Synthesis And Thermoelectric Transport Properties Of Co-based High Entropy Oxide Ceramic

Thermoelectric materials can achieve the direct conversion between the thermal and electrical energy without gas emissions and high maintenance costs,and show great potential to improve the energy efficiency and energy recycling.Oxide thermoelectric materials have potential advantages on high-temperature thermoelectric application due to their stable physical and chemical properties at high temperature and safe and non-toxic properties,but the low thermoelectric values restrict further development.The concept of high entropy can effectively regulate the thermoelectric transport properties,and are widely used in the design of high-performance thermoelectric materials.In this paper,we introduced the selective high entropy design into the Co-based oxide thermoelectric ceramic materials Ca₃Co₄O₉ and LaCoO₃ system,to regulate their microstructures and thermoelectric transport properties,and obtained the following results:A novel layered(Ca_0.35Sr_0.2Ba_0.15Na_0.2Bi_0.1)₃Co₄O₉ high-entropy ceramics was obtained for the first time by selective high entropy design in the Ca site of Ca₂Co O₃ insulating layer based on the layered Ca₃Co₄O₉ oxide.The detail microstructure characterizations show that the multi-component cations introduced into the Ca sites and forms a single-phase homogeneous solid solution with c-axis preferred orientation and multi-scale structural defects.The textured high-entropy ceramic,compared to the pristine Ca₃Co₄O₉ sample,shows a significantly increased electrical conductivity due to the higher carrier mobility derived from the c-axis selective orientation,and obtains an enhanced power factor of 0.27 m W·m^-1·K^-2.Besides,high entropy induced multi-dimensional lattice defects can act as the phonon scattering centers to effectively suppress the lattice thermal conductivity,and achieves an ultralow thermal conductivity of 0.87 W·m^-1·K^-1 as well as a peak figure-of-merit of 0.3 at 973 K,which is about 2.5 times larger than that of Ca₃Co₄O₉.These results not only show that the high entropy design has a positive impact on the thermoelectric properties of the material,but also provide a good research foundation for future the design and understanding of high-performance high entropy thermoelectric materials.A series of single-phase cubic structure(La_0.25Sr_0.25Ba_0.25Ca_0.25)Co O₃,(La_0.25Nd_0.25Sr_0.25Ba_0.25)Co O₃ and(La_0.2Nd_0.2Sr_0.2Ba_0.2Ca_0.2)Co O₃ ceramics were obtained by introducing high entropy into the La site of perovskite-type LaCoO₃ oxide.All samples exhibit uniform distribution of elements without segregation and aggregation.The electrical transport measurements show that the three samples have low resistivity and exhibit the semiconductor-metal transition,the transition temperature decreases with the increase of the average valence state of the A-site cations.This may be because the increasing average valence in A-site cations leads to the decrease of the activation energy E_a,and improved the free conducting carriers,as well as the electrical conductivity.The thermal transport data show the(La_0.25Sr_0.25Ba_0.25Ca_0.25)Co O₃ have the dominated lattice thermal conductivity,and the lowest thermal conductivity is about 1.25 W·m^-1·K^-1（937 K）.In contrast,(La_0.25Nd_0.25Sr_0.25Ba_0.25)Co O₃ and(La_0.2Nd_0.2Sr_0.2Ba_0.2Ca_0.2)Co O₃ are dominated by electronic thermal conductivity,exhibiting a thermal conductivity of 1.52 W·m^-1·K^-1 at the same temperature.This suggests that the lattice distortion resulting from A-site disorder forms multi-scale phonon scattering centers and inhibits the lattice thermal conductivity,while increasing the average valence state of cations can enhance electronic thermal conductivity.These results show that the entropy engineering strategy can effectively regulate the electrical and thermal transport properties or behaviors of thermoelectric materials,and induce some novel physical phenomena.