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Energetic Materials: Part 1. Decomposition, Crystal and Molecular Properties PDF

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PREFACE Our own involvement in the area of energetic materials stems from the second half of a New Orleans Saints football game in 1979, which one of us (P.P.) was attending together with Rod Bartlett (a contributor to these volumes). The Saints were losing by a large score, and with interest in the game waning, Rod spent much of the second half expounding, to a very receptive listener, the opportunities in energetic materials research. Whether this area has since been advanced or set back as a result of the Saints' ineptness that afternoon is for others to decide. What is certain, however, is that during these years we have been extremely fortunate in having the assistance, encouragement and support of a very fine group of project officers: Dick Miller and Judah Goldwasser, ONR; David Squire, Ron Husk and Bob Shaw, ARO; Larry Davis, AFOSR; Jack Alster and Frank Owens, ARDEC; Bob McKenney, Norm Klausutis and Paul Bolduc, AFATL; Horst Adolph, NSWC; and Leonard Caveny, BMDO. We are grateful to all of them. In putting together these two volumes, we have greatly appreciated the enthusiastic encouragement and suggestions that we received from Betsy Rice and her willingness to prepare the overview that begins each volume, in addition to her other contribution. Finally, we want to thank Mick Coleman and Bob Murray for their much-needed help with various aspects, often tedious, of the editing process. Peter Politzer and Jane S. Murray P.A. Politzer and J.S. Murray (Editors) Energetic Materials, Part :1 Decomposition, Crystal and Molecular Properties Theoretical and Computational Chemistry, Vol. 21 (cid:14)9 2003 Elsevier B.V. All rights reserved. Overview of Research in Energetic Materials Betsy M. Rice U. .S Army Research Laboratory, Aberdeen Proving Ground, MD 21005-5069 Energetic materials encompass different classes of chemical compositions of fuel and oxidant that react rapidly upon initiation and release large quantities of force (through the generation of high-velocity product species) or energy (in the form of heat and light). Energetic materials are typically classified as explosives, propellants, pyrotechnics or incendiaries, and distinctions among the classes are usually in terms of the types of products generated and rates of reactions. These particular features have been advantageously employed in a wide variety of industrial and military applications, but often these utilizations have not been fully optimized, mainly due to the inability to identify and understand the individual fundamental chemical and physical steps that control the conversion of the material to its final products. The conversion of the material is usually not the result of a single-step reaction, or even a set of a few simple consecutive chemical reactions. Rather, it is an extremely complex process in which numerous chemical and physical events occur in a concerted and synergistic fashion, and whose reaction mechanisms are strongly dependent on a wide variety of factors. For instance, the performance of a material is a strong function of the temperature and pressure of its environment and of its mechanical properties. The rate of energy release can be modified by additives or by varying the concentrations of fuel and oxidizer. The response of the material can be affected by the manner in which it was processed. The properties and behavior of the material can change over time. The rate of the conversion of the material is strongly dependent on the conditions of the initiation. For example, an explosive can be ignited to bum, a process that can be thought of as a reaction wave that proceeds through the material at subsonic speeds. Or it could be shock-initiated to detonation, where detonation can be described as a reaction wave that proceeds through the material at supersonic speeds. For these cases, the product concentrations and species are not the same. Also, these processes often occur under extreme conditions of temperature and pressures, making experimental measurement difficult. These are but a few of the complexities associated with studies of reactions of energetic materials that make resolving the individual details so difficult. They are also the reason that research in the field of energetic materials is so exciting! These difficulties have required the development of a variety of innovative theoretical methods, models and experiments designed to probe details of the various phenomena associated with the conversion of energetic materials to products. Probably the majority of experimental and theoretical efforts in energetic materials research to date have been directed toward assessing performance. The focus is often on quantifying the amount of energy that is released, identifying key reaction mechanisms, or investigating ways in which the energy release can be manipulated and controlled. However, increased environmental and safety concerns are placing new emphases on understanding other aspects of energetic materials, such as toxicity, processing emissions, combustion emissions, wear and erosion on combustion fixtures from reaction products, contamination of ground water or soil, environmentally-friendly synthesis and processing, destruction or disposal, life-span (chemical stability), storage, handling, or vulnerability to a variety of external stimuli. Naturally, models are emerging that can be used to investigate these aspects of energetic materials. The high time and pecuniary costs associated with the synthesis or formulation, testing and fielding of a new energetic material has called for the inclusion of modeling and simulation into the energetic materials design process. This has resulted in growing demands for accurate models to predict properties and behavior of notional energetic materials before committing resources for their development. For example, in earlier times, extensive testing and modification of proposed candidate materials for military applications could take decades before the material was actually fielded, in order to assure the quality and consistent performance of the material in the desired application. Predictive models that will allow for the screening and elimination of poor candidates before the expenditure of time and resources on synthesis and testing of advanced materials promise significant economic benefit in the development of a new material. The growing demands for predictive models of energetic materials have not eliminated the need for experimentation on energetic materials; rather, they call for increasingly detailed experimental studies to allow critical assessment, correction and enhancement of the models. This is a particularly challenging requirement, since the time and length scales of most experiments are often many orders of magnitude larger than those of some of the models (i.e. atomistic). Nonetheless, advances are being made in designing experiments to probe behavior of energetic materials at ever- decreasing time and spatial scales; some of these are described in these volumes. The contributions to these volumes will highlight challenges faced by researchers in the energetic materials community, as well as describe research activities directed toward fundamental characterization of properties and behavior of these interesting materials. An assortment of theoretical studies of energetic materials processes at different time and length scales and under different conditions is presented. Accompanying these are discussions of the complexity, assumptions and levels of empiricism of the models used in these studies. Also included, in order to provide perspective, are surveys of experiments designed to investigate the complex and unique chemical and physical processes occurring in energetic materials. It is hoped that the works presented in these volumes will stimulate further investigations and advances in the area of energetic materials research. P.A. Politzer and J.S. Murray (Editors) Energetic Materials, Part :1 Decomposition, Crystal and Molecular Properties Theoretical and Computational Chemistry, Vol. 21 (cid:14)9 2003 Elsevier B.V. All rights reserved. Chapter 1 A survey of the thermal stability of energetic materials Jimmie C. Oxley Chemistry Dept; University of Rhode Island; Kingston, RI 02881 .1 INTRODUCTION It is reported that the Chinese discovered black powder as early as 200 AD, but the earliest surviving record is in a Chinese military manual "Wu Jing Zong Yao" dated 1044 A.D. The Chinese used this new found powder for warfare, attaching bamboo tubes of gun powder to arrows to make "rocket arrows" and sometimes bundling many rocket arrows onto launchers which were hauled into battle on wheelbarrows, also a Chinese invention. The Chinese formula reached Europe in the 13th century, and the Europeans were firing bombards by 1327 A.D. Sir Francis Bacon, who published the formula in the West, was so impressed by its awesome power that he speculated man would give up making "war when he saw the terror of black powder 1 . Black Powder Chinese Bacon present 200 AD 1242 AD day saltpeter KNO3 72.5% 37.5% 74% charcoal 19.0% 31.25% 15.6% sulfur 8.4% 31.25% 10.4% Man did not give up war, nor did he find a peaceful use for black powder until the 1600's. Then, for the first time in Hungary, black powder was used for mining. Tenny L. Davis credits black powder as one of the three inventions responsible for ending the Middle Ages 1 . (The other two were the printing press and the discovery of the New World.) The first commercial production in the United States was by DuPont (1802)2. Worm War I saw the maximum use of black powder. To function as good explosives, formulations must maximize expansion volume (i.e. density and gas formation) and heat release. For a substance to function as an explosive, its molecules must (a) be broken down to smaller molecules without high energy costs (weak linkages) so that the net decomposition reaction is exothermic; (b) contain sufficient oxygen to quickly oxidize most of the molecule to gas--carbon and hydrogen to CO, 2OC and H20; )c( undergo oxidative breakdown rapidly enough that the energy release supports the shock wave. It was nearly four hundred years after the Western world had adopted black powder before explosives using these principles, the first "high explosives," appeared. By the mid- to late nineteenth century, Lavoisier had "fathered" modem chemistry and died in the French revolution; Bertholet had published his famous treatise on chemistry; and nitration reactions had been discovered: alcohols with mixed acid (sulfuric and nitric) made highly flammable, if not explosive, species 3. Most military explosives satisfy requirements (a) and (b) by containing only carbon (C), hydrogen (H), oxygen (O), and nitrogen (N). The oxygen is found usually in nitro groups since it is relatively easy to break the 2ON-X bond, and the nitrogen can convert to nitrogen gas, while the oxygen aids the gasification of carbon and hydrogen. Chronologically, nitrate esters (O- 2ON e.g. nitrocellulose, nitroglycerin) were the first high explosives discovered in the mid-nineteenth century. Nitroarenes (C-NO2, e.g. picric acid, TNT) came into use at the end of the nineteenth century. Nitramines ,2ON-N( e.g. RDX), though discovered about the same time as TNT, did not come into their own until World War II and later. In modem explosive design requirements (a) and (b) still dictate that compounds be designed to be dense and have positive heats of formation. High oxygen content is less important in high nitrogen/low carbon containing materials. In the United States about five billion pounds of explosives are used annually. Most are used for commercial purposes and are ammonium-nitrate based formulations 2,4. There are less than a dozen chemical explosives that are manufactured in bulk quantities. These were "discovered" in the fifty-year period between 1850 and 1900 5,6,7,8,9,10. New explosives have been synthesized, yet optimization of synthesis, study of stability and sensitivity, and optimizing the formulations takes decades. This problem is not new; the development of nitrocellulose and nitroglycerin as useful explosives also took decades. Thermal stability must be evaluated and quantified, not only under the intended parameters of manufacture or storage, but outside these limits where a catastrophic upset might occur. Often, as in the case of nitrocellulose, the intrinsic thermal stability is low or easily deteriorated by contamination so that a stability enhancement agent is necessary. To find a stabilizing agent, the mechanism of decomposition must be thoroughly understood. Thus, thermal stability must be evaluated over a wide range of temperatures and the decomposition mechanism(s) determined before stabilizing agents can be chosen rationally. 2. NITRATE ESTERS Nitrocellulose or "gun cotton" was discovered by Braconnot (France, 1833) and patented by Schonbein (1846) (Fig. 1). It is speculated that in order to determine whether the action of nitric acid was a reaction or merely a sorption into fibrous material, Sobrero (1846) treated glycerin, a liquid, with nitric acid and found a true reaction took place. Hence, nitrocellulose and nitroglycerin were discovered within a decade of each other, but neither found widespread use until the 1860's when methods of stabilizing them were devised. Between 1865 and 1868, Abel patented improved preparations of nitrocellulose. He found that pulping allowed impurities, such as residual acid, to be more easily washed out by "poaching" and resulted in improved stability. The Abel stability test is named after him5. 2ON O2NG I 2ONO- /X 2ONO O2~ ONO2 nitroglycerin NTEP nitrocellulose 9NO2 ONO2 CHiONO 2 H2cH CH2~H2 O2NO "CH2CH CH 3 C "~("3H CH 2"ONO 2 0 2 ONO 2 CH 2 -ONO 2 BTTN PGDN NTEMT O2NO CH2cH20 CH2cH20 CH2cH2ONO 2 NDGET .giF 1 Nitrate Esters While complex nitrate esters, such as nitrocellulose and nitroglycerin, were the first to find application as explosives, understanding the mechanisms of nitrate ester decomposition was accomplished through the study of simpler compounds 11-16. Ethanol nitrate was examiaed by numerous researchers, and they concluded the first, and rate-determining, step was revers~le loss of NO2. RCH20-NO2 <=> RCH20" + 2ON" Griffiths, Gilligan, and Gray studied 2-propanol nitrate pyrolysis and found it exhibited more 3-cleavage than primary nitrate esters 17. It yielded nearly equal amounts of 2-propanol nitrite and acetaldehyde, as well as small amounts of acetone, nitromethane, and methyl nitrite. Homolytic cleavage of the RO-NOz bond formed the 2-propoxy radical, which subsequently reacted with nitric oxide to produce 2-propanol nitrite, or it could elimiaate methyl radical to form acetaldehyde and methyl-derived products; or it could be converted to acetone by oxidization or loss of a hydrogen atom. Dinilrates of butanediol were investigated by Powling and Smith 18. They found that 1,4- butanediol dinitrate, following cleavage of one NO2 moiety, decomposed to formaldehyde, nitrogen dioxide, and ethylene, whereas 2,3-butanediol dinitrate gave acetaldehyde ni place of formaldehyde and ethylene. A simihr intramolecular decomposition to gaseous products has been proposed for nitroglycerin 19. Ng, Field, and Hauser used time-of-flight mass spectrometry to identify the decomposition products of pentaerythritol tetranitrate (PETN); based on their results they posttdated the decomposition involved the formation of the tert~y tris-(nitroxymethyl)methyl radical 20. A comprehensive study examined the thermal stability and decomposition mechanism of a large number of primary, secondary, and tertiary mononitrates: n-pentanol, 3-buten- 1-ol, ethanol, neopentanol, 2- phenylethanol, 2-propanol, cyclohexanol, 2-methyl-2-propanol, and 2-methyl- 2-butanol nitrates. Hydrogen-donating solvents, capable of cappiag radical intermediates, were employed to stabilize intermediates for identification and to divert the oxides of nitrogen from further complicating the course of degradation. The reversibility of 2ON homolysis was demonstrated by solvent cage effects and isotopic labeling experiments. To shed fight on the timing of the loss of NO2 and CH20 from primary alkanol nitrates, certain nitrate esters leading to stabilized alkyl radicals were designed to favor concerted fragmentation. The effect of -substitution was studied by using nitrate esters of varyiag degrees of substitution. Nitrate esters with multiple nitrate ester moieties were also examiaed: 1,4-butanediol dinitrate; 1,5-pentanediol dinitrate; 2, 2- dimethyl- 1,3-propanediol dinitrate; 1,1,1-tris(hydroxymethyl)- ethane trinitrate; and PETN. These thermal decomposition studies illustrated three general principles 21 . a) The rate-determining step in nitrate ester thermolysis si usually homolytic cleavage of the RO-NO2 bond but tertiary nitrates can undergo 1E elimiaation. b) The presence of radical-stabilizing substituents on the l-carbon determines the rate and extent of ~3-scission and elimiaation of formaldehyde. c) In compounds containing more than one nitrate ester, the structural orientation has a marked effect on the reaction products. The thermolysis of compounds of this sort can result in ring closure or sequential elimination of NO2 and CH20 if the nitrate esters are in close proxinity (Fig. 2). Most nitrate esters are in the same physical state at ambient temperature as their parent alcohol. With few exceptions (e.g. highly symmetric PETN and the polymeric nitrocellulose), most are liquids. Many nitrate esters find use in propellants. Present-day studies on nitrocellulose primarily focus on its reactions during combustions 22,23. The Navy uses a variety of liquid nitrate esters as plasticizers and propellants trimethylolethane trinitrate (TMETN), triethylene glycol dinitrate (TEGDN), 1,2-propylene glycol dinitrate (PGDN), 1,2,4 butanetriol trinitrate (BTTN). Brill et al have examined the thermal decomposition of some of these nitrate esters using T- jump (heating at 2000~ FT-IR. They reported the relationship between the nitrate ester and its pyrolysis products was straightforward. The amounts of NO2 and CH20 produced were proportional to the number of -NO2 and-CH20- groups in the starting material 24. Less conventional uses of nitrate esters include the use of ethyl hexyl nitrate ester as a cetane modifier in diesel fuel 25 and isopropyl nitrate ester as the principle ingredient in a number of Russian enhanced-blasted explosive devices, such as the RPO-A 26. 01 2ONO/ \ON2O I ~ON\ 0 2 ON ~O (cid:12)9 2ON- N2O O/ ,ON2O /'~', -CH20 ON2O ~ .~ 2ONO /,v O 671~ -NO 2 O2N) 2ONO _No~ O /\. 0 / k.i-~o. -CH20 LNO2 LO N 20 -NO 2 // O , I 0 X 0 Fig. 2 Proposed Decomposition Pathway of PETN (Ref. 21)

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