ABSTRACT
The oxidation of aluminum is highly exothermic and, as a consequence, an ignition front has been observed passing over the sample’s surface during firing. An ignition wavefront is undesirable for the RBAO process because both thermal and chemical stresses are developed. The thermal stresses are transitory and are caused by the large temperature difference between the hot reaction zone and the cooler unreacted zone. The chemical stresses are caused by the 28% volumetric expansion associated with the oxidation of aluminum. The chemical stresses are not transitory, but remain in the wake of the ignition wave-front, as is evident from the steep composition gradients in the radial direction, shown in Figure 12.1. The depth of the reacted shell is restricted by the radial diffusion of oxygen through the pores of the rod. Sample failure is usually observed soon after ignition.
