Interfacial stability of CoSb3 in contact with chromium: Reactive diffusion and microstructure evolution

Abstract Reliability of thermoelectric devices strongly depends on the stability of structural materials at the interfaces. In this work reactivity and diffusion in a Cr–CoSb3 couple under vacuum were systematically studied. Diffusion couples were produced by magnetron sputtering of chromium onto CoSb3 substrates. The experimental procedure involved isothermal annealing of the diffusion couples sealed in the evacuated quartz ampules in the temperature range 500 °C–600 °C for time intervals up to 288 h. Different techniques of electron microscopy and spectroscopy were employed to evaluate the extent of reactive diffusion. Transport processes and chemical reactions involved in the formation of an interdiffusion zone were assessed. In all samples this zone was layered and composed of Co–Cr antimonides with different proportions of transition metals. To visualize the complex structure of the interdiffusion zone, a 3D model was created using focused ion beam – scanning electron microscopy (FIB-SEM) tomography. It was found that its growth kinetics followed a parabolic rate law. The calculated parabolic rate constants, k, for the innermost layer composed of (Co,Cr)Sb2 were 4.3·10−13cm2s−1, 1.5·10−12cm2s−1 and 3.6·10−12cm2s−1, and for the outermost layer composed of (Cr,Co)Sb 1.2·10−13cm2s−1, 4.4·10−13cm2s−1 and 9.4·10−13cm2s−1 at 500 °C, 550 °C and 600 °C, respectively. Activation energies, Q, for the layer growth of (Co,Cr)Sb2 and (Cr,Co)Sb were 118.7 kJ·mol−1 and 114.5 kJ·mol−1, respectively. Finally, consecutive stages of the interdiffusion zone formation were explained.