Patent Publication Number: US-2007113939-A1

Title: High energy blast explosives for confined spaces

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation-in-part of U. S. patent application Ser. No. 10/779,545, filed Feb. 11, 2004, now U.S. Pat. No. 6,955,732, and U.S. patent application Ser. No. 10/779,548, filed Feb. 11, 2004, which is a divisional of U.S. patent application Ser. No. 10/326,958 filed on Dec. 23, 2002, now U.S. Pat. No. 6,969,434. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
      The invention described herein may be manufactured and used by or for the government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.  
     FIELD OF THE INVENTION  
      The invention disclosed herein relates to explosive formulations with improved combustion efficiency. More particularly, the explosive formulations of the invention are capable of maintaining a relatively high blast pressure in an oxygen poor environment, such as a tunnel or other confined spaces.  
     BACKGROUND OF THE INVENTION  
      There is a long history of studying blast explosives, reactive metals and associated metal combustion technologies. The success of the development of Solid Fuel-Air-Explosive (SFAE) has been demonstrated providing 30-40% increased internal blast over a conventional explosive. SFAE is a singular event with combined mixing and initiation of the reaction. In confined spaces, transition to full detonation is not required for enhanced blast, if the solid fuel is ignited early in the dispersion process. A series of reflective shock waves generated by the detonation mixes the hot detonation gases with metal particles and compresses the metal particles at the same time. These actions provide the chemical kinetic support to maintain a hot environment, causing more metal to ignite and burn. This late time metal combustion process produces a significant pressure rise over a longer time duration (10-50 msec). This is a phase generally referred to as after burning or late-time impulse which can occur outside of where the detonation occurred, resulting in more widespread damage.  
      Aluminum has been used as the metal of choice, due to high heat of combustion, cost and availability. Billets of SFAE made with Al, provide savings in volume with increased fuel mass for blast performance. However, combustion efficiency has been an issue, especially in the event that the fuel content (35-60 wt %) is high with respect to the total weight of explosive composition. Poor combustion efficiency is often observed in many of the thermobaric warhead tests, which causes the severe ineffectiveness of the weapon. This is due to the high ignition temperature, 2200 K, typically required for proper combustion of AL. During the burning of Al, heat is produced and aluminum oxide is formed. However, the burning of all the metal to completion requires maintaining the hot environment. This environment can be best maintained if it is supported chemically by the combustion of other oxidizer species (i.e. AP or nitrate ester liquid, IPN (isopropyl nitrate)) that are much easier to ignite (AP has an ignition temperature of 250 C and IPN has a low flash point of 22 C). The combustion of these additives produce the hot gases to support the burning of metal, thus 100% combustion efficiency can be obtained. Metal composites, metal and oxidizer combined granules, produced from coating of particles with a binder, can be made easily with techniques well known in the art.  
      Another combined approach to further improve the metal combustion efficiency is to use a more reactive metal as part of or as the entire metal fuel component. New reactive metal materials such as nano-sized aluminum to increase the reactivity, titanium and boron alloy to increase the thermal output, and magnesium/aluminum alloy to lower the ignition temperature are among the most promising approaches to increase the metal combustion efficiency. More powerful explosives such as CL-20 that are capable of raising the detonation pressure and temperature are also extremely beneficial.  
      There exists a need in the art for new explosive formulations with new reactive metal and metal composites to have 50-100% higher blast energy than those by the baseline composition such as Tritonal or PBX N109. Further, the new formulations coupled with new warhead designs will have the potential to form one of the most powerful thermobaric warheads, when compared to the weapon systems that currently exist.  
     SUMMARY OF THE INVENTION  
      The present invention relates to a metal composite that combines a binder, a reactive metal and an oxidizer. In an embodiment of the present invention, a plasticizer and a catalyst are also included. In yet another embodiment of the present invention, the binder includes polymers capable of coating the reactive metal and oxidizer powder. Two embodiments include methods to produce the compositions of the present invention: 
      (1) The coated powder forms the fuel charge through pressing, combining this fuel charge with a high explosive charge (HMX, RDX or CL-20 based PBX&#39;s) in an annular design to make up the fill for the warhead.     (2) Using metal or metal/oxidizer powders in a mixing, casting and curing process to combine with high explosive to form castable PBX&#39;s. The reactive metal contains ingredients that are intrinsically reactive with the reaction products of high explosive and oxidizer with or without the presence of high concentration of oxygen.    

      An embodiment of the present invention discloses a metal composite comprising about 60 to about 96 weight % of at least one reactive metal, about 4 to about 10 weight % of at least one binder and about 0 to about 36 weight % of an oxidizer. The reactive metal includes, but not limited to at least one of nano-sized metal particles, metastable mechanical alloy and any combination thereof. More specifically, the reactive metal includes, but not limited to at least one of nano-sized aluminum, nano-sized boron and nano-sized titanium, nano-sized magnesium, and nano-sized Al—Mg, Al—Mg—H, B—Mg, Al—B and Ti—B mixtures. The binder includes, but not limited to at least one of copolymer of vinylidine fluoride hexafluoropropylene, nitrocellulose, GAP and Zeon.  
      Embodiments of the present invention relating to castable compositions disclose an explosive having an annular construction. The explosive includes a cylindrical shell of solid fuel air explosive surrounding a cylindrically shaped high explosive. In other embodiments the solid fuel air explosive includes at least one of reactive metal and metal composite. The metal composite including about 60 to about 80 weight % of at least one reactive metal, about 4 to about 8 weight % of at least one binder and about 0 to about 36 weight % of an oxidizer. The reactive metal includes, but is not limited to at least one of nano-sized metal particles, metastable mechanical alloy and any combination thereof. More specifically, the reactive metal includes at least one of nano-sized aluminum, nano-sized boron and nano-sized titanium, nano-sized magnesium, and nano-sized Al—Mg, Al—Mg—H, B—Mg, Al—B and Ti—B mixtures, H-2 (2 μm spherical aluminum) and H-5 (5 μm spherical aluminum). The oxidizer includes, but is not limited to at least one of ammonium perchlorate, ammonium dinitramide and ammonium nitrate.  
      The present invention is to provide an explosive with enhanced combustion efficiently capable of sustaining a high pressure over a period of time in a confined environment with a limited oxygen supply.  
      The present invention is to provide an explosive capable of maintaining a relatively high pressure (30-60 psi) for up to 50 msec in an environment characterized with high rate of thermal quenching (cold air), this environment has a profound adverse effect for metal combustion, which is the main cause for combustion efficiency.  
      Further, when very small metal particles in the order of twenty nano-meters are utilized as the reactive metal in an explosive device made in accordance with the present invention blast effectiveness is increased by a factor of two over more conventional explosive devices and  
      Additionally, embodiments the present invention is to provide an explosive with increased reactivity, increased thermal output and lower ignition temperatures.  
      Embodiments the present invention are also to provide thermobaric explosive formulations with reactive metals and metal composites which have a 100% higher blast energy than compositions such as Tritonal and PBX N109.  
      It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not to be viewed as being restrictive of the present invention, as claimed. These and other objects, features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiments and the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       FIG. 1  is a sectional view of a typical explosive having an annular construction; and  
       FIG. 2  is a plot illustrating the blast effectiveness of an explosive which includes a reactive metal of nano-meter sized metal particles. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      The invention disclosed herein relates to an explosive capable of enhanced combustion efficiently capable of sustaining a high pressure over a period of time in a confined environment, such as an air tight room or a cave, where oxygen may be in limited supply.  
      The reactive metal used in an embodiment of the present invention includes nano-sized metal particles, metastable mechanical alloys and any combination thereof. The metal fuel in these explosive formulations of the present invention incorporates nano-sized aluminum, including, for example, Alex®, boron, manganese and titanium, those having a size of about 20-500 nm. The metastable mechanical alloys include nano-sized Al—Mg, Al—Mg—H, B—Mg, Al—B,Ti—B, mixtures and H-2 and H-5 made from high energy milling. The metastable mechanical alloys include nano-crystalline metastable phases with particle sizes of about 1-50 μm. The reactive metal used also includes Ti, B or Mg. In another embodiment of the present invention, the reactive metal includes about 60-80 weight % of the total metal composite, or at about 74 weight %.  
      The thermobaric explosive formulations of the present invention incorporate high energy explosive material including, but not limited to hexa-nitro-hexa-aza-isowurtzitane (CL-20), cyclotrimethylenetrinitramine (RDX) and cyclotetramethylene tetranitramine (HMX). The powerful oxidizers, including ammonium perchlorate (AP), ammonium dinitramide (ADN), ammonium nitrate (AN) and barium nitrate are selected to be used in the metal composite or castable PBX&#39;s. Another embodiment of the present invention uses ammonium perchlorate (AP) particles, or about 11-100 μm in size. The oxidizer includes about 12-36 weight % of the total metal composites, or at about 20 weight %.  
      The binder includes polymers capable of coating the reactive metal and high explosive powder. The binder includes, but is not limited to at least one of copolymer of vinylidine fluoride hexafluoropropylene, including Viton®, nitrocellulose, glycidyl azide polymer (GAP) or an acrylic acid ester polymer, including Zeon®. In another embodiment of the present invention, the binder includes about 4-6 weight % of the total metal composites, or at about 4 weight % for the total metal composite. The binders used for castable PBX&#39;s include, for example, hydroxy-terminated polybutadienes (HTPB), hydroxy-terminated polycaprolactone (PCP), hydroxy-terminated polyesters, hydroxy-terminated polyethers (HTPE), Glycidyl azide polymer (GAP), trifluoroethyl-terminated poly (1-cyano-1-difluoramino)-polyethylene glycol (PCDE) and any combination thereof. Typically, 5 to 7 weight % is used for castable PBX embodiment.  
      In other embodiments, a plasticizer and a bum rate catalyst is added. The plasticizer includes bis-(2,2-ro-2-fluoroethyl) formal (FEFO). However, other plasticizers utilized, include energetic plasticizers selected from those compounds, which are liquids and contain energetic moieties or groups in their chemical structures. These moieties include, but not limited to nitro or nitrate ester groups, azido groups, or nitramino groups. Suitable plasticizers include TEGDN (triethyleneglycol dinitrate), or Butyl NENA (n-butyl-2-nitratoethyl-nitramine). Other suitable plasticizers include DEGDN (diethyleneglycol dinitrate), TMETN (trimethylolethane trinitrate), and BTTN (butanetriol trinitrate). These plasticizers are used independently or in combination. Other fluoramino groups including bis-(2,2-ro-2-fluoroethyl) formal (FEFO) and bis-[2,2-bis(difluoramino)-5,5-dinitro-5-fluoropentoxy] methane (SYFO) could also incorporated into the formulations. In other embodiments of the present invention, the plasticizer include about 4 weight % of the formulations.  
      Iron oxide (Fe 2 O 3 ), nano-sized is a suitable burn rate catalyst and is optional to exotic burn rate catalysts including superfine iron oxide, chromic oxide, catocene, or carboranes. In other embodiments aluminum oxide is also used. In embodiments of the present invention, the burn rate catalyst comprises about 1 weight % of the total metal composites. Tables I and II disclose a number of the formulations of the present invention.  
               TABLE I                          Chemical Composition of Metal Composite Coated by Various Binders                                             Plas-           Reactive Metal   Oxidizer   Binder   ticizer   Catalyst               80% H-5   14% AP, 11 μm   6% Viton ®   None   None       60% H-5, 20%   14% AP, 11 μm   6% Viton ®   None   None       Al/Mg       alloy, 28 μm       80% H-5   12% AP, 11 μm   6% Viton ®   None   1% Fe 2O   3 ,                       nano-sized       74% H-5   20% AP, 11 μm   6% Viton ®   None   1% Fe 2O   3 ,                       nano-sized       37% Ti, 44 μm   21% AP, 11 μm   6%   None   None       37% B, 0.6-7       Nitrocellulose       μm       74% Ti—B,   21% AP, 11 μm   6%   None   None       20 μm       Nitrocellulose       74% Mg—B,   21% AP, 11 μm   6%   None   None       20 μm       Nitrocellulose       50% H-5 24%   20% AP, 11 μm   5%   None   1% Fe 2 O 3 ,       Alex ® 0.2 μm       Nitrocellulose       nano-sized       50% H-5 24%   20% AP, 11 μm   4%   4%   1% Fe 2 O 3 ,       Alex ® 0.2 μm       Nitrocellulose   FEFO   nano-sized       74% Alex ®   20% AP, 11 μm   5%   None   1% Fe 2 O 3 ,       0.2 μm       Nitrocellulose       nano-sized       40% Flake Al,   36% AP, 100 μm   4% Viton ®   None   None       20% Al/Mg       alloy                 Note:            Al/Mg milled in batch MA020129-01,            Ti—B milled in batch MA020317-01, and            Mg—B milled in batch MA020319-01 at New Jersey Institute of Technology, Newark, New Jersey.             
 
     
       
         
           
               
             
               
                 TABLE II 
               
               
                   
               
               
                   
               
               
                 Typical Composition of Castable PBX&#39;s Containing Reactive Metal and 
               
               
                 AP Oxidizer 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Reactive 
                   
                   
                   
                 Plasticizer &amp; 
               
               
                 Metal 
                 Oxidizer 
                 Binder 
                 High Explosive 
                 Catalyst 
               
               
                   
               
               
                 20-40% 
                 15-35% AP, 
                 10-15% HTPB 
                 30-55% HMX 
                 4-6% 
               
               
                   
                 11-100 μm 
               
               
                   
               
               
                 Metal 
               
               
                 Composite 
                   
                 Binder 
                 High Explosive 
                 Plasticizer 
               
               
                   
               
               
                 40-60%* 
                 None 
                 10-15% HTPB 
                 30-45% HMX 
                 None 
               
               
                   
                   
                   
                 or 30-50% 
               
               
                   
                   
                   
                 HMX 
               
               
                 40-60%* 
                 None 
                 10-15% LMA 
                 30-45% 
                 None 
               
               
                   
                   
                   
                 HMX 
               
               
                 30-55%* 
                 None 
                 10-15% HTPB 
                 35-60% CL-20 
                 None 
               
               
                 20-24%* 
                 15-35% AP 
                 10-15% HTPB 
                 30-55% HMX 
                 None 
               
               
                   
               
               
                   Note:    
               
               
                   *metal composite contains oxidizer    
               
            
           
         
       
     
      The novel thermobaric explosives of the present invention are spherical particles of composite material containing high explosive, oxidizer, reactive metal and binder. Plasticizer and burn rate catalyst are added to manipulate performance. A method of making the novel thermobaric explosives described herein is disclosed in U.S. Pat. No. 5,750,921 issued to Chan et al. on May 12, 1998, hereby incorporated herein by reference.  
      In an embodiment of the present invention, a solid fuel-air explosive annular construction is used as shown in  FIG. 1 . In a typical annular construction, a cylindrical shell of solid fuel air explosive (SFAE)  22  surrounds the high explosive  21 . As a matter of preference, the shapes of the high explosive charge are include, but not limited to spherically or cylindrically symmetric, to provide a uniform dispersion pattern. Solid metal casings  23  are typically pressed from reactive metal powder or metal composite (listed in Table 1) as SFAE. These solid metal casings are typically machined from stock into billets, but are also manufactured by other methods including casting or forging. The SFAE is then pressed into solid billets with a density (preferred to be 80-90% TMD) applicable to the particular use. The annular construction uses flake alumninum as the reactive metal. The SFAE billets are then placed in the warhead and the explosive is cast or pressed into place. The final SFAE fuel to explosive ratio is dependent upon the size and configuration of the warhead. PBX N112 consists of 89% HMX (high explosive) and 11% LMA (lauryl methacrylate). The PBX N112/reactive metal weight ratio includes the range of about 0.66 to about 1.45, or the ratio of about 1.  
      Embodiments of the compositions of the present invention are formed into a unicharge. The unicharge construct uses spherical aluminum as the reactive metal. Table II discloses ranges of ingredients for the formulations of the unicharge embodiment. As noted previously, a plasticizer and/or a burn rate catalyst are added to the formulations to tailor the formulations to particular needs. Although specific binders are listed, any of the binders previously noted are also used in the formulations. Similarly, any of the oxidizers previously noted are also substituted for AP and any of the high explosives previously noted are substituted for HMX.  
      Referring to  FIG. 2 , there is shown a plot illustrating the blast effectiveness of an explosive which includes a reactive metal of nano-meter sized metal particles. The metal particles used in the explosive have sizes of twenty nano-meters and two hundred nano-meters. For the first few micro-seconds the plot represented by the reference numeral  25  is virtually identical for the twenty nano-meter and two hundred nano-meter metal particles. After about 5-10 msec., the plots vary substantially. The plot for the twenty nano-meter metal particles (represented by the reference numeral  28 ) represents a blast effectiveness which is two times that of the two hundred nano-meter metal particles (represented by the reference numeral  26 ). As shown in  FIG. 2 , the time required for a complete release of energy can be reduced by substantially more than 50 msec and can be as much as 100 msec. Further, by using twenty nano-meter sized metal particles in the explosive device, the time required for total energy release is reduced to approximately 20 msec. which is genrally the maximum time during which total energy release by the explosive device should occur.  
      It should be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof will be suggested to persons skilled in the art and are to be included within the spirit and purview of this application and the scope of the appended claims.