Preparation And Characterization Of Telluride Based Thermoelectric Materials And Devices

These days,broad societal needs have forced attention on technologies that can reduce ozone depletion,greenhouse gas emissions,and fossil fuel usage.Thermoelectric(TE)materials can realize heat and electricity transformation directly,and are increasingly being seen as potential clean energy materials.Telluride based TE materials are famous class of TE materials due to their high TE performance.In this study,bismuth telluride(Bi2Te2),lead telluride(PbTe),and tin telluride(SnTe)were synthesized by mechanical alloying(MA)combined with spark plasma sintering(SPS).Their TE transport properties were improved by element doping and nanoscale composition fluctuations,respectively.,along with the simulation and fabrication of telluride based TE modules.Bi2Te3 is not only an excellent TE materials used near room temperature,but also a topological insulator.In 2013,QiKun Xue et.al.reported that quantum anomalous Hall effect was observed in magnetic elements(Cr/Fe)doped Bi2Te3 film,which is beneficial to obtaining high carrier mobility.In this study,a series of Fex(Bi0.15Sb0.85)2-xTe3 were synthesized.The results indicate that Fe3+ would enter into the Bi/Sb site when x?0.05.Further increasing x over 0.10 produces the formation of a second phase FeTe2.The carrier concentration was reduced and the mobility was improved by doping Fe.The highest power factor(PF)of 45?Wcm-1K-2 was obtained at 323 K when x=0.05.Owing to the synergy effect of PF and thermal conductivity(Ktotal),a maximum ZT value of 1.2 was obtained at 323 K when x=0.05.SnTe is a potential mid-temperature TE materials due to its cubic phase structure and similar band structure with PbTe.However,its TE performance is inferior owing to its vast of intrinsic Sn vacancy.In this study,a series of AlxSn1.03-xTe were fabricated,the lattice constant was expanded firstly and then shrunken after Al doping,,which can be ascribed to the compensation of Al to Sn vacancy firstly and then the substitution of Al to Sn.With increase of x,the carrier concentration and mobility were reduced simultaneously.The room temperature PF increases with increase of x.When x=0.05,the room temperature PF can reach to 11?Wcm-1K-2.The maximum PF of 18.7 ?Wcm-1K-2 was obtained at 623 K when x=0.05.Combined with lower electrical thermal conductivity,a highest ZT is 0.65 at 823 K for x=0.05 sample.Its maximum efficiency(?max)can reach 5.6%at the range of 323-773 K,which is 2.4 times than that of pristine SnTe sample.AgPbmSbTem+2(abbreviated as LAST),belongs to a branch of PbTe based TE materials.LAST possesses high TE performance owing to the formation of nano-scale composition fluctuation after annealing or second-milling processing.Hence optimization of the processing condition especially sintering temperature(TSPS)should be conducted before other approaches like compositional modification or nanostructuring to boost the ZT values.In this study,the significant enhancement of TE performance can be attributed to nanoscale composition fluctuation,which was appeared by only adjusting TSPS.When TSPS=923 K,the peak ZT achieved to 1.28 at 773 K.The highest ZTeng value of 0.26 and the corresponding ?max value of 5.8%were obtained at the range of 323-723 K,which are almost twice than those of the other counterparts.LAST is a typically high ZT material,while the corresponding TE module was rarely reported so far.How to utilize LAST to fabricate high efficiency TE modules therefore remains a central problem.In this study,two-stage PbTe/Bi2Te3 TE modules were developed with an enhanced efficiency through a comprehensive study of device structure design,module fabrication,and performance evaluation.Finite element simulation indicates that the temperature stability of the two-stage module for LAST is better than that of two-segmented module.Compared to Cu,Ni,and Ni-Fe alloys,Co-Fe alloy is an effective metallization layer for PbTe due to its low contact resistance(<1.5 m?cm2)and thin diffusion layer(<1 ?m).By sintering a slice of Cu(50?M)on TE legs,the pure tinfoil can be used as a common welding method for mid-temperature TE modules.A ?max of 9.5%was achieved in the range of 303-923 K in an optimized PbTe/Bi,Te3 based two-stage module,which is highest value for the LAST based TE module reported so far.Furthermore,two-segmented/multi-segmented TE modules were designed and simulated via optimizing TE leg dimensions and TE materials ratio,in which the telluride based TE materials with high performance optimized in this work and by our group were utilized.Based on the assumption of adiabatic boundary condition,a ?max,of 15.57%was achieved at the range of 323-823 K for the telluride based multi-segmented TE module by using Bi2Te3/LAST as n-type and Bi2Te3/GeTe/SnTe as p-type TE materials.