Reaction Interface for Heterogeneous Oxidation of
Aluminum Powders
- Publication date
- Publisher
Abstract
Heterogeneous
oxidation of aluminum is rate limited by diffusion
through a growing aluminum oxide layer. If inward diffusion of oxygen
ions is faster than outward diffusion of aluminum, the reaction will
occur at the inner interface of the oxide. Conversely, the reaction
will occur at the outer oxide surface if outward diffusion of aluminum
is faster. The location of the heterogeneous reaction is identified
processing results of thermogravimetric measurements for two oxidizing
spherical aluminum powders with different but overlapping particle
size distribution. For each experiment, the measured weight gain is
distributed among particles of different sizes assuming that the rate
of oxidation is proportional to the reactive interface area. Different
models are considered to determine the interface area. For a ductile
oxide shell, when there is no void between oxide and aluminum, two
cases with the reaction occurring at both inner and outer surfaces
of the shell were evaluated. In addition, a case with the reaction
at the outer surface of a rigid oxide shell is considered, for which
a void inside the particle forms when the aluminum core is shrinking.
Oxidation weight gains for the same size particles present in different
aluminum powders are expected to be identical to each other when the
calculated reactive interface area reflects the true oxidation mechanism.
It is concluded that the reaction at the outer surface of a rigid
oxide shell describes the experiments most accurately. Thus, the outward
diffusion of aluminum ions controls the rate of heterogeneous oxidation
of aluminum in a wide range of temperatures of approximately 400–1500
°C. The conclusion is further supported by the electron microscopy
of particles quenched at different temperatures, showing oxide surface
features consistent with the identified reaction mechanism