The possibility of combustion synthesis of perovskite-oxide thermoelectric materials with the attendant saving of energy and time and without deterioration in the thermoelectric properties was investigated by evaluating the thermoelectric properties of lanthanum-doped strontium titanate (Sr 1Àx La x TiO 3 , 0 x 0:1). The materials were successfully combustion synthesized and spark plasma sintered with 98.0-99.6% of true density, and their thermoelectric properties were evaluated from room temperature to 850 K. The optimal lanthanum doping amount ratio x in the considered temperature range was from 0.06 to 0.08, in which Sr 0:92 La 0:08 TiO 3 sample showed the maximum ZT of 0.22 at 800 K. This value was close to the highest recorded ZT at the same temperature up to now, and the ZT of most samples are higher than those synthesized by the conventional solid state reaction method. Thus, combustion synthesis is promising for producing perovskite-oxide thermoelectric materials for high-temperature application.
Thermoelectric properties of Al-doped ZnO prepared by solution combustion synthesis using urea as fuel and sintered by spark plasma sintering were investigated for developing an energy-and time-saving synthesis method to decrease its thermal conductivity without a significant deterioration in other thermoelectric properties. The desired materials were successfully synthesized and sintered. The thermoelectric properties of the synthesized products subjected to planetary ball milling (PBM) treatment before sintering were compared with those of synthesized products not subjected to PBM treatment; the results showed that the former products had a larger power factor and higher thermal conductivity than the latter products. The thermal conductivity of all as-synthesized products was in the range of 8.3-19.7 WÁm À1 ÁK À1 at room temperature, which was significantly lower than that of the products synthesized by a conventional solid-state reaction method. (Zn 0:99 Al 0:01 )O obtained by PBM had the highest dimensionless figure of merit ZT of 0.050 at 863 K. From these results, it is inferred that solution combustion synthesis is an effective method for synthesizing Al-doped ZnO with relatively low thermal conductivity for high-temperature thermoelectric applications.
Thermoelectric properties of La-doped SrTiO 3 were investigated in order to determine the optimum sintering temperature for its fabrication by using a combination of combustion synthesis and spark plasma sintering. Combustion-synthesized samples (Sr 1Àx La x TiO 3 , x ¼ 0:08) were subjected to spark plasma sintering at temperatures from 1513 to 1663 K. The average grain size of sintered Sr 0:92 La 0:08 TiO 3 enlarged as sintering temperature rose up. The maximum average grain size was 23.5 mm for a sintering temperature of 1663 K. The thermoelectric properties of sintered Sr 0:92 La 0:08 TiO 3 were measured from room temperature to 1073 K. The optimum sintering temperature in the experimental sintering temperature range was 1633 K. Among the samples, the Sr 0:92 La 0:08 TiO 3 sample sintered at 1633 K showed the maximum power factor of 1:51 Â 10 À3 Wm À1 K À1 at 375 K. Further, we investigated the effects of pressing direction during sintering on the thermoelectric properties of combustion-synthesized Sr 0:92 La 0:08 TiO 3 . The combustion-synthesized samples were sintered well along the pressing direction during sintering; therefore, the electric conductivity measured along the pressing direction during sintering was more than twice that measured along the direction perpendicular to the pressing direction during sintering. Thus, we concluded that pressing direction during sintering affected the electric property of Sr 0:92 La 0:08 TiO 3 .
This paper describes the design of two-and three-stage cascaded oxide thermoelectric generators (TEGs) for high-temperature heat recovery using reported data to optimize energy conversion efficiency. We used the general intermetallic compounds Bi 2 (Se,Te) 3 and (Bi,Sb) 2 Te 3 for the low-temperature stages and oxides of TiO 1:1 , La-doped SrTiO 3 , Na x Co 2 O 4 , and Al-doped ZnO for the higher-temperature stages. A two-stage TEG with TiO 1:1 as the p-type material and La-doped SrTiO 3 as the n-type material was found to have the highest efficiency at heat-source temperatures below 852 K, while the three-stage TEG was slightly more efficient than the two-stage TEG for heat-source temperatures above 852 K. For the three-stage TEG, the optimal boundary temperature of the second and third stages was calculated to be 698 K; at this temperature, the maximum energy conversion efficiency, 13.5%, was obtained at a heat-source temperature of 1223 K. The results showed that the designed two-and three-stage cascaded oxide TEGs have high potential for heat recovery from high-temperature waste.
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