Mechanisms of Low Temperature Oxidation for Metal Particles down to the Nano-Scale and their Influence to Propellant Combution

摘要

Particles of various sizes down to nano-scale were studied by TG, SEM and in-situ X-ray diffraction. Al particles contain a 2-4 nm oxide passivating layer the thickness of which was found from the weight increase on complete oxidation. The oxidation occurs at least in two steps. The first step forms a layer of 6 to 10 nm thickness composed of crystallites of the same size independent on the initial particle size. Chemical kinetics controls this step and converts an essential amount of the particle for sizes below 1 μm, which could strongly the ignition and combustion behavior. The second step occurs by combined diffusion and chemical reaction and proceeds therefore slowly, the slower the bigger the particles are. The kinetic parameters of the steps were found by applying an adequate model. The particle size also influences the phases of the oxides formed as shown by x-ray diffraction. This first oxidation step obviously leads to the increased burning rate of composite rocket propellants. It manifests its impact on the burning behavior by an increased heat release close to the burning surface with higher temperatures measured close to it confirmed by a simplified combustion modeling.