Solution chemical synthesis of nanostructured thermoelectric materials.

1. Introduction

The performance of a thermoelectric (TE) material is evaluated by its figure of merit Z = [[alpha].sup.2]/[rho][kappa], where [alpha] is the Seebeck coefficient, [rho] is the electrical resistivity, and [kappa] is the thermal conductivity, which includes the contributions from both carriers and phonons, i.e. [kappa] = [[kappa].sub.[epsilon]] + [[kappa].sub.l]. A nanostructure, as indicated by its prefix, has one of its crucial dimensions on the order of 1-100 nm. In recent years, low dimensional or nanostructured thermoelectric materials has become an active research field of TE study, mostly motivated by the presence of increased electron density of states at the Fermi level in nanostructured materials and the possibility of exploiting boundary scattering to reduce the thermal conductivity (1). Theoretical calculations of low dimensional TE materials have predicted a significant enhancement in ZT as the film thickness in two dimensional (2D) systems (2) or the wire diameter in one-dimensional (1D) systems (3) is decreased, benefiting from both quantum confinement effects to carriers and pronounced phonon scattering at the boundaries. Besides low-dimensional thin films or nanowire arrays, which might be mainly used in a micro-electro-mechanical system (MEMS), bulk TE materials are of more interest for their broad applications in commercial Peltier modules and power generation devices. The decision to pursue the concept of bulk nano-composite thermoelectrics comes primarily from the heat transport management point of view as a way to further improve the figure of merit of the existing materials, as the bulk nanocomposite approach has the advantage of working in a dual manner (4). Moving to lower dimensions could improve the electronic properties via quantum effects, notably the Seebeck coefficient, while introducing nanoparticles should lower the lattice thermal conductivity by effectively reducing the phonon mean free path. Using modern rapid sintering techniques such as spark plasma sintering (SPS), it is possible to form nanostructured bulk materials from nanopowders without significant grain growth by reducing both time and temperature needed for the densification. However, to make bulk nanocomposite, a practical question would be how to grow various thermoelectric nano powders with an acceptable yield? A variety of techniques are available to produce nanostructured powders. Means like rapid solidification (5), atomization (6) and high energy ball milling (7) could be categorized as physical techniques. Other than these physical techniques, chemical approaches, such as Chemical Vapor Deposition (CVD), reduction & code-position, sol-gel, and so on, are all effectual nano preparation routes. Among which, hydrothermal / solvothermal synthesis, are attracting more and more attentions on the synthesis of nanostructured powders due to their advantages of low cost, very high yield, low energy consumption, short duration and most important, versatility, which will be demonstrated in this paper.

Hydrothermal synthesis is one of the important methods for producing fine powder of oxides. A hydrothermal system is usually maintained at a temperature beyond 100[degrees]C and the autogenous pressure of water exceeds the ambient pressure, which is favorable for the crystallization of products. Research indicates that the hydrothermal method is also a practical means for preparing chalcogenide and phosphide nano materials. Similar to hydrothermal synthesis, in a solvothermal process (8), a nonaqueous solvent, which is sealed in an autoclave and maintained in its superheated state, is the reaction medium, where the reactants and products are prevented effectively from oxidation and volatilization and the reaction and crystallization can be realized simultaneously. Furthermore, organic solvents may be favorable for the dispersion of non-oxide nano-crystallites and may stabilize some metastable phases. Hydrothermal and solvothermal syntheses are both commonly used methods to prepare nanostructured powders with controllable size and morphology these days.

Concerning our own work, we've been applying solution chemical methods, including hydrothermal and solvothermal approaches to prepare nano thermoelectric materials for several years. By utilizing and continuously developing these techniques, various nanoscale state-of-the-art thermoelectric compounds, such as [Bi.sub.2.][Te.sub.3], [CoSb.sub.3], PbTe & PbSe, Bi2S3, Bi-Sb-Te & Bi-Te-Se, etc., with multiple nanostructures, have been successfully synthesized. In this paper, some of these synthesizing works, along with our lately ...