Entropy of conduction electrons from transport experiments

The entropy of conduction electrons was evaluated utilizing the thermodynamic definition of the Seebeck coefficient as a tool. This analysis was applied to two dierent kinds of scientific questions that can-if at all-be only partially addressed by other methods. These are the field-dependence of meta-magnetic phase transitions and the electronic structure in strongly disordered materials, such as alloys. We showed that the electronic entropy change in meta-magnetic transitions is not constant with the applied magnetic field, as is usually assumed. Furthermore, we traced the evolution of the electronic entropy with respect to the chemical composition of an alloy series. Insights about the strength and kind of interactions appearing in the exemplary materials can be identified in the experiments

TThermodynamics of the thermoelectric working fluid close to the superconducting phase transition

The bottleneck in state-of-the-art thermoelectric power generation and cooling is the low performance of thermoelectric materials. While the adverse effects of lattice phonons on performance can be mitigated, the main difficulty remains to obtain a large thermoelectric power factor as the Seebeck coefficient and the electrical conductivity cannot be increased independently. Here, relating the thermoelastic properties of the electron gas that performs the thermoelectric energy conversion, to its transport properties, we analyze theoretically whether an electronic phase transition can enhance thermoelectric conversion and at what cost. More precisely, we consider the metal-to-superconductor phase transition in a model two-dimensional system, and we seek to quantify the contribution of the 2D fluctuating Cooper pairs to the power factor in the close vicinity of the critical temperature $T_{\rm c}$. In addition, we provide experimental evidence of the rapid increase of the Seebeck coefficient without decreasing the electrical conductivity near $T_{\rm c}$ in a 100-nm Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ thin film with high structural quality resulting in a power factor enhancement of approximately 300. This level of performance cannot be achieved in a system with low structural quality as shown experimentally with our sample degraded by ion bombardment as defects preclude the strong enhancement of the Seebeck coefficient near the phase transition. Finally, we theoretically discuss the ideal thermoelectric conversion efficiency (i.e. disregarding adverse phonon effects) and show that driving the electronic system to the vicinity of a phase transition may be an innovative path towards a strong performance increase but at the cost of a narrow temperature range of use of such materials.Comment: Submission to SciPos

Electronic Entropy Change in Ni-doped FeRh

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 ΔSel≈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% from 0.1 to 6 T. © 2019 Elsevier Ltd.The authors thank Dr. Sebastian Fahler for insightful discussions. TU Darmstadt acknowledges funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (grant no. 743116 project Cool Innov)

Unveiling the phonon scattering mechanisms in half-Heusler thermoelectric compounds

Half-Heusler (HH) compounds are among the most promising thermoelectric (TE) materials for large-scale applications due to their superior properties such as high power factor, excellent mechanical and thermal reliability, and non-toxicity. Their only drawback is the remaining-high lattice thermal conductivity. Various mechanisms were reported with claimed effectiveness to enhance the phonon scattering of HH compounds including grain-boundary scattering, phase separation, and electron–phonon interaction. In this work, however, we show that point-defect scattering has been the dominant mechanism for phonon scattering other than the intrinsic phonon–phonon interaction for ZrCoSb and possibly many other HH compounds. Induced by the charge-compensation effect, the formation of Co/4d Frenkel point defects is responsible for the drastic reduction of lattice thermal conductivity in ZrCoSb1−xSnx. Our work systematically depicts the phonon scattering profile of HH compounds and illuminates subsequent material optimizations