Michihiro Ohta

Thermoelectric properties of lanthanum sesquisulfide with Ti additive

The electrical resistivity, thermopower, and thermal conductivity have been measured for the lanthanum sesquisulfide (La2S3) of which the crystal phase is controlled by the Ti additive. In all the samples, the thermopower is negative between 300 and 1000 K. The sample with 8 wt % Ti, which consists almost of the cubic γ phase, behaves as a degenerate semiconductor. The thermoelectric figure of merit ZT increases with increasing temperature, reaching a value of 0.21 at 1000 K. In contrast, the sample with 2 wt % Ti consists almost of the tetragonal β phase. The transport mechanism can be well explained by the model of the Anderson localization. The ZT value increases abruptly with increasing temperature. At 1000 K, this ZT value is comparable with that of the sample with 8 wt % Ti.

Thermoelectric properties and phase stability of La/sub 10/S/sub 14/O

The electrical resistivity, thermopower, thermal conductivity measurements have been made on tetragonal La/sub 10/S/sub 14/O. The electrical resistivity was measured during heating and successive cooling through 300 and 1000 K to investigate the high-temperature phase stability. The measurements were carried out in the vacuum chamber pumped down to 1.0 Pa. The electrical properties of La/sub 10/S/sub 14/O are changed dramatically through heat treatment. As annealing time increases, the electrical resistivity increases abruptly and then decreases gradually. However, it seems that this phase can be stabilized by a small addition of Ti. In the electrical resistivity of La/sub 10/S/sub 14/O with Ti, the cooling curve agrees with heating curve. Moreover, the ZT value increases abruptly with increasing temperature, reaching a value of 0.18 at 1000 K. The improvement with respect to the thermoelectric properties of the La/sub 10/S/sub 14/O is realized by a small addition of Ti.

Influence of phase on thermoelectric properties in lanthanum sesquisulfide doped with titanium

The thermoelectric power and the electrical resistivity in lanthanum sesquisulfide (La/sub 2/S/sub 3/) doped with Ti at 300 K were measured over the dopant Ti content ranging from 0.5 wt %. to 15 wt %. The tetragonal (/spl beta/) phase powder which contains impurity oxygen of about 5 atomic % was used as, the starting material. The samples, were prepared by vacuum hot-pressing at the temperature range from 1323 K to 1773 K for 3.6 ks. The phase of the compacts obtained changes with the amount of the dopant Ti content and the sintered temperature from the /spl beta/ phase to the cubic (/spl gamma/) phase. Moreover, the LaS phase is formed at the initial stage in the phase change. The absolute value of the thermoelectric power and the electrical resistivity decrease with the phase change. The compacts which are mainly composed of the /spl gamma/ phase exhibit a large power factor.