Several experiments were conducted to improve our understanding of the properties of aerosol particles generated by detonation of conventional explosive and explosives prepared from nanophase materials. Initial number concentrations (∼ 106−107 cm−3) of particles produced by detonations of the nano-explosives were comparable to that produced by conventional explosive. In general, data taken by a time-of-flight aerodynamic sizer and a scanning differential mobility analyzer for the first sample indicate a multi-modal distribution that there were a peak between 0.7 and 0.9 μm, and one and/or two peaks smaller than 100 nm depending upon the explosive used. The material properties and formulation of the explosive appear to play a significant role in the enhanced particle growth and increased deposition velocity leading to a higher reduction rate of total particle concentrations. Furthermore, the high level of ultrafine particles and nanoparticles in addition to the enriched toxic metals, the biological properties (e.g., the cellular toxicity) of the detonation particles need to be investigated in the near future.

Oxidation of Al-Ti mechanical alloys with Ti concentrations from 5 to 25 at% was studied and compared to the oxidation of Al powders using thermal analysis in the temperature range of 300–1500 °C in oxygen. Differential scanning calorimetry (DSC) and differential thermal analysis (DTA) with simultaneous thermo-gravimetric analysis (TGA) were used to monitor phase changes and oxidation reactions. Intermediate reaction products were recovered at different temperatures and analyzed using x-ray diffraction (XRD) and scanning electron microscopy (SEM). Oxidation of all samples occurred stepwise. The temperature of the first oxidation step for Al-Ti mechanical alloys correlates with the exothermic formation of the Al3Ti intermetallic at T>700 °C and has a higher rate and greater degree of oxidation than the first oxidation step observed for pure aluminum. The second and third oxidation steps in mechanical alloys and aluminum occur at higher temperatures and both appear to be controlled by changes in the permeability of the Al2O3 films. The effect of the Al2O3 film becomes less noticeable at increased Ti concentrations.

It has been well documented that the reactivity of nano-powders as well as bulk properties of materials derived from nano-particulates might be significantly different than those obtained from micron-sized grains. This paper addresses several aspects related to the characterization of coated and uncoated aluminum nanopowders, mixing of binary nanopowders, and the reactivity of nanosize aluminum with copper oxide under unconfined and confined conditions. It was found that the volumetric method is more versatile method than TGA or calorimetry for determination of reactive aluminum in coated aluminum nanopowders. It was shown that wet mixing in the presence of dispersants is essential to obtain mixtures with a high level of concentration uniformity. The combustion front velocity can be increased both by using an excess of aluminum in MIC systemand by coating of aluminum nanopowders or addition of dispersants during wet mixing. Protective organic coating with hydrophobic groups is essential to protect aluminum nanopowder from the reaction with moisture at higher relative humidity levels.

Advancements in nanotechnology for material processing have spurred the development of superalloys that provide improved protection against corrosion and wear. Nano-scale reactant particles offer unique thermal properties and increased homogeneity that may improve the microstructural features and macroscopic properties of the final product. In this study up to 10-wt% nano-scale molybdenum tri-oxide (MoO3) particles were added to micron scale nickel (Ni) and aluminum (Al). The goal was to produce a superalloy by generating excessively high heating rates and adding an oxidizer that would produce small quantities of Al2O3 (a strengthening agent) within the microstructure of the alloy. Experiments were performed on pellets pressed to 60% theoretical maximum density. Ignition and flame propagation were examined using a CO2 laser and imaging diagnostics that include a copper-vapor laser coupled with a high-speed camera. Product microstructure was examined using scanning electron microscopy. Abrasion testing was performed to evaluate the wear resistance properties of the superalloy. Results show that adding MoO3 increases the flame temperature and produces greater ignition sensitivity. Also, small quantities of MoO3 produce a more homogeneous microstructure and increase the overall wear resistance of the product.

The structural properties of several nanoparticles of 2,4,6,8,10,12-hexanitrohexaazaiso-wurtzitane, HNIW or CL-20, are studied by using molecular dynamics simulations. The internal structure of the CL-20 molecule is held rigid and the intermolecular interactions in the nanoparticles are taken from a previously developed force field. [Sorescu et al., J. Phys. Chem. B, 102, 948 (1998)] Molecular dynamics simulations of solid-like and annealed nanoparticles with 48 and 88 CL-20 molecules have been carried out in the solid-state range of temperatures from 50 to 500 K. The center-of-mass to center-of-mass radial distribution functions, dipole-dipole correlation function, the orientations of the surface dipoles, and the density of the nanoparticles were calculated at fixed temperatures for the nanoparticles.

As part of a cooperation between several TNO institutes, including TNO Prins Maurits Laboratory, recently a new initiative on nanotechnology was started. The research subjects within this initiative can be roughly divided into two areas: (1) Instrumentation for analysis and manufacture at nano-scale and (2) Nanoscale engineering techniques to create materials and components (including their applications). Currently the research at TNO Prins Maurits Laboratory is focusing on the application of reactive nanomaterials to decontaminate surfaces from e.g. bacteria or toxic chemicals, the use of plasma's to generate nanomaterials like carbon nanotubes, and the application and characterization of nanomaterials in energetic formulations (e.g. explosives, propellants and pyrotechnic compositions). In this paper results on the latter subject will be presented in more detail. Also results will be included of other research projects involved with energetic/reactive nanomaterials.

The characteristics of TNT (trinitrotoluene) crystals in the fracture surface of Composition B (a melt-cast mixture of TNT and RDX) have been studied using atomic force microscopy (AFM). The size of TNT crystals has been examined by analyzing the surface structure that is exhibited after mechanical failure of the Composition B. The failure occurs when the material is subjected to high acceleration in an ultracentrifuge and the shear or tensile strength is exceeded. AFM examination of the topography of the Composition B fracture surface reveals fracture across columnar grains of the TNT. The width of the columnar TNT grains ranges in size from ∼ 1 μm to ∼ 2 μm. Their height ranges in size from ∼ 50 nm to ∼ 300 nm. Flat TNT columns alternate with TNT columns containing river patterns that identify the direction of crack growth. Steps in the river patterns are a few nanometers in depth. The TNT constitutent fracture surface morphology is shown to occur on such fine scale, beginning from adjacent columnar crystals only 1–2 μm in width, and including river marking step heights of only a few nanometers, that AFM-type resolution is required.

An energetic material has been routinely manufactured from nano-metric powders of aluminum (Al) and molybdenum trioxide (MoO3). When optimized, the burn-rate of these materials (∼400 m/s) exceeds that of conventional thermites (based on micron-sized powders), but is less than that of conventional explosives. Similar burn-rates around 350 m/s are measured for these “super-thermites” using n-Al powder in the size range between 30 and 90 nm in diameter (20–60 m2/g, 60–80 wt%Al) and an oxygen to fuel (O/F) mass ratio of 1.4. The burn-rate decreases when the surface area of the MoO3 is decreased from 64 to 40 m2/g, or when O/F is changed from 1.2. Thus, for each average particle diameter, there is an optimum burn-rate at an O/F ratio that depends on the wt%Al present in the material and the particle size distribution of the powder. The burn-rate is dependent on several materials and processing factors such as the quality of the nano-metric ingredients, the processing method, and exposure to air and light, so the effect of aging and environmental exposure on the individual ingredients has been investigated. The results of this powder aging study suggests that the surface area of n-MoO3 can decrease two-fold within 10–12 days, and the Al-metal content in n-Al can decrease as much as 50% over two years. Adequate handling and storage procedures must therefore be followed for the effective use of nano-metric powders and their super-thermite mixes.

We report the catalytic effect on the synthesis of multiwalled carbon nanotubes (CNTs). The CNTs were grown vertically aligned on the iron (Fe), cobalt (Co), and nickel (Ni) catalytic nanoparticles deposited on alumina substrates by thermal chemical vapor deposition (CVD) of acetylene in the temperature range 900–1000 °C. We also synthesized them on the silicon oxide substrates by pyrolyzing iron phthalocyanine (FePc), cobalt phthalocyanine (CoPc), and nickel phthalocyanine (NiPc) at 700–1000 °C. In both syntheses, the CNTs grown using Fe exhibit about 2 times higher growth rate than those using Co and Ni. As the temperature rises from 700 to 1000 °C, the growth rate of CNTs increases by a factor of 45. The Arrhenius plot of growth rates provides the activation energy 30 ± 3 kcal/mol for all three catalysts, which is similar with the diffusion energy of carbon in bulk metal. It suggests that the bulk diffusion of carbon would play a decisive role in the growth of CNTs. The diameter of CNTs is in the range of 20–100 nm, showing an increase with the temperature. As the diameter is below 30 nm, the CNTs usually exhibit a cylindrical structure. The CNTs were intrinsically doped with the nitrogen content 2–6 atomic%. The degree of crystalline perfection of the graphitic sheets increases with the temperature, but depends on the catalyst and the nitrogen content. The graphitic sheets of CNTs grown using Fe are better crystalline than those grown using Co and Ni. As the nitrogen content increases, the degree of crystalline perfection decreases and the structure becomes the bamboolike structure probably due to a release of strains.

In the field of composite energetic materials, properties such as ingredient distribution, particle size, and morphology, affect both sensitivity and performance. Since the reaction kinetics of composite energetic materials are typically controlled by the mass transport rates between reactants, one would anticipate new and potentially exceptional performance from energetic nanocomposites. We have developed a new method of making nanostructured energetic materials, specifically explosives, propellants, and pyrotechnics, using sol-gel chemistry. A novel sol-gel approach has proven successful in preparing metal oxide/silicon oxide nanocomposites in which the metal oxide is the major component. Two of the metal oxides are tungsten trioxide and iron(III) oxide, both of which are of interest in the field of energetic materials. Furthermore, due to the large availability of organically functionalized silanes, the silicon oxide phase can be used as a unique way of introducing organic additives into the bulk metal oxide materials. As a result, the desired organic functionality is well dispersed throughout the composite material on the nanoscale. By introducing a fuel metal into the metal oxide/silicon oxide matrix, energetic materials based on thermite reactions can be fabricated. The resulting nanoscale distribution of all the ingredients displays energetic properties not seen in its microscale counterparts due to the expected increase of mass transport rates between the reactants. The synthesis and characterization of these metal oxide/silicon oxide nanocomposites and their performance as energetic materials will be discussed.

This paper reviews the techniques that have been developed at the Cavendish Laboratory for the study of the mechanical and ignition properties of energetic materials.

Ultrafine aluminum powders have been identified as very promising fuels as part of energetic materials formulations. However, the large specific surface area that gives these powders a high reactivity also makes them particularly difficult to maintain in an unoxidized state. They also agglomerate easily during compounding processes. The coating of the particles by a polymer could offer a solution to these problems. We carried out the in-situ synthesis of high density polyethylene on the surface of aluminum nanoparticles by the Ziegler-Natta process. The process was found to be flexible enough to control the amount of polyethylene grafted on the powders. Ultrasounds were applied during the polymerization process to enhance the dispersion of the particles in the solvent. The resulting characteristics of the coated powders are discussed by means of thermogravimetric characterizations and electronic microscopy.

Continual advances in laser technology lead to shorter pulses and higher energies. As the duration of a laser pulse shortens, different physical mechanisms become important in determining the thermo-mechanical response of an absorbing particle. These thermo-mechanical responses fall into the general category of thermal heating (temperature rise), vaporization, and shock wave formation. Our theoretical work has produced a computational model that allows the quantitative calculation of all of these responses for a laser of any pulse duration or energy, absorbed by a particle of any size. We find that for relatively long pulses, particle damage occurs most easily, i.e. at the least pulse energy, due to thermal effects. As the pulse duration shortens, explosive vaporization can dominate as the primary damage mechanism. For short pulses, shock wave production becomes the dominant damage mechanism. We describe how the relative terms of “short” and “long” pulse duration can be determined from knowledge of the thermo-mechanical properties of the absorber. Conversely, when the thermo-mechanical properties are not known, we explain how our theoretical work leads suggests an experimental technique that allows measurement of these absorber properties. This technique is applicable to extremely small particles that present difficulties for thermo-mechanical measurements. Finally, we show computational evidence of chaotic behavioral response of the absorber. This results in some laser pulse durations and energies that cause anomalously small shock waves, whereas other durations and energies cause surprisingly large and damaging responses.

The utilization of sol-gel chemical methodology to prepare nanostructured energetic materials as well as the concepts of nanoenergetics is described. The preparation and characterization of two totally different compositions is detailed. In one example, nanostructured aerogel and xerogel composites of sol-gel iron (III) oxide and ultra fine grained aluminum (UFG Al) are prepared, characterized, and compared to a conventional micron-sized Fe2O3/Al thermite. The exquisite degree of mixing and intimate nanostructuring of this material is illustrated using transmission and scanning electron microscopies (TEM and SEM). The nanocomposite material has markedly different energy release (burn rate) and thermal properties compared to the conventional composite, results of which will be discussed. Small-scale safety characterization was performed on the nanostructured thermite. The second nanostructured energetic material consists of a nanostructured hydrocarbon resin fuel network with fine ammonium perchlorate (NH4ClO4) oxidizer present.

Based on the analysis of extensive experimental data, we have formulated basic criteria necessary for the synthesis of a variety of oxides in the combustion mode, and defined optimum conditions for the production of high-surface area, well-crystalline nano-powders of desired phase composition and purity. Also, for the first time, detailed chemical mechanisms of interaction for various systems are identified, outlining specific roles of different fuels, oxidizers and thermal conditions

Described in this paper are ongoing research efforts at ATK Thiokol on new formulations of gun propellants and explosives containing nano-sized materials. Funding for this work was provided both internally and by ARDEC. The nano-material properties as well as selected properties of intermediate compositions are reported.

The explosives section of this paper focuses on the theoretical evaluation of nano-aluminum in explosives. The theoretical cylinder expansion energy of two common aluminized explosives, PAX-3 and PAX-28, are examined. The comparison examines the effect of aluminum content in small aluminum particles on metal pushing performance at V/V0 = 6.5.

ATK Thiokol and TACOM-ARDEC have teamed together to study the effect of nano-sized materials on advanced ETPE gun propellants. The gun propellant section of this paper focuses on the impact of nano-sized additives, specifically TiO2 and aluminum, on selected ETPE propellants. The safety properties of the nano-aluminum and nano-TiO2 analogs of two previously developed gun propellants, TGD-031 and TGD-035, are presented.

Energetic nano-sized particles have been shown to have a great potential for use in the aerospace propulsion applications. Some of the unique combustion properties of nano-particles such as very rapid ignition and short combustion times make them particularly valuable for propulsion systems; they can be included in solid fuels, solid propellants, or even as energetic gellant in liquid systems. However, due to the novelty of the application and rapid development of production techniques, there is no comprehensive understanding of what characteristics of a nano-sized particle are important in contributing to desirable performance and ease of processing into a final usable form. Previous studies have shown that HTPB-based solid fuels containing various types of nano-sized particles showed differing performance results when tested in the same hybrid rocket motor under identical conditions. Many of these particles have data available only on the basic composition (aluminum, boron, boron carbide, etc.), average diameter, and/or BET surface area. In order to better understand and correlate observed combustion behavior with intrinsic material properties, the particles of interest need to be better characterized. A variety of standard particle characterization techniques were applied to the fifteen types of particles examined in this study and the results tabulated. Some of the parameters measured were average particle diameter, specific surface area, amount of active content, and oxide layer thickness. Trends in propulsion performance measured using a parameter of great interest to the hybrid rocket community (fuel mass burning rate) in general matched trends in particle characteristics (i.e. active content, surface area), but there were some noticeable exceptions. This study indicates that there is still much more to learn about the correlation between physical and chemical properties and measured combustion performance. Other parameters that should be examined in the future include particle size distribution, degree of agglomeration, reactivity and thermal effects (oxidation rate, onset temperature for oxidation exotherm, heat release associated with any excess stored energy), etc.

We report a volume combustion synthesis study of Al/Ni multilayers. The alternating layers of pure Al and pure Ni were grown on silicon and glass substrates using electron beam evaporation. Layers of equal thickness and with bilayer period of 50 nm were grown with total thickness to 1 μm. We focus in this study on the interface properties of as deposited and following anneals up to 660°C with 100°C intervals. Anneal duration was generally 10 min with longer anneals to verify consistency with previously published results. Interface structure was probed using X-ray reflectivity (XRR). Based on interface roughness, we identify three temperature ranges. In the as-deposited to 260°C range the interface properties remain intact, as seen from the XRR interference fringes beyond the critical angle. From 360 to 460°C fringes disappear, indicating the loss of smooth interface morphology. The third temperature range is 500 to 560°C in which the XRR interference fringes are clearly seen, signifying the presence of layering. We describe these results based on a model in which alloy domains form at the interface and grow with temperature and time. Differences in lateral and vertical growth rates of alloy domains are naturally taken into account. At 660°C, the melting temperature of Al, and above the fringes completely vanish. X-ray diffraction is used to identify the nickel aluminide compounds formed upon anneal.

This study is concerned with the development of a modified sol-gel synthesis of Fe2O3 xerogels that would allow the design and control of the interfacial area between the oxidant iron oxide matrix and the metal reducing agent, thus optimizing the energetic yield of these highly energetic reactions. The modification consisted in the addition of a new class of di-functional template molecules, such as diamines or di-acids, as gelation agents. pH profile measurements indicated that the mechanism of reaction of propylene oxide and of succinic acid as the gelation agents was fundamentally different. Propylene oxide acts as a proton scavenger, reducing the hydrated iron species to Fe2O3, thus reducing the concentration of protons in the reaction mixture leading to an increase in pH. When succinic acid is used as the gelation agent, a decrease in pH versus time during the reaction indicates the formation of carboxylate ions, thus creating reactive molecules that are capable of stabilizing the Fe2O3 clusters during the growth process. Infrared spectra of the products in both reactions support presence of carboxylate groups in the Fe2O3 xerogels. X-ray diffraction analyses revealed low levels of crystallinity in both products, and the presence of different phases of Fe2O3.