Explosive characteristics of aluminized HMX-based nanocomposites

The explosive characteristics of HMX compositions doped with 15% Al (by weight) were studied experimentally. The detonation velocity, pressure and temperature profiles, the velocity of endwise acceleration of plates, and the heat of explosion of dense pressed samples were measured. The results were compared for compositions based on mechanical mixtures of initial micron-size particles of HMX with aluminum powders of various sizes and for nanocomposites. The addition of nanoaluminum reduces the detonation velocity to a greater degree than the addition of micron-size aluminum. The mechanical mixtures have close detonation velocities, whereas in composites containing different types of nanoaluminum, they differ by almost 200 m/sec. For all compositions, except for the most homogeneous nanocomposite, two-peak pressure profiles are observed. For charges of a composite and a mechanical mixture with nanoaluminum of the same type, the second peak pressures almost coincide but are reached in different times. At the same time, the peak pressure increases with decreasing aluminum particle size. The temperature profiles agree qualitatively with the pressure profiles. The velocity of endwise acceleration of plates depends linearly on the activity of the aluminum powder used. Nanocomposites and mechanical mixtures containing the same aluminum powder have close heats of explosion. Nanoaluminum is almost completely oxidized during calorimeter bomb tests, and the major factor determining the heat of explosion of the compositions with nanoaluminum is also the content of active metal in the aluminum powder. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 2, pp. 85–100, March–April, 2008.     

Combustion of Nanoaluminum and Water Propellants: Effect of Equivalence Ratio and Safety/Aging Characterization

The deflagration and combustion efficiency of 80 nm aluminum/ice (ALICE) mixtures with equivalence ratios of ϕ=1.0, 0.75, and 0.67 were experimentally investigated. We find that pressure exponent and burning rate vary little between these three mixtures, with the exponent varying only from 0.42 to 0.50 and burning rate at 6.9 MPa varying from 2.05 to 2.10 cm s⁻¹. However, reducing the equivalence ratio from 1.0 to 0.67 surprisingly increases combustion efficiency from 70 % to 95 % with unburned aluminum agglomerates visible in electron microscopy photographs of 70 % combustion efficiency (ϕ=1.0) products. Our findings suggest that nanoaluminum/water combustion is diffusionally limited for all conditions considered. Aging tests on the propellant show that storage at −30 °C essentially stops the Al/H₂O reaction such that little nanoaluminum degradation occurs after 200 days. Electrostatic discharge (ESD), shock initiation, and impact sensitivity tests indicate that the propellant is insensitive to ignition by these stimuli. Specifically, while neat nanoaluminum powders are highly ESD sensitive (ignition threshold 0.3–14 mJ), nAl/H₂O mixtures are insensitive to ESD and have ignition thresholds in excess of 400 mJ. Likewise, nAl/H₂O mixtures are insensitive to impact ignition, having an ignition threshold in excess of 2.2 m. Propellants containing 80 nm or larger average particle size aluminum were also found to be insensitive to shock initiation.     

爆炸热作用所致的铝粉颗粒温度响应

采用高速运动分析系统研究了硝基甲烷爆轰产物在空气中的膨胀飞散过程,并利用热渗透理论计算了硝基甲烷爆炸热作用下铝粉颗粒温度响应。结果表明,硝基甲烷液体炸药爆轰产物膨胀初期阶段对铝粉颗粒的热作用明显优于其爆轰区阶段的热作用;而在许多研究中往往忽略了爆轰区对铝粉热作用的影响,小尺寸铝粉颗粒在炸药爆轰区也能够迅速被加热至活化温度而可能参与化学反应。     

The Effect of Additives on the Burning Rate of Silicon‐Calcium Sulfate Pyrotechnic Delay Compositions

The effect of fuel particle size as well as the influence of inert and reactive additives on the burning rate of the Si‐CaSO₄ composition was evaluated. The burning rate decreased with increase in fuel particle size, while the enthalpy remained constant. Addition of fuels to the base composition increased the burning rate, with an increase from 12.5 mm ⋅ s⁻¹ to 43 mm ⋅ s⁻¹ being recorded upon 10 wt‐% Al addition. Ternary mixtures of silicon, calcium sulfate, and an additional oxidizer generally decreased the burning rate, with the exception of bismuth trioxide, where it increased. The Si‐CaSO₄ formulation was found to be sensitive to the presence of inert material, addition of as little as 1 wt‐% fumed silica stifled combustion in the aluminum tubes.     

Nanometric Aluminum in Explosives

Aluminum nanopowders, because of their larger surface area, can increase the burning rate of propellants. It has been suggested that the powders could also enhance the detonation properties of certain explosives. For these reasons, an experimental study was undertaken to compare the performance of nanometric and micrometric aluminum in various explosives. No enhancement of performance was found in plastic‐bonded explosives. In fact, a reduction of the detonation velocity was found in plastic‐bonded explosives that are based on an energetic binder system. No increase of the detonation velocity was found in mixtures of aluminum and either Composition B or Ammonium Nitrate Fuel Oil, but a small increase in the heat of detonation was measured. The mixture of TNT and nano‐aluminum demonstrated higher detonation velocities and heats of detonation. The increase was higher at small charge diameters. Nanometric aluminum was shown to reduce the critical diameter of such mixtures, and it is concluded that the nano‐aluminum reacts faster than regular micron‐size particles in TNT/Al compositions.     

Ignition and combustion of disperse and nanodisperse gas suspensions under dynamic conditions

Results of mathematical modeling of propagation of heterogeneous detonation waves in a mixture of fine aluminum particles in oxygen and ignition of fine metal particles in reflected and transmitted shock waves and high-temperature gas flows, which were obtained at the Khristianovich Institute of Theoretical and Applied Mechanics of the Siberian Branch of the Russian Academy of Sciences, are briefly reviewed. One-dimensional steady flows and one-dimensional and two-dimensional unsteady flows of reactive mixtures are considered. Flows in the form of weak and overdriven detonation waves and the Chapman-Jouguet detonation are found. Stability of the Chapman-Jouguet flow and weak detonation to interactions with rarefaction waves is demonstrated. Regimes of strong and weak initiation are determined; the calculated and experimental values of the initiation energy are found to be in reasonable agreement. Laminar detonation and cellular detonation for monodisperse and polydisperse mixtures are found within the framework of two-dimensional unsteady detonation flows. New numerical codes are developed for solving nonlinear-boundary-value problems of ignition of individual particles. Particular attention is paid to the accuracy of reproduction of integral parameters (burning time) of some experimental data, depending on the ambient temperature, etc. A solution of a single-phase Stefan problem of nanosized particle melting is described.     

Detonation sensitivity of liquid nitromethane doped with methyl nitrite

It has been suggested, by other workers, that the isomerization of normal nitromethane (NM CH₃NO₂) to methyl nitrite (CH₃ONO) is an important first step in the chemical kinetics of liquid NM detonation. We examine this idea by studying the effect of direct methyl nitrite addition on NM's detonation sensitivity. Failure diameter is used as the measure of sensitivity. A comparison is made between the effect of methyl nitrite addition and that of a known sensitizer of NM i.e., the NM aci ion (CH₂NO₂⁻). Methyl nitrite is found to be an ineffective sensitizer relative to NM's aci ion.     

Explosive generation of heterogeneous detonation waves in aerocolloids of a unitary fuel

The process of explosive initiation of spherical, heterogeneous detonation waves in monodisperse, homogeneous aerocolloids of a unitary fuel is modeled mathematically. It is shown that regimes of decaying and detonating combustion of the reacting disperse mixtures are possible, depending on the initial mass concentration and initial particle size in the mixture. It is established that the laws governing the mass transfer of burning particles have a significant influence on the patterns of explosive generation and propagation of detonation waves in unitary fuel aerocolloids. The critical (maximum) diameter of the unitary fuel particles, below which the shock initiation of heterogeneous combustion is possible, is determined as a function of the particular value of their relative mass concentration in the mixture.Institute of Mechanics of Multiphase Systems, Siberian Branch of the Russian Academy of Sciences. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 3, pp. 83–91, May–June, 1995.     

On the Oxidation and Combustion of AlH3 a Potential Fuel for Rocket Propellants and Gas Generators

Aluminum hydride is a promising candidate for application in energetic materials and hydrogen storages. E.g. an AP/HTPB rocket propellant filled with alane was calculated for a 100 N s kg⁻¹ higher specific impulse compared to the same concentration of aluminum. Different investigations on α‐AlH₃ polyhedra using thermoanalytical methods and X‐ray diffraction were performed to receive a better understanding of dehydration at about 450 K, passivation of the remaining porous aluminum particles and further oxidation. A modeling approach to describe these conversions including diffusion processes, Avrami‐Erofeev mechanism and Arrhenius type reaction steps of n‐th order were introduced. Results were discussed in comparison to experimental investigations under pressure with model propellants on the base of gelled pure nitromethane and also filled with alane or pure aluminum in concentrations of 5%, 10% and 15%. Both alane and aluminum increase the burning rate on a factor of two correlated with a temperature increase up to 500 K and more. A mesa burning effect at 6 to 10 MPa was indicated by the mixtures with alane.     

Axisymmetric expanding heterogeneous detonation in gas suspensions of aluminum particles

A problem of expansion of heterogeneous detonation in a suspension of aluminum particles in gaseous oxygen from a circular tube and its propagation in a semi-bounded or unbounded space is studied by numerical methods. The effects of the particle diameter in monodisperse suspensions and of the composition of bidisperse suspensions on detonation propagation regimes are studied. The calculated results are compared with data on heterogeneous detonation of gas suspensions in a plane channel and on gas detonation. The critical values of the channel width and the tube diameter are found to differ by a factor of 2–2.5, as it is also observed in gas detonation. However, the ratio of the critical diameter to the detonation cell size in the case of heterogeneous detonation can be smaller than that in gas mixtures by an order of magnitude.     

Rheology and curing kinetics of fumed silica/cyanate ester nanocomposites

A low‐viscosity bisphenol E cyanate ester (BECy) monomer was combined with fumed silica with average primary particle diameters of 12 and 40 nm to form high‐temperature adhesives with processability at ambient temperatures. Rheological evaluation revealed that for silica loadings below 15 vol%, suspensions of both particle sizes exhibited shear thinning and thixotropic behavior. Samples with high silica loadings (>15 vol%) of 40‐nm silica also showed intense shear thickening at shear rates above 10 s^?1. Thixotropy was most pronounced for the 12‐nm silica, but the formation of a gel was slow, indicating that the polar nature of the BECy monomer was responsible for disrupting hydrogen bonds between silica particles. Rheokinetic evaluation of catalyzed samples showed that increasing silica content reduced gel time and increased gel viscosity, and this effect was most pronounced for the 12‐nm silica. Differential scanning calorimetry confirmed that the silica's hydroxyl groups have a minor catalytic effect on the polymerization kinetics, such that the activation energies of the catalyzed suspensions were decreased with increased nanoparticle loading and decreased particle size. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Particle Sizes of Fumed Silica

Fumed silica is a synthetic amorphous silicon dioxide produced by burning silicon tetrachloride in an oxygen-hydrogen flame. Surface areas range from 50–400 m²/g. Using particle sizing techniques, fumed silica shows micron sized particles leading to surface areas markedly lower than expected. Fumed silica appears as a fluffy solid with bulk densities down to 0.03 g/cm³, being invariant over the wide range of surface areas. Attempts to relate the variation of the surface area directly to the performance of fumed silica in technical applications, such as its thickening efficiency in fluids, mainly fail and remain ambiguous.     

Rheological behavior and network dynamics of silica filled vinyl‐terminated polydimethylsiloxane suspensions

Present article reports the rheological properties and network dynamics of fumed silica filled vinyl‐terminated polydimethylsiloxane suspensions. The results reveal that as filler loading increases, the span of the linear viscoelastic region with constant dynamic storage modulus is narrowed with increase in strain amplitude while the relaxation time of the compounds get shifted to longer time scales. The dynamics of filler‐network indicated significant Payne effect due to fumed silica incorporation into the PDMS matrix. Further, strain‐induced agglomeration of fumed silica particles, characterized by a peak in the dynamic loss modulus curve could also be observed. High loss‐tangent was observed for lower contents of filler in the suspension, an effect with an apparent relationship to the loosely formed filler‐network. The formation of a saturated network structure of fumed silica particles was evident from the dynamic modulus and complex viscosity data, that remained unaffected with frequency till a critical amount of fumed silica loading. Han plots (storage modulus versus loss modulus) revealed the microstructural changes for various filled systems that was attributed to build‐up of the filler‐network causing an apparent evolution of yielding phenomenon. Van Gurp‐Palmen plots (complex modulus versus phase lag) showed that flow behavior of the filled PDMS suspensions resembled to that of typical viscous fluids. POLYM. ENG. SCI., 57:973–981, 2017. © 2016 Society of Plastics Engineers     

Manganese as Fuel in Slow‐Burning Pyrotechnic Time Delay Compositions

Manganese metal was evaluated as a fuel for slow‐burning delay compositions press‐filled in aluminium or compaction‐rolled in lead tubes. Oxides of antimony, bismuth, copper, manganese and vanadium were considered as oxidants. Measured burn rates for binary mixtures varied between 5 and 22 mm s⁻¹ but slower burning ternary and quaternary compositions were also found. The addition of fumed silica to the Mn/MnO₂ system had little effect on the propagation rate but a low level addition of hollow glass sphere significantly reduced the burn rate. Mn MnO₂ mixtures showed reliable burning over a wide stoichiometric range. In this system the fuel and the oxidant share a common metal. They combine to form the more stable intermediate oxide (MnO) releasing considerable quantities of heat in the process.     

Detonation and Blast Wave Characteristics of Nitromethane Mixed with Particles of an Aluminium–Magnesium Alloy

Investigation of detonation parameters, blast wave characteristics and quasi‐static pressures (QSPs) for the mixtures of nitromethane and particles of an aluminium and magnesium (Al₃Mg₄) alloy was carried out. The mixtures of gelled nitromethane containing 15–60 wt.‐% Al Mg alloy were tested. Detonation velocity and Gurney energy were determined. Parameters of blast waves produced by charges of the investigated explosives were measured. QSP measurements were conducted in a steel chamber of 0.15 m³ volume filled with air. Thermochemical and gasdynamical calculations were also performed. The degree of combustion of the metallic addition with the gaseous products during detonation and expansion is discussed.     

Exit of a Heterogeneous Detonation Wave into a Channel with Linear Expansion. II. Critical Propagation Condition

Propagation of a detonation wave in monodisperse suspensions of reacting particles (based on the model of the suspension of aluminum particles in oxygen) in channels with linear expansion is studied within the framework of mechanics of heterogeneous reacting media. Reduced kinetics is described with allowance for the transitional (from diffusion to kinetic) regime of combustion of micron-sized and submicron-sized spherical aluminum particles. The effects of the channel width, particle diameter, and expansion angle on propagation conditions and detonation regimes are determined. The critical channel width is found to be a nonmonotonic function of the expansion angle, which is associated with qualitatively different wave patterns behind an oblique step. Flow charts are constructed, and the results are compared with solutions of problems of heterogeneous detonation wave propagation in channels with a backward-facing step and with sudden expansion.     

Mathematical modeling of detonation suppression in a hydrogen-oxygen mixture by inert particles

The paper addresses the problem of searching for methods that can control, suppress, and attenuate explosive and detonation processes in homogeneous and heterogeneous media (mixtures of reactive gases and inert species). The analysis is performed by analytical and numerical methods. The problem of detonation suppression in a mixture of reactive gases and inert species (argon and sand particles) in a one-dimensional unsteady flow is formulated, and its solution is given. The effect of the particle diameter and concentration on the detonation velocity is determined; the parameters of the detonation wave in a stoichiometric hydrogen-oxygen mixture diluted by a chemically inert gas (argon) and particles is determined. The influence of the initial parameters of the mixture on the possibility of detonation suppression by inert particles is studied. It is shown that the detonation velocity substantially decreases with increasing volume fraction of particles. A decrease in the particle size with an unchanged volume fraction is also found to reduce the detonation velocity.     

Synergistic effects of fumed silica on intumescent flame‐retardant polypropylene

The synergistic effects of fumed silica on the thermal and flame‐retardant properties of intumescent flame retardant (IFR) polypropylene based on the NP phosphorus‐nitrogen compound have been studied by Fourier transfer infrared (FTIR) spectroscopy, cone calorimeter test (CCT), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), limiting oxygen index (LOI), and UL‐94 tests. The LOI and UL‐94 data show that when ≤1 wt % fumed silica substituted for the IFR additive NP can increase 2 to 4% LOI values of the PP blends and keep the V‐0 rating. The data obtained from the CCT tests indicate the heat release rates (HRR) reduce by about 23% for the PP/NP sample with 0.5 wt % fumed silica, whereas the mass loss rates (MLR) and total heat release (THR) values are much lower than those of the PP/NP samples without fume silica. The TGA data demonstrate that a suitable amount of fumed silica can increase the thermal stability and charred residue of the PP/IFR/SiO₂ blends after 500°C. The morphological structures of charred residues observed by SEM give positive evidence that a suitable amount of fumed silica can promote the formation of compact intumescent charred layers and prevent the charred layers from cracking, which effectively protects the underlying polymer from burning. The dynamic FTIR spectra reveal that the synergistic flame‐retardant mechanism of a suitable amount of fumed silica with IFR additive is due to its physical process in the condensed phases. However, a high loading of fumed silica restricts the formation of charred layers with P? O? P and P? O? C complexes formed from burning of polymer materials and destroys the swelling behavior of intumescent charred layers, which deteriorates the flame retardant and thermal properties of the PP/IFR blends. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Preparation and characterization of novel polydimethylsiloxane composites used POSS as cross‐linker and fumed silica as reinforcing filler

A series of novel polydimethylsiloxane (PDMS) composites were prepared using octa[(trimethoxysilyl)ethyl]‐POSS (OPS) as cross‐linker and fumed silica as reinforcing filler. The cross‐linked networks, morphologies, thermal and mechanical properties of these novel PDMS composites were examined by attenuated total reflection infrared spectroscopy and the extraction/swelling experiment, scanning electron microscope, thermogravimetric analysis, and universal tensile testing machine, respectively. It was found that both the resistance to thermal degradation and mechanical properties of the novel PDMS composites were improved greatly by adding fumed silica. The prominent improvements in resistance to thermal degradation and mechanical properties of novel PDMS composites were likely attributed to the enhanced interaction of PDMS chains and aggregated particles resulted from synergistic effect between POSS and fumed silica. Meanwhile, we also found that the resistance to thermal degradation of the PDMS composites was lowered slightly with the further increment in loading fumed silica, but their mechanical properties were enhanced. The slight decrease in trend of the resistance to thermal degradation of the novel PDMS composites was likely ascribed to the increasing amount of hydroxyl groups resulting from fumed silica. And the improving mechanical properties were mainly attributed to the increasing interaction of PDMS chains and aggregated particles originated from synergistic effect between POSS and fumed silica. POLYM. COMPOS., 34:1041–1050, 2013. © 2013 Society of Plastics Engineers     

Agglomeration Characteristics of Aluminum Particles in AP/AN Composite Propellants

Most solid rockets are powered by ammonium perchlorate (AP) composite propellant including aluminum particles. As aluminized composite propellant burns, aluminum particles agglomerate as large as above 100 μm diameter on the burning surface, which in turn affects propellant combustion characteristics. The development of composite propellants has a long history. Many studies of aluminum particle combustion have been conducted. Optical observations indicate that aluminum particles form agglomerates on the burning surface of aluminized composite propellant. They ignite on leaving the burning surface. Because the temperature gradient in the reaction zone near a burning surface influences the burning rate of a composite propellant, details of aluminum particle agglomeration, agglomerate ignition, and their effects on the temperature gradient must be investigated. In our previous studies, we measured the aluminum particle agglomerate diameter by optical observation and collecting particles. We observed particles on the burning surface, the reaction zone, and the luminous flame zone of an ammonium perchlorate (AP)/ammonium nitrate (AN) composite propellant. We confirmed that agglomeration occurred in the reaction zone and that the agglomerate diameter decreased with increasing the burning rate. In this study, observing aluminum particles in the reaction zone near the burning surface, we investigated the relation between the agglomerates and the burning rate. A decreased burning rate and increased added amount of aluminum particles caused a larger agglomerate diameter. Defining the extent of the distributed aluminum particles before they agglomerate as an agglomerate range, we found that the agglomerate range was constant irrespective of the added amount of aluminum particles. Furthermore, the agglomerate diameter was ascertained from the density of the added amount of aluminum particles in the agglomerate range. We concluded from the heat balance around the burning surface that the product of the agglomerate range and the burning rate was nearly constant irrespective of the added amount of aluminum particles. Moreover, the reduced burning rate increased the agglomerate range.