Preparation And Optimization Of Low Thermal Conductivity Thermoelectric Tellurides

Thermoelectric (TE) materials can be used for the direct conversion between electricity and thermal energy. There are mainly two effects during the energy conversion process, namely Seebeck effect and Peltier effect. Based on these, TE materials can be used as power generator, cooler and also precise temperature controller. Due to the advantages TE devices exhibiting, such as small and lightweight, maintenance-free, no moving parts, acoustically silent and electrically quiet etc., they have been widely utilized in many different fields, for example, the power generator in spaceflights, infrared-seeking missiles, laser diode coolers, low noise amplifiers and also wine cabinets. However, the low energy conversion efficiency is still the neck, which limited the application of TE devices. The optimization of TE materials with higher TE performance is very essential for the further development of TE industry.Based on the definition of thermoelectric figure of merit, ZT=α2CσT/κ. The TE performance is proportional to the electrical conductivity, a and Seebeck coefficient, a and inverse proportional to the thermal conductivity,κ. Most of the researches today focus on the reduction of thermal conductivity in the TE materials with high electrical transport properties. However, there is a minimum limitation for the thermal conductivity, which is called the amorphous limitation. So in this study, we focus our attention on the TE materials with intrinsically low thermal conductivity. In-situ precipitates, solid solution, boundary clearance and doping were performed on different low-κTE systems to enhance the electrical transport properties, while maintain or further decrease the thermal conductivity to final improvement of the TE properties. The conclusions are listed below:1. The crystalline kinetics theory has been used to analyze the crystallization process of SiTe-based amorphous. The activation energies of crystallizationΔEa have been calculated to be 198-204 kJ/mol and 236-244 kJ/mol for Si15Te85 and Si20Te80, respectively. By the analysis of related data, Si2oTego amorphous has been found with lower crystallization ability but higher thermal stability comparing with Si15Te85 And the crystallization processes for these two amorphous both indicated a mixed mechanism of two and three-dimensional growth.2. Low temperature transport properties measurements and related theories were combined together to analyze the phonon transport properties in Se doped GeTe-based amorphous. The low temperature specific heat measurements identified some localized low-frequency oscillation modes (Einstein modes) in conjunction with a Debye-like behavior. The thermal conductivity of all Ge20Te80-xSex exhibited typical amorphous heat conduction behavior, which has been discussed in connection with the small phonon mean free path. All these studies provided the theoretical basis for the afterwards study on the low-κTE materials.3. In-situ nanocomposites with different precipitations (Ag2Te or Sb2Te3) have been obtained in AgSbTe2 system by tuning the ratio of Ag2Te to Sb2Te3. It proved that these in-situ precipitates can contribute to the improvements of Seebeck coefficient and electrical conductivity at the same time based on the energy filtering effect. On the other hand, the precipitates of Ag2Te nanodots and nano-lamellae structures are much more effective for the optimization of thermoelectric properties comparing with Sb2Te3 precipitates. The maximum ZT value of 1.53 has been obtained in AgiTe precipitated samples at 673 K with the enhancement of～40% comparing with the single phased samples.4. The boundary clearance process has also proved to be very effective to (GeTe)85(AgSbTe2)15 (TAGS-85). By changing the ball-mill atmosphere from air to Argon, clean boundaries and smaller grains have been obtained. The ZT value has been improved from 1.4 to 1.8, which was sintered by SPS after the ball-mill process in air and Argon, respectively.5. Non-stoichiometric Ag8GeTe6 based alloys have been successfully synthesized. The Ag self-doping has proved to be very effective for the enhancement of the thermoelectric properties due to the improvement in Seebeck coefficient, while maintaining the intrinsically low thermal conductivity. The maximum ZT of 0.86 has been obtained in Ag7.99GeTe6 at 623 K which is more than 30% improvement comparing with the stoichiometric AggGeTe6.