Radioactive battery

Aug. 18, 1959 a. thomas 2,900,535

RADIOACTIVE BATTERY

Filed June 28, 1956

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Tnuentor, Alexander Thomas,

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United States Patent Office

2,980,535

Patented Aug. 18, 1959

2,900,535

RADIOACTIVE BATTERY

Alexander Thomas, Weston, Mass., assignor to Tracerlab, Inc., Boston, Mass., a corporation of Massachusetts

Application June 28,1956, Serial No. 594,564

Claims. (Cl. 310—3)

This invention relates to the generation of electrical energy, and more particularly to novel methods of and apparatus for converting the energy of radioactive radiations directly to electrical energy. This application represents an extension of the principles disclosed in application Serial No. 385,612, filed October 12, 1953 by Roger Sweetser and applicant and assigned to Tracerlab, Inc.

In the above-identified application there is disclosed apparatus for converting radiations into useable electric power comprising, in general, a casing in which are stacked a plurality of spaced bi-metallic electrodes, one of the metals being of high work function and the other of appreciably lower work function. The electrodes are surrounded by a common volume of an ionizable gas, and in the preferred embodiment, beta-emitting tritium gas is mixed with the ionizable gas to uniformly ionize the same. The ions formed in the gas under the influence of the radiation are collected by the field produced by the contact potential difference of the high and low work function metals. Output terminals are connected to the high work function metal of the electrode at one end of the stack and to the low work function metal of the electrode at the other end of the stack, the open circuit potential of the battery being substantially the product of the contact potential difference of the metals used multiplied by the number of electrodes in the stack.

The apparatus disclosed in the earlier application is, on the whole, satisfactory, but difficulty has been experienced in the fabrication of the bi-metallic electrodes, particularly in getting them flat when of desired thinness. Also, the plastic insulating spacers employed in the battery were difficult to fabricate and assemble, were subject to objectionable ingassing, and proved to be somewhat unstable under continued radiation bombardment. Further, it has been observed that, the electrical characteristics of the battery are somewhat unstable with time, due, it is believed, to deterioration of the surfaces of the electrodes by oxidation caused by water vapor present in the filling gas.

It is, accordingly, an object of the present invention to provide an improved nuclear battery of the contact potential difference type.

Another object of the invention is to provide a nuclear battery’having good electrical stability with time.

Another object of the invention is to provide an improved method of fabricating electrodes for a nuclear battery to permit ease of handling, to insure flatness, and to provide stability over long periods of time.

The novel features considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof will best be understood by reference to the following description and to the drawings, in which:

Fig. 1 is an elevation cross-section of the battery constructed in accordance with the invention;

Fig. 2 is an exploded view illustrating the manner in which the cells of the battery are assembled; and

Fig. 3 is a fragmentary cross-section, greatly enlarged, illustrating the details of construction of the battery.

Referring now to Fig. 1, the battery comprising a plurality of cells cascaded within a common container, comprises an outer cylinder 10, preferably formed of stainless steel, within which is fitted a sleeve 11 of inorganic insulating material, an insulator known as “Alsimag 243,” 10 consisting of aluminum, silicon and magnesium, and distributed by American Lava Company, Chattanooga, Tennessee, having been found particularly satisfactory for the purpose. To provide a sealed fit between the outside of sleeve 11 and the inside of cylinder 10, the 15 outer surface of sleeve 11 is preferably coated with a glaze 12 of lead borosilicate, and the interior surface is also similarly glazed, as indicated at 13 to eliminate the possibility of ingassing. Cylinder 10 is slightly longer than sleeve 11, providing a step at one end to receive a 20 disc 14 of insulating material, which, like sleeve 11, is preferably formed of inorganic “Alsimag 243.” Disc 14 is of a size to fit snugly within cylinder 10, and to seal the insulator in place, insulator 14 is glazed and a bead 15 is applied around its periphery. To stabilize 25 its insulating properties, insulator 14 is coated on the exterior surface with a thin coating of silicone, and while not to be construed as limiting, may consist of 2% by weight of Dow Corning 200 Fluid, Viscosity Grade 100, distributed by the Dow Corning Corporation 30 of Midland, Michigan, in perchlorethylene sprayed onto the insulator and the baked. It has been observed that when thus coated, the physical and electrical properties of insulator 14 are unaffected by wide variations in temperature, and smearing with fingerprints did not lower 35 the resistance. The inner surface of disk 14 is glazed simultaneously with the interior of sleeve 13, sealing the two parts together and providing an interior surface of uniform characteristics. Insulator 14 is provided with a circular aperture 14« in which is firmly sealed a con40 ducting cylinder 16, having a central recess 16a in which is positioned a fine spring 17, the functions of which will be later described.

Stacked inside sleeve 11 are a plurality of electrode plates 20, one surface of each of which is of a relatively 45 high work function material and the other of low work function material, the number of plates employed depending upon the desired open-circuit voltage of the battery. The plates 20 are uniformly spaced apart, by a plurality of annular shaped spacers 21, formed of inorganic in⁵⁰ sulating material, such as mica. It has found that by splitting mica sheets, reasonably uniform thicknesses can be achieved, it having been found satisfactory to space plates 20 apart about .004 inch. Mica is preferred 5g over polystyrene, or other plastic insulators, since it is ⁰ not subject to absorption of gas as is plastic, and is not distorted at the soldering temperatures involved in the fabrication of the battery. Plates 20 are centered in sleeve 11 by a similar plurality of centering rings 22, 60 also preferably formed of mica for the reasons stated above, and of a thickness comparable to that of plates 20. Alternatively, centering rings 22 and spacers 21 may be formed of “Fiberglas” cloth coated with resin and flaked mica, these shapes being readily stamped from a sheet 65 of the material, and the somewhat tedious hand splitting of mica is eliminated.

As best seen in Fig. 3, plates 20, shown greatly enlarged, each comprise a thin supporting disc 20« of metal, preferably type 304 stainless steel, which is available in 70 very small thicknesses, e.g., .0015 inch, and one of the few materials on which surfaces having suitable work functions can be applied. In accordance with the in2,900,535 vention, instead of spot welding two dissimilar metals together to form the couple, dissimilar metals are deposited on the opposite faces of disc 20α, the latter functioning only to support the dissimilar materials and not otherwise contributing to or affecting the electrical characteristics of the battery. Thus, each plate may be made extremely thin, and when stacked in sleeve 13 has the desirable physical characteristics of flatness and rigidity. The disc 20α is coated on one face with a suitable high work function material, such as lead dioxide, and is coated on the other face with a suitable low work function material, such as magnesium or zinc.

In the fabrication of electrode plates 20, a pair of sheets of type 304 stainless steel are first cleaned in a solution consisting of 7 parts of concentrated sulphuric acid to one part of saturated potassium dichromate in water, the surfaces being wiped with glass wool while in this solution. The sheets are then washed in tap water, and may be stored indefinitely in distilled water. For deposition of the lead dioxide coating, a pair of sheets are taped together and immersed in a stainless steel tank containing 2.8% lead nitrate in water solution. Using a potential source of 5 volts, with the positive terminal connected to the sheets and the negative terminal connected to the tank, anodization to lead dioxide surfaces 206 results in five minutes at 0.54 milliampere per square centimeter. The black lead dioxide coating appears only on the outer surfaces of the sheets, the inner surfaces being unaffected. The thus anodized sheets may also be stored indefinitely in distilled water preparatory to application of the other surface.

The low work function surface is applied by evaporation, the anodized surfaces of two sheets being faced together and held flat in a vacuum evaporator above a crucible containing, for example, reagent grade magnesium turnings. The crucible is heated by an electrical heating element, the heat being adjusted to give approximately .0002 inch evaporated coatings in a half hour. Immediately following evaporation, the magnesium coating has a dull white appearance and relatively high work function, but upon polishing with dry glass wool it becomes lustrous metallic and exhibits the same low work function of pure magnesium sheet metal. Thus, upon completion of the evaporation step, the stainless steel 20α is coated on one side with a very thin layer 206 of lead dioxide and on the other side with a thin layer 20c of magnesium (or other low work function material which can be evaporated, such as zinc). The prepared sheet is cut into strips, which are then placed between paper and the electrode plates 20 punched out. The paper keeps the surfaces clean and prevents the electrodes from contaminating each other. The contact potential difference of lead dioxide and magnesium is about 1.8 volts; accordingly, each cell of the battery has an open circuit voltage of about 1.8 volts, and by stacking a plurality of cells, the potential is increased substantially in proportion to the number of cells whereby a battery of reasonable voltage is attainable.

In the assembly of the battery, with insulator 14, terminal 16 and spring 17 in place, and the interior glazed as described above, a centering ring 22 is first dropped into the bottom of the sleeve 12, and an electrode plate 20 centered therein with the lead dioxide surface 206 thereof directed downward. Thereafter, a_; spacer ring 21 is dropped in, then another centering ring, another electrode, and so on, the parts being conveniently handled with tweezers. All of the electrode plates 20 are directed the same way, so that after assembly, the high work function surface of one opposes the low work function of tire adjacent electrode plate, and the upper end plate 20' has its low work function surface directed upward. As illustrated in Fig. 3, there is a small clearance (exaggerated in the drawing for clarity) between the outer periphery of the electrode plates and the inner periphery of centering rings 22, between the outer peripheries of spacers 21 and rings 22 and the glaze 13, and between the adjacent surfaces of spacers 21 and rings 22. These clearances are sufficient to permit gas to diffuse through the battery from one cell to the next upon introduction 5 at one end of the stack, but the overlap of the spacer rings 21 on the peripheries of the electrode plates effectively minimizes gaseous electrical leakage from one side of an electrode plate to the other. There is a certain amount of resiliency to the thus stacked electrodes and 10 spacers, and when the container is filled to the top of sleeve 11, a fine wire helical-spiral spring 24 is positioned centrally of electrode plate 20', and the end closed by stainless steel end plate 25. Plate 25 is of a diameter equal to the inside diameter of cylinder 10, and is pro15 vided with a circular groove 26, the battery being sealed by “spinning” the upper edge of cylinder 10 into the groove as shown, and thereafter soldered. A tubulation 27, preferably of copper, is soldered in a central opening in end plate 25, this tubulation being used for pumping 20 the battery and filling with proper gases (to be described) and thereafter sealed off as shown. Electrical contact is made between the upper surface 20c of plate 20' and plate 25 through spring 24, and conducting cap 28 threaded to a boss 29 on plate 25, encloses the tubula²⁵ tion 27 and provides a convenient external terminal for the battery. Spring 17 makes essentially point contact with the under surface of the lower plate, and with cylinder 16, forms the other terminal of the battery.

With the battery assembled as above-described, i.e.·, ³⁰ with the high work function lead dioxide surface 206 directed downward and the low work function surface 20c directed upward, terminal 16 becomes the negative terminal and cap 28 becomes the positive. It will be understood, however, that should the application require, the 35 polarity of the battery may be reversed by simply re. versing the electrode plates 20 during assembly.

After assembly of the battery as described (save for the sealing of tubulation 27) air and other gases are thoroughly removed by connecting a vacuum pump to 40 tubulation 27, and applying heat to the battery during the pumping operation. Pumping is continued until a pressure gauge, such as a Pirani gauge, indicates negligible out-gassing when the battery is isolated from the pump. After thorough evacuation, which may require pumping 45 for about 18 hours at 80-95° C., the battery is filled with tritium gas diluted with hydrogen, and argon to provide a medium for gaseous ionization to take place. The tritium, hydrogen and argon may be premixed prior to filling, or they may be introduced separately, the natural con50 strictions inherent in the construction of the battery serving admirably in effecting admixture of the gases within the battery. In order to prevent contamination and deterioration of the battery with time, spectroscopically controlled hydrogen and argon are preferably employed. ⁵⁵ The electrical characteristics of the battery are to some extent dependent upon the pressure and molecular weight of the ionizable gas, it having been found, however, that the use of argon as the fill gas at a pressure of about six to nine atmospheres or more is necessary for the elec60 trode spacings employed.

The number of available radioisotopes suitable for a battery of this type is quite limited, and of those investigated, tritium, which has been described above, appears to be the most desirable. This isotope is readily and con⁵ veniently introduced into the battery, has a reasonably long half-life of 12.4 years, has short range beta particles, and the maximum beta particle energy of 18 kev. is sufficiently low that none of the radiation escapes to the 70 exterior of the battery.

It will, of course, be understood that the battery in accordance with the invention may be constructed of any physical size, the length, however, being largely controlled by the voltage desired and the thickness of elec75 trode plates 20 and spacers 21. By way of example,

2,900,535 applicant has constructed a battery having 59 cells, the spacing between electrodes 20 being about .005, and the low and high work function surfaces being evaporated and abraded magnesium and anodized lead dioxide, respectively. The battery has a diameter of 2.3 cm. and is 5 2.3 cm. long, the volume therefore being about 0.6 cubic inch. The average electrode contact potential difference with these surfaces is 1.8 volts, the open circuit voltage of the battery being approximately 115 volts. The saturation current of the battery is of the order of 1400 10 micro-microamperes.

While lead dioxide and magnesium have been found satisfactory as the electrode materials for general application, the use of other materials is, of course, within the contemplation of the invention. Investigation has indi- 15 cated, for example, that at low temperatures the battery is somewhat sluggish in its operation, due, it is believed, to the formation of an insulating magnesium dioxide layer on the magnesium surface. Since zinc has less spontaneous reaction with air than magnesium, it is promising 20 as a low work function material which can be expected to give less sluggishness in current response and faster voltage recovery at low temperature. With the stainless steel plate thoroughly cleaned, as above-described, and protecting the cleaned surface between cleaning and coat- 25 ing, it is possible to evaporate zinc onto the stainless steel in a hard and tightly adherent coating which may be buffed to a lustrous finish.

Although the high work function lead dioxide surface has long time chemical stability in air at room tempera- ³³tures and up to temperatures to which the battery might be exposed in use, there is the possibility that the surface might deteriorate over long periods of time (10-15 years) in the highly ionized atmosphere of the battery. Such deterioration has not been observed in the period of ³³investigation, but with higher activity per cell and elapse of more time, it may occur. A promising alternative, which appears not to have this drawback, is one of the oxides of nickel. The nickel oxide can be anodized, using a suitable plating solution, onto thoroughly cleaned 40 stainless steel to form a hard surface.

From the foregoing, it is apparent that applicant has provided a gaseous ionization contact potential difference battery having practical electrical characteristics of convenient dimensions. The use of evaporated low work 45 function electrodes greatly improves the flatness and uniformity of electrode couples without sacrificing the high potential difference when used with lead dioxide anodized on the other side. The use of inorganic insulation instead of plastic increases the useful temperature range, de- 50 creases ingassing of the insulators, and reduces the temperature coefficient of voltage. The “Alsimag” sleeve glazed with lead borosilicate and the terminal insulator have high temperature stability and low electrical conductivity, and the silicone coating on the terminal insulator 55 protects it from electrical leakage and is not affected by extremes in temperature. While two materials have been suggested for each of the high and low work function surfaces, it can be expected that others might be found by ones skilled in the art which may be respectively 60 plated and evaporated onto a supporting plate. A specific embodiment has been described, including examples of suitable dimensions, but it will be understood that this description is intended to be illustrative only, and that the invention be limited only by the appended claims. 65