A comparative study on combustion synthesis of Nb-B compounds

A comparative study on the preparation of specific niobium borides (including Nb3B2, NbB, Nb5B6, Nb3B4, and NbB2) 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 Nb3B4 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 NbB2, respectively. In contrast, multiphase compounds consisting of Nb3B4, NbB, and NbB2 were synthesized from the powder compacts with starting stoichiometries Nb:B = 3:4 and 5:6. However, it was found that two boride phases Nb3B2 and Nb5B6 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 NbB2 were determined to be 151.8 and 132.4 kJ/mol, respectively.