Preparation And Properties Of High-entropy Oxides

High-entropy materials are a new class of materials consisting of a variety of principal elements in equimolar or near equimolar ratios.Due to their unique properties relative to traditional single-principal materials,they have recently attracted the attention of researchers in the field of inorganic non-metals.In this paper,we studied the phase transition process and micro-nano surface structure of the earliest developed(Mg0.2Co0.2Ni0.2Zn0.2Cu0.2)O series rocksalt structure entropy-stabilized oxide under heat treatment conditions.n this basis,a new type of(Ce0.2 Zr0.2Hf0.2Sn0.2Ti0.2)O2 series fluorite structure entropy-stabilized oxide was proposed and discussed systematically.Firstly,(Mg0.2Co0.2Ni0.2Zn0.2Cu0.2)O samples were treated under different thermal conditions,and the samples produce Jahn-Teller distortion under annealing and slow cooling conditions.This distortion occurs during the cooling process.Extending sintering time of the ceramics can promote the Jahn-Teller distortion.The internal stress has no significant effect on the Jahn-Teller distortion,but it significantly inhibits the phase decomposition of ceramics at a low temperature.The samples produce a large amount of holes due to high temperature oxidation under slow cooling condition,resulting in an increase in conductivity.The quaternary(Mg0.25Co0.25Ni0.252n0.25)O can also form a single phase,but can't show "entropy stabilization".Then,the surface structure changes of the ceramic discs,powders and porous ceramics of(Mg0.2Co0.2Ni0.2Zn0.2Cu0.2)O entropy-stabilized oxide were studied under thermodynamic driving.(Mg0.2Co0.2Ni0.2Zn0.2Cu0.2)O entropy-stabilized oxide ceramics have a lamellar grain shape,and the surface can grow different micro-nano precipitates through heating treatment at 750? for different times.The surface structure of ceramics can be reversibly transformed between high and low temperatures.Using molten salt method can observe a single powder crystal grain,but it is difficult to observe the lamellar structure of the crystal.The micro-nano surface structure of the porous ceramic is formed in substantially the same way as the ceramic discs,but it's formation speed is faster.A phase-reversible fluorite-type(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)O2 entropy-stabilized oxide was designed and successfully synthesized.The oxide is metastable at temperatures below 11 00? until the temperature rises to 1 500? to become a single fluorite phase with a low thermal conductivity of 1.28 W m-1 K-1.On this basis,new system of fluorite structure high entropy oxides were initially synthesized by using other valence materials or reducing the number of principal components.Based on this,two high-entropy oxides of perovskite structure were successfully synthesized,which proved that the "high-entropy effect" can indeed promote the formation of a single phase.