Nanoscale Engineering for the Design of Efficient Inorganic-Organic Hybrid Thermoelectrics
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Research aimed at enhancing the thermoelectric performance of semiconductors comprised of only earth-abundant elements has recently come under renewed focus as these materials systems offer a cost-effective path for scavenging waste heat. In light of the prediction that nanostructuring could increase the thermoelectric performance of materials; semiconductor nanowires comprised of non-toxic, low cost, and earthabundant elements were synthesized and studied for their thermoelectric performance. For accomplishing this task, zinc phosphide (Zn_(3)P_(2)), zinc antimonide (Zn_(4)Sb_(3)), and zinc oxide (ZnO) nanowires were synthesized on the gram-quantity scale. Post-synthesis decomposition techniques were developed to controllably reduce the nanowire diameter and to create nanotubes. This decrease in nanowire diameter comes at an additional cost of an exponential decrease in surface stability. To combat this, a vapor phase surface passivation technique was developed to protect the nanowire surfaces from degradation and agglomeration. Finally, gram quantities of both functionalized and unfunctionalized nanowires were compressed into dense nanobulk pellets and characterized for their thermoelectric performance. The reactive vapor transport synthesis technique resulted in gram-quantities of single-crystalline nanowires with consistent 20 nm diameters. The nanowire diameters were further reduced to create sub-5 nm quantum wires and nanotubes using controlled decomposition. A self-consistent mechanism to describe this phenomenon was proposed. The nanowires were further surface-functionalized with various organic molecules to prevent surface degradation and to control the interfacial transport properties within the consolidated nanowire-pellets. The stability enhancement of the nanowires using this vapor-phase self-assembled monolayer technique was shown using traditional organic characterization techniques and suspension stability. Zn_(3)P_(2) and ZnO nanowires were then hot-pressed and spark plasma sintered, respectively, into nanobulk pellets. It was observed that the nanowires in the Zn_(3)P_(2) pellet did not break upon compaction, but bent elastically to achieve their sintered density; this was confirmed using a single nanowire inside a TEM. The thermoelectric performance of the functionalized nanowires was shown to be 3-fold higher than that of unfunctionalized nanowires due to less nanowire surface oxidation. Finally, the ZnO nanowire-bulk pellets were optimally-doped resulting in a 30% decrease in thermal conductivity compared to the bulk and the highest reported n-type oxide zT to date of 0.60.
Brockway, Lance Robert (2014). Nanoscale Engineering for the Design of Efficient Inorganic-Organic Hybrid Thermoelectrics. Doctoral dissertation, Texas A & M University. Available electronically from