1. Field of the Invention
The present invention relates to methods for preparing and using energetic fills containing crystalline high explosive materials.
2. Related Art
The basic standard methods for loading energetic or explosive materials into munitions are press-loading, and cast loading (whether using melt-cast or cast-cure techniques). With the relatively recent emergence of the production of smart weapon systems that are lighter and smaller and have greater lethality and survivability, the need exists for smaller, reliable Safe and Arm (S&A) devices for activating the explosive train of the explosive device. The challenges in producing Micro-Energetic Initiators (MEI) for Micro Electro-mechanical Mechanisms (MEMS) as safe and arm devices, involve the need to introduce the energetic materials into extremely small volumes and to have the energetic materials function properly after such introduction. MEIs for safe and arm devices will necessarily be smaller in size and weight than traditional fuzing devices, and will permit a larger loading of the energetic fill of the end item, thereby resulting in increased lethality. The standard loading methods mentioned above cannot be used to load the very small (microliter) volumes contained in these devices.
Considering the latter point in more detail, as indicated previously, the standard methods for loading an energetic fill into a munition are press-loading and cast-loading. With respect to the former, delivering the material to the fixture, followed by consolidation thereof by pressing, presents difficulties because of the very small required volume of the solids. Further, because of the delicateness of the materials of construction of the critical fixture, press loading of the energetic fill into the fixture is not a viable option. One potential approach would be to prepare a pellet of the energetic material externally of the fixture, and then load the pellet into the fixture. To complete the process, in order to maintain the pellet in place, some kind of adhesive would have to be applied to the pellet, e.g., on the side thereof, or to the wall of the fixture. It will be appreciated that such a process would be cumbersome and relatively costly.
As was also mentioned previously, casting of an energetic fill into a fixture can be done either by melt casting and cast curing. Melt casting basically entails heating a substance to a temperature above its melting point, adding any needed ancillary materials to the melt, pouring the mixture into the volume to be filled, and allowing the fill to solidify in place. Among other problems with this approach, because of the very small delivery volumes involved in producing MEIs for safe and arm devices, heat loss to the ambient environment would be a problem and, in this regard, could cause the energetic material to solidify before being emplaced.
Cast curing basically entails mixing the substance to be cast in a liquid polymer mixed with a cross-linking reagent. The resultant cast mixture has a finite “pot life” after which the viscosity of the mixture increases because of the chemical crosslinking process. This change in rheological properties may cause difficulty in the delivery into the fixture of energetic material prepared in this way.
There are, of course, a number of state-of-the-art delivery devices for the delivery of small volumes of materials including ink jet printing. The latter is a mature technology that can be used to accurately deliver small volumes of material. However, the present technology is unsuitable for delivering energetic materials for two reasons. First, most inks used for ink jet printing are dye-based, i.e., the colorant dye is dissolved in the fluid medium, and although there are pigment-based ink jet inks available wherein the colorant is an undissolved crystalline material, the undissolved solids are of a sub-micron particle size. Important secondary high explosives such as CL-20 (epsilon HNIW) are not presently available in a sub-micron particle size. Further, in an ink jet printer, the ink is typically delivered from the print head by a piezoelectric discharge that ejects droplets of ink at elevated pressure and temperature onto the printing substrate; the combination of an electric discharge and high temperature/pressure may be a safety hazard when attempting to deliver energetic materials using ink jet printing.