Empirical methods have been developed to predict density and heat of formation of proposed high explosives. These methods are parameterized in terms of molecular constituents consisting of atoms in particular bonding environments. These methods, along with the atomic composition of the molecule, are then used in an empirically based method to predict the performance of the explosive.

Together, these methods form a unified mechanism of relating molecular structure to explosive performance. Indeed, this effort, along with the synthesis effort at Los Alamos, have led to the identification of a new class of high-nitrogen molecules that may prove to be high performance explosives.

The effects of physical properties and molecular conformation on the thermal decomposition kinetics of several cyclic nitramines are examined and compared to the decomposition of RDX. The compounds used in the study are: octahydro-1,3,5,7-tetranitro- 1,3,5,7-tetrazocine (HMX), hexahydro-l-nitroso-3,5-dinitro-s-triazine (ONDNTA), 1,3,5- trinitro- 1,3,5-triazacycloheptane (TNCHP), and 2-oxo-1,3,5-trinitro-1,3,5-triazacyclohexane (K6). The decomposition pathways of HMX in the liquid phase are similar to the four parallel decomposition pathways that control the decomposition of RDX in the liquid phase. The products formed during the thermal decomposition of ONDNTA arise from multiple reaction pathways. The identities and temporal behaviors of the ONDNTA decomposition products are discussed. TNCHP is thermally stable in the liquid phase. The decomposition products from TNCHP are formed via multiple reaction pathways. One decomposition pathway for TNCHP is through its mononitroso intermediate. TNCHP does not form a stable product that is analogous to oxy-s-triazine (OST) formed in RDX or the smaller ring fragments formed in the liquid-phase decomposition of HMX. K6 is less thermally stable and the decomposition mechanism is much simpler than that of RDX, HMX and TNCHP. The thermal decomposition of K6 occurs between 150 and 180 °C. The products formed during the decomposition of K6 are mainly CH2O and N2O with minor amounts or HCN, CO, NO, and NO2.

High explosives, blasting agents, propellants, and pyrotechnics are all metastable relative to reaction products and are termed energetic materials. They are thermodynamically unstable but the kinetics of decomposition at ambient conditions are sufficiently slow that they can be handled safely under controlled conditions. The ease with which an energetic material can be caused to undergo a violent reaction or detonation is called its sensitivity. Sensitivity tests for energetic materials are aimed at defining the response of the material to a specific situation, usually prompt shock initiation or a delayed reaction in an accident. The observed response is always due to a combination of the physical state and the molecular structure of the material. Modeling of any initiation process must consider both factors. The physical state of the material determines how and where the energy is deposited in the material. The molecular structure in the solid state determines the mechanism of decomposition of the material and the rate of energy release. Slower inherent reaction chemistry leads to longer reaction zones in detonation and inherently safer materials. Slower chemistry also requires hot spots involved in initiation to be hotter and to survive for longer periods of time. High thermal conductivity also leads to quenching of small hot spots and makes a material more difficult to initiate. Early endothermic decomposition chemistry also delays initiation by delaying heat release to support hot spot growth. The growth to violent reaction or detonation also depends on the nature of the early reaction products. If chemical intermediates are produced that drive further accelerating autocatalytic decomposition the initiation will grow rapidly to a violent reaction.

In this paper we focus on the relation between impact sensitivity and energy transfer rates. When a crystal receives an impact, low frequency lattice vibrations (called phonons) are excited. Typical phonon frequencies are 0-200 cm-1. This energy must then be converted to vibron frequencies (1000–2000 cm-1) before bond breaking can occur. We derive a simple formula for the energy transfer rate in terms of the density of vibrational states and the vibron-phonon coupling. We are able to estimate the phonon upconversion rate in widely varying energetic materials such as TATB, HMX, and Pb styphnate by examining existing inelastic neutron scattering data. We find that the estimated energy transfer rates in pure unreacted material are strongly correlated with impact sensitivity.

The radiation stabilities of the nitramine explosives 1,4-dinitroglycolurile (DINGU), 1,4-dimethyl-2,5-dinitroglycolurile (DMD) and hexanitrohexaazaisowurtzitane (HNIW) have been determined using XPS. Samples were exposed to x-rays for times up to eight hours while photoelectron spectra were recorded in the carbon, oxygen, and nitrogen Is energy regions and mass spectra were recorded of gases evolved during the decomposition process. These data are compared to the previously determined stabilities for cyclotrimethylene trinitramine (RDX) and cyclotetramethylene tetranitramine (HlMX). The N1s spectra of the irradiated materials show the NO2 peak decreases relative to the total nitrogen signal while low binding energy peaks grow. The rate of loss of the NO2 peak was fit to first order kinetics and the rate constants obtained show some correlation with the N-N bond strength as measured by the average N-N bond distance and the average NO2 asymetric stretch frequency. Despite the differences in structure of these molecules (DINGU and DMD are bicyclic rings, RDX and HMX are rings and HNIW is a cage) the radiation stability appears to be controlled by the strength of the N-N bond.

Thin films of RDX (1,3,5-trinitro-1,3,5-triazine) have been prepared by vapor deposition onto a 77 K substrate window and pyrolyzed with a pulsed CO2 laser. Each sample is rapidly quenched after the laser pulse by heat conduction into the cold substrate, and the initial reaction products are trapped on the window for examination by transmission FTIR spectroscopy. We have detected N2O4, the dimer of nitrogen dioxide, as an initial condensed phase pyrolysis product, confirming that scission of one of the N-N bonds is the first step in the reaction mechanism. No evidence was found for formation of methylene nitramine via a proposed concerted depolymerization pathway.

Free-radical thermal and photochemical decomposition products of ammonium dinitramide (ADN), an acyclic nitramine, are compared to that of cyclic nitramines (RDX, HMX, and HNIW) and to ammonium perchlorate (AP). Photochemical formation of NO2 from uvphotolysis of ADN at 77 K is found to follow first-order kinetics; whereas, zero-order NO2 formation is observed from the cyclic nitramines under the conditions of this experiment. Mechanisms are suggested for ADN decomposition. A general trend of cyclic nitramines to thermally decompose forming nitroxide radicals is supported by 15N-ring-labeled HNIW results. ADN thermally decomposes at 19° C to form free-radical reaction products in solution with tetrahydrothiophene-1,1-dioxide.

The initiation of chemical reaction in explosive crystals during shock or impact is shown to be a solid state, quantum mechanical, process not accessible by a classical continuum approach. The process by which dislocations move in a solid is analyzed in terms of quantum mechanical tunneling. The motion of these dislocations significantly perturbs the lattice and creates phonons which, for low dislocation velocities can cause local ignition sites and for high dislocation velocities, can have energies sufficient to directly pump the internal vibrational modes of the explosive molecules and cause rapid molecular dissociation.

Sterically hindered shear in a plane shock wave in an explosive crystal is shown to be important in determining the shock sensitivity of pentaerythritol tetranitrate and nitromethane. The steric hindrance is analyzed at the intermolecular level in the unit cell, and compared to the experimental evidence.

Energetic materials are of significant interest for scientific and practical reasons in the extraction (mining) industry, space propulsion, and ordnance. The nature of the fracture process of such materials under high acceleration is of particular interest, especially in ordnance. This paper describes new experimental and analysis techniques that allow us to characterize quantitatively and to compare the fracture surfaces of different energetic materials, and to deduce the specific fracture mechanisms. The techniques are widely applicable to other composite systems. In the materials discussed herein, topographical profiles spaced 1.0 mm apart across the fracture surfaces of two types of Octol have been obtained with a diamond stylus profilometer. Spatial power spectra (wavelengths of 1.0 μm 1.0 cm) have been calculated using a prolate spheroidal data window in the horizontal space domain prior to using a fast Fourier transform algorithm. The spatial power density of the fracture surface profiles is found in general to decrease with increasing spatial frequency over the region of interest, ≈ 1 mm-1 to ≈ 1 cm-1. Quasi-periodic peaks corresponding to HMX particle sizes are observed in the Octol spatial power spectra. These peaks indicate the inhomogeneous HMX grain size distribution in the Octol fracture surfaces. Peaks in the Octol spectra indicate that intergranular fracture often occurs between the TNT and HMX grains. Fractal analysis of the Octol power spectral slopes indicates the regions of deterministic, intergranular failure and the regions of the nondeterministic, trans-granular failure through TNT or HMX grains. This non-deterministic (fractal) failure is chaotic and may indicate the origin of failure in the sample.

The metal azides (or azido complexes) of La, Ce, Ti, Fe(III), Co, Ni, Pd, Ag, Tl, Cd, Sn, Sb and Bi have been synthesised and subjected in situ within a high resolution transmission electron microscope, to electron-beam fluxes (ca 0.1–1.0 A/cm2) to induce rapid decomposition. The materials, which are highly energetic and some are reportedly explosively sensitive, were carefully handled during synthesis to avoid mechanical shock, and only microgram quantities were decomposed whilst on holey carbon support films. In all cases beam-induced decomposition was very rapid. The exothermicity of the reaction induced in the beam promotes vigorous melting which either precedes or is accompanied by radiolysis. The net result is a rapid evolution of nitrogen (product) gas, which causes atomisation and spallation of the metal products. These molten metal particles are hard landed on a 300–500 μm 2 area of the support film with diameters in the nanometre size range. Data is presented on the synthesis of the azides, the nature of the beam-induced process as well as representative electron micrographs of some of the pristine starting materials and the nanoparticulate products.

Deformation of cyclotrimethylenetrinitramine (RDX) and ammonium perchlorate (AP) crystals at low strain rates was studied by diamond pyramid (Vickers and Knoop) microindentation hardness testing. RDX is two to three times harder than AP and has relatively limited slip system activity. While both crystals readily crack, cracking did not reduce hardness in RDX but did in AP. Strain fields surrounding the hardness impressions in RDX were extremely localized while in AP they extended well beyond the impressions. Shock experiments were conducted on large (5–9 mm), single crystals in a fluid-filled tank designed to permit high-speed photography and sample recovery. Reaction threshold was obtained by varying the shock pressure entering the crystals. Shock-entry orientation and large hardness impressions were used to alter microstructural responses. High-speed photography showed luminous crack propagation and reaction in both materials and the same slip deformation in AP as from hardness testing. Orientation affected the microstructural response and reaction threshold for AP, and hardness impressions sensitized chemical decomposition far from the impressions. Recovered AP crystals were much more plastically deformed than RDX crystals and were often still transparent in the region opposite shock entry. Recovered RDX crystals, at even the lowest shock pressure of 8.6 kbar, were uniformly white from a high density of fine cracks. RDX reaction threshold was ∼62 kbar versus 17 to 24 kbar for AP, depending on crystal orientation to the shock wave.

Some molecular modeling of slip using both rigid and flexible molecules of PETN in perfect, but finite, lattices has been performed. Studies of rigid displacement show slip on (001)<010> to be relatively unhindered. The shock orientation with a maximum resolved shear stress in this direction would, therefore, be expected to be insensitive. Molecular mechanics calculations have also been performed. The results show that it is likely that molecular deformations occur and have an important effect in determining the shear strength of crystalline PETN.

The impact sensitivity of energetic materials is frequently measured in small-scale drop-weight tests. Plastic deformation of the sample leads to heating, the onset of chemical reaction, and eventual ignition. An analytic model provides deformation dependent estimates of the energy delivered to the sample, its stress state, and its temperature distribution. Results for hydoxy-terminated polybutadiene (HTPB/AP) propellants are interpreted in terms of two initiation criteria: critical decomposition temperature and critical thickness for shear banding. The influence of friction, impact velocity, and sample size on initiation give possible explanations of experimental results. Sample material properties variations are studied for both initiation criteria. The model includes energy localization by varying the thickness of shear layers at the sample/machine interface.

The role of impurity atoms and vacancy defects in the heating of hot spots in a 2D, fcc, monatomic lattice, subjected to rapid, 1D compression, has been studied by means of molecular dynamics. The interaction potential of the impurity atoms, their mass, and size were allowed to differ from those of the atoms in the host lattice. Several configurations of impurity and vacancy clusters were examined. The mechanisms of heating observed here were similar to those found earlier in a bcc lattice with vacancy defects but without impurities.

We present a comparative study of two different chemically-sustained shock waves. One shows behavior expected from the Zel'dovich, von Neumann, and Doering (ZND) continuum theory of planar detonations. The other exhibits the complexity of a split shock wave resulting from the presence of a polymorphic phase transition. This comparative study demonstrates the importance of carefully considering the high-pressure characteristics of the model in developing potentials for simulating detonations. This comparative study also raises the fascinating possibility of a first-order phase transition accompanying a condensed-phase detonation.

An experimental methodology for recording femtosecond time-resolved observations of irreversible change in solids is described. The central problem posed is that the trime-dependent evolution must be observed on a single-shot (i.e. real-time) basis since the sample may be permanently altered after each excitation event. Preliminary demonstrations of real-time femtosecond spectroscopic observations are presented. In addition, one-shot data acquisition techniques open up the possibility of excitation intensities that greatly exceed optical damage threshholds of most samples. Since only one excitation pulse is used, cumulative damage mechanisms may be circumvented. Even if the sample is damaged in a single shot, in some cases the events of interest may be observed before damage occurs. The use of timed sequences of high-intensity excitation pulses to drive large-amplitude, coherent lattice vibrations is discussed. If successful, such large-amplitude lattice vibrations could assist crystalline chemical reactions or structural phase transitions.

More than 100 years ago Worthington [1,2] reported his observations on drops exploding on impacting rigid surfaces they did not wet. Mercury was his favorite fluid, since mercury does not wet most solids. For water drops he had to especially “smoke” his surfaces to make them non-wetting. In his day high speed photography had not been developed, but he had learned to “stop” his falling drops by spark illumination. His drops had radii of typically 1 mm, fell from a height of ∼10 cm and ejected always an even number of spikes, typically ∼24. These spikes would shoot out from the impacted area at velocities several times higher than the impact velocity.

An embedded molecular cluster model is used to study the electronic structure of cyclotrimethylene trinitramine(RDX) molecular crystal. In this model, a molecular cluster describing a local region of the crystal is treated in the Hartree-Fock approximation. The embedding lattice is represented by multipoles that are determined quantum-mechanically. Cluster-lattice orthogonality is achieved by transforming canonical orbitals to orbitals which are localized on individual molecular sites by means of a self-consistent localization potential. Results for the free molecule and the perfect crystal demonstrate the domination of three N-NO2 groups on intermolecular properties. For the defective crystal, results show a significant distortion in the electronic structure caused by local deformations in the lattice that may account for the formation of hot spots.

We examine shock induced detonation in a three-dimensional model of a nitromethane crystal. The crystal may contain a defect in the form of a small void. Three regimes are identified: the shock can be weak enough that no chemical bonds are broken; the shock can be so strong that a detonation front is established in the perfect crystal; or the shock can be of intermediate strength, where chemical activity requires the existence of the defect. In all regimes, the defect increases the reaction rate and causes a hot spot to appear.