Optimization Of Electrial/Thermal Transporting Properties Of BiCuSeO Based Thermoelectric Materials

BiCuSeO based thermoelectric material is one of the most promising thermoelectric materials with oxygen because of its special natural superlattice structure,high S,lowκ,good thermal and chemical stability.However,its low intrinsic carrier concentration and very low carrier mobility have limited the improvement of its thermoelectric conversion efficiency.At present,the research focuses on how to ensure the intrinsic lowκand improve the electrical transporting performance at the same time.Aiming at the above problems,this work takes p-type BiCuSeO based thermoelectric materials as the research object,and uses mechanical alloying method and high-temperature solid-state reaction method to prepare BiCuSeO combined with spark plasma sintering,and regulates its thermoelectric properties by means of process optimization,single element doping,double element doping,grain refinement,modulation structure,nanocomposite,etc.The specific results and discussion are shown below:（1）Single-phase BiCuSeO powder was prepared by mechanical alloying and high-temperature solid-state reaction respectively with Bi₂O₃,Bi,Cu and Se as raw materials,and bulk BiCuSeO was prepared by spark plasma sintering.Compared with mechanical alloying,when BiCuSeO was prepared by high-temperature solid-state reaction,the synthesis efficiency was higher than that of mechanical alloying,the synthesis time was shorter,but the grains were coarser,and the thermoelectric properties were lower than that of mechanical alloying sample.The grain size of the mechanical alloying sample is smaller,and the grain size is further refined with the increase of the mechanical alloying strength.The results show that the samples with high carrier concentration,high conductivity and lowκhave a best ZT value of 0.62,which is 44%higher than that of annealed samples at 873 K.（2）In the aspect of single element doping,Bi site was doped by Sb/Sm/Er/Pb to regulate its electronic structure and improve its thermoelectric properties.It is found that substitution of Sb for Bi position will result in shorter Cu-Se bond,stronger hybridization between Cu 3d and Se 4p,narrower band gap,higher carrier concentration and higherσ.The best value of 0.73 was obtained from 5%Sb doped sample at 873 K.The effect of Sm doping is similar.The density of Sm 4f orbital near the top of valence band（-2 eV）is higher,which significantly enhances the transfer of carriers from the storage layer to the conductive layer,and leads to the increase ofσ.The highest ZT value was obtained from6%Sm doped samples at 873K.It is worth noting that Er doping results in a significant broadening of the band gap and a decrease in the carrier concentration,but because the energy difference between the heavy and light bands is smaller,the carrier mobility andσare significantly improved.The best value of ZT is 0.83,which is obtained from 8%Er doped sample at 873K.Pb doping exhibits a strong acceptor effect,and the efficiency of introducing holes is much higher than other doping elements.Theσreaches～1000 Scm-1at room temperature,and the ZT value increases significantly in the whole temperature range.The best value of 0.92 is obtained from 8%Pb doping sample at 873K.（3）BiCuSeO was prepared by mechanical alloying and spark plasma sintering.Bicuseo was doped by Ba/Pb,Ba/Te and Sb/Te to improve its thermoelectric performance.The substitution of Ba²⁺/Pb²⁺for Bi³⁺can enhance the acceptor effect and increase theσ.After Ba/Te doping,because Ba²⁺replaces Bi³⁺,the acceptor effect will be enhanced,and Te replaces Se,the band gap will be narrowed,thus increasing the concentration of hole carriers and the conductivity correspondingly.The best ZT value of 1.07 is obtained at873K.After Sb/Te doping,substitution of Sb for Bi and Te for Se will narrow the band gap,which will increase the carrier concentration andσ.The change of latticeκis complex.On the one hand,because Sb（122）is"lighter"than Bi（209）,the substitution of Bi will lead to the reduction of heavy elements and higher latticeκ;on the other hand,Te（128）is“heavier”than Se（79）,the enhancement of heavy elements will correspondingly cause stronger phonon scattering and lower latticeκ.At the same time,the substitution of Sb/Te for Bi/Se will lead to the enhancement of scattering of phonons by point defects.The best ZT value of 1.04 is obtained at 873 K.The doping efficiency of double element doping is higher than that of single element doping.On the one hand,it can increase theσand PF by adding two elements.On the other hand,it can effectively inhibit the latticeκdue to more kinds and quantities of defects introduced by double element doping.（4）The grains of BiCuSeO were refined by increasing the mechanical alloying strength.The results showed that,with the increase of milling time,the milling strength increased,and the grains decreased obviously.The average grain size of the samples milled for 5 h is～1445 nm,and the corresponding average grain size of the samples milled for 8 h,12 h and 16 h is～971 nm,～487 nm and～381 nm respectively.The grain size of BiCuSeO was obviously refined by mechanical alloying.Theσof all the doped samples decreased with the increase of temperature,showing metalloid transporting behavior.With the increase of milling time,theσof the samples increased in the whole temperature range.The increase ofσis due to the increase of mechanical alloying strength,which will cause the increase of Cu vacancy concentration and the obvious enhancement of acceptor effect,so as to increase the concentration of hole carrier.Due to the increase of carrier concentration,the S shows a downward trend as a whole.The increase ofσin high temperature region overshadowed the decrease of S,and the PF increased obviously.The highest PF of 0.721 mWm^-1K^-22 was obtained at 873 K by ball milling for 16 h with Bi_0.92Sb_0.08CuSe_0.92Te_0.08O.With the increase of milling time,the overallκshows a downward trend.So the decrease ofκis caused by the decrease of latticeκ.With the increase of milling time and mechanical alloying strength,the plastic deformation of powder will be more intense,and more micro defects will be formed when the grains are refined,which will cause the enhancement of phonon scattering and the decrease of latticeκ.In the middle and high temperature range,theκdecreased while the PF increased,and the ZT value increased greatly.The best value of 1.19 was obtained by ball milling at 873 K at Bi_0.92Sb_0.08CuSe_0.92Te_0.08O for 16 h.（5）The modulation structure is formed by mixing the undoped sample with the heavily doped sample.The average composition of the modulation doped sample is the same as that of the uniform doped sample,but theσis higher than that of the uniform doped sample in the whole temperature range.This is because the mixed samples are the mixture of undoped samples and heavily doped samples.The undoped samples have low carrier concentration but high carrier mobility,while the heavily doped samples have low carrier mobility and high carrier concentration.When the two samples form a macro uniform modulation structure,on the one hand,the mixed samples have relatively high carrier concentration,on the other hand,the carriers tend to move towards the high-mobility region.In this way,the sample maintains a high mobility while maintaining a high carrier concentration,and the electrical transport performance is improved.The S of the modulation doped sample is slightly higher than that of the uniformly doped sample,which may be due to the introduction of more interfaces in the modulation mixed structure of the heavily doped and undoped samples,which enhances the scattering of carriers,correspondingly improves the effective quality,and then improves the S.The best PF is0.980 mWm^-1K^-2（873K）.Due to the enhancement of carrierκand latticeκ,theκof the modulation doped samples is higher than that of the uniform doped samples in the whole range.The ZT value decreased greatly in the whole temperature range,with the best value of 1.17 was obtained at 873 K.（6）The composite of nano SiC and Bi_0.92Pb_0.08CuSeO matrix was realized by ball milling and spark plasma sintering.The results showed that nano Si C was granular and partially agglomerated,and dispersed on the surface and grain boundary of the matrix.With the increase of nano SiC content,theσshows a downward trend.This is due to the decrease of carrier concentration caused by the lowσof SiC and the decrease of carrier mobility caused by the scattering of nano SiC.With the increase of SiC content,S shows an upward trend.This is due to the decrease of carrier concentration.Nano SiC can cause energy filtration effect,enhance potential barrier and filter out low energy carriers.There is a negative correlation between the low energy carrier and the S,and a positive correlation between the high energy carrier and the S.With the increase of the content of nano SiC,theκbasically shows a downward trend.On the one hand,the carrierκis positively correlated with the conductivity,and decreases with the decrease of conductivity.On the other hand,it is due to the decrease of latticeκ.Nano SiC will introduce a large number of interfaces and enhance the interface scattering.In addition,in the process of ball milling and spark plasma sintering,the shape and size of nano SiC will change,and there will be a small range of polymerization phenomenon,which is conducive to scattering a wider range of intermediate frequency phonons,thus reducing the latticeκ.In addition to the 10%nano SiC composite samples,the ZT values of all other samples increased greatly in the whole temperature range due to the increase of PF and the decrease ofκ.The best value of 1.32 was obtained from 8%nano SiC composited Bi_0.92Pb_0.08CuSeO samples at 873 K.With the increase of the content of nano SiC,the mechanical properties of the samples are improved obviously,and the rejection rate is greatly reduced,which should be due to the dispersion strengthening of nano SiC.