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 CaB₆-SrB₆ diffusion couples, showing that the diffusivity of Ca²⁺ and Sr²⁺ 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 Ca²⁺ and Sr²⁺ 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 Ca²⁺ diffusion in SrB₆ and Sr²⁺ diffusion in CaB₆. We determine an enhancement of 3.8× for Ca²⁺ into SrB₆ versus an enhancement of 1.8× for Sr²⁺ into CaB₆. No new phases are formed at the interface between CaB₆ and SrB₆, 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.