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

Abstract

Critical experiments were performed on Al and MoO 3 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 oxidation͒ are suggested. An equation for the flame velocity versus Al nanoparticle geometrical parameters, thermomechanical properties, and synthesis parameters is formulated.

Comment on “In situ imaging of ultra-fast loss of nanostructure in nanoparticle aggregates” [J. Appl. Phys. 115, 084903 (2014)]

“…There is some qualitative data that is inconsistent with the diffusion mechanism, such as the increase (decrease) of the flame rate with increase (decrease) of the shell thickness, 16 reduction in flame rate with increasing mass density of the reactive mixture for nanoparticles and opposite trends for micron scale particles (which cannot react by MDM), 17 suppressing effect of the damage of the shell on flame propagation, 4,16 much smaller particle size after reaction, 9,10 and some other results presented in Refs. 4-6, 10, and 12.…”

Mechanochemical mechanism for reaction of aluminium nano- and micrometre-scale particles

“…At the same time in experiments at 900 • C, reaction time for even bare Al particles with a diameter d = 20 nm exceeds 1 s [7]. In addition, the flame speed and ignition time delay became independent of particle size for d < 120 nm [1,3,8]; for the diffusion oxidation mechanism, they are a power function of the diameter [3,9].…”

Stress relaxation in pre-stressed aluminum core–shell particles: X-ray diffraction study, modeling, and improved reactivity

“…Numerous confirmations of the MDM, mostly in flame tube experiments [5][6][7][10][11] but also in flash ignition experiments [12], have been obtained for nano-and micron-scale particles.…”

“…and the ratio M=R/δ of the core radius R to the oxide shell thickness δ [5][6][7][10][11]. Such an analytical quantitative model provides methods to control (increase) particle reactivity and, consequently, the flame speed [5][6][7][10][11]13,14].…”

“…Our focus here is on the very high heating rate regimes for which mechanochemical MDM is claimed [5,6]. Initially [5,7], this mechanism was suggested to rationalize extremely high flame speed V (up to 1 km/s) observed for combustion of Al nanoparticles [7][8][9]. Later [10,11], the MDM was generalized for few-micron-scale Al particles.…”