SOLID HYDROGEN FUEL ELEMENTS AND METHODS OF MAKING THE SAME

Abstract

A hydrogen fuel element (10, 110, 210) includes a heat-generating pyrotechnic charge (12) which comprises any suitable pyrotechnic material and an ammonia borane encasement (16, 116). The encasement partly (encasement 16) or wholly (encasement 116) encases the pyrotechnic charge (12). An ignition train (14) is powered by electrical leads (28a, 28b) to ignite pyrotechnic charge (12) to heat both the ammonia borane binder it contains and the encasement (16, 116), which itself includes or is made entirely of ammonia borane. Hydrogen is evolved from the heated ammonia borane binder and encasement. The hydrogen fuel element (10, 110) may be encased within a suitable housing (30) which may be made of a carbon open-cell foam.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns ammonia borane solid fuel elements which provide a source of hydrogen gas. In particular, the present invention provides a solid fuel element comprising a heat charge and an ammonia boron encasement for the pyrotechnic charge.

2. Related Art

The use of binders in energetic materials is well known in the art, including the use of polybutadienes, fluoroelastomers, polyesters and copolymers of the foregoing. Some binders are reactive and some are non-reactive, the latter having the effect of diluting the energetic material so that a lesser energetic output per weight of bonded (binder-containing) energetic material is attained. Thermosetting binder systems are mixed as liquid monomers with chemical crosslinking agents poured into molds containing the energetic material and then heated and cured. Powder-based binders are dissolved in an appropriate solvent and the binder precipitates out of solution such that it coats the particles of energetic material. Polytetrafluoroethylene (“PTFE”) provides a solventless system in which the PTFE liquefies and flows under pressure to coat and bond the particles of energetic material.

In some cases it is desired to embed particles of energetic material within the binder to form a hydrogen solid fuel element as a coherent body comprising a binder matrix having particles of energetic material disposed therein. In other cases it is desired to encapsulate the reactive material within an encasement comprised of the binder. The amount of binder used determines the desired mechanical properties of the resulting product. In any case, whether a reactive or non-reactive binder is utilized, the binder is not a source of hydrogen and consequently reduces the fuel element's gravimetric efficiency (weight of hydrogen produced per unit weight of the fuel element). In the case of hydrogen-generating solid fuel cartridges, a pyrotechnic or other activating charge is juxtaposed with a hydrogen source, such as a pyrolytic hydride, e.g., ammonia borane, so that ignition of the activating charge decomposes the pyrolytic hydride to release hydrogen gas. Those skilled in the art will appreciate that in many applications it is highly desirable to maximize the hydrogen output per unit weight of cartridge, that is, to provide a hydrogen-generating material of high gravimetric efficiency. In such cases, it is necessary to minimize the amount of material in the cartridge which does not generate hydrogen. As noted above, conventional prior art binders, whether reactive or unreactive, do not generate hydrogen and of course inert containers used to house the solid fuel cartridge produce no hydrogen and further decreases gravimetric efficiency.

Published U.S. Patent Application US 2003/0180587 A1 of Peter Brian Jones et al. for “Portable Hydrogen Source” discloses hydrogen-generating elements comprising a pellet holder provided with one or more recesses, within which recesses hydrogen-generating pellets are retained. The pellets may comprise a mixture of ammonia borane and hydrazine bis-borane and optionally other compounds. See page 3, paragraph [0045]. Page 5, paragraph [0063] discloses a combination of LiAlH₄ and NH₄Cl in one layer and ammonia borane in another layer of the hydrogen-generating pellets.

U.S. Pat. No. 4,315,786 of William D. English et al. for “Solid Propellant Hydrogen Generator” issued on Feb. 16, 1982, discloses a particulate metal reactant comprised of at least two metals which undergo an exothermic reaction to form an intermetallic compound in quantity to sustain decomposition of a borane reactant to yield hydrogen (or deuterium). The example at column 3 of the application discloses pressing into a pellet a mixture of fine (micron-sized) nickel and aluminum powders and ammonia borane. A pellet was pressed from this composite mixture and ignited by electrical leads wrapped around the pellet.

U.S. Pat. No. 3,666,672 of Ralph H. Hiltz for “Hydrogen Generating Compositions” issued on May 30, 1972, discloses an autogeneously combustible composition which liberates hydrogen on burning. The composition contains an alkali metal borohydride and hydrazine sulfate in specified proportions. The mixture is “compressed to form a coherent compact” (column 1, lines 18-22).

U.S. Pat. No. 4,157,927 of W. M. Chew et al. for “Amine-Boranes as Hydrogen Generating Propellants” issued on Jun. 12, 1979, and is typical of a plethora of prior art disclosing the generation of hydrogen or deuterium upon combustion of a mixture containing an amine-borane or a derivative thereof and a reactive heat-producing compound or a heat-producing mixture. At column 2, there is disclosed the preparation of such a mixture of fine powders and, after mixing, “the mixed powder is then pressed into pellets using pressure from about 500 to about 10,000 pounds total load.” (column 2, lines 25-28).

SUMMARY OF THE INVENTION

The present invention is based, at least in part, on the discovery that ammonia borane can be utilized both as an encasement for the pyrotechnic charge used for heating hydrogen-generating materials to release hydrogen gas therefrom and as a binder for such pyrotechnic, e.g., thermite, charges. Ammonia borane itself is a pyrolytic hydride which, as is well known to those skilled in the art, evolves hydrogen when heated. As a result, the ammonia borane binder or encasement itself evolves hydrogen when heated by reaction of the pyrotechnic material.

Generally, in accordance with the present invention, ammonia borane is utilized as an encasement for a pyrotechnic activating charge, to provide a hydrogen-generating fuel element suitable for use in a hydrogen cartridge, which fuel element comprises an encasement which generates hydrogen upon initiation of the pyrotechnic activating charge.

The present invention further contemplates utilizing ammonia borane as a binder for the pyrotechnic material used to heat hydrogen-generating materials used in fuel elements of, e.g., hydrogen generation cartridges. Because the ammonia borane binder itself is a source of hydrogen, the gravimetric efficiency of the fuel element, and therefore of the hydrogen-generating device, is increased.

Specifically, in accordance with the present invention there is provided a solid hydrogen fuel element comprising a pyrotechnic charge and a coherent, self-sustaining ammonia borane encasement which partly or fully encases the pyrotechnic charge. The pyrotechnic charge, e.g., a thermite, may optionally include a binder such as ammonia borane which, upon being heated to its activation temperature, releases hydrogen.

Another aspect of the present invention provides that the ammonia borane encasement is made by a process of molding ammonia borane into a hollow shape by placing the ammonia borane into a suitably shaped mold and applying sufficient pressure to the ammonia borane within the mold to render the encasement as a coherent, self-sustaining body, and placing the pyrotechnic charge within the encasement.

One aspect of the present invention provides for applying to the ammonia borane a pressure of at least about 2,000 psi, e.g., a pressure of from about 2,000 to about 10,000 psi.

Another aspect of the present invention provides that the pyrotechnic charge is made by a process of admixing an incoherent pyrotechnic charge, e.g., a powder or granular pyrotechnic charge, with a binder, e.g., ammonia borane, to form an admixture of the binder and the incoherent pyrotechnic charge. A related aspect of the invention provides that the binder is present in the admixture in a quantity which is sufficient, upon application to the admixture of suitable pressure, typically at least about 2,000 psi, e.g., from about 2,000 to about 10,000 psi, to render the incoherent pyrotechnic charge into a coherent, self-sustaining body. The quantity of binder present, however, must not be so great as to preclude reliable ignition and burning of the pyrotechnic charge.

Another aspect of the present invention provides a solid hydrogen fuel element as described above having an ignition train positioned in energy transfer relationship with the pyrotechnic charge.

Yet another aspect of the present invention provides for a hydrogen fuel element further comprising a housing enclosing the ammonia borane encasement and the pyrotechnic charge, which housing is pervious to hydrogen gas generated by the fuel element.

A method aspect of the present invention provides for malting a solid hydrogen fuel element comprising subjecting ammonia borane to a pressure of at least about 2,000 psi, e.g., from about 2,000 to about 10,000 psi, to form a coherent, self-sustaining hollow encasement of ammonia borane, and at least partly encasing a pyrotechnic material within the encasement. Optionally, the method may comprise fully encasing the pyrotechnic material within the ammonia borane encasement.

Another method aspect of the present invention comprises admixing a binder with the pyrotechnic material, the binder comprising a pyrolytic hydride, e.g., ammonia borane, characterized by evolving hydrogen at least when heated to a temperature sufficiently high to evolve hydrogen from ammonia borane.

The present invention also provides a method of making a solid hydrogen fuel element further comprising mounting an ignition train in signal transfer communication with the pyrotechnic charge of the fuel element.

Related aspects of the present invention provide one or more of the following: the ignition train may be mounted within the ammonia borane encasement; the ignition train has an output end which may contact the pyrolytic material; and the encasement and the pyrotechnic charge may be enclosed within a housing which is pervious to the flow of hydrogen gas from the interior to externally of the housing.

As used herein and in the claims the term “pyrotechnic material” means any non-explosive energetic material which may be initiated to evolve at a temperature high enough to release hydrogen from ammonia borane. Preferably, the pyrotechnic material will attain temperatures high enough when initiated to drive off all the hydrogen from a molecule of ammonia borane.

As used herein and in the claims, the term “coherent, self-sustaining” encasement or body means an encasement or body which has been formed into a given shape and is able to retain that given shape during normal handling and filling in a manufacturing method as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a hydrogen fuel element suitable for use in a hydrogen cartridge in accordance with one embodiment of the present invention;

FIG. 1A is a view, enlarged relative to FIG. 1, of the portion of FIG. 1 enclosed by area A;

FIG. 2 is a schematic cross-sectional view in elevation of a hydrogen fuel element suitable for use in a hydrogen cartridge in accordance with a second embodiment of the present invention;

FIG. 3 is a schematic cross-sectional elevation view of a hydrogen fuel element suitable for use in a hydrogen cartridge in accordance with a third embodiment of the present invention; and

FIGS. 4A-4E schematically illustrate a method of manufacture of a solid hydrogen fuel element in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION AND SPECIFIC EMBODIMENTS THEREOF

As described in the above-mentioned U.S. Published Patent Application 2003/0180587 A1, a hydrogen-generating device may comprise a pellet holder provided with a plurality of recesses within which hydrogen-generating pellets are retained. In other types of devices, the hydrogen-generating fuel may be sized or shaped differently from pellets. Such hydrogen-generating fuel elements (sometimes below referred to simply as “hydrogen fuel elements” or “fuel elements”) comprise a heat-generating or other activating charge and a solid source of hydrogen, which source may be ammonia borane (NH₃BH₃). A hydrogen fuel element may be equipped with an initiation device to initiate the heat-generating or other activating charge, and a suitable container.

A typical hydrogen fuel element contains an ignition train comprised of an initiation element, a pick-up charge and an ignition charge. The ignition train is placed in energy-transfer relationship, e.g., in contact with or embedded at least partly within, a main pyrotechnic charge. A solid hydrogen fuel charge is in energy-transfer relationship with the main pyrotechnic charge. A housing at least partly encloses the components. Each of the initiation element, pick-up charge, ignition charge and main pyrotechnic charge self-propagates a reaction before igniting the next material in the train, ultimately the main pyrotechnic charge whose heat output generates hydrogen gas from the solid hydrogen fuel. In general, the energetic materials must remain in close proximity, if not in intimate contact, in order that each element ignite the next adjacent element to fully function the device. The housing performs several functions including, but not limited to, providing structural support to the components, isolating the energetic materials from their surroundings, and participating in the functional output. Housings can be machined, molded or formed from almost any suitable solid material.

Referring now to FIG. 1, a hydrogen fuel element 10 comprises a heat-generating pyrotechnic charge 12, an ignition train 14 and an ammonia borane encasement 16 which partially encases heat-generating pyrotechnic charge 12. Encasement 16 (like encasement 116 of FIG. 2) preferably may be made entirely of ammonia borane in order to enhance gravimetric efficiency (weight of hydrogen produced per unit weight of encasement 16 or 116). Encasement 16 (and 116) may include other materials provided that such other materials are not present in such amounts as to prevent forming the encasements into coherent, self-sustaining bodies. If such other materials are included in encasements 16 or 116, they are preferably other pyrolytic hydrides so that they too generate hydrogen upon being heated in order to contribute positively to gravimetric efficiency. Ignition train 14 is comprised (FIG. 1A) of an ignition charge 18 in contact with pyrotechnic charge 12, a pick-up charge 20 in contact with ignition charge 18, a semiconductor bridge igniter 22 mounted on a standard TO-46 header 24 and embedded within pick-up charge 20. These components are all contained within a charge holder 26. A pair of electrical leads 28a, 28b are connected to header 24 to supply electrical power to semiconductor bridge igniter 22 from a source not shown.

Pyrotechnic charge 12 may comprise any suitable pyrotechnic material, such as a thermite. Thermite is a mixture of a reactive metal, usually aluminum, although other reactive metals such as manganese could be used, and an oxide such as iron oxide (Fe₃O₄ or Fe₂O₃). Aluminum has significant advantages as the reactive metal, including cost and ease of handling. The reactive metal, e.g., aluminum, and the metal oxide, e.g., Fe₂O₃, are usually in powder form and are often mixed with a binder to prevent separation of the powders. When heated to reaction temperature the aluminum metal is oxidized in an aluminothermic reaction which produces aluminum oxide, heat at high temperatures (of up to about 2,500° C. for aluminum/iron III oxides) and the metal, e.g., iron, of the metal oxide. Generally, such thermite materials when ignited release heat at temperatures on the order of about 2,000° C. to 2,500° C., have a very high caloric output to weight ratio and produce little or no gas when burned. Other pyrotechnics may be utilized such as, for example, various combinations of fuel and oxidizers. Exemplary fuel-oxidizer combinations are aluminum/cupric oxide; aluminum/ferric oxide; silicon/cupric oxide and silicon-boron/ferric oxide. Other combinations of pyrotechnics can be utilized employing one or more fuels, for example, aluminum, boron, silicon, titanium, zirconium and molybdenum in combination with one or more oxidizers such as cupric oxide, ferric oxide (Fe₂O₃), tin dioxide and titanium dioxide.

Ammonia borane is a white powder which, when placed under sufficient pressure, e.g., when subjected to a pressure of about 2,000 pounds per square inch (“psi”) or more, will consolidate and retain its shape under its own weight. Unless specifically otherwise stated, all pressures given herein or in the claims are given in pounds per square inch absolute (“psi”). Ammonia borane pellets so produced have been handled and dropped without deformation. In addition, ammonia borane can be added, in various weight percentages, to a pyrotechnic material to provide different physical characteristics. With a smaller weight percentage of ammonia borane, the mix with a pyrotechnic material, e.g., a thermite, tends to be crusty and brittle. As the weight percentage of ammonia borane increases, the formed material becomes more ductile. When the pyrotechnic charge 12 contains sufficient ammonia borane as a binder it can be formed into a coherent, self-sustaining body. The amount of ammonia borane binder thus determines the mechanical properties of pyrotechnic charge 12. Higher levels of binder impart greater elasticity over a wide temperature range. Lower levels of binder can be useful to assist in granulation or pelletization. Properties such as crush strength, granule size, stress, strain, and modulus are strongly influenced by the amount of ammonia borane admixed with the pyrotechnic material of pyrotechnic charge 12. Ammonia borane works well as binder and when used as part of a pyrotechnic charge 12 will produce hydrogen when the pyrotechnic charge 12 burns, thereby improving the gravimetric efficiency of the fuel element. The proportion of ammonia borane binder used in the pyrolytic charge should not, however, be so great as to render ignition, burning and attainment of desired heat output and temperature of the pyrotechnic charge problematic. The upper limit of ammonia borane used as binder will vary depending on the ignition and burning characteristics of the particular type of pyrotechnic material used in the pyrotechnic charge; the lower limit will depend on the desired properties as noted above, (crush strength, granule size, etc.) of the pyrotechnic charge.

The pyrotechnic charge 12 must retain its coherence and position within the fuel element during manufacturing, storage, and handling. This may be accomplished by enclosing an incoherent (e.g., powder or granular) pyrotechnic mixture within a suitable encasement, such as encasement 16 described below. In addition, the pyrotechnic charge 12 may itself be rendered into a coherent body, or at least a less incoherent body, by admixing with it a sufficient quantity of a binder, preferably a hydrogen-producing binder, for example, ammonia borane, which will increase coherence of the pyrotechnic charge. Suitable processing may be employed to treat the admixture of binder and pyrotechnic charge, for example, subjecting the ammonia borane to a pressure of at least about 2,000 psi will form it into a coherent body. If ammonia borane is used in sufficient quantity as the binder in the admixture of binder and pyrotechnic charge, subjecting the admixture to a pressure of, for example, 2,000 to 10,000 psi, will render the admixture as a coherent body. If the amount of ammonia borane binder in the admixture is insufficient to render the admixture as a coherent, self-sustaining body it will at least be a less incoherent mass, i.e., it will tend to agglomerate rather than flow or separate into particles or granules.

A cavity (not shown) that retains its shape may be formed in the pyrotechnic charge 12 to allow ignition train 14 to be at least partly embedded within pyrotechnic charge 12. Alternatively, ignition charge 18 may simply be placed in abutting contact with the surface of pyrotechnic charge 12. Ignition train 14 is required to start combustion of pyrotechnic charge 12. Adding a binder to pyrotechnic charge 12 is one means of ensuring that the pyrotechnic charge 12 is a coherent body which can be retained within the fuel element and against or within which an ignition train 14 can be mounted. The use of mechanical means such as a metal or plastic (synthetic organic polymer) housing or container or the like for such purpose is not practical for devices requiring high gravimetric efficiency (mass of hydrogen produced relative to overall mass of the device). One of the biggest drawbacks of using inert or reactive but non-hydrogen producing binders or mechanical housings in this device, is that such binders or housing would reduce the gravimetric efficiency of the system by adding to the system mass which does not produce hydrogen. Another drawback is that such expedients reduce hydrogen generating efficiency by absorbing heat needed to pyrolyze the hydrogen solid fuel, e.g., ammonia borane. Many non-reactive binders can also produce gaseous species (chlorine- and fluorine-based compounds) that are harmful, e.g., to a fuel cell system to which the evolved hydrogen is supplied. Reactive binders used with pyrotechnic materials produce gaseous products (carbon monoxide and nitric oxides) that are also harmful to fuel cells. Although a few energetic binders exist, the majority of the chemical binders absorb heat from the pyrotechnic reaction. All such chemical binders add mass which does not produce hydrogen.

Encasement 16 preferably is comprised of substantially pure ammonia borane and may be formed into a coherent, self-sustaining cup-like body of sufficient structural strength to contain pyrotechnic charge 12 with ignition train 14 mounted thereon. The cold flow properties of ammonia borane permit forming it into a coherent, self-sustaining encasement without need for additives of any kind, thereby providing an encasement of high gravimetric efficiency. Although not preferred, in other embodiments, encasement 16 may comprise ammonia borane admixed with other materials, preferably with other hydrogen-evolving materials such as other pyrolytic hydrides, e.g., hydrazine bis-borane, N₂H₄(BH₃)₂. Encasement 16 contains at least a sufficient percentage by weight of ammonia borane to insure that encasement 16 is capable of being formed into a coherent, self-sustaining body of sufficient structural strength and coherence to serve as an encasement for pyrotechnic charge 12.

The ammonia borane encasement 16 produces hydrogen when the hydrogen fuel element 10 is functioned. Encasing the pyrotechnic charge 12 in ammonia borane optimizes the use of heat from the pyrotechnic charge in that pyrotechnic-generated heat must flow through ammonia borane.

Utilizing ammonia borane as the binder for pyrotechnic charge 12 and as the sole or major component of encasement 16 will contribute to the overall production of hydrogen while generating few destructive gaseous species. Combined with a suitable pyrotechnic, e.g., a thermite in pyrotechnic charge 12, the ammonia borane binder material is heated to temperatures high enough to remove hydrogen from the ammonia borane. The intimate admixture of ammonia borane binder and the pyrotechnic material particles of pyrotechnic charge 12 insures good heat transfer to the ammonia borane upon ignition of the pyrotechnic charge, which facilitates heating the ammonia borane to a temperature high enough to attain removal of all three mols of hydrogen from the ammonia borane. As those skilled in the art will appreciate, removal of the third mol of hydrogen requires attainment of a higher temperature than that required to remove the first two mols of hydrogen, and the presence of an appropriate proportion of ammonia borane binder in intimate admixture with a suitable pyrotechnic, e.g., thermite, facilitates the attainment of such high temperature by the ammonia borane. The ammonia borane used to form the pellet-like pyrotechnic charge 12 and encasement 16 is also a, or the, primary reactant for the hydrogen generation. The unique “cold flow” properties of ammonia borane allow not only for forming a coherent, self-sustaining encasement, but also allows for pelletization of a mixture of ammonia borane particles with particles of a suitable pyrotechnic material, in cases where solid hydrogen fuel elements having the form of pellets having good mechanical properties are desired.

In operation of the solid hydrogen fuel element 10 of FIG. 1, the semiconductor bridge igniter 22 is supplied with electrical power through leads 28a, 28b and generates a plasma which ignites pick-up charge 20. The pick-up charge 20 in turn ignites the ignition charge 18 which ignites the pyrotechnic charge 12. As the pyrotechnic charge 12 burns, its energy is transferred to the ammonia borane (a pyrolytic hydride) contained in pyrotechnic charge 12 and in encasement 16, causing the ammonia borane to decompose and give off hydrogen.

Referring now to FIG. 2, there is shown an embodiment of the invention in which a hydrogen fuel element 110 comprises an encasement 116 of ammonia borane which entirely encloses the pyrotechnic charge 12. (Components of the several embodiments illustrated in the Figures which are identical are identically numbered; corresponding components which are not identical are numbered by adding 100 to the counterpart element of the other embodiments.) The only opening in encasement 116 is closed by an ignition train 14; the passage of ignition train 14 (or any other device) through encasement 116 and into contact with pyrotechnic charge 12 is not deemed to change the fact that encasement 116 “fully encases” pyrotechnic charge 12. The quoted term is deemed to embrace structures in which the encasement is penetrated by an ignition train or the like. Ignition train 14, which may be identical to that of FIG. 1A, is, in the illustrated embodiment, mounted in contact with pyrotechnic charge 12 to insure that ignition train 14 is in signal transfer communication with pyrotechnic charge 12. The term “signal transfer communication” in this context means merely that functioning of ignition train 14 will ignite pyrotechnic charge 12. Ignition train 14 has an output end (provided in the illustrated embodiment of FIG. 1A by ignition charge 18) which contacts and which optionally may penetrate into pyrotechnic charge 12 so as to be partly embedded therein, to promote good signal transfer to, and ignition of, pyrotechnic charge 12. Electrical leads 28a, 28b protrude from encasement 116. This fully-encased arrangement improves functional reliability while improving the gravimetric hydrogen production efficiency by including additional ammonia borane.

The encasement 116 of ammonia borane will also keep the ignition train 14 in compression, thus keeping all energetic interfaces (between semiconductor bridge igniter 22, pick-up charge 20, ignition charge 18 and pyrotechnic charge 12) in intimate contact with each other. Even small gaps (0.005″ or less) between these elements can cause disruptions in the propagation and cross propagation of the reaction between adjacent ones of the different energetic materials.

Electrical leads 28a, 28b protruding from the encasement 116 allow an electrical signal to be sent to the ignition train 14 to start the production of hydrogen. The ammonia borane encasement 116 (like encasement 16 of FIG. 1) provides structural support during manufacturing, storage and operation and provides containment which keeps the pyrotechnic charge 12 in intimate contact with the ignition charge 18 of ignition train 14.

Referring now to FIG. 3, there is shown an embodiment of the present invention in which the solid hydrogen fuel element 210 further comprises a housing 30 within which is contained a fuel element similar or identical to element 110 of FIG. 2. Housing 30 may be made of any suitable material having sufficient structural strength and permitting the passage therethrough, via openings (not shown) or via a porous or foraminous structure, of hydrogen gas generated by functioning of hydrogen fuel element 210. For example, housing 30 may comprise a high temperature refractory carbon foam comprised of open-cell pores. Such material not only will allow generated hydrogen to flow through the porous structure of housing 30, but will capture particulate and liquid phase reaction products, and retain heat from the pyrotechnic charge 12 while slowly distributing heat to control the temperature within the housing 30. The foam has a low thermal conductivity, a pore size capable of retaining solids and liquid (formed during the functioning of the device), allows hydrogen to freely flow through it, and has a low density. The foam material is machinable but can be molded to shape during manufacturing and/or machined to final dimensions and configuration. Carbon foams have a lower density relative to other solid porous materials and so the weight penalty imposed by this non-hydrogen generating material is limited. The foam manufacturing process can be tailored to change the pore size and the thermal insulating properties of the foam. Fibers can be incorporated into the carbon foam and these fibers act as condensing surfaces for liquid reaction products. Many of the condensed reaction products can still give off hydrogen if sufficient heat is evolved by pyrotechnic charge 12 to maintain a sufficiently high temperature.

Generally, it is seen that the hydrogen fuel element comprises a pyrotechnic charge, an ignition train in energy-transfer relationship with the pyrotechnic charge, which is wholly or partly encased within an ammonia borane encasement and may include an ammonia borane binder. The hydrogen fuel element may also include a suitable outer casing or housing, such as a carbon foam housing.

The fuel elements 10, 110 and 210 may be manufactured by molding ammonia borane powder at a pressure of from about 2,000 to 10,000 psi to form a coherent, self-sustaining body generally in the form of an open cup as illustrated in FIG. 1. A pyrotechnic charge 12 may then be placed within the formed cup of ammonia borane. The ammonia borane may be left within the mold if it is desired to press the pyrotechnic material to compact it. The pyrotechnic charge 12 may contain a pyrolytic hydride binder, such as ammonia borane. For a full ammonia borane encasement as illustrated in FIG. 2, the open ammonia borane cup is closed by pressing a layer of ammonia borane over the open end, thereby enclosing pyrotechnic charge 12 within the full encasement. In this embodiment, the ignition train 14 may be disposed within the closing layer of ammonia borane. In all cases, the ignition train is in signal transfer communication with, preferably in abutting contact with or slightly embedded within, pyrotechnic charge 12 to insure good contact.

For production of the hydrogen fuel element as illustrated in FIG. 3, a suitable housing, which may be a high-temperature refractory carbon foam as described above, may be formed by any suitable means to enclose ammonia borane encasement 116.

Housing 30 provides a more robust structure as well as enhancing a reliable contact between elements of the ignition train 14. The housing 30, however, unavoidably reduces the gravimetric efficiency of the hydrogen fuel element 210. The ammonia borane encasement 116 and pyrotechnic charge 12 can be pressed into or poured into the housing 30. When the materials are ammonia borane-containing powders which are pressed into housing 30, the powder will bind within the housing 30, creating a residual stress at the interface of the walls of housing 30 and the pressed powder. The residual stress keeps the material intact within the housing 30, thus preventing the materials from moving during storage, transportation, or operation. For pressed powders, the housing provides structural support during the pressing process.

FIGS. 4A-4E illustrate steps in the manufacture of solid hydrogen fuel element 10 of FIG. 1. A mold 32 has a charge 34 of ammonia borane placed therein as shown in FIG. 4A. FIG. 4B shows a die 36 inserted within mold 32 to apply sufficient pressure, e.g., a pressure of about 2,000 to about 10,000 psi, to form the incoherent charge 34 into a coherent, self-sustaining encasement 16 within mold 32, as shown in FIG. 4C. A charge 38 of pyrotechnic material is introduced into encasement 16 while the latter is still retained within mold 32, as shown in FIG. 4D. A suitable binder, preferably a pyrolytic hydride binder, most preferably ammonia borane, is included in charge 38 so that upon application of pressure by die 36 to charge 38 to form pyrotechnic charge 12 as illustrated in FIG. 4E, the coherence of pyrotechnic charge 12 is increased, preferably to the point of rendering it as a coherent body. In some cases, the pyrotechnic charge may be left in a partly agglomerated state, i.e., may not be formed into a coherent, self-sustaining body. In other cases, pyrotechnic charge 12 may be left in a granular or powder form.

If it is desired to manufacture the solid hydrogen fuel element 110 of FIG. 2, another processing step (not illustrated) is utilized to encase the exposed surface of pyrotechnic charge 12 (FIG. 4E) with a coherent layer of ammonia borane. Upon the application of suitable pressure, a closing layer of ammonia borane can be sealed to the lip of the “cup” of ammonia borane encasement 16 while the latter is in the mold 32. A suitable opening may be left in, or made in, the closing layer of ammonia borane to receive an ignition train such as ignition train 14 of FIG. 1A. Alternatively, some other suitable cover may be applied. It is preferred, however, to provide an encasement made entirely of ammonia borane as illustrated in FIG. 2 for enhanced gravimetric efficiency. Subsequent manufacturing steps (not illustrated) are utilized to add to hydrogen fuel elements 10, 110 or 210 an ignition train 14 and, in the case of hydrogen fuel element 210, a housing 30.

Generally, any suitable pyrotechnic material can be utilized to provide heat to the pyrolytic hydride. The pyrotechnic material should have a high energy density, be ignitable by a semiconductor bridge device (or other energetic charge), yield insignificant gas production, be environmentally green (before and after combustion), burn in an inert atmosphere at atmospheric pressure, and retain its shape during manufacturing, storage and operation. A number of energetic materials (thermites, heat powders and intermetallic mixes) exist which have a high energy density, low gas output, manageable ignition properties, are capable of sustaining combustion in an inert atmosphere at atmospheric pressure, and are environmentally acceptable. These materials require binders or high pressure to create the configurations shown in the figures.

The ammonia borane will produce hydrogen as the pyrotechnic charge burns. Due to the very high temperature (greater than 2,000° C.) generated during the combustion of the pyrotechnic charges, and the close proximity of the ammonia borane to the pyrotechnic charge, e.g., thermite, a high proportion of the hydrogen contained in the ammonia borane is released.

While the invention has been described in detail with respect to a specific embodiment thereof, it will be appreciated that the invention has other applications and may be embodied in numerous variations of the illustrated embodiment.

Claims

1. A solid hydrogen fuel element comprising a pyrotechnic charge and a coherent, self-sustaining ammonia borane encasement at least partly encasing the pyrotechnic charge.

2. The fuel element of claim 1 wherein the encasement fully encases the pyrotechnic charge.

3. The fuel element of claim 1 or claim 2 wherein the pyrotechnic charge includes a binder which upon being heated to its activation temperature releases hydrogen.

4. The fuel element of claim 3 wherein the binder comprises ammonia borane.

5. The fuel element of claim 1 or claim 2 wherein the ammonia borane encasement is made by a process of molding ammonia borane into a hollow shape by placing the ammonia borane into a suitably shaped mold and applying sufficient pressure to the ammonia borane within the mold to render the encasement as a coherent, self-sustaining body, and placing the pyrotechnic charge within the encasement.

6. The fuel element of claim 5 including applying a pressure of at least about 2,000 psi.

7. The fuel element of claim 5 including applying a pressure of from about 2,000 to 10,000 psi.

8. The fuel element of claim 5 wherein the pyrotechnic charge is made by a process of admixing an incoherent pyrotechnic charge with a binder to form an admixture of the binder and the incoherent pyrotechnic charge.

9. The fuel element of claim 6 wherein the binder is present in the admixture in a quantity which is sufficient, upon application of sufficient pressure to the admixture, to render the incoherent pyrotechnic charge as a coherent, self-sustaining body, but which quantity is not so great as to preclude reliable ignition and burning of the pyrotechnic charge.

10. The fuel element of claim 9 including applying to the admixture a pressure of at least about 2,000 psi.

11. The fuel element of claim 10 wherein the binder comprises ammonia borane.

12. The fuel element of claim 8 wherein the pyrotechnic composition comprises one or more fuel/oxidizer couples and wherein the fuel is selected from the group consisting of one or more of aluminum, boron, silicon, titanium, zirconium and molybdenum, and the oxidizer is selected from one or more of cupric oxide; Fe3O4; Fe2O3; tin dioxide and titanium dioxide.

13. The fuel element of claim 8 wherein the pyrotechnic composition is selected from the group consisting of one or more of the following fuel/oxidizer couples: aluminum/cupric oxide; aluminum/Fe3O4; aluminum/Fe2O3; silicon/cupric oxide; silicon-boron/Fe3O4 and silicon-boron/Fe2O3.

14. The fuel element of claim 8 wherein the pyrotechnic material comprises a thermite.

15. The solid hydrogen fuel element of claim 1 or claim 2 further comprising an ignition train positioned in energy transfer relationship with the pyrotechnic charge.

16. The fuel element of claim 15 wherein the ignition train is embedded in the encasement.

17. The fuel element of claim 15 wherein the ignition train is positioned within the pyrotechnic charge.

18. The fuel element of claim 15 wherein the ignition train is embedded within the encasement and extends into contact with the pyrotechnic charge.

19. The fuel element of claim 15 further comprising a housing enclosing the ammonia borane encasement, which housing is pervious to hydrogen gas generated by the fuel element.

20. A method of making a solid hydrogen fuel element comprising subjecting ammonia borane to a pressure sufficient to form it into a coherent, self-sustaining hollow encasement of ammonia borane, and at least partly encasing a pyrotechnic material within the encasement.

21. The method of claim 20 including fully encasing the pyrotechnic material within the ammonia borane encasement.

22. The method of claim 20 or claim 21 wherein the pressure is at least about 2,000 psi.

23. The method of claim 20 or claim 21 wherein the pressure is from about 2,000 to about 10,000 psi.

24. The method of claim 20 or claim 21 further comprising admixing a binder with the pyrotechnic material, the binder comprising a pyrolytic hydride characterized by evolving hydrogen at least when heated to a temperature sufficiently high to evolve hydrogen from ammonia borane.

25. The method of claim 20 or claim 21 further comprising mounting an ignition train in signal transfer communication with the pyrotechnic charge.

26. The method of claim 25 wherein the ignition train is mounted within the ammonia borane encasement.

27. The method of claim 25 wherein the ignition train has an output end and mounting the output end in contact with the pyrolytic material.

28. The method of claim 25 further comprising enclosing the ammonia borane encasement within a housing which is pervious to the flow of hydrogen gas from the interior to externally of the housing.