High Thermoelectric Power Generation below Room Temperature by TiS₂ Compact Pellet

The highly anisotropic single crystalsof titanium disulfide (TiS2) have an n-type semiconductingproperty with attractive thermoelectric(TE) performance above room temperature (RT). However, the TE propertiesand power generation below the RT of TiS2 have not yetbeen reported. In this work, the TE performance of the TiS2 powder compact in a pellet form was investigated in the temperaturerange of 233-323 K. Moreover, the power generation characteristicswere measured under the temperature gradient of & UDelta;T = 25 and 45 K either below or above RT. The electrical conductivityof the powder compact decreased by heating, while the Seebeck coefficientincreased. Annealing the compact pellet was also performed and foundto significantly enhance its TE performance. The resulting power factorrecorded & SIM;540 & mu;W/mK(2) at T = 233 K and stayed almost constant despite a temperature change.Similar trends were also observed in the charge carrier density andHall mobility, with a slight decrease during heating. On the otherhand, the measured in-plane thermal conductivity of a TiS2 compact is 3.92 W/mK at T = 298 K and increasedto 4.44 W/mK at T = 373 K. The resulting figure ofmerit, zT, was 0.041-0.048 at 298-373K. The generated maximum power of a TiS2 single leg modulerecorded 1.24 and 4.3 & mu;W at & UDelta;T = 25 and45 K (cold side temperature T (1) and hotside temperature T (2) are both > RT),respectively,above RT. Surprisingly, the generated TE power below RT was foundto be several times higher than the generated power above RT withrecorded values of 10.2 and 26.5 & mu;W at & UDelta;T = 25 and 45 K (T (1) and T (2) are both < RT), respectively. This remarkable resultfirmly indicates that comparatively costless TiS2 compactscomposed only of nontoxic elements should be useful to generate TEpower in cold environments below RT utilizing the waste heat froma warm system.