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2 New Energetic Materials CURRENT RESEARCH FOCUS In presentations made to the committee, the commitment of the U.S. Air Force, U.S. Army, and U.S. Navy to new energetic materials research on CHNO/F compounds was emphasized. Representatives of each service stated that new CHNO/F compounds will play a vital role in improving the capability of existing and planned systems. Each service presented research efforts focused on essentially unique suites of CHNO/F compounds. They all agreed that CHNO/F compounds continue to be the central and core ingredients for the vast majority of explosive and propellant formulations for the foreseeable future.) As such, this specialized field of synthesis needs to be a dynamic element of any initiative for meeting the emerging performance goals of future military ordnance.2 It is important to note that the committee's task did not extend to verifying these service requirements presented to it. The U.S. effort in the synthesis of energetic materials at present involves approximately 24 chemists, several of whom are approaching retirement. Few chemists are being trained to replace them.3 The committee considers these scientists to be a national resource whose productivity in terms of new energetic compounds has been very high. If the level of effort that these scientists have contributed is not fostered and maintained, the United States will lose the technological edge that it has gained as a result of their work. Attracting top synthetic chemistry talent to energetic materials research is possible only if the field is perceived to be scientifically exciting and financially stable. It has been argued that expansion of the synthesis effort is easily justified with respect to U.S. 2 R.S. Miller. 1995. Research on new energetic materials. Pp. 3-14 in Proceedings of the Materials Research Society, Vol. 418: Decomposition, Combustion, and Detonation Chemistry of Energetic Materials, T. B. Bril l , T. P. Russell, W.C. Tao, and R. B. Ward le. eds. Warrendale, Pa .: Materials Research Society. J.M. Goldwasser, ONR, 2001, presentation to the committee, July 31; J.A. Lannon, RDC/Picatinny, 2001, presentation to the committee, July 31; M. Berman, AFOSR, 2001, presentation to the committee, July 31; D. Woodbury, DARPA, 2001, presentation to the committee, July 31; K. Kim, DTRA, 2001, presentation to the committee, July 31. 3 T. Highsmith,Thiokol. 2002. Presentation tothe committee. April 17 8 .
NEWENERGETIC MATERIALS competitiveness with other countriesfor example, Russia and China, where the perception is that hundreds of capable scientists actively work in this area.4 These numbers would suggest that the United States and Western nations should have faced major technological surprises from this large community investigating new compounds. While there have been notable exceptions, such as with ammonium dinitramide and aluminum hydride (AIHs, or alane), the numbers and impact of foreign-generated new energetic materials have been comparatively small, calling into question the validity of this argument. Bigger is not necessarily better. This discussion does not, however, dilute the case for a continued, viable U.S. energetic materials synthesis program. The need for a strong synthesis program today is inherently based on the new critical 9 tactical requirements of the battlefield. These requirements are a result of new mission profiles and rapid turnover in other weapons system components and tactics that will fundamentally alter the mission requirements for new energetic materials. Using yesterday's energetic materials exclusively in today's (or even more so, tomorrow's) battlefield systems would be as effective as trying to run a Ferrari on kerosene. While it is generally accepted that new CH NO/F molecules will offer only incremental improvements to the currently employed materials, these improvements will lead to significant cumulative weapons system performance enhancement on target when coupled to technological advances in targeting, lethality, survivability, and advanced fusing, to cite a few areas. TRANSITION BARRIERS It must be pointed out that over the past several decades, the products of the research of the energetic materials synthesis community have not successfully made the transition to military applications. One of the greatest barriers to capitalizing on current efforts is the lack of adequate and stable resources (including personnel) for synthesizing new materials and for shifting the most promising materials from the laboratory into fielded systems. Historically, the transition period from discovery of a new material to its availability in the field has been several decades. Very few materials complete that transition owing to the large number of requirements that a material must meet. These include the need to achieve high density, good mechanical properties, low sensitivities, good stability, low cost, ease of manufacture, and environmental acceptability.5 While the synthesis of new molecules is relatively inexpensive, full characterization, scale-up, and other processes necessary to introduce a new material into the military inventory require significantly more resources. In the current acquisition process, program managers cannot assume the inherent risk associated with research materials, since there is a good chance that major stumbling blocks will be encountered and system developers do not have the charter, or the resources, to invest in the development of new materials. Moreover, research managers are similarly resource-constrained. They cannot afford to support full characterization of emerging materials, which in the past has been the responsibility of the applied and advanced development community (i.e., the underfunded 6.2 and 6.3 program elements, respectively). The Department of Defense (DoD) is essentially the only customer for these energetic materials. There is no question that the nation's capability to discover and to utilize new energetic materials is in decline. A significant, defense-funded energetic materials program would need to be implemented to stop this decline. Such a program should do the following: 4 H. Shechter, OSU. 2001. Synthesis of 1,2,3,4-Tetrazines Di-N-Oxides, Pentazole Derivatives, and Pentazine Poly-N-Oxides. Presentation to the committee. December 13. 5 A. Sanderson. 1995. Proceedings of the NIMIC (NATO Insensitive Munitions Information Center) Workshop on What Makes a Useable New Energetic Material. MIMIC TR19950061. Listed online at http://www.nato.int/related/nimic/reports/limited/limited.htm.
10 ADVANCED ENERGETIC MATERIALS · Become closely coupled to future weapons systems needs; · Address the full spectrum of research, advanced scale-up, and characterization of advanced energetic materials; · Focus on the qualification of new energetic materials for service use; and · Train tomorrow's workforce. CU RRENT CH NO/F EN ERG ETIC MATERIALS RESEARCH The national energetic materials synthesis programs all have a common theme: beating the performance of the current, most energetic materials deployed in today's arsenalnamely, the nitramine explosives cyclotrimethylenetrinitramine (RDX) and cyclotetramethylenetetranitramine (HMX).6 The goal of new materials synthesis is generally focused on performance improvement. In most cases, target molecules are chosen only after theoretical predictions from extensively calibrated, empirically based computer codes indicate that the substance, if synthesized, will significantly improve performance in weapons applications. This approach to new conventional CHNO/F energetic materials can be characterized by recent U.S. successes, as detailed below. The chemical and molecular structures of such materials are shown in Figure 1-1 in Chapter 1. CHNO/F Targeted Energetic Materials Synthesis Programs Caged Nitramines In the early 1970s, the Research Department of the then-Naval Weapons Center (NWC) in China Lake, California, conducted a short-term effort to synthesize hexanitrobenzene (HNB). This work was funded by Lawrence Livermore National Laboratory (LLNL). The successful synthesis of HNB catalyzed a multiyear effort of new CHNO (carbon- hydrogen-nitrogen-oxygen) compound synthesis at China Lake that culminated in 1987 with the synthesis of the caged nitramine explosive hexanitrohexaazaisowurtzitane (CL-201.7 The caged nitramine effort was funded over almost a 15-year period by a number of sources, including the Office of Naval Research (ONR) Mechanics Division, internal NWC 6.1 (basic researchy, and 6.2 (applied research) funding.89 CL-20 has the highest density of all currently known stable nitramine explosives. (Density is an important physical property that couples directly to improved performance.) Lawrence Livermore National Laboratory has developed and fully characterized the performance and safety properties of a new explosive formula, Livermore Explosive Formulation 19 (LX-19), using CL-20 as the energetic component. CL-20, the single CHNO explosive currently in the transition process for qualification as an explosive and propellant ingredient, shows great promise. ATK Thiokol Propulsion has developed the scale-up processing protocol under DoD ManTech funding for large-scale synthesis of CL-20, and CL-20 is readily available for explosive and propellant developers to employ in future military applications. 6 See Figure 1-l in Chapter 1 for the molecular structures of RDX and HMX. 7 See Figure 1-l for the molecular structure of HNB and CL-20. A.T. Nielsen, A.P. Chafin, S.L. Christian, D.W. Moore, M.P. Nadler, R.A. Nissan, D.J. Vanderah, R.D. Gilard i, C. F. George, and J.L. Flippen-Anderson. 1998. Polyazapolycycl ic caged polynitra m ines. Tetrahedron 54:11793-11812. 9 A.T. Nielsen, ed. 1995. Nitrocarbons. Weinheim, Germany: VCH Publishing.
NEWENERGETIC MATERIALS Octanitrocubane 11 Researchers at the University of Chicago recently published the successful synthesis of one such target compound, octanitrocubane.~° ii The work was supported by the U.S. Army's Armament Research, Development, and Engineering Center (ARDEC) and by the ONR l\/lechanics Division. A tour de force of modern synthetic chemistry, this work is illustrative of the value of sufficient and sustained support. However, octranitrocubane currently does not exhibit the predicted high density, and it is not yet clear whether cubane-based energetics will find a practical niche. In addition, the extremely long synthetic route of this material implies a high production cost, which may affect its application. Nitrogen Fluorine Substituted Nitramines In several classes of explosives, the replacement of oxygen with fluorine may enhance many desired properties. For example, the introduction of difluoroamine groups into HMX may increase its density, performance, and specific impulse. Particular advantages may be noted when the formulation includes metal ingredients. These expectations suggest that similar modifications should be investigated for a selected few of the more promising CHNO/F compounds that have been synthesized. A comparison of two of the more common compounds can be seen in Table 2-1. TABLE 2-1 Comparison of HMX and HNFX Compound Density (g/cm3) P (GPa) Isp (s) HMX 1.90 37.4 272 H N FX 1.gga 47.4a 285a a calculated The ONR Mechanics Division continues to fund synthesis efforts at the Research Department, Naval Air Warfare Center-Weapons Division (NAWC-WDy, China Lake, and at the Naval Surface Warfare Center-lndian Head Division (NSWC-IH), pursuing fluorine analogs of HMX. In an essentially one-person effort over the last decade, a family of difluoramine (NF~) substituted cyclic nitramines was synthesized, having calculated densities, heats of formation, and performance equal to or greater than those of HMX.~2-~4 The first scale-up of these materials was initiated in 2002 with a small commercial contract as one task in the Advanced En ergetics Initiative (AEI).~5 The goal of this project is to prepare sufficient gram LOP. Eaton and M.X. Zhang. 2002. Octanitrocubane: A new nitrocarbon. Propellants, Explosives, Pyrotechnics 27:1-6. itM.X. Zhang, P. Eaton, and R. Gilardi. 2000. Hepta- and octanitrocubanes. Angewandte Chemie, I nternational Edition 39:401-404. i2 R.D. Chapman, M.F. Welker, and C.B. Kreutzberger. 1998. Difluoramination of heterocyclic ketones: Control of microbasicity. Journal of Organic Chemistry 63:1566-1570. i3 R.D. Chapman, R.D. Gilardi, M.F. Welker, and C.B Kreutzberger. 1999. Nitrolysis of a highly deactivated amide by protonitronium. Synthesis and structure of HNFX1. Journal of Organic Chemistry 64:960-965. i4T. Axenrod, X-P Guan, J. Sun, L. Qi, R.D. Chapman, and R.D. Gilardi. 2001. Synthesis of 3,3- bis~d if l uorami noyoctabyd ro-1,5,7,7-tetranitro-1,5-diazocine (TN FX), a diversified energetic heterocycle. Tetrahedron Letters 42:2621-2623. i5The Advanced Energetics Initiative was proposed by the Office of the Secretary of Defense for maturing the fundamental technologies required to transition the next generation of energetics materials into field use.
12 ADVANCED ENERGETIC MATERIALS quantities of HNFX, a gem-difluoronitramine substituted HMX analog, to confirm calculated physical and performance properties. TNAZ Other work funded by ARDEC led to the synthesis and process for the commercial scale-up of 3,3,1-trinitroazetidine (TNAZ), a strained ring Heterocyclic nitramine. TNAZ is one of the few new energetic materials found to be thermally stable above its melting point. However, in formulations studies, it has been found that TNAZ has high volatility that will severely inhibit its utility in military explosive and propellant applications. Further limitations to its use include the processing, polymorph, and material costs. High-Nitrogen-Con ten t Heterocyclic Molecules Significant progress toward enhanced performance and increased stability is being made in the synthesis of high-nitrogen-content Heterocyclic molecules. This area of synthesis is being funded in the Department of Energy (DOE) Laboratories, LLNL, and Los Alamos National Laboratory as components of DoD and ONR programs, and at SRI International and at the Rocket Propulsion Laboratory, Edwards Air Force Base, by the Defense Advanced Research Projects Agency (DARPA). While a plethora of new molecules has been synthesized, none has yet been prepared in sufficient quantity or purity for extensive evaluation. Theoretical calculations on some target molecules suggest that materials with greater performance than that of HMX, and a higher heat of formation than that of either HMX or CL-20, may exist in this class of energetic materials.~7 Highsmith,~s Shechter'49 Hiskey,20 and Koppes2i touched on several examples in their presentations to the committee. Many of these compounds, or relatives thereto, were initially discovered and reported in the open literature by scientists in the former Soviet Union.22-3i To date, no OK. Anderson, J. Homsy, R. Behrens, and S. Bulusu. 1998. Modeling the thermal decomposition of TNAZ and NDNAZ. Pp.239-247 in Proceedings of the Eleventh International Detonation Symposium, August 31-September 4, 1998, Snowmass, Colo. ~ 7 R.J. Bartlett, University of Florida. 2002. Presentation to the committee. April 18. JET. Highsmith, Thiokol. 2002. Presentation to the committee. April 17. is H. Shechter, OSU.2001. Synthesis of 1,2,3,4-Tetrazines Di-N-Oxides, Pentazole Derivatives, and Pentazine Poly-N-Oxides. Presentation to the committee. December 13. 20 M.A. Hiskey, LANL.2002. Presentation to the committee. April 18. 2i W. Koppes, NSWC-IH. 2002. Presentation to the committee. April 18. 22S.A. Shevelev, l.L. Dallinger, T.K. Shkineva, and B.l. Ugrak. 1993. Nitropyrazoles,7. Nitro derivatives of hi-, term, and quaterpyrazoles. Russian Chemical Bulletin 42:1857-1861. 23 I.L. Dallinger, T.l. Cherkasovaa, and S.A. Shevelev. 1997. Mendeleev Commun.58. 24S.Sh. Shukurov and M.A. Kukaniev. 1993. A new synthesis of 3-alkyl-6-alkylthio-1,2,4-triazolo t3,4- b] 1,3,4-thiadiazoles, Russian Chemical Bulletin 42:1860-1861. 25S.A. Shevelev, V.M. Vinogradov, l.L. Dallinger, B.l. Ugrak, A.A. Fainzilberg, and V.l. Fillipov. 1991. Reaction of NH-Azoles with fluorosulfonyl-N, N-difluorohydroxylamine. Synthesis of N- Fluorosulfonylazoles. English translation of Izv. Akad. Nauk Ser. Khim. 10:2419-2429. 263~51-Amino-4-nitropyrazole: Convenient synthesis and study of nitration. 1993. Russian Chemical Bulletin 42:1861-1864. 27 H. Piotrowski, T. Urbanski, and K. WeJrochmatacz. 1971. Reaction of 2,2-dinitropropane-1,3-diol with 1,3,5-trialkylhexabydro-s-triazines. Bull. Acad. Sci. France 359-362. 280.V. Zavarzina, O.A. Takitin, and L.l. Khemlnitskii. 1994. Substitution of the nitro group in chloronitrofuroxan by N- and O-trimethyl derivatives. Mendeleev Commun. 135. 29 I.B. Starchenkov, V.G. Andrianov, and A.F. Mishev. 1998. Chemistry of furazano t3,4-d~pyrazine 6. 1,2,3-triazolot4,5-d~furazanot3,4-bipyrazines. Chemistry of Heterocyclic Compounds 34:1081-
NEW ENERGETIC MA TERIALS evidence exists which suggests that any of these materials reported in the former Soviet Union were moved into military systems. Investment in the category of high nitrogen compounds appears at this time to have the potential for generating significant midterm application. Additionally, new energetic materials efforts over the past 10 to 15 years funded by the DoD Office of Munitions, in partnership with the national laboratories, have resulted in the synthesis of many polycyclic nitrogen-containing heterocyclic materials of potential military application. More recently, DARPA has initiated continuation funding for energetic materials work in this field at SRI International.32 13 All-Nitrogen Materials In the general area of high-density energetic materials, the syntheses and reduction to practice of all-nitrogen compounds are high-risk endeavors. Theoretical calculations predict that many of the all-nitrogen compounds will have higher positive heats of formation (the calculated heat formation of the unknown compound, No, is 753,120 J/mol, whereas the heat formation of HMX is 75,019 J/moly, higher densities (the calculated density of N4, is 2.757 g/cm3, whereas the density of HMX is 1.905 g/cm3y, lower combustion signatures, good ca Icu lated propel la nt cha racteristics, a nd perha ps lower sensitivities the n those of materials in the arsenal.33 These properties have yet to be verified by experiment.34 The heat of explosion for these all-nitrogen compounds relies solely on the endothermicity of these molecules, as they have no constituents that will oxidize binder, metal, or fuel to contribute to Isp, or detonation pressure. The recently synthesized N5+ cation is highly reactive and only relatively stable when associated with a large polyfluoro-element anion.35 Ideally, based on ionization potential and electron affinity calculations, the imaginary N5- species is a likely candidate to form a stable, high-energy compound when combined with the N5 cation. The probability of a functionally fielded, all-nitrogen compound is very low, even in the long term. While theorists may predict that a variety of all-nitrogen species should exist, e.g., N~, Ns~, N~-, Ns, and N~o, the synthetic routes to these materials will certainly be a long time in coming. Syntheses of the all-nitrogen compounds should be a far-term goal at best. Nevertheless, this highly innovative research effort should be continued. It must be noted that all of the new energetic molecules discussed above are essentially legacy molecules that resulted from sustained, concerted multiyear or even multidecade efforts, and that the funding of these materials synthesis programs has essentially dwindled to near zero. These legacy materials are in no way ideal, however, and some degradation and decomposition must be expected. The Advanced Energetics Initiative has begun to address the current funding deficiencies, but it is manifestly clear that a significant infusion of resourcesboth funding as well as new trained personnelwill be required to reestablish this critical technology base. 1085. 30V.A. Tatakovsky. 1996. The design of stable high nitrogen systems. Pp. 15-36 in Proceedings of the Materials Research Society, Vol. 418: Decomposition, Combustion, and Detonation Chemistry of Energetic Materials, T.B. Brill, T. P. Russel l , W.C. Tao, and R. B. Ward le. eds. Warrendale, Pa.: Materials Research Society. 3iY. Yongzhong and S.Z. Huang. 1989. Synthesis of polynitrocompounds from nitroguanidine. Propellants, Explosives, and Pyrotechnics 14:150-152. 32J. Bottaro. 2001. Presentation to the committee. December 14. 33R.J. Bartlett, University of Florida. 2002. Presentation to the committee. April 18. 34R.J. Bartlett, University of Florida. 2002. Presentation to the committee. April 18. 35K.O. Christe, USC. 2001. Presentation to the committee. December 14.
14 ADVANCED ENERGETIC MATERIALS CURRENT TRANSITION TO APPLICATIONS Of all the new energetic materials synthesized over the past 20 years or soand there have been literally hundredsCL-20 is unique in that it has shifted to significant commercial production. It is now in exploratory and advanced development for a variety of defense applications. CL-20 has also received considerable interest in the Free World, and extensive vvork on this material has been and is being conducted in Sweden, France, Great Britain, and elsewhere. ATK Thiokol Propulsion markets CL-20 in the United States. BOFORS/Celsius of Sweden and Societe National de Poudre et Explosivs of France are also commercial manufacturers of CL-20. It must be emphasized that the slow transition by CL-20 bodes poorly for other promising materials. The research effort that led to the synthesis of CL-20 spanned a period of approximately 15 years, culminating in its synthesis in 1987. Its transition to commercial production has taken another 15 years; it is currently available commercially from U.S. and foreign vendors. All other energetic materials CHNO/F compounds, that is, high explosives .. .. . . . . . . . . . . . . as well as other materials, are in early stages of research and exploratory development and are, at a minimum, 5 to 10 years from potential utilization. Most of these materials will need a similar investment in order to reach their commercial potential, but it is unlikely that they will receive such an investment.36 The 6.1, 6.2, and 6.3 funding for new energetic materials synthesis has been significantly reduced across the board at all DoD laboratories performing energetic materials research and development.37 Although the transition history of CL-20 is long, it is still shorter than the norm for new energetic molecules currently in the U.S. arsenal. An examination of energetic materials fills currently in use in the modern U.S. weapons arsenal reveals that the principal ingredients for explosive and propellant applications remain TNT (a World War I explosive) and the nitramines HMX and RDX (World War 11 explosives). The same materials are the preponderant ingredients for foreign military applications as well. These highly energetic CHNO compounds are the choice of weapons designers because they are relatively inexpensive and available, and they meet the extensive and stringent list of requirements imposed for performance, safety, reliability, compatibility, lifetime, environmental impact, and life-cycle cost, to list just a few characteristics.38 Consideration of all of these properties is critical before a promising new material can be moved into production. In order to adequately address them, substantially more time and effort will be needed. Unfortunately, today's funding environment does not support the requisite transition program for potentially viable new energetic molecules. 36The only "material" that does not fall into this category is the thermobaric fill demonstrated by a Defense Threat Reduction Agency effort. This is termed a new material, but it was devised simply through formulation, using currently employed energetic materials to mimic an explosive fill first demonstrated by the former Soviet Union. 37 It is important to note that the sponsor's principal charge to the committee was to find, if it existed, the "low hanging fruit" generated by the synthesis/energetic materials community. The criteria for this goal were such that if a significant investment was made in the near termthat is 1 to 3 years a particular material could be brought to maturity for insertion into weapons use. Unfortunately, no low hanging fruit was found to exist in any of the technologies that the committee was charged to examine. 38 A. Sanderson. 1995. Proceedings of the NIMIC (NATO Insensitive Munitions Information Center) Workshop on What Makes a Useable New Energetic Material. MIMIC TR19950061. Listed online at http://www. nato. i nt/ rel ated/n i m i c.
NEWENERGETIC MATERIALS FINDINGS AND RECOMMENDATIONS FINDINGS With regard to the research and development of new energetic materials, the committee found that: . . . 15 CHNO/F (carbon-hydrogen-nitrogen-oxygen compound with fluorine) compounds will continue to be the central and core energetic ingredients for the vast majority of explosive and propellant formulations for the foreseeable future. As such, this specialized field of synthesis will be a dynamic element of any initiative for meeting the emerging performance goals of future military ordnance. One of the greatest barriers to capitalizing on current efforts is the lack of adequate and stable resources (including personnel) for continued synthesis of new materials and for supporting transition development studies of the most promising materials from the laboratory into fielded systems (from 6.1 fbasic research] to 6.2 Applied research] and 6.3 Advanced technology development] and beyond). Expansion of the scale-up and properties characterization program is imperative to move the most promising materials from 6.1 to the 6.2 and 6.3 levels. · The anticipated smaller, internally carried ordnance with a concomitant requirement for higher performance will require new explosive formulations with higher energy content. These new critical tactical requirements of the battlefield mandate a strong synthesis program. · Many of the current synthesis efforts are essentially one-person efforts or are led by very senior scientists. Funding these first-class synthetic chemists at a continuous, high level so that they are able to develop the next generation of energetic materials scientists is of utmost importance. The future of energetic materials syntheses and development rests on this group. · CL-20 is the only new CHNO explosive compound that is currently available for large- scale synthesis and qualification in new military explosive and propellant formulations. The classical organic synthesis of new energetic molecules has low risk, yet a disproportionately high payoff. Performance and property enhancements available from new materials will be the stepping-stones to improved weapons effectiveness. Current productivity in the area of organic synthesis has been quite high, in spite of a relatively small annual investment. Recommendations To conclude, energetic materials synthesis has prov ided the only "low hanging fruit" identified by the committee, and the Department of Defense should invest in the continued discovery, characterization, and development of such materials. The committee recommends that: . An investment strategy be implemented that emphasizes not only the development of new energetic materials, but also their characterization and scale-up. · Investment be made in formulation technology to facilitate the transition of new compounds. It is important that this effort be funded to the point at which a weapons system designer can be assured that these new formulations have sufficiently low risk for implementation because they ensure improved performance against targets.