Effects of boron oxide composition, structure, and morphology on B4C formation via the SHS process in the B2O3-Mg - C ternary system

This study investigates the formation of B4C in the B2O3-Mg-C ternary system via a magnesiothermic reduction process using two kinds of boron oxide (B2O3) - the commercial B2O3 and that synthesized from boric acid via the coupled chemical-thermal process. In addition to the raw materials, the products were subjected to XRD, FTIR, SEM, and FESEM analyses to determine the effects of microstructural and morphological properties of boron oxide as an important raw material, on B4C formation in the combustion synthesis process. For this purpose, powder mixtures of B2O3:Mg:C were prepared at a stoichiometric molar ratio of 2:6:1 and compacted into pellets using a uniaxial hydraulic press, which were subsequently subjected to the combustion synthesis process based on the self-propagating high-temperature synthesis (SHS). Finally, the samples thus obtained were leached in an aqueous hydrochloric acid solution. Analysis of the commercial B2O3 revealed the presence of large amounts of such by-products as magnesium borate (Mg3B2O6) and magnesium oxide (MgO) along with relatively small amounts of boron carbide after the leaching process, while those obtained for the chemically-thermally synthesized B2O3 showed a relatively large amount of B4C (from micron-sized particles to nanoparticles) together with a remaining carbon phase and very small amounts of magnesium borate as by-products. It can be, therefore, concluded that the changes in chemical composition and introduction of a hydrous HBO2 phase in the boron oxide in the B2O3-Mg-C mixture as well as its varied microstructure, morphology, and particle size have significant effects on the efficiency of B4C production through the SHS process.