Optimizing Synergistically The Microstructure And The Thermoelectric Properties Of Bi CuSeO Via Doping With The Rare-earth Variable-valence Element Yb

Thermoelectric materials do not require mechanical transmission and can directly convert heat into electricity,which has great application potential.BiCuSeO is a p-type semiconductor thermoelectric material with a layered structure,has a high Seebeck coefficient and a low thermal conductivity,but the hole carrier concentration is not high and the effective mass is large,resulting in low conductivity,limits its thermoelectric performance improvement.In this paper,pure-phase Bi_1-xYb_xCuSeO nanopowders were synthesized by two-step solid-state reaction and high-energy ball milling.Influence of Yb doping on the microstructure,carrier characteristics,band structure and thermoelectric properties of BiCuSeO doped with under pressureless sintering and hot-pressing sintering were investigated.Finally,the related mechanism of doping could be revealed.After introducing nanostructures by high-energy ball milling in the powder synthesis process,it was found that,a large number of grain boundaries and dislocations are generated during sintering of the bulk.These crystal defects increase phonon scattering and reduce the lattice thrmal conductivity of Bi_1-xYb_xCuSeO samples.Therefore,the thermal transport properties of BiCuSeO was effectively improved.A high-density bulk sample can be prepared by cold isostatic pressing and pressureless sintering.After the cold isostatic pressing,the samples show much stronger diffraction intensities at?00l?Bragg directions.In order to complete the densification process,the pressureless-sintered Bi_1-xYb_xCuSeO samples were agglutinated between the powder particles,and there were no chemical reaction to produce second phase impurities.Yb doping can effectively refine the BiCuSeO grains and compact the microstructure.At the same time,the electrical conductivity and power factor of BiCuSeO were effectively improved by Yb doping.The maximum power factor was obtained when x=0.06,which increases from 33?Wm^-1K^-2?pure phase?to 110?Wm^-1K^-2@843 K of Bi_0.94Yb_0..06CuSeO.Compared with pressureless sintering,the hotpress-sintered bulk samples could have a better performance.With x less than 0.10,Bi_1-x-x Yb_x CuSeO are pure phase.However,the nano-second phase Yb₂O₃ could be found when Yb heavily doped?x?0.15?.Yb₂O₃ is diffusely distributed and high-density phase boundaries and dislocations are introduced into the sample,which can enhance phonon scattering and reduce thermal conductivity.Through X-ray photoelectron spectroscopy,we could find that there wsa two valence states Yb²⁺and Yb³⁺,which can simultaneously increase the carrier concentration and the carrier mobility of Bi_1-xYb_x CuSeO.Substituting Yb²⁺for Bi³⁺introduce a hole and increase hole carrier concentration.The first-principles calculation results show that Yb doping can adjust the energy band structure,and the heavy hole band gradually approaches the light hole band,which reduces the effective mass of hole carriers and improves the carrier mobility.The thermoelectric figure of merit reaches a maximum of 0.62@873 K for Bi_0.7Yb_0.3CuSeO.