Access Type

Open Access Dissertation

Date of Award

January 2016

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Stephanie L. Brock

Abstract

The overall purpose of this work is to address several of the roadblocks to use of thermoelectric materials for generation of electricity, namely inefficient processing of materials and low performance, commonly rated by the figure of merit, ZT=T2/tot. The ZT includes  as the Seebeck coefficient,  as electrical resistivity, T as the average temperature, and tot as total thermal conductivity. tot is the sum of electronic charge carrier (C) and lattice (L) contributions to thermal conductivity. Attempts to increase ZT in the literature to values >1 have focused on decreasing the thermal conductivity via nanostructuring or optimizing the electrical conductivity and Seebeck coefficient by doping. In this work, two separate approaches are taken to tackle these issues: (1) Target higher ZT by assembling lead telluride (PbTe) nanoparticles from a multi-gram synthesis utilizing ligand stripping techniques or deliberately including discrete lead sulfide (PbS) NCs. (2) Develop a rapid, convenient synthesis of tetrahedrite (Cu12Sb4S13).

Approach (1): Nanostructuring of PbTe and PbTe–PbS. Nanostructured PbTe and nanocomposites of PbTe–PbS are hypothesized to increase ZT by lowering thermal conductivity, while ligand stripping of PbTe NCs by sulfide or iodide is expected to increase ZT because it has been demonstrated to increase electrical conductivity in thin films of PbS. A new synthesis is in demand because mixing PbTe and PbS NCs requires that the PbTe be dispersible, and literature syntheses of such NCs suffer from small yields (< 200 mg). Thus, applications of dispersible PbTe NCs are largely limited to thin films. The ZT values of these thin films are not reported due to difficulty in quantifying thermal conductivity. In the dissertation research, nanostructured PbTe pellets are prepared by hot-pressing PbTe NCs after either mixing with PbS NCs by incipient wetness, or ligand stripping with sulfide salt, iodide salt, or both. The PbTe NCs themselves are prepared in multi-gram quantities by hot-injection methods in solution. The NCs are characterized for crystallinity by powder X-ray Diffraction (XRD). The size and morphology of the NCs are probed via Transmission Electron Microscopy (TEM), and their composition is determined by Energy Dispersive Spectroscopy (EDS). The thermoelectric properties are studied on hot-pressed pellets of each sample.

Approach (2): Developing a facile route to tetrahedrite and doped derivatives. Tetrahedrite is exciting the thermoelectric community due to its lack of rare or toxic elements, the tunability of its electronic properties by doping, the ability to dope by ball-milling with the plentiful natural mineral, and the ability to achieve a ZT of unity. However, the natural mineral is unsuitable on its own due to an excess of natural dopant, and reported tetrahedrite syntheses require heating at high temperature 650 °C in a three day process followed by two weeks of heating at 450 °C. This work establishes a new synthesis amenable to industrial production that reduces the heating time from over 2 weeks to 2 days for simultaneous batch production at moderate temperature (155 °C for one day and 430 °C for 30 min, cooling naturally). The tetrahedrite powder is prepared from chloride-free metal salts and thiourea by solvothermal methods and characterized by XRD for crystallinity. The composition is determined by Inductively Coupled Plasma analysis. Products from multiple batches are mixed by ball-milling alone or combined with the natural mineral as a means to dope with Zn2+ as a solid solution. The resulting powder is then hot-pressed to pellet form for thermoelectric characterization. The tetrahedrite is also doped in-situ by zinc over a range of 0.79 to 1.40 mol equivalents using chloride-free metal salts.

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