Synthesis of a high-boron aluminum boride via borothermic reduction of alumina

Abstract

The reaction between alumina and boron in vacuum was studied between 800 and 1500‡C at different compositions of the starting mixture. The reaction products were analyzed chemically and by x-ray diffraction, optical, and microstructural analysis techniques. The compositions of the reaction intermediates and products were determined, which made it possible to propose a reaction scheme which differs significantly from the commonly accepted one and assumes that not only B2O3 but also AlO are removed in the course of the reaction. It is shown that only 60% of the starting Al is incorporated into the forming phase, AlB18-31 (Β-B hexagonal structure,a = 1.0970 nm,c = 2.3780 nm). No aluminum dior dodecaboride was detected in the reaction products. The proposed mechanism of the process explains why the borothermic reduction of various metal oxides typically yields only the highest boron phases.