Nanocomposites for thermoelectrics and thermal engineering
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Introduction The possibility of converting thermal energy directly into electricity holds promise for improving the efficiencies of current power systems and developing future sustainable energy systems.1 Solid-state thermoelectric (TE) materials2,3 hold this promise through the well-known Seebeck effect, whereby a temperature gradient drives the thermal diffusion of charge carriers, giving rise to a counterbalancing electrical voltage. The reverse (Peltier) effect, whereby a drift electrical current carries heat, can deliver cooling power without any moving parts or (potentially hazardous) working fluids.4 Attractive as it is, the application of TE materials has long been limited by their low efficiency. The efficiency of a TE device is proportional to the dimensionless figure of merit
zT ≡ S 2σT /(κe + κp ),
(1)
where S is the Seebeck coefficient defined as the ratio of the open-circuit voltage to the applied temperature difference (S ≡ –ΔV/ΔT); σ is the electrical conductivity (the combination S2σ is usually called the power factor); T is the absolute temperature; and κe and κp are the electron and phonon contributions, respectively, to the thermal conductivity. Before the
development of nanostructured thermoelectrics starting in the 1990s, the highest zT value achieved was around 1, while a zT of at least 3 is needed for thermoelectrics to compete with conventional power generation and refrigeration technologies.5 The low zT, and thus the low efficiency, stems from the fact that these material properties are intertwined and usually show opposite trends in a single material.5–7 The idea of making composites has long been exploited in materials research for achieving balanced material properties. Classical modeling of composites usually invokes the “effective medium theory,” which dates back to the works of Lord Rayleigh,8 Maxwell,9 and Maxwell Garnett,10 with the essential assumption that the local material properties of a composite take on values of the corresponding bulk constituent. This assumption is valid when the sizes of individual constituent domains are large, so that classical/quantum size effects11,12 and certain interfacial phenomena can be safely ignored and the local equilibrium distributions of carriers can be established. The effective medium theory for macrocomposites has been applied to thermal transport, for example, by Nan et al.,13 taking into account both the spatial distribution of the material properties and the Kapitza thermal interface resistance.
Bolin Liao, Department of Mechanical Engineering, Massachusetts Institute of Technology, USA; [email protected] Gang Chen, Department of Mechanical Engineering, Massachusetts Institute of Technology, USA; [email protected] DOI: 10.1557/mrs.2015.197
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MRS BULLETIN • VOLUME 40 • SEPTEMBER 2015 • www.mrs.org/bulletin
© 2015 Materials Research Society
NANOCOMPOSITES FOR THERMOELECTRICS AND THERMAL ENGINEERING
In the case of thermoelectrics, the analysis of macrocomposites can be traced back to Herring14 and was later extended by Bergman and Levy.15
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