Theoretical Analysis of Passivating Pyrophoric Nanopowders: A Macrokinetics Approach

Abstract

In this paper we use a macrokinetics approach to propose and develop a mechanism for the ignition and passivation of a pyrophoric nanopowder layer. Assuming that the oxidizer diffusion is the rate-limiting step in the wave-propagation mechanism of passivation, we are able to determine the dependence of the maximum temperature of the nanopowder passivation on key parameters. As a result, two-stage passivation with an increasing oxidant concentration in the gas phase at the second step is proposed. We have shown that, at an allowable warm-up level, the two-stage process reduces the time required for the passivation of a nanopowder layer to be completed by several times. The minimum time of the transition to the second stage at a given rate of temperature growth has been analytically predicted. We have also made numerical simulatons that show a good agreement with the results of our approximate calculations, additionally supporting the conclusions based on the theoretical analysis used. The macrokinetic approach is successfully applied to adeqautely described the passivation of pyrophoric nanopowders when it is strictly limited due to small particle sizes by the diffusion transfer of the passivating gas into the backfill.