Effects Of Variable-valence Elements Eu-/Sm-Doping On The Thermoelectric Performance Of BiCuSeO

Thermoelectric material,as a new kind of functional material,can realize the conversion between heat and electrical power.It has many advantages,such as no noise,safety,high reliability and low pollution to the environment.And it has a number of potential applications in the fields of thermoelectric power generation,thermoelectric cooling and so on.BiCuSeO with a stacking layered structure,as a typical P-type thermoelectric material,has attracted more and more attentions since its high Seebeck coefficient and low thermal conductivity.However,a high electrical resistivity of BiCuSeO lead to its low thermoelectric properties.In this study,rare-earth variable-valence elements Eu-or Sm-doped BiCuSeO-based thermoelectric materials were prepared by combining a two-step solid-state reaction with spark plasma sintering?SPS?.The effects of Eu-or Sm-doping on the phase composition,microstructures,carrier concentrations and mobilities,thermoelectric properties and band structure of BiCuSeO were investigated by XRD,SEM,Hall measurement,LSR-3,LFA457 and first-principles density functional theory?DFT?calculations,respectively.The single-phase Eu-doped Bi_1–xEu_xCuSeO thermoelectric materials have been prepared successfully and two valence states of Eu²⁺and Eu³⁺coexist in the samples.Eu-doping not only improves the carrier concentrations,but also modifies the band structure of BiCuSeO,resulting in an effective improvement of electrical transport properties.The electrical conductivity is enhanced from 16 Scm^-11 of pristine BiCuSeO to 98 Scm^-1of Bi_0.85Eu_0.15CuSeO at 823 K.A maximum power factor of 0.32 mWm^-1K^-2 is achieved for Bi_0.975Eu_0.025CuSeO at 823 K owing to the moderate electrical conductivity and relatively high Seebeck coefficient,which is 1.8times as high as that of the undoped sample.The ZT value of 0.47 is obtained for Bi_0.975Eu_0.025CuSeO,which is higher than that of the pristine BiCuSeO.The single-phase Sm-doped Bi_1-xSm_xCuSeO thermoelectric materials have been prepared successfully and no noticeable impurity phases are observed.Sm-doping can effectively refine the grains of BiCuSeO and increase the density of samples.Two valence states Sm²⁺and Sm³⁺are detected by XPS analysis in Bi_1-xSm_xCuSeO samples.The holes are introduced into the conducting layers by Sm²⁺substituting for Bi³⁺and the carrier concentration of samples is increased.The first-principles calculation results show that the energy band structure of samples is changed by Sm isovalent substituting for Bi.As the Sm-doping content increases from x=0to 0.025,the band gap slightly decreases.The carriers will be more easily excited across the band gap,and thus increases the hole carrier concentration.With the further increase of Sm-doping content from x=0.025 to 0.1,the band gap broadens and the energy difference between the heavy hole band and light hole band lowers.In this case,two hole bands become converged gradually,the average band effective mass thus decreases and the carrier mobility increases.Because Sm is lighter than Bi,faster sound velocities and larger mean free paths for the Sm-doped samples are achieved,leading to a slight enhancement of?_lat.However,the total thermal conductivity of Bi_1-xSm_xCuSeO remains a relatively low value(?0.8 Wm^-1K^-1)over the whole temperature measurement range.The enhancement of power factor offsets the slight increase of thermal conductivity,and the greatest ZT=0.65 is achieved at 823 K for the Bi_0.975Sm_0.025CuSeO sample,which is 1.25 times as high as that of pristine BiCuSeO.