Development of FGM thermoelectric materials in Japan-the state of the art

Two times higher performance than a traditional thermoelectric material can be expected if the proper carrier concentration gradient is tailored to fit the temperature gradient. Performing a stepwise change of carrier concentration is also a method for practical application. That is a fundamental concept of energy converting functionally graded materials (FGM). It is essential to choose a proper material for each part to fit the temperature gradient. The proper material is a material with the proper carrier concentration and a proper compound to match the temperature of each part along the temperature gradient. Joining of these FGM materials and fitting electrodes with FGM interfaces are also core techniques, because thermal stress relaxation caused by the difference of thermal expansion coefficients is important at a high temperature. Joining two Bi₂Te₃ samples with carrier concentrations n_c of 1.0 and 4.5×10²⁵ was done by the ordinal soldering technique or diffusion bonding. The specific temperature range of the Seebeck coefficient α for the joined Bi₂Te₃ is extended from 50 to 100 K, and the value of α at the valley between the two materials with different n_c was higher than both materials. The sintered n-type PbTe FGM with 3 layers of n_c =3.51, 2.60 and 2.26×10²⁵ was prepared by hot pressing. The effective maximum power P_max of the FGM at the temperature difference of ΔT=310 K is 150 Wm/m² and is about 7% larger than that of the layer with n_c=3.51×10 ²⁵ whose P_max is the greatest in all layers