Fabrication and characterization of nanowire array composites for thermoelectric power generators

Thermoelectric devices can be used to harvest waste heat energy lost during the operation of mechanical and electrical systems by converting it into usable electrical energy. The primary challenge in realizing efficient thermal-to-electrical energy converters is developing materials with good thermal, electrical and thermoelectric properties. Recent efforts in this area have primarily focused on nanostructured thin film based thermoelectric materials that have exhibited thermoelectric figures-of-merit(ZT)exceeding those of similar materials in bulk form. However, the viability of these thin-film structures for device purposes is limited by the scalability of the growth techniques, and by the elastic constraints imposed by thin-film epitaxy of lattice mismatched materials on a macroscopic substrate. In the present work, I explore Bi2Te3 and Bi 2(Te,Se)3 materials systems for fabrication of high volume fraction nanowire arrays for use as thermoelectric materials. An electrochemical synthesis method that is scalable to thicknesses in the range of 10-100 μm is employed, as this thickness range is optimal for energy harvesting. The nanowire morphology offers elastic lateral relaxation of lattice misfit strain to accommodate nanoscale composition modulation without losing lattice coherency. In this work, various challenges involved in realizing a nanowire array based thermoelectric module were systematically addressed and resolved, including: (1) fabrication of dense, crystalline Bi2Te3 nanowire arrays with 110 fiber texture, a crystallographic texture that yields high ZT; (2) fabrication of compositionally modulated Bi2Te3-x Sex multilayer nanowire arrays with varying Se content from a single electrochemical bath; (3) process flow for PAA matrix replacement with a low thermal conductivity polymer, SU-8 (0.2 W/m-K); (4) fabrication of novel branched porous anodic alumina templates (B-PAA) that serve as a sacrificial framework for synthesis of self-supporting, 3-D interconnected nanowire arrays; (5) demonstration of an order of magnitude reduction in thermal conductivity in Bi2Te3-xSex nanowire arrays through nanostructuring. Through the demonstrated hybrid nanostructuring, it is possible to engineer the materials bandgap as well as scatter phonons over a wide range of wavelengths, leading to enhanced ZT\u27s that would have significant transformative impact in the field of thermoelectrics. This work was supported by a grant from the Office of Naval Research (N000140610641)