Preparation And Thermoelectric Properties Of BiSbTe With Te/CdTe/ZnTe Incorporating

Recently,issues pertaining to the energy crisis and environmental pollution caused by burning of fossil fuels have attracted worldwide attention that has motivated searches for alternative renewable sources of energy and the development of related conversion technologies.Thermoelectric materials,which enable direct conversion between heat and electrical energy,show great prospects in the area of industrial waste heat recovery,including capturing and recycling heat from the exhaust systems of gasoline and diesel powered cars and trucks.Thermoelectric energy conversion generates no emissions,is quiet and exceptionally reliable.Bismuth telluride-based materials possess superior performance near room temperature,and zone-melted forms of bismuth telluride have been successfully used in commercial modules operating as thermoelectric refrigerators as well as in the automotive exhaust heat recovery.However,to capture a significantly larger fraction of the energy market,particularly as far as large-scale commercial applications are concerned,further improvements in the performance of Bi2Te3-based materials,including their heat stability,are urgently needed.The target of this study is p-type BiSbTe based compounds.Aiming at the core theme of enhancing the thermoelectric properties,defects and boundaries engineering were chosen as the key stratagem to optimize the transport performance of carriers and phonons,based on the conventional quenching and melt spinning method combined with plasma activated sintering technique.The roles of introducing Te,Zn and Cd on the defetes structure,phases composition and thermoelectric properties of BiSbTe were systematically investigated,the main contents and results are listed as follows.Te is distributed uniformly at the grain boundaries of the rapidly solidified ribbons.During the SPS process,melting and squeeze out of Te promotes a directional alignment,which improves the orientation factor F and the electrical conductivity perpendicular to the pressing direction.As a result,a sample sintered from melt-spun ribbons containing 25 wt%excess of Te has a higher orientation factor F=0.17 and electrical conductivity?=8.56×10⁴ S/m than stoichiometric Bi_0.5Sb_1.5Te₃?F=0,?=6.70×10⁴ S/m?.The perpendicular lattice thermal conductivity,although rather small at 0.65 Wm^-1K^-1 for a sample with 25 wt%excess of Te,remains significantly higher than claimed in ref.16.Measured in the parallel orientation the thermal conductivity is much lower(0.74 Wm^-1K^-1 for the sample with 25 wt%excess of Te)because it is more difficult for phonons to propagate across the van der Waals gaps of the crystal structure.The maximum value of ZT we have measured reached 1.24 at 350 K for samples with20 and 25 wt%excess of Te,representing about 15%increase over the figure of merit of identically prepared Bi_0.5Sb_1.5Te₃ having no excess of Te.We demonstrate a thermoelectric figure of merit ZT of 1.4 at 400 K,realized in Zn-containing BiSbTe alloys by integrating defect complexity with nanostructuring.We have succeeded in creating nanostructured BiSbTe alloys containing ZnTe nanoprecipitates.We present a melt-spinning-based synthesis that forms in-situ ZnTe nanoprecipitates to produce extremely low lattice thermal conductivity of⁰.35 Wm^-1K^-1 at around 400 K,approaching the amorphous limit in the Bi_1-xSb_x Te₃ system,while preserving the high power factor.These samples show excellent repeatability and thermal stability at temperatures up to 523 K.Theoretical calculations and experimental results show that Zn is inclined to form dual site defects,including two substitutional defects Zn'_Bi/Sb and a Te vacancy,to achieve full charge compensation,which was further explicitly corroborated by Positron annihilation measurement.The strong enhancement of thermoelectric properties was validated in a thermoelectric module fabricated with the melt-spun p-legs?ZnTe-nanostructured BiSbTe?and zone-melt n-legs?conventional BiTeSe?which achieved a thermoelectric conversion efficiency of5.0%when subjected to a temperature gradient of 250 K,representing about 40%improvement compared with a commercial zone-melt-based module.The pseudo ternary phase diagram for Cd-Bi/Sb-Te was systematically studied,which proved that the solid solution limit of Cd in the BiSbTe matrix is around for sample of nominal Cd dopping and CdTe incorporating.The carrier concentration increases with the increasing Cd content for both series samples.For samples of nominal Cd doping,Cd serves as the acceptor dopant contributing holes,while for samples of CdTe incorporating,the existence of Te vancanies will promotes the formation of Sb/Bi_Te antisite defects,the combined effects of three types of defects devote to the increase of hole concentration.The lattice thermal conductivity of samples reduces significantly with the increasing Cd content and even closing to 0 Wm^-1K^-1assuming a single parabolic band model.Due to the enhancement of power factor and reduction of lattice thermal conductivity the maximum ZT value can reach 1.15 at 424K for both series of samples.The orientation factor of?00l?crytal interface is about0.25 and 0.02 measured parallel and perpendicular to the pressure direction respectively,however the orientation factor varies little with the Cd content.When the Cd content is relative low,the ZT value measured in parallel direction is higher than that of perpendicular direcion for the lattice thermal conductivity decreases more obviously than the deterioration of power factor,and it can reach 1.25 at 425 K when x=0.01.However,when the Cd content increases,the carrier mobility in perpendicular direction is much higher which would strongly scatter the phonons,therefore,the lattice thermal conductivity is lower than that of the parallel direction.The melting spinning method was introduced to further refine the grain size and decrease the lattice thermal conductivity,a higher ZT of 1.30 at 400 K was realized when nominal CdTe content x=0.005.Based on the constitution with nomnal CdTe x=0.06 prepared by traditional method,we regulate the carrier concentration by changing the Bi/Sb ratio and Se solid solution,the maximum ZT values can reach 0.72 and 0.66 at 523 K when Bi content x=0.70 and Se content x=0.30 respectively.