Institute for Metallic Materials - Recent Highlights

Thermomagnetic generator with a pretzel-like magnetic flux topology

Anja Waske, Daniel Dzekan, Kai Sellschopp, Dietmar Berger, Alexander Stork, Kornelius Nielsch & Sebastian Fähler
Nature Energy 4, pages68–74 (2019)
DOI: 10.1038/s41560-018-0306-x

To date, there are very few technologies available for the conversion of low-temperature waste heat into electricity. Thermomagnetic generators are one approach proposed more than a century ago. Such devices are based on a cyclic change of magnetization with temperature. This switches a magnetic flux and, according to Faraday’s law, induces a voltage. Here we demonstrate that guiding the magnetic flux with an appropriate topology of the magnetic circuit improves the performance of thermomagnetic generators by orders of magnitude. Through a combination of experiments and simulations, we show that a pretzel-like topology results in a sign reversal of the magnetic flux. This avoids the drawbacks of previous designs, namely, magnetic stray fields, hysteresis and complex geometries of the thermomagnetic material. Our demonstrator, which is based on magnetocaloric plates, illustrates that this solid-state energy conversion technology presents a key step towards becoming competitive with thermoelectrics for energy harvesting near room temperature.

Integrated microthermoelectric coolers

Guodong Li, Javier Garcia Fernandez, David Alberto Lara Ramos, Vida Barati, Nicolás Pérez, Ivan Soldatov, Heiko Reith, Gabi Schierning & Kornelius Nielsch
Nature Electronics 1, 555-561 (2018)
(DOI: 10.1038/s41928-018-0148-3)

Microthermoelectric modules are of potential use in fields such as energy harvesting, thermal management, thermal imaging and high-spatial-resolution temperature sensing. In particular, microthermoelectric coolers (µ-TECs)—in which the application of an electric current cools the device—can be used to manage heat locally in microelectronic circuits. However, a cost-effective µ-TEC device that is compatible with the modern semiconductor fabrication industry has not yet been developed. Furthermore, the device performance of µ-TECs in terms of transient responses, cycling reliability and cooling stability has not been adequately assessed. Here we report the fabrication of µ-TECs that offer a rapid response time of 1 ms, reliability of up to 10 million cycles and a cooling stability of more than 1 month at constant electric current. The high cooling reliability and stability of our µ-TEC module can be attributed to a design of free-standing top contacts between the thermoelectric legs and metallic bridges, which reduces the thermomechanical stress in the devices.

Mixed axial–gravitational anomaly in the Weyl semimetal NbP

Johannes Gooth,  Anna C. Niemann, Tobias Meng, Adolfo G. Grushin, Karl Landsteiner, Bernd Gotsmann, Fabian Menges, Marcus Schmidt, Chandra Shekhar, Vicky Süß, Ruben Hühne, Bernd Rellinghaus, Claudia Felser, Binghai Yan & Kornelius Nielsch
Nature  547, pages 324–327 (20 July 2017)
DOI: 10.1038/nature23005

The conservation laws, such as those of charge, energy and momentum, have a central role in physics. In some special cases, classical conservation laws are broken at the quantum level by quantum fluctuations, in which case the theory is said to have quantum anomalies1. One of the most prominent examples is the chiral anomaly2,3, which involves massless chiral fermions. These particles have their spin, or internal angular momentum, aligned either parallel or antiparallel with their linear momentum, labelled as left and right chirality, respectively. In three spatial dimensions, the chiral anomaly is the breakdown (as a result of externally applied parallel electric and magnetic fields4) of the classical conservation law that dictates that the number of massless fermions of each chirality are separately conserved. The current that measures the difference between left- and right-handed particles is called the axial current and is not conserved at the quantum level. In addition, an underlying curved space-time provides a distinct contribution to a chiral imbalance, an effect known as the mixed axial–gravitational anomaly1, but this anomaly has yet to be confirmed experimentally. However, the presence of a mixed gauge–gravitational anomaly has recently been tied to thermoelectrical transport in a magnetic field5,6, even in flat space-time, suggesting that such types of mixed anomaly could be experimentally probed in condensed matter systems known as Weyl semimetals7. Here, using a temperature gradient, we observe experimentally a positive magneto-thermoelectric conductance in the Weyl semimetal niobium phosphide (NbP) for collinear temperature gradients and magnetic fields that vanishes in the ultra-quantum limit, when only a single Landau level is occupied. This observation is consistent with the presence of a mixed axial–gravitational anomaly, providing clear evidence for a theoretical concept that has so far eluded experimental detection.

Cover pages