Unfilled Co4Sb12 skutterudites typically exhibit n-type conductivity at room temperature due to native donor-type defects associated with off-stoichiometric compositions arising from Sb loss during high-temperature synthesis. Achieving stable p-type conductivity in the absence of rare-earth fillers or transition-metal substitution at the Co site remains a significant challenge. Here, we report an in situ composite engineering approach in which elemental Bi (x = 0, 3, 6, 11 wt.%) is incorporated into Co4Sb12 via a melting–quenching technique followed by ball milling and reactive spark plasma sintering. Phase composition and microstructural analyses confirm that Bi does not occupy the skutterudite icosahedral void sites and instead promotes the formation of Bi1−xSbx and Co4Sb2 secondary phases, whose volume fractions increase systematically with nominal Bi content. Notably, the sample with 11 wt.% Bi exhibits an essentially complete switch from n- to p-type conduction across the entire temperature range studied, suggesting effective mitigation of Sb loss through Bi incorporation into Co4Sb12. The room-temperature total thermal conductivity decreases from ~6 W m−1 K−1 for pristine Co4Sb12 to ~4.7 W m−1 K−1 for the 11 wt.% Bi-added sample, primarily due to enhanced phonon scattering at grain boundaries and interfaces. Additionally, the room-temperature electrical conductivity of this sample increased by more than one order of magnitude relative to the Bi-free sample, an effect most likely attributable to the combined influence of modified defect equilibria in the skutterudite matrix and the presence of conducting Bi1−xSbx phase. While the maximum thermoelectric figure of merit (zT of ~0.11 at 573 K) remains modest, the demonstrated conduction-type switching mechanism, achieved without rare-earth elements or intentional void filling, offers a rare-earth-free strategy for tailoring conductivity type in skutterudite-based thermoelectrics through reactive composite formation.
Raut et al. (Mon,) studied this question.