Defect Engineering For Modulating The Electronic-Phonon Structures And Electrical-thermal Transport Behaviors In Thermoelectrics

In inorganic functional solids（IFS）,electrons and phonons co-exist and inherently coupled with each other,thus determining their intrinsic physical and chemical properties,and endowing them with plenty of novel physical phenomena and diverse applications.Therefore,modulation studies of the electron-phonon structures and their transport behavior in IFS can not only realize the flexible adjustment of their intrinsic physicochemical properties,but also direct the exploration of novel physical-chemical phenomena and design of new functional materials,such thermoelectric material,which has been the hot topic in the energy conversion materials research field due to its ability to achieve reversible conversion between electricity and heat on the basis of thermoelectric effect（for specifically,Seebeck effect and Peltier effect）.However,due to the inverse coupling between the three thermoelectric parameters,the thermoelectric figure of merit,ZT value,as well as the energy conversion efficiency of thermoelectric devices,is hitherto limited at low values,which strongly hinder the application and commercialization of thermoelectric technology.Theoretical analysis has shown that the electrical and thermal transport properties in thermoelectric solids are codetermined by their inherent electronic-and phonon-structures,which often couple with each other mediated by the ubiquitous lattice defects.Meanwhile,lattice defects in the IFS will inevitably affect the electronic and phonon structures and their electron-thermal transport behavior due to the periodic destruction.Based on the analysis above,it is natural to come up with the idea that we can use defect engineering to optimize thermoelectric properties via intentional manipulation of the type,size,concentration,and spatial distribution of defects,which can serve as a scaffold for engineering the intrinsic electron’ and phonon’ structures and their transport behaviors to synergistically optimize the thermoelectric parameters.In this dissertation,based on the idea of defect engineering,we introduced crystal defects like substitution atoms and vacancies,into the target thermoelectric systems to carry out a deep study of their effects on the electron-phonon structures as well as the electrical-thermal transport behaviors,and used it to modulate and optimize the thermoelectric properties.Meanwhile,by virtue of defect characterization and theoretical calculation,we analyzed the intrinsic mechanism of defect modulation and elucidated an interlayer-charge-transfer effect in superlattice structure,and established the basic structure-property relationship between defect structure and thermoelectric property.At the end of this dissertation,the author re-investigated the problem of the modulation degree of freedom caused by defect engineering strategy,and put forward a concept of "multiple degrees of freedom synergistic modulation" to enable the full potential of defect engineering for boosting thermoelectric performance.The main contents of this dissertation include the following aspects:1.In this chapter,taking the natural superlattice structure BiCuSeO as a sample,we presented an equivalent doping strategy to reduce the lattice thermal conductivity with no deterioration in the electrical transport performance（i.e.thermoelectric power of factor,PF）,which finally gave rise to an optimized thermoelectric ZT value.By virtue of their unique superlattice structure stacking by alternating insulating[Bi₂O₂]²⁺layers and conductive[Cu₂Se₂]^2-layers along the c axis in BiCuSeO,corresponding equivalent mono-and dual-doping samples can be achieved by substitution of Bi and Cu atoms in the respective sublayers with equivalent La and Ag atoms.Electrical-thermal transport characterization results demonstrated that,compared to that of pristine sample,both La and Ag doping can increase the electrical conductivity significantly,which compensated the decrease of the Seebeck coefficient and eventually acquired an optimized power of factor（PF=σS2）.Meanwhile,benefiting from the phonon scattering effect by these point defects,the thermal conductivity was further reduced,which contributes to an ultimate improved ZT performance.For example,the ZT value of La-Ag dual-doped sample Bi_0.98La_0.02Cu_0.98Ag_0.02SeO reached 0.46 at 755 K,which is the highest among the three doped samples and is increased by 70%compared to that of the pure sample（0.27 at 755 K）.Present work indicates that dual-doping with equivalent atoms in superlattice BiCuSeO can simultaneously optimize its electrical and thermal transport properties.What’s more,this unique superlattice structure with alternately stacked sublayers also provides us with an ideal platform for clear study of the dual-or multi-doping effects in thermoelectric property modulation.2.In previous chapter,due to the equivalent state of dopants to that of the host atoms,the increase in electrical conductivity is limited,and thus the PF value is overall low.Regarding to this problem,in this chapter,we proposed to take the vacancy-type point defect as the new source to modulate BiCuSeO’s thermoelectric property.Meanwhile,considering the possible dilemma of deterioration in electrical conductivity in conventional mono vacancy strategy,a new type of vacancy defect with simultaneous deficiencies of two host atoms,i.e.dual-vacancies,was designed.By means of coinstantaneous deficiencies of Bi and Cu atoms in the respective sublayers,Bi/Cu dual vacancies were successfully introduced into the system and resulted in the synergistic optimization of electrical and thermal parameters.In vacancy-modulated Bi1-xCu1-ySeO,while the Bi/Cu vacancies and grain boundaries are favorable for short-and long-wavelength phonon scattering,the superlattice interfaces between adjacent[Bi₂O₂]²⁺ and[Cu₂Se₂]^2-layers can strongly scatter the midwavelength phonons,achieving an all-length-scale phonon scattering.Meanwhile,as compared to its pristine and monovacancy samples,the dual-vacancies sample further increase the phonon scattering,which brought about a maximum reduction of the lattice thermal conductivity and results in an ultralow thermal conductivity of 0.37 Wm-1K-1 at 750 K.Most importantly,the clear cut evidence in positron annihilation unambiguously confirms the interlayer charge transfer between these Bi/Cu dual vacancies,which results in the significant increase of electrical conductivity with relatively high Seebeck coefficient.As a result,BiCuSeO with Bi/Cu dual vacancies shows a high ZT value of 0.84 at 750 K,which is superior to that of its native sample and monovacancies-dominant counterparts.These findings undoubtedly elucidate a new strategy and direction for rational design of high performance thermoelectric materials.3.In this chapter,we proposed the vacancy engineering strategy for adjusting the electrical transport properties in I2-Ⅱ-Ⅳ-VI4-type wide-bandgap semiconductor thermoelectric system and achieved optimization of its thermoelectric performance.Taking Cu₂ZnSnSe₄ as an example,the pristine and vacancy-doped samples（Cu,Zn,and Sn vacancy,respectively,all with a concentration of 2 mol%）were successfully synthesized by a convenient solid-state reaction and characterized with their thermoelectric performances.Results have shown that all samples had intrinsic Se vacancies and void defects,while in vacancy-doped samples vacancies existed as vacancy clusters containing Se vacancies and cation vacancies,leading to the occurrence of disordered crystalline domains in part grains.The introduction of the vacancies increases the carrier concentration of the material,thus improving the electrical conductivity.What’s more,benefitting from the strong phonon scattering effect of vacancy clusters and crystal voids,as well as the effects of the disordered domains,the lattice part as well as the total thermal conductivities were both reduced,achieving a simultaneous optimization of the electrical and thermal conductivities,which compensated the deterioration in Seebeck coefficient,and finally contribute to significantly improved ZT performance in all three vacancy-doped samples compared to that of the pristine one.For example,For example,the ZT value of Sn vacancy sample Cu₂ZnSn_0.98Se₄ at 750 K reached 0.44,which is extraordinarily enhanced by 2 times compared to that of pristine Cu₂ZnSnSe₄ sample（ZT=0.14）.In addition,considering the high abundance and low/non-toxicity of the constituent elements in these Ⅰ2-Ⅱ-Ⅳ-Ⅵ4-type semiconductors,present studies opens up a new direction for exploration of economic and non-toxic high-performance thermoelectric materials.