Kinetic mechanism in the process of carbothermal reduction of ferrum niobate

Abstract

Ferrum niobate was synthesized by solid-phase sintering method in a vacuum carbon tube furnace at 1 300 °C for 180 min. The phase transformation of ferrum niobate carbothermal reduction process was studied by XRD. The reduction reactions of ferrum niobate in different temperature stages were determined by the TG-DSC curve. Meanwhile, according to the TG curve, the reaction kinetics parameters were calculated by A.W.Coats integration and the control steps in different temperature stages were ascertained. The results showed that the reduction of ferrum niobate starts at the temperature of 1 000 °C, and the reduction process carries out in two steps according to sintering temperature.In a temperature range of 1 000-1 238 °C (the first step), the main reduction products are NbO₂ and Fe; the kinetic equation of initial stage is [-ln(1-α)]⁴=kt, controlled by nucleation growth, and the apparent activation energy is 388 kJ/mol; with the temperature increasing, the kinetic equation is α+(1-α)ln(1-α)=kt, which is the Valensi two-dimensional diffusion kinetic equation, and the apparent activation energy is 264.4 kJ/mol. The main reaction in a range of 1 238-1 344 °C(the second step) is the reduction of NbO₂ to NbC, the kinetic equation is [(1-α)^-1/3-1]²=kt, which is controlled by the three-dimensional diffusion, and the apparent activation energy is 482.7 kJ/mol.