Invited: Large-Scale Production and Assembly of Nanowires for the Fabrication of Highly Efficient Thermoelctrics
Authors: Vaddiraju Sreeram 
Published: February 2020

Energy usage in the United States is predicted to increase exponentially over the foreseeable future. However, data from the Energy Information Administration (EIA) indicates that 56-57% of all energy produced each year in the US is still rejected as waste heat. For example, a major portion of the energy generated by burning gasoline in automobiles (approximately 70%) is lost either in the engine coolant or through the exhaust. Therefore, a possible pathway for meeting the ever-increasing demand for energy is through the recovery of waste heat. Solid-state thermoelectric modules are useful for this purpose. The lack of any moving parts, portability, very long operational lifetimes, and minimal maintenance requirements of these solid-state thermoelectric modules make them very attractive for waste heat recovery. Current state-of-the-art thermoelectric modules are not highly efficient. This limits their use to specialty application (e.g., for powering up satellites). The lack of a pathway for precisely and independently tuning the thermal and electrical transport through materials employed in the fabrication of thermoelectrics is the primary reason for their low efficiencies. Recent theoretical and experimental studies have indicated that nanostructuring of materials serves as a route for independently tuning the thermal and electrical transport through them, and further enhance their heat-to-electricity conversion efficiencies. Of all the forms of nanomaterials, nanowires hold the most promise for the fabrication of efficient thermoelectric modules. The two different dimensions of the nanowires, namely the diameter and the length, offer a convenient pathway for independently tuning the electrical and thermal transport through them. This fact is supported by a few recent experimental studies on individual nanowires that indicated that nanowire form of materials exhibit enhanced thermoelectric performance, compared to their bulk counterparts. As the large-scale deployment of thermoelectrics for terrestrial use is a bulk application, it is required to ensure that enhanced thermoelectric performance observed in individual nanowires is also extendable to assemblies composed of a multitude of nanowires. This further requires strategies for the synthesis and assembly of nanowire powders on an industrial scale. Furthermore, any assembly strategy developed should offer the ability to tune the thermal and electrical transport through the interfaces between the nanowires. In short, the fabrication of next-generation, highly-efficient thermoelectrics requires the large-scale synthesis and interface-engineered assembly of nanowires (1-3). The aim of this presentation is to showcase the progress that has been made in this research area by our group. More specifically, the strategies useful for the bulk production of nanowire powders and their in-situ functionalization with organic molecules (1), together with the strategies useful for assembling them in an interface-engineered manner on a large-scale into functional thermoelectric devices (2,3), will be addressed in this talk (Figure 1). Example illustrations of how the interface-engineered assembly affords thermal and electrical transport control of the interface between the nanowires in the assembly will be presented. Novel strategies for imparting the nanowires resistance against air-, moisture- and acid-assisted degradation will also be presented. Specific examples that will be addressed include the fabrication and characterization thermoelectric devices based on bulk assembled Zn3P2 and ZnO nanowire powders (2,3). In addition, use of phase transformation as a strategy for the simultaneous synthesis (4) and assembly via welding of metal silicide nanowire powders will also be presented. The thermoelectric performance of metal silicide nanowire assemblies will be discussed.
ACKNOWLEDGEMENTS
Financial support from NSF/DOE thermoelectrics Partnership (NSF CBET # 1048702) is gratefully acknowledged.
REFERENCES

L. Brockway, M. Van Laer, Y. Kang, S. Vaddiraju, Physical Chemistry Chemical Physics, 15, 6260-6267, (2013).

L. Brockway, V. Vasiraju, S. Vaddiraju, Nanotechnology, 25, 125402 (2014).

L. Brockway, V. Vasiraju, H. Asayesh-Ardakani, R. Shahbazian-Yassar, S. Vaddiraju, Nanotechnology, 25, 145401 (2014).

Y. Kang, L. Brockway, S. Vaddiraju, Materials Letters, 100, 106-110 (2013).