Flame propagation experiments of non-gas-generating nanocomposite reactive materials

Nanoenergetic composites are being examined for their use as energy storage and delivery materials. For this application, non-gas-generating mixtures that produce enough energy to sustain reaction propagation are ideal. A potentially promising formulation for this application is comprised of manganese (Mn) powder mixed with manganese dioxide (MnO₂) because thermoequilibrium calculations predict that the Mn/MnO₂ reaction is non-gas-generating and highly exothermic. The objectives of this study were to examine the flame propagation behavior of various mass percentages of Mn and MnO₂ and evaluate the influence of fuel particle size. Results showed that conduction is the dominant energy-transfer mechanism for the reaction even in the loose powder configuration, regardless of the fuel particle size. Combustion wave speed (CWS) was measured as a function of the composition and Mn particle diameter. Nanometric Mn particles (referred to as nano-Mn) produced higher CWS than micrometric Mn particles (referred to as micro-Mn) for all of the mass percentages of Mn studied. This behavior was attributed to the manganese(II,III) oxide (Mn₃O₄) passivating shell around Mn particles, which created an oxygen-rich environment for the reaction zone. This finding is in contrast to nanometric aluminum (Al) particle (referred to as nano-Al) combustion studies, in which energy transfer is dominated by conduction (i.e., highly consolidated nanocomposite). For nano-Al, the alumina (Al ₂O₃) passivation shell does not participate in the oxidation reaction and, instead, acts to retard energy propagation as a heat sink. As a result, reducing the particle size of Al produces lower CWS. The CWS was found to be at a maximum of 12.01 mm/s for the nano-Mn/MnO₂ reaction at a Mn mass percentage of 29%. Flammability limits are also defined.