Synthesis of niobium carbide and nitride in air from mechanically activated Nb-C powder mixtures

Powders of Nb and C mixed in various mole fractions (C=10-80 mol%) have been ground in a planetary ball mill. When thus mechanically activated mixtures were transferred into a graphite crucible and exposed to air, they ignited spontanteously and self-propagating high-temperature synthesis (SHS) took place. NbC, Nb sub 2 C and Nb sub 2 N were obtained as the main products in the samples with C=40, 30 and 10 mol%, respectively. From measurement of the lattice constant and XPS spectra, the dissolution of nitrogen into NbC obtained from the mixtures with below C=50 mol% was observed, while almost stoichiometric NbC formed for the mixtures with C=50-80 mol%. A possible reaction mechanism for the formation of niobium carbides and nitrides is discussed.