9712141 Takacs This grant investigates the mechanochemical processes involved in the initiation of combustion by ball milling. When a highly exothermic powder mixture is milled, the mechanical agitation ignites a self-sustained combustion after an activation time. Milling is the ideal tool with which to study the mechanism of mechanochemical reactions because the ignition phase is a well-defined critical state in the activation process. The theoretical modeling includes consideration of the initiation of combustion by single collisions of the milling balls and the propagation of the combustion front away from the impact sites. The modeling of ignition is supported by experimental studies of the interrupted combustion effect, the mutual suppression of combustion in a mixture of combustive powders, and the repeated combustion effect. Reactions with titanium, zirconium, and hafnium and the formation of chalcogenides are among the reactions to be examined. Measurements include the temperature of the milling vial and activation times to combustion. X-ray diffraction and electron microscopy are used to characterize the powder at different stages of the process with emphasis on samples taken just prior to combustion. The thermal ignition of combustion and the relaxation of activated samples are studied by differential thermal analysis and heating curve measurements. %%% The initiation of chemical reactions by ball milling is used in several technologies, including the preparation of metastable alloys, refractory powders, and nanocrystalline materials. Further advancements depend on improved fundamental understanding of the complex processes involved. ***
