Abstract:
A fuze includes a housing having a first end and an opposing second end and defining an elongate channel extending between the first and second ends; and a mechanically activated reactive material comprising at least first and second elements disposed within the channel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Provisional Patent Application No. 61/473,552, filed Apr. 8, 2011, which application is hereby incorporated by reference along with all references cited therein. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with government support under Contract/Grant No. N00014-07-1-0969 awarded by the Office of Naval Research, and under Contract/Grant No. HDTRA1-08-1-0006 awarded by the Defense Threat Reduction Agency. The government has certain rights in the invention. 
    
    
     SUMMARY OF THE INVENTION 
     Generally speaking, a gasless ignition system (or fuze or delay element) includes a housing having a first end and an opposing second end and defining an elongate channel extending between the first and second ends; and a mechanically activated reactive material comprising at least first and second elements disposed within the channel. 
     It is an object of the present invention to provide an improved gasless ignition system. 
     Other objects and advantages of the present invention will be more apparent upon reading the following detailed description in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side, cross-sectional view of a gasless ignition system  10  with tunable delay in accordance with one embodiment of the present invention. 
         FIG. 2  is a side, cross-sectional view of a gasless ignition system  20  in accordance with another embodiment of the present invention. 
         FIG. 3  is an enlarged micrograph of a non-mechanically activated reactive mixture  14  comprising aluminum powder and nickel powder of the ignition system  10  of  FIG. 1 . 
         FIG. 4  is an enlarged micrograph of a mechanically activated reactive mixture  14  of aluminum and nickel, with a further enlarged portion of one of the mechanically activated particles. 
         FIG. 5  is a sketch depicting the mechanically activated reactive material  41  of  FIG. 4 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated herein and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described processes, systems or devices, and any further applications of the principles of the invention as described herein, are contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Generally speaking, a tunable gasless fuze is provided that has no gas byproduct during its combustion and can be tuned to ignite upon introduction to a substantially precise level of input energy and can be tuned to burn at a selected rate. 
     Referring to  FIG. 1 , there is shown a gasless ignition system (used interchangeably herein with “fuze” or “delay element”)  10  with tunable delay in accordance with one embodiment of the present invention. Fuze  10  generally comprises a housing  11  and a reactive material  12  that burns without a gas byproduct and acts, upon ignition at a first end  15  to ignite a combustible material  17  located at its opposite, second end  16 . In one embodiment, the reactive material  12  comprises aluminum and nickel that has been mechanically activated. 
     Housing  11  is any appropriate material that will hold the reactive material  12  in its physical condition throughout it combustion, such as metal, paper, ceramic, etc. 
     Mechanical activation is achieved by subjecting the reactive material to high energy ball milling, as discussed in J. D. E. White et al., “Thermal Explosion in Al—Ni System: Influence of Mechanical Activation,”  J. Phys. Chem. A , Vol. 113, No. 48, pp. 13541-13547, which is hereby incorporated by reference in its entirety. The reactive (the reactive material of a fuze) is typically a mixture of metal-metal or metal-non metal powders capable of producing a heterogeneous intermetallic material (NiAl, NiTi, etc.) or a refractory inorganic compound (TiC, TiB 2 , SiC, etc.) upon reaction. Both stoichiometric and non-stoichiometric mixtures of these powders, as well as mixtures that include other additives, are contemplated to function well as the reactive material. Many of the materials that are appropriate for the mechanical activation described herein also provide a benefit in being of a type that are environmentally benign and less or not hazardous to human or animal health. 
     In  FIG. 3  there is shown one example of a non-mechanically activated reactive material  14  comprising a mixture of 325 mesh aluminum powder and 3-7 μm nickel powder. Subjecting mixture  14  to mechanical activation reconfigures the materials into mechanically activated reactive material (ARM)  41 , comprising alternating strata, as shown in  FIG. 4  and depicted in  FIG. 5 , with the aluminum stratum  18  and the nickel stratum  19  having been forced together to form an extraordinarily high degree of interfacial, reactionary surface area. Here, a mechanically activated reactive material means a composition containing at least two elements that, upon heating to an ignition temperature (T IG ), will burn in the absence of other elements or externally provided heat, and wherein such two or more elements have been mechanically acted upon to deform into configurations wherein the specific area of interfacial contact among such two or more elements is considerably (i.e., more than 10 times) greater than before such mechanical activation. 
     Alternative embodiments are contemplated wherein the mechanically activated reactive material  14  is deformed to achieve a combined area of interfacial contact among such two or more elements that is less than 10 times that of non-mechanically activated reactive materials (such as powders), but still operates with a higher desired combustion rate and/or a decreased ignition temperature than that of non-mechanically activated reactive materials. 
     In one embodiment, the ball milling machine comprises a generally circular bowl, into which is deposited the aluminum and nickel powders along with ball bearings or similar agitating members. A lid covers the bowl, and the bowl is mounted in a machine that rotates the bowl about multiple axes to cause the ball bearings to act upon the powders, as described herein. Alternative embodiments are contemplated wherein the mechanical activation is accomplished in other ways that results in essentially the same physical reconfiguration of the reactive materials (e.g., Ni and Al) to substantially increase the interfacial contact among such reactive materials. 
     The delay of fuze  10  is the time it takes for fuze  10  to burn from its point of ignition  15  essentially to its opposite end  16  adjacent or proximal to the corresponding combustible material  17 , whereupon the energy output of fuze  10  ignites combustible material  17 . The delay of a fuze, in general, is a function of the material comprising the fuze, the cross-sectional size and shape of the fuze, the length of the fuze and the extent of limitation of heat loss from the fuze during its combustion. 
     More selective variation of the delay is achieved in fuze  10  by varying the extent of mechanical activation applied to the reactive material. That is, mechanical activation increases the area of contact between the reactive materials (Al and Ni), and by varying the extent of mechanical activation, the amount of such interfacial contact area is varied. The extent of mechanical activation can be varied, inter alia, by varying the milling intensity or the overall milling duration. Increasing the milling intensity and/or milling duration acts to further compress the materials and increase the number of strata, which increases the overall specific contact surface between reactives. Increasing the milling intensity is done by increasing the rotational speeds of the ball milling machine, varying the number and/or size of milling media (e.g., ball bearings), and by other means known in the art of such machine. 
     As such contact area is increased, the duration of the delay is shortened and the burning rate of the ARM is increased. Alternatively, decreasing the extent of mechanical activation will result in a slower burning rate material and a longer delay fuze. Furthermore, because of the increased interfacial contact areas, ARMs have a higher burning rate than non-mechanically activated materials and can be expected to propagate in a small channel better than non-mechanically activated compositions and can be expected to propagate in smaller channels where a non-mechanically activated reactive material would otherwise not be able to propagate. For example, in the reaction
 
Ni+Al→NiAl
 
the adiabatic reaction temperature (T ad ) will be about 1900K; and, for powders, the ignition temperature (T IG ) is about 930K, while for an ARM the ignition temperature (T IG ) is between 500-700K, depending on the extent of mechanical activation. However, the rate of combustion for the ARM, owing to the increased interfacial contact area, will be two to three times that of the powder mixture, and likely considerably more. As a particular reactive mixture  12  burns within the fuze housing  11 , heat losses through housing  11  can exceed the rate of heat generated by combustion of the reactive material  12 . If the channel  19  defined by housing  11  is too small for a given length, the heat generated by the slower burning powder material will not be enough to sustain combustion, and the powder based reaction will quench, while the ARM will continue to burn. Thus, the ARM will effectively burn and ignite the combustible material  17  in a smaller fuze channel  19  where the powder material will not.
 
     Current delay compositions rely heavily on the use of barium chromate and potassium perchlorate, which are both toxic and environmentally hazardous, while Al and Ni compositions are considerably more environmentally friendly and less or non-toxic. 
     In manufacture, the ARM  12  is packed into channel  19  of housing  11  with the appropriate force to keep ARM  12  in place and to achieve the desired material burn in a fashion similar to non-mechanically activated reactive materials. 
     The operation of the fuze begins with an input energy source  18  that ignites the mechanically activated material  12  in the delay element  10 . The delay element  10  then burns at a rate dependent on its mechanical activation level, providing the desired delay time. After the delay element combustion is complete, the heat release from the mechanically activated material ignites the combustible material (main charge)  17 . 
     In another embodiment shown in  FIG. 2 , a gasless ignition system  20  provides precise timing control and tunable ignition energy. Gasless ignition system  20  includes a housing  21 , a material charge  22 , and an initiator element  23 . The housing  21  is here a metal cup, but can be any appropriate structure configured to hold reactive material  22  in the desired shape and position adjacent a corresponding combustible material  26 . The initiator element is an Explosive Bridge Wire (EBW), semiconductor bridge (SCB) or hot wire, or any suitable initiating device for delivering a desired energy input to material charge  22 . 
     Like reactive material  14 , material charge  22  comprises a mass of mechanically activated gasless heterogeneous reactive material  28 , that is, a reactive material such as a metal-metal (e.g., Ni—Al) combination that has been subjected to mechanical activation (high energy ball milling or a similar process), as discussed in J. D. E. White et al., id. The mechanical activation process (high energy ball milling of the reactive material) allows the sensitivity of the gasless reactive mixture to its initiator energy input to be controlled. That is, the level of energy required to initiate exothermic reaction of the reactive material  28  can be selectively determined by varying the extent of mechanical activation. The extent of mechanical activation can be varied, inter alia, by varying the milling intensity or the overall milling duration. Igniter  20  can also include a small mass of oxidizable metal powder (not shown), like Zr or Ti positioned in one or more isolated areas and/or intermixed with the mechanically activated material  28 . The material charge  22  of gasless ignition system  20  could also contain another highly exothermic gasless heterogeneous mixture (not shown) that is not mechanically activated, but is ignitable with the mechanically activated mixture  28 . 
     The gasless ignition system  20  is initiated through an electric pulse sent from an external power supply to initiator element  23 , which then provides energy to the mechanically activated reactive material (ARM)  28 . When the energy thus imparted to ARM  28  reaches the desired (and specifically determined) threshold level, the ARM  28  ignites and burns from its point of ignition  31  and outwardly therefrom until it reaches its opposite end  33  proximal the corresponding combustion material  26 , against which it has been positioned. The combustion of ARM  28  releases heat completely through gasless reaction processes, which avoids the otherwise attendant volumetric gaseous expansion and explosion of the igniter  20 . The heat released by igniter  20  at its opposite end  33  ignites the combustible material  26 , as desired. The heat release also ignites the optional metal powder, which reacts exothermically with any pore gases, acting as a gas scavenger. The mechanically activated material  28  provides high heat release and can be tuned to ignite at different energy inputs, allowing for selection of appropriate EBW, SCB, or hot wire to provide the necessary resistance to inadvertent discharge. 
     This invention provides relatively precise timing control through use of the EBW or SCB, and it is a truly gasless system. Other ignition systems using a hot wire to provide the initial energy input do not inherently provide such precision in the ignition timing control as the required heating time in such systems will depend on the environmental heat loss conditions experienced by the igniter system. Use of mechanically activated reactive material will allow the use of a hot wire as a precise timing initiator, as the mechanically activated material can be made sufficiently sensitive to ignite at a desired controlled thermal inputs. For example, where it may be desired that ignition be achieved at with a specific low thermal input threshold, the mechanically activated reactive material can be processed to ignite at that threshold. 
     The invention also provides a completely gasless heat source, in contrast to other EBW or SCB initiated ignition systems. Such other systems utilizing EBWs or SCBs use a sensitive primary material, like lead azide, to initiate the igniter. Such materials upon ignition produce significant amounts of gas, and such igniters cannot, therefore, be considered gasless. 
     The invention has application for use with initiation of rocket motors, explosives, pyrotechnics (including fireworks), and expendable heat sources, but is not limited to these applications. Any application requiring an application of heat without a gas byproduct that could overpressurize the corresponding container, object or system would benefit from this invention. 
     Alternative embodiments are contemplated wherein ignition system  10  comprises a reactive material that does not result in gasless ignition, but that is tuned as described for ignition system  10  by being mechanically activated. While not gasless and thus not appropriate for uses where “gasless” ignition is required or desired, such non-gasless reactive material may nevertheless benefit from being tuned to have a more precise ignition temperature, a lower ignition temperature, a higher combustion rate, and/or the ability to propagate better in a smaller channel. 
     Alternative embodiments are contemplated wherein the mechanical activation is not “high-energy”, but is of any energy level, extent, duration or other characteristic that achieves the desired level of mechanical activation to deform and reconfigure the constituent reactive materials into strata or shapes that significantly increase the total contact surface area among the reactive materials to achieve a more precise ignition temperature, a lower ignition temperature, a higher combustion rate, and/or the ability to propagate better in a smaller channel. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.