Kinetics of rapid high-temperature reactions: Titanium-nitrogen system

We have developed a computer-assisted electrothermography method to determi ne the intrinsic kinetics of reactions under conditions similar to those re alized during combustion synthesis of materials. In this method, metal wire s are heated in a reactive gas atmosphere in a controlled manner by passing electric current, and when time-dependence characteristics of electric pow er applied to the wire are measured, the reaction kinetics can be extracted . To illustrate the technique, it is applied to investigate the kinetics an d other features associated with the reaction of titanium with nitrogen at 1 atm of pressure. The temperature range 1400-2300 K and heating rates 10(4 )-10(5) K/s are studied. It is shown that at temperatures below the melting point of titanium, the reaction follows a parabolic rate law, correspondin g to a sharp-interface model with nitrogen diffusion through the developing titanium nitride layer as the rate-controlling step. The obtained activati on energy value (230 kJ/mol) is in good agreement with this model. At highe r temperatures, however, the reaction mechanism is different because of fas ter diffusion of nitrogen in molten titanium. In addition, the reaction rat e is shown to depend strongly on the heating rate and on the heat pretreatm ent of titanium.