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Melting of aluminum nanoparticles within alumina shell at high heating rates

YONG SEOK HWANG (Iowa State University), Valery Levitas (Iowa State University)

Mechanics of Phase Transforming and Multifunctional Materials

Tue 4:20 - 5:40

CIT 219

Melting of aluminum nanoparticles within alumina shell at high heating rates was examined using phase field approach coupled to mechanics. A coherent solid-liquid interface with stress relaxation and possibility of surface premelting and melting were utilized. Finite element method and COMSOL code have been used for numerical simulations. Mesh- and time step dependence of the solution was studied and parameters for objective solutions have been chosen. Effects of the heating rate, surface energy, aluminum core size, and alumina shell thickness on melting behavior was explored. While nanosize of particles leads to reduction of the melting temperature below the bulk melting temperature of 933 K, superheating up to 1250 K was observed in the case of 40nm aluminum core radius, 2nm alumina shell thickness, and 10^12 K/s heating rate. This superheating was resulted from two factors: high internal pressure (several GPa) induced by restriction of alumina shell and fast heating rate. Melting suppression due to high internal pressure is explained by Clausius-Clapeyron relation. We found that a solid-melt interface speed was not fast enough to ignore heating rate when heating rate exceeds 10^10 K/s. Thus, before solid-liquid interface reaches the center of particle, temperature grows significantly. Such a superheating depends on the particle size and reduction in aluminum-alumina surface energy during melting. Obtained details of the melting and stress development are important for optimization of the melt-dispersion mechanism of combustion of aluminum nanoparticles. In this mechanism, high pressure due to melting breaks and spallates an alumina shell. Unloading spherical wave propagates to the particle center and creates high tensile pressure (3-8 GPa), which disperses molten particles into small clusters. Reaction of such clusters is not limited by diffusion through oxide shell, which explains extremely high reactivity of aluminum nanoparticles.