Branched and interconnected bismuth telluride nanowire arrays for thermoelectric power generation

Pranati Tewari, Purdue University

Abstract

Thermoelectric generators can be used to extract waste heat energy and convert it into usable electrical energy. For decades, the thermoelectric figure of merit (ZT), which is a function of the Seebeck coefficient, electrical conductivity and thermal conductivity, has been limited to values of about 1 for practical bulk thermoelectric materials because in bulk thermoelectric materials, the parameters of ZT are interdependent. The challenge is to find materials whose thermal and electrical properties can be altered independently. Recent progress shows that in advanced bulk materials and low dimensional systems, the electrical and thermal parameters can be modulated independently allowing us to attain ZT values >1. In principle, the nanowire topology allows manipulation of the power factor in addition to reduction in thermal conductivity by increased phonon scattering from free surfaces. The focus of this work is the fabrication of nanowire arrays that can be used to form a thermoelectric device in order to exploit the benefits offered by nanowires and increase the figure of merit of the device. Emphasis is placed on resolving challenges faced during fabrication of templates as well as the nanowires, and to achieve scalable synthesis at low cost, comparable to bulk, while retaining nanoscale control of composition. 300-350 μm thick, Branched Porous Anodic Alumina (BPAA) templates were fabricated for subsequent electrodeposition of bismuth telluride nanowires. The nanowires of diameter ∼100nm, length exceeding 100μm were fabricated using potentiostatic pulsed electrodeposition (2s-6s pulses) providing good filling factors and uniform deposition. The Bi2Te3 nanowire core was annealed in Se to form a Bi2(Se,Te)3 shell, after equilibration of point defects by annealing in a Te vapor overpressure. This structure is designed to scatter long-wavelength phonons, lowering the thermal conductivity, while electrostatically repelling electrons from the free surfaces to reduce defect scattering and enhance mobility. Thus, it is anticipated that the resulting material may exhibit enhanced ZT, forming the n-type leg of a bulk thermoelectric device.

Degree

M.S.E.C.E.

Advisors

Sands, Purdue University.

Subject Area

Electrical engineering|Energy

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