Thermoelectric Modeling of Si-Si 1-x Ge x Ordered Nanowire Composites
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Thermoelectric Modeling of Si-Si1-xGex Ordered Nanowire Composites Ming Y. Tang1, Mildred S. Dresselhaus1,2, Ronggui Yang3, and Gang Chen3 1 Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA 02139 2 Department of Physics, Massachusetts Institute of Technology, Cambridge, MA 02139 3 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT Thermoelectrics have always been attractive for power generation and cooling because of power reliability and environmentally friendly issues. However, this concept remains noncompetitive due to the limitation in the efficiency of available thermoelectric materials and device designs [1]. In the 1990s, Hicks and Dresselhaus predicted the possibility of a dramatic enhancement in thermoelectric performance based on the special behavior of low dimensional materials [2, 3]. This enhancement is in part due to the increase in quantum confinement effects, the increase in electronic density of states at specified energies, and the increase in the phonon interface scattering for low dimensional structures. Nanowires and core-shell nanowires can be considered to be model systems to illustrate representative behavior in low dimensional thermoelectric materials. It is expected that a system made out of nanowires or core-shell nanowires would have a higher thermoelectric performance than its bulk counterpart due to an increase in the number of interfaces. The interfaces that are introduced must be such that phonons are scattered more strongly than are electrons. Theoretical studies have been carried out to better understand the transport properties of Si-Si1-xGex ordered nanowire composites. The composite is modeled as having Si wires embedded in a Si1-xGex host matrix. Thus, core-shell Si/Si1-xGex nanowires can be considered as a building block of the composite. The effect of the wire diameter and the shell alloy composition on ZT is presented. INTRODUCTION In the 1990s, using low dimensional physics concepts, Hicks and Dresselhaus predicted that a dramatic enhancement in thermoelectric performance was possible through the use of quantum wells and quantum wires [2, 3]. In the late 1990s, Chen also predicted that an enhancement in ZT could result from the significant reduction in the thermal conductivity in low dimensional system [4]. At the same time, nano-fabrication technology rapidly improved during the last decade. With advances in technology and in new low dimensional concepts, increases in the thermoelectric figure of merit ( ZT = S 2σ T /(κ e + κ ph ) where σ is the electrical conductivity, S is the Seebeck coefficient, κ e and κ ph are respectively the electrical and lattice thermal conductivity, and T is the temperature) have been demonstrated [5]. The generation of electricity from heat is one of the major thermoelectric applications, especially for deep space exploration where the thermal energy is supplied by a radioactive source. until today, thermoelectrics
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