An evaluation was made of whether T-jump/FTIR spectroscopy could be used to determine the decomposition kinetics (Ea and ln A) and thermochemical (ΔHd) constants of an energetic material at high temperature and high heating rate. Polystyrene peroxide was chosen because of its known, simple, decomposition process. The kinetic constants are reasonable for O-O bond homolysis as the rate determining step: Ea = 39 kcal/mol, ln (A, s−1 ) = 21.5. Significant uncertainty exists, however, in the estimation of ΔHd.

A number of efforts are currently underway in the U.S. to model the self-sustained combustion of solid energetic materials at the level of fundamental physical processes and elementary chemical reactions. Since many of these materials bum, at least at some pressures, with a liquid surface layer, these models must address phenomena in three physical phases. Most of these modeling efforts have focussed on pure RDX as a prototype. From a systems viewpoint this was a natural choice since it, or materials like it, is one of the components of modem propellants. It is argued here that, from a scientific viewpoint, RDX is already too complex with too many uncertain mechanisms and unavailable supporting data to serve this role effectively. We discuss here a number of issues related to the condensed phase and interfacial phenomena which have not been previously identified and which warrant more detailed research. In this paper frozen ozone is adopted as a more suitable prototypical 3-phase system. In this paper progress toward addressing this system is presented. The first new process to be modelled was a gas/surface reaction A detailed analysis was performed on the reaction mechanisms at play in the ozone flame and how they are affected by the heterogeneous reaction. Other new mechanisms are associated with the multicomponent nature of the liquid surface layer and will be addressed in future work.

The reactions of BF + OH and BO + HF are critical reactions in the reaction mechanism of fluorine-enriched boron combustion.1 In this study, ab initio multiconfigurational methods have been used to calculate energies, optimized geometries, harmonic vibrational frequencies and zero-point energies for reactants, products and intermediates of these reactions. Results for the following reactive pathways will be discussed:

Boron ignition delay times for 24 μm diameter particles have been measured behind the reflected shock at a shock tube endwall in reduced oxygen atmospheres and in a combustion bomb at higher pressures in the products of a hydrogen/oxygen/nitrogen reaction. The shock tube study independently varies temperature (1400 – 3200 K), pressure (8.5, 34 atm), and ignition-enhancer additives (water vapor, fluorine compounds). A combustion chamber is used at a peak pressure of 157 atm and temperature in excess of 2800 K to study ignition delays at higher pressures than are possible in the shock tube.

The enthalpy of oxygen dissolution in liquid zirconium occurring during the combustion of Zr particles is estimated. The analysis presented uses direct experimental measurements of the temperature, size, and composition histories of burning Zr particles. The dissolution enthalpy is limited by the range of 700–830 kJ/mol and is somewhat less than that of Zr oxidation. This enthalpy determines the rate of the heat release during Zr combustion until stoichiometric ZrO2 forms out of the supersaturated Zr/O solution. Stoichiometric ZrO2 is formed near the end of combustion and the additional energy released at that time causes a rapid temperature increase which can trigger a particle explosion.

A large population of combustion-generated soot aggregates (more than 3,000 samples) was thermophoretically extracted from a variety of laminar and turbulent flames and analyzed using transmission electron microscopy (TEM). It was shown that the scaling structural properties of these fractal aggregates cannot be exclusively characterized by a single mass fractal dimension. Asymmetric properties of the aggregates were considered here by first assuming and then demonstrating their self-affinity via. an affinity exponent reflecting scaling with respect to the length and width of the aggregate projections. In addition to the conventional fractal dimension, Df, determined by using the geometrical mean of the longitudinal and transverse sizes as the characteristic length, the affinity exponent, H, and two complementary fractal dimensions, one longitudinal, DL =[(1+H)/2 ]Df, and one transverse, DW =[(1+H)/2H]Df, were introduced. By fitting TEM data for the entire population of aggregates, the values Df = 1.75 and H = 0.91 were obtained. This more complete description of aggregate morphologies in terms of the self-affine scaling is expected to lead to a better understanding of the transport properties and restructuring kinetics of flame-generated aggregates.

Planar Laser Induced Fluorescence (PLIF), UV/Vis Absorption, and thermocouple measurements were done for HNF, RDX, HMX, and XM39 deflagration with and without CO2 laser-support. RDX and especially HNF have very short self-deflagration flame length scales. HMX and XM39 have taller self-deflagration flames. XM39 has a marked dark zone with plateau temperature about 1400K. RDX's dark zone, present under laser supported deflagration, collapses when the external laser flux is removed. PLIF was used to measure the 2D NH, OH, and CN species profiles for these materials and OH temperature profile for RDX and HNF under non-laser supported conditions. The best spatial resolution for the RDX PLIF was about 4μm. Sandwiches of HNF and various binders were studied with PLIF and while obvious diffusion flames were present at low pressure, they are weak and are not expected to be burn rate controlling.

We have investigated the combustion chemistry of nitric oxide doped into premixed 10 Torr flames of CH4/O2/N2, C2H6/O2/N2, C2H2/O2/N2, and C2H2O2N2 which had similar peak temperatures. Profiles of OH, CH, 3C2, CN, NCO, NH, and NO were recorded by laser-induced fluorescence and compared among the different fuels, indicating differences in hydrocarbon flame structure and NO reactivity in these environments. Concentrations of nitrogen intermediates are comparable for the methane, ethane, and ethylene flames, but are approximately a factor of three higher with acetylene, indicating much greater NO reactivity in this flame.

Flash pyrolysis of polyethyleneglycol by T-Jump/FTIR spectroscopy to temperatures of the surface during combustion reveals that volatile products arise from approximately equal amounts of C-O and C-C homolysis. Nine volatile products are discussed. The average number of repeating units in the volatile oligomers is 2.5. A shift in product distribution occurs at 420–480°C resulting from a change in the polymer structure. Below 420°C, di- and mono-ether oligomers and diethyleneglycol dominate. Above 480°C, the mono-ethers and ethyleneglycol dominate. The Arrhenius constants for decomposition reflect this difference: Ea=8.8 kcal mol−1, In (A, s−1) =2.0 at 370–420°C and Ea=19 kcal mol−1, In (A, s−1)=10 at 480–550°C.

The principal binder of many LOVA propellants is cellulose acetate butyrate (CAB). By the use of T-Jump/FTIR spectroscopy, CAB was flash-pyrolyzed to set temperatures in the 465–600°C range, while rapid-scan IR spectra were used to identify the main decomposition products and to measure the rate of formation of each product as a function of temperature. Eleven specific products, which include oligomers of CAB, acids, aldehydes, ketenes, esters, CO2 and CO, were quantified by chemometric procedures. The ketenes are the most novel products. The Arrhenius parameters reveal that below 510 ± 20°C, the rate of product evolution is controlled mainly by condensed phase reactions. Above 510 ± 20°C, the rate of product evolution is controlled by desorption/evaporation of the volatile products.

A new class of epoxy resins having N-N bonds in the backbone has been synthesized with a view to explore their properties as energetic binders. The N-epoxidation of bis-dicarbonylhydrazones of adipic, azelaic and sebacic dihydrazides results in the formation of viscous resins having epoxide end groups. The resins have been characterized by the elemental and end group analyses, IR and NMR spectra. Relevant properties for their use as binders in solid propellants, such as thermal stability, heat of combustion, burn rate and performance parameters of AP-based propellant systems, have been evaluated. A significant increase in the burn rate of AP-based propellants noticed, is perhaps related to the exothermicity of the binder decomposition and the reactivity of N-N bonds with perchloric acid formed during the combustion of AP.

The methods of probing mass spectrometry (PMS) for diagnostic of flames and for the study of kinetics and mechanism of the thermal decomposition products of energetic materials (EM) are described. Several types of instruments based on microprobe and molecular beam mass spectrometric sampling have been developed. Time of flight mass spectrometer has been used. Apparatuses for high (10 atm) and low (<1 atm) pressure have been developed for the study of combustion and decomposition of EM by PMS “in situ”. Several examples are presented to demonstrate application of PMS method for the study of EM flame structure, thermal decomposition and dynamic of ignition. Experimental data on decomposition of double base propellants ammonium dinitramide, ammonium perchlorate are presented.

An atomic force microscope (AFM) has been used to reveal residual sub-micron sized decomposition sites in drop weight impacted RDX and laser irradiated AP crystals. In impacted RDX, the small and early reaction sites observed are hemispherical craters, ranging in size from 20–300 nm. The smallest reaction site encompassed about 10,000 molecules with an expected energy evolution of 2 × 10−14 J. On a somewhat larger scale hillocks of 200–800 nm were observed, their shape giving evidence of internal reaction and hot spot melting. Dislocation densities as high as 5 × 1012 per cm2 were observed in sub-ignited RDX. High resolution AFM images of the RDX lattice structure indicate molecular rotation as well as displacements at dislocation sites. In AP, after nanosecond pulsed laser irradiation, reaction sites were trumpet shaped with a smallest size of approximately 50 nm. Most sites contained a crystallographically oriented central square lid formed above the surrounding crystal surface, probably relating to the orthorhombic to cubic phase transition documented in micron scale cracking patterns observed at the laser heated sites.

Modem quantum chemistry and molecular dynamics computer codes are powerful tools with which to study the physics and chemistry of energetic materials at the molecular level. Quantum chemistry calculations, on one or two energetic molecules, can give valuable information about the initial steps in their decomposition. Molecular dynamics calculations, even with empirical potentials, can yield important information about the physical processes involved in the initiation and growth of reaction of energetic materials. The combination of Molecular dynamics and quantum chemistry techniques offers the potential to probe energetic material reaction chemistry in real systems, in some detail, in the near future. Such an approach is vital if we are to be able to create new realistic macroscopic models within hydrocodes that can describe the initiation and growth of reaction in explosives. This paper gives an overview of the approach being adopted at DRA Fort Halstead to understanding energetic materials at the molecular level. In particular, the use of quantum chemistry and Molecular dynamics to help construct new macroscopic models will be discussed.

Because of its importance in designing safer, more reliable explosives the shock to detonation transition in condensed phase energetic materials has long been a subject of experimental and theoretical study. This transition is thought to involve local hot-spots which represent regions in the material which couple efficiently to the shock wave leading to a locally higher temperature and ultimately initiation. However, how at the atomic scale energy is transferred from the shock front into these local “hot spots” remains a key question to be answered in studies of the predetonation process. In this paper we report results of molecular dynamics simulations that suggest that even nanometer scale defects can play an important role in the shock to detonation transition.

Sharing of the kinetic energy among the translational-rotational-vibrational (T-R-V) degrees of freedom in a molecular crystal, following excitation of the T degrees of freedom, has been studied by means of molecular dynamics. The model was similar to that employed in [2]. The Tdegrees of freedom were energized by adding a velocity distribution to the (100) molecular lattice planes so as to excite certain normal modes in the [100] direction. The kinetic energy components in TR, V and the rates of energy sharing between T-V and T-R were determined. The results were then compared with those from rapid compression of the same model, with or without defects.

Solid energetic substances have long played an important technological role as explosives and also as fuels. Much of the research on the solid phases has been concentrated on ground state properties, and also on the chemistry of the molecules comprising the solid. In addition, significant understanding of the detonation properties has been obtained by semi-empirical continuum mechanical modeling. Traditional solid state studies of this important class of materials have mostly been ignored. This may be due to the apparent success of the semi-empirical models in describing the detonation properties, as well as the practical difficulties in performing band theoretical studies. Recent interest in more fundamental questions relating to the basic properties of these systems as materials, coupled with a desire to probe fundamental questions relating to the initiation and sustaining of the chemical reactions leading to combustion/detonation is generating significant interest in the basic solid state properties of such energetic systems. In particular, recent analysis of detonation by J. J. Gilman emphasizes the need to include excitation of the electronic system in obtaining an understanding. In this manuscript, the basic solid state properties of PETN are considered.

We present the results of molecular dynamics calculations concerning the structure and the thermodynamic state of the reaction zone in the liquid explosive NO under shock sollicitation.We show that the compression of explosive is due to inertial shock followed by endothermic decomposition. Then exothermic reactions occur and cause a slow decreasing of pressure until the end of the reaction zone. Analysis of particle velocities distribution and interatomic forces shows that only a partial equilibrium is reached in the reaction zone. As the relaxation time of particles velocities is very small compared to the characteristic time of chemical reactions, thermal equilibrium is always reached and the various molecular species are isothermal. It is not the case of interparticle forces and pressure in the usual thermodynamic way cannot be defined: the reaction zone is not isobaric. Our conclusion is that an equation of state cannot be defined in the reaction zone.

The short length and time scales associated with chemical detonations make these processes excellent candidates for study by MD simulation. Potentials used in these simulations must have sufficient flexibility to describe gas-phase properties of isolated reactant and product molecules, high density material generated under shock compression, and allow smooth adjustment of bonding forces during chemical reaction. The REBO formalism has been shown to provide these characteristics and allow the treatment of a sufficient number atoms for sufficiently long times to demonstrate a chemically-sustained shock wave (CSSW). In this paper we present a REBO potential describing the oxygen system for use in MD simulation of detonation in an ozone molecular solid. The potential reproduces spectroscopic properties of isolated gas-phase O2 and O3. It also describes an ozone molecular solid with density and speed of sound within physical norms. We observe detonation characteristics that depend on crystallographic orientation in simulations using a three dimensional ozone molecular crystal.

To investigate the role of exothermicity on the properties of a chemically sustained shock waves, a series of two-dimensional molecular dynamics simulations was carried out in which the exothermicity was systematically varied. The simulations were based on a model diatomic system which has been previously shown to produce reasonable values for shock wave properties. A decrease of 33% in the amount of energy released in the reaction produced a significant decrease in detonation front velocity and an increase in the impact energy necessary to initiate a sustained shock wave. Redistribution of energy between the reaction products at constant total exothermicity had a much smaller effect on the properties of the detonation front.