Modulation Of Thermoelectric Performance And Electrical-thermal Transport In Liquid-like Compound AgCrSe₂

Waste heat recycling is one of the effective approaches to meet the ever-rising issue of environmental pollution.Thermoelectric materials have been under the spotlight for several decades owing to the capability of direct conversion between heat and electricity without environmental emission.Plus,they are scalable to meet the application demand for industrial waste heat cycling,deep space exploration,portable power sources etc.However,the widespread applications of thermoelectric materials are restricted by the relatively low energy conversion efficiency,which depends on the dimensionless figure of merit,z T=S²T/ρ（κ_e+κ_L）,where S,T,ρ,κ_e,κ_L are the Seebeck coefficient,the Kelvin temperature,the electrical resistivity,the electronic and lattice components of thermal conductivity,respectively.Synergistical modulation of these parameters for high z T is at the core of the study of thermoelectric materials.Among these parameters,lattice thermal conductivity is the only independent parameter which plays an important role in enhancing z T.Therefore,compouds with intrinsically low lattice thermal conductivity are attractive for the investigation of thermoelectric materials.As a typical representative,the superionic conductor AgCrSe₂is considered as a promising thermoelectric material due to its intrinsically ultralow lattice thermal conductivity.AgCrSe₂ shows a layered structure with alternating Ag layers and CrSe₆ octahedral layers repeating along the c axis.Such a crystal structure is beneficial to Ag-ion conduction.It is reported that at～450 K,AgCrSe₂ undergoes a superionic transition owing to the significant increase of ionic conduction and effectively suppresses the transport of transverse acoustic（TA）phonon.Besides,strong phonon scattering is also triggered by the disorder occupation in the superionic phase.Both above-mentioned effects lead to the ultralow lattice thermal conductivity for AgCrSe_2.Unfortunately,the inferior electronic performance severely limited the further improvement of z T in AgCrSe₂.What role the defects can play in modulating thermoelectric performance for AgCrSe₂remains a mystery as yet.Therefore,understanding the electronic transport properties,especially for clarifying the contribution of various kinds of impurities is of critical importance for improving the thermoelectric performance of AgCrSe₂.In this work,firstly,thermoelectric parameters such as carrier concentration,mobility,Seebeck coefficient,lattice thermal conductivity for AgCrSe₂were systematically studied though modulating composition,doping,formation of solution and preparation of composite.Secondly,the influence of these strategies on electrical and thermal transport property were analyzed by the models,and then the key factors of improving the thermoelectric property of AgCrSe₂ were pinpointed.The main results are listed as below:（1）Here,the thermoelectric properties of AgCrSe₂with various intrinsic defects through modulating Ag content are systematically investigated.It is found that the carrier concentration can be modulated in a broad range by inducing deep level impurity states,enabling a metal-insulator-like transition and a reliable evaluation on the thermoelectric transport properties.The single parabolic band（SPB）model gives a good clarification regarding the increased electronical performance and some key imformation,such as effective mass,scattering mechanism.Furthermore,the enhancement of the liquid-like effect triggered by the Ag vacancies contributes to the ultralow lattice thermal conductivity.Eventually,a maximum figure of merit z T of～0.6is realized in Ag_0.97CrSe₂ at 750 K.The present findings provide novel insights into the improvement of thermoelectric performance of AgCrSe₂ materials.（2）It is shown from last section that low mobility become a bottleneck to improve the electrical property for AgCrSe₂.In this section,the Sb/Bi elements are doped at Cr sites in Ag_0.97CrSe₂,i.e.,Ag_0.97Cr_1-x（Sb/Bi）_xSe₂,aiming at producing a better overlap of electronic orbits between different atoms for altering the valence band flatness which enables low effective mass to achieve higher carrier mobility.In comparison with the pristine Ag_0.97CrSe₂,a considerable improvement（>50%）in power factor of～400μW·m^-1·K^-2at 750 K is realized upon 3%Sb doping.The single parabolic band model clarifies that the decreased effective mass and optimized carrier concentration are responsible for the enhanced electronic performance.Furthermore,an ultralow lattice thermal conductivity of～0.2 W·m^-1·K^-1 is well-maintained for the sample with 3%Sb doping due to the nearly unchanged superionic conduction which validate the dominance of the liquid-effect.Eventually,a high peak figure of merit z T of～0.7 is obtained in Ag_0.97Cr_0.97Sb_0.03Se₂ at 750 K.The current finding provides an excellent avenue for advancing thermoelectrics in AgCrSe₂ materials.（3）Another method to achieve high electrical property is the improvement of Seebeck coefficient.In this section,it is found that the metallic nano-inclusion forming in situ is beneficial to achieve higher Seebeck coefficient,leading to nearly twofold increase in average power factor over the temperature range for Ag_0.97CrSe₂(Cr_2/3Te)_0.03compared with pristine AgCrSe₂.The selective carrier scattering from these nano-inclusions enabling higher Seebeck coefficient is demonstrated through the carrier transport property.Higher average power factors testify to the effectiveness of proper barrier potential in filtering low-energy carrier.Besides,the lattice thermal conductivity at room temperature is further reduced to～0.3 W·m^-1·K^-1by additional phonon scattering of these nano-inclusions.In the final analysis,a high peak figure of merit z T of～0.8 is achieved in Ag_0.97CrSe₂(Cr_2/3Te)_0.03at 750 K,while the average figures of merit for samples with nano-inclusions are also higher than that of pristine AgCrSe₂.The selective scattering is rendered highly effective in enhancing thermoelectric performance for AgCrSe₂ by these in situ nano-inclusions.（4）In this section,guided by the intention of optimizing the thermal property for AgCrSe₂,（AgCrSe₂）_x（CuCrSe₂）_1-x composite materials are synthesized though solid reaction.It is found that power factor and thermal conductivity are gradually improved with the increase of CuCrSe₂composition.The Seebeck coefficient and resistivity both satisfy the results predicted by general effective medium theory（GEMT）when the factor is set as 8.However,the thermal conductivity is lower than the results predicted by model when x reaches the percolation threshold.Therefore,higher z T is realized near the percolation point stemmed from the fact that electrical and thermal transport are out of sync.Besides,the stability and repeatability are improved because of the introduce of CuCrSe₂.In the end,thermoelectric performances are investigated by combining three above-mentioned strategies.It is found that quality factor as an indicator plays a key role in how to synergize thermoelectric property of AgCrSe₂.the peak z T of～0.8,average z T of～0.5 and efficiency～9%is realized because of higher quality factor in Ag_0.977Cr_0.97Sb_0.03Se₂(Cr_2/3Te)_0.03.These results pave the way for the thermoelectric application of AgCrSe₂.The innovations of our work are embodied as follows:（1）The influence of intrinsic defects on the thermoelectric performance.The transition from shallow level defect to deep level defect is triggered when the Ag content is increased.It is found that deep-level intrinsic defects promote the the thermoelectric performance of AgCrSe₂ in the high temperature region because of the gradual ionization with the rise of temperature.The revolution of intrinsic defect is successfully exhibited through the energy level of defects,which provide different idea for the realization of high z T of AgCrSe₂.（2）Modulation of effective mass by the modulation of energy band.It is not easy to alter the mobility which is subject to the energy band.Attempt to elevate mobility by broadening energy band succeed when Ag_0.97Cr_1-x（Sb/Bi）_xSe₂is formed.As a result,carrier mobility is elevated because of low effective mass.It is experimentally and theoretically found that power factor is also improved due to the low effective mass.（3）Selective scattering of carrier by in-situ nano-inclusion.Given that the Seebeck coefficient is square term,the enhancement of Seebeck coefficient can compensate the loss of mobility due to the inclusions,leading to the increase of power factor.It is found that Seebeck coefficient can be improved by the selective carrier scattering by metallic inclusions because the low-energy carriers which have a negative effect on the Seebeck coefficient are filtered out.The influence of selective scattering is also expounded through the electric transport model.（4）Thermal and electrical transport for composite are synergistically modulated.Both electrical and thermal properties for composite are usually consistent with the proportion of secondary phase.However,it is found that there is a deviation from the results predicted by model for thermal conductivity at the percolation point,which decouple the the electrical and thermal transport.Therefore,formulation of composite is an effective strategy to modulate thermoelectric performance.To sum up,thermoelectric performance for AgCrSe₂ is successfully modulated by intrinsic defects,solid solution,in-situ inclusion and composite,aimed at tuning carrier concentration,mobility,Seebeck coefficient and lattice thermal conductivity.The mechanisms of electrical and thermal transport are analyzed through the corresponding models.The thermoelectric parameters and the characteristics of electrcical-thermal transport for AgCrSe₂ are obtained in parallel,which pave the way for the practical application and thermoelectric enhancement of the counterpart of AgCrSe₂.