Synthesis and Thermal Oxidation Stability of Nanocrystalline Niobium Diboride

A method has been developed for the synthesis of nanocrystalline NbB2 powder with an average particle size of 65 nm. The material has been prepared by reacting Nb and amorphous B powders (1 : 2 ratio) in Na2B4O7, KCl, and KBr ionic melts after pretreatment with hydrogen and activation in a high-energy planetary mill for 40 min. The synthesis process was run for 32 h at 800 degrees C in argon at a pressure of 4 MPa. The results demonstrate that, independent of the composition and nature of the melt, the process yields NbB2 nanoparticles with hexagonal symmetry (sp. gr . P6/mmm) and unit-cell parameters a = 0.3100-0.3108 nm and c = 0.3278-0.3298 nm. The products of the oxidation of the NbB2 nanoparticles with atmospheric oxygen during heating to 1000 degrees C and isothermal oxidation at 400, 450, 500, 550, and 600 degrees C have been characterized by thermal analysis, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray microanalysis, X-ray photoelectron spectroscopy, and IR frustrated total internal reflection spectroscopy. The rate constants for the oxidation of NbB2 nanoparticles at these temperatures have been determined to be 0.0013, 0.045, 0.47, 2.61, and 8.83 h(-1), respectively. The oxidation onset temperature has been determined to be 310 degrees C. The effective activation energy evaluated for the oxidation of Nb-2 nanoparticles from the temperature dependence of rate constants is 220 +/- 8 kJ/mol.

Automated summary

A method was developed to synthesize nanocrystalline NbB2 powder with specific structure and parameters, oxidation rate and activation energy were experimentally determined under certain temperature and pressure conditions.