Furukawa Co Ltd developed a thermoelectric conversion material that shows a high performance in the medium temperature range (room temperature-600°C).
The company developed a skutterudite-based thermoelectric conversion material consisting of iron (Fe), cobalt (Co), antimony (Sb) and rare earth elements. A skutterudite-based material is a crystal structure of a compound, which is typified by CoSb3. Both the p- and n-types have excellent properties.
The keys to achieve high performance in thermoelectric conversion material are good electrical properties and a low thermal conductivity. Good electrical properties mean that a high electromotive force is generated when a temperature difference is applied to the material and that the material itself has a high electric conductivity. Skutterudite-based materials have the problem of relatively high thermal conductivity, despite their excellent electrical properties.
To reduce the thermal conductivity without sacrificing the electrical properties of skutterudite-based materials, the company filled a skutterudite with lanthanum (La), barium (Ba), ytterbium (Yb) and calcium (Ca) along with other elements.
The company used the Group XIII elements including aluminum (Al), gallium (Ga) and indium (In), as well as the Group IV elements including titanium (Ti), zirconium (Zr) and hafnium (Hf) to fill the skutterudite. The company significantly lowered the thermal conductivity by using more than three kinds of elements at a time. The company believes that, when the skutterudite is filled with these elements, the crystal lattice vibrates irregularly to prevent the heat conduction.
The performance of a thermoelectric conversion material is usually indicated by the non-dimensional performance index (ZT). For both p- and n-type materials, the value ZT = 1 is regarded as the measure of commercialization. As a result of reducing the thermal conductivity, the ZT value of (La, Ba, Ga, Ti)0.9(Fe, Co)4Sb12, which is a p-type material, rose from 0.5 to 1.1.
The ZT value of (Yb, Ca, Al, Ga, In)0.9(Co, Fe)4Sb12, which is an n-type material, increased from 0.7 to 1.3. From a practical standpoint, the material is expected to be applied to wider areas because the ZT values of both p- and n-types exceed 1 in a wide temperature range of 350-550°C.
Furukawa used the latest material to prototype a thermoelectric conversion module measuring 50 (W) x 50 (D) x 8mm (H). The company evaluated the module under conditions where its upper surface (higher temperature side) reached 720°C, while the lower surface (lower temperature side) is 50°C. With respect to the thermoelectric conversion performance, the results of the experiment indicated that the module has a thermoelectric conversion efficiency of 7% and an output of 33W (power density of 1.3W/cm2).