Pyrotechnic Mixtures Research Articles

Influence of interface conditions on laser diode ignition of pyrotechnic mixtures: application to the design of an ignition device

This paper treats of numerical modelling which simulates the laser ignition of pyrotechnic mixtures. The computation zone is divided into two fields. The first is used to take account of the heat loss with the outside. It can represent an optical fibre or a sapphire protective porthole. The second field represents the reactive tablet which absorbs the laser diode's beam. A specific feature of the model is that it incorporates a thermal contact resistance R c between the two computation fields. Through knowledge of the thermal, optical and kinetic properties, this code makes it possible to compute the ignition conditions. The latter are defined by the energy E 50 and the time t i of ignition of any pyrotechnic mixture and for various ignition systems. This work was validated in the case of an ignition system consisting of a laser diode with an optical lens re-focussing system. The reactive tablet contains 62% by mass of iron and 38% by mass of KClO 4. Its porosity is 25.8%. After an evaluation of the laser's coefficient of absorption, the variations of the ignition parameters E 50 and t i are studied as a function of the thermal contact resistance R c . Temperature profiles are obtained as a function of time and for various values of the thermal contact resistance R c . More fundamental observations are made concerning the position of the hot spot corresponding to priming. From this study, which concerns the heat exchange between the two media, several practical conclusions are given concerning the design of an ignition device. By evaluation of the thermal contact resistance R c , comparison with test results becomes possible and the results of the computations are in reasonable agreement with the test measurements.

Thermal properties and firing characteristics of the Zr/KClO4/Viton A priming compositions

Zirconium powder is a powerful reducing agent that reacts with oxidizer at elevated temperature, releasing enough heat to ignite pyrotechnic mixtures. In this work, the thermal properties of Zr/KClO4/Viton A priming compositions made by different preparation methods were first investigated using thermal analysis techniques. Then these priming compositions were pressed into the charge holders of pressure cartridges. The firing characteristics of the pressure cartridge were evaluated by using the Bruceton method. The data indicate that the thermal behavior and firing characteristic of Zr/KClO4/Viton A priming compositions were changed because of the different methods of preparation.

A Safe Way of Performing Some Dangerous Experiments. II. Construction of a Safety Dropper

Due to the high safety risks, chemistry instructors avoid demonstrating many remarkable experiments based on the addition of a liquid to a solid. Well-known examples of such demonstrations are various pyrotechnic mixtures of potassium chlorate and sugar (sucrose), which are usually activated with a drop of concentrated sulfuric acid. Other attractive demonstrations are the addition of water to freshly prepared magnesium phosphide and addition of water to burning magnesium. In all of these demonstrations the reaction that takes place immediately is very vigorous and can be hazardous for the instructor. Because chemistry teachers and instructors usually try to avoid performing experiments that include a hazard, a number of highly attractive experiments may remain unknown to the public. Using a simple homemade device called a safety dropper, one can perform all of these experiments with complete safety, both for the audience and the demonstrator. Details for performing some of these experiments as well as for the construction of the safety dropper are given in this paper. Video clips of demonstrations are included as an aid for inexperienced instructors.

Study of the Radiation Emitted During the Combustion of Pyrotechnic Charges. Part I: Non Stationary Measurement of the Temperature by Means of a Two-Color Pyrometer

Ignition and combustion of pyrotechnic mixtures in a closed volume were carried out by squib type devices. Measurements and numerical simulations in a previous study showed that the phenomenon is very fast with durations of about 200 μs. The objective of this work is to present temperature measurements obtained by means of a two-color pyrometer. A specific pyrometric device was built to carry out these measurements, analyzing the radiation emitted by the pyrotechnic flame. It was concluded that the phenomenon is primarily controlled by the emission of particles. The radiation peculiar to the gases, inside the flame, is negligible. From these assumptions, a method can be established enabling to process the voltage signals from the pyrometer. The measuring apparatus was validated for very short response times in the order of μs. The temperature measured this way seems to reach a constant level for the first 50 μs of combustion. The luminance signals then continue to increase and a decrease in the temperature which reaches approx. 3000 K is observed.

Linear Stability of a Plane Front in Solid Combustion at Large Heat of Reaction

The existence and linear stability of a uniformly propagating plane front in solid combustion are considered under the assumption of a large heat of reaction, which is of interest in pyrotechnic mixtures. The Zel'dovich number is not necessarily large, and the chemical reaction is not confined to a reaction sheet for typical values of the parameters. Thus the effective calculation of the fronts and the analysis of their stability rely on qualitative methods and on some numerics. It is seen that there is always a minimum burning rate and that the front is stable only if the heat of reaction is sufficiently large. A comparison is made with analytical results obtained from a phenomenological delta-function model.

Investigation of the thermophysical properties of metal - polytetrafluoroethylene pyrotechnic compositions

Thermophysical data (enthalpy, specific heat, thermal diffusivity, and density) for pyrotechnic mixtures have been determined. The materials under tests were pyrotechnic compositions containing a pure metal or a metal alloy and polytetrafluoroethylene (PTFE). The enthalpy and thermal characteristics of specific heat were studied by microcalorimetric (differential scanning calorimeter, DSC) measurements. Thermal diffusivity was investigated with a laser-flash apparatus. Results of investigations on three selected specimens containing PTFE with 76.7, 56.7, and 54.5 wt% Zn and 54.5 wt% Al 3 Mg 4 are discussed in detail; data obtained for nine other specimens are reported in brief.

Optimization of a Smoke Producer Composition by Experiment Design

This paper deals with the work carried out to define new formulations used to increase the infrared effectiveness of a reference smoke producer composition. This pyrotechnic mixture is constituted by a combination of oxidizing agents and fuels generating mainly carbon but also chlorinated metallic salts which are essential for camouflage by combustion. First, for several compositions, the 3–5 μm and 8–12 μm extinction coefficients are correlated to the proportions of Magnesium and (Mg+MgCl), respectively (determined by thermochemical computations with the Bagheera code). Then, to widen the investigations on smoke producing compositions, this correlation was studied by associating two experiment designs, an equiradial matrix and a Scheffe mixing matrix. We would point out that using this experimental strategy makes both possible, the minimization of the number of experiments – and therefore the cost factor – and the modelling of the results which allows to gain the properties of potential formulations on the whole experimental field.

Optimization of Interface Conditions in the Case of Laser Diode Ignition of Pyrotechnic Mixtures

Optimization of Interface Conditions in the Case of Laser Diode Ignition of Pyrotechnic Mixtures

The effect of different zirconium on thermal behaviors for Zr/KClO 4 priming composition

Zirconium powder is a powerful reducing agent and reactive with an oxidizer at elevated temperature to release enough heat to ignite pyrotechnic mixtures. The auto-ignition temperature, ignition temperature within 5 s of zirconium powder and the heat of explosion of the Zr/KClO 4 priming composition with different zirconium powders were investigated using thermal analysis techniques. The Zr/KClO 4 priming compositions, with different zirconium powders, were pressed into the change holder of a pressure cartridge. The firing characteristics of a pressure cartridge were evaluated by the Bruceton test and its statistical calculation. The data indicate that thermal behavior of Zr/KClO 4 priming composition were varied when using different zirconium powders.

Use of Moire Tomography to Measure the Temperature Field of the Flame of a Pyrotechnical Composition from Its Infrared Radiation

Measuring the temperature field of the flame of a burning pyrotechnical composition from its infrared radiation is a difficult problem because of the high luminosity and high concentration of the condensed‐phase particles. In the present work, moire deflection tomography was used to measure the instantaneous temperature field during combustion of a pyrotechnical mixture. The boundaries of the burning region were clearly observed. The distribution of the refractive index of the axisymmetric burning region was reconstructed using numerical inversion of the Abel integral equation, and the temperature distribution in the horizontal section was obtained. The maximum value of the flame temperature was determined.

Recording local fluctuations of physicochemical parameters in a combustion wave in condensed systems

A method for recording fluctuations of physicochemical parameters in a combustion wave in condensed systems is proposed that makes it possible to estimate the frequency of these fluctuations experimentally. Experiments with a Zr+WO3 pyrotechnic mixture using this method show that in the combustion wave there are fluctuation modes with frequencies ≈200 and ≈20 Hz corresponding to characteristic dimensions of heterogeneities of the system of order 0.1 and 1 mm, respectively.

A new mechanism for deflagration-to-detonation in porous granular explosives

An investigation has been carried out into the differences between the deflagration-to-detonation (DDT) process as it occurs in low density [∼30% theoretical maximum density (TMD)] columns of conventional grain size (∼180 μm) pentaerythritol tetranitrate (PETN) and in ultrafine PETN with a grain size ∼1 μm. The principle technique for observing the process utilized charges confined within a steel housing fitted with a polycarbonate slit window. This allowed direct recording of the transition using high speed streak photography. The explosive was thermally ignited using a pyrotechnic mixture with low gaseous emission to minimize any prepressurization of the charge. In addition to the photographic records of the events, the outputs of photodiodes along the length of the column were monitored in order to determine the rate at which the reaction proceeds. The results obtained show that the DDT process in the larger grain PETN at low density was similar in structure to the DDT process at higher densities. In contrast a different mechanism leads to detonation in columns composed of the smaller grain size PETN when packed to densities less than 50% TMD. After ignition hot gases propagate along the column both compacting and igniting material as they pass. After the gases have reached the downstream end of the column, the column continues to burn and the pressure and temperature increase. Some time later initiation takes place at a point along the burning column, and detonation waves propagate in both directions from this point. The detonation waves propagate from the initiation point at speeds that would normally be associated with material compacted to around 60% TMD. The process appears to be in effect a deflagration-to-localized thermal explosion detonation transition.

Construction and evaluation of ion selective electrodes for perchlorate with a summing operational amplifier: application to pyrotechnics mixtures analysis

The construction and evaluation of an electrode selective to perchlorate with improved sensitivity, constructed like a conventional electrode (ISE), but using an operational amplifier to sum the potentials supplied by four membranes (ESOA) is described. The two types of electrodes, without an inner reference solution, were constructed using octylammonium chloride as sensor, a mixture of dibutylphthalate and o-nitrophenyloctyl ether as solvent mediator and PVC as plastic matrix, the membranes being obtained by direct application onto a conductive epoxy resin support. After the comparative evaluation of their working characteristics they were used in the determination of perchlorate in propellants, fulminating powders and fireworks. On all occasions, the results obtained by the ESOA were found to be similar to those obtained by the conventional ISE, but with higher precision. The limit of detection of the direct potentiometric method developed was found to be 0.1 g kg–1, and the precision and accuracy of the method, when applied to eight different samples of pyrotechnic mixtures, expressed in terms of mean relative standard deviation and average percentage of spike recovery were 0.4% and 100.5%, respectively. Adequate agreement was found between the results obtained by the potentiometric method and the reference methods, since the calculated relative errors ranged from +0.4% to –0.5%.

Ignition of pyrotechnic mixture by means of a laser diode. Part II: Experimental study

AbstractAn experimental device using a laser diode is described. The laser diode is coupled with a 100 μm diameter optical fiber. Lasering times are about one ms and the energy of the pulse ranges from 0.5 mJ to 5 mJ. Output power provided by optical fiber is about 0.5 W and the wavelength of radiation is 820 nm. Before each shot the pulse energy is measured by means of a joule meter. It allows a very accurate determination of the amount of energy transferred to the solid explosive. A small reactor containing about 40 mg of explosive is used. It allows to initiate combustion with a direct contact between the output of an optical fiber and the solid explosive. Three pyrotechnic compounds were tested and tests have shown that it is possible to ignite Zr/KClO4 and Zr/PbCrO4 mixtures. Experimental results are discussed and compared with numerical simulations presented in Part I.

Prediction of shock response of pyrotechnic mixtures from thermal analysis

The thermal behavior of metal, oxide, and oxidizer mixtures, some with fluorine compound additions, has been studied in order to examine the influence of thermal properties on the initiation conditions for chemical reaction of pyrotechnic powders under dynamic loading conditions. The autoignition energies of the mixtures obtained from thermal analyses were compared with shock initiation energies, determined from planar shock measurements. Although some mixtures showed an approximate equivalency between the energies obtained from the two different experiments, the experimental results indicated that any comparison should be made with great care.

Ignition of pyrotechnic mixture by means of a laser diode. Part I: Numerical modelling

AbstractA numerical model of the heating and ignition of a reactive solid by a laser beam has been developed. A transient, two‐dimensional heat equation was solved numerically using an explicit scheme. A nonlinear source term (Arrhenius equation) complicates the analytical resolution of this type of problem. Laser beam absorption is considered in a few micrometer depth. Influence of depth absorption coefficient is investigated. Influence of laser power density, lasering time and thermal diffusivity on ignition are examined by this model. The developed numerical model has been used to design a laser ignition system for explosive substances. It has been shown that ignition by a 0.6 W laser diode at a fiber optics output is feasible.

Pressure measurements at the combustion front of gas-free pyrotechnic mixtures with low gas permeability

It is shown that the method of continuous measurement of pressure at the combustion front using an axial-force gauge is applicable to gas-free pyrotechnic systems producing solid reaction products and characterized by low gas permeability of specimens in the initial state. Small inclusions of a high-energy material producing gas combustion products are placed in the specimens to check the reliability of the result. These inclusions are located near the inlet to the main line going to the pressure gauge. The results of experiments on Ti+C+20% TiC specimens containing inclusions of pyroxylin powder are given.

The effect of heat and mass addition on the base pressure of bodies of revolution at supersonic speeds

The results of an experimental investigation of the influence of mass addition and afterburning of combustion products of pyrotechnic mixtures in the base region of axisymmetric bodies in supersonic air flow at Mach numbers of 1.2–3.0 are reported. It is shown that the base-pressure increment increases monotonically to a maximum value with an optimum injected-mass flow rate. The value of this increment reduces as the Mach number decreases. A dependence generalizing the experimental data is presented.

Deflagration-to-detonation transition in granular pentaerythritol tetranitrate

The deflagration-to-detonation transition process has been observed in pressed granular columns of the explosive pentaerythritol tetranitrate. Charges were confined within a steel housing which had been fitted with a polycarbonate window to allow direct recording by high-speed streak photography. The explosive was thermally ignited by a gasless pyrotechnic mixture to minimize pre-pressurization of the charge. The results indicate that upon ignition, early choked flow of the combustion gases prevents the continued propagation of the combustion via a convective heat transfer mechanism and that the propagation of reaction becomes governed by a leading compaction wave which causes ignition through the mechanical formation of hot spots. Detonation finally occurs when the leading front of an accelerating plug of highly compacted material (density close to the theoretical maximum) formed between the reaction front and the leading compaction wave, attains the critical pressure necessary for shock-to-detonation transition.

Detonation and Deflagration Properties of Pyrotechnic Mixtures

AbstractTheroretical calculation of detonation and deflagration properties of pyrotechnic mixtures have been performed including report charges and display charges. Calculation were performed with the KHT (Kihara-Hikta-Tanaka) code[l]. KHT results are compared with a modified version of the TIGER code[2] which allows calculation with 900 gaseous and 600 condensed product species at high pressure. Detonation properties computed by KHT and BKWS (Becker-Kistiakowskii-Wilson) give favorable agreement with experimental results of detonation velocity measurements. Hydrodynamic computation by one dimensional Lagrangian hydrodynamic code using the isentrope given by KHT constant volume explosion, indicated that experimental results for blast wave measurement for 30kg and 50kg of report charge were an incomplete reaction. Underwater detonation experiments with explosive charge of 25g, however, indicates a more energetic nature than the KHT prediction. This scale effect indicates complicated slow reactions and a number of condensed phase deflagration products of powder mixtures such as aluminum or titanium with oxidizers such as potassium perchlorate or nitrate salts as suggested by Hobbs et al[3].