The interactions of shocks with defects in two-dimensional square and hexagonal lattices of particles interacting through Lennard-Jones potentials are studied using molecular dynamics. In perfect lattices at zero temperature, shocks directed along one of the principal axes propagate through the crystal causing no permanent disruption. Vacancies, interstitials, and to a lesser degree, massive defects are all effective at converting directed shock motion into thermalized two-dimensional motion. Measures of lattice disruption quantitatively describe the effects of the different defects. The square lattice is unstable at nonzero temperatures, as shown by its tendency upon impact to reorganize into the lower-energy hexagonal state. This transition also occurs in the disordered region associated with the shock-defect interaction. The hexagonal lattice can be made arbitrarily stable even for shock-vacancy interactions through appropriate choice of potential parameters. In reactive crystals, these defect sites may be responsible for the onset of detonation. All calculations are performed using a program optimized for the massively parallel Connection Machine.

An important goal in the detonation physics and chemistry community is to design explosives with high-performance capabilities, yet sufficiently low impact and shock sensitivities. The traditional ways of developing increasingly energetic explosives have led to an increase in sensitivity. It is, therefore, of practical and theoretical importance to design, from molecular level considerations, polycrystalline explosive systems which clearly exhibit directional sensitivity and initiation properties, and which approach the insensitivity of homogeneous or single-crystal explosives. Theoretical studies indicate that such systems may be constructed from special materials such as high-quality pyrolytic, layered, hexagonal boron nitride (BN) crystals. Intercalation of the crystals with various oxidizing agents can yield an explosive system with the desired properties. Under ordinary laboratory conditions and with modest pressures, the intercalate molecules readily enter the host lattice by exploiting the weak bonding between the layers, resulting in denser, metastable crystals. High energy release is achieved when products containing B2O3 are formed.

This paper discusses experimental techniques and modelling tools used to characterize energetic solids subjected to dynamic deformation and shock. Critical experiments have been designed to study shock response and impact sensitivity of energetic materials. For example, a simplified two dimensional experiment has been developed to study the critical phenomena involved in delayed detonation reactions (XDT). In addition, wedge tests are used to obtain equation-of-state data. Coupled with hydrocodes, these experiments give us an in-depth understanding of the response of energetic materials subjected to shock loading. A coupled methodology using both experimental and modelling tools is presented. Consisting of three parts, it addresses all possible responses to fragment impact. The three parts are: (1) Fragment impact modelling (hydrocodes and empirically based codes); (2) Experiments to obtain accurate data for predicting prompt detonation; and (3) Tests with planar rocket motor models to explore mechanisms related to bum reaction thresholds and degree of violence. This methodology is currently being used in weapon design and munitions hazard assessments.

We present a simulation studying the initiation of a model energetic molecular solid. For low-velocity, massive flyer-plate impact, the model detonation initiates behind a compaction shock. This detonation front subsequently overtakes the compaction shock exhibiting a behavior qualitatively similar to that observed in experminental studies of homogeneous initiation and delayed detonation from low velocity impact.

Electronic excitations which precede and can facilitate chemical reactivity of explosive molecules during shock passage will be discussed. The structurally similar family of explosives, TATB, DATB, Picramide and TNB were the focus of this work. It has been found that the energy of shake-up transitions observed in x-ray photoelectron spectra of these molecules shows a linear relation with the explosive sensitivity. To the contrary, optical absorption maxima, dependant upon dipole selection rules display a trend opposite that intuitively expected in relation to ease of initiation. The shake-up transitions are governed by monopole selection rules, so that the transitions between initial and final states of the same symmetry are allowed. The forbiddeness of the excited state may favor proton transfer, bond breakage and free radical formation relative to de-excitation by returning to the ground state. Electronic excitations of this type are believed to be important to multiphonon up pumping. The Is core hole resulting from photoionization creates a central coulomb force that shrinks the orbitals, simulating conditions similar to those of high pressure or shock passage. These factors may be at the heart of the correlation between the shake-up promotion energy and explosive sensitivity.

Pressure/temperature/reaction phase diagrams for several nitramine compounds, including hexanitrohexaazaisowurtzitane (HNIW), 1,3,5-trinitrohexahydro-l,3,5-triazine (RDX), ammonium dinitramide (ADN), and p-nitroaniline (PNA) are presented. A diamond anvil cell was used in conjunction with optical polarizing light microscopy (OPLM), Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray diffraction (EDXD), and micro FT-Raman spectroscopy to determine these diagrams. A description is given of the diamond anvil cell and the associated techniques employed.

A study has been made of the crystal growth behaviour and perfection of TNT. Depending on the purity and the solvent used, crystallisation from solution yields material of monoclinic, twinned monoclinic or orthorhombic forms. Of these only the monoclinic form was suitable for detailed study by X-ray topographic techniques. The crystals of the orthorhombic form were too small and imperfect for examination.

Seeded growth from ethyl acetate solution yielded lamellar twinned crystals in which the twin density was low. Under ideal conditions, relatively perfect samples could be obtained in which the dominant residual growth defects were stacking faults of Burgers vector b ½ [001] and dislocations.

We have studied the (210) surface of ammonium perchlorate (NH4ClO4) with an atomic force microscope (AFM) in air. Large scale images show monomoreculat steps that are often aligned with major crystalline directions. Surface modification is clearly observed with forces as low as 30 nN, but is usually limited to the bottom and the sides of the field of view, where the tip scans for a long time and where it turns around, respectively. At the molecular scale we have observed structures with substantial order, with minimum lateral sizes of 3 Å. The identification of these features is difficult due to the large unit cell of this compound (40 atoms per unit cell), and the lack of perfect order in the images. We use the SARCH/LATUSE progrmn developed by Van Hove and Hermann to analyze this structure. Ammonium perchlorate is hygroscopic; it is therefore possible that surface modification is enhanced by the moisture present in air. Experiments in a dry atmosphere have confirmed this explanation. We have therefore designed and built a UHV (ultra-high vacuum) AFM for these studies. The first images from this instrument are presented here.

The occurrence of [100] direction slip on the (021) slip plane of orthorhombic RDX (cyclotrimethylenetrinitramine, (CH2·N·NO2]3) is shown to favor the formation of 1,3-dinitroso-5-nitro-l,3,5-triazacyclohexane, as detected in drop-weight impact tests at sensitivity height levels near to those measured for initiation. Also, the reported observation of deformation twinning in the chemically-related monoclinic HMX (cyclotetra-methylenetetranitramine, (CH2·N·NO2 ]4) crystal lattice is explained on the basis of the greater flexibility of the larger HMX molecule allowing a number of bond rotations that are required to produce a relatively unusual Type II deformation twinning structure. These examples give support to the consideration that on the molecular level involved in deformation-induced decompositions there may be a direct mechanical force aspect that is additional to the established importance of “hot spot” heating, say, as described for dislocation pile-up avalanches in RDX.

Even before the ‘Buckey-ball’ (or fullerene) era, organic chemists were interested in, and attracted to, the synthesis of compounds containing many rings fused together - especially the symmetric ones. By virtue of their balanced linkages, such compounds may be kinetically stable despite inclusion of massive deviations from normal bond distances and angles. These distortions, in compounds such as cubanes, tetrahedranes, dodecahedranes and prismanes, comprise a storehouse of chemical potential energy known to the chemist as ‘strain energy’ (unrelated to overall crystal strain discussed in many materials studies).

Besides the obvious benefit of including more potential energy in an energetic material, the arrangement of atoms in ring assemblies (rather than chains) leads to a material with a high intrinsic crystal density - a sine qua non for an explosive or propellant. It has long been recognized that the next generation of energetics - better in performance and/or safety - will contain many polycyclic compounds. Hundreds of model compounds were synthesized in the last decade, and their molecular structures identified or verified at the Naval Research Lab. Some of the most promising, beautiful, and unusual ones, including many energetic cubanes, will be discussed.

NTO is an explosive of current interest. It has been evaluated as an insensitive component to replace RDX in the bomb fill, and as a major ingredient for the auto air bag system. The crystal structure of the β from of NTO has been determined by single-crystal X-ray methods. The unit cell is monoclinic, space group P21/c, with a = 9.326, b = 5.515, c = 9.107 Å, β = 100.77°. There are four molecules in the cell, density 1.878 g/cm3. Infinite extension of H-bonding in two-dimensional sheets occurs in the monoclinic form. Bond lengths and angles all have normal values.

Efforts have been made to determine the crystal structure of α-NTO. However, a structure refined only to R = 17% was obtained, probably due to some kind of twinning about the crystal needle axis. The unit cell is triclinic, space group P1, with a = 5.12, b = 10.30, c = 17.9 Å, α = 106.7°, β= 97.7°, γ = 90.2°. There are eight molecules in the cell, density 1.92 g/cm3. Ribbons of NTO molecules formed by a relatively strong network of hydrogen bonds are observed. It was found that α-NTO is the stable, dominating form. A variety of techniques have been chosen to identify the two polymorphs.

Large single crystals of PETN, RDX, and TNT can be grown easily from evaporating ethyl acetate solutions. The crystals all share a similar type of defect that may not be commonly recognized. The defect generates conical faces, ideally mosaic crystals, and may account for the “polymorphs” of TNT and detonator grades of PETN.

TATB crystals manufactured by the amination of trichlorotrinitrobenzene in dry toluene entrain two forms of ammonium chloride. One of these forms causes “worm holes” in the TATB crystals that may be the reason for its unusually low failure diameter.

Strained HMX crystals form mechanical twins that can spontaneously revert back to theuntwinned form when the straining force is removed. Large strains or temperatures above 100°C lock in the mechanical twins.

The reaction chemistry of energetic materials is often considered only with respect to the types and quantities of gaseous products formed. For a thorough understanding of the initi reaction steps, that largely determine the sensitivity of the material, both gaseous and solid products must be determined. In addition, it is the solid state partial decomposition intermediates remaining in a damaged material that can lead to increased sensitivity. The preference for the initial reactions to take place at the surface of particles and the low concentrations of intermediates formed demands the use of highly sensitive surface specific chemical probe techniques. State of the art surface chemical techniques will be discussed, focusing on x-ray photoelectron spectroscopy and surface mass spectrometry. Principles of operation and performance will be highlighted, comparisons will be made to bulk chemical analysis, and examples of applications will be presented.

The chemistry at the surface of a burning propellant plays a major role in the burnrate, the stability of the flame, and the potential for a run-away hazardous event. Chemical details can be determined experimentally by simulating the burning surface with a thin film of material heated rapidly to the surface temperature. T-Jump/FTIR spectroscopy is designed to create these conditions. The dominant, initial, heat-producing reactions in the surface zone of NH4ClO4, NH4NO3, NH4[N(NO2)2], HMX, RDX, and DNNC are elucidated at high temperatures representative of the burning surface.

We present a theoretical approach aimed at elucidating the microscopic dynamics of energetic materials fragmentation. Two interdependent components are combined to bridge the atomic and bulk realms. A hierarchy of quantal multiple-scattering expansions is first introduced to decompoce the complex reaction dynamics into coupled sequences of fewatom interactions, each of which is more amenable to calculation. This microscopic component is complemented by a continuum-mechanical process to account for energy flow into the as yet unreacted bulk material. Such a dual approach allows for quantum-mechanical treatment of coupled atomic-bulk dynamics in a self-consistent way that incorporates large thermal gradients. The analysis, in terms of coupled few-atom interactions, also yields additional insight into the various pathways for reactivity and energy transfer.

The NMR chemical shift anisotropy (CSA) has a temperature dependence as a result of thermal motions of the NMR site. For slow motions of a polymer chain at or near the glass transition, Tg, the CSA begins to decrease, with the characteristic powder pattern features, shoulders and divergence, approaching one another. This motional narrowing can be interpreted to yield the correlation time of the thermal motions. If Δ is the rigid-lattice shoulder to shoulder chemical shift anisotropy and τ is the correlation time of the slowmotion of the polymer chains, the total fractional shift, δ/Δ, for the general asymmetric case is found to be

derived using Lee's theory. In this work, Nitrocellulose isotopically highly enriched with 15N was studied at four temperatures between 27° and 120° Celsius. To circumvent signal-to-noise problems in obtaining CSA data from a polycrystalline/amorphous sample, we used the Magic Angle Spinning (MAS) technique of Herzfeld and Berger. The principal values of the chemical shift tensor were then obtained from the relative intensities of the spinning sidebands in the MAS NMR spectrum. Correlation times were found to be of the order of 400 ms at 70°C, and 17 ms at 120°C.

Interest in the mechanisms by which hot spots either grow to sustained reaction or are quenched results from the observation that the energy required to ignite a propellant or explosive can be significantly less than that needed to bulk heat a test specimen uniformly to its ignition temperature. This result is independent of the original form of non-thermal energy and has been used to interpret data for shock, impact, friction and electrostatic discharge (ESD) stimuli. We present new flowcharts which include 1) events resulting in hot spot formation and 2) subsequent pathways which lead to sustained reaction or quenching. The mechanism appears capable of categorizing and demonstrating the similarities and differences between hot spot growth or quenching, for impact and ESD stimuli. Sample confinement and temperature and stimulus duration are the independent variables whose roles are particularly clarified in the mechanism.

A small-scale laboratory experiment in which the detonation products are confined was designed such as to yield for small charges (as little as 1/2 gram) gas expansion rates comparable to those due to the underwater detonation of large-scale charges. The resulting slow expansion allows the aluminum and the other non-ideal components typically used in underwater explosives to react to completion within the time frame of the experiment. Both ideal and non-ideal aluminized explosives were tested. The traditional Jones-Wilkins-Lee (JWL) equation of state reproduced the measurements in the case of the ideal explosives. An extended JWL equation of state in which the time-dependent late energy release is introduced was adjusted until it reproduced the measurements of the non-ideal explosive tested. This derived time-dependent equation of state also reproduced the data of large-scale cylinder tests and underwater detonations using the same non-ideal explosive.

Clouds of dispersed explosive or combustible solid particles are detonable and such systems may exhibit self-sustained detonation [1–8]. However, the method by which individual particles in the explosive cloud interact to sustain detonation is not well understood. The similar case of liquid fuel/air explosives was investigated in detail during the 1960's and 1970's. For these systems, it was established that incident shock waves serve to shatter large liquid droplets into a mist of micro-droplets. These microdroplets are almost instantaneously accelerated to the convective flow velocity behind the shock wave. The energy released upon ignition of the micromist supports the shock wave and selfsustained detonation results [9–11].

Time dependence of the energy release of metallized explosives has been observed and must be accounted for in predicting their performance. However, there is no complete theoretical description of this phenomenon. A preliminary investigation of the combustion of aluminum indicates that it probably takes place behind the reaction zone. A flexible time dependent model for the reaction of Al in explosive mixtures has been implemented in EPIC2, a Lagrangian material dynamics computer program. Exploratory calculations for nitromethane/PMMA/Al mixtures indicate that close in pressure measurements could reveal the presence of Al reaction behind the detonation front.