Mechanical-thermal synthesis of NbB2 powder from Mg/B2O3/Nb powder mixture

Abstract

Abstract Three different approaches were used for the synthesis of niobium boride using a stoichiometric ratio of Mg–B2O3– Nb powder mixture: (i) thermal ignition, (ii) mechanochemical and (iii) a combined mechanical and thermal activation process. Phase transformation analysis and structural evaluation were carried out by means of differential thermal analysis techniques, X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. During the thermal synthesis, the reaction between Mg and B2O3 partially occurred (liquid state reaction). The desired final phase (NbB2) was poorly obtained. Phase analysis revealed that the NbB2 phase was synthesized after 10 h of high energy ball milling. A pre-milling treatment of the powder mixture up to 2 h significantly reduced the reactions temperature (580 °C). Consequently, NbB2 was prepared simultaneously involving magnesiothermic reduction of B2O3 and reaction between elemental of Nb and boron. Mg3(BO3)2, as major by-product, was formed in noticeable amounts during the thermal process, while its quantity significantly decreased during mechanochemical synthesis and completely eliminated by using the combined mechanical and thermal synthesis.