Thermoelectric Transport Properties And Performance Enhancement Of Bi₂Te₃-Based Materials

Thermoelectrics show great potential in energy harvesting and electronic cooling.On the materials level,increasing efficiency requires enhancing the thermoelectric figure of merit ZT.Bi₂Te₃-based alloys,as the state-of-the-art low temperature thermoelectric materials,have been put into industry for cooling devices for decades.On the other hand,since over 60%of the energy is wasted as heat in industry and in which the heat under300°C counts 50%can not be recovered by heat engine,how to push the high performance of Bi₂Te₃-based thermoelectrics to mid-temperatures is also highly desired.Advanced powder processes are advantageous for microstructure engineering and have achieved increasing attention in thermoelectrics.Aimed at enhancing the ZT values and push them to higher temperatures of both p-type and n-type alloys,this study is focued on how to use mechanical alloying based synthesis methods,chemical defects modulation,compositions optimization,microstructures engineering and etc.to comprehensively tune the thermoelectric performance.By comparing the thermoelectric properties between the samples fabricated by ball milling/mechanical alloying?BM/MA?and spark plasma sintering?SPS?and the zone melted ingots,it is found that for p-type?Bi,Sb?₂Te₃ the electrical properties are only slightly changed between different fabrication methods,while thermal conductivity is greatly reduced,the ZT is enhanced for BM and SPS samples.However,for the n-type Bi₂?Te,Se?₃,the electrical performance is greatly deteriorated,showing much lower carrier mobility.Due to the fact that the reduction of thermal conductivity cannot compensate the decrement of power factor,ZT of n-type Bi₂?Te,Se?₃ is decreased.Then to improve the ZT of n-type BM and SPS samples,further point defect engineering and Se alloying were applied to tune the carrier concentration along with thermal conductivity,resulting in a ZT of 0.82 at 473 K for the composition of Bi₂Te_2.2Se_0.8.SiC nanoparticles dispersion is applied to enhance the mechanical properties of Bi₂Te_2.2Se_0.8.And it is found that the low temperature ZT values are also improved by SiC dispersion.Further Cu/I doping make a self-tuning behavior of charge carriers,due to the temperature dependent solubility of Cu/I in Bi₂Te_2.2Se_0.8-SiC composites.Since optimal ZT at higher temperature needs higher carrier concentration,such self-tuning behavior result in a ZT plateau being very different from the commonly single ZT peak.The plateau of ZT=0.86 over a wide temperature range from 200-300°C is of great significance for low and mid-temperatures waste heat recovery.In the view of microstructure engineering,nanoscale defecs and microscale texture structure are well modulated by repeated SPS hot-forging-like process in Bi₂Te_2.2Se_0.8.High texture degree can maintain the high electric performance,while nanostructures dispersion could impede phonons transport and result in reduced thermal conductivity.By optimizing the hot forging temperature,the texturing degree,grain size and nanostructures are balanced,leading to an comprehensive enhancement of electrical performance and a decrease of thermal conductivity.A high ZT of 1.1 at 473 K is hence achieved.For p-type?Bi,Sb?₂Te₃,large porosity and microscale dislocations are introduced by using an unconventional melt-centrifuge technique.Lacking conduction medium at the pore sites decrease the thermal conductivity and the electrical conductivity equally,while pore interfaces additionally scatter the phonons.Dislocations target mid-frequency phonons which cannot be scattered by interfaces and point defects,making it more efficient to reduce the thermal conductivity.Both porosity and dislocations can lower the thermal conductivity and contribute 50%and 50%respectively for the reduction according to the systematic analysis based on Debye-Callway model and effective medium theory.A ZT of 1.2 at 373 K is obtained for the centrifuged Bi_0.5Sb_1.5Te₃.