A comparative study on combustion synthesis of Nb–B compounds

Abstract

A comparative study on the preparation of specific niobium borides (including Nb 3B 2, NbB, Nb 5B 6, Nb 3B 4, and NbB 2) in the Nb–B system was experimentally conducted by self-propagating high-temperature synthesis (SHS) from elemental powder compacts of their corresponding stoichiometries. Effects of the sample green density, preheating temperature, and starting stoichiometry on combustion characteristics, as well as on product composition were studied. Experimental evidence indicated that except for the samples of Nb:B = 3:2, upon ignition a planar and self-sustained combustion front was established and proceeded throughout the entire sample in a steady manner. However, combustion of the samples with Nb:B = 3:2 was characterized by a localized reaction zone propagating along a spiral trajectory, due largely to the low combustion temperatures which further resulted in a poor degree of phase conversion with a significant amount of Nb left unreacted. The incomplete reaction in the Nb:B = 3:2 powder compact produced boride compounds Nb 3B 4 and NbB in minor quantities. Reactant compacts of Nb:B = 1:1 and 1:2 were shown to yield practically single-phase monoboride NbB and diboride NbB 2, respectively. In contrast, multiphase compounds consisting of Nb 3B 4, NbB, and NbB 2 were synthesized from the powder compacts with starting stoichiometries Nb:B = 3:4 and 5:6. However, it was found that two boride phases Nb 3B 2 and Nb 5B 6 did not appear in the end products from any of the reactant compacts. Based upon the temperature dependence of combustion wave velocity, the activation energies associated with combustion synthesis of NbB and NbB 2 were determined to be 151.8 and 132.4 kJ/mol, respectively.