Simultaneous reduction nitridation for the synthesis of tungsten nitrides from Ni–W–O precursors

Abstract

Tungsten nitrides were synthesised from NiO–WO₃ and NiWO₄ precursors at 973–1273 K in a flow of H₂–N₂ gas mixture. The reduction–nitridation reactions were carried out isothermally in fluidised bed reactor, and the off-gas from the reactions was continuously analysed by gas chromatography. The effect of reaction temperature and precursor composition on the rate of formation of Ni–W nitrides was studied. The different phases developed during the reduction–nitridation reactions were identified by X-ray diffraction analysis technique. The morphology and the grain structure of the precursors were examined by SEM, and the elemental composition in the structure was analysed by electron dispersive spectrometry. The results showed that the reduction of Ni–W–O precursors proceeded in a stepwise manner (NiWO₄→Ni–WO₃→Ni–WO₂→Ni–W). Tungsten nitrides (WN and WN₂) were formed from the reaction of the freshly reduced W metal with N₂ gas and WN was the predominant phase detected at higher temperatures. The reaction mechanisms were elucidated from the apparent activation energy values and the application of different formulations derived from the gas–solid reaction model at early and later stages of reactions. It was concluded that the interfacial chemical reaction is the rate determining step at initial stages, while a combined effect of gaseous diffusion and interfacial chemical reaction controlled the reaction at later stages. At final stages, the nitridation reactions contributed to the reaction mechanism leading to produce tungsten nitrides.

Keywords:

• Gas–solid reaction
• Nickel tungstate
• Tungsten nitrides
• Kinetics and mechanism