ABSTRACT
The reaction of Co with gaseous BBr3 in a temperature range of 700 to 1000 °C was studied using the hot-wire method with an experimental set-up reminiscent of the van Arkel-de Boer method. The borides Co2B und CoB form as layers on the surface of elemental cobalt. The influence of pressure, temperature and time on the reaction rate and on the composition of the borides was investigated. The reaction rate is significantly decreased by small amounts of an inert gas. The adjustment of reaction conditions allows to obtain single-phase and well-crystallized bulk materials of Co2B or CoB.
ABSTRACT
The crucible-free reaction between heated tungsten wires and gaseous boron tribromide yields different tungsten borides. The experiments were carried out at various reaction temperatures and times, revealing the formation of W2 B, WB and WB2 phases. The underlying reactions were analyzed by using thermodynamic model calculations. The stability of the gaseous tungsten bromides was evaluated using quantum chemical methods. While the developed synthesis of phase-pure borides is only possible to a limited extent, it offers a potential route for a formation of protective coatings with high chemical and thermal resistance.
ABSTRACT
This study gives an account of an innovative, crucible-free technique for the synthesis of single-phase borides at relatively moderate temperatures. A metal wire heated by an electrical current reacts with a chosen gaseous boron halide in a gas/solid reaction yielding a single-phase, oxygen- and carbon-free product, as evidenced by X-ray powder diffraction and chemical analysis. This method is demonstrated using the example of hafnium reacting with boron tribromide. Preliminary thermodynamic considerations show that this kind of crucible-free synthesis specifically enables the preparation of borides of transition metals and similar elements.
