A slice of an aluminum particle: Examining grains, strain and reactivity

Abstract

Micron-scale aluminum (Al) particles are plagued by incomplete combustion that inhibits their reactivity. One approach to improving reactivity is to anneal Al particles to increase dilatational (volumetric) strain which has also been linked to increased combustion performance. While optimal annealing temperatures have been identified (roughly 300 °C), little is known about cooling rate effects on particle combustion performance. This study examines the effect of quenching after annealing Al microparticles to 100, 200 and 300 o C on intra-particle dilatational strain and reactivity. Synchrotron X-ray diffraction analysis of the particles reveals the cooling rates in the range from 0.007 to 0.38 K/s have little effect on the dilatational strain of the aluminum-core, alumina-shell particles. The annealed and quenched Al particles were then combined with a metal oxidizer (copper oxide) to examine reactivity. Flame propagation experiments follow the same trend: flame speeds are unchanged until a critical annealing temperature of 300 °C is reached and performance is maintained for each annealing temperature regardless of cooling rate. These results show that altering the mechanical properties and combustion performance of Al particles is strongly dependent on the annealing temperature and unchanged with variation in cooling rate. The contributions from elastic and plastic deformation mechanisms on strain are also considered and additional experimental results are shown on the microstructure of an Al particle. Focused ion beam milling of an Al particle to electron transparency was combined with transmission electron microscope imaging in order to examine the microstructure of the Al particles. This confirmed that the Al microparticles have a polycrystalline structure shown by grains all exceeding 100 nm in size.

Dropping the hammer: Examining impact ignition and combustion using pre-stressed aluminum powder

“…Note that these numbers were in quantitative agreement with theoretical predictions based on the melt dispersion mechanism. 1,2 Similar results on increased reactivity were obtained in McCollum et al 5 In particular, they found pre-stressed (PS) Al powder annealed at or above 300 C showed the highest increase in reactivity.…”

“…Both micron and nanometer scale Al particles are examined and material preparation and characterization was accomplished using established protocols. 5 Impact ignition analyses were accomplished by designing a pressurized reaction cell coupled with a traditional drop weight impact tester. The pressurized reaction cell is equipped with sensors to monitor the transient pressure and light intensity within the closed cell upon sample impact.…”

Highly reactive energetic films by pre-stressing nano-aluminum particles

“…The results in eqn (19) show residual compressive stress and strain in the core at annealing temperature are much smaller than in the unrelaxed state at room temperature. Now we can rewrite eqn (13)- (15) for new reference temperature T a ¼ 573 K at which there is no creep strain in the shell (s h ¼ 0) and stress and strain in the core are determined by eqn (19). These new equations are shown in eqn (20)- (22) and indeed, for 3 h c ¼ 0 and T ¼ T a we obtain s h ¼ 0 and eqn (19).…”

“…Measurements from this beamline that quantify dilatational strain in mAl powder have been reported previously. [14][15][16][17] In contrast to previous measurements on mAl powder, data on the nAl particles were taken with a 10Â 2m 12 keV monochromatic beam because the size from the white beam focus (around 1 m) was not sufficiently small to resolve the nAl particles. For the nAl particles, data were taken in 'powder diffraction pattern' mode and dilatational strain was derived from the shis in 2q values of the powder rings with respect to their 'unstrained' positions.…”

Impact ignition and combustion of micron-scale aluminum particles pre-stressed with different quenching rates

“…This change in stress state is linked to changes in reactivity. [11][12][13][14] In order to quantify the stress state within pre-stressed (PS) Al particles, several studies measure dilatational strain within the core [11][12][13][14] (e.g., the shell is amorphous, and the strain cannot be directly measured via X-ray diffraction). The primary diagnostic to resolve dilatational strain in the Al particle core-shell structure is synchrotron X-ray diffraction (XRD) using the combined white and monochromatic microbeam approach at the Advanced Light Source (ALS) 15 Particles annealed to 573 K (300 C) and quenched at moderate rates (i.e., <100 K/min) led to a significant increase in dilatational strain, corresponding to a tensile stress in the Al core and compressive stress in the Al 2 O 3 shell.…”