M Sato

M. Hamabe

S. Yamaguchi

National Institute for Fusion Science (NIFS-Repository)

§1O. Nernst-Seebeck Element in High Magnetic Field Hamabe, M., Yamaguchi, S. (Opt. Electric Eng., Chubu Univ.) Sato, M. Thennoelectric generator has the advantage that it exchanges heat energy directly to electric energy. We have proposed the use of the magnetic field effect as a new approach to enhance conversion efficiency of the thennoelectric gcncrator. 1l The following magnetic field effect were observed for the polycrystalline bismuth (Bi): [1] increase of the electric resistivity, [2] increase of the Seebeck coefficient, [3] decrease of the thermal conductivity, and [4] occurrence of the high Nemst coefficent.') Effect [2]-[4] acts as the positive effect for the thennoelectric element, though effect [1] is negative. In the Seebeck element, the Peltier heat flux flows to the same direction as the thennal conductive heat flux, and then the Peltier heat flux increases the total heat flux. In the Nemst element, the Ettingshausen heat flux flows to the opposite direction to the thennal conductive heat flux, and then the Ettingshausen heat flux reduces the total heat flux; therefore, the conversion efficiency of the Nemst element can approach to the CamO! efficiency at the lower figure-of-merit ZT than that of the Seebeck element. 1) Unlike the Seebeck element, the heat leakage occurs in the Nemst element along the electrodes which are equipped on the side wall of the element. This heat leakage causes the difficulty in utilizing the Nemst element. On the Nemst-Seebeck element, two electrodes (A and B in Fig. 1) are equipped on the diagonal comers. These electrodes defect the heat leakage on the electrodes and enhances the output voltage of the thermoelectric generator by using the sum of the Seebeck electric field and the Nemst electric field in the magnetic field. Dependence of the conversion efficiency of the Nemst-Seebeck element on ZT is considered to be drawn between that of the Nemst element and that of the Seebeck element, since the Nemst-Seebeck element also uses the Ettingshausen heat flux. To consider the high field limit of the Nemst-Seebeck element, the thermopower measurement and the electric resistance measurement were carried out in a high magnetic field within ±8 T. Figure I shows the geometry of the Nemst-Seebeck sample. The material was the polycrystalline Bi since the polycrystalline Bi showed good magnetic field effects as the low-temperature Nemst-Seebeck element in the weak magnetic field. 2) The Nemst voltage is proportional to (width)/(Iength) of the sample. The geometry of 5 mm in length and 10 mm in width was chosen to attain the large Nemst effect. Figure 2 shows the magnetic field dependence of the net thennopower (VN+S/I!.1) of the Nemst-Seebeck element at 100 - 287 K. Except at 100 K, the net thennopower increased almost linearly to the magnetic field and reached -800 ~ V IK at 150 K. The values in the high magnetic field correspond to that of the materials like BiTe used for 238 commercial thermoelectric device. Compared with the thermopowers at -8 T, the thermopowers in the zero magnetic field were sufficiently small; the Nemst-Seebeck element using polycrystalline Bi corresponded at -8 T with the Nemst element concerning the thermopower. On the other hand, ZT of the thermoelectric generator is inversely proportional to the electric resistance of the material. The electric resistivity of Bi decreases with decreasing temperature, whereas it increases remarkably in the magnetic field at lower temperature. For example, the electric resistance between A and B in Fig. 1 increased at 150 K from 0.00232 Q at 0 T to 0.151 Q at±8 T and at 200 K from 0.00279 Q to 0.0643 Q. However, ZT is proportional to (net thermopower)'- As a result, the increase of the thennopower exceeded the increase of the electric resistance; ZT at -8 T increased by 15.9 times at 150 K and by 18.8 times at 200 K, compared with ZT in the zero magnetic field. Reference 1) Yamaguchi, S., loumal of Plasma and Fusion Research 78, (2002) 19 (in Japanese) 2) Hamabe, M., et aI., Proc. 22nd Int. Coni on Thermoelectrics, La Grande MOlle. France, 2003 (IEEE, Piscataway, NJ, 2003) 567. f 8(0) tl!.T \0 (unit; mm) Fig. I. Schematic diagram of the Nemst-Seebeck element (thickness was 2 mm). Here, I!.T is temperature difference between the heat source and the heat sink. -800 , -600 , -100K , , - - 150 K , SZ -400 , ... - 200 K '> , -----277 K ~ ·200 ~ • 0 0 0. -"-0 200 E • , , ~ 400 , , t- , , 600 , , 800 , ·10 -5 0 5 10 Magnetic field [T] Fig. 2. Magnetic field dependence of the net thermo -power of the Nemst-Seebeck element. 

Nernst-Seebeck Element in High Magnetic Field

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