Thermoelectric power generation has been attracting attention as a technology for waste heat utilization in which thermal energy is directly converted into electric energy. It is well known that layered cobalt oxide compounds such as NaCo2O4 and Ca3Co4O9 have high thermoelectric properties in p-type oxide semiconductors. However, in most cases, the thermoelectric properties in n-type oxide materials are not as high. Therefore, n-type magnesium silicide (Mg2Si) has been studied as an alternative due to its non-toxicity, environmental friendliness, lightweight property, and comparative abundance compared with other TE systems. In this study, we fabricated π-structure thermoelectric power generation devices using p-type NaCo2O4 elements and n-type Mg2Si elements. The p- and n-type sintering bodies were fabricated by spark plasma sintering (SPS). To reduce the resistance at the interface between elements and electrodes, we processed the surface of the elements before fabricating the devices. The end face of a Mg2Si element was covered with Ni by SPS and that of a NaCo2O4 element was coated with Ag by silver paste and soldering.
The thermoelectric device consisted of 18 pairs of p-type and n-type legs connected with Ag electrodes. The cross-sectional and thickness dimensions of the p-type elements were 3.0 mm × 5.0 mm × 7.6 mm (t) and those of the n-type elements were 3.0 mm × 3.0 mm × 7.6 mm (t). The open circuit voltage was 1.9 V and the maximum output power was 1.4 W at a heat source temperature of 873 K and a cooling water temperature of 283 K in air.