Vacuum-free arc synthesis and characterization of crystalline molybdenum borides as instrumental material

Transition metal borides are a fascinating class of materials that have garnered significant attention in various fields due to their unique properties and diverse applications. Here the possibility of obtaining crystalline phases in the molybdenum-boron system by direct current (DC) arc discharge method under ambient air conditions is presented for the first time. The synthesis process can be carried out in an open-air environment, which significantly simplifies the design of electric arc reactors and increases the efficiency of desired materials. The influence of different parameters, such as the power supply current, arc exposure time, the Mo:B ratio in the raw powder material on the phase composition of the synthesis products is studied. As a result, almost all known phases of molybdenum borides, namely I4/m$c$m-Mo₂B, $I{4}_1/amd$-MoB (α-MoB), $\mathrm{Cmcm}$-MoB (β-MoB), $P1$-Mo₇B₁₁, $R\overline{3}m$-MoB₂, $P$6₃/$\mathrm{mmc}$-MoB₃, as well as practically unexplored higher boride $P$6₃/$\mathrm{mmc}$-MoB_5-x are synthesized. Convolution neural network (CNN) trained on the computational data of known molybdenum borides allows the analysis of experimental X-ray diffraction data and identification of individual phases in the synthesized powders. The optimal parameters of synthesis are determined to obtain samples with ∼70 wt% of higher molybdenum boride MoB_5-x: current strength of 200 A, arc exposure time of 40  s, molybdenum to boron atomic ratio of 1:17. Synthesized powder is then used to sinter ceramic samples to measure the mechanical properties, such as hardness and elastic modulus. The findings of this study are much more extensive than those presented here. They offer a potential for rapid and cheap synthesis of instrumental materials based on molybdenum borides, which possess high mechanical properties.