| Budget Amount *help |
¥14,300,000 (Direct Cost: ¥14,300,000)
Fiscal Year 2006: ¥3,700,000 (Direct Cost: ¥3,700,000)
Fiscal Year 2005: ¥4,600,000 (Direct Cost: ¥4,600,000)
Fiscal Year 2004: ¥6,000,000 (Direct Cost: ¥6,000,000)
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| Research Abstract |
Rare-earth sesquisulfides R_2S_3 (R : La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Yb, and Lu) were synthesized by the reaction of rare-earth oxides with CS_2 gas. With the exception of Lu, synthetic powders devoid of intermediate products such as an oxysulfide were obtained. A monosulfide was obtained in the case of Eu. The thermoelectric properties were improved by the optimization of the ratios of rare-earth metal to sulfur. For a γ-GdS_x compact, the thermoelectric figure of merit ZT increased with a decrease in the sulfur content and reached a value of 0.16 at x = 1.43. Moreover, the thermoelectric properties were measured for the γ-LaS_<1.5> compact whose crystal phase was controlled by the addition of Ti to a synthetic powder. The transformation from the tetragonal β-phase to the y-phase is affected by the oxygen impurities. The β-phase is in fact an oxysulfide La_<10>S_<14>(O_<1-x>S_x) with 0【less than or equal】x【less than or equal】1. While the presence of oxygen stabilizes the tetragonal form over a wide temperature range, the β-phase with a low oxygen content undergoes further transformation to the y-phase at around 1600 K. We observe that the Ti additive promotes the transformation from La_<10>S_<14>O to the γ-phase. In the γ-phase with 8 wt. % Ti, ZT increases with the temperature and reaches a value of 0.21 at 1000 K. The Ti additive is suitable for improving the high-temperature thermoelectric properties because it promotes the formation of the y-phase with high thermoelectric performance. Moreover, the β-phase with Ti is a candidate material for high-temperature thermoelectric conversion. The magnitude of the Seebeck coefficient increases with the temperature, while the electrical resistivity decreases. Consequently, ZT increases abruptly from 0.013 at 300 K to 0.18 at 1000 K.
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