Ignition Of Energetic Materials Research Articles

Research Progress in the Flight Characteristics of Flyers Driven by Different Technologies

Flyers driven by various technologies have been widely used as effective shock loading techniques in the ignition of energetic materials, space debris impact modeling, equation of state measurement, etc. However, their flying characteristics have the greatest influence on application reliability. The flying velocity, as well as the flyer’s planarity and integrity, are crucial characteristics. Therefore, mainstream flyer-driving technologies are examined in this study, and research progress in investigation approaches on flyer velocity and morphology are reviewed. Furthermore, for each flyer-driving technique, the affecting factors of flyer velocity and morphology are summarized. Current research deficiencies are highlighted, and future research directions are discussed, including the parallel coupling of various driving principles, quantitative characterizations of the flyer’s planarity and integrity, and the physical model matching the flying process.

Al/SnO2 Nanothermite ESD Desensitization by Means of the Elaboration of Tailored SnO2-Polypyrrole Composites.

Incidents can occur when handling energetic materials (EMs) due to their sensitivity to external stimuli. Mechanical (friction, impact) and electrical (electrostatic discharge (ESD)) stimuli can trigger involuntary ignition of EMs. Nanothermites, defined as highly reactive mixtures of a metal and a metal oxide, show interesting reactive properties (high temperature/amount of heat). However, their extreme sensitivity to ESD is a significant drawback in their development. With a sensitivity threshold of lower than a millijoule and considering the human body capacity discharge is a few tens of millijoules, the handling of such energetic mixtures is extremely hazardous. ESD desensitization is therefore of crucial importance for the use of these materials. To achieve this goal, polypyrrole (PPy) conductive polymer was used as an additive within an Al/SnO2 energetic formulation. This polymer is able to easily conduct the electrostatic charges, which is useful to avoid an ignition after a spark stimulus. PPy was added to the nanothermite through the elaboration of SnO2-PPy composites. Delaying the SnO2 introduction time during pyrrole's polymerization (t = 0, 1, 24 h) changed the microstructures of the composites. The various SnO2-PPy composites highlighted that the additive's distribution within the composite is a key component for the modulation of ESD sensitivity. For example, this threshold is higher for a nanothermite formulated with the composite elaborated at t = 0 h (96.5 mJ with 7.5 vol % of polymer) than a composition based on the composite at t = 24 h (16.9 mJ with 8.2 vol % of polymer). Meanwhile, the reactive properties are decreased. The loss of reactivity (e.g. combustion speed) was explained by the thermal insulating property of the additive. Overall, the aim of this study is to help in the understanding of ESD desensitization of nanothermites in order to make them safer for operators and adapted to the required applications.

Finite Volume Method for the Launch Safety of Energetic Materials

It is important to examine the ignition of energetic materials for launch safety. Given that there is a paucity of experimental tests, numerical simulations are important for analysing energetic materials. A computer program based on the finite volume method and viscoelastic statistical crack mechanics model is developed to study the ignition of energetic materials. The trends of temperature and stress of energetic materials subjected to projectile base pressure are studied by numerical examples. The results are compared with those in an extant study, which verified the correctness of the proposed method. Additionally, the relationships between the temperature increase and nonimpact ignition of energetic materials were analysed. The results show that when the temperature at the bottom of the explosive rises to a certain value, it will cause the explosive to ignite. This research has significance to the study of the base gap of explosives, and it provides a reference for launch safety evaluation of energetic materials.

Conductive Polymer Spark Gap Igniters

AbstractElectrodes fabricated from electronically active polymers offer some benefits over those fabricated with common metals, including an appreciable resistance to corrosion and lower density. These benefits, and potentially others, manifest the advantages of using conductive polymers in lieu of metals for the mass production of ignition systems. In this work, spark gap igniters were fabricated using a potentially low‐cost, roll‐to‐roll compatible process utilizing the doped conductive polymer polyaniline as the electrode material. Subsequently, nanothermite was printed on the spark gap igniters between the organic leads and ignited using a high voltage source to demonstrate the effectiveness of the device. All of the igniters successfully fired with a mean spark over voltage of 3.14 kV. In addition, all of the igniters with deposited nanothermite successfully ignited the material. Accordingly, this work outlines the materials and processes required to develop organic‐based spark gap igniters and establishes a baseline for future work related to the metal‐free ignition of energetic materials.

Energy dissipation in polymer-bonded explosives with various levels of constituent plasticity and internal friction

The ignition of energetic materials (EM) under dynamic loading is mainly controlled by localized temperature spikes known as hotspots. Hotspots occur due to several dissipation mechanisms, including viscoplasticity, viscoelasticity, and internal friction along crack surfaces. To analyze the contributions of these mechanisms, we quantify the ignition probability, energy dissipation, damage evolution, and hotspot characteristics of polymer-bonded explosives (PBXs) with various levels of constituent plasticity of the energetic phase and internal crack face friction. Using PBX9501 consisting of HMX (Octahydro-1,3,5,7-Tetranitro-1,2,3,5-Tetrazocine) and Estane as a reference material, we analyze variants of this material with several values of the yield stress of the energetic phase and coefficients of internal crack face friction, while other parameters are kept unchanged. The impact loading involves piston velocities between 200 and 1200 m/s. The analysis uses a Lagrangian cohesive finite element framework that explicitly accounts for finite-strain elastic-viscoplastic deformation of the grains, viscoelastic deformation of the binder, arbitrary crack initiation and propagation in the grains and the binder, debonding between the grains and the binder, contact between internal surfaces, friction and frictional heating along internal surfaces, heat generation resulting from inelastic bulk deformation, and heat conduction. To determine the ignition status of the material or “go” or “no-go” state, we use a criterion based on a criticality threshold obtained from chemical kinetics calculations. For PBX with various levels of HMX plasticity and friction, the probability of ignition, the evolution of dissipation caused by plasticity and friction, the density of cracks, and the locations of cracks are quantified. Results show that samples with higher levels of constituent plasticity (lower yield strengths) or lower levels of internal friction are less likely to ignite. The relative importance of plasticity and friction depends on load intensity, with frictional heating decreasing as load intensity increases. Although the overall viscoplastic heating outweighs the overall frictional heating, friction plays a very important role in hotspot development at all load intensities analyzed, owing to the fact that frictional heating is more localized than viscoplastic heating. The predicted thresholds and ignition probabilities are expressed in a load intensity-load duration relation for PBX with different constituent properties.

Generation of Electrohydraulic Shock Waves by Plasma-Ignited Energetic Materials: II. Influence of Wire Configuration and Stored Energy

Electrohydraulic shock waves (SWs) have been used for shale gas exploitation in recent years. The traditional method to generate SWs is underwater pulsed discharge with a water gap load or an electrical wire (EW) load, but the SW energy is limited heavily by the stored energy. To obtain stronger SW, the EW explosion ignited energetic material (EM) load is proposed here. To obtain a good ignition performance, the influence of capacitor charging voltage, stored energy, wire diameter, and wire material (Cu and Mo) were studied. The results show that the higher the charging voltage is, the more effective the ignition is; liquidation and vaporization will affect the discharge but will not ignite the EM explosion effectively; the EMs given in this paper are more sensitive to the arc plasma. Besides, the nonrefractory copper is more suitable for the EM ignition than the refractory molybdenum.

Ignition of energetic materials by a high-current electron beam and the effect of irradiation on their thermal sensitivity

The ignition of a powdered nitrocellulose–nitroglycerin composition and its mixtures with lead oxide and zirconium in different ratios by a high-current electron beam is studied. Experimental results on assessing the influence of electron beam irradiation on the thermal sensitivity of the studied energetic materials are reported. It is shown that the key role in the ignition of the materials is played by the cathode flare generated at the cathode after the electron beam.

Mechanism of the thermal ignition of organic explosives by an electron beam

A numerical simulation of the ignition of organic explosives (PETN, HMX, RDX, TATB) by an electron beam is performed. A criterion for the ignition of energetic materials with a melting point below the ignition temperature is obtained. The results of numerical calculations of the critical energy density of the electron beam are consistent with the criterion of ignition. The calculated critical energy density of PETN ignition is in good agreement with the available experimental data. The most sensitive HE is PETN, whereas the most heat-resistant is TATB.

Time-Resolved Spectroscopy of Initiation and Ignition of Flash-Heated Nanoparticle Energetic Materials

Nanotechnology has brought a great deal of excitement to research in energetic materials (EMs). Nanoparticle EMs have high densities of stored energy and the potential for multifunctionality. Here we discuss research on fundamental mechanisms of initation and ignition of EM with Al or B fuel nanoparticles and TeflonAF or nitrocellulose (NC) oxidizer. Polybutadiene (PB) was also used as an inert. The thin-film samples were confined between two windows and were activated by flash-heating the metal nanoparticles with picosecond laser pulses. Reactions of isolated nanoparticles with their surroundings were studied by measuring ablation thresholds. A shock-induced polymer dissociation model was needed to explain the growth of the reaction volume surrounding a flash-heated fuel particle. Thicker oxide passivation layers confined the nanoparticle allowing the pressure to build up to higher values during flash-heating. Initiation, as the onset of chemical reactivity, was probed using time-resolved Raman or infrared to monitor disappearance of nitrato (ONO2) of NC or CF2, CF3,or CFO of TeflonAF. Ignition, the onset of energy-releasing processes, was studied by analyzing time-dependent spectra of emission bursts. In flash-heated Al/Teflon, initiation occurs in ∼50 ps and ignition occurs in ∼100 ps. In B/Teflon, ignition occurs in ∼200 ps. Transient opacity measurements of Al/Teflon suggest that chemical reactivity beyond the initial exothermic formation of AlF occurs within ∼5 ns.

Nonuniform laser ignition in energetic materials

Laser ignition of energetic materials is typically described in terms of a one-dimensional homogeneous ignition model. However, the Gaussian energy distribution from a laser can induce multidimensional effects on ignition. A two-dimensional numerical model that simulates radiant heating and subsequent ignition of energetic materials was developed. A laser with a Gaussian energy distribution was used as the external energy source applied to the flat surface of a cylindrical pellet. The nonuniform behavior of thermal ignition leads to radially dependent ignition times, temperatures, and energy associated with the front surface of the pellet. Existing experimental ignition data was compared to the numerical results. The model shows the importance of a two-dimensional analysis for ignition because the nonuniform external heat source can result in the formation of localized heating that will develop into nonplanar flame propagation. The model is compared to experimental results that further validate the importance of a two-dimensional ignition model and show that ignition occurs first in the center of the pellet where the Gaussian beam intensity is greatest.

Influence of the Metal Particle Size on the Ignition of Energetic Materials

A theoretical study of multi-particle ignition uses a hot spot model which calculates the temperature evolution of individual hot spots in an energetic material. It indicated that ultra-fine hot particles would be very effective in igniting energetic materials if impinging and penetrating the solid propellant. Igniting mixtures were prepared containing ultra-fine Ti particles which react fastest compared with coarse particle mixtures, standard B/KNO3 and black powder. The ultra-fine particles, however, are obviously oxidized or gasified too fast as to reach the energetic material to be initiated and longer ignition delays are found mainly compared with coarse particle mixtures. An optimized mixture of coarse and ultra-fine particles would give an improvement of ignition delay times.

A new ignition criterion based upon a critical thermal energy density

It has long been known that various ignition criteria of energetic materials have been limited in applicability to small regions. In order to explore the physical nature of ignition, we calculated how much thermal energy per unit mass of energetic materials was absorbed under different external stimuli. Hence, data of several typical sensitivity tests were analyzed by order of magnitude estimation. Then a new concept on critical thermal energy density was formulated. Meanwhile, the chemical nature of ignition was probed into by chemical kinetics. Finally, it was considered that perhaps the critical thermal energy density was an important characteristic parameter for energetic material ignition.

Two-stage ignition of energetic materials with a liquid surface layer

Two-stage ignition regimes of energetic materials with a liquid surface layer were revealed by mathematical modeling of transient combustion processes. First, under the action of a radiant flux, the regime of forced gasification of a condensed phase with a degree of its surface depletion of 0.1–0.3 was observed. Gas-phase combustion occurs in the blow-off regime. As the radiant flux decreases, the gas flame approaches the surface and becomes determining, and the degree of condensed-phase depletion decreases.

Ignition of energetic materials under conditions of complex heat exchange

The ignition of energetic materials under conditions of complex heat exchange between energy source and ignited substance surface is studied. Expressions are obtained for the parameters of the ignition of energetic materials by transient heat fluxes. The results are generalized, and an approximate method for calculating the ignition parameters using a universal curve is suggested. It is shown theoretically and supported by experimental results that when heat-flux intensity increases dramatically in time, ignition delay depends weakly on kinetic and thermal parameters. A particularly interesting case, the ignition of energetic materials exposed to two-phase flows (gas + solid particles), is considered in detail. It is shown that in this case the penetration of solid particles into the surface layer of the reacting substance is essential. An approximate mathematical model of ignition that includes this effect is proposed. Nomenclature a = thermal diffusivity b = series coefficient c = specific heat D = sample diameter D(6,, x) = auxiliary function d = particle diameter E = activation energy k = power coefficient