(218b) Bottom-up Assembly of Metal Silicide Nanowires into Highly Efficient Bulk Thermoelectrics

Increasing the energy efficiencies of the existing processes and systems through waste heat scavenging could be employed to partially offset the projected increase in the future energy demand. The importance of waste heat recovery for energy generation becomes more apparent, when automobiles are considered as an example. In automobiles, approximately 70% of energy generated by burning gasoline is lost either in the engine coolant or through the exhaust, and waste heat scavenging vastly improves their fuel efficiency. Use of solid-state thermoelectric modules is a minimally invasive method for scavenging waste heat from such systems and processes. The lack of any moving parts, portability, very long operational lifetimes, and minimal maintenance make thermoelectrics attractive for waste heat recovery.

The lack of the requisite materials for achieving a good $/watt heat-to-electricity conversion metric is currently preventing the large-scale deployment and use of thermoelectrics in terrestrial applications. A factor responsible for this bottleneck is the low efficiencies of the current thermoelectric materials, which in turn could be attributed to the lack of a pathway for precisely and independently tuning the thermal and electrical transport through them. Recent theoretical and experimental studies have demonstrated that nanostructuring of materials serves as a route for independently tuning the thermal and electrical transport. Most of these demonstrations were made using individual suspended nanowire devices as testpads. However, these studies do not delineate a pathway for extending enhanced thermoelectric performance exhibited by individual nanowires to bulk devices composed of a multitude of nanowires. Metal silicide nanowires are a class of inexpensive and non-toxic materials useful for the fabrication of thermoelectrics, provided their efficiencies are improved beyond that is currently possible. Considering all these facts together, a fundamental question that needs to be answered is the following: How can metal silicide nanowires be mass produced and assembled into devices in a manner that offers precise control over the chemical composition, thermal and electrical transport of the interfaces between the nanowires after their assembly? In this context, we have developed a solid-state phase transformation strategy useful not only for converting pre-synthesized silicon nanowires into metal silicide nanowires, but also for assembling the formed metal silicide nanowires via welding. Assembly of metal silicide nanowires in a manner that makes the chemical composition of the bridges between the nanowires the same as that of the nanowires was accomplished. These results will be illustrated using magnesium silicide (Mg₂Si) nanowire synthesis and assembly as an example. The thermoelectric performances of welded Mg₂Si nanowire assemblies and their relationship to the dimensions of the nanowires and the bridges between the nanowires will be discussed. The thermoelectric performances of welded Mg₂Si nanowire assemblies will be compared to their unwelded counterparts.