Enhancing the Thermoelectric Performance of Hydrothermally Synthesized Porous n-Type Co_{4–<i>y</i>–<i>z</i>}Ni_<i>y</i>Fe_<i>z</i>Sb₁₂ Skutterudites via Solid–Vapor Indium Incorporation

Conventionally, incorporating indium into skutterudites has been accomplished through energy-intensive melting processes. However, hydrothermal synthesis alone cannot facilitate the incorporation of indium. Our study presents an approach for synthesizing indium-filled n-type Co4–y–zNiyFezSb12 skutterudites by combining hydrothermal synthesis with a solid–vapor reaction process. Our innovative method overcomes the limitations of hydrothermal synthesis, resulting in a significant 73% improvement in the thermoelectric dimensionless figure-of-merit compared to an indium-free counterpart. Analyses reveal the in situ formation of InSb and In2O3 phases, which significantly influence the transport properties and thermoelectric performance. The dimensionless figure-of-merit (zT) is achieved at 685 K with a peak value of 0.96. This improvement can be attributed to a notable reduction in thermal conductivity while maintaining a favorable power factor. A trade-off relationship between the Seebeck coefficient and electrical conductivity was observed upon indium addition, which is attributed to the interfacial low-energy electron filtering effect facilitated by the in situ formed nanosized InSb and In2O3 phases at grain boundaries.