Electrodeposition of bismuth:tellurium nanowire arrays into porous alumina templates for thermoelectric applications

Bismuth telluride is a well-known thermoelectric material for refrigeration applications. Thermoelectrics possess several advantages over conventional refrigeration and power generation devices, yet are not widely-used due to low efficiencies. It has been predicted and shown experimentally that the efficiency of thermoelectric devices increases when the semiconducting materials have reduced dimensions. Therefore, the aim of this research was to show enhanced thermoelectric efficiency in one-dimensional nanowires.;The nanowires were synthesized via electrochemical deposition into porous alumina templates. Electrodeposition is a versatile technique that ensures electrical continuity in the deposited material. The nanowire templates, porous alumina, were made by the double anodization of high-purity aluminum foil in oxalic acid solutions. This technique produces parallel, hexagonally packed, and nanometer-range diameter pores that can reach high aspect ratios (greater than 2000:1). The main anodization variables (electrolyte concentration, applied potential, 2nd anodization time, and temperature) were studied systematically in order to deconvolute their effects on the resulting pores and to obtain high aspect ratio pores. The porous alumina is of great importance because the pore dimensions determine the dimensions of the electrodeposited nanowires, which influence the thermoelectric performance of the nanowire arrays.;Nanowire arrays were characterized in several ways. Powder X-ray diffraction was used to assess crystallinity and preferred orientation of the nanowires, revealing that the nanowires are highly crystalline and grow with strong preferred orientation such that the material is suited for optimal thermoelectric performance. Scanning electron microscopy was used to evaluate the nanowire nucleation percentage and growth-front uniformity, both of which were enhanced by pulsed-potential electrodeposition. Compositional analysis via electron microprobe indicates that the as-grown nanowires are Te-rich or Bi-deficient, which agrees with Seebeck coefficient data showing the arrays are n-type semiconductors. In collaboration with Marlow Industries, the thermoelectric performance of the arrays was gauged. The nanowire arrays were successfully contacted with robust nickel layers as revealed by the low AC resistances of the arrays. One array was incorporated into a hybrid thermoelectric device and a DeltaT of 14.8°C was measured, indicating that the measurement and electrical contact approaches were successful despite further optimization being needed.