"In situ" borothermic route to Group 4 nanosized metal borides particles

An "in-situ" borothermal synthesis was investigated to achieve submicron sized ZrB2 particles at temperature <1000°C in flowing of Ar/H2 atmosphere. We propose the use of sodium borohydride (NaBH4) as a reducing agent as an alternative to elemental boron or boron carbide. Advantages of sodium borohydride with respect to the latters are a lower price, higher purity and lower melting point. Furthermore, the reaction byproducts are easily removed by washing with cold diluted HCl solution. The powders, e.g. ZrO2 and NaBH4, were dry mixed and thermally treated at increasing temperatures (600, 900, 1100°C) and B:Zr ratio (4:1, 3:1, 2:1) to study the reaction yield. X-Ray diffraction and FE-SEM and STEM techniques were used to analyses reagents and products before and after synthesis. For the sake of comparison, parallel experiments were also conducted with elemental boron using the classical borothermal reaction in the same conditions. It was found that the reaction of ZrO2 with NaBH4 was completed after 30 min at 900°C, with a 4:1 B:Zr ratio, e.g. at 200°C lower than typical synthesis temperature via the borothermal reaction. Microstructural observation revealed ZrB2 particles with dimensions around 100 nm. After successful synthesis of ZrB2, work is in progress to extend the reaction to other refractory metal oxides, such as HfO2 and TiO2.