Realizing Enhanced Thermoelectric Performance and Hardness in Icosahedral Cu5FeS4−xSex with High‐Density Twin Boundaries

Bornite (Cu5FeS4) is an Earth‐abundant, nontoxic thermoelectric material. Herein, twin engineering and Se alloying are combined in order to further improve its thermoelectric performance. Cu5FeS4−xSex (0 ≤ x ≤ 0.4) icosahedral nanoparticles, containing high‐density twin boundaries, have been synthesized by a colloidal method. Spark plasma sintering retains twin boundaries in the pellets sintered from Cu5FeS4−xSex colloidal powders. Thermoelectric property measurement demonstrates that alloying Se increases the carrier concentration, leading to much‐improved power factor in Se‐substituted Cu5FeS4, for example, 0.84 mW m−1 K−2 at 726 K for Cu5FeS3.6Se0.4; low lattice thermal conductivity is also achieved, due to intrinsic structural complexity, distorted crystal structure, and existing twin boundaries and point defects. As a result, a maximum zT of 0.75 is attained for Cu5FeS3.6Se0.4 at 726 K, which is about 23% higher than that of Cu5FeS4 and compares favorably to that of reported Cu5FeS4‐based materials. In addition, the Cu5FeS4−xSex samples containing twin boundaries also obtain improved hardness compared to the ones fabricated by melting‐annealing or ball milling. This work demonstrates an effective twin engineering‐composition tuning strategy toward enhanced thermoelectric and mechanical properties of Cu5FeS4‐based materials. Cu5FeS4−xSex (0 ≤ x ≤ 0.4) icosahedral nanoparticles, containing high‐density twin boundaries, have been synthesized by a colloidal method. The peak power factor and zT value of Cu5FeS3.6Se0.4 reach 0.84 mW m−1 K−2 and 0.75 at 726 K, respectively. Our twin boundary‐containing Cu5FeS4 also obtains a high Vickers hardness of 182.