Mechanical activation of the solid-state reaction between Al and TiO2

Titanium dioxide and aluminium have been mechanically milled together for times up to 100 h. The onset temperature of the main reaction was observed to decrease from 1050 to 660 degrees C after 5 h of milling, increasing the milling time to 100 h decreased the onset to 560 degrees C. Elemental maps showed that the major decrease was due to increased mixing of the phases. Beyond 5 h, the temperature at which the reaction was half completed was found to be a function of log (milling time) and seemed to be correlated with the decrease in crystallite size. There seemed to be three different rate determining steps, the importance of each changing with milling time. Long range solid state diffusion was predominant for milling times < 5 h, short range diffusion for 5-50 h and, possibly, diffusion along lattice defects after 100 h milling. An onset temperature of 560 degrees C was observed for milling times of greater than or equal to 10 h. This was thought to be due to a change in the rate determining step from mass transport to chemical reaction control. (C) 1998 Elsevier Science S.A. All rights reserved.