Nanoscale Engineering for the Design of Efficient Inorganic-Organic Hybrid Thermoelectrics
Abstract
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.
Citation
Brockway, Lance Robert (2014). Nanoscale Engineering for the Design of Efficient Inorganic-Organic Hybrid Thermoelectrics. Doctoral dissertation, Texas A & M University. Available electronically from https : / /hdl .handle .net /1969 .1 /152687.