Investigation On The Thermoelectric Properties Of Tellurides And Fabrication Of Thermoelectric Devices

Thermoelectric technology has drawn increasing attention due to its particular ability of direct energy conversion between heat energy and electricity.Bi₂Te₃-based alloys are the most well-known thermoelectric materials for room-temperature application.However,the maximum z T values of Bi₂Te₃-based materials are usually achieved near room temperature.The z T values dramatically drop above 400 K because of material's intrinsic thermal excitation in Bi₂Te₃-based materials,which seriously restricts their applications as thermoelectric power generators.As the high performance tellurides during the medium temperature,Pb Te is the research focus of thermoelectric materials,but the toxicity of Pb element limits its application.Belonging to IV-VI compounds,Ge Te is becoming the most promising thermoelectric material to replace Pb Te due to its excellent electrical performance.However,the thermoelectric performance of Ge Te is severely restricted by its extremely high carrier concentration and lattice thermal conductivity.Moreover,its phase transition and special band structure also brings huge challenge to enhance the thermoelectric performance of Ge Te.So far,the reports about the design and fabrication of Ge Te-based thermoelectric devices are absen,which seriously hindered the application of Ge Te-based materials.This dissertation firstly tried to dope Ag in Bi_0.5Sb_1.5Te₃ to increase the carrier concentration and suppress the intrinsic excitation,and detailedly studies the electrical and thermal transport mechanisms for the materials having the bipolar effect.Then we focus on the enhancement of thermal performance of Ge Te-based material via doping different elements and investigates the influence on the electrical and thermal transport,band structure and crystal structure of Ge Te.Based on the optimized Ge Te-based material,the Ge Te-based thermoelectric device is successfully designed and fabricated and its output performance and reliability are both evaluated.The main achievements are describled as follows:1.By doping tiny amount of Ag into Bi_0.5Sb_1.5Te₃,we successfully suppressed the intrinsic excitation in p-type Bi₂Te₃-based materials and shifted the ZT peak temperature to high temperatures.Eventually,due to the enhanced power factor and greatly depressed bipolar thermal conductivity at high temperature,a maximum ZT of1.25 at 400 K was obtained in Ag_0.002Bi_0.5Sb_1.498Te₃,and an average ZT value of approximately 1.03 was achieved between 300 and 600 K.The theoretical energy conversion efficiency of TE module fabricated with Ag_0.002Bi_0.5Sb_1.498Te₃ achieves a maximum value of 11.0%when?T=300 K,which makes p-type Bi₂Te₃-based devices attractive for the applications in TE power generation.2.Ge Te is not sensitive to the chemical composition,allowing for a small amount of Ge excessive and Te absence.Adjusting the content of Ge or Te has little impact on the electrical and thermal transport of Ge Te.The carrier concentration of Ge Te is effectively reduced only via doping Sb and Bi.Moreover,doping Sb and Bi also can lead to the increase of crystal symmetry degree of Ge Te,resulting in the improved electrical performance.At the same time,additional point defects are introduced by Sb and Bi-doping,which greatly enhances the phonon scattering and thus suppresses the lattice thermal conductivity.Mg doping into Ge Te realizes the convergence of valence bands and increase the band degeneracy,which is beneficial to the electrical performance.Mg doping also enlarges the band gap of cubic Ge Te and suppresses the intrinsic excitation at elevated temperature.Mn doping into Ge Te converts the crystal structure from the rhombohedral phase into cubic phase.Yb cannot be largely doped into Ge Te,and most of them precipitate in the form of the second phase Yb Te,which deteriorate the electrical performance of Ge Te.3.Co-doping of?Bi,Sb?can not only effecticely optimize the carrier concertration but also introduce more lattice distortions and defects,which significantly decrease the lattice thermal conductivity.The maximum z T value of 1.90 at 700 K has been achieved via Bi and Sb co-doping.Co-doping?Mg,Sb?in Ge Te successfully suppresses the lattice thermal conductivity via introduce additional mass and strain field fluctuations.Moreover,the coupling effect of Mg and Sb dopants yields smooth volume variation around the phase transition temperature.Co-doping?Bi,Mg?introduces the low-lying phonon modes and produces strong interactions with the acoustic phonons to reduce the lattice thermal conductivity.Finally,a peak z T of 2.52 at 700 K and an average z T of 1.34 in 300–800 K have been achieved,an increase of 140%compared to Ge Te.4.Based on the three-dimensional multi-physical field thermoelectric coupling model,the optimization design is conducted on the Ge Te/skutterudite thermoelectric device,which achieves the optimal scheme for the maximum output power and conversion efficiency.Furthermore,Ni and Mo are screened out to be the electrode and barrier layer.The Ge Te/skutterudite thermoelectric device has been successfully fabricated and its output performance has been evaluated.Under the temperature difference of 500?,the maximum output power 2.0 W and the maximum conversion efficiency 7.8%is reached.However,after multiple thermal recycle,the cracks presence in the Ge Te-based uni-leg.To solve the cracking problem,the p-type uni-leg is fabricated using Ge_0.85Mg_0.05Sb_0.1Te,showing quite good service stability after 450thermal cycles.