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30 December 2005 Evaluation of flexible Bi2Te2.5Se0.5 and Bi0.5Sb1.5Te3 thermopile generator
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Thermoelectric conversion based on the thin film of thermoelectric materials has the advantages of integration, flexibility and miniaturization. A thin-film flexible thermopile power generator has practical application in a free-form surface due to its flexibility. Such a generator also has the potential to be used as a wearable power-source. We proposed a thermopile composed of Bi2Te2.5Se0.5 and Bi0.5Sb1.5Te3 as thermoelectric materials formed on a polyimide sheet with a heat absorber sheet and a heat sink sheet. BiTe was used as the thermoelectric material, because it has highest performance around room temperature. Thin films of the thermoelectric materials were deposited on a polyimide sheet by the DC sputtering and lift-off process. There were 380 thermocouples fabricated on the sheet. A flexible thermopile power generator was formed when a substrate polyimide sheet bent into a wavy form and the set between a flexible heat absorber and sink sheets. Hot and cold junctions were formed when the substrate was bent into a wavy form. Due to this wavy form and slits in the thermopile sheet, heat absorber and sink sheets, the thermopile generator could be bent in two orthogonal two directions. The fabricated thermopile had a length of 66 mm, a width of 38 mm and a height of 2.5 mm. The average output voltage was 160 μV/K per thermocouple. This result was about 41% of the theoretical value. However, a high-resistivity of p-type BiTe thin-film was obtained because of a poor crystalline structure. The achieved resistance of 110mΩ•cm was 130 times greater than that of the sputtering target. In addition, the low continuity of brittle thermoelectric materials leads to low flexibility. Therefore, we investigated this problem by using x-ray diffraction and electron probe microanalysis. We propose a novel structure and fabrication technique that solves these problems and enhances the potential use of flexible thermopile generator in practical applications.
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M. Shiozaki, S. Sugiyama, N. Watanabe, H. Ueno, and K. Itoigawa "Evaluation of flexible Bi2Te2.5Se0.5 and Bi0.5Sb1.5Te3 thermopile generator", Proc. SPIE 6037, Device and Process Technologies for Microelectronics, MEMS, and Photonics IV, 603714 (30 December 2005);

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