Michelle L. Pantoya

Combustion velocities and propagation mechanisms of metastable interstitial composites

Combustion velocities were experimentally determined for nanocomposite thermite powders composed of aluminum (Al) fuel and molybdenum trioxide (MoO3) oxidizer under well-confined conditions. Pressures were also measured to provide detailed information about the reaction mechanism. Samples of three different fuel particle sizes (44, 80, and 121nm) were analyzed to determine the influence of particle size on combustion velocity. Bulk powder density was varied from approximately 5% to 10% of the theoretical maximum density (TMD). The combustion velocities ranged from approximately 600 to 1000m∕s. Results indicate that combustion velocities increase with decreasing particle size. Pressure measurements indicate that strong convective mechanisms are integral in flame propagation.

Melt dispersion mechanism for fast reaction of nanothermites

An unexpected mechanism for fast oxidation of Al nanoparticles covered by a thin oxide shell (OS) is proposed. The volume change due to melting of Al induces pressures of 0.1–4GPa and causes spallation of the OS. A subsequent unloading wave creates high tensile pressures resulting in dispersion of liquid Al clusters, oxidation of which is not limited by diffusion (in contrast to traditional mechanisms). Physical parameters controlling this process are determined. Methods to promote this melt dispersion mechanism, and consequently, improve efficiency of energetic nanothermites are discussed.

Mechanochemical mechanism for fast reaction of metastable intermolecular composites based on dispersion of liquid metal

An unexpected mechanism for fast reaction of Alnanoparticles covered by a thin oxide shell during fast heating is proposed and justified theoretically and experimentally. For nanoparticles, the melting of Al occurs before the oxide fracture. The volume change due to melting induces pressures of 1–2 GPa and causes dynamic spallation of the shell. The unbalanced pressure between the Al core and the exposed surface creates an unloading wave with high tensile pressures resulting in dispersion of atomic scale liquid Al clusters. These clusters fly at high velocity and their reaction is not limited by diffusion (this is the opposite of traditional mechanisms for micron particles and for nanoparticles at slow heating). Physical parameters controlling the melt dispersion mechanism are found by our analysis. In addition to an explanation of the extremely short reaction time, the following correspondence between our theory and experiments are obtained: (a) For the particle radius below some critical value, the flame propagation rate and the ignition time delay are independent of the radius; (b) damage of the oxide shell suppresses the melt dispersion mechanism and promotes the traditional diffusive oxidation mechanism; (c) nanoflakes react more like micron size (rather than nanosize) spherical particles. The reasons why the melt dispersion mechanism cannot operate for the micron particles or slow heating of nanoparticles are determined. Methods to promote the melt dispersion mechanism, to expand it to micron particles, and to improve efficiency of energetic metastable intermolecular composites are formulated. In particular, the following could promote the melt dispersion mechanism in micron particles: (a) Increasing the temperature at which the initial oxide shell is formed; (b) creating initial porosity in the Al; (c) mixing of the Al with a material with a low (even negative) thermal expansion coefficient or with a phase transformation accompanied by a volume reduction; (d) alloying the Al to decrease the cavitationpressure; (e) mixing nano- and micron particles; and (f) introducing gasifying or explosive inclusions in any fuel and oxidizer. A similar mechanism is expected for nitridation and fluorination of Al and may also be tailored for Ti and Mg fuel.

EFFECT OF AL PARTICLE SIZE ON THE THERMAL DEGRADATION OF AL/TEFLON MIXTURES

Abstract Reactive mixtures of aluminum (Al) and polytetrafluoroethylene (PTFE or Teflon) have applications in propellants, explosives, and pyrotechnics. This study examines the thermal degradation behavior of Teflon and nanometer scale Al particles compared with micron-scale Al particles. Differential scanning calorimetry and thermo-gravimetric analyses were performed in an argon environment on both nanometer and micron scale particulate mixtures revealing lower ignition temperatures and larger exothermic activity for the nanometer Al/Teflon mixture. This increased ignition sensitivity and exothermicity is caused by a pre-ignition reaction unique to the nano-Al mixture. Experiments suggest that the pre-ignition reaction mechanism is controlled by the fluorination of the Al particle passivation shell. Micron-scale Al particles have a lower specific surface area and therefore the influence of the passivation shell promoting a pre-ignition reaction is reduced. Chemical kinetics are discussed along with particle morphology to explain the thermal degradation process of the mixtures. These results are helpful in the fundamental understanding of Al particle size effects on the reactivity of Al/Telfon composites.

Melt dispersion versus diffusive oxidation mechanism for aluminum nanoparticles: Critical experiments and controlling parameters

Critical experiments were performed on Al and MoO3 thermites. The diameter and alumina shell thickness of the Al nanoparticles were varied, and flame propagation velocities were measured. The results strongly support the melt-dispersion mechanism and contradict the diffusion oxidation mechanism. The parameters that control the oxidation rate and flame velocity are justified and directions for the synthesis of Al nanoparticles (which are opposite to the current directions based on diffusion oxidation) are suggested. An equation for the flame velocity versus Al nanoparticle geometrical parameters, thermomechanical properties, and synthesis parameters is formulated.

Ignition dynamics and activation energies of metallic thermites: From nano- to micron-scale particulate composites

Ignition behaviors associated with nano- and micron-scale particulate composite thermites were studied experimentally and modeled theoretically. The experimental analysis utilized a CO2 laser ignition apparatus to ignite the front surface of compacted nickel (Ni) and aluminum (Al) pellets at varying heating rates. Ignition delay time and ignition temperature as a function of both Ni and Al particle size were measured using high-speed imaging and microthermocouples. The apparent activation energy was determined from this data using a Kissinger isoconversion method. This study shows that the activation energy is significantly lower for nano- compared with micron-scale particulate media (i.e., as low as 17.4 compared with 162.5kJ∕mol, respectively). Two separate Arrhenius-type mathematical models were developed that describe ignition in the nano- and the micron-composite thermites. The micron-composite model is based on a heat balance while the nanocomposite model incorporates the energy of phase transformat...

Combustion Behaviors Resulting from Bimodal Aluminum Size Distributions in Thermites

Studies that replace a portion of the micron-size aluminum (Al) with nano-Al particles in an energetic formulation demonstrate significant performance enhancement. Little is known, however, about the critical level of nano-sized fuel particles needed to enhance the performance of the energetic composite. Ignition sensitivity and combustion velocity experiments were performed using a thermite composed of Al and molybdenum trioxide (MoO 3 ). Both loose powders and compressed pellets were examined. A bimodal Al particle size distribution was prepared using 4 or 20-μm-diam Al fuel particles that were replaced in 10% increments by 80-nm-diam Al particles until the fuel was 100% nano-AI. Results show that with only 20% nano-Al content, the mixtures showed reduced ignition delay times by up to 2 orders of magnitude. The combustion velocity was shown to dramatically increase as more nano-Al particles replace micron-Al particles within the mixture. This increasing trend was attributed to incomplete reactions of the micron-Al particles or significantly slower reactions such that the micron-Al particles promote cooling or quenching of the reaction.

Effect of Bulk Density on Reaction Propagation in Nanothermites and Micron Thermites

The thermite reaction of nanoscale aluminum and molybdenum trioxide particles has revealed a paradoxical relationship between Al particle size and mixture bulk density. Specifically, with micron-scale Al particles, the thermite demonstrates an expected growth in flame speed with increased density, but nanoscale-Al-particle mixtures exhibit an opposing trend. This paper presents new experimental measurements of the thermal properties of this thermite as a function of Al particle size and applies a new oxidation mechanism in an effort to explain the paradoxical results between Al particle size and mixture bulk density. Results show that the nanocomposites behavior is consistent with a new melt-dispersion oxidation mechanism and convective mode of flame propagation. Compaction-induced damage of the oxide shell and distortion of the shape of spherical particles, as well as reduced free space around Al nanoparticles suppress the melt-dispersion mechanism and reduce flame speed. An additional mode of energy transfer is proposed that is associated with molten Al clusters from the melt-dispersion mechanism that advance faster than the flame velocity. Micron-scale particle reactions may be governed by diffusion such that increased bulk density coincides with increased thermal properties and increased flame speeds.

Impact ignition of aluminum-teflon based energetic materials impregnated with nano-structured carbon additives

The inclusion of graphene into composite energetic materials to enhance their performance is a new area of interest. Studies have shown that the addition of graphene significantly enhances the thermal transport properties of an energetic composite, but how graphene influences the composite’s ignition sensitivity has not been studied. The objective of this study is to examine the influence of carbon additives in composite energetic material composed of aluminum and polytetrafluoroethylene (Teflon™) on ignition sensitivity due to low velocity, drop weight impact. Specifically, three forms of carbon additives were investigated and selected based on different physical and structural properties: spherically shaped amorphous nano particles of carbon, cylindrically shaped multi walled carbon nanotubes, and sheet like graphene flakes. Results show an interesting trend: composites consisting of carbon nanotubes are significantly more sensitive to impact ignition and require the lowest ignition energy. In contrast,...

Melt-dispersion mechanism for fast reaction of aluminum particles: Extension for micron scale particles and fluorination

The theoretically predicted relationship for the relative flame rate versus relative particle size based on the melt dispersion mechanism (MDM), which was previously confirmed for oxidation of 40–120nm diameter aluminum particles, is found to be in agreement with experiments for 1–3μm diameter Al particles and fluorination. The main physical parameters for MDM (pressure in molten particle, cavitation threshold, and nanoclusters’ velocity) have been estimated for micron scale particles. The results suggest parameters that could be controlled during particle synthesis that would enable micron scale Al particles to react and achieve the performance of nanoscale Al particles.