Changing the reaction pathway of the CaAl₂ oxidation using ball milling

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.