Abstract:
Lithium fuel pellets are coated with a fluorinated oil, rather than a fluorinated polymer or telomer. The lithium pellets are coated by placing the pellets inside the lithium reaction vessel and then pouring a fluorinated oil into the reaction vessel. The reaction vessel is rotated in order to evenly coat the lithium pellets. The oil adheres to the lithium pellets and does not settle to the low point of the boiler cavity.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to a power source in which an alkali metal is oxidized to provide heat. More specifically, it relates to lithium pellets that are coated with a fluorinated oil in a lithium reaction vessel. 
         [0002]    In certain situations, it is necessary to provide propulsion in an environment where oxygen is not available. Underwater vehicles, such as torpedoes, are one such situation. Torpedo propulsion systems frequently use a vaporized fluid (such as water) to drive a turbine, and the turbine then drives a suitable propulsor. Typically, water is vaporized in a boiler that is heated by an intensely exothermic chemical reaction, such as metallic lithium oxidized with sulfur hexafluoride. 
         [0003]    The lithium used in such a boiler may be either a solid piece of lithium or lithium pellets coated with a fluorinated polymer. However, both of these forms of lithium fuel have certain drawbacks. 
         [0004]    For example, using a solid block of lithium involves using aluminum potassium perchlorate as a thermal starting device to heat the boiler and the lithium inside it to operating temperature. The aluminum potassium perchlorate generates high temperatures, typically in the range of about 3000 to 4700° C., and must be formed into pellets that are closely packed together in order to initiate the system. To achieve this, core holes are formed in the lithium, and the core holes are filled with the aluminum potassium perchlorate. A squib is also provided within the core hole, and the core is then sealed with a lithium plug. Substantial pressures are generated during ignition of the aluminum potassium perchlorate, requiring the boiler structure to be very strong. Moreover, if the aluminum potassium perchlorate, while undergoing oxidation, contacts the boiler surfaces, it can burn through parts of the boiler and damage the system. 
         [0005]    To avoid these problems associated with using a solid lithium core, encapsulated lithium pellets are more commonly used. The lithium pellets are generally spherical in shape and have diameters in the 1-25 millimeter range. The pellets are of varying sizes to allow for close packing of the pellets. In addition, each pellet is provided with a coating of a fluorine-substituted hydrocarbon material, typically a polymer or telomer. Commercially available materials suitable for such a coating include products sold under the trademarks Teflon® and Vydax®. The coated lithium pellets are placed inside a boiler and a starter squib is fired, which melts the pellets adjacent to the starter squib. The melted lithium pellets react with their coatings, generating sufficient heat to propagate the reaction throughout the boiler. Sulfur hexafluoride is supplied to the boiler in a controlled fashion to maintain the reaction at a desired rate. 
         [0006]    However, coating the lithium pellets with a fluorinated hydrocarbon is a complex, multi-stage process. For example, a typical process begins with placing the lithium pellets in a sealed tumbler in which they can be agitated. The interior of the tumbler is filled with an inert gas, such as argon. The coating material is dispersed in a liquid, and the resulting slurry is sprayed on the pellets inside the tumbler. The pellets are agitated during the spraying in order to provide an even coating. After the desired build-up of coating is achieved on the lithium pellets, the spraying is stopped and a slight vacuum is formed in the interior of the tumbler in order to evaporate the liquid applied to the lithium pellets within the tumbler. After the liquid has evaporated, the process is repeated several times until the desired thickness of coating is achieved. 
         [0007]    Alternatively, lithium pellets can also be coated inside of the boiler. Lithium pellets are placed inside of the boiler, and a fluorinated hydrocarbon dissolved or suspended in a solvent is poured into the boiler. The solvent is evaporated under a vacuum using heat. The process is repeated, if necessary, until a sufficiently thick layer of fluorinated hydrocarbon coats the lithium pellets. 
         [0008]    Coating lithium pellets with a sufficiently thick layer of fluorine-substituted polymer or telomer may require many layers to be applied, so the process can take several days to complete. Thus, there is an additional need for a simple, low-cost method of applying a coating to lithium pellets. In addition, the fluorine-substituted telomers commonly used to coat lithium pellets are no longer available, having been removed from the market because of environmental concerns. Thus, there is a need for a commercially available, off-the-shelf material that can be used to coat lithium pellets for use in a lithium reaction vessel. Both of these needs should be satisfied without significantly diminishing the advantages obtained by using coated lithium pellets in a lithium reaction vessel. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present invention is a lithium fuel pellet that is coated with a fluorinated oil, rather than a fluorinated polymer or telomer, and a process for applying the coating to a lithium pellet. An appropriate fluorinated oil is simply poured into the boiler after it is loaded with a binary mixture of lithium pellets. The oil adheres to the lithium pellets and does not settle to the low point of the boiler cavity. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a cross-sectional view of a portion of a lithium boiler utilizing the invention. 
           [0011]      FIG. 2  is a flow diagram illustrating the manner of coating lithium pellets with a fluorinated oil for use in a lithium boiler. 
           [0012]      FIG. 3  is a block diagram of the propulsion system with a lithium boiler utilizing the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  shows an embodiment of the invention. A mixture of lithium pellets including large lithium pellets  11  and small lithium pellets  12  is placed into boiler  20  of a lithium reaction vessel. Large lithium pellet  11  is roughly spherical in shape, with a diameter of about 1.25 centimeters (cm). Small lithium pellet  12  is roughly spherical or cylindrical in shape, with a diameter of about 1.5 millimeters (mm) and, for a cylindrical pellet, a length of about 1.5 mm. Because of their smaller size, small lithium pellets  12  react more readily than large lithium pellets  11 , so small lithium pellets  12  are used to initiate a reaction in boiler  20 . Large lithium pellets  12 , in turn, are used to maintain the reaction for the desired period of time. Those skilled in the art will recognize that the size and shape of lithium pellets used in the mixture of pellets is a design choice, and the invention is not limited to any particular size and shape of lithium pellets. Similarly, while lithium is used as an example because it is the metal most commonly used as fuel, other alkali metals, particularly sodium and potassium, may also be used. 
         [0014]    After large lithium pellets  11  and small lithium pellets  12  have been placed in boiler  20 , fluorinated oil  30  is poured into boiler  20 . Fluorinated oil  30  is a liquid, and it fills the spaces between and around large lithium pellets  11  and small lithium pellets  12 . Commercially available fluorinated oils that can be used in connection with the invention include oil and grease products sold under the trademark Krytox® by E.I. du Pont de Nemours and Company, and particularly the Krytox® GPL 100-107 series. Krytox® is a perfluoropolyether (PFPE), also called perfluoroalkylether (PFAE) or perfluoropolyalkylether (PFPAE). Krytox® fluorinated oils are a series of low molecular weight, fluorine end-capped, homopolymers of hexafluoropropylene epoxide. The polymer chain is completely saturated and contains only the elements carbon, oxygen and fluorine; hydrogen is not present. On a weight basis, Krytox® contains 21.6% carbon, 9.4% oxygen and 69.0% fluorine. 
         [0015]    After the fluorinated oil  30  is introduced into boiler  20 , boiler  20  is rotated or otherwise agitated in order to evenly coat large lithium pellets  11  and small lithium pellets  12  with fluorinated oil  30 . Fluorinated oil  30  adheres to the surface of large lithium pellets  11  and small lithium pellets  12 , forming a coating  32  on the lithium pellets. The period of agitation is relatively short, typically only a few hours. 
         [0016]    Lithium boiler  20  is now fueled and ready for use. A reaction may be initiated in any conventional way, such as using a squib and a detonation cord (not shown) to ignite small lithium pellets  12 . The detonation cord will raise the temperature of small lithium pellets  12  above their melting point, and small lithium pellets  12  will react with coating  32 . This reaction is exothermic and releases sufficient heat to melt large lithium pellets  11 , causing them to react with coating  32  and fluorinated oil  30 . Thus, the reaction spreads throughout boiler  20 . An oxidizer, such as sulfur hexafluoride, is then injected into boiler  20 . The lithium reacts with the sulfur hexafluoride in an intensely exothermic manner, raising boiler  20  to its operating temperature of about 1100° C. Boiler  20  then heats a working fluid, such as water, above its boiling point, and the resulting steam is used to turn a turbine. 
         [0017]      FIG. 2  is a flow diagram illustrating the manner of coating lithium pellets with fluorinated oil for use in a lithium reaction vessel. The process  100  begins at Start box  110 . Process  100  then moves to box  120 , where a binary mixture of lithium pellets is introduced into the lithium reaction vessel of the boiler. As discussed previously, the invention is not limited to any particular size of lithium pellets. Different sizes and shapes of lithium pellets are commonly mixed together when lithium pellets are used as fuel. Optimally, the mixture of lithium pellets used at box  120  is a binary mixture of pellets similar to large lithium pellets  11  and small lithium pellets  12 , which were discussed with respect to  FIG. 1 . 
         [0018]    Next, at box  130 , fluorinated oil sufficient to coat the lithium pellets is added to the binary mixture of lithium pellets in the boiler. As discussed with respect to  FIG. 1 , commercially available fluorinated oils that can be used in connection with the invention include oil and grease products sold under the trademark Krytox® by E.I. du Pont de Nemours and Company, and particularly the Krytox® GPL 100-107 series. 
         [0019]    At box  140 , the boiler is agitated to uniformly coat the mixture of lithium pellets with the fluorinated oil. Typically, this agitation involves rotating the boiler containing the lithium pellets and fluorinated oil. However, any sort of agitation sufficient to coat the lithium pellets with fluorinated oil could take place at this step. 
         [0020]    The process ends at End box  150 . 
         [0021]    The present invention can be used in any application in which lithium is used as fuel. The oxidation of lithium with sulfur hexafluoride does not require oxygen, so it can occur in places where oxygen is not available, such as underwater and outer space. Most commonly, lithium reaction vessels are used as a heat source for propulsion in underwater devices, particularly torpedoes, and this invention can certainly be used in that application. Lithium reaction vessels have also been considered for use as a heat source for emergency power supplies for space vehicles, and the present invention could also be used to fuel a lithium reaction vessel used in that manner. 
         [0022]      FIG. 3  is a block diagram showing one application of the invention. Propulsion system  300  includes boiler  310 , turbine  320  and propulsor  330 . Boiler  310  corresponds to boiler  20  in  FIG. 1  and is filled with a mixture of lithium pellets that are coated with a fluorinated oil, as described in detail with respect to  FIG. 1 . Boiler  310  heats a working fluid, such as water, and the resulting steam is used to turn turbine  320 . Turbine  320  is connected to propulsor  330 , and propulsor  330  turns along with turbine  320  to generate propulsion. 
         [0023]    The present invention is a lithium pellet coated with a fluorinated oil for use as fuel in a lithium reaction vessel and a process for coating the pellets. The invention can use readily-available commercial materials and takes a small fraction of the amount of time needed to coat lithium pellets according to the prior art. 
         [0024]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.