Feasibility study for methods of determining crystal phase of aluminum oxide (AL2O3) rocket exhaust particles

Aluminum-oxide particles collected from sol id-propellant rocket exhausts apparently occur in at least two distinct crystalline phases: alpha, a hexagonal crystal with specific gravity of 3.98; and gamma, a cubic crystal with specific gravity of 3.5. The occurrence of two phases in the exhaust is generally thought to be a rate-controlled process determined by the temperature-time history of the particles. A quantitative verification of this idea will require extension of kinetic rate data for the AI₂O₃ system to higher temperatures. As a first step in understanding the origin of the two distinct phases, however, the present research investigates the feasibility of a simple analytic technique, based upon infrared spectroscopy, for use as a quantitative determination of the mass ratio of the two phases in typical rocket exhausts. This potential technique was applied to nine particle samples collected from rocket motor firings at Arnold Engineering Development Center (AEDC). The method yielded relative masses of the alpha to the gamma phase; hence, the method appears viable as a technique for phase identification. Of great interest was the fact that by using this method, all samples were found to be predominantly gamma phase, with gamma mass fractions ranging from 64 to 93 percent. This is in contradiction to analysis of particle samples reported in the literature. Analyses performed by Sandia Laboratories (AEDC samples) and NASA (STS SRB samples) using x-ray diffraction and acid solubility techniques, repectively, are believed to be inapplicable to the AI₂O₃ particle sizes and the thermal environment that exists in the exhaust plumes of solid rocket motors tested at AEDC.