Indexes of the Proceedings for the Ten International Symposia on Detonation 1951-93

The Proceedings of the ten Detonation Symposia have become the major archival source of information of international research in explosive phenomenology, theory, experimental techniques, numerical modeling, and high-rate reaction chemistry. In many cases, they contain the original reference or the only reference to major progress in the field. For some papers, the information is more complete than the complementary article appearing in a formal journal; yet for others, authors elected to publish only an abstract in the Proceedings. For the large majority of papers, the Symposia Proceedings provide the only published reference to a body of work. This report indexes the ten existing Proceedings of the Detonation Symposia by paper titles, topic phrases, authors, and first appearance of acronyms and code names

Effects of microstructure and chemistry on the ignition sensitivity of PBX under shock loading

The ignition sensitivity of heterogeneous energetic materials subject to shock loading is analyzed using both a Lagrangian and Eulerian computational framework. The specific focus here is on the various microstructure heterogeneities (including cracks, granular anisotropy, voids, and aluminum additives) and their relative contributions to the development of critical hotspots and macroscale detonation behavior characteristics, such as the run distance to detonation. A probabilistic approach is developed by generating statistically equivalent microstructure sample sets (SEMSS) and measuring the ignition behavior of each one under similar impact conditions. By varying the material and microstructural characteristics in a controlled fashion, the contribution to ignition of each specific type of microstructural defects is rank-ordered. The Lagrangian-based cohesive finite element method (CFEM) is used to track material response prior to the onset of chemical reaction. A probability threshold is proposed based on a modified form of the Hugh James and Walker-Wasley energy-based ignition criterions. The computations focus on both 100% packed energetic grains (HMX) as well as aluminized polymer-bonded explosives (APBXs). The exact physical mechanisms governing the development of hotspots are quantified, and the friction is found to be the dominant dissipation mechanism. The Sandia National Laboratories Eulerian hydrocode, CTH, is then used to simulate the entire shock to detonation transition (SDT) of pressed HMX. The run-to-detonation distance is predicted as a function of shock pressure. The initial probability analysis is expanded upon to generate a predictive map of the SDT threshold for both 2D and 3D samples. The probability thresholds proposed in this study serve as a useful design metric and may directly influence future shock experimentation as well as the development of new insensitive high explosives design metric and may directly influence future shock experimentation as well as the development of new insensitive high explosives.Ph.D

Probabilistic relations between thermo-mechanical response and microstructure of heterogeneous energetic materials for shock/nonshock loading

An approach is developed to predict the ignition sensitivity of heterogeneous energetic materials under shock and nonshock loading as a function of microstructure. The underlying issue of impact-induced initiation of chemical reactions is driven by the deposition of mechanical work into energetic materials in the form of localized heating or the development of hotspots. These hotspots govern the ignition of energetic materials. The aim of this study is to understand the mechanisms of hotspot evolution, computationally predict the ignition sensitivity, and analyze the effects of loading and microstructural attributes on hotspot development and material ignition sensitivity. A computational framework based on a Lagrangian cohesive finite element method (CFEM) is developed. This framework is used to statistically analyze the material sensitivity, accounting for microstructural attributes in terms of morphology, constituent properties, inclusions, and defects. Multiple samples with statistically similar microstructural attributes are generated in a controlled manner and used to obtain a quantitative measure for the statistical variation in ignition behavior due to material heterogeneity. To relate loading and microstructure to the onset of chemical reaction, a hotspot-based criticality criterion is established. The analysis involves the quantification of hotspots via the CFEM simulations. The approach yields criticality conditions in terms of the critical impact velocity, critical time required for ignition, and total energy required for ignition under a given loading rate. The stochasticity of the material behavior is analyzed using a probability distribution as a function of microstructural attributes including grain volume fraction, grain size, amount of metallic inclusions, and specific binder-grain interface area. A probability superposition model is proposed to delineate the effects of different sources of stochasticity. The ignition threshold for granular explosives (GXs) and polymer-bonded explosives (PBXs) under shock and nonshock loading are predicted. The particular thresholds predicted are the James-type ignition threshold and the Walker-Wasley ignition threshold. The dependence of the ignition probability on material and microstructure is analyzed for a wide range of loading conditions. The microstructure – ignition threshold relations with the probability envelopes developed in this study provide a guide for the design of new energetic materials.Ph.D

Meso-Scale Analysis of Deformation Induced Heating in Granular Metalized Explosives by Piston Supported Waves

Shock sensitivity of heterogeneous explosive composites is dependent on the formation of hot-spots which are small regions of elevated temperatures within the material. Changes in the initial meso-structure (i.e. packing density, composition, particle size and shapes) of the explosives can significantly alter the hot-spot fields in the material and thereby affect its shock sensitivity. In this study, an explicit, 2D, Lagrangian finite and discrete element technique is used to numerically simulate the deformation induced heating of granular mixtures of explosive (HMX), and metal (Al) particles due to piston supported uniaxial deformation waves (400 ≤ Up ≤ 800 m/s). A number of simulations are performed by systematically varying the effective initial packing densities φs, metal mass fractions λm, and particle size distributions. Emphasis is placed on charactering how the inclusion of metal (Al) affects both the effective wave end states (Hugoniots) and the hot-spot fields within the explosive (HMX) component relative to neat HMX. Variations in hot-spot volumetric quantities such as number density and volume fraction are characterized since these quantities can be used in the ignition and growth models to describe macro-scale material sensitivity. Predictions indicate that porosity has a leading order effect on the shock sensitivity of the material due to enhanced dissipation resulting from plastic pore collapse. For a fixed porosity and piston speed, inclusion of metal is found to enhance the effective plasticity in the material due to higher pressures. This leads to larger hot-spots within the metalized formulations. However, due to the high thermal conductivity of the metal, frictional induced hot-spots are suppressed within the material since most of the frictional dissipation at the Al-HMX interfaces is absorbed by the metal. Additionally, hot-spot formation is found to have a highly non-linear dependence on Al particle size with a substantial decrease in hot-spot number density and volume fraction predicted with increasing metal particle size. Meso-structural stochasticity arising due to random seeding of particles, and/or large particle clustering were found to affect the hot-spot statistics minimally

Deep learning for synthetic microstructure generation in a materials-by-design framework for heterogeneous energetic materials

The sensitivity of heterogeneous energetic (HE) materials (propellants, explosives, and pyrotechnics) is critically dependent on their microstructure. Initiation of chemical reactions occurs at hot spots due to energy localization at sites of porosities and other defects. Emerging multi-scale predictive models of HE response to loads account for the physics at the meso-scale, i.e. at the scale of statistically representative clusters of particles and other features in the microstructure. Meso-scale physics is infused in machine-learned closure models informed by resolved meso-scale simulations. Since microstructures are stochastic, ensembles of meso-scale simulations are required to quantify hot spot ignition and growth and to develop models for microstructure-dependent energy deposition rates. We propose utilizing generative adversarial networks (GAN) to spawn ensembles of synthetic heterogeneous energetic material microstructures. The method generates qualitatively and quantitatively realistic microstructures by learning from images of HE microstructures. We show that the proposed GAN method also permits the generation of new morphologies, where the porosity distribution can be controlled and spatially manipulated. Such control paves the way for the design of novel microstructures to engineer HE materials for targeted performance in a materials-by-design framework

Acceleration and Heating of Metal Particles in Condensed Matter Detonation

For condensed explosives containing metal particle additives, interaction of the detonation shock and reaction zone with the solid inclusions leads to non-ideal detonation phenomena. Features of this type of heterogeneous detonation are described and the behaviour is related to momentum loss and heat transfer due to this microscopic interaction. For light metal particles in liquid explosives, 60-100% of the post-shock velocity and 20-30% of the post-shock temperature are achieved during the timescale of the leading detonation shock crossing a particle. The length scales corresponding to particle diameter and detonation reaction-zone length are related to define the interaction into three classes, bound by the small particle limit where the shock is inert, and by the large particle limit dominated by thin-detonation-front diffraction. In particular, the intermediate case, where the particle diameter is of similar order of magnitude to the reaction-zone length, is most complex due to two length scales, and is therefore evaluated in detail. Dimensional analysis and physical parameter evaluation are used to formalize the factors affecting particle acceleration and heating. Examination of experimental evidence, analysis of flow parameters, and thermochemical equilibrium calculations are applied to refine the scope of the interaction regime. Timescales for drag acceleration and convective heating are compared to the detonation reaction time to define the interaction regime as a hydrodynamic problem governed by inviscid shock mechanics. A computational framework for studying shock and detonation interaction with particles is presented, including assumptions, models, numerics, and validation. One- and two-dimensional mesoscale calculations are conducted to highlight the fundamental physics and determine the limiting cases. Three-dimensional mesoscale calculations, with up to 32 million mesh points, are conducted for spherical metal particles saturated with a liquid explosive for various particle diameters and solid loading conditions. Diagnostic measurements, including gauges for pressure, temperature, and flow velocity, as well as mass-averaged particle velocity and temperature, are recorded for analysis. Mesoscale results for particle acceleration and heating are quantified in terms of shock compression velocity and temperature transmission factors. In addition to the density ratio of explosive to metal, the solid volume fraction and the ratio of detonation reaction-zone length to the particle diameter are shown to significantly influence the particle acceleration and heating. A prototype heterogeneous explosive system, consisting of mono-disperse spherical aluminum particles saturated with liquid nitromethane explosive, is studied to develop fitting functions describing the shock compression transmission factors. Results of the mesoscale calculations are formulated into a macroscopic physical model describing an effective shock compression drag coefficient and Nusselt number. The novel models are explored analytically and are then applied to two challenging sets of test cases with comparison to experiment. Heterogeneous detonation is considered for aluminum particles saturated with liquid nitromethane, and inert particle dispersal is studied using a spherical explosive charge containing steel beads saturated in nitromethane. Finally, discussion of practical considerations and future work is followed by concluding remarks

Feedstock powders for reactive structural materials

Metals as fuels have higher energy density per unit mass or volume compared to any hydrocarbon. At the same time, metals are common structural materials. Exploring metals as reactive structural materials may combine their high energy density with attractive mechanical properties. Preparing such materials, however, is challenging. Requirements that need to be met for applications include density, strength, and stability enabling the component to sustain the structure during its desired operation; added requirements are the amount and rate of the energy release upon impact or shock. Powder technology and additive manufacturing are approaches considered for design of reactive structural materials. Respectively, feedstock powders are of critical importance. These feedstock powders must have the chemical composition ensuring, along with mechanical characteristics, a rapid initiation of the reactive material upon impact or shock, and high total energy release. They also must have the morphology suitable for processing. In this work, several powders designed to serve as feedstock for manufacturing reactive structural materials are prepared, tuned, and characterized. High-energy mechanical milling is the common manufacturing approach for such powders in this study. The materials include elemental metals, such as aluminum, with the narrowly sized spherical porous powder and magnesium, with custom powder coating. Composite powders combining metals and metalloids, e.g., boron-titanium and boron-zirconium, with different structures and morphologies are also prepared and characterized. Milling conditions are varied and it is shown that the structures, sizes, porosities, and shapes of the produced powder particles can be adjusted through such variation. The experimental work includes characterizing ignition and combustion of the prepared powders. Custom experiments employing an electrically heated wire are used with all prepared materials. Particle combustion experiments, quantifying the particle burn time and temperatures are performed with selected materials. Additionally, thermal analysis is used extensively in addition to electron microscopy and x-ray powder diffraction. Microcalorimetry in oxidizing gas serves to quantify stability of the selected materials. Nitrogen adsorption is used for many prepared powders to characterize their specific surface area and respective porosity. Prepared powders combine unique morphological properties making them amenable to additive manufacturing, in particular, with high reactivity and stability. It is expected that using them as feedstock will lead to design of a new generation of reactive structural materials

Fluorine-based inorganic oxidizers for use in metal-based reactive materials

This work explores inorganic fluorides as oxidizers for fuel-rich reactive materials. A preliminary assessment of metal fluorides accounting for their enthalpy of formation points to bismuth (III) fluoride, BiF3 and cobalt (II) fluoride, CoF2 as oxidizers of interest. Initially, composite powders of aluminum with chosen fluorides at 50-50 wt. % are prepared by arrested reactive milling. Despite an increase in reactivity and lowtemperature ignition, the prepared composite powders are insensitive to initiation by electro-static discharge (ESD), making them attractive alternative to analogous thermites having very high ESD sensitivity. In air, the composite powder particles burn faster than reference aluminum particles of the same size. Very high combustion temperatures are observed suggesting gasification of a significant fraction of the fluorinated combustion products. However, in hydrogenated environments, fluorination of the fuel is hindered due to cannibalistic side-reaction between fluoride and water vapor; as a result the burn rates for composite particles are the same or even lower than for pure Al. Further, nickel (II) fluoride is considered as an oxidizer in more aluminum-rich compositions. Milling protocol is refined to achieve low ignition temperatures for the selected composition. Similar fast burn times and low ignition temperatures in air is achieved with only 30 wt. % of NiF2 suggesting it is possible to prepare even more fuelrich composites with attractive reactivities by further refining the mixing scale between fuel and oxidizer. The aerosol of aluminum-nickel fluoride composite burns with higher efficiency than spherical aluminum powder with comparable size distribution. Boron-based compositions with 50 wt. % of both fluoride oxidizers are similarly prepared and characterized. The nascent hydrated boron oxide layer is found to react with the fluoride and initiates low-temperature ignition. The composites burned in air faster than boron yielding gaseous reactive fluorinated products of interest to chemical and biological agent-defeat applications. The fluoride content is reduced to characterize the effect of composition to develop boron-replacement fuel. Additionally, solvent-based nanometric BiF3 coating is deposited on boron particle to homogenously disperse smaller quantity of fluoride. It is observed that only 10 wt. % of fluoride is sufficient for both milled, and coated boron powders to ignite readily and burn much faster than boron in air. During combustion, the reduced metal, Bi, functions as a catalytic oxygen-shuttle accelerating the particle burn rate. Finally, silicon-based compositions with the same fluoride oxidizers are prepared and characterized. For all the three fuels, ignition is found to be driven by lowtemperature oxidation initiated by fluorination. The fluorination mechanism is based on multiple factors such as fluoride stability in air, fuel reactivity and alloying tendency between metal fuel and metal reduced from the fluoride. Fluoride decomposition-driven ignition is observed in boron and silicon-based composites for different fluorides. For composites of Al with CoF2 and NiF2, fluorination occurs through redox-reaction; for Al·BiF3, the reaction was driven by decomposition of BiF3. Directions of possible future work are outlined based on the results and properties of different inorganic fluorides

Modeling, Numerical Analysis, and Predictions for the Detonation of Multi-Component Energetic Solids

Metal powders are often used as an additive to conventional high explosives to enhance the post-detonation blast wave. Piston-impact simulations are commonly utilized to predict performance metrics such as detonation speed and strength, as well as assessing the impact and shock sensitivity of these materials. The system response is strongly influenced by the initial particle size distribution and material composition. Multiphase continuum models have been routinely applied at the macroscale to characterize the detonation of solid high explosives over engineering length scales. Current models lack a description of the physically permissible constitutive relations for mass transfer due to general chemical reactions between multiple components. The model developed in this study is a major extension of one formulated for an inert mixture to include these reactions, which features a rigorous analysis of the energetic processes that identically satisfy the Second Law of Thermodynamics. Additional features of the model include evolutionary equations which predict phase temperature changes due to individual dissipative heating processes. Macroscale models often include nonconservative source terms that prevent the system of evolutionary equations from being posed in divergence form. A significant challenge in the development of numerical methods to solve these model equations is the proper inclusion of discretizations for the nonconservative sources. In the present work a novel modification of a centered finite-volume scheme is formulated, which is a rigorous extension of a conservative method to include nonconservative sources. This numerical scheme was used to perform a parametric study of metalized explosives containing the high explosive HMX (C4H8N8O8), with both inert and reactive aluminum. Wave speeds, structures, and energetics were shown to exhibit a strong dependence on metal grain size, with reactive aluminum significantly accelerating the detonation speed for the mixture above that of pure HMX for d_m \u3c 500 nm

ENHANCING THE COMBUSTION CHARACTERISTICS OF ENERGETIC NANOCOMPOSITES THROUGH CONTROLLED MICROSTRUCTURES

Metastable Intermolecular Composites (MIC’s) are a relatively new class of reactive materials which, through the incorporation of nanoscale metallic fuel and oxidizer, have exhibited multiple orders of magnitude improvement in reactivity. Although considerable research has been undertaken, their reaction mechanism is still poorly understood, primarily due to the complex interplay between chemical, fluid mechanic and thermodynamic processes that happen rapidly at nanoscale. For my dissertation, I have attempted to tackle this problem by employing controlled nanomaterial synthesis routes and optical diagnostics to identify the dominant underlying mechanisms. I begin my investigation by examining the nature of metal nanoparticle combustion wherein, I employed laser ablation to generate size- controlled aggregates of titanium and zirconium nanoparticles and studied their combustion behavior in a hot oxidizing environment. The experiments revealed the dominant role of rapid nanoparticle coalescence, before significant reaction could occur, resulting in a drastic loss of nanostructure. The large-scale effects of sintering on MIC combustion was explored through a forensic analysis of reaction products. Electron microscopy was employed to evaluate the product particle size distributions and focused ion beam milling was used to expose the interior composition of the product particles. The experiments established the predominance of condensed phase reaction at nanoscale and the interior composition revealed the poor extent of reaction due to rapid reactant coalescence before attaining completion. In light of such limitations, the final part of my dissertation proposes a solution to counteract rapid, premature coalescence through the synthesis of smart nanocomposites containing gas generating (GG) polymers. The GG acts as a binder as well as a dispersant, which disintegrates the composite into smaller clusters prior to ignition, thereby avoiding large scale loss of nanostructure. High speed optical diagnostics including an emission spectrometer and a high-speed color camera pyrometer were developed to quantify the enhanced combustion characteristics which indicate an order of magnitude improvement in reactivity over counterparts using commercial nanomaterials. Moreover, thermal pretreatment as a possible bulk processing strategy to improve nanoaluminum reactivity in a MIC is examined, where a 1000% increase in reactivity was observed compared to the untreated case. Finally, composites of nanoaluminum and reactive fluoropolymers (PVDF) are examined as a possible candidate for energetic material additive manufacturing (EMAM) and its viability is demonstrated by 3D printing and characterizing reactive multilayer films