High efficient program and carrier concentration FGM thermoelectric materials in the intermediate temperature region

As a part of the national project in Japan, the experimental examination and investigation have been carried out on PbTe compounds and their functionally graded material (FGM) with a view to an enhancement of thermoelectric energy conversion efficiency. The experiments consist of preparation and thermoelectric characterization of n-type PbTe melt-grown and sintered non-FGM and FGM. The electron concentration in the non-FGM single crystals was controlled in the range from 5/spl times/10/sup 23/ to 5/spl times/10/sup 25//m/sup 3/ by doping 700-6000 molppm PbI/sub 2/. The FGM structure for PbTe was obtained through the unidirectional solidification in a special growth ampule by doping 1500 and 3400 molppm PbI/sub 2/, in which carrier concentration varied continuously along the growth direction in one order of magnitude. An electron concentration at the bottom end of the FGM ingot doped with 1500 molppm PbI/sub 2/ was 0.82/spl times/10/sup 25//m/sup 3/, and the figure of merit was estimated to be 2/spl times/10/sup -3//K at 410 K. A 3-stage segmented FGM was prepared by hot-pressing subsequently the layers with electron concentrations of 3.6, 2.6 and 2.2/spl times/10/sup 25//m/sup 3/. It is found that the temperature dependence of electric figure of merit (/spl alpha//sup 2//spl sigma/) for the sintered FGM showed a broad peak and never came lower than those for the component non-FGM. The results indicate that high efficiency in thermoelectric energy conversion will be expected for well-designed FGM structure.