RESUMO
Bornite (Cu5 FeS4 ) is an Earth-abundant, nontoxic thermoelectric material. Herein, twin engineering and Se alloying are combined in order to further improve its thermoelectric performance. Cu5 FeS4-x Sex (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 Cu5 FeS4-x Sex colloidal powders. Thermoelectric property measurement demonstrates that alloying Se increases the carrier concentration, leading to much-improved power factor in Se-substituted Cu5 FeS4 , for example, 0.84 mW m-1 K-2 at 726 K for Cu5 FeS3.6 Se0.4 ; 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 Cu5 FeS3.6 Se0.4 at 726 K, which is about 23% higher than that of Cu5 FeS4 and compares favorably to that of reported Cu5 FeS4 -based materials. In addition, the Cu5 FeS4-x Sex 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 Cu5 FeS4 -based materials.
RESUMO
GeSe was theoretically predicted to have thermoelectric (TE) performance as high as SnSe. However, the relatively high TE performance was not achieved experimentally in doped GeSe samples with an original orthorhombic structure but observed in Ag(Sb,Bi)(Se,Te)2 alloyed samples that crystalize in either a rhombohedral or cubic structure. Herein, to clarify the crystal structure-dependent properties, the electrical and thermal transport properties of GeSe1-xTex (0 ≤ x ≤ 0.5), where orthorhombic, hexagonal, and rhombohedral phases are stable at room temperature for different Te content, have been studied, without any intentional manipulation on carrier concentration. It is found that the three phases show intrinsically different hole concentrations: â¼1016 cm-3 for the orthorhombic phase but as high as 1021 cm-3 for the hexagonal and rhombohedral phases. Ge-rich status in the orthorhombic phase and Ge-poor status in hexagonal and rhombohedral phases may be responsible for the huge difference in hole concentrations. The rhombohedral phase shows a much higher Seebeck coefficient than the hexagonal phase with similar hole concentration, indicating that the profile of valance band maximum for the rhombohedral structure is more favorable for high TE performance than the hexagonal phase in GeSe1-xTex. The highest zT of 0.69 has been obtained in GeSe0.55Te0.45 at 778 K, at which temperature the rhombohedral phase has already transformed to a cubic phase; however, a zT value of 1.74 at 628 K is predicted by the quality factor analysis for rhombohedral GeSe0.55Te0.45 if optimum hole concentration can be achieved.
