Changing the reaction pathway of the CaAl2 oxidation using ball milling

Abstract

As previously shown, CaAl2 can be oxidized using elemental O2 to form CaAl2O4. This reaction, however, proceeds via Ca12Al14O33 and elemental Al as intermediates which are subsequently transformed into the stoichiometric reaction product. High-energy ball milling is known to decrease the crystallite size of a material and to significantly produce defects enabling different reaction pathways compared to a highly crystalline bulk material. In this subsequent study, a different oxidizing agent (H2O) as well as the ball milling behavior of CaAl2 and the consecutive oxidation via elemental O2 were studied. While the use of H2O as the oxidizing agent showed only minor differences in the reaction products, ball milling of CaAl2 decreases, as expected, the crystallite size of the material and introduces defects. This is visible both in the powder X-ray diffraction patterns and in the 27Al solid-state MAS NMR spectra. In the subsequent steps, the ball milled material was oxidized in an STA system. Already 5 min of ball milling significantly changes the energy pattern of the reaction. Powder X-ray diffraction studies on the oxidized material clearly indicate that a different reaction pathway occurs. Samples ball milled for 180 min even get pyrophoric.