COMPOSITIONS AND METHODS FOR PREPARING MATERIALS HAVING CONTROLLED REACTIVITY

The present invention was made with U.S. Government support, under grant numbers FA8651-04-C-0227 and N00178-04-C-3068 awarded by the United States Air Force and the United States Navy, respectively. Accordingly, the U.S. Government may have certain rights in the invention described herein.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon, and claims the benefit of priority from, previous U. S Provisional Application No. 60/584,933, entitled Compositions and Methods for Preparing Materials Having Controlled Reactivity filed on July 1, 2004, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to reactive composite materials having structural integrity, and to methods of manufacturing such materials. More particularly, the present invention relates to reactive composite materials having fine architectures and exhibiting controlled reactivity under desired conditions.

BACKGROUND OF THE INVENTION

It has been known for a considerable period of time that exothermic reactions between a metal and a metal oxide and between metallic elements are extremely useful sources of energy production. For example, a thermite welding process using aluminum and iron oxide has been in use for over a hundred years. Indeed, this process is still the most frequently used method for the in-situ welding of railroad tracks.

Thermite mixtures, intermetallic reactants and metal fuels have desirable properties including high energy density, gas production and being inert until initiated. Accordingly, they have found application in a wide variety of fields, including underwater torches, airbag inflation mechanisms, pyrotechnic matches, welding systems and as additives for explosives.

A discussion of the properties of various thermites can be found in "Theoretical energy release of thermites, intermetallics, and combustible metals", Fischer et al, presented at the 24 ^th

International Pyrotechnics Seminar, Monterey, California, July 1998, the entire contents of which are incorporated herein by reference.

Whilst thermites, intermetallic reactants and combustible metals have been applied to a wide range of industrial applications, their further application has been limited by their lack of structural integrity. Thus, they have found application in uses only where structural integrity (for example, strength) is not required. Furthermore, because of their reactive nature, processing of these materials must be performed carefully under controlled conditions such that the risk of the materials becoming initiated during the processing is minimized. This can result in complicated and expensive processing procedures.

Consequently, there is a need to provide reactive composite materials having structural integrity. In addition, there is a need to provide manufacturing processes for the safe production of such materials. Furthermore, there is a need to provide cost efficient manufacturing processes for the safe production of such materials.

SUMMARY OF THE INVENTION

The present invention overcomes the problems encountered with conventional reactive mixtures, and with the processes for producing those mixtures, by providing efficient, cost effective processes for preparing reactive composite materials having structural integrity, reactivity and stability. More specifically, the invention provides a fibrous monolith processing technique for producing reactive composite materials having structural integrity, fine architectures and exhibiting controlled reactivity under desired conditions. Furthermore, the invention provides a method for casting such reactive composite materials. Still furthermore, the invention provides a method for forming such reactive composite materials from metallic foams comprising reactive compositions.

Advantageously, the reactive composite materials have structural integrity and can be processed to form complex 3-D shapes as articles of manufacture. In addition, by careful selection of the reactive combinations used in the processing, the reactive composite materials can be formed with desired qualities of tensile strength, reactivity, stability and density, among others. In this regard, reactivity is the amount of energy released per unit mass or per unit volume when the composite material is initiated. Stability refers to the activation energy needed to initiate the composite material. The higher the activation energy, the more difficult it is to initiate the reaction between the reactive constituents of the composite material. Thus, a

composite material having a high activation energy will be inert unless a large amount of energy is imparted to it, for example by a thermal pulse or shock initiation.

Reactive composite materials according to the invention can be used in a wide range of applications, for example as electric matches for pyrotechnic displays; as tailored welding rods with regulated burn rates that can be switched on and off during use; as thermal lances with controlled burn rates; as thermic systems for biological and chemical hazard disposal; for controlled optical spectra generation; as remotely detonated devices; as part of kinetic energy impact devices; as advanced solid propellants; as initiators such as pressure cartridges and impulse cartridges; as cartridge actuated devices such as explosive bolts and nuts, cutters and guillotines, and piston actuators; and as propellant actuated devices such as start cartridges and rocket motors.

Accordingly, an object of the present invention is to provide versatile manufacturing processes for the safe production of reactive composite materials, wherein the properties of the materials can be tailored to particular requirements depending upon the particular application of the materials.

Accordingly, another object of the present invention is to provide reactive composite materials having relatively high strength, stability below a particular activation energy and a required reactivity.

These and other object advantages and features will be set forth in the description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the processing steps used to make fibrous monolith composites;

FIG. 2 is a perspective cross-sectional view of a uni-axial reactive composite formed by fibrous monolith processing in accordance with the invention;

FIG. 3 illustrates "second pass" extrusion processing of filaments to form a reactive composite in accordance with the invention;

FIG. 4 shows a second pass composite material formed with an aluminum shell and a PTFE core;

FIG. 5A shows a second pass composite material formed with an Fe ₂ O ₃ core and an Al shell;

FIG. 5B shows a detail of the composite material of FIG. 5 A in which fine features can be seen;

FIG. 6A shows a sample of a reactive composite material having a 50%/50% nylon/EEA polymer blend;

FIG. 6B shows samples of a reactive composite material having a 66%/33% nylon/EEA polymer blend;

FIG. 7A shows a 3" by 3" uni-axial plate formed from a reactive composite material having a Bi ₂ O ₃ core and an Al shell;

FIG. 7B shows a detail of the second pass feed-rod of the reactive composite material of FIG. 7A;

FIG. 8 shows a 3" by 3" plate formed with 0 degrees, +45 degrees and -45 degrees lay- up from a 340 micron extruded filament having Fe ₂ O ₃ AAl as the reactive combination;

FIG. 9 shows a 3" by 3" plate formed from the same filament as in FIG. 7, but with the plate being formed by random chopped fiber lay-up;

FIG. 10 is a photograph of a cross-section through a reactive composite material comprising Al and Bi ₂ O ₃ ;

FIG. HA shows a coupon made from 0.340 micron filaments laid up in a 0/45 degrees alternating pattern and pressed into a 3" x 3" x .250" coupon;

FIG. 1 IB shows a front (impact) surface of the coupon of FIG. 1 IA, after impact by a 0.270 caliber round;

FIG. HC shows a rear (exit) surface of the coupon of FIG. HA, after impact by the 0.270 caliber round;

FIG. 12A shows the microstracture of a porous metallic (Al) foam in cross-section; FIG 12B shows a detailed view of the porous metallic (Al) foam of FIG. 12A;

FIG. 13 is a graph showing the calculated energy release for a 2 kg mass at different impact speeds, the energy release depending on the reactive composition of the mass.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to reactive composite materials having structural integrity and to processes for manufacturing such composite materials. A table of suitable reactive elements for use in embodiments of the present invention may be found in "Theoretical energy release of thermites, intermetallics, and combustible metals", Fischer et al, presented at the 24 ^th International Pyrotechnics Seminar, Monterey, California, July 1998, the entire contents of which are incorporated herein by reference. Any reactive material or combination of materials that is reactive known to those of skill in the art may be used in accordance with the various embodiments of the invention.

According to a first embodiment of the invention, a process known as fibrous monolith processing, devised by the present assignee, is used as a basis for producing reactive composite materials having structural integrity. A detailed description of fibrous monolith processing can be found in U.S. Patents Nos. 6,740,286; 6,797,220; 6,803,003; 6,805,946; 6,847,699; and 6,709,737; the entire contents of each of which are incorporated herein by reference.

FIG. 1 illustrates a processing system used to make fibrous monolith composites. The processing begins with the milling of a first powder to obtain a mechanically activated and substantially agglomerate-free first powder product or particulate material. The first powder product is then blended with melt-extrudable polymer binders and plasticizers, using a high shear mixer, to form a substantially smooth, uniformly suspended first blend 20. The first blend is then pressed into a feed-rod 24. A second powder is milled to obtain a mechanically activated and substantially agglomerate-free second powder product. The second powder product is then blended with melt-extrudable polymer binders and plasticizers also, using a high shear mixer, to form a second blend 22. The second blend is then pressed into a thin shell 26.

The thin shell 26 is designed to fit tightly around the feed-rod 24, and thin shells are pressed around the feed-rod to form a composite feed-rod 28. The composite feed-rod 28 is then extruded through a ram-extruder to form a bi-component or filament 32. The filament is flexible and can be woven, wound, braided, chopped and pressed, or laid-up to produce a near net shape pre-form. In addition, a post-forming operation may be performed to realize the required tolerances.

According to the first embodiment of the invention, reactive starting materials are selected for use in the fibrous monolith processing. The reactive starting materials are selected to have a desired reactivity and a desired activation energy (stability). The starting materials

may include, but are not limited to, a metal such as aluminum (Al), magnesium (Mg), lithium (Li), beryllium (Be), titanium (Ti) or tantalum (Ta) and a compatible metal oxide selected from a group comprising oxides of: bismuth (Bi), boron (B), calcium (Ca), carbon (C), cerium (Ce), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lanthanum (La), lithium (Li), manganese (Mn), nickel (Ni), praseodymium (Pr), sulfur (S), tantalum (Ta), titanium (Ti), vanadium (V) and zirconium (Zr). In addition, oxides of platinum and palladium may be used. Furthermore, a combination of aluminum and PTFE may be used. Still furthermore, a combination of aluminum and tungsten oxide (WO ₃ ) may be used, or a combination of aluminum and molybdenum oxide (MoO ₃ ) may be used. Furthermore, any suitable combination from the above referenced paper by Fischer et al may be used.

As stated above, the particular reactive starting materials to be used are selected in dependence upon the required reactivity, strength and activation energy of the reactive composite. In this regard, reactive energies by mass range from about 75 cal/gram for Ti + SiO ₂ to about 2134 cal/gram for Mg + B ₂ O ₃ . Reactive energies by volume range from about 243.1 cal/cm ³ for Ti + SiO ₂ to about 6387 cal/cm ³ for Be + PbO ₂ . A high reactive energy per unit volume may be particularly desirable where there is a limited payload or where space is at a premium. The oxidizer is selected in excess by amount. Preferably, there is up to ten times as much oxidizer.

Density is also an important consideration for a number of applications. Densities range from about 0.52 g/cm ³ for Li + Fe ₂ O ₃ to about 13.5 g/cm ³ for Ta + WO ₂ . The former could be used for flight systems, and the latter could be used for penetrator applications.

After the reactive materials have been selected, one of the materials may be milled into the first powder product, and then blended with melt-extrudable polymers and plasticizers to form the first blend. The other material may be milled into the second powder product and then blended with melt-extrudable polymers and plasticizers to form the second blend.

The first blend is then formed into feed-rods and the second blend is formed into thin shells, with the thin shells subsequently being pressed around the feed-rods to form a plurality of composite feed-rods. The composite feed-rods are then extruded to form filaments, and a plurality of filaments may be bundled together and disposed within another shell. The filaments may have diameters ranging from about 0.34 to about 5 mm. This combination of filaments is a reactive composite material 10 having a macro-architecture as shown in FIG 2. Thus, the reactive composite material 10 comprises filaments 12 having a primary phase in the form of

cells or cores 14 and a secondary phase in the form of cell boundaries or shells 16. In other words, the reactive composite material has a "honeycomb" configuration of shells and cores. In this embodiment, the cores contain a first reactive material, such as a metal, and the shells contain a second reactive material, such as a metal oxide. Of course, alternatively, the cores may contain the metal oxide and the shells may contain the metal. In each of the cores and the shells, the reactive material is blended with one or more polymers to provide strength to the composite material.

Preferably, the filaments are extruded at about 2mm and are formed into a second feed- rod for a "second pass" extrusion, as shown in FIG. 3. By performing two extrusion passes, fine features may be fabricated in the reactive composite material. FIG. 4 shows a second pass composite material formed with an aluminum shell and a PTFE core. Similarly, FIG. 5A shows a second pass composite material formed with an Fe ₂ O ₃ core and an Al shell. FIG. 5B shows a detail of the composite material of FIG. 5 A in which the fine features can be seen.

The composite material of FIGS. 5A and 5B has, by volume, 82.5% core (Fe ₂ O ₃ ) and 17.5% shell (Al). However, architectures having other volume ratios may be fabricated. For example, a composite material may be produced having a core/shell volume ratio of 69%/31% or 50%/50%. Thus, core/shell volume ratios are preferably in the range of about 50%/50% to about 82.5%/17.5%. The optimum core/shell volume ratio for a particular application will depend on the particular reactive materials used and the desired properties of the system. For example, the core/shell volume ratio may be chosen to maximize the reaction between the reactive elements in the core and the reactive elements in the shell. Furthermore, the core/shell volume ratio has a significant influence upon the tensile strength of the reactive composite material. Thus, if the shell volume is made too small, the tensile strength of the material becomes unacceptably low.

Although the invention is described with reference to generally cylindrical-shaped fibrous monolith (FM) filaments that are bundled together to form FM reactive composite materials, wherein the shape of the filaments become essentially hexagonal in cross-section as a result of processing, other configurations are contemplated, as will be appreciated by those skilled in the art. For example, filaments having square, rectangular, oblongs, or triangular cross-sections may be obtained by varying the shape of the extrusion die accordingly. Thus, different shapes and configurations of filaments in the composite materials may be obtained, which may impact the resultant mechanical properties of those composite materials.

To obtain as high a reactivity as possible from particular reactants, it is desirable to have as large a solids loading fraction as possible in the first and second blends. Thus, a solids loading fraction of between 45% and 75% is desirable. However, as the solids loading fraction increases, so the processing, particularly extrusion, becomes more difficult. Therefore, it is further desirable to have a solids loading fraction of between about 50% and about 60%, more preferably about 55%.

According to the first embodiment of the invention, a reactive composite material having a core/shell architecture with a desired density, reactivity and stability can be produced. Furthermore, the reactive composite material is structurally integral as a result of the fibrous monolith processing. In this regard, the structural integrity of the material is influenced by the polymers that are blended with the reactive materials, hi addition to by the core/shell volume ratios discussed above. Examples of polymers that may be blended with the reactive materials include ethylene ethyl acetate (EEA) co-polymers, which are commercially available as DPDA- 618NT from Union Carbide, nylon (for example nylon 6, 6), PEEK, ethylene vinylacetate (EVA), which is commercially available as ELVAX 470 from E.I. DuPont Co., and Acryloid Copolymer Resin (B-67), which is commercially available from Rohm and Haas, Philadelphia, Pa. In addition, plasticizers may also be used in the blending. Examples of plasticizers include heavy mineral oil (HMO) commercially available as Mineral Oil White, Heavy, Labguard.RTM. and methoxy polyethyleneglycol having a molecular weight of about 550 (MPEG-550) commercially available from Union Carbide.

Also, a combination of different polymers may be used. For example, two separate samples of EEA having different melt flow indices may be blended together, or EEA may be blended together with nylon 6, 6. For example, a 50/50 mixture of EEA and nylon may be used, as shown hi FIG. 6A, or a 33/66 mixture as shown in FIG. 6B. The use of nylon is desirable because of its strength. However, other alternate, high strength thermoplastics may be used instead or as well. The use of EEA is desirable because of the flexibility that it gives to the extruded filament, enabling the reactive composite material to be formed into a desired shape.

Furthermore, the polymers play an important role in ensuring that the reactive composite material is inert unless severely impacted. In other words, the polymers help to determine the activation energy of the composite. Still furthermore, the use of polymers in the blend facilitates manufacture of the material because thermoplastic encapsulation of the discrete reactive

particles generally renders the particles non-reactive during processing. Thus, the polymers help to ensure that the manufacturing process is safe.

Table 1 below shows a Brabender recipe for production of a reactive composite material according to an example of the first embodiment of the invention. According to this example, the reactive composite material has an aluminum (Al) shell and a bismuth oxide (Bi ₂ O ₃ ) core.

The polymer system used in this example comprises a mixture of two different melt flow indices (MFI 1.5 and MFI 20) of ethylene ethyl acetate (EEA), which are blended to form an extrudable batch. In addition, an additive of stearic acid is added.

Although specific examples of materials that may be used have been set forth above, any combination of materials that is capable of being initiated under select conditions and that provides a desired strength to weight ratio for the intended use, can be used. This includes any suitable combination set forth in "Theoretical Energy Release of Thermites, Intermetallics, and Combustible Materials", by Fischer and Grubelich.

The reactive composite materials may be formed into particular desired shapes from the extruded filaments. For example, a 3" by 3" plate has been formed by extruding a blended system of Fe ₂ O ₃ /Al and polymer into 340 micron filament, followed by winding and oriented lay-up. In addition, the scrap filaments were randomly chopped and compacted and formed into a 3" by 3" plate as well.

FIG. 7A shows a 3" by 3" uni-axial plate formed from a reactive composite material having a Bi ₂ O ₃ core and an Al shell. FIG. 7B shows a detail of the second pass feed-rod of the composite material, which underwent two extrusion passes. FIG. 8 shows a 3" by 3" plate formed with 0 degrees, +45 degrees and -45 degrees lay-up from a 340 micron extruded filament

having Fe ₂ O ₃ ZAl as the reactive combination. Also, FIG. 9 shows a 3" by 3" plate formed from the same filament, but with the plate being formed by random chopped fiber lay-up.

FIG. 10 is a photograph of a cross-section through a reactive composite material comprising Al and Bi ₂ O ₃ . Each of the hexagonal cells of the material shown in FIG. 10 is approximately 100 microns in diameter. In other examples, each of the cells may be about 1.5 mm in diameter.

According to a second embodiment of the invention, the reactive materials are blended together in at least one of the first and second blends. In other words, an FM reactive composite material is produced having a core/shell architecture in which the core and/or the shell comprises a mixture of reactive materials. For example, the blend from which the core is formed could include both a metal powder (e.g. aluminum) and a metal oxide powder (e.g. Fe ₂ O ₃ ), such that the core then includes both those reactants, together with one or more polymers. The shell could then be made inert, or could also be formed from a blend containing a metal powder and a metal oxide powder. These powders may or may not include the same elements as the metal and metal oxide powders used for the core. Alternatively, the shell could contain a combination of reactive elements (e.g. aluminum and PTFE) and the core could be inert.

The particular configuration of the core and shell constituents depends upon the application for which the reactive composite material is intended. Including both reactive elements (e.g. a metal and a metal oxide) within the core (or the shell) will reduce the activation energy required to initiate the material, because of the close proximity and homogenous mixture of the elements. This may be desirable, for example, if the reactive composite material is to be detonated remotely, so as to destroy a compromised hardware device, for example.

In addition, making one of the core or shell inert enables the materials for that portion of the composite material to be selected to optimize a particular system parameter, without having to ensure reactivity of that portion. For example, an inert shell could comprise high strength thermoplastics to help optimize the tensile strength of the system.

Thus, the processing technique for producing reactive composite materials using the fibrous monolith approach is versatile in function, and can be used to produce a variety of different composite materials. In this regard, compositions comprising the core (cell phase) will differ from those comprising the shell (boundary phase) in order to provide the benefits generally associated with FMs. For example, the compositions may include formulations of different compounds (e.g., metal oxide for the core and metal for the shell) or formulations of

the same compounds but in different amounts (e.g., a metal/metal oxide combination for the core and the same metal/metal oxide combination for the shell, but with the core and shell having different metal/metal oxide ratios or different polymer blends) as long as the overall properties of the compositions are not the same. For example, the compositions can be selected so that no excessively strong bonding occurs between the two phases.

The shell (cell boundary phase) may be selected to create pressure zones, microcrack zones, ductile-phase zones, or weak debond-type interfaces in order to increase the toughness of the composite.

Samples of tensile bars comprising Bi ₂ O ₃ cores and Al shells, blended with EEA copolymers, have been strength tested, and the results are shown in Table 2 below. By using a polymer blend including nylon, tensile strengths in the range of about 24 to about 32 MPa are obtainable, more particularly, tensile strengths of about 28 MPa are obtainable.

Furthermore, a coupon comprising a reactive combination of Al and Fe ₂ O ₃ has been tested to check that it is inert unless a significant minimum activation energy is imparted to it. The coupon was made from 0.340 micron filaments laid up in a 0/45 degrees alternating pattern and pressed into a 3" x 3" x .250" coupon, and is shown in FIG. 1 IA.

A cartridge was fired at the coupon from a distance of 50 yards. The cartridge was a 0.270 caliber, 130 grain, copper jacketed lead bullet. At the distance at which it was fired, the projectile was traveling at 2900 feet per second at impact. The impact energy was 2497 foot pounds. These figures are based on published data from the manufacturer.

The bullet impacted the coupon in the upper half and produced a fracture similar to that seen for ceramic body armor failure, as shown in FIG. 1 IB. The impact zone shows some of the outer layers blown out. The exit zone, seen in FIG. HC, shows a typical failure with the

damage zone developed as an expanding cone in the outer layers. However, no melting was seen on the residue still attached to the coupon, and no flaming or burning was observed where the bullet impacted. Thus, there was no melting or initiation of the reactive material in the area of the bullet penetration, showing that the material is inert under normal circumstances. Hence, this type of reactive composite material could be used in an application requiring the material to remain inert unless a relatively large activation energy is applied to it, for example by a substantial kinetic impact.

Reactive composite materials produced according to the methods of the second and third embodiments of the invention may be formed around a core of another material. For example, they may be formed around a core of tungsten carbide to provide a strong, shaped object. In addition, the reactive composite materials may be fabricated into complex 3-D shapes.

According to a third embodiment of the invention, an alternative process for the production of reactive composite materials having structural integrity is provided. This process uses non-aqueous gel casting to produce the materials. Non-aqueous gel-casting was developed as a technique for green forming ceramic powders that were sensitive to water.

The gel casting process of the third embodiment of the invention begins with a "slip", as would be used for the slip casting of ceramic mugs. Powders of selected reactive materials, such as metal oxides and metals, are weighed and blended in the correct proportions for the thermite reaction. Suitable combinations of metals and metal oxides may be selected from the above referenced paper by Fischer et al, and the above listed combinations may be used. Polymers used for gel casting are then weighed and blended with the powder in a mill jar with mixing media to aid in blending. Once the powders and chemical blends have thoroughly mixed, the resulting slurry is cast into a shaped die to achieve the desired shape, size and details. The shaped die can be made from easily manufactured aluminum, molding wax or any materials with no surface porosity. The only major limitations are the ability of the slurry to evenly fill the die and the ability to remove trapped air or gas after casting. Bubbles weaken the structure and could possibly interfere with initiation performance.

Next, the slurry is de-gased to remove trapped air. The cast slurry, whilst still in the die, is then placed in a warm oven (for example, at about 85 ⁰ C) for between about 30 minutes and several hours to solidify. Once the casting has solidified, it is removed from the die and placed in melted wax or a proprietary solvent to prevent cracking during the balance of the curing process.

Furthermore, once the cast part has solidified, it can be machined to a desired size and/or shape or article. The strength of the cast part is similar to that of other green ceramic materials. It is strong enough to be handled, but would probably break or chip if dropped.

Thus, according to this embodiment, an efficient and cost effective process for fabricating reactive composite materials having structural integrity is provided.

In a fourth embodiment of the invention, the gel casting according to the third embodiment is combined with a metallic foam. Methods for preparation of metallic foam products are described, for example, in U.S. patents 6,524,522 and 6,852,272, the entire contents of each of which are incorporated herein by reference.

According to one such method of preparing a metallic foam, a structure-forming powder, a binder and a pore-forming compound are mixed to provide a flowable composition. The pore- forming compound is generally immiscible with the structure forming powder and binder. The flowable composition is then gelled in a mold to form a composite object. Next, the composite object is heated and maintained at a raised temperature for a period of time long enough to remove at least some of the pore forming compound, such that a sintered object is formed having internal porosity.

The structure-forming powder may be a metal powder, a ceramic powder, a cermet powder or a combination thereof. Examples of suitable metals include, but are not limited to, Al, Cu, Mg, Sn, Ti, Zn, Co, Ni, Mo, Nb and alloys and combinations thereof. Ceramic powders that can be used include, but are not limited to, any carbide, nitride or oxide compound such as SiC, Si ₃ N ₄ , alumina, ZrO ₂ , ZrC, HfC, Si-Al-O-N, WC-Co, and the like, and combinations thereof.

Combinations of metallic and ceramic powder may be used to fabricate cermet foams. As used herein, "cermet" refers to compositions that include both ceramic and metallic powders. Any thermodynamically compatible metallic and ceramic powders, including the previously listed ceramic and metallic powders, can be combined to fabricate cermet foams.

Preferably, thermoplastic and thermoset polymer binders may be used, and are selected based on several factors, including the powder used, the desired processing to create and finish the desired object, etc. The amount of polymer binder needed will depend on the density of the ceramic, metallic, or cermet powder. Preferably, the mixture contains binder in an amount from about 40% to about 60% by volume, and more preferably from about 45% to about 50% by

volume based on the foam composition. The polymer binder may be selected to be compatible with use in an injection molding process, extrusion, or gel casting processes.

Thermoplastic polymer binders that can be used include, but are not limited to, PMMA (polymethyl methacrylate), EVA (ethyl vinyl acrylate), EEA (polyethylene ethacrylate), PEOx (poly-2-ethyl-2-oxazoline), PEG (polyethylene glycol), polystyrene, microcellulose, and the like, and combinations thereof. Thermoset polymer binders that can be used include, but are not limited to, BLO (butrylactone, which is commercially available from Aldrich Chemical Company in Milwaukee, Wis.), HODA (hexanedioldiacrylate, which is also commercially available from Aldrich Chemical Company in Milwaukee, Wis.), and the like and combinations thereof.

Thus, according to the process of the fourth embodiment of the invention, firstly a porous metallic foam is produced, as outlined in U.S. Patents Nos. 6,524,522 and 6,852,272, and then gel casting is performed on the foam.

To perform the gel casting, a slurry mixture comprising one or more reactive materials and one or more polymers is cast into the open pores of the porous metallic foam, and allowed to solidify in the open pores to form a reactive composite material. The slurry mixture may be cast into the open pores of the metallic foam by immersing the foam into the slurry mixture. The slurry mixture may be de-gased as in the third embodiment, and the metallic foam may be placed in a warm oven to allow the slurry mixture to solidify. Structures containing about 50% solids, by volume, can be obtained in these filled foams. The volume of the foam can be about 30%.

In addition, in a preferred variation of the fourth embodiment, the metallic foam is engineered to participate in the thermite reaction, either directly or indirectly. The metallic foam can participate directly through the reaction of metals such as Al and Ni, or can participate indirectly through the reaction with additional metal oxide powder, such as iron oxide powder, in the gel cast slurry mixture.

FIG. 12A shows the microstructure of a metallic (Al) foam in cross-section. It can be seen that there are discrete pores which are filled with the reactive materials. FIG. 12B shows another view of the Al foam.

Hence, the fourth embodiment of the invention provides a simple and effective process for forming a reactive composite material having structural integrity. The metallic foam

provides structural strength for the material and, advantageously, can participate in the thermite reaction, thus increasing the reactive energy produced. The foams tend to have tensile strengths in the range of 30 to 40 MPa, more particularly about 35 MPa. Furthermore, the foams can advantageously be fabricated into complicated 3-D shapes, allowing reactive composite materials having useful shapes to be formed.

The reactive composite materials formed by the metallic foam gel casting of the fourth embodiment tend not to initiate as well as the reactive composite materials formed by the fibrous monolith approach when localized initiation is performed using a torch (e.g. propane torch) or electronic initiator. This is because the foam inhibits propagation of the reaction to a certain extent.

The various processing methods according to the embodiments of the invention enable the production of reactive composite materials having structural integrity, and having other selected properties of stability, reactivity and density. These structural composite materials may be particularly useful in kinetic applications, where energy is released as a result of an impact. FIG. 13 is a graph showing the calculated energy release for a 2 kg mass at different impact speeds. The lowest line (a), linked by circles, shows the energy that would be released by an inert 2 kg mass on collision at speeds of between 2 and 8 kft per second. The line above (b) shows the energy release that would be obtained if the 2 kg weight comprised an aluminum/nickel (Al/Ni) composition of 50% by weight (i.e. 1 kg). It can be seen that the predicted energy release obtained is significantly higher than that for the inert solid. The next line (c) shows the predicted energy release if the 2 kg solid contained 50% by weight Bi ₂ O ₃ ZAl, and the line above (d) shows the predicted energy release if the 2 kg solid contained 60% by weight Bi ₂ O ₃ /Al. The lines (a) to (d) represent structures formed by the fibrous monolith processing technique. The line (e) represents an Al/Ni porous metallic foam comprising a Bi ₂ O ₃ /Al reactive mixture cast into the open pores, the foam comprising 50% by weight of the 2 kg mass. The dashed line (f) represents the theoretical energy that could be released if the 2 kg mass contained 100% Bi ₂ O ₃ . However, such a mass would, of course, have no structural integrity.

Thus, it can be seen from the figure that a substantial increase in energy release can be obtained by forming a solid mass that comprises reactive materials. For example, taking an impact speed of 4 kft/sec, it can be seen that the energy release of the inert 2 kg mass (line a) is about 1.5 MJ, whilst the energy release of the 2 Kg mass comprising 60% Bi ₂ O ₃ /Al (line d) is

about 4 MJ. Hence, in this instance, the energy release of the 2 kg mass would be increased by over 2 ¹ A times.

Accordingly, the embodiments of the invention enable the safe and versatile production of reactive composite materials having structural integrity. Furthermore, the embodiments of the invention provide reactive composite materials having structural integrity, and selected degrees of reactivity and stability. The density of the materials may also be varied in accordance with the particular use for which they are intended. Advantageously, the invention provides reactive composite materials having structures such that they are inert during normal handling, but which provide large amounts of energy once activated beyond their activation energies. Furthermore, the reactive composite materials have tensile strengths enabling them to be used in a wide range of applications that thermite materials have not previously been suitable for. Also, because the fabrication processes enable complex 3-D shapes comprising the reactive materials to be formed, the materials can be used in a wide range of applications.

Numerous modifications and variations of the invention are possible to provide reactive composite materials having desired characteristics. Thus, modifications and variations in the practice of the invention will be apparent to those skilled in the art upon consideration of the foregoing detailed description of the invention. Although preferred embodiments have been described above, and illustrated in the accompanying drawings, there is no intent to limit the scope of the invention to these or other particular embodiments. Consequently, any such modifications and variations are intended to be included within the scope of the following claims.