Patent Application
20100058945
1. Field of the Invention
This invention relates to explosives and their trigger mechanisms and more particularly to triggers that are activated by exposure to electromagnetic radiation.
2. Description of the Related Art
An explosive device includes a mass of explosive and a trigger to initiate detonation of the explosive. Historically, initiation is achieved through direct contact between the trigger and the explosive. For example, a trigger based on a chemical process may burn a length of combustible material from a free end into the explosive device to ignite the explosive material causing the explosive, otherwise known as a “fuze”. A trigger based on an electrical process applies a direct electrical current to the explosive, or possibly a smaller booster explosive, to initiate detonation. Electrical triggers are commonly used in warheads.
More recently, the use of microwave radiation with a frequency in the range from about 1,000 MHz to about 30,000 MHz has been reported to ignite high explosives such as PETN, RDX, HMX and TNT (see Kazuo Hasue, Masami Tanabe, Nobutune Watanabe, Shoji Nakahara, and Fumiaki Okada, “Initiation of Some Energetic Materials by Microwave Heating,” Propellants, Explosives, Pyrotechnics, vol. 15, pp. 181-186 (1990), incorporated by reference herein). When these explosives are exposed to microwave energy, their temperatures increase as they absorb the microwaves until the exothermic reaction takes over and they ignite. Hasue reported delays of more than 70 seconds to ignite the explosive, attributable to the low dielectric loss of the explosives.
U.S. Patent Application Pub. No. US 2006/0011083 entitled “Microwave Heating of Electric Materials” by William L. Perry et al. published Jan. 19, 2006, which is hereby incorporated by reference, describes a process for mixing the explosive with materials (“sensitizers”) that readily absorb microwaves. When exposed to microwave energy, the sensitizer creates “hot spots”, and ignition occurs at many locations through the entire explosive. The addition of sensitizers reduces both the amount of microwave energy required to ignite the explosive and the ignition delay from tens of seconds to milliseconds. Effective sensitizers have a dielectric loss that ranges from one to several orders of magnitude greater than that of the explosive. Examples of sensitizers include carbon nanotubes, metal powder, semiconductor powder and mixtures thereof. In one reported example, a charge of HMX (0.5 gram) mixed with carbon nanotubes (1 percent by mass) ignited with 7.5 joules at an average rate of 750 W for 10 milliseconds. To raise a charge of the same mass of neat HMX to an autoignition temperature of 200 degrees Celsius would require much more energy (about 110 joules) for a longer duration about 150 milliseconds at most.
The present invention provides a process for the formation of a more efficient and controllable triggering mechanism for solid explosives ignited through the application of electromagnetic energy. This is accomplished with an electrophoresis process for dispersing and aligning nanostructures in-situ in the explosive or ex-situ in a pre-preg layer (or layers) that is then introduced to the explosive.
In an embodiment of the in-situ approach, a mass of conductive nanostructures is placed in an uncured explosive solution. An electric field is applied to disperse and align the nanostructures in the uncured explosive with the field lines via electrophoresis. While the electric field is applied, the solution is cured to form a solid explosive including an aligned nanostructure trigger. The electrophoresis process must be controlled to allow electrophoretic migration of the nanostructures while not inducing a reaction (such as an explosion) until the migration is complete. The process can be repeated with the same or a different electric field to layer the explosive in order to better distribute the aligned nanostructure triggers through the volume of the explosive.
In an embodiment of the ex-situ process, a mass of conductive nanostructures is placed in an uncured pre-preg host solution. An electric field is applied to disperse and align the nanostructures in the uncured host solution with the field lines via electrophoresis. While the electric field is applied, the host solution is cured to form a pre-preg layer including an aligned nanostructure trigger. The pre-preg layer is placed in an uncured explosive solution. The explosive is heated to melt the pre-preg layer and remove the pre-preg host. The explosive is cured to form a solid explosive with an impregnated aligned nanostructure trigger. The viscosity of the uncured pre-preg host is low enough to allow migration of the nanostructures in the applied electric field while the viscosity of the uncured explosive is high enough to prevent reaggregation of the nanostructures due to Van der Waals forces once the pre-preg host is removed prior to curing the explosive. Multiple pre-preg layers having the same or different aligned nanostructures may be distributed throughout the volume of the uncured explosive solution.
In another embodiment of the ex-situ process, multiple pre-preg layers are used to form a laminate in the uncured explosive. The laminate may be formed by shifting and rotating the same pre-preg layer, by modifying the electrophoresis process to create pre-preg layers with different patterns of aligned nanostructures or by a combination of the two approaches. Removal of the pre-preg host causes the aligned nanostructures in each layer to move to a common plane to form a more complex trigger. This more complex trigger can be designed to efficiently couple to a particular electromagnetic signal to initiate detonation. This has the dual benefit of being very sensitive to a specified trigger signal while rejecting false trigger signals or random electromagnetic radiation. Multiple laminate structures may be distributed throughout the volume of the uncured explosive solution.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:
a and 4b are process flow diagrams for the in-situ process;
a through 6d are process flow diagrams for the ex-situ process
a and 7b are diagrams of an embodiment for distributing multiple aligned nanostructure pre-preg layers throughout the volume of the explosive;
a through 8g are process flow diagrams for forming different pre-preg layers via the electrophoresis process and laminating them to form a more complex aligned nanostructure trigger in the explosive;
a and 9b are diagrams illustrating the translation and rotation of pre-preg layers in a laminate to form a more complex aligned nanostructure trigger in the explosive; and
The present invention provides a process for the formation of a more efficient and controllable triggering mechanism for solid explosives ignited through the application of electromagnetic energy.
Perry's “mixing” of CNTs with the explosive will leave a mass of CNTs that are tangled together with random orientations. The CNTs will not disperse uniformly through the explosive material in response to mixing. The Van der Waals forces that hold CNTs together are very strong. Dispersal is desired for more uniform volumetric triggering. Also randomly oriented CNTs have relatively poor coupling efficiency to the incident electromagnetic radiation. Aligned structures improve coupling efficiency.
The present invention provides a process for dispersing nanostructures throughout the explosive (or a portion thereof) and aligning them to form an aligned nanostructure trigger or ‘antenna’ with improved coupling efficiency to the incident electromagnetic radiation. Any aligned structure is an “antenna”; more complex aligned structures can be more discriminating in that they provide high coupling efficiency to desired radiation (e.g. a specific band) and low coupling efficiency to undesired signals to reduce the risk of accidental or malicious triggering of the explosive.
An electrophoresis process is used to overcome the Van der Waals forces and disperse and align the nanostructures to the field lines of an applied electric field in-situ in the explosive or ex-situ in a pre-preg layer (or layers) that is then introduced to the explosive. The in-situ process requires fewer steps but is more constrained in the trigger structures that can be formed and care must be taken not to detonate the explosive during the electrophoresis process. The ex-situ process requires additional steps but provides greater flexibility to create trigger structures and the electrophoresis process is performed on a lower viscosity non-explosive material.
In the ex-situ approach, a mass of conductive nanostructures is placed in an uncured explosive solution. An electric field is applied to disperse and align the nanostructures in the uncured explosive with the field lines via electrophoresis. While the electric field is applied, the solution is cured to form a solid explosive including an aligned nanostructure trigger. The electrophoresis process must be controlled to allow electrophoretic migration of the nanostructures while not inducing a reaction (such as an explosion) until the migration is complete. The process can be repeated with the same or different electric field to layer the explosive in order to better distribute the aligned nanostructure triggers through the volume of the explosive.
In the ex-situ approach, electrophoresis is used to form aligned nanostructures in a pre-preg layer. The pre-preg host has a low viscosity that allows the nanostructures to disperse and align to the field lines and will not explode. The electric field is maintained while the pre-preg host is cured. The pre-preg layer (or layers) is placed in the uncured explosive solution. Multiple layers may be dispersed throughout the volume of the explosive to improve the uniformity of the volumetric explosion and/or laminated to form a more complex antenna. The solution is kinetically heated to remove the pre-preg host. The high viscosity of the uncured explosive is used to advantage to hold the nanostructures in their dispersed and aligned pattern preventing them from reaggregating under the influence of Van der Waals forces. The explosive is then cured to provide a solid explosive with an impregnated aligned nanostructure trigger. This trigger is more sensitive to the desired electromagnetic radiation and provides more uniform volumetric detonation.
As shown in
The system includes an electromagnetic source 20 that generates electromagnetic radiation 22. The source may be positioned proximate the explosive and itself triggered by external means (e.g. coded detonation signal). The radiation regions of interest are the long wave RF (50 Hz to 1 GHz), microwave (1 GHz to 30 GHz) and terahertz (30 GHz to 10 THz) as these regions interact with and rapidly heat nanostructures. The nanostructures have a conductivity or dielectric loss at least 10× that of the explosive materials and typically higher. Radiation 22 passes from the source through a waveguide 24, also known as an applicator, to solid explosive 12. The waveguide has the typically desirous effect of applying a uniform field to the explosive so that the aligned nanostructures are heated simultaneously to produce a uniform volumetric explosion. In some applications, the waveguide may be formed around only a portion of the explosive for a more directional explosion or the waveguide may be omitted entirely.
Aligned nanostructure trigger 14 will more efficiently couple radiation 22 than randomly oriented and clumped CNTs. The effect is to reduce the required energy from the source to initiate detonation. The trigger can be designed to preferentially absorb radiation 22 in certain bands. This allows the explosive to be configured so that it is only triggered by the certain band of radiation, known only to those in control of the explosive, and to reject or at least be less sensitive to other bands of radiation. This reduces the likelihood of a false detonation due to random environmental radiation or deployed countermeasures.
As used herein, “nanostructures” are conductive materials that have at least one dimension in the nanometer scale (i.e. less than 1 micrometer). All three dimensions may be less than 1 micrometer. This definition of nanostructures encompasses nanotubes and nanowires and fullerenes and nanopowders. The nanotube may be single-walled nanotube (SWNT) or a multi-walled nanotube (MWNT). Carbon nanotubes (CNTs) are one commonly found version of a nanostructure. Other conductive materials such as Nitrogen, Boron, Titanium, Silicon, Germanium, Aluminum and Gallium may also be used to form nanotubes. The nanotubes or fullerenes may or may not be themselves impregnated with explosive materials (see related co-pending application U.S. Ser. No. 11/530,081 entitled “Improved Explosive Materials By Stabilization In Nanotubes”).
Nanostructures may interact with electromagnetic energy through different pathways including conduction, resonances and bond vibrations. The specific pathway of absorption isn't as important as the rate. The absorption rate of these structures to radiation in the long wave RF, microwave and THz ranges can be very high. This high rate allows them to heat up very rapidly and produce an efficient volumetric explosion. Nanostructures such as nanotubes or nanowires that have an aspect ratio (length/diameter) greater than 1 and typically greater than 10 generally exhibit a higher absorption rate. These structures are more conductive than fullerenes and powders and tend to have induced and permanent dipole moments that enhance the resonant coupling between structures. These structures also send to exhibit better migration during electrophoresis.
The nanotubes of the present invention may be prepared by any known method, and some are commercially available. In general, each of these synthesis processes produces CNTs as a tangled mess. The CNTs are held together by various forces, most noticeably Van der Waals forces. A wide variety of methods have been devised for producing CNTs since the early disclosures by Iijima et al., including “Helical microtubules of graphitic carbon”, NATURE, 354, 56 (1991) and “Single-shell carbon nanotubes of 1-nm diameter”, NATURE, 363, 605-606 (1993). For example, a number of methods are mentioned in U.S. Pat. No. 7,052,668, the disclosure of which relating to preparation of SWCNTs is incorporated herein by reference. SWCNTs are commercially available presently in small commercial quantities. Various methods are known for synthesis of carbon nanotubes, and presently there are three main approaches. These include the laser ablation of carbon (Thess, A. et al., SCIENCE 273, 483 (1996)), the electric are discharge of graphite rod (Journet, C. et al., NATURE 388, 756 (1997)), and the chemical vapor deposition of hydrocarbons (Ivanov, V. et al., CHEM. PHYS. LETT. 223, 329 (1994); Li A. et al., SCIENCE 274, 1701 (1996)). The production of multi-walled carbon nanotubes by catalytic hydrocarbon cracking is conducted on a commercial scale (U.S. Pat. No. 5,578,543), while the production of single-walled carbon nanotubes was still in a gram scale (as of 1998) by laser (Rinzler, A. G. et al., APPL. PHYS. A. 67, 29 (1998)) and are (Haffner, J. H. et al., CHEM. PHYS. LETT. 296, 195 (1998)) techniques. The nanotubes of the present invention may be prepared by any of the variety of techniques known in the art, assuming the proper purity and defect requirements can be met. Such defects include known nanotube defects, such as holes or openings in the nanotube wall or walls caused by one or more missing atoms. As known in the art, such defects can often be removed by irradiation of the nanotubes.
In the process of the present invention, electrophoresis is used to overcome the Van der Waals forces and disperse and align the nanostructures to the field lines of an applied electric field to form an aligned nanostructure trigger. Electrophoresis is the most well-known electrokinetic phenomenon. It was discovered by Reuss in 1807. He observed that clay particles dispersed in water migrate under influence of an applied electric field. Generally, electrophoresis is the motion of dispersed particles relative to a fluid under the influence of an electric field that is space uniform.
Electrophoresis occurs because particles dispersed in a fluid almost always carry an electric surface charge. An electric field exerts electrostatic Coulomb force on the particles through these charges. Recent molecular dynamics simulations, though, suggest that surface charge is not always necessary for electrophoresis and that even neutral particles can show electrophoresis due to the specific molecular structure of water at the interface.
The electrostatic Coulomb force exerted on a surface charge is reduced by an opposing force which is electrostatic as well. According to double layer theory, all surface charges in fluids are screened by a diffuse layer. This diffuse layer has the same absolute charge value, but with opposite sign from the surface charge. The electric field induces force on the diffuse layer, as well as on the surface charge. The total value of this force equals to the first mentioned force, but it is oppositely directed. However, only part of this force is applied to the particle. It is actually applied to the ions in the diffuse layer. These ions are at some distance from the particle surface. They transfer part of this electrostatic force to the particle surface through viscous stress. This part of the force that is applied to the particle body is called electrophoretic retardation force.
There is one more electric force, which is associated with deviation of the double layer from spherical symmetry and surface conductivity due to the excess ions in the diffuse layer. This force is called the electrophoretic relaxation force.
All these forces are balanced with hydrodynamic friction, which affects all bodies moving in viscous fluids with low Reynolds number. The speed of this motion v is proportional to the electric field strength E if the field is not too strong. Using this assumption makes possible the introduction of electrophoretic mobility μe as coefficient of proportionality between particle speed and electric field strength:
This process is governed by the set of equations with FL being the Lorentz force, q is the charge carried by the body under movement, and E is the electric field:
FL=qE
The electrophoretic migration caused by the passing of electricity is slowed by the forces of friction and in general terms how quickly a host material such as an explosive or pre-preg material allows this migration is proportional to the intensity and how uniform in a constant homogeneous field as well as the viscosity of the uncured host materials or:
FL=vf or qE=vf
where v is velocity and f is the coefficient of friction. The viscosity of uncured pre-preg host materials is lower than the viscosity of uncured explosive materials (typically at least a factor of ten), and thus more conducive to electrophoretic migration.
The electrophoretic mobility (sometimes written as μc) is written mathematically as
μc=v/E=q/f
This simplified equation applies only to non-conductive idealized matrices. More specifically the mobility is given by:
μc=(εε0ζ/η)
ε is the dielectric constant of the matrix (in liquid or viscous form), ε0 is the permittivity of free space, ζ is the surface potential of the particle reinforcement in the nanocomposite case and η is the viscosity of the matrix.
An embodiment of the in-situ approach using electrophoresis to impregnate an aligned nanostructure trigger in a solid explosive is illustrated in
In an embodiment the conductive nanostructures comprise carbon nanotubes of ˜1.0 nm in diameter and 100 s of micrometers in length. These carbon nanotubes consist of roughly ˜0.1 volume percent. The electric field used is typically in the range of 0.5V/cm and is applied across the entire cure time of the matrix. This time will vary depending on the host material. Once migration is complete, the host material is cured preventing the nanotubes from moving in the matrix to unwanted positions.
An embodiment of an ex-situ approach using electrophoresis to impregnate an aligned nanostructure trigger in a solid explosive is illustrated in
If multiple pre-preg layers 104 are to be introduced to the explosive (step 106) steps 80, 90 and 102 are repeated. There are several options. First, the same pre-preg layer with the same aligned nanostructure pattern can be formed. These layers can then be distributed throughout the volume of the explosive to improve the uniformity of volumetric explosion. Alternately, the same layers can be laminated by translating and rotating the layers to form a more complex antenna structure. Another approach to forming the laminate is to modify the electric field (step 108) used to disperse and align the nanostructures in the pre-preg layer. These pre-preg layers can be registered to from the laminate and the more complex antenna structure.
Once the pre-preg layer(s) are formed, they are placed in an uncured explosive solution 110 (step 112) in a mold 114. The explosive solution is kinetically heated 116 to melt the pre-preg layer and remove the host (step 118) leaving the aligned nanostructure trigger suspended in the explosive solution. For example, if the vapor point of the pre-preg material is lower than that of the explosive solution, the pre-preg host will evaporate. Typical explosive materials include PETN, RDX, HMX and TNT. The viscosity is sufficiently high to inhibit reaggregation of the nanostructures. The explosive solution is cured (step 120) to form a solid explosive 122 with an impregnated aligned nanostructure trigger 124. The apparatus (e.g. a gas chamber) and methods for curing explosives are well-known.
As shown in
In an embodiment the conductive nanostructures comprise carbon nanotubes of ˜1.0 nm in diameter and 100 s of micrometers in length. These carbon nanotubes consist of roughly ˜0.1 volume percent. The electric field used is typically in the range of 0.5V/cm and is applied across the entire cure time of the matrix. This time will vary depending on the pre-preg material. Once migration is complete, the pre-preg material is cured preventing the nanotubes from moving in the matrix to unwanted positions. Pre-preg material is a process by which a resin and curing agent mixture are put into the mixture thus reinforcing the composite. These impregnated reinforcements take three forms industry wide and any are useful for this process. These are woven fabrics, Roving and Unidirectional Tape. Fabrics and tapes are continuous rolls typically with widths up to 72 inches and can measure 100 s of feet long. Impregnated roving is wound into cores or bobbins and is a filament winding placed into prepositioned places. These materials are sacrificial and removed from elevated heating. There are 100 s of materials which are typically used as pre-preg materials.
Any aligned nanostructure will form an “antenna” that more efficiently couples to incident electromagnetic radiation than random nanostructures. Within a given pre-preg layer the nanostructures are constrained to align themselves with uniform electric field. Consequently, a single layer antenna is a fairly rudimentary structure. A more complex antenna that combines different structures can exhibit preferential coupling to certain specified bands in the RF, Microwave or THz spectrums. This may be desirable to both reduce the amount of energy in the specified band required to trigger detonation of the explosive and to inhibit unintended trigger due either to random electromagnetic signals or third-party signals that are endeavoring to maliciously trigger the explosive.
A more complex antenna structure can be formed by laminating multiple pre-preg layers one on top of the other in the explosive solution. Once the pre-preg host is removed the pressure induced by the explosive material and/or pressure in a gas chamber in which the solid explosive is formed will cause them to move to one plane to form the antenna. The antenna is held in place by the high viscosity of the explosive solution while the explosive is cured.
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While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.
This application claims benefit of priority under 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/095,730 entitled “Efficient Triggers Stable Explosives Using Aligned Nanostructures and Alignment Technique Therefore” and filed on Sep. 10, 2008, the entire contents of which are incorporated by reference. This patent is related to a co-pending application U.S. Ser. No. 11/530,081 filed Sep. 7, 2007, entitled “Improved Explosive Materials By Stabilization In Nanotubes”.
| Number | Date | Country | |
|---|---|---|---|
| 61095730 | Sep 2008 | US |
