(Invited) Nanowire-Based Bulk Thermoelectrics
Authors: Vaddiraju Sreeram 
Published: February 2020

Theoretical and experimental studies over the last two decades have indicated that nanostructuring could be employed to overcome the constraints imposed by Wiedemann-Franz law and thereby allow for independently tuning thermal and electrical transport through materials. Such precise and independent control over electrical and thermal transport through materials allows for tuning their thermoelectric performances, which is typically estimated using the figure of merit, zT. The figure of merit, zT, of thermoelectric materials is given by the following: zT=S
2
σT/(κ
e+κ
l). Here, S is the Seebeck coefficient of the material, σ is the electrical conductivity of the material, and κ
e and κ
l are the thermal conductivities of the material from electronic and lattice contributions, respectively. In addition to lowering the κ
l of materials through nanostructuring, power factor (S
2
σ) enhancement could also be performed for enhancing their zT values. Strategies such as dopant optimization and resonant scattering aid in enhancing the power factor values of materials. Consequently, multiple nanostructured morphologies of materials could be employed in the fabrication of thermoelectrics, including nanoparticles and quantum dots, superlattices and nanowires. However, the enhanced electrical conductivities expected of single crystal nanowires makes them preferable over nanoparticles. Similarly, incoherent phonon scattering for reducing the thermal conductivity in superlattices is partially masked by coherent phonon scattering. Hence, lowest possible thermal conductivities may not be achieved in superlattices. Furthermore, relative to the mass production of nanowires, mass production of superlattices is difficult. As research in the field of nanowire mass production progresses rapidly, it is entirely possible to mass produce and deploy terrestrial thermoelectrics for not only scavenging waste heat, but also the generation of electricity from renewable sources (e.g., solar thermal energy).

Translating recent successes in the laboratories in enhancing the thermoelectric performances of nanowires to industrial-scale production and terrestrial deployment of nanowire-based devices requires, first and foremost, extending enhanced energy conversion performances achieved in individual nanowires and small-scale nanowire arrays and mats to devices composed of large-scale nanowire assemblies. This in turn requires not only the mass production of nanowires, but also their integration into macroscale devices in a manner that offers the ability to engineer the interfaces between the assembled nanowires. In addition, ensuring reliable thermoelectric performances necessitates that the nanowires and the interfaces comprising the devices retain their respective chemical compositions over extended periods of time. So, nanowires need to be thermally inert and remain unreactive with air and moisture. In other words, convergence of the correct electrical, thermal, mechanical and chemical properties is essential to ensure fabrication of highly efficient bulk thermoelectrics based on nanowires.

In this invited talk, the strategies developed by our group for the mass production of nanowires that involve the direct reaction of component elements will be presented (1). Strategies for mass producing nanowires in a by-product free manner will be also be presented and discussed (2). In-situ methods useful for decorating nanowires with inorganic/organic molecules during/immediately after their synthesis for imparting them thermal and chemical stability will also be discussed (1,3-6). Implementation of pressure-assisted and shear-assisted strategies for the interface-engineered assembly of nanowires into either randomly oriented nanowire assemblies or aligned nanowire assemblies will be discussed (4-5,7-9). Illustrations of how nanostructuring, doping optimization and resonant scattering were employed to optimize the thermoelectric performances of bulk nanowires assemblies will be discussed in detail using Si, Zn3P2, ZnO and Mg2Si nanowire systems as illustrative examples (4-5,7-8).

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