This group studies electronic and thermal transport characteristics mainly of semiconducting materials of interest for thermoelectric applications. Materials are studied in the form of bulk, nanostructure or thin films. Fundamental properties are studied on single crystalline specimens. The impact of defect formation and nanostructure on the thermoelectric properties of such materials is investigated in composites and polycrystals. This group is the host of the “PPMS Lab” facility, a service unit for scientists of the Institute for Metallic Materials.
Head of Research Group "Transport in Thermoelectrics"
Room: A 3E.07
Phone: +49 351 4659 833
N. Pérez, M. Sahoo, G. Schierning, K. Nielsch, G. S. Nolas, Materials 15, 3439 (2022).
The temperature- and field-dependent, electrical and thermal properties of inorganic clathrate-VIII Eu8Ga16Ge30 were investigated. The type VIII clathrates were obtained from the melt of elements as reported previously. Specifically, the electrical resistivity data show hysteretic magnetoresistance at low temperatures, and the Seebeck coefficient and Hall data indicate magnetic interactions that affect the electronic structure in this material. Heat capacity and thermal conductivity data corroborate these findings and reveal the complex behavior due to Eu2+ magnetic ordering and clustering from approximately 13 to 4 K. Moreover, the low-frequency dynamic response indicates Eu8Ga16Ge30 to be a glassy magnetic system. In addition to advancing our fundamental understanding of the physical properties of this material, our results can be used to further the research for potential applications of interest in the fields of magnetocalorics or thermoelectrics.
D. M. Smiadak, R. Claes, N. Perez, M. Marshall, W. Peng, W. Chen, G. Hautier, G. Schierning, A. Zevalkink, Mat. Today Phys. 22, 100597 (2022).
Crystals with anisotropic thermoelectric transport coefficients can yield a high figure-of-merit along the direction with the highest electronic mobility, provided the Seebeck coefficient and thermal conductivity are relatively isotropic. In this study, we combine experiment and theory to investigate the anisotropic properties of the quasi-1D Zintl phase Ca5In2Sb6. Ca5In2Sb6 is predicted by ab initio calculations to have extremely anisotropic p-type electrical conductivity and power factor, arising from light effective mass parallel to its ladder-like polyanionic chains. In contrast, the Seebeck coefficient and lattice thermal conductivity are predicted to be relatively isotropic. The latter is evidenced by the nearly isotropic computed speed of sound tensor and experimentally obtained thermal expansion coefficients. In order to characterize the anisotropic electrical conductivity, Ca5In2Sb6 single crystals were grown from an IneSb rich molten flux and measured both parallel and perpendicular to the polyanionic chains. Due to the small crystal cross-sections, measurements perpendicular to the growth direction demanded a novel photolithography methodology whereby micro-ribbons were extracted using focused ion beam milling, and processed using laser photolithography to deposit contacts for electrical resistivity and Hall coefficient measurements. The conductivity parallel to the growth direction was found to be nearly 20x higher than the perpendicular direction, in agreement with our theoretical predictions. This study represents one of the first experimental confirmations of highly anisotropic electrical conductivity in Zintl thermoelectrics.
S. Bayesteh, S. Sailler, H. Schlörb, R. He, G. Schierning, K. Nielsch, N. Pérez, Mat. Today Phys. 24, 100669 (2022).
We report on the effect of artificially generated textures of Bi2Se3 in thermoelectric performance and low-temperature magnetoresistance. A set of texturized nanograined Bi2Se3 samples was investigated, ranging from predominantly c-axis texture to random texture. c-axis oriented layered domains rendered the samples highly conducting due to drastically enhanced mobility, up to 1600 cm2V−1s−1 at low temperature, and enhanced both carrier concentration and electrical conductivity. The largest power factor of 800 μWm−1K−2 and highest zT ≈ 0.14 both at 300K were observed in a sample with a predominantly layered and c-axis oriented texture. The random texture reduced the thermal conductivity, while the Seebeck coefficient showed no particular correlation with the texture. We have shown that the milling procedure generated a higher degree of disorder by increasing the milling frequency. As a result, the carrier scattering mechanism in the samples changed from mostly electron-phonon interaction at 5Hz to disorder-related scattering at 10Hz and 20Hz milling frequency. The weak antilocalization effect was observed in the magnetoresistance of pressed samples with different textures, pointing towards surface-like transport channels. The Hikami-Larkin-Nagaoka (HLN) model was used to evaluate the phase coherence length, resulting in a high value of roughly 600 nm regardless of the texture. However, a larger number of surface-like transport channels was obtained for the samples with random texture.
N. Pérez, A. Chirkova, K.P. Skokov, T.G. Woodcock, O. Gutfleisch, N.V. Baranov, K. Nielsch, G. Schierning, Mat. Today Phys. 9, 100129 (2019).
The net entropy change corresponding to the free charge carriers in a Ni-doped FeRh bulk polycrystal was experimentally evaluated in a single sample using low temperature heat capacity experiments with applied magnetic field, and using Seebeck effect and Hall coefficient measurements at high temperatures across the first order phase transition. From the heat capacity data a value for the electronic entropy change ∆Sel ≈ 8.9 J kg−1K−1 was extracted. The analysis of the Seebeck coefficient allows tracing the change of the electronic entropy jump with applied magnetic field directly across the transition. The difference in electronic entropy contribution obtained is as high as 10 percent from 0.1 to 6 T.
