A comparative study on combustion synthesis of Nb–Si compounds

A comparative study on the preparation of niobium silicides (such as Nb3Si, Nb5Si3, and NbSi2) in the Nb–Si system was conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of different stoichiometries. Effects of the sample green density, preheating temperature, and starting stoichiometry on combustion characteristics, as well as on product composition were studied. Test specimens with five different compositions including Nb:Si = 3:1, 5:3, 3:2, 1:1, and 1:2 were employed in this study. Experimental observations indicated that except for the sample of Nb:Si = 3:1, upon ignition self-sustained combustion was well established and proceeded throughout the entire sample. Measured results showed that the reactant compact of Nb:Si = 5:3 had the highest flame-front propagation velocity, followed sequentially by the powder compacts with Nb:Si = 3:2, 1:1, and 1:2. Variation of the combustion temperature with sample initial stoichiometry is in a manner consistent with that of the flame-front velocity. On account of the close atomic ratios, the test specimens made up of either Nb:Si = 5:3 or 3:2 yielded a single-phase silicide Nb5Si3 in both α and β forms. As identified by the XRD analysis, it was found that α-Nb5Si3 was dominant in the final product from the Nb:Si = 5:3 compact, but β-Nb5Si3 was the major composition formed by the Nb:Si = 3:2 compact. Because of the nonexistence of a silicide phase with the atomic ratio Nb/Si = 1, a multiphase product consisting of α-Nb5Si3 and NbSi2 was synthesized from the reactant compact of Nb:Si = 1:1. For the samples with Nb:Si = 1:2, the disilicide NbSi2 was monolithically produced along with trivial amounts of unreacted Nb and Si. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with combustion synthesis of Nb5Si3 and NbSi2 were determined to be 259.2 and 160.9 kJ/mol, respectively.