Towards high performance bulk thermoelectric materials with enhanced mechanical properties by Severe Plastic Deformation (SPD)

Résumé

Nowadays, one of the most promising strategies to produce highly efficient thermoelectric (TE) materials is to reduce the lattice thermal conductivity by introducing phonon scattering centres (such as submicron sized grain boundaries, second-phase nano-particles, and point defects) at different length scales. For highly anisotropic crystals such as Bi2Te3-based thermoelectrics, the combination of nanosized grain structures (to improve phonon scattering) together with strong crystallographic texture (to exploit the anisotropic properties of the crystal) is not readily accessible by the standard high energy ball-milling and powder consolidation techniques. This work presents a novel technique that permits to obtain, simultaneously, highly textured submicron grained structures in Sb2-xBixTe3 thermoelectric material. The severe plastic deformation (SPD) induced by high pressure torsion (HPT) of Sb2-xBixTe3 leads to fully dense disk-shaped samples with stable homogeneous grain sizes of ~100 nm and a strong crystallographic texture with the basal plane preferentially oriented perpendicular to the torsion axis. Such combination has a synergistic effect on the enhancement of the thermoelectric performance. For instance, after HPT processing, Sb1.6Bi0.4Te3 compound displays a good TE performance in a wide range of temperatures and shows a maximum zTRA (i.e. PF measured in-plane and κ out of plane) of ~2 at 375 K, (zTRR~1.5 estimated in-plane). Moreover, HPT improves significantly the hardness of the processed samples, although thetheir strong crystal texture is detrimental for their flexural strength. HPT has also been successfully applied to pure PbTe. The results confirm HPT processing as a promising alternative to spark plasma sintering to process mechanically improved PbTe-based thermoelectric compounds with all-scale hierarchical architectures.