Optimization of thermoelectric efficiency in graded materials
Z. Dashevsky1, Y. Glebstein1, I. Edry1 I. Drabkin2, M.P. Dariel1
1Department of Materials Engineering, Ben-Gurion University,
Beer-Sheva 84105, Israel
2Institute of Chemical problems for Microelectronics, Moscow, Russia
In order to achieve high thermoelectric conversion efficiency, it is necessary to make use of materials with a maximal figure of merit Z =S2´ s /k (S is the Seebeck coefficient, s and k are the electrical are the electrical and thermal conductivity, respectively) over a wide temperature range. AIVBVI (chalcogenides of group IV elements) semiconductors are well known materials that have found widespread applications in the thermoelectric energy converters. The thermoelectric properties of semiconductors may be improved by designing crystals with a gradient of charge carrier concentration that is commensurate with the temperature gradient that prevails along the axis in an actual thermoelectric device. A graded carrier concentration can be generated by building an appropriate profile of doping impurities.
Optimization of the carrier concentration profile along the thermoelectric leg requires determining the temperature profile that is set up in the semiconductor placed between the high temperature and the low temperature reservoirs. The temperature distribution along a leg was calculated from the heat flux equation under steady state conditions for the 50-6000C temperature range. In the calculation, account was taken of the temperature dependence of S, s and k, respectively, for n-type PbTe crystals doped with iodine.
These calculations allow determining the dopant concentration profile along the thermoelectric leg that provides optimal efficiency. The results suggest that possibility, in principle, of achieving conversion efficiency up to 15 % for an operating temperature range of 50-6000C.