Thermoelectric Transport Properties Of P-type BiCuSeO-based Oxyselenides

Thermoelectric materials,which are capable of directly converting waste heat into usable electricity,have received considerable amount of scientific attention.Oxide thermoelectric materials are expected to be potential candidates for thermoelectric applications owing to their excellent thermal and chemical stability at high temperature,low cost and nontoxicity.Specifically,BiCuSeO-based oxyselenides with unique crystal structure have become one of the current hot research topics.The BiCuSeO superlattice is composed of insulating?Bi2O2?2⁺ oxide layers alternately stacked with conductive?Cu2Se2?2-selenide layers along the c axis of the tetragonal crystal:The former layers with ionic bonds act as charge reservoirs while the latter layers with covalent bonds provide the conduction pathway for carrier transport.Moreover,the weak bonding between the layers,heavy component elements and unique crystal structure of BiCuSeO can produce strong scattering of phonons,thus resulting in an intrinsically low thermal conductivity.In this work,Bi1-xSbxCuSe1-yTeyO,Bi1-xZnxCuSeO1-xSx,BiCu1-xAgxSeO and Bi1-2xMgxPbxCuSeO compounds are prepared by combing a traditional two-step solid state reaction and inductively hot-pressing sintering.The effects of different doping or substituting methods on the thermoelectric transport properties of BiCuSeO-based oxyselenide are systematically investigated.Despite the specially layered crystal structure of BiCuSeO,the thermoelectric transport properties can be assumed to be isotropic.All the isovalent substitutions have a significant effect on the electrical transport properties.A maximum power factor of?3.8 ?Wcm-1K-2 for Bi_0.98Sb0.02CuSeO at 750 K is obtained where the Sb substitution increases the carrier concentration and the substitution of Te produces an opposite effect.In addition,no noticeable effect of isovalent substituting on reducing the total thermal conductivity is observed.As a result,a maximum dimensionless figure of merit ZT of 0.56 is achieved for BiCuSe0.978Te0.025O at 750 K,which is about 1.44 times as large as that for the pristine BiCuSeO.Upon Zn and S dual-site substitution,the electrical conductivity is enhanced from 28.9 S/cm for pure BiCuSeO to 43.3 S/cm for Bi0.975Zn0.025CuSeO0.975S0.025 at 750 K and the largest Seebeck coefficient is 360?V/K for Bi0.95Zn0.05CuSeO0.95S0.05.A maximum power factor of 4.6 ?Wcm-1K-2 is achieved for Bi0.95Zn0.05CuSeO0.95S0.05 at 750 K due to its moderate electrical conductivity and high Seebeck coefficient.As the enhanced power factor compensates for the slight increase in the total thermal conductivity,the dimensionless figure of merit ZT reaches a maximum value of 0.68 for Bi0.95Zn0.05CuSeO0.95S0.05 at 750 K,which shows a 70%enhancement compared with that of pristine BiCuSeO.Upon replacing Cu⁺ by Ag⁺,maximum values of electrical conductivity of 36.6 Scm-1 and Seebeck coefficient of 350 ?VK-1 are obtained in BiCu_0.98Ag0.02SeO and BiCu0.92Ago0.08SeO,respectively.Nevertheless,a maximum power factor of 3.67?Wcm-1K-2 is achieved for BiCu0.95Ag0.05SeO at 750 K owing to the moderate electrical conductivity and Seebeck coefficient.Simultaneously,BiCu0.95Ag0.05SeO exhibits a thermal conductivity as low as 0.38 Wm-1K-1 and a high ZT value of 0.72 at 750 K,which is nearly 1.85 times as large as that of the pristine BiCuSeO.The enhancement of thermoelectric performance is mainly attributed to the narrowed band-gap and increased density of states near the Fermi level as indicated by the calculated results.Single BiCuSeO phase is obtained for Bi1-2xMgxPbxCuSeO samples with negligible preferential growth in the crystallites.X-ray photoelectron spectroscopy results indicate that both Mg and Pb dopants are in the 2⁺ oxidation state whereas other than the expected Bi3⁺,higher-oxidation-state Bi ions also exist in both the pristine and doped BiCuSeO compounds.The dual doping yields a remarkable enhancement in the electrical conductivity coupled with a moderate Seebeck coefficient.A pronounced increment in power factor is achieved from 2.54?Wcm-1K-2 for the pristine BiCuSeO to 11.1?Wcm-1K-2 for Bi0.88Mg0.06Pb0.06CuSeO at 750 K.In addition,dual doping further reduces the lattice thermal conductivity owing to the Mg and Pb dopant-induced point defect scattering of phonon.As a result,a maximum ZT value of 1.19 is obtained for Bi0.88Mg0.06Pb0.06CuSeO at 750 K,which is about 3.1 times as large as that for the pristine BiCuSeO.