Pyrophoricity of nascent and passivated aluminum particles at nano-scales

Pyrophoricity of nascent and passivated nano-aluminum particles in air is studied theoretically using energy balance analyses. The work incorporates size-dependence of physicochemical properties of particles at nano-scales, and considers free-molecular and radiation heat exchange with the surrounding environment. The heterogeneous oxidation process is modeled using the Mott–Cabrera mechanism. Nascent aluminum particles with diameters lower than 32 nm are predicted to be pyrophoric. The critical diameter for particles passivated with 0.3-nm thick oxide layer is calculated as 3.8 nm. Particles with oxide layers thicker than 0.3 nm are found to be non-pyrophoric. The sensitivity analysis suggests that the model results are significantly affected by the choice of physicochemical properties, polymorphic state of the oxide layer, parameters of the Mott–Cabrera oxidation kinetics, and heat-transfer correlation. The critical particle size increases by 40%, when bulk material properties calculated at room temperature are used and the oxide layer is assumed to be in a crystalline form. It decreases by 43%, when free-molecular effects are neglected.