Branched nanowire arrays as bulk-like thermoelectric materials
Thermoelectric generators (TEG) and coolers (TEC) consist of a plurality of n-type and p-type semiconductor TE leg elements, which can be utilized to convert heat into electricity, or conversely, create temperature gradients. The efficiency of a TE device is directly related to the material's dimensionless thermoelectric figure of merit, ZT = S2σT/ K, where S is the Seebeck coefficient, sigma is the electrical conductivity, T is the absolute temperature, and k is the thermal conductivity. Bulk-alloy bismuth telluride (Bi2Te3) as the most widely used TE material has a room temperature ZT around unity. Recent research progress in nanostructured materials and composite materials enlightened the path to enhance the ZT above 1. Nanowire array topologies offer promise for engineering the electrical conductivity and phonon scattering effects, both of which have been studied extensively since the 1990s. Conventional porous anodic alumina (PAA) was used for successful fabrication of Bi2Te3 nanowire arrays, but with array thicknesses limited to tens of microns, too thin for high performance TE devices. The parallel nanowire arrays also suffer from mechanical fragility, low Bi2Te3 volume fraction, limited thermal conductivity suppression, vulnerability to shear load and slow template fabrication. In this work, I investigated the Bi2Te3 branched nanowire array (BNA) topology that features nanostructure-to-bulk-like materials integration and radially modified nanowire compositions, allowing for thermal conductivity reduction, multiscale nanostructure creation, preservation of power factor, strain relaxation and shear compliance. Branched Bi2Te 3 nanowire arrays were fabricated from the bottom up by electrochemically depositing Bi2Te3 into 300-500 micron thick branched porous anodic alumina (BPAA) templates followed by vapor annealing. The annealing process homogenizes the native point defects and can be used to introduce Se as an axially-graded dopant, which also forms nanocrystalline Bi2 SeTe2 phonon scatterers. The thickness of the nanowire array device can be fabricated up to 350 micron (wire aspect ratio > 1000:1), which is approaching commercialized bulk-form TE elements. Various characterization techniques including XRD, FESEM, HRTEM, EDS, AFM, etc. were employed to analyze the morphologic, structural and stoichiometric properties of the material. At room temperature, characterization of electrical, thermal and Seebeck properties were performed to demonstrate the device-level fabrication viability. At 300 K, thermal conductivity was measured to be 0.5 W/m-K. Electrical conductivity was measured at ∼1.43x104 S/m. An effective bulk ZT of 0.31 was measured and the microscopic corrected value was 0.39. To demonstrate the p-type TE counterpart, pulsed laser assisted electrodeposition was used to fabricate Bi0.5Sb1.5Te3 nanowires. In addition, in order to calculate the optimum thermoelectric element leg length under different heat transfer conditions, a numerical model of thermoelectric device for both n- and p-type legs was developed in Matlab and powered by Rappture on nanoHUB.org. The demonstration of Bi2Te3 branched nanowire array structures enables a nano-to-bulk, composition-modulated and potentially scalable method to fabricate thermoelectric materials. The BNA structure maintains the nanowire electrical conduction continuity by electrochemical deposition, and offers extensive control over the wire radial composition and nanostructure. Moreover, the BNA's phonon scattering effect is attributed to nanowire array branching, high surface-to-volume ratios, grain boundaries, Bi2SeTe 2 nanocrystals and atomic alloying, covering length scales ranging from tens of microns down to less than a nanometer. Overall, the branched Bi 2Te3 nanowire array as a bulk-like thermoelectric material is a promising structure for enhancing the figure of merit, ZT, and has promise to lead to the development of superior engineered nanomaterials as a substitute for bulk Bi2Te3 alloy materials. This work was partially supported by Office of Naval Research (contract No. N00014-09-1-0486).
Sands, Purdue University.
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