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Engineering thermoelectric energy conversion by heterogeneity

Modulating electrons and phonons constitutes the fundamental route to enhancing thermoelectric figure of merit or ZT, a quantity that characterizes the efficiency of conversion of waste heat into electricity. The fundamental question is however how do we engineer their transport properties. Nanostructuring and alloying have widely been studied in the context of phonons. Nonetheless, strain and disorder effects are usually neglected in computing electronic or phononics contributions to energy transport (or in the context of thermoelectrics, energy conversion). Using first-principles calculations, we study the effect of heterogeneity on thermoelectric properties of alloys, taking into account a full description of the electronic bands and strain and disorder effects. For example, study on electron-dependent properties shows that alloying substantially affects thermopower[1].


Fig. It is found that configuration (b) is conducive for thermopowe, however, (a) has substantial advantage over reducing conductivity, thereby increasing ZT.

We also explore the influence of deformation on phonons, which play a critical role in modulating the denominator of ZT. The objective of this study is to identify how deformation and material heterogeneity couple with phonons and modulate its energy carrying capacity.

Fig. Decomposition of phonon modes and their dependence on strain is reported in [2].

Further reading  
             [1] Alloying and nanostructuring effects on electron dependent thermoelectric properties in Si/Si1-xGex
             M. Z. Hossain and H. T. Johnson, Applied Physics Letters, (in press), 2012

            
[2] Effects of composition, strain, and atomic disorder on optical phonon frequencies in Si1-xGex
             M. Zubaer Hossain and H. T. Johnson, Journal of Applied Physics 107, 073515 (2010)