32Research Digest Fig. 1. Difference between SE in thermoelectric materials and ANE in magnetic materials, as well as the corresponding designs of heat flux sensors. The photograph is a prototype ANE-based heat flux sensor, fabricated on a flexible polyimide sheet.Fig. 2. Magnetic bulk hybrid materials with enhanced electrical power output due to STTG (up). Closed-circuit structure for direct electrical probing of αxy (down).References1) W. Zhou and Y. Sakuraba, Appl. Phys. Express 13, 043001 (2020).2) W. Zhou, K. Yamamoto, A. Miura, R. Iguchi, Y. Miura, K. Uchida, and Y. Sakuraba, Nat. Mater. 20, 463 (2021).3) W. Zhou, A. Miura, T. Hirai, Y. Sakuraba, and K. Uchida, Appl. Phys. Lett. 122, 062402 (2023).4) W. Zhou, A. Miura, Y. Sakuraba, and K. Uchida, Phys. Rev. Appl. 19, 064079 (2023).Transverse Thermoelectric Generation for Heat FluxSensingWeinan ZHOU1. Outline of ResearchIn order to establish the interconnection between individuals and physical objects for the Internet-of-Things, a variety of sensors are needed to gather various types of information. An important component of sensors for this purpose is the heat flux sensor. Different from a thermometer measuring the temperature, a heat flux sensor directly measures the thermal energy density passing through; it is inherently sensitive to the direction of heat flow and fast responsive. Currently, the commercially available heat flux sensors are based on the Seebeck effect (SE), where the generated electric field (E) is parallel to the temperature (∇T). As a result, the current design of heat flux sensors is usually a complicated 3D structure, consisting of an array of thermoelectric pillars with positive and negative SE thermopower, which largely contributes to the high thermal resistance, high cost, and low flexibility of the sensors [Fig. 1]. In the recent years, transverse thermoelectric generation is gathering increasing interests, for its capability to convert ∇T in one direction into E perpendicular to that direction. The anomalous Nernst effect (ANE) observed in magnetic materials is a well-known example. The orthogonal relationship between ∇T and E allows the heat flux sensor to be in a simple 2D form made of connecting wires on a surface [Fig. 1], which has various advantages for practical application comparing to the traditional design [1]. However, the transverse thermopower of ANE is still much smaller comparing to the SE thermopower in thermoelectric materials. For the new design to have sensitivity comparable to that of heat flux sensors available on the market, it is crucial to realize significant enhancement in the transverse thermopower.2. Research ActivitiesRecently, we proposed an approach to enhance the transverse thermopower by forming a closed circuit comprising of magnetic and thermoelectric materials, which converts the large SE thermopower in the thermoelectric material to transverse direction by the anomalous Hall effect of the magnetic material, called the Seebeck-driven transverse thermoelectric generation (STTG) [2]. The proof-of-concept demonstration of STTG was carried out on samples made of magnetic thin films and bulk thermoelectric slabs. In 2022, we showed that STTG can be realized even in all-bulk hybrid materials, where the magnetic material is also bulk and has a similar size as the thermoelectric material [3]. The samples were fabricated by electrically connecting bulk Co2MnGa slabs and n-type Si substrates to form a closed circuit. We observed the large transverse thermopower up to 16.0 μV K−1 at room temperature due to the STTG contribution, which is several times larger than the transverse thermopower of Co2MnGa slab (6.8 μV K−1). We also experimentally demonstrated that magnetic hybrid bulk materials are suitable for extracting electrical power owing to their small internal resistance [Fig. 2]. On the other hand, based on the closed-circuit structure but with a non-magnetic conductor, we proposed and demonstrated a method to estimate the anomalous Nernst conductivity (αxy) of a magnetic material with fewer parameters needing to be measured [Fig. 2] [4].
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