Research On Thermoelectric Properties Of Layered Oxygen-Containing Material Bi₂O₂Se

Increasingly serious energy problems and environmental crises are inspiring researchers to explore clean and sustainable energy technologies.Thermoelectric materials can directly convert waste heat into useful electricity,which can play an important role in energy solutions.High-performance thermoelectric compounds usually contain rare,toxic or volatile metal elements,limiting their broad applications,especially in high-temperature environments.In this case,oxide-based semiconductors with low toxicity,high chemical stability and thermal stability have attracted attention.It is reported that the quaternary layered compound BiCuSeO exhibits excellent p-type thermoelectric performance and is considered as a promising medium and high temperature thermoelectric material.Bi₂O₂Se can be viewed as an n-type reference for BiCuSeO.Bi₂O₂Se is composed of alternately stacked insulating layers[Bi₂O₂]²⁺and conductive layers[Se]^2-.The layered structure weakens the chemical interaction between the interlayer atoms,hinders the phonon transmission in the compound,and enhances the phonon scattering at the interlayered interface.Therefore,Bi₂O₂Se has inherently low thermal conductivity(0.7-0.8 W m^-1 K^-1).These alternately stacked conductive layers and insulating layers form a natural superlattice,leading to the two-dimensional confinement effect of charge carriers,which favors a relatively high absolute value of Seebeck coefficient(?500?V K^-1).Therefore,Bi₂O₂Se is considered as a potential n-type thermoelectric material.This paper systematically studied the effects of anisotropy,texture and doping on the thermoelectric properties of n-type Bi₂O₂Se bulk materials.By comparing the thermoelectric transport performance of the material perpendicular to the pressure direction and parallel to the pressure direction,the value of the thermoelectric figure of merit ZT shows no essential difference within the error range,but the electrical conductivity perpendicular to the pressure direction is significantly higher than the value in the parallel direction.Introducing Te to ealize a liquid phase sintering mechanism can strengthen the texture of the bulk material.Nb doping and W doping at Bi site can improve the thermoelectric figure of merit.The main results and conclusions are as follows:(1)Pristine Bi₂O₂Se bulk samples were prepared by ball milling,solid-phase reaction and hot-press sintering.The thermoelectric properties of bulk samples perpendicular to the pressure direction and parallel to the pressure direction were characterized.At 823 K,the electrical conductivity perpendicular to the direction of pressure reached 3.44 S cm^-1,which is higher than that parallel to the direction of pressure at 2.33 S cm^-1.No difference in Seebeck coefficients at high temperature is identified.Thus,the power factors perpendicular to the pressure direction higher than the power factor parallel to the pressure direction are high than the power factors parallel to the pressure direction at high temperatures.Our experimental results show that,in the whole temperature range,the thermal conductivity in the direction perpendicular to the pressure decreases from 1.5 to-0.8 W m^-1 k^-1,and that in the direction parallel to the pressure decreases from 1.2 to-0.66 W m^-1 k^-1.The thermal conductivity in the perpendicular direction is generally higher than that in the parallel direction.Therefore,the ZT values are almost the same.In the following section,samples perpendicular to the direction of pressure were selected for characterization of thermal and electrical transport properties.We intend to adding 2wt%Te to strengthen the texturing through repetitive hot pressing process,It is found that the lattice thermal conductivity of Bi₂O₂Se+2wt%Te decreases with the increase of the number of cycles,and the lattice thermal conductivity decreases from 0.83 W m^-1 K^-1 dropped to 0.77 W m^-1 K^-1.At the same time,the electrical transport performance is improved,and the final ZT peak value reaches the maximum value of 0.21 at the first cycle at 823 K.(2)Low intrinsically electrical conductivity restricts the thermoelectric figure of merit for Bi₂O₂Se.Therefore,the carrier concentration was increased from 10¹⁶ cm^-3 to10¹⁸ cm^-3 by Nb doping with Bi.Thereby the electrical conductivity increases at room temperature from 0.0143 S cm^-1 to 71.13 S cm^-1,since Nb acts as an electron donor,which can introduce addition electrons,to increase the carrier concentration.The power factor of Bi_1.94Nb_0.06O₂Se reached 2.19?W cm^-1 K^-2 at 823K.The final ZT value is0.195 at 823 K,which is 325%higher than the undoped sample.(3)Based on the significant effect of n-type doping,we consider the introduction of W at Bi site to further improve the thermoelectric performance of Bi₂O₂Se.The carrier concentration is effectively increased by W doping.At 823 K,the electrical conductivity of Bi_1.92W_0.08O₂Se reaches 43.16 S cm^-1,which is nearly 12 times higher than the electrical conductivity of the pure sample at high temperature of 3.75 S cm^-1.Thus,the power factor is increased from 0.74?W cm^-1 K^-2 to 1.74?W cm^-1 K^-2.At 823K,Bi_1.98W_0.02O₂Se has a maximum thermoelectric figure of merit of 0.247,which was nearly 4 times higher than that of the pristine sample.