Study On Microstructure Modification,Thermoelectric Properties And Mechanical Properties Of Cu₂Se Based Thermoelectric Materials

The efficient and green energy system is a vital prerequisite for human beings to achieve the long-term sustainable development,where improving efficiency of energy utilization is an indispensable part for constructing the system.As a new energy material,thermoelectric materials can effectively convert the waste heat generated by human activities to usable electricity,thus receiving widespread attentions.Specifically,Cu₂Se-based thermoelectric material shows great potential for its earth abundant constituent element and excellent transport properties.Thermoelectric materials suitable for commercial applications features at massive microstructures to achieve high thermoelectric performance,as well as excellent thermal stability and mechanical properties to ensure long-term stable service.However,the method for microstructure modulation of high-thermoelectric-performance Cu₂Se materials is limited,the thermal stability is unclear and the mechanical behaviours is unveiled.In this work,the effect of ultrasonic treatment（UT）on the phase composition and microstructure of Cu₂Se powders synthesized by self-propagation high-temperature synthesis（SHS）and melting（M）is compared.Followed by the spark plasma sintering（SPS）,the microstructural differences of Cu₂Se polycrystalline bulks and the corresponding effects on transport properties has been investigated,and a novel methodology to prepare high-thermoelectric-performance Cu₂Se materials with refined microstructure through UT process for the presence of residual stress in the grains has been developed.Basing on the high-performance bulks,the evolution of phase and microstructure,and the transport properties under different annealing temperatures（773 K,873 K and 973 K）have been symstematically studied,which reveals the reason of functional failure for Cu₂Se materials under long-term high-temperature service.What’s more,the relationship between room-temperature elastic parameters,mechanical properties and microstructure of Cu₂Se bulks prepared by varied ways have been summaried,and the mechanism for structural failure is analyzed.Finally,the behavior of plastic deformation for Cu₂Se material at high temperature has been explored,and this phenomenon is explained from the perspective of microstructure.The main research work and conclusions are as follows:（1）Microstructural modulation through the combination of SHS and ultrasonic treatment to enhance transport properties of Cu₂Se materials.The Cu₂Se bulks composited with multiscale pores have been efficiently fabricated by the combination of self-propagating high-temperature synthesis,ultrasonic treatment and SPS.Owing to the residual stress introduced by the highly non-equilibrium combustion synthesis in the grains,the ultrahigh-velocity fluid and shock waves generated by sudden collapse of bubbles breaks the grains into the structure with reduced size.The rapid enclosure of refined SHS-UT powders with high surface energy during sintering process result in massive pores in the SHS-UT-SPS bulks.The in-situ formed pores ranging from several to hundreds of nanometers fiercely scatter the medium and long wavelength phonons undisturbed by the migration of Cu⁺,and the thermal conductivity of pore-incorporated Cu₂Se bulks are reduced to0.40～0.45Wm^-1K^-1at 873 K.The maximum ZT value is increases to 1.71 for the SHS-UT-SPS Cu₂Se bulks,and the averaged ZT value reaches 1.10 in the temperature interval from 473 K to 873 K for the sample ultrasonicated under 2.50 W/cm²for 10min（SU-2.50-10）,which is improved remarkably when compared to that of the sample without ultrasonication.As a comparison,an attempt to refine the melting powders so as to modulate microstructure to enhance thermoelectric performance of Cu₂Se material through ultrasonic cavitation has been made and proved unsuccessful.Whatever ultrasonic conditions is,the transport properties of the Cu₂Se bulks are almost unchanged with and without ultrasonic treatment.Thus,the existence of residual stress in the initial powders plays the crucial role in whether the ultrasonication can modulate microstructures to optimize transport properties of Cu₂Se materials.（2）The evolution of phase and structure of high-performance Cu₂Se thermoelectric material during long-term service at high temperature.The high-thermoelectric-performance SU-2.50-10 sample has been selected as research object to explore the evolution of constituent phase,microstructure under different temperatures（773 K,873 K and 973 K）and the corresponding influence on transport properties.After annealing at temperature up to 873 K for a week,the excellent thermoelectric performance is well maintained in the Cu₂Se bulk.And the 873 K annealed sample achieves the ZT_max1.85 at 873 K.However,with further increasing the annealing temperature to 973 K,the Cu₂Se material suffers a lot from instability.A large number of pores formed by the the precipitation of Cu and volatilization of Se result in the significant decrease in electrical conductivity,and the ZT value deteriorates immensely.（3）The relationship between room temperature mechanical properties and microstructure of Cu₂Se materials.The Cu₂Se bulks were prepared by varied methods including melting,SHS,and a combination of SHS and UT,followed by SPS.It is found that the elastic modulus(longitudinal modulus C₁₁,shear modulus G,Young’s modulus E,volume modulus B and Poisson’s ration)tends to increase with reducing the grain size.Meanwhile,the high density grain boundaries in SHS-UT-SPS bulk effectively hinder the propagation of cracks,thus improving the mechanical strength distinctly.The characteristic value of Weibull analysis for Vickers hardness,bending strength and compressive strength of the sample ultrasonicated under 2.50 W/cm²for 10 minutes are 0.474 GPa,53.6 MPa,and 148MPa,respectively,enhanced by 33.5%,14.3%and 18.4%when compared with the melting sample.While for ability to resist thermal stress,the thermal shock resistance of SHS-SPS and SHS-UT-SPS bulks is inferior to that of melting bulk,which can be mainly attributed to the reduced thermal conductivity.（4）Large deformation and mechanism for plastic behavior of Cu₂Se material under high temperatures.The deformation behavior of Cu₂Se material under different temperatures and Cu_2-xSe（0≤x≤0.25）materials at room temperature have been investigated systematically by unidirectional compression test with a large coefficient of stress state softness,and it’s found that the plastic behavior is closely related to the constituent phase of the material.In the interval from room temperature to 423 K,the Cu₂Se material gradually completes the transformation into a high-temperature cubicbphase.Accordingly,the Cu₂Se bulk undergoes mechanical failure when the compressive strain reaches about 5%at room temperature,and for423 K,the failure strain under compressive test increases to more than 20%,while increasing the ambient temperature to 573 K,no failure has been observed when the compressive strain exceeds 50%.And Cu_1.80Se and Cu_1.75Se bulks with cubic phase structure can maintain structural integrity when the compressive strain exceeds 20%at room temperature.Comparing the deformation behavior of Cu₂Se bulks with different grain sizes at 423 K,the compressive failure strain of the coarse-grain sample is close to that of the fine-grain sample.The results of refined microstructure for Cu_2-xSe bulk after mechanical test and molecular dynamics simulations ofb-Cu₂Se material at 400 K demonstrate that the easily triggered slip deformations along（111）/[112]and（111）/[11 0]directions in the cubic phase are underlying mechanism for the ability of large deformation in Cu₂Se material at high temperature.