Decoupling Seebeck coefficient and resistivity, and simultaneously optimizing thermoelectric and mechanical performances for n-type BiTeSe alloy by multi-pass equal channel angular extrusion

[Display omitted] •Decoupling Seebeck coefficient and resistivity.•The mechanism of grain refinement was investigated by experiment and simulation.•Increasing the number of extrusion pass could not refine the grain already refined. Equal channel angular extrusion (ECAE) has become a new process for both preparing ultrafine grained uniform microstructure and minimizing contamination. In the present work, the BiTeSe polycrystalline bulk alloys with uniform microstructure was obtained by applying ECAE method to extruding the cast ingot directly. The lamellae coarse grains (hundreds microns to several millimeters) of initial cast ingot were efficiently and uniformly refined to about 18 ~ 24.7 μm. Experiments combining with numerical simulation confirmed that the uniformity of the grain size distribution was remarkably improved by increasing the number of extrusion pass owing to the continuous refinement of grains in the unrefined region and the retainment of grains in the refined region. The Vickers Hardness was doubled at 4th pass in this connection. On the other hand, it is inspiring that applying ECAE to the initial cast ingot not only decoupled the Seebeck coefficient and resistivity, but also simultaneously optimized the electrical and thermal transport performances. For this reason, the dimensionless figure of merit (ZT) increased rapidly with the increase of the number of extrusion pass. Finally, BiTeSe billet extruded for 4 passes achieved the maximum ZT, which was far higher than that of the initial state.