EFFECT OF METAL-PARTICLE MORPHOLOGY ON THE COMBUSTION OF REFRACTORY-METALS IN NITROGEN

The synthesis of niobium nitride by combustion of niobium metal powder under nitrogen atmosphere was investigated. Ignition and propagation characteristics were studied in the region of nitrogen pressure between 5 and 100 atmospheres and solid-phase dilution up to 30 wt%. Within this region of operating parameters, the combustion reaction proceeds without any significant liquid-phase formation, thus exhibiting many features common with the combustion of tantalum in nitrogen. However, the effect of metal particle size on the combustion characteristics was different for the two metal systems. This was attributed to the difference in morphology and microstructure between the niobium and tantalum reactant particles. The operating parameters for efficient synthesis of nearly stoichiometric cubic NbN were determined. Results from appropriately designed experiments under conditions where the filtration limitations were minimized were used to extract the apparent activation energy of the combustion nitridation process, which was found to be 40 809 cal/mol.