 
produces 9 volt DC from a foil .045 mm thick. The current is determined by the isotope used and the surface 
area of the foil. This foil may be rolled into tubes, spirals or be cut into virtually any shape. Cdculations show 
a specific power of 24 watts per kilogrm with a 10 year working life is achievable for a 10 watt device (NASA 
1991). Based on recent experimentation the efficiency is estimated to be on the order of 25% tlhermd to ekdric, 
which is three times greater than the 8% capability of current thermoelectric systems. 
The REC is superior to betavoltaic devices of the p-n junction type because the REC desigra does not subject 
the voltage mechanism to radiation damage; superior to the contact potential &fference converter because 
without a gas the REC is not susceptible to space charge effects while ion recombination is h i t e d  by the nmrow 
media gap; and superior to the RTG by operating at low temperatures with greater efficiency, lower weight, and 
seduced health dangers. 
The possibility of using relatively simple manufacturing techniques to fabricate these miniature, thin-film 
RECs at Isw cost suggests a variety of applications. Here at IsoGen, we are investigating how to deposit these 
devices directly onto integrated circuits to provide onsite power for computer memory chips. 
In concept, the REC offers several advantages over batteries made with liquid electrolytes. The REC will 
last much longer due to the fact that radioisotopic energy is several orders of magnitude greater than chemical 
energy. They also operate over a broader temperature range than batteries with electrolytes made from liquids, 
which may freeze in extreme cold or boil in extreme heat. Some batteries on the market use a gel or a solution 
suspeloded in plastic as their electrolyte, but none is totally solid state. The REC may be the first totally solid 
state battery. 
"Smart" credit cards equipped with a tiny computer chip to keep track of a user's banking records as well 
as such other information as medical history are being tested in Europe and Japan. The REC could run smart 
credit cards, cardiac pacemakers, possibly even artificial hearts or other electronics that do not require large 
amounts of power. New applications will be developed around this new long life portable power source that we 
can only dream of today. 
Beta-conductivity is a phenomenon evidenced by the increase in electrical conductivity of a material after the 
absorption of ionizing radiation. This effect is attributed to the increased number of free electrons generated 
by absorption of the ionizing radiation. Of course, recombination of the charge carriers does occur, however, 
during the time these carriers are free in the material, the conductivity can be greatly enhanced. We minimize 
recombination by using very thin layers of the ionized material in the REC. 
For a material in which one type of carrier predominates, for example electrons, the change in conductivity 
with iirradiation can be expressed as (Brown 1990b): 
h e r e  Au is conductivity change, 
Ae is change in free carrier density, 
e  is electronic charge, 
p is carrier mobility, and 
Ap is change in carrier mobility. 
"Beta-conductivity gainn may be defined as the number of inter-electrode transits that can be made by an 
electron in a conductor before the beta-generated "holen is eliminated by recombination. For this case where 
one type of carrier predominates, the gain is expressed as (Brown 1990b): 
Were  x is carrier lifetime, 
Vis applied voltage, and 
h is spacing between electrodes. 
It is presumed that free electrons are retained in a metal by the forces of attraction bemeen two unlike 
charges, since there must be as many positively charged atoms as there are free electrons. An mount  of work 
must be performed to remove an electron from the surface of any metal called the work function of the material, 
and of course, this value may be calculated. But for this discussion it is sufficient to say that the work function 
of most elements, including the metals have been measured and tabulated in suitable reference books. ?rpica11y, 
work functions range from 1 eV (easiest to remove) to 5.5 eV (most difficult) for metals. 
Wrhen two metals of different work functions, say cesium and platinuoo, are brought into electricd contact 
(by direct contact or through a conductor) an electrical potential difference is established as discwered by Volta 
in 1797. This contact potential difference is the result of the difference between the work functions of the two 
metals. The sign is determined by the fact that the metal with the largest work function gives up electrons the 
least readily and becomes the more electronegative. 
An mount of work must be performed on a neutral atom to remove electrons (ionize the atom). This work 
manifests itself as increased potential energy and may be utilized to do work before allowing the electron and 
ion to recombine. 
The Betavoltaic Effect may simply be defined as the conversion of ionizing radiation to electrical eneru by 
a material or combination of materials. Radiation that is absorbed in the vicinity of any potential barrier wi11 
generate separate electron-hole pairs which in turn flow in an electric circuit due to the Voltaic Effect. Of 
course this occurs to a varying degree in different materials and geometries. 
Figure 4 is a representation of a basic betavoltaic converter and demonstrates the characteristics of the REC. 
Electrode A has a positive potential while electrode B is negative with the potential difference p r o ~ d e d  by the 
Contact Potential Difference of the dissimilar metals. An electric field exists between the electrodes, a zone we 
will call the junction. The junction between the two electrodes is thus comprised of a suitably ionizable mediaam 
exposed to decay particles emitted from a radioactive source. 
I 
Potential 
Difference 
FIGURE 4. Betavoltaic Converter. 
In general, the introduction of ions from any source into an electric field will generate electriciq in 
accordance with well known physical principles and may be satisfactorily explained in terms commonly associated 
with the Volta Effect. The energy contributed to such a circuit does not come from the ions themselves but 
rather from the work done on the circuit to generate the ions, known as the ionization potential of tbat particular 
material. 
Neither the electric field, the electrodes or the medium between the electrodes contribute any energy in the 
Voltaic Effect. The energy is contributed by the ion generator whether this mechanism is chemical, 
electromapetic or nuclear is irrelevant. Modeling for the REC has been generated and published (Brown 
4Wa). 
CONCLUSION 
The REC is a current generating device including a pair of electrodes of electro-chemidy dissimilar 
materids sepaated by a space filled with a solid medium having a relatively high dielectric constant and a 
rehtively low ionization potential. Current flows in an external circuit coupling the two electrodes together when 
the soEd me&um is ionized. A suitable radioactive material is mixed or dispersed in the solid medium to provide 
the ionizhg flu. 
The REC could be a less toxic, longer lasting alternative to conventional chemical batteries, and most likely 
to compete with button cell batteries to power small electronic items. Such a device could have a broad range 
of applications, from portable power supplies for integrated circuits to information processing, and nonpolluting 
generators in portable phones. The RECs lightweight, durability, and lack of corrosiveness also make it a 
natural c;am&date for space flight. 
Market surveys have been conducted by the nuclear industry in the past and the conclusion has been that 
there is a need for a long-lie radioisotope nuclear battery. Economic studies indicate that the REC could be 
economicJ~y competitive with chemical batteries for applications requiring lifetimes of over two years at remote 
locations where the expense of charging or chanhg  batteries is si@cant. Applications where the inaccessibility 
after implantation is a consideration that leads to the selection of an REC due to its superior reliability and life. 
This work was performed at the IsoGen Incorporated Radioisotopic Research Laboratory under internal 
fundixng support. 
Brown, P. (1Wa) "Radioisotopic Generators: Beta Voltaic Effect Modeling," R a w  & ; k i t  Magazine, 2(1):81- 
82. 
Brom, P. ( l w b )  "The Beta Voltaic Effect Applied to Radioisotopic Power Generation," h e r i c a n  Nuclear 
Society h u d  Meeting, Nashville, TN. 
 row^, Pq (11992a) "Isotopic Electric Generators: Conception to Application," IsoGen, Inc., Portland, OR. 
Bras-, P. (IW2b) "The Contact Potential Isotopic Generator," Raum & Zeit Magazine 3 (3):72-73. 
Linder, E. (1956) "Direct Conversion of Radiation into Electrical Energy," Proceedings of the International 
Conference on the Peaceful Uses of Atomic Energy, 15:283-290, United Nations, NU. 
NASA, (9991) "Eetavoltaics of Increased Power," NASA Tech Briefs, 5(8):32 
Olsen, L, (1979 "Betavoltaic Energy Conversion," Energy Conversion 13117 
Onsen, L,. (11974) "Advanced Betavoltaic Power Sources," 9th Intersociety Energy Conversion Engineering 
Conference 


Solid-state Isotopic Power Source for Computer Memory Chips

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