Radiative properties of MoO₃ and Al nanopowders from light-scattering measurements

The combustion behavior of nanometer-scale energetic materials is much different than micron size or larger materials. Burning rates up to 950 m/s have been reported for a thermite composition of nanosized aluminum and molybdenum trioxide. The energy transport mechanisms in the reactive wave are still uncertain. The relative contribution of radiation has not yet been quantified. To do so analytically requires dependent scattering theory, which has not yet been fully developed. Radiative properties for nanoaluminum and nanomolybdenum-trioxide were obtained experimentally by comparing light scattering measurements on a one-dimensional slab of powder with multiple-scattering simulations using Monte Carlo and discrete ordinate methods. The equivalent isotropic-scattering extinction coefficient for close-packed molybdenum trioxide (MoO₃) nanopowder was found to be 5900 ± 450 cm^-1; the equivalent isotropic-scattering albedo was 0.97 ± 0.035. Aluminum (Al), which proved to be more difficult to work with, had an albedo of 0.35 and 0.38 from two tests. The radiative conductivity based on the MoO₃ results is two orders of magnitude less than the diffusive thermal conductivity, indicating that radiation is not a dominant heat transfer mode for the reactive wave propagation of nanothermites under optically thick conditions.