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
Sebastian Sailler, Ruben Bueno Villoro, Samaneh Bayesteh, Heike Schlorb, Magdalena Ola Cichocka, Kornelius Nielsch, Siyuan Zhang, Nicolas Perez*, Materials Today Physics 2024, 46, 101477, https://doi.org/10.1016/j.mtphys.2024.101477
The addition of Fe3O4 nanoparticles to Bi2Se3 nanocomposites resulted in a significant increase of the thermo[1]electric figure of merit zT at room temperature from 0.14 in Bi2Se3 to 0.21 in Bi2Se3 with added 5 % volume of Fe3O4 nanoparticles. The main contributor to the improved zT is the large increase in carrier concentration, hence increased electrical conductivity. Electron microscopy investigation revealed chemical changes in the Bi2Se3 leading to segregation of Bi to the interface with the Fe3O4 nanoparticles. The fact that the altered Bi/Se proportion was constrained close to the grain boundaries resulted in a mild reduction of the Seebeck coefficient, less than what is expected from simple band structure models for the same increase in carrier density. The addition of Fe3O4 in relatively small volume fraction allowed to both decrease the thermal conductivity and increase the power factor in a Bi2Se3 material, the latter more intensely than usual substitution methods.
Keywords: Thermoelectricity, Nanocomposite, Performance, Power factor
Alejandra Ruiz-Clavijo, Nicolás Pérez*, Olga Caballero-Calero, Javier Blanco, Francesca Peiró, Sergi Plana-Ruiz, Miguel López-Haro, Kornelius Nielsch, and Marisol Martín-González*, ACS Nano 2023, 17, 16960, https://doi.org/10.1021/acsnano.3c04160
The resistance of an ordered 3D-Bi2Te3 nanowire nanonetwork was studied at low temperatures. Below 50 K the increase in resistance was found to be compatible with the Anderson model for localization, considering that conduction takes place in individual parallel channels across the whole sample. Angle-dependent magnetoresistance measurements showed a distinctive weak antilocalization characteristic with a double feature that we could associate with transport along two perpendicular directions, dictated by the spatial arrangement of the nanowires. The coherence length obtained from the Hikami−Larkin− Nagaoka model was about 700 nm across transversal nanowires, which corresponded to approximately 10 nanowire junctions. Along the individual nanowires, the coherence length was greatly reduced to about 100 nm. The observed localization effects could be the reason for the enhancement of the Seebeck coefficient observed in the 3D-Bi2Te3 nanowire nanonetwork compared to individual nanowires.
KEYWORDS: metamaterials, bismuth telluride, 3D-AAO, nanonetwork, surface transport, localization effects.
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.
