Metal-based reactive nanomaterials

Recent developments in materials processing and characterization resulted in the discovery of a new type of reactive materials containing nanoscaled metal components. The well-known high oxidation energies of metallic fuels can now be released very rapidly because of the very high reactive interface areas in such metal-based reactive nanomaterials. Consequently, these materials are currently being examined for an entire range of applications in energetic formulations inappropriate for conventional, micron-sized metal fuels having relatively low reaction rates. New application areas, such as reactive structural materials, are also being explored. Research remains active in manufacturing and characterization of metal-based reactive nanomaterials including elemental metal nanopowders and various nanocomposite material systems. Because of the nanometer scale of the individual particles, or phase domains, and because of the very high enthalpy of reaction between components of the nanocomposite materials, the final phase compositions, morphology, and thermodynamic properties of the reactive nanocomposite materials may be different from those of their micron-scaled counterparts. Ignition mechanisms in such materials can be governed by heterogeneous reactions that are insignificant for materials with less developed reactive interface areas. New combustion regimes are being observed that are affected by very short ignition delays combined with very high metal combustion temperatures. Current progress in this rapidly growing research area is reviewed and some potential directions for the future research are discussed.     

Application of Cerimetric Methods for Determining the Metallic Aluminum Content in Ultrafine Aluminum Powders

Ultrafine (nano‐sized) aluminum is a promising component for solid propellants, gel propellants, explosives, etc. The work is focused upon a problem of determining active (metallic) aluminum content in nanoAl powders as one of the characteristics of their reactivity. It is shown that the high reactivity, the presence of either the gases adsorbed or coating matter on the particle surface restricts the traditionally used permanganatometric and volumetric analytic methods. A new technique determining the active aluminum content is presented. This is the adaptation of known cerimetric method based on the analytical reaction Ce⁴⁺+e⁻=Ce³⁺. The method can be applied for analysis of metallic aluminum in the probes of nanoAl (including ones with organic coating), as well as micron sized aluminum, and their condensed combustion products.     

Combustion wave speeds of nanocomposite Al/Fe2O3: the effects of Fe2O3 particle synthesis technique

Combustion wave speeds of nanoscale aluminum (Al) powders mixed with iron oxide (Fe₂O₃) were measured as a function of Fe₂O₃ synthesis technique and fuel/oxidizer composition. Three reactant synthesis techniques were examined; two focus on sol–gel processing of nanoscale Fe₂O₃ particles and the third utilizes commercially available nanoscale Fe₂O₃ powder. Nanoscale aluminum particles (52 nm in diameter) were combined with each oxidizer in various proportions. Flame propagation was studied by igniting low-density mixtures and taking data photographically with a high-speed camera. Both open and confined burning were examined. Results indicate that the combustion wave speed is a strong function of the stoichiometry of the mixture and a slightly fuel-rich mixture provides an optimum combustion wave speed regardless of oxidizer synthesis technique. Oxidizers processed using sol–gel chemistry originally contained impurities which retarded the combustion wave speeds. When the same oxidizers are annealed at moderate temperatures, the new heat-treated oxidizer shows a dramatic improvement, with combustion wave speeds on the order of 900 m/s.     

Effect of Nano‐Aluminum and Fumed Silica Particles on Deflagration and Detonation of Nitromethane

The heterogeneous interaction between nitromethane (NM), particles of nanoscale aluminum (38 and 80 nm diameter), and fumed silica is examined in terms of the deflagration and detonation characteristics. Burning rates are quantified as functions of pressure using an optical pressure vessel up to 14.2 MPa, while detonation structure is characterized in terms of failure diameter. Nitromethane is gelled using fumed silica (CAB‐O‐SIL^®), as well as by the nanoaluminum particles themselves. Use of nanoaluminum particles with fumed silica slightly increases burning rates compared to the use of larger diameter Al particles; however distinct increases in burning rates are found when CAB‐O‐SIL is removed and replaced with more energetic aluminum nanoparticles, whose high surface area allows them to also act as the gellant. Mixtures including fumed silica yield a reduced burning rate pressure exponent compared to neat NM, while mixtures of aluminum particles alone show a significant increase. Failure diameters of mixture detonations are found to vary significantly as a function of 38 nm aluminum particle loading, reducing more than 50% from that of neat nitromethane with 12.5% (by mass) aluminum loading. Failure diameter results indicate a relative minimum with respect to particle separation (% loading) which is not observed in other heterogeneous mixtures.     

Combustion of nanoaluminum at elevated pressure and temperature behind reflected shock waves

This study presents experimental measurements on the combustion of nanoaluminum particles behind reflected shock waves in a shock tube. These experiments were performed at elevated pressures (4-32 atm) and temperatures (1200-2100 K) in the oxidizers oxygen and carbon dioxide, with nitrogen also present. The light emission from the reacting particles was monitored. For all cases, a brief period of intense light emission was observed soon after exposure to the reflected shock conditions. The time scales of this emission event are quantified by the 10-90% integrated emission intensity method to yield a reaction time for this rapid exothermic process. The duration of the emission is found to be 50-500 μs for the conditions tested here. Reaction times in 50% O₂ and 50% N₂ were shown to decrease significantly with ambient temperature, with Arrhenius-type exponentials fitting reasonably well to the observed experimental data. The reaction times were also dependent on pressure, with the timescales decreasing by 1.6-4 times as the pressure was increased from 8 to 32 atm over the range of temperatures in the experiments. In 50% CO₂ and 50% N₂, the reaction occurs in two sequential stages, with more of the emission at earlier times under higher-temperature conditions. Particle temperatures were also measured. During the bright emission event, the temperature rises above the ambient and then cools to near the ambient as the emission event ends. The peak temperature of the particle varied with ambient temperature, pressure, and oxidizer, with high ambient temperatures (2000 K), high pressures (32 atm), and high oxygen mole fractions (50%) giving the highest particle temperatures (∼3500 K). Conversely, 50% CO₂ atmospheres produced particle temperatures just slightly above the ambient. The spectral output of the light emission was shown to be dominated by broadband emission. At high temperatures and pressures in oxygen, weak emission from the AlO B-X transition was observed.     

Combustion characteristics of nanoaluminum, liquid water, and hydrogen peroxide mixtures

An experimental investigation of the combustion characteristics of nanoaluminum (nAl), liquid water (H₂O_(l)), and hydrogen peroxide (H₂O₂) mixtures has been conducted. Linear and mass-burning rates as functions of pressure, equivalence ratio (Φ), and concentration of H₂O₂ in H₂O_(l) oxidizing solution are reported. Steady-state burning rates were obtained at room temperature using a windowed pressure vessel over an initial pressure range of 0.24 to 12.4 MPa in argon, using average nAl particle diameters of 38 nm, Φ from 0.5 to 1.3, and H₂O₂ concentrations between 0 and 32% by mass. At a nominal pressure of 3.65 MPa, under stoichiometric conditions, mass-burning rates per unit area ranged between 6.93 g/cm² s (0% H₂O₂) and 37.04 g/cm² s (32% H₂O₂), which corresponded to linear burning rates of 9.58 and 58.2 cm/s, respectively. Burning rate pressure exponents of 0.44 and 0.38 were found for stoichiometric mixtures at room temperature containing 10 and 25% H₂O₂, respectively, up to 5 MPa. Burning rates are reduced above ∼5 MPa due to the pressurization of interstitial spaces of the packed reactant mixture with argon gas, diluting the fuel and oxidizer mixture. Mass burning rates were not measured above ∼32% H₂O₂ due to an anomalous burning phenomena, which caused overpressurization within the quartz sample holder, leading to tube rupture. High-speed imaging displayed fingering or jetting ahead of the normal flame front. Localized pressure measurements were taken along the sample length, determining that the combustion process proceeded as a normal deflagration prior to tube rupture, without significant pressure buildup within the tube. In addition to burning rates, chemical efficiencies of the combustion reaction were determined to be within approximately 10% of the theoretical maximum under all conditions studied.     

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.