The microstructural stability and thermoelectric properties of Mm0.9Fe3.5Co0.5Sb12-based skutterudites

Recent efforts to scale the synthesis of skutterudite thermoelectric materials have identified microstructural inhomogeneities as a potential cause for diminished mechanical stability above 873 K. Specifically, this work evaluated the effect of un-reacted Sb on the dimensional stability of p-type skutterudite of nominal composition Mm0.9Fe3.5Co0.5Sb12 between 773 and 973 K. It is believed that residual Sb resides at grain boundaries, thus upon heating above the melting temperature of Sb (904 K), the microstructure softens and expands: (i) to relieve residual stress associated with hot pressing and/or (ii) due to vaporization of elemental Sb. Two approaches for mitigating the effects of un-reacted Sb were conducted. First, the effect of dimensional stability was evaluated based on Sb concentration between x = −1.0 (deficient) and 1.0 (excess) in Mm0.9Fe3.5Co0.5Sb12+x. When x > 0 the samples heated to 973 K exhibited four times the linear dimensional change (ΔL/L) compared to all other samples. Second, the effect of dimensional stability was evaluated using excess Fe as an Sb scavenger where y = 0.0 to 0.32 in Mm0.9Fe3.5+yCo0.5Sb12. For y ≥ 0.06, the dimensional change was significantly lower for samples heated to 873 and 973 K. Additionally, comprehensive thermoelectric property assessment indicated that the compositions resulting in improved dimensional stability had a negligible effect on ZT where the nominal ZT was approximately 0.9 at 773 K.

► Excess Sb reduces the thermal stability of p-type skutterudite.
► The microstructure of Sb-deficient skutterudite is more stable.
► Excess Fe acts as an Sb scavenger to improve thermal stability in skutterudite.
► Improved thermal stability while maintaining thermoelectric properties.
► Reducing excess Sb can improve consistency in processing and characterization.