(Invited) Thermoelectric Materials and Modules for High Temperature Application
In order to improve thermoelectric conversion efficiency, new thermoelectric materials with higher ZT than the above oxides are indispensable. We are developing some new homologous oxides composed of Ba and Co. These layered oxides show low thermal conductivity.
A silicide material with a good n-type thermoelectric property has been discovered. This silicide possesses a composition of Mn3-xCrxSi4Al2 (x = 0-0.7) and hexagonal CrSi2 structure. The dimensionless thermoelectric figure of merit ZT reaches 0.21 at 773 K for a non Cr substituted sample at the Mn site and 0.30 at 573 K for a Cr substituted one with x = 0.3. Since oxide passive layer is formed on the surface, electrical resistivity measured at 873 K is constant for two days in air, which indicates good oxidation resistance in air of this material. Thermoelectric modules consisting of 64 pairs of legs have been fabricated using MnSi1.7 and Mn3Si4Al2 devices as p- and n-type legs, respectively. Output power reaches 9.4 W, which corresponds to 2.3 kW/m2 of power density against surface area of the substrate at the heat source temperature of 873 K in air. The Ni–B plating can improve the degradation of the output power for the silicide modules significantly at the heat source temperature of 773 K. Also, the Ni–B plating can decrease the contact resistance at the junctions. Mixing Pt or Pd into the Ag paste effectively improves the durability at the heat-source temperature of 773 K. The output power can be estimated to hold more than 85 % of the initial value after 10 years with current generation of 1A.