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-SrB diffusion couples, showing that the diffusivity of Ca and Sr 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 and Sr 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 diffusion in SrB and Sr diffusion in CaB. We determine an enhancement of 3.8× for Ca into SrB versus an enhancement of 1.8× for Sr into CaB. No new phases are formed at the interface between CaB and SrB, 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.