Solvothermal Preparation And Performance Of Chalcogenide Thermoelectric Materials

Thermoelectric (TE) materials, which can directly convert thermal to electrical energy, provide a possible solution for carry out energy crisis. Thus they have a wide range of applications in all-solid-state power generation and semiconductor refrigeration. Ⅳ-Ⅵ and Ⅴ-Ⅵ chalcogenide compounds are one class of the most important thermoelectric materials near room temperature and in mid-temperature zone. Bi2Te3and its alloys are the best TE materials available near room temperature currently. Nanostructured bulk TE materials have a significant improvement in both mechanical properties and TE performance of these traditional materials. For the bottom-up approach preparation of nanostructured bulk TE materials, the solution chemistry method has unique advantages in obtaining nano powders of10nm. Especially, hydro/solvothermal method has attracted extensive attention over the last decade.This paper aims at controllable synthesis of thermoelectric materials nano powders and optimizing TE performance of the micro-nano composite bulk materials. The ethylene glycol (EG) reduction solvothermal synthesis of Bi2Te3and other IV-VI and V-VI chalcogenide nano powders was systematically researched. The preparation and thermoelectric properties of Bi2Te3based micro-nano composite bulk thermoelectric materials were also presented. Some important results of the work are listed below.1. This paper developed an EG reduction solvothermal method for controllable synthesis of Bi2Te3nano powders. EG was used as both reducing agent and solvent, and no other reducing agents were added. The mild reductive EG and low temperature (180℃) were used to control the nucleation and growth of the Bi2Te3nano crystals with hexagonal morphology and uniform size. The reaction temperature and alkaline environment play important roles in the formation of Bi2Te3nano crystals, and the morphology of the products can be influenced by the added surfactants (PVP). The thickness of the nanosheets can be controlled by adjusting the concentration of NaOH. The growth of Bi2Te3nanosheets mainly follows a diffusion controlled growth mechanism.2. Binary Sb2Te3, Bi2Se3, Sb2Se3, PbTe, SnTe, and PbSe nanocrystals and ternary Bi0.5Sb1.5Te3and Bi2Te2Se nanopowders were also successfully prepared by similar EG reduction solvothermal route. It is proved that this EG reduction solvothermal method has good process universality and compatibility for the preparation of IV-VI and V-VI chalcogenide nanomaterials3. Micro-nano composited Bi2Te3bulk samples were prepared by using vacuum hot pressing method. The impact of nano powders content on the thermoelectric properties of these bulk samples was also systematically researched. The increaseof the nano powder content resulted in a significant reduction in thermal conductivity of the micro-nano composite bulk samples. The electrical properties of the micro-nano composite bulk samples were affected not only by the scattering at grain boundaries, but by the point defects such as Te vacancy, anti-site defects and donor effect. More work is needed to understand the role of various types of point defects in the carrier transport behavior in micro-nano composite bulk samples. In this work, the maximum ZT of the micro-nano composite bulk samples was0.8(at427K), with a nano-powder mass fraction of85%.