US2913510A

US2913510A - Radioactive battery

Info

Publication number: US2913510A
Application number: US499543A
Authority: US
Inventors: John H Birden; Kenneth C Jordan
Original assignee: Individual
Current assignee: Individual
Priority date: 1955-04-05
Filing date: 1955-04-05
Publication date: 1959-11-17
Anticipated expiration: 1976-11-17

Description

1959 J. H. BIRDEN ET AL 2,913,510

RADIOACTIVE BATTERY Filed April 5, 1955 Mr r OHM LOAD INVENTOR.

John H. Birden and Kenneth 0. Jordan ATTORNEY nited tates atent Ofiice RADIOACTIVE BATTERY John H. Birden and Kenneth C. Jordan, Dayton, Ohio,

assignors to the United States of America as represented by the United States Atomic Energy Commission Application April 5, 1955, Serial No. 499,543

2 Claims. (Cl. 136-4) The present invention relates to the generation of electrical energy from radioactivity, and more especially to a novel battery or cell wherein the energy of the charged particles emitted by a radioactive isotope is converted into heat energy and then is converted into electricity, in amounts for doing useful work, by means of a thermopile.

It has been heretofore proposed that very small electrical currents could be generated by collection of the beta particles emitted from a radioactive isotope on a charged surface, that small currents may be generated between two electrochemically dissimilar electrodes separated by an ionizable gas by forcibly ionizing the gas with radiation and connecting a load across the electrodes, and that electrons from radioactive strontium be utilized to bombard a semi-conductor having a large junction formed by an impurity therein. None of the proposed devices have proved entirely satisfactory for practical power packs, principally becausethey are voltage sources of relatively high internal impedances. Where large currents are required, the internal power loss due to the in ternal impedance is so great that the batteries must be made undesirably large in physical size to overcome their ineificient operation. Moreover, relatively small currents have been obtained from prior radioactive batteries, and they have been costly and diflieult to construct. Great care must be taken to reduce the radiation hazard to the user or, associated equipment, and the amounts of radioactive material which can be used must accordingly be kept rather small.

With a knowledge of the difficulties associated with constructing radioactive batteries of the types known to the prior art, the inventors have for a primary object of their invention production of a novel radioactive voltage source of inherently low internal impedance, so that large amounts of useful power may be delivered by a source of reasonable physical size. A further object is to provide a radioactive battery which creates no radiation hazard of any kind to the user or to equipment. Yet another object is to provide a source that is relatively simple to construct with available structural materials. Other objects and advantages of the present invention will become ap parent from the following detailed description of certain preferred embodiments thereof, when read in connection with the appended drawings, wherein:

Figure 1 illustrates one embodiment of our novel electric cell;

Figure 2 shows an alternative, preferred form of the active portion of our cell;

Figure 3 shows the outer envelope enclosing the portion shown in Figure 2;

Figure 4 illustrates schematically the electrical circuit of our novel cell; and

Figure 5 shows schematically the thermal circuit of our cell.

According to our invention, electrical energy is generated by converting the energy of radioactive decay to 314 heat energy and then converting the heat energy to electrical energy. A radioactive material of high specific activity is sealed inside a suitable container which contacts thermally the hot junctions of a thermopile-21 plurality of series-connected, alternately hot and cold thermojunctions. The cold junctions of the thermopile are thermally insulated from the hot junctions and from the inner container and are electrically connected in series relationship with the hot junctions. Preferably, for development of maximum power, our cell is so designed that one-half the heat generated by radioactive decay of the source is transferred to the outer container by way of equal-resistance thermopile leads, and the internal series resistance of the thermopile is made equal to the load resistance to be connected thereto.

Referring now to Figure 1, in one form our cell may comprise a spherical capsule 1 containing an intensely radioactive material therein, a thermopile of dissimilar metal leads 2, 3 provided with hot junctions 4 and cold junctions 5, an outer container 6 enclosing the assembly, and thermal insulating material, not shown in the interest of clarity, interposed between capsule 1 and container 6. Each hot junction 4 is placed in thermal contact with capsule 1, while each cold junction 5 contacts container 6. The thermopile leads are brought out through tube socket or base 8, which fits tightly into the bottom of container 6. Suitable materials of construction for this cell are shown in Table II, column 1.

Referring now to Figures 2 and 3, a preferred cell construction comprises a small cylindrical metal capsule 21 containing a source of highly radioactive material; two groups 22, 23 of thermocouples, each couple having a hot junction in thermal contact with capsule 21 and a cold junction formed externally to one of the insulator plates 24, 25; an outer sleeve 31 surrounding the assembly for protection and insulation from the outside air; end caps 32, 33 closing the ends of the sleeve; and mechanical spacers 28 to separate the closely-spaced wires to prevent contact between wires. The thermal insulator, preferably a light powder filling the space between capsule and container, is not shown for clarity. The entire space between plates 24, 25 is normally filled with the insulator, which is not electrically conductive. External leads 26, 27 are provided from opposite terminals of the thermopile, the two groups 22, 23 of couples being electrically connected in series. Materials used in construction of the battery illustrated are shown in Table II, column 2. The thermocouple wires extend through parallel rows of holes in the base plates 24, 25, each plate being formed of two half-discs held together by straps near their periphery. The end caps may preferably be hermetically sealed to sleeve 31 and leads 26, 27 brought out through vacuumtight seals 34, 35. Each cold junction may be cemented to the outer container by an electrically-insulating, thermally conductive cement to provide a large surface area for cooling of the cold junctions. The spacers 28 are not required if sufiicient tension is put on the wires to keep them separate.

Referring now to Figure 4, the simple electrical circuit of the thermopile comprises a plurality of resistances, (r r represent the electrical resistance per lead of each type of thermocouple wire in ohms and r is the resistance per couple). The leads are connected in series forming 11 hot or cold junctions and the opposite ends being connected to a load represented by n (1 +r ohms. We have found that maximum power will be developed from our radioactive battery when it is connected to a load resistance equal to the internal resistance of the battery. The power developed W will be given by the expression (ne) /4nr, where e is the voltage generated per couple. The efliciency of the radioactive battery may be found from the expression [Ef1=100 W /W =25 ne /W ml where W is the heat generated by the radioactivity withm the capsule, and r is the series resistance of both leads forming a couple. v I We have found that for maximum efiiciency, the leads of our thermopile should have a thermoresistance equal to the sum of the thermoresistances of the space between the capsule and the outer container and the thermoreslstance from the capsule to the hot junctions. We. have further found that the diameter of each type of wire in the thermopile should be so chosen that the thermoresistance of each lead is the same, for maximum efliciency. 7 Referring now to Figure 5, the heat W generated by radioactivity within the capsule, travels from the capsule across the thermoresistance R from the capsule to the hot junctions, which are at temperature T From the hot junctions the heat travels along all the wires between 'hot and cold junctions, the thermoresistance of all the wires being represented by R to the cold junctions, which are at temperature T Some of the heat from the capsule also flows through the space between the capsule and the outer container, the thermoresistance of that space being represented by R ,All of the heat from the source except that converted to electrical power flows through the network shown in Figure 5. L

The electrical resistivity pe and thermal resistivity p of the wires in the thermopile 'are related by the Wiedemann-Franz constant If the diameter of each wire is chosen so that the thermal resistance r of each lead is the same, the thermal re- V sistance R, of the n thermally parallel leads is given by 7 2n For maximum efficiency, R should equal the sum of R +R It may be seen that the maximum efliciency depends upon and varies inversely with Wiedemann-Franz constants of the thermopile wires selected. Therefore, the couple PI'OVIdlIlg the greatest thermopower is not necessarily the best choice for the radioactive battery. If

R R +R then'it can be shown thatthe maximum efficiency 1 2) 1+ 2)( 1+ z) where B is the thermoelectric power of each thermocouple 1n volts/ C. and m is the ratio of R to R T and T are in degrees kelvin.

The efiiciency of the battery does not depend upon the number of junctions; that is, increasing the number of junctions does not provide more elficient utilization -of the heat. Moreover, the thermoresistance between the capsule and the hot junctions will increase slightly with the number of junctions, thus actually decreasing the efficiency. The internal resistance of the battery will increase as the square of the number of junctions, while the total voltage developed will be proportional to the number of junctions employed.

In designing the radioactive battery for meeting specified requirements, the type of thermocouple to be used should be selected first. Since eificiency is proportional to the square of the thermoelectric power, selection of materials will be a most important factor. However, as above stated, the Wiedemalnn-Franz constant and the resistivity of the thermocouple wires must be considered, as should be the melting points and the welding or soldering properties of the wires. Chromel-constantanand iron constantan have high thermoelectric power an'd are entirely satisfactory in other respects. With a fixed number of junctions, and for maximum efficiency, the radius of the wire to be used depends upon the Wiedemann- Franz constant and the resistivity, so that a Chromel wire, because of its much higher resistivity must be substantially larger than the iron wire, Although the large most purposes the battery efiiciency with a Chromel junction will be higher than with an iron junction.

The heat source should be next considered, important factors being the availability of highly active materials, the specific activity of the isotope used, the relative ease of handling, including the dangerous radiations given oil, and the half life of the radioactive material. Any radio active isotope could be used, but strontiumand polonium-210 appear to be most suitable from a cost per curie standpoint, and require a minimum of radiation shielding. Polonium-208 is ideally suited for battery use except for its excessive cost. In construction of the source, polonium may be volatilized intov a capsule or container which is then closed with a plug and coated with nickel. Polonium has a-high specific activity and gives off 5.4 mev. energy per disintegration. This permits the use of heat sources whose size and heat loss is determined only by mechanical considerations, and results in a. minimum number of curies of activity required for a given quantity of heat produced. Strontium-90 is available in sealed containers and is better suited for a longlived battery in that the useful life would be from 60 to 70'times the life of the P0 battery.

If a battery is to be designed to deliver a maximum power W into a load R with a load current 1;, and a developed voltage of V the equations for optimum battery design based on our battery having an efficiency of 0.2 percent, are: i

ohms

I milliamperes V 9.4 10- when N is the number of junctions required, P0 is the curies of polonium required, and Sr is the curies of strontium required.

Table I lists two examples of our novel-batteries calculated from these equations, based upon an efficiency of 0.2 percent attained by a battery constructed and tested.

TABLE I Po curies It is apparent that we 'have for the first time provided suitable and practical batteries for electric generation capable of furnishing suflicient amounts of electrical power to be commercially useful, yet which are smaller, lighter, and can do more work than some dry cells. We have demonstrated that with relatively small physical dimensions we can provide an electric cell having extremely long life, even while delivering full rated current, and which can produce sufficient amounts of current to operate, for example, a transistorized radio circuit such .as that shown in Radio and Television News, February 1953, page 37. Moreover, the output of our cells is not affected by temperature of its environment so adversely as are dry cells. 1

It will be apparent to those skilled in theart that the source of radioactivity might be incorporated into the hot junction itself, rather than placed in a container and thermally connected to the junction. Itwill be further apparent that isotopes which emit only alpha particles are to be preferred, since the problem of shielding the dangerous radiations therefrom is easily met, whereas sources which emit hard gamma and beta rays produce dangerous in... iii-0 radiation hazards. For maximum efi'iciency, the source material shown should provide sufiicient energy per unit volume, and should give maximum energy per disintegration. Mechanical requirements presently set a minimum size for the source capsule. The volume of such capsule is larger than the volume of polonium-ZIO required, but substantially that required for strontium-90.

Since the hot junctions must not make electrical contact with each other, but must all be thermally connected to the source, heavy insulating cement such as Sauereisen should be used for the hot and cold junctions. The cement should provide at least fair heat conduction, good TABLE II Battery No. l

Radioactivity- Capsule containing PO L 57 curies, P0 Sphere, O.D. 0.4 in

Battery No. 2

146 curies, Po Cylinder O.D. 0.21 in.,

length 0.45 in.

Material of capsule 0.047 in. cold-rolled steel with 0.02 in. cold-rolled steel with 0.02 in. nickel coating. 0.02 in. of nickel coating.

Thermocouples Silver-soldered Chromel-con- Welded Chromel-constantan.

stantan.

Number of jimctinn 40.

Length of1eads 1.2 cm 1.3 cm.

Wire Siz S {B and S No. 18 Chromel. B and S No. 29 Chromel.

e B and S No. c0nstantan B and S No. 30 constantan. Insulation between junctions and capsule Sauereisen cement Sauereisen cement. Estimated m.-- 0.3.-.

Insulating material Santocel Santocel.

Outside containen- Lucite cylinder. Aluminum cylinder.

Internal resistance 0.25 ohm 15 ohms.

Voltage at no load.-- 42 millivolts 750 millivolts.

Te(%1pe{ a)ture rise from hot to cold junctions 42/7X7.7 10- =78 C 750/X7.7X10- =244 C.

Temperature of cold junction (T2) C 80 0.

Temperature of capsule (based on m above) 146 C 373 Max. power delivered 9.4 mllliwatts.

Activity of P0 in watts 4.65 watts.

Elficiencyuu 0.20 percent.

Wei ht 1 gins.

Work capacity 7.7 10f joules.

Current at max. power- 85 milliamps 2-5 Imlhamps.

electrical insulation, must be temperature-stable at very high temperatures, and must provide good adhesion for structural purposes.

If hermetically sealed, and evacuated cells are not provided, a good thermal insulator such as Santocel, a silica aerogel should be provided in the space between the source and the outer container.

Having thus described our invention, we claim:

1. A radioactive cell comprising an evacuated outer envelope, a thermally-conducting capsule containing radioactive material characterized by emission of radiation selected from the group consisting of alpha particles and beta rays disposed therewithin, and a thermopile having two groups of alternate junctions and provided with a pair of output terminals forming the cell output, one group of junctions being in direct thermal contact with said capsule and electrically insulated therefrom and the other group being in thermal contact with said envelope whereby a temperature difference is maintained across said thermopile.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Chemical and Engineering News, vol. 32, No. 42, Oct. 18, 1954, pages 4183-4184.

RCA Atomic Battery.

Claims

1. A RADIOACTIVE CELL COMPRISING AN EVACUATED OUTER ENVELOPE, A THERMALLY-CONDUCTING CAPSULE CONTAINING RADIOACTIVE MINERAL CHARACTERIZED BY EMISSION OF RADIATION SELECTED FROM THE GROUP CONSISTING OF ALPHA PARTICLES AND BETA RAYS DISPOSED THEREWITHIN, AND A THERMOPILE HAVING TWO GROUPS OF ALTERNATE JUNCTIONS AND PROVIDED WITH A PAIR OF OUTPUT TERMINALS FORMING THE CELL OUTPUT, ONE GROUP OF JUNCTIONS BEING IN DIRECT THERMAL CONTACT WITH SAID CAPSULE AND ELECTRICALLY INSULATED THEREFROM AND THE OTHER GROUP BEING IN THERMAL CONTACT WITH SAID ENVELOPE WHEREBY A TEMPERATURE DIFFERENCE IS MAINTAINED ACROSS SAID THERMOPILE.