(Invited) Thermoelectric Materials and Modules for High Temperature Application

Oxide materials are considered to be promising ones because of their durability against high temperature, low cost for producing etc. The misfit CoO₂ compounds show high thermoelectric efficiency at high temperature in air. Thermoelectric modules using p-type Ca₃Co₄O₉, one of the CoO₂ compounds and n-type CaMnO₃ have been produced. The maximum power density against area of the substrate of the module reaches 4.3 kW/m² at 973 K of the heat source temperature. Cascade modules piled up the oxide and Bi₂Te₃ modules have been prepared. The maximum power density of 7.8 kW/m² has been obtained at 1000 K of the heat source temperature. Testing of the cascade systems on industrial furnace and incinerators makes clear the next goals from viewpoints of efficiency and economy for the application of the waste heat recovery by thermoelectrics.

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 Mn_3-xCr_xSi₄Al₂ (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 MnSi_1.7 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.