Kinetics of solid vanadium-nitrogen reactions at high temperatures

Abstract

Thermogravimetric studies were made to determine the kinetics of the reaction between 99.83 pct vanadium sheet and purified nitrogen gas in the temperature range of 1173 to 1723 K, at pressures of 9.3 × 104 to 9.3 Pa (700 to 7 × 10-2 torr) and times up to 160 h. Metallographic and X-ray diffraction techniques were used to examine the reaction products. The data followed either parabolic, cubic, or linear kinetics. In general it was observed that the initial kinetics were parabolic. At high temperatures and near atmospheric pressures, the reaction kinetics changed from parabolic to cubic at longer times. At the lowest pressure the preparabolic kinetics were linear. Weight gains in the parabolic region were mainly due to solution and diffusion of nitrogen in vanadium with the weight gain shown by the equation (ΔW/A)2 = 3.2 × 10-4 Te162-000/RTt. The cubic region was related to the formation of an outer layer of VN over an intermediate layer of V3N and a core of the saturated solution of nitrogen in vanadium with the approximate weight gain shown by the equation (ΔW/A)3 = 1.24 × 10-3Te-208,000/RTt Linear weight gains at low pressure were presumed to be controlled by the rate of nitrogen adsorption.