We have synthesized undoped, Co-doped (up to 5%), and Se-doped (up to 4%) FeS2 materials by mechanical alloying in a planetary ball mill and investigated their thermoelectric properties from room temperature (RT) to 600 K. With decreasing particle size, the undoped FeS2 samples showed higher electrical conductivity, from 0.02 S cm−1 for particles with 70 nm grain size up to 3.1 S cm−1 for the sample with grain size of 16 nm. The Seebeck coefficient of the undoped samples showed a decrease with further grinding, from 128 μV K−1 at RT for the sample with 70-nm grains down to 101 μV K−1 for the sample with grain size of 16 nm. The thermal conductivity of the 16-nm undoped sample lay within the range from 1.3 W m−1 K−1 at RT to a minimal value of 1.2 W m−1 K−1 at 600 K. All doped samples showed improved thermoelectric behavior at 600 K compared with the undoped sample with 16 nm particle size. Cobalt doping modified the p-type semiconducting behavior to n-type and increased the thermal conductivity (2.1 W m−1 K−1) but improved the electrical conductivity (41 S cm−1) and Seebeck coefficient (-129 μV K−1). Isovalent selenium doping led to a slightly higher thermal conductivity (1.7 W m−1 K−1) as well as to an improved electrical conductivity (26 S cm−1) and Seebeck coefficient (110 μV K−1). The ZT value of FeS2 was increased by a factor of five by Co doping and by a factor of three by Se doping.
