Thermoelectric Properties of Sulfur-Filled and Iron-Substituted Co₄Sb₁₂

Filling the voids of a skutterudite material using suitable electropositive elements majorly affects the power factor by altering the carrier concentration. The mass difference of the filler with constituent elements leads to a lower lattice thermal conductivity and consequently increases the figure of merit. The combination of doping and filling can further improve thermo-electric properties. Usually, the type of bonding between the fillers and host atoms is decided by the difference in their electro-negativity values. Electropositive elements have been successfully used as n-type fillers in Co4Sb12. The doping of electron donors at the Co or Sb site helps to incorporate electronegative elements into the voids by the charge compensation between a n-type dopant and a p-type filler. In this study, a p-type dopant Fe has been substituted in the Co site of S0.15Co4Sb12. A series of S0.15FexCo4-xSb12 (x = 0.05-0.9) samples were prepared by a solid-state reaction. CoSbS, Sb secondary phases were noticed in the samples with x = 0.05 and 0.1 and FeS, Sb secondary phases were observed in the samples with x >= 0.2. The filling content of S into the void was similar to 0.04 for S0.15Co4Sb12, which decreased with the increasing Fe content. The Seebeck coefficient was positive, indicating that holes are the majority of charge carriers. An increase in carrier concentration was observed with the increase in the Fe content, which reduced the Seebeck coefficient and electrical resistivity. The thermal conductivity was reduced for the samples with x = 0.1 and 0.9 compared to the sample S0.15Co4Sb12 due to Fe doping in the Co site and S filling in the void site. The highest zT of similar to 0.30 at 773 K was obtained for S0.15Fe0.9Co3.1Sb12.