GASLESS COMBUSTION FRONTS WITH HEAT LOSS

For a model of gasless combustion with heat loss, we use geometric s ingular perturbation theory to show existence of traveling combustion fr onts. We show that the fronts are nonlinearly stable in an appropriate sense if an Evans fun ction criterion, which can be verified numerically, is satisfied. For a solid reactant and exot hermicity parameter that is not too large, we verify numerically that the criterion is satisfi ed

Stability of traveling waves in partly parabolic systems

Abstract. We review recent results on stability of traveling waves in partly parabolic reactiondiffusion systems with stable or marginally stable equilibria. We explain how attention to what are apparently mathematical technicalities has led to theorems that allow one to convert spectral calculations, which are used in the sciences and engineering to study stability of a wave, into detailed, theoretically-based information about the behavior of perturbations of the wave

The Energy-Efficient Processing of Fine Materials by the Micropyretic Synthesis Route

Energy-efficient processing of TiB compound with nanowhiskers by micropyretic synthesis is investigated in this paper. Micropyretic synthesis not only offers shorter processing time but also excludes the requirement for high-temperature sintering and it is considered as the one of the novel energy-saving processing techniques. Experimental study and numerical simulation are both carried out to investigate the correlation of the processing parameters on the microstructures of the micropyretically synthesized products. The diffusion-controlled reaction mechanism is proposed in this study. It is noted that nanosize TiB whiskers only occurred when the combustion temperature is lower than the melting point of TiB but higher than the extinguished temperature. The results generated in the numerical calculation can be used as a helpful reference to select the proper route of processing nanosize materials. The Arrhenius-type plot of size and temperature is used to calculate the activation energy of TiB reaction. In addition to verifying the accuracy of the experimental measures, the reaction temperature for producing the micropyretically synthesized products with nanofeatures can be predicted

Review of gasless pyrotechnic time delays

Gasless pyrotechnic delay compositions for time‐sequencing energetic events are reviewed. They are mixtures of powdered fuels and oxidants capable of a highly exothermic oxidation‐reduction reaction. Trends favor ‘green’ compositions targeted to replace compositions containing perchlorates, chromates, lead and barium. Thermite‐based reactions dominate but intermetallics (especially multi‐layered versions) and hybrids appear promising considering progress in self‐propagating high temperature synthesis technology. Improving computer modelling will require better description of condensed phase reactions. Progress was made with the development of “hot spot” models and expressing reactivity in terms of the number of contact points (or contact surface area) between particles. Promising processing advances include mechanochemical synthesis of reactive particle composites by arrested milling or comminution of cold‐rolled multilayer intermetallics. Dry mixing of reactive powders has made way for slurry mixing followed by spray drying.AEL Mining Services (AEL/UP‐4400021845) and by the National Research Foundation (NRF) of South Africa (Grant 83874).http://www.pep.wiley-vch.de2020-01-01hj2019Chemical Engineerin

Self-Propagating Reactive Fronts in Compacts of Multilayered Particles

Reactive multilayered foils in the form of thin films have gained interest in various applications such as joining, welding, and ignition. Typically, thin film multilayers support self-propagating reaction fronts with speeds ranging from 1 to 20 m/s. In some applications, however, reaction fronts with much smaller velocities are required. This recently motivated Fritz et al. (2011) to fabricate compacts of regular sized/shaped multilayered particles and demonstrate self-sustained reaction fronts having much smaller velocities than thin films with similar layering. In this work, we develop a simplified numerical model to simulate the self-propagation of reactive fronts in an idealized compact, comprising identical Ni/Al multilayered particles in thermal contact. The evolution of the reaction in the compact is simulated using a two-dimensional transient model, based on a reduced description of mixing, heat release, and thermal transport. Computed results reveal that an advancing reaction front can be substantially delayed as it crosses from one particle to a neighboring particle, which results in a reduced mean propagation velocity. A quantitative analysis is thus conducted on the dependence of these phenomena on the contact area between the particles, the thermal contact resistance, and the arrangement of the multilayered particles

The role of microstructure on the combustion and impact behavior of mechanically activated nickel/aluminum reactive composites

Metal-based reactive composites are a class of materials that consist of at least one metals, such as Ni/Al, that have high-energy densities and can produce significant energy output during exothermic reaction after thermal or mechanical initiation. However, conventionally these materials typically have slow reaction rates and are difficult to ignite at typical micron particle size ranges limiting their application. Therefore, mechanical activation techniques have been used to create materials with high surface areas and smaller characteristic dimensions in order to increase combustion velocity and ignition sensitivity. Their combustion and mechanical impact behavior is being studied to develop the understanding needed so that the materials can be ultimately developed for applications such as multi-functional energetic materials, blast enhancement and synthesis of novel metastable non-equilibrium materials. ^ In this work Ni and Al powder is mixed by High-Energy Ball Milling (HEBM) to produce a mechanically activated (MA) Ni/Al reactive composite. A two-step process is adopted that includes dry milling followed by wet milling using hexanes as a process control agent. The microstructure of the resulting powder contains layered Ni and Al laminates that have micron to nano-scale dimensions depending upon the dry milling time and particle size of the material. The mechanical impact response and combustion behavior of these materials was studied through a series of experiments. ^ Mechanical impact experiments were performed using a modified Asay shear experiment where properties such as mechanical impact ignition threshold, ignition delay time, and combustion velocity were identified. It was found that the mechanical impact ignition threshold decreases as the dry milling time increases. The material with the longest dry milling time considered (97% tcr where tcr is the critical milling time that results in combustion during milling, which was 17.5 minutes, ignited at impact energy of about ∼50 J or higher (projectile speed of about ∼65 m/s). Ignition delays due to the formation of hotspots ranged from 1.2 to 6.5 msand were observed to be in the same range for all milling times considered less than tcr. ^ Combustion velocities ranged from 25-31 cm/s for impacted samples at an impact energy of 200-250 J. Combustion experiments on MA Ni/Al pressed into cylindrical pellets shows that the combustion velocities increase as the milling time increases from ∼9.4 cm/s at 25% tcr to ~20 cm/s at a milling time of 97% tcr. Maximum combustion temperatures were measured to be ∼1870 ± 35 K for samples milled up to 50% tcr, whereas combustion temperatures for materials milled for 97% tcr were on average of 100 K lower. It was also shown that hydrocarbon contaminants are milled into the MA Ni/Al composite particles during the wet milling step and result in the expansion of the pellets during combustion. It was shown that the concentration of hydrocarbon contamination decreased as the dry milling times increased, which suggests particle structure and mechanical property evolution during dry milling also play a role in contamination during wet milling. ^ Mechanically activated Ni/Al composite powders were also annealed at two different temperatures to observe the effect of intermetallic formation and strain relaxation on the reaction kinetics. Williamson-Hall analysis suggests that recovery occurs at an annealing temperature of 403K, resulting in lower strains for both Al and Ni. At an annealing temperature of 460 K, both recovery and Al grain growth was observed along with the growth of NiAl3 phase, which was also detected using scanning electron microscopy. The morphology and characteristic laminate dimensions were not affected significantly by annealing. The 403 K annealing had little effect on combustion velocities and temperature, but the 460 K annealing significantly reduced the combustion velocities and temperatures. This shows that the combustion velocities and temperatures are not significantly affected by strain relaxation but are largely influenced by the formation of NiAl3 during annealing. ^ Lastly, it was shown that cleaning the milling jar with hexanes, as opposed to water, decreased the amount of cold-welding on the milling jar walls and media. This reduced the finial yield of fine particles (\u3c 106 μm) and increased the concentration of solid solutions, and intermetallics. As a result of this loss in available heat release, combustion temperatures, and combustion velocities decreased

Dynamics of Patterns

Patterns and nonlinear waves arise in many applications. Mathematical descriptions and analyses draw from a variety of fields such as partial differential equations of various types, differential and difference equations on networks and lattices, multi-particle systems, time-delayed systems, and numerical analysis. This workshop brought together researchers from these diverse areas to bridge existing gaps and to facilitate interaction

Couplages multi-physique et multi-échelle pour la modélisation prédictive de la combustion de matériaux énergétiques intégrés dans leur environnement

Cette thèse présente le développement et l'exploitation d'un modèle qui simule à la fois l'initiation et la réaction complète de propagation de nanothermites à base de poudre avec la seule prise en compte de mécanismes en phase condensée. Trois objectifs ont été poursuivis. - Un modèle prédictif de l'initiation et de la propagation de la réaction nanothermite pour des systèmes à base de poudres, en mettant en œuvre le mécanisme découvert récemment de "fusion réactive" sous différentes rampes de chauffe. - Une étude de l'influence de nombreux paramètres clés tels que la taille du système expérimental, la taille des particules, le rapport stœchiométrique des matériaux ou le taux de compaction, sur les deux aspects de la réaction : initiation et propagation. Cela permet à la fois d'explorer les effets de ces facteurs pour la conception du système et d'accélérer les étapes de conception par des comparaisons systématiques théorie/expérience. - Une comparaison de ce modèle avec une approche purement en phase gazeuse pour expliquer les travaux expérimentaux récents explorant l'importance de la "fusion réactive" et pour contribuer à la discussion dans le domaine sur les mécanismes fondamentaux de la réaction de combustion des nanothermites. La réalisation de ces objectifs est détaillée dans ce manuscrit, organisé en deux chapitres, complétés par deux articles publiés. Dans un premier chapitre, un bref état de l'art des nanothermites est présenté pour donner la motivation et le contexte scientifique de ce projet. De nombreux travaux expérimentaux sont cités pour résumer les méthodes de fabrication et de synthèse des nanothermites, leurs principales caractéristiques et l'effet de la nanostructure sur les performances en termes de vitesse de combustion et de délais d'initiation. Cela inclut ensuite un aperçu des arguments actuels dans le débat sur les mécanismes fondamentaux qui dominent la combustion. Nous poursuivons par une présentation des approches de modélisation existantes, leurs objectifs, leurs formulations et leurs limites. Le chapitre 2 présente la base théorique de notre modèle qui s'appuie sur une formulation de mécanismes de combustion liés à la phase condensée, développée spécifiquement pour une application aux thermites à nanopoudres. Une première formulation propose l'initiation de deux nanoparticules (un combustible, un oxydant) en contact, couplées à une équation thermique. Ce modèle est ensuite étendu à la dimension de la propagation, qui comprend les différents éléments du transfert de chaleur macroscopique. Ainsi, la formulation finale du modèle combine les mécanismes hétérogènes à l'échelle nanométrique, les réactions chimiques aux interfaces et la propagation du front global de combustion à l'échelle de l'observation macroscopique. Nous ajoutons à ces deux chapitres un article publié dans Journal of Applied Physics, où nous nous concentrons sur le modèle de combustion élémentaire, c'est-à-dire à l'échelle d'un couple de nanoparticules en contact, afin d'étudier l'importance des mécanismes en phase condensée sur l'initiation des thermites. L'article porte particulièrement sur le processus et l'impact sur la combustion de la fusion réactive, pour différents couples de thermites et différentes vitesses de chauffe. Les résultats sont d'abord validés par comparaison avec des récents travaux expérimentaux puis sont comparés avec une simulation considérant seulement la phase gazeuse comme médiatrice de la combustion. Nous y ajoutons également un deuxième article, qui porte sur l'exploitation du modèle complet de propagation auto-entretenue, avec une discussion des facteurs probables qui influencent la vitesse de réaction. L'article se termine par une discussion sur la propagation du front de combustion fonction de la formulation du terme de conductivité thermique, en correspondance avec les bases théoriques discutées dans le chapitre 2.This thesis presents the development and exploitation of a model that simulates both the initiation and propagation reaction of powder-based nanothermites with purely condensed phase mechanisms. Three main goals have been targeted: - A predictive model of both the initiation and propagation of the nanothermite reaction for powder-based systems implementing the recently discovered "reactive sintering" mechanism under different external heating regimes at low computational cost with flexibility to adapt to newly interesting materials. - A study of the influence of numerous key parameters such as the size of the experimental apparatus, particle size, stoichiometric ratio of materials, or the amount of compaction on both aspects of the reaction. This permits an exploration of the effects of these factors for system design, as well as acts as a method of validation through comparison with experimental studies. - A comparison of this model with a purely gas phase approach to expound on the recent experimental works exploring the importance of reactive sintering and to contribute to the discussion within the domain on the fundamental driving mechanisms of the nanothermite combustion reaction. The fulfillment of these goals is detailed in this manuscript, organized into five chapters. In a first chapter, a state of the art of nanothermites is presented to outline the motivation and scientific context of this project. Numerous experimental works are cited to summarize the methods of manufacturing and synthesis of nanothermites, their principal characteristics, and the effect of the nanostructure on performance in terms of the burn rate and initiation delays. We include an overview of the current arguments in the debate on the fundamental driving mechanisms of reaction, followed by a presentation of the existing modelisation approaches, their aims, formulations, and limitations. Chapter 2 presents the theoretical basis for the proposed model exclusively based on a condensed state formulation of the combustion, developed specifically for application to nanopowder thermites. A first formulation considers a base model of the initiation of two nanoparticles (one fuel, one oxidizer) sintered into contact, coupled to a thermal equation. Then, the base model is expanded into a full propagation model, which includes theoretical constructions of different macroscopic heat transfer mechanisms that were compared to find the best reproduction of experimental results. Thus, the final iteration combines the heterogeneous nanoscale mechanisms and chemical reactions with the overall macroscale propagation reaction. In Chapter 3, the base model is utilized to investigate the importance of condensed-phase mechanisms on the initiation of these thermites, with particular insights into reactive sintering for different thermite couples and different heating rates, and comparison with recent experimental works in addition to a purely gas phase simulation. This is supplemented by a benchmark study of the initiation of nanothermites with different varying parameters including the particle size, stoichiometric ratio, native oxide thickness, and the fuel and oxide material species. Chapter 4 continues with the exploitation of the full self-sustaining propagation model with discussion of the probable factors that most influence the reaction rate. This begins with a presentation of the basic results for an Al:CuO system for each of the three model formulations presented in Chapter 2. Once the chosen system was validated, this version was then used to test the effect of varying important parameters such as the compaction rate, the material species', and the different heat transfer mechanisms also discussed previously. Finally, this is followed by a general conclusion to summarize this work and its implications, as well as explore the perspectives for future work. A Software Architecture Document is available in Appendix A

A study of ignition and propagation of combustive synthesis reaction between titanium and carbon

Combustive Synthesis or Self-Propagating High-Temperature Synthesis (SHS), is an energy-efficient combustion method of producing metallic, ceramic and composite materials from their constituent powders. This thesis presents the results of an experimental and numerical evaluation of the propagation velocity for the SHS solid-solid reaction of titanium and carbon, as well as a study of the ignition process for the reaction. The experimental results show the dependency trend of the wave propagation speed on various parameters: diameter of the reactant compact, density of the compact, reactant mixture composition, and dilution of the reactant mixture with the inert product TiC. Conditions at which the reaction ceases to propagate in a self-supporting manner are also identified. This thesis attempts to generalize the existing experimental observations of the gasless SHS process by means of a dimensional analysis, thus offering a mechanistic framework within which future developments can be correlated. The implementation of the new reaction kinetics model of Kanury and some suitable dimensionless variables permit the main factors affecting the process to be embedded in a single key parameter, the Da number. This parameter includes the overall effects of thermal properties, stoichiometry of the reaction, carbon particle size, a process constant, a compression effect and the diffusion of one reactant through an intermediate complex. The study of propagation covers a broad range of possible Da numbers that could arise for different conditions found in experiments. A section in numerical calculations of the preheated length is included as well. Comparison of the numerical and experimental results for propagation are found to be in reasonable agreement, thus validating the suitability of the analytical model. The numerical study includes an examination of the ignition problem for a stoichiometric mixture, using a prescribed surface temperature boundary condition. For this condition, an ignition threshold curve is determined above which ignition will always occur and below which no ignition is possible

Influence of initial temperature and convective heat loss on the selfpropagating reaction in Al/Ni multilayer foils

A two-dimensional numerical model for self-propagating reactions in Al/Ni multilayer foils was developed. It was used to study thermal properties, convective heat loss, and the effect of initial temperature on the self-propagating reaction in Al/Ni multilayer foils. For model adjustments by experimental results, these Al/Ni multilayer foils were fabricated by the magnetron sputtering technique with a 1:1 atomic ratio. Heat of reaction of the fabricated foils was determined employing Differential Scanning Calorimetry (DSC). Self-propagating reaction was initiated by an electrical spark on the surface of the foils. The movement of the reaction front was recorded with a high-speed camera. Activation energy is fitted with these velocity data from the high-speed camera to adjust the numerical model. Calculated reaction front temperature of the self-propagating reaction was compared with the temperature obtained by time-resolved pyrometer measurements. X-ray diffraction results confirmed that all reactants reacted and formed a B2 NiAl phase. Finally, it is predicted that (1) increasing thermal conductivity of the final product increases the reaction front velocity; (2) effect of heat convection losses on reaction characteristics is insignificant, e.g., the foils can maintain their characteristics in water; and (3) with increasing initial temperature of the foils, the reaction front velocity and the reaction temperature increased