Enhanced Thermoelectric Performance Of Ternary Compound Cu₃PSe₄ And CoSbS By Defect Engineering

Thermoelectric technology is able to convert heat into electricity in solid state,including waste heat recovery and solid state refrigeration technology.Thermoelectric conversion devices,which attracts intensive attention due to its high power density,no moving parts and high reliability,has become one of the hot topics for solid state physics.Good thermoelectric materials require high conductivity,low thermal conductivity and high Seebeck coefficient.The coupling of these parameters makes it impossible to optimize one of these parameters alone without degrading the other two parameters,which is a challenge in both physics and material science.Judging from the development of thermoelectric materials,we find that the progress of thermoelectrics mainly originates from the advancement of semiconductor technology and theory.In the1950s,thanks to the research and application of semiconductors,the interest of thermoelectric study is shifted from metal to semiconductor.During this period,researchers were more concerned with elementary or binary thermoelectric materials,for example,high temperature thermoelectrics SiGe alloy,medium temperature PbTe and room temperature Bi₂Te₃.In the 1990s,G.Slack purposed an idealized design concept"phonon glass-electron crystal"for thermoelectrics,which requires excellent electrical properties like crystals and low thermal conductivity like glasses.This novel concept motivates the rapid development of thermoelectrics in last decade.In the same time,more and more binary and ternary thermoelectric compounds were discovered,expanding the thermoelectric family.Among them,ternary copper-based sphalerite-derived diamond-like compounds and Co-based filled-skutterudites derived compounds have been intensively studied owing to their diversified element combinations and high performance.In addition,these ternary compounds hold great promise for practical application due to a variety of advantages,such as high stability,good mechanical properties,easy preparation,low cost,non-toxic and earth abundance.In this work,we focus on ternary diamond-like compound Cu₃PSe₄ and Co-based compound CoSbS made by traditional solid-state reaction combined with spark plasma sintering,and strategies of carrier concentration optimization and defect engineering are employed to improve the thermoelectric properties.The main contents are as follows:（1）Polycrystalline Cu_3-xPSe₄（x=0,0.03,0.06,0.09,0.12）samples were synthesized by traditional solid reaction method combined with spark plasma sintering.Although pristine Cu₃PSe₄ sample has low lattice thermal conductivity,its thermoelectric performance is limited by low carrier concentration and poor conductivity.Herein,for the first time,we attempt to optimize its thermoelectric performance by means of copper vacancy engineering to enlarge the hole concentration.It is found that introducing Cu vacancy into Cu_3-xPSe₄ can effectively promote the carrier concentration and enhance the electrical properties.By analyzing the relationship of carrier concentration and mobility with respect to temperature,it is found that the carrier concentration increases by two orders of magnitude,when the vacancy content is higher than x=0.03.However,high concentration of impurities leads to strong ionized impurity scattering,which largely degrades carrier mobility at room temperature while mobility is maintained at high temperature when acoustic phonon scattering dominates.In all,the raised carrier concentration increases the electrical conductivity and the power factor increases significantly from 0.13 mW/mK² to 0.55 mW/mK² at x=0.12 at630 K.Combined with the intrinsic low thermal conductivity of Cu_3-xPSe₄（x=0,0.03,0.06,0.09,0.12）samples,a maximum zT of 0.62 is attained at 680 K for x=0.12sample and the zT_ave is also markedly increased.Therefore,for Cu₃PSe₄ compounds with intrinsically low thermal conductivity,Cu vacancy engineering is an effective method to optimize the thermoelectric properties.（2）The Co_1-xZn_xSbS_0.85Se_0.15（x=0.0,0.02,0.05,0.08）bulks were prepared via the conventional solid state reaction method combined with spark plasma sintering.The microstructures,thermal properties and electrical properties of the samples were studied.It is an effective way to reduce the lattice thermal conductivity in Co_1-x-x Zn_xSbS_0.85Se_0.15compounds by the substitution of Zn for Co.The effect of the point defect scattering on the reduction in lattice thermal conductivity is explained successfully by the Debye-Callaway-Klemens model.It is identified that the main reason for such noticeable reduction in lattice thermal conductivity is stress field fluctuation caused by the difference of and atomic radius between Co and Zn atom,.Such reduction in the thermal conductivity leads to a maximum zT of 0.34 at 875 K for Co_0.95Zn_0.05SbS_0.85Se_0.15 at x=0.05,100%enhancement as compared to the CoSbS0.85Se0.15.