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Tuesday, March 26
GTE in "Graded Porous Micro-Nanostructured" Thermoelectric MaterialsDimitris Niarchos, National Centre of Scientific Research , Athens, Greece
Measurement Science and Systems Engineering Division
Nanosystems and Structures Group Seminar
1:00 PM — 2:00 PM, Research Office Building (5700), Room L-204
Contact: Panos Datskos (email@example.com), 865.574.6205
AbstractThe efficiency of thermoelectric materials can be described in terms of the dimensionless figure-of-merit ZT = σS2T/K, where σ is the electrical conductivity, S the Seebeck coefficient, T the temperature, and K the total thermal conductivity. The key ideas for improving the efficiency of thermoelectric devices are connected with the enhancement of the power factor P=sS2 and reduction in the thermal conductivity K=KL+Kel, where Kel is the contribution of the free charge carriers and KL is the lattice thermal conductivity (LTC). Among various types of thermoelectric materials, porous media play an important role and represent a highly dynamic research area that aims at the creation of the next generation of efficient solid-state thermoelectric energy conversion devices.
In this work we report of opportunities of improving the ZT by a) the reduction in LTC of "graded porous nanostructures" with inhomogeneous porosity b) decrease by a lower rate of the electrical conductivity, in such a way that the ratio s/K to increase by almost an order of magnitude. We employ the effective medium theory to calculate the LTC of a micro-nanoporous media with spherical or cylindrical hole pores of variable radius and show that the filling fraction and the pore diameters play a major role in the thermal conductivity reduction and to a lesser extend of the electrical conductivity. This reduction is caused by enhanced scattering of thermal phonons with the pore boundaries. Thus, high frequency phonons are scattered by small pores while low frequency phonons having longer wavelengths are scattering by bigger pores. It means that porous materials with inhomogeneous porosity are expected to have stronger reduction in thermal conductivity than that with pores of equal sizes The variation of the KL ,, s and S as a function of porosity, pore sizes and the number of pore groups with different sizes have been studied. A reduction of K can reach values of more than 100, without too much reduction of the electrical conductivity. A small enhancement of the Seebeck coefficient is also calculated based on the reduction of the charge carriers. A condition is found at which the reduction in KL of a composite with ordered inhomogeneous porosity is much more effective than that in the case of random homogeneous porosity. We have used data from literature in order to validate our results and we found excellent agreement.
About the Speaker Dr. D. Niarchos obtained his Ph.D in Materials Science from Athens University(Greece) in 1978 and an MBA of R&D from Loyola University in 1985. From 1978 to 1981 he was IAEC Distinguish Post-Doctoral Researcher at Argonne National Lab. From 1981 to 1985 he was Assistant Professor at the Illinois Institute of Technology, Chicago, then moved to the Institute of Materials Science of the NCSR “D” in 1985. From 1994-1999 he served as the Director of IMS and from 1996-1999 as Vice President of the NCSR “Demokritos”. From 2005-2010 he was elected as Director and President of the Board of the NCSR “Demokritos”. He developed the lab for thin film growth of magnetic and superconducting/oxide multilayers using sputtering and laser ablation . He is the author and co-author of more than 400 publications with more than 4050 citations and has managed more than 35 National and EU projects with a budget of approximately 11 MEURO. He had served as advisor for the Greek Goverment , NATO and the EC.