Giant Thermoelectric Effect in Graded Micro-Nanoporous Materials
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Giant Thermoelectric Effect in Graded Micro-Nanoporous Materials Dimitrios G. Niarchos1,*, Roland H. Tarkhanyan1 and Alexandra Ioannidou1 1
Institute for Advanced Materials, Physicochemical Processes, Nanotechnology & Microsystems, Department of Materials Science, NCSR “Demokritos”, Athens, Greece ABSTRACT In this work we report on opportunities for a colossal reduction in lattice thermal conductivity (LTC) of graded micro-nanoporous structures with inhomogeneous porosity which leads to the considerable improvement in thermoelectric figure of merit ZT. We employ the effective medium theory to calculate the LTC of a porous media with hole pores of variable radius and show that porous materials with inhomogeneous porosity are expected to have stronger reduction (about 30 times!) in thermal conductivity than those with pores of equal sizes. Such a reduction is caused by enhanced scattering of thermal phonons with the pore boundaries. We have studied the variations of the LTC as a function of porosity, pore sizes, geometry and the number of pore groups with different sizes. Our theoretical results show excellent agreement with experimental data. INTRODUCTION In the last two decades, a great effort has been made to enhance the range of highperformance thermoelectric (TE) materials for industrial applications [1,2]. The key ideas for improving the efficiency of TE devices are connected with the enhancement of the power factor P = σ S 2 ( σ - electrical conductivity, S - Seebeck coefficient) and reduction in the thermal conductivity K=Ke+KL, where Ke is the contribution of the free charge carriers and KL is the lattice thermal conductivity (LTC). Recent theoretical and experimental results [3-7] show that the LTCs of thin nanocomposite films can be reduced by orders of magnitude with respect to bulk material values. At present, semiconductor based quantum well superlattice structures and nanowires exhibit highest power factor and lowest LTC which leads to the significantly higher thermoelectric figure of merit ZT = PT / K compared to that of the bulk materials [8-10]. However, in the technology of thermoelectric materials for general applications, the devices based on the bulk materials are more preferable. In recent work of M. Dresselhaus et al [11] was theoretically proposed and experimentally proved the enhancement of the power factor (but not the thermal conductivity) in modulation doped silicon germanium alloy nanocomposites leading to ZT of 1.3 at 900oC. They have used Si95Ge as the matrix and Si70Ge30P3 as the nanaparticles. Modulation-doping approach in thermoelectrics has been introduced in a previous work of the same group of authors [12]. This approach is based on well known in semiconductor physics phenomenon of spatial separation of ----------------------------------------------------------------------* Presenting Author. E-mail: [email protected]
the charge carriers from their ionized parent atoms which leads to the reduction of the charge scattering and consequently to higher electrical conducti
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