John J. Granier

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.

Nonuniform laser ignition in energetic materials

Laser ignition of energetic materials is typically described in terms of a one-dimensional homogeneous ignition model. However, the Gaussian energy distribution from a laser can induce multidimensional effects on ignition. A two-dimensional numerical model that simulates radiant heating and subsequent ignition of energetic materials was developed. A laser with a Gaussian energy distribution was used as the external energy source applied to the flat surface of a cylindrical pellet. The nonuniform behavior of thermal ignition leads to radially dependent ignition times, temperatures, and energy associated with the front surface of the pellet. Existing experimental ignition data was compared to the numerical results. The model shows the importance of a two-dimensional analysis for ignition because the nonuniform external heat source can result in the formation of localized heating that will develop into nonplanar flame propagation. The model is compared to experimental results that further validate the importance of a two-dimensional ignition model and show that ignition occurs first in the center of the pellet where the Gaussian beam intensity is greatest.

The effect of size distribution on burn rate in nanocomposite thermites: a probability density function study

Burn rates of thermites are typically calculated in terms of an average particle size that characterizes the bulk mixture. As the particle diameter approaches the nano-scale the burn rate calculation becomes increasingly sensitive to changes in the particle diameter. In this study, burn rate estimates for nanoscale particle composite thermites are statistically evaluated in terms of an integral that employs a probability density function (pdf) for particle size distribution and a diameter dependent burn rate equation. It is shown that the burn rates depend sensitively on the mean particle diameter and the particle size distribution. Both single mode and bimodal particle size distributions were studied. The analysis shows that as the particle size is reduced to the nano-scale, the size distribution, rather than the average particle size alone, becomes increasingly important. Large variability in burn rate is associated with large standard deviations in particle size. Combining nano-scale with bulk-scale particles in a bimodal distribution does not significantly increase the burn rate as compared to a composite consisting of strictly nanoparticles. The results presented here suggest that better reproducibility of the burn rate may be achieved experimentally by selecting a material with a narrow particle size distribution.

Ignition and Combustion Behaviors of Nanocomposite Al/MoO 3

Flame propagation in Al/MoO 3 thermites is measured as a function of bulk density and initial sample temperature. The composites are composed of nano-scale reactant particles mixed and pressure molded to between 49 and 73% of the theoretical maximum density. A relationship between cylindrical die pressure and final pellet density is derived. Experiments are also performed by initially pre-heating samples to a uniform temperature ranging from 20 to 200 °C. Ignition sensitivity was determined by measuring the ignition delay time and temperature using a 50-W CO 2 laser and thermocouples, respectively. Combustion wave speeds were measured using high-speed imaging diagnostics. Results allow comparison of combustion behaviors associated with nano- vs. micron-scale particle composites. The nano-scale particle composites are extremely sensitive to ignition, especially when initially preheated. Combustion wave speeds for the compressed nano-composites were found to double when compared to the micron-scale composites.

COMBUSTION BEHAVIORS OF PRE-HEATED NANOCOMPOSITE THERMITES

The burn rates and ignition and flame temperatures of compressed Al/MoO3 pellets were measured. The composites are composed of nano-scale reactant particles mixed and pressed to 52 % theoretical maximum density. Experiments were performed by initially pre-heating samples to a uniform elevated temperature ranging from 20 to 200 oC. Additional experiments were performed on samples initially at ambient conditions and with varied laser powers ranging from 10 to 60 W. Results allow comparison of combustion behaviors associated with nano- vs. micron-scale particle composites. The nano-scale particle composites are extremely sensitive to ignition when initially preheated. Burn rates for the compressed nano-composites were found to double when compared to the micron-scale composites.

MODELING LASER IGNITION AND HEAT PROPAGATION IN NANOCOMPOSITE THERMITES

Nanocomposite energetic materials are a mixture of nano-scale fuel and oxidizer particles. When these compositions are ignited, reactions are highly exothermic and can exhibit widely tunable energy release rates. These materials show great potential as igniters, propellants and for energy generation applications. However an understanding of the ignition and thermal propagation of these materials has not been fully developed. The objective of this study is to model the heat transfer in a composite energetic material and identify the influence of particle size on the ignition energy, time and temperature. A two-dimensional, transient numerical model is described for the radiant heating and subsequent combustion. The numerical model is based on finite difference nodal equations and is calculated in terms of thermal resistances and capacitances. Boundary conditions are predetermined, including a laser energy source applied to a single surface. Interstitial heat generation is based on laser absorption and chemical kinetics. The effect of particle size on the transient heating and energy release rate has also been considered. The model uses a 250 W CO 2 laser to ignite a Magnesium/ Sodium-Nitrate pyrotechnic sample. Results also show the 2-D nature of heat propagation that results from the Gaussian energy distribution from the laser beam.