Reactive growth of niobium silicides in bulk diffusion couples

The diffusion-controlled growth of niobium silicides (NbSi2 and Nb5Si3) was studied in Nb/Si and Nb/NbSi2 bulk diffusion couples annealed at 1200-1350 degreesC for 2-24 h. Both compounds were found to grow as parallel layers, according to the parabolic rate law. The concept of the integrated diffusion coefficient is used to describe the growth kinetics of the two silicides. The corresponding activation energy is 263 kJ/mol for Nb5Si3 and 304 kJ/mol for NbSi2. The activation energy (in eV) scales as 0.98T(m)(K)/1000 for Nb5Si3 and as 1.4T(m)(K)/1000 for NbSi2 in agreement with the general behavior observed for many transition metal silicides. The position of the Kirkendall plane inside the Nb5Si3 layer developed in Nb/NbSi2 couples indicates that, in the present temperature range, the diffusion of Si in Nb5Si3 is considerably faster than that of Nb.