Tristan Barbier

Up-scaled synthesis process of sulphur-based thermoelectric materials

The scale up of Spark Plasma Sintering (SPS) for the consolidation of large square monoliths (50 × 50 × 3 mm3) of thermoelectric materials is demonstrated and the properties of the fabricated samples compared with those from laboratory scale SPS. The SPS processing of n-type TiS2 and p-type Cu10.4Ni1.6Sb4S13 produces highly dense compacts of phase pure material. Electrical and thermal transport property measurements reveal that the thermoelectric performance of the consolidated n- and p-type materials is comparable with that of material processed using laboratory scale SPS, with ZT values that approach 0.75 and 0.35 at 700 K for Cu10.4Ni1.6Sb4S13 and TiS2, respectively. Mechanical properties of the consolidated materials show that large-scale SPS processing produces highly homogeneous materials with hardness and elastic moduli that deviate little from values obtained on materials processed on the laboratory scale. More generally, the process described in this paper is a promising way to produce high performance thermoelectric materials with square geometry, specifically required for thermoelectric device production.

Structural and thermoelectric properties of n-type isocubanite CuFe2S3

An effective strategy to enhance thermoelectric performance consists of scattering phonons by point defects, such as the ones intrinsic to the n-type isocubanite CuFe2S3. In this structure, the TEM evidences a random distribution of vacancies, and copper and iron ions of the 4c and 4d sites naturally reduce the phonon lifetime. Furthermore, orthorhombic and cubic cubanite are usually found in their natural states intimately intergrown with other sulfides such as chalcopyrite (CuFeS2) and pyrrhotite (Fe7S8). We performed comprehensive studies of Rietveld analysis of X-ray diffraction, transmission electron microscopy, and thermoelectric transport measurements of the n-type isocubanite CuFe2S3, and we obtained a maximum ZT value of 0.14 at 700 K in this environmentally friendly compound. Our electronic structure calculations support the fact that chalcopyrite CuFeS2 is an antiferromagnetic insulator, together with a higher degree of covalency of sulfide compared to oxides. The weak temperature dependence of the resistivity points towards strong disorder in the 4c and 4d sites of the isocubanite.