Changing the reaction pathway of the CaAl oxidation using ball milling.

As previously shown, CaAl can be oxidized using elemental O to form CaAlO. This reaction, however, proceeds CaAlO 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 (HO) as well as the ball milling behavior of CaAl and the consecutive oxidation elemental O were studied. While the use of HO as the oxidizing agent showed only minor differences in the reaction products, ball milling of CaAl 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 Al 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.