Kinetics of NiCr ₂ O ₄ Formation and Diffusion of Cr ³⁺ Ions in NiO

The reaction kinetics for NiCr 2 O 4 formation and the diffusion of Cr 3+ ions into single‐crystal NiO were studied between 1300° and 1600°C in air. The experimental activation energy for NiCr 2 O 4 formation was about 83 kcal/mol. After incubation, NiCr 2 O 4 formed by a diffusion‐controlled process. The origin of pores at the NiO/NiCr 2 O 4 interface is discussed. The concentration profiles of Cr 3+ in NiO were linear because the interdiffusion coefficient was directly proportional to the mol fraction Cr 3+ . Theoretical considerations indicate that the interdiffusion coefficient equals 3/2 the self‐diffusion coefficient of Cr 3+ , which is rate‐determining. The interdiffusion coefficient at 1 mol% Cr 2 O 3 can be expressed as D̄=4×10 −3 exp (−55,000/RT) cm 2 s −1 .