Energetic Materials (eBook)

Part 2. Detonation, Combustion

JS Murray, Peter Politzer (Herausgeber)

eBook Download: EPUB

2003 | 1. Auflage
474 Seiten
Elsevier Science (Verlag)
978-0-08-053091-8 (ISBN)

Infos

This volume provides an overview of current research and recent advances in the area of energetic materials, focusing on explosives and propellants. The contents and format reflect the fact that theory, experiment and computation are closely linked in this field.

The challenge of developing energetic materials that are less sensitive to accidental stimuli continues to be of critical importance. This volume opens with discussions of some determinants of sensitivity and its correlations with various molecular and crystal properties. The next several chapters deal in considerable detail with different aspects and mechanisms of the initiation of detonation, and its quantitative description. The second half of this volume focuses upon combustion. Extensive studies model ignition and combustion, with applications to different propellants. The final chapter is an exhaustive computational treatment of the mechanism and kinetics of combustion initiation reactions of ammonium perchlorate.

Overall, this volume illustrates the progress that has been made in the field of energetic materials and some of the areas of current activity. It also indicates the challenges involved in characterizing and understanding the properties and behaviour of these compounds. The work is a unique state-of-the-art treatment of the subject, written by pre-eminent researchers in the field.

- Overall emphasis is on theory and computation, presented in the context of relevant experimental work
- Presents a unique state-of-the-art treatment of the subject
- Contributors are preeminent researchers in the field

Inhalt

Cover 1
Copyright Page 5
Preface 6
Table of Contents, Part 2 14
Part 2: Overview of Research in Energetic Materials 22
Chapter 1. Sensitivity Correlations 26
1. Introduction 26
2. Background 28
3. Sensitivity Correlations 29
4. TATB: A Case Study 31
5. Electrostatic Potential 33
6. Summary 38
Chapter 2. A Study of Chemical Micro-Mechanisms of Initiation of Organic Polynitro Compounds 46
1. Introduction 46
2. Data Sources 48
3. Basic Mechanisms of Thermal Decomposition of Organic Polynitro and Polynitroso Compounds 56
4. Initiation of Polynitro Compounds 57
5. Conclusions 67
Chapter 3. Dynamics of Energy Disposal in Unimolecular Reactions 74
1. Chemical Issues in the Initiation of Detonations 75
2. The Key Role of Unimolecular Reactions 75
3. Quantum Chemistry Provides Potential Energy Surface 77
4. Computing the Reaction Path 78
5. The Reaction Hamiltonian 82
6. Methylene Nitramine Decomposition 85
7. Concluding Remarks 89
Chapter 4. Initiation and Decomposition Mechanisms of Energetic Materials 92
1. Introduction 92
2. Initiation Models 93
3. Nonradiative Energy Transfer in Nitromethane 94
4. Effects of Pressure and Vacancies 96
5. Decomposition of HMX 108
Chapter 5. Initiation due to Plastic Deformation from Shock or Impact 122
1. Introduction 122
2. AFM and STM Observations of the Microscopic Processes of Plastic Deformation 124
3. Theoretical Developments 129
4. Calculations 134
5. Conclusions 141
Chapter 6. Fast Molecular Processes in Energetic Materials 146
1. Introduction 146
2. The Phenomenology of Energetic Materials 147
3. Molecular Level Structure of Energetic Materials 164
4. Up-pumping, Sensitivity and Ignition 174
5. Hot Spot Formation in Porous Materials 189
6. Molecular Response in Detonation 193
7. Fast Processes in Nanometric Energetic Materials 196
8. Concluding Remarks 200
Chapter 7. The Equation of State and Chemistry of Detonation Products 214
1. Introduction 214
2. Computational Method 219
3. Fluid Equations of State 221
4. Condensed Equations of State 228
5. Application to Detonation 230
6. Experimental 231
7. Results and Discussion 234
8. Conclusions 242
Chapter 8. Combustion Mechanisms and Simplified-Kinetics Modeling of Homogeneous Energetic Solids 246
1. Introduction 247
2. Mathematical Model of Macroscopically Steady Combustion 248
3. Results for Macroscopically Steady Combustion 270
4. Quasi-Steady Theory of Unsteady Condition 294
5. Results for Quasi-Steady, Oscillatory Combustion 299
6. Intrinsic Stability 309
7. Concluding Remarks 311
Chapter 9. Modeling of Nitramine Propellant Combustion and Ignition 316
1. Introduction 318
2. Theoretical Formulation 323
3. Numerical Method 335
4. Discussion of Model Results 336
5. Concluding Remarks 367
Chapter 10. Use of Kinetic Models for Solid State Reactions in Combustion Simulations 372
1. Introduction 372
2. Model 377
3. Results and Discussion 381
4. Conclusion 390
Chapter 11. Towards Reliable Prediction of Kinetics and Mechanisms for Elementary Processes: Key Combustion Initiation Reactions of Ammonium Perchlorate 394
1. Introduction 395
2. Computational Methods 396
3. Results and Discussion 400
4. Concluding Remarks 457
Index for Parts 1 and 2 466

Textauszug

Chapter 1

Sensitivity Correlations

Peter Politzer; Jane S. Murray    Department of Chemistry, University of New Orleans, New Orleans, LA 70148, USA

1 INTRODUCTION

A continuing major objective in the area of energetic materials is to achieve diminished sensitivity, i.e. to reduce vulnerability to detonation initiated by unintentional external stimuli; these can include, for example, impact, shock, heat, friction and electrostatic charge [1,2]. The importance attached to this issue is seen in the establishment, by the North Atlantic Treaty Organization, of the Munitions Safety Information and Analysis Center (MSIAC, formerly NIMIC) [http://hq.nato.int/related/nimic]. This Center promotes efforts to decrease sensitivity and disseminates relevant information.

In general, vulnerabilities to the various types of stimuli follow roughly similar trends, although there are certainly deviations. Thus, Storm et al obtained satisfactory correlations, for diverse groups of compounds, between measured shock and impact sensitivities and between shock sensitivity and the critical temperature at which thermal decomposition becomes self-sustaining [3]. Zeman et al found somewhat more equivocal relationships involving impact and electrostatic spark sensitivity [4].

Energetic compounds are metastable. The introduction of external energy can lead to rapid decomposition, the first phase of which is endothermic but which subsequently becomes highly exothermic, releasing a great deal of energy and gaseous products at high temperatures, and giving rise to a large pressure gradient (shock front) that propagates through the compound at supersonic velocity, producing continuing self-sustaining exothermal decomposition (detonation) [1,2,5,6].

How readily this sequence of events will occur for any given compound (i.e. its sensitivity) depends of course upon its molecular structure and composition, as well as its crystal properties and physical form. Achieving greater insight into molecular factors offers a route to designing new less-sensitive energetic materials; understanding crystal and designing new less-sensitive energetic materials; understanding crystal and physical effects may permit improvement of existing ones, e.g. by modification of crystallization techniques and conditions. For instance, the β polymorph of HMX (1) is more stable toward impact than is the δ [7], and both are made somewhat less sensitive by grinding the crystals. Appropriate alterations of crystal shapes have diminished the shock sensitivities of several explosives, including RDX (2), HMX and CL-20 (3) [8,9].

Over a period of years, considerable effort has gone into developing relationships that correlate (and hence can predict) sensitivity in terms of some molecular or crystal property, usually the former. The first problem is obtaining reliable and reproducible experimental data. These correlations most often focus upon impact sensitivity, which is normally determined by measuring the height from which a given mass, dropped on the material, has a 50% likelihood of causing detonation [1–3,10,11]. The greater is this height, designated h50, the lower is the sensitivity. The results of this “drop-weight” test are very dependent upon the exact procedure that is followed and the condition of the sample, and are frequently difficult to reproduce.

A more serious problem, however, is that the initiation of detonation, as already mentioned and to be further discussed, depends upon a complex interplay of various molecular, crystal and physical factors. It can therefore be viewed as remarkable that relatively good correlations have been established between impact sensitivity and a number of different properties, although they are generally limited to a particular class of compounds, e.g. nitroaromatics. The existence of these relationships certainly does not mean that all of these properties (or perhaps any of them) play important roles in detonation initiation, as was pointed out by Brill and James [12,13]. Many of them may be symptoms of some more fundamental factor; others may happen to correlate with a more relevant property.

In this chapter, we shall present an overview of some of these sensitivity correlations. We shall try, as much as possible, to relate them to a conceptual framework.

2 BACKGROUND

A key concept, with respect to detonation initiation, is that of hot spots, proposed by Bowden and Yoffe [14,15]. These are small regions in the crystal lattice in which is localized some portion of the energy introduced by, for example, impact or shock. If this energy is sufficiently channeled into appropriate molecular vibrational modes, it may result in the endothermic bond-breaking or other step that leads to exothermic decomposition and detonation. For this to happen, hot spots must be large enough and hot enough that they are not prematurely dissipated by thermal diffusion. Bowden and Yoffe estimated their dimensions to be of the order of 1(10− 6 m, temperatures > 700 K and lifetimes of 10− 5 to 10− 3 s [14,15]. Later work, cited by Tarver et al [16], indicated that this description roughly fits shock-induced hot spots but that those due to impact are larger and longer-lived, by factors of about 103. More recently, however, atomic force microscopy studies of RDX have shown that the latter can be as small as ~ 10− 8 m [17]. Using a combination of techniques (scanning electron microscopy, x-ray photoelectron spectroscopy and chemical-ionization mass spectrometry), it has been possible to identify, associated with hot spots, some of the decomposition products of TATB (4) and TNT (5) [18]. Tarver et al have modeled hot spots and the ignition of exothermic decomposition in HMX and TATB [16]. They found that as the hot spot becomes larger, the temperature required for ignition decreases but the time increases.

The formation of hot spots is generally attributed to the presence of lattice defects [11,17,19–23], which could include vacancies, voids, dislocations, misalignments, cracks, impurities, etc. One explanation is that defects induce strain in the lattice, which is relieved, via structural relaxation, by the externally-introduced energy; this results in a disproportionate localization of energy in the neighborhood of the defect, a portion of it being in lattice vibrations [21,22]. The thermal energy of hot spots must be efficiently transferred to appropriate molecular vibrational modes, a process called “up-pumping” [24–27], if bond-breaking and subsequent exothermic decomposition and detonation are to be achieved. The term “trigger linkage” has been applied to the key bond or bonds that are initially ruptured [28]. Any dissipation of hot spot energy, for instance by diffusion, will lessen the likelihood of these processes. Thus Kamlet suggested that free rotation around the trigger linkage can have a desensitizing effect, since it uses energy that could otherwise go into bond-breaking vibrational modes [28,29].

The fact that solids composed entirely of very small particles are less sensitive [7,30] can now be explained on the grounds that these will have smaller hot spots and thus, as shown by Tarver et al [16], require higher temperatures for ignition. Once this has been achieved, however, these higher temperatures will result in more rapid reaction, as has been observed [31].

It should be noted that initiation of detonation can occur even in a homogeneous (defect-free) solid [6,24,32–36] (although this is of less practical significance since energetic compounds typically do contain lattice defects of various sorts). This can occur, for example, if there is efficient anharmonic coupling to channel energy from lattice into the critical molecular vibrations [24,25,36].

3 SENSITIVITY CORRELATIONS

The concepts that were outlined in the last section have suggested several approaches to developing sensitivity correlations. A popular one has been to focus upon the trigger linkage. Kamlet viewed C–NO2 and N–NO2 bonds as playing this role in H/C/N/O explosives, and scission of these bonds, which tend to be the weakest in the molecule [13,37–39], has indeed been shown experimentally to be the first step in the thermal decompositions of many of them [12,13,37,40–42]. Accordingly there have been a number of attempts to correlate sensitivity with properties of C–NO2 and N–NO2 bonds, generally some measure of their stabilities [39,43–46]. On the whole, these have been quite successful, although they are usually limited to a given class of compounds, e.g. nitramines, nitroaromatics, etc. However Owens was able to correlate with impact sensitivity the computed C–NO2, N–NO2 and O–NO2 bond energies of 11 molecules of different types: nitroaromatics, nitramines and nitrates [39].

An. interesting variation of this emphasis upon the trigger linkage was offered by Kohno et al...

Produkt-Details

• Erscheint lt. Verlag: 21.11.2003
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Chemie ► Physikalische Chemie
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Thermodynamik
• Themenwelt: Technik
• ISBN-10: 0-08-053091-5 / 0080530915
• ISBN-13: 978-0-08-053091-8 / 9780080530918