Single Step Process for Crystalline Ni-B Compounds.

Crystalline Ni2B, Ni3B, and Ni4B3 are synthesized by a single-step method using autogenous pressure from the reaction of NaBH4 and Ni precursors. The effect of reaction temperature, pressure, time, and starting materials on the composition of synthesized products, particle morphologies, and magnetic properties is demonstrated. High yields of Ni2B (>98%) are achieved at 2.3⁻3.4 MPa and ~670 °C over five hours. Crystalline Ni3B or Ni4B3 form in conjunction with Ni2B at higher temperature or higher autogenous pressure in proportions influenced by the ratios of initial reactants. For the same starting ratios of reactants, a longer reaction time or higher pressure shifts equilibria to lower yields of Ni2B. Using this approach, yields of ~88% Ni4B3 (single phase orthorhombic) and ~72% Ni3B are obtained for conditions 1.9 MPa < Pmax < 4.9 MPa and 670 °C < Tmax < 725 °C. Gas-solid reaction is the dominant transformation mechanism that results in formation of Ni2B at lower temperatures than conventional solid-state methods.

Synthesis of Magnesium Nickel Boride Aggregates via Borohydride Autogenous Pressure.

We demonstrate synthesis of the ternary intermetallic MgNi3B2 using autogenous pressure from the reaction of NaBH4 with Mg and Ni metal powder. The decomposition of NaBH4 to H2 and B2H6 commences at low temperatures in the presence of Mg and/or Ni and promotes formation of Ni-borides and MgNi3B2 with the increase in temperature. MgNi3B2 aggregates with Ni-boride cores are formed when the reaction temperature is >670 °C and autogenous pressure is >1.7 MPa. Morphologies and microstructures suggest that solid-gas and liquid-gas reactions are dominant mechanisms and that Ni-borides form at a lower temperature than MgNi3B2. Magnetic measurements of the core-shell MgNi3B2 aggregates are consistent with ferromagnetic behaviour in contrast to stoichiometric MgNi3B2 which is diamagnetic at room temperature.