Preparation And Properties Of GeTe Rich Sb₂Te₃?GeTe?_n Based Thermoelectric Materials

Thermoelectric?TE?materials,which can convert heat into electricity?or vice versa?directly,contain unique advantages when applying in the new energy field.In particular,the medium-temperature TE materials have drawn the attentions of worldwide researchers for their promising application prospects in recycling waste heat power or supplying for self-driven electronic devices.The core task for TE materials research is to improve the ZT value,leading to a marvelous conversion efficiency at the application temperature range.In this thesis,the p type medium-temperature TE materials,Ge Te-rich Sb₂Te₃?GeTe?_n,were found to have promising TE performance,however,limited by their intrinsically high carrier concentration.Thus,we enhanced the electrical and thermal properties by vacancy engineering,donor doping or equivalent doping method and investigated the mechanisms about structure characteristics and performance optimization.Additionally,the unique structure of Sb₂Te₃?GeTe?_n was further fabricated in Sb2Te3?Sn Te?n sample and the TEM analysis techniques were successfully applied to the Mn4Si7 sample in this work.The main achievements can be summarized as follow:A series of Sb₂Te₃?GeTe?_n samples with different components were prepared by melting followed spark plasma sintering?SPS?method,confirming that the ZT value of n=17 could reach about 1.9 at 773 K,which depicted the best performance in these compounds.With suitable thermal treatments,it was reported for the first time that the Ge vacancy defects in Sb₂Te₃?GeTe?₁₇ samples preferred to migrate,reconstruct and form long-term gap defects during this long annealing process,leading to a better thermoelectric performance.Finally,the ZT value of Sb₂Te₃?GeTe?₁₇ sample with 7 days' annealing was up to 2.4 at 773 K,and a record ZTave value of 1.5 was achieved from 323 K to 773 K.Owing to the systematic studying of Bi doped Sb₂Te₃?GeTe?₁₇ samples and Bi I3 doped Sb₂Te₃?GeTe?₁₇ samples,the carrier concentration could be suppressed effectively to 2.5×1020 cm-3 by the donor doping method.Additionally,vast anion vacancy clusters were also indicated by Cs-corrected transmission electron microscopy?TEM?in the Bi I3 doped samples.This kind of defects could effectively scatter phonons,reducing the lattice thermal conductivity from 1.0 Wm^-1K^-1 to 0.8 Wm^-1K^-1.Eventually,a high ZT value of ?2.2 at 723 K and a large average ZT value of ?1.4 between 323 and 773 K were achieved in the 1.5 at% Bi I3-doped Sb₂Te₃?GeTe?₁₇ sample.Doping Yb into Sb₂Te₃?GeTe?₁₇ matrix could reduce the Ge vacancy formation energy,resulting in large amounts of Ge vacancy defects,so that the lattice thermal conductivity was reduced from 0.75 Wm^-1K^-1 at x=0 to about 0.42 Wm^-1K^-1 at x=0.005.Eventually,a maximum ZT value of about 2.4 at 773 K,and a high ZTave value of 1.5 from 323 K to 773 K were obtained.Moreover,a further investigation showed that the Yb doped sample contained excellent repeatability and thermal stability,which was suitable for thermoelectric devices.Finally,when the temperature gradient was 423 K,the measured conversion efficiency is about 8% and the output power is about 3W.Additionally,Doping S into Sb₂Te₃?GeTe?₁₇ matrix could increase the Ge vacancy formation energy,thereby reducing the carrier concentration.Finally,when the doping content x=0.1,the peak ZT value reached 2.2 at 773 K.To expand the performance advantages of Sb₂Te₃?GeTe?_n brought by the microstrucutre,the Sn Te-rich Sb2Te3?Sn Te?n samples were firstly synthesized in this work,and dense Sn vacancy defects and gap-like structures were directly observed in our Sb2Te3?Sn Te?8 sample.As the same as the Sb₂Te₃?GeTe?_n sample,this gap structure was a kind of two-dimensional defect formed by the migration and reconstruction of cation vacancies.Attributing to this extra defect,the lattice thermal conductivity decreased from 3 Wm^-1K^-1 of Sn Te to 0.8 Wm^-1K^-1 of Sb2Te3?Sn Te?8 sample.Finally,the maximum ZT value could reach 1.12 at 723 K.Moreover,for the optimization of high manganese silisides which also contained a kind of speicial microstructure,here,the aberration-corrected scanning transmission electron microscopy high-angle annular dark field?STEM-HAADF?technique was used to identify the crystal structure of Mn4Si7,and the accurate ratio between Mn atom layers and Si atom layers could be directly obtained.Finally,in the B-doped Mn4Si7 sample,the peak ZT value of 0.55 at 773 K,and the ZTave value of 0.4 from 300 K to 823 K could be achieved at x=0.04.