Effect of Current on Diffusivity in Metal Hexaborides: A Spark Plasma Sintering Study

We present the effect of pulsed direct current on metal ion diffusion in CaB 6 –SrB 6 diffusion couples, showing that the diffusivity of Ca 2+ and Sr 2+ across the diffusion couple interface is higher toward the positive electrode when subjected to a current flow of 2.2 kA at a temperature of 2007 K. We attribute this enhanced mobility to the movement of negatively charged metal vacancies toward the positive electrode in the system. Energy-dispersive spectroscopy is used to map the concentration of Ca 2+ and Sr 2+ in the region near the interface, and diffusion profiles are fitted with error functions. The concentration curves display concentration-dependent Boltzmann–Matano diffusivity. Total dopant values ( Q ) have been used to numerically compare the differences between Ca 2+ diffusion in SrB 6 and Sr 2+ diffusion in CaB 6 . We determine an enhancement of 3.8× for Ca 2+ into SrB 6 versus an enhancement of 1.8× for Sr 2+ into CaB 6 . No new phases are formed at the interface between CaB 6 and SrB 6, since hexaboride compounds readily form solid solutions. The results elucidate the role of pulsed direct current on the diffusion of metal ions in hexaboride compounds.