Abstract
The influence of shear strain on the microstructural, physical, and mechanical properties was studied on large bulk samples (diameter: 30 mm, thickness: 1 or 8 mm), which were consolidated by high-pressure torsion (HPT) from a commercial powder DD0.7Fe3CoSb12. Particularly, the thick sample (mass �53 g) allowed measuring the thermoelectric (TE) properties with respect to various orientations of the specimen in the sample. All data were compared with those of a hot-pressed (HP) reference sample, prepared with the same powder. Transmission electron microscopy, as well as X-ray powder diffraction profile analyses, Hall measurements, and positron annihilation spectroscopy, supported these investigations. Furthermore, synchrotron data for the temperature range from 300 to 825 K were used to evaluate the changes in the grain size and dislocation density as well as the thermal expansion coefficient via the change in the lattice parameter during heating. In addition, hardness and direct thermal expansion measurements of the HPT samples were performed and compared with the HP reference sample's values. With the increase of the shear strain from the center to the rim of the sample, the electrical resistivity becomes higher, whereas the thermal conductivity becomes lower, but the Seebeck coefficient remained almost unchanged. For the thin as well as thick samples, the enhanced electrical resistivity was balanced out by a decreased thermal conductivity such that the maximum ZT values (ZT = 1.3-1.35 at 856 K) do not vary much as a function of the shear strain throughout the sample, however, all ZTs are higher than that of the HP sample. The thermal-electric conversion efficiencies are in the range of 14-15 (for 423-823 K). With similar high ZT values for the n-type skutterudites, fabricated in the same fast and sustainable way, these p- and n-type skutterudites may serve as legs for TE generators, directly cut from the big HPT bulks. ©
Highlights
Thermoelectricity is a simple and green technology for the direct conversion of heat into electricity, and vice versa
From synchrotron radiation patterns (Figure 4), one can see that with increasing temperature the peak maxima are shifted to lower 2θ values and the peaks become slimmer, indicating growing lattice parameters and grain growth, respectively
This paper demonstrates that severe plastic deformation (SPD) via high-pressure torsion (HPT) enables a successful production of bulk samples of p-type skutterudites (DD0.7Fe3CoSb12) with a diameter of 30 mm and thicknesses of 1 mm and 8 mm from commercial raw powder
Summary
Thermoelectricity is a simple and green technology for the direct conversion of heat (i.e., any exhaust heat, geothermal, solar heat, or even body heat) into electricity, and vice versa. ACS Applied Energy Materials conductivity λph.[29−32] This can be achieved by applying severe plastic deformation (SPD) via high-pressure torsion (HPT) With this technique severe torsional strain, increasing along the disc radius, is introduced, resulting in ultrafine or even nanocrystalline materials and in a high density of crystal lattice defects, especially dislocations and grain boundaries with high angles of misorientation (ref 33 and references therein). In contrast to the thin samples, the marginal strain gradient in the case of thick HPT discs was utilized to compare the properties of specimens cut parallel and perpendicular to the rotation axis It appears that ZTs are all higher than the ZT of the HP reference sample.
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