Microstructure Study Of High-entropy Fluorite Oxide(Zr_0.2Ce_0.2Hf_0.2Y_0.2Al_0.2)O_2-δ At Different Temperatures And Atmospheres

High-entropy fluorite oxides(HEFOs)have a wide range of elemental composition and excellent crystal structure stability,which shows great application prospects in high temperature martials.At present,the development of high entropy fluorite oxides has focused on the addition of different species of rare earth elements,with less research on other elements,and microstructural studies at different temperatures and atmospheres have not yet been carried out.Therefore,a new HEFO(Zr0.2Ce0.2Hf0.2Y0.2Al0.2)O2-δ has been designed and prepared in this work,and the micro structure of powder was investigated at different temperatures and atmospheres.(Zr0.2Ce0.2Hf0.2Y0.2Al0.2)O2-δ powder was obtained by a sol-gel method combined with heat treatment,using citric acid,metal nitrate and chloride salts as the main raw materials.Firstly,a comprehensive micro structure study of powder at room temperature was carried out.Secondly,aberration corrected scanning transmission electron microscopy was used for atomic-scale observation of the crystal structure,and the strain of lattice distortion was quantified by geometric phase analysis.Finally,the thermal behavior,chemical valence,crystal structure,lattice parameter and thermal expansion coefficient(TEC)of the powders were investigated at the low temperature range(room temperature to 1100℃)in an inert atmosphere and the high temperature range(1100 to 1500℃)in the air,respectively.The main conclusions are summarized as follows:(1)After heat treatment at 1500℃ for 4 h,the powder has been completely crystallized and stabilized,and the crystal structure is nearly a pure cubic fluorite crystal phase.The particle size distribution of the powder ranges from 0.5 to 4 μm,mainly concentrated in 1 to 2 μm,with an average particle size of 1.8 μm.The five metallic elements Zr,Ce,Hf,Y,and A1 show a uniform distribution.The content of Ce3+is about 33.84%,and δ is 0.234 at room temperature.The ideal configurational entropy of the anion and cation is 1.609 R and 0.722 R,respectively,yielding the total configurational entropy of 2.331 R(R is the ideal gas constant),and the standard composition is(Zr0.2Ce0.2Hf0.2Y0.2Al0.2)O1.766.After heat treatment from 900 to 1500℃ for 4 h,the powders were cooled by water quenching,air quenching and slow cooling,respectively.The tested powders are able to maintain the cubic fluorite structure,indicating the excellent crystal structure stability.(2)The radius and brightness of the atomic column projections exhibit slight variations owing to the random distribution of the cations,indicating differences in the relative contents of the five elements in each atomic column,resulting to the changes of crystal plane spacing.The oxygen ions are influenced by both the random distribution of the cations and the large number of oxygen vacancies,leading to M-OM(M=Zr,Ce,Hf,Y,and Al)bond angles ranging from 80 to 105°,mainly from 85 to 100°.The projected diameters of the cationic atomic columns in the crystal structure are distributed from 0.115 to 0.185 nm,showing an approximately normal distribution,mainly from 0.125 to 0.165 nm,with an average value of 0.146 nm.The distributions of strains εyy and εxx(along the lattice spacing direction and vertical direction)within the crystal structure are mainly centred within±4%,and there is a tendency for alternating positive and negative values in strains εyy and εxx maps without significant aggregations.(3)As the temperature increases from 300 to 800℃ under vacuum,the nominal composition changes from(Zr0.2Hf0.2Ce0.2Y 0.2 Al0.2)O1.765 to(Zr0.2Hf0.2Ce0.2Y0.2Al0.2)O1.711,due to partial valence reduction of Ce4+,Zr4+,and Hf4+.44.59%of Ce,20.28%of Zr and 23.93%of Hf show+3 valence at 800℃.Valence reduction occurres only during heating.Although a number of Zr3+,Ce3+and Hf3+ions are present,(Zr0.2Ce0.2Hf0.2Y0.2Al0.2)O2-δ maintains the high-entropy fluorite crystal structure.The TEC increases from 9.38 to 13.12×10-6 K-1 at 300 to 1100℃and decreases from 10.71 to 9.15×10-6 K-1 at 1100 to 300℃.The valence reduction reactions of partial Ce,Hf and Zr in the crystal structure and the change in the standard composition due to oxygen overflow in the lattice together determine the difference in the TEC during heating and cooling.(4)As the temperature increases from 1200 to 1300℃ in the air,partial of Ce4+in the crystal structure is reduced to Ce3+,which has an increasing effect on the lattice parameter,and the TEC increases from 9.58 to 9.84 and then decreases to 9.59 × 10-6 K-1 with the enhancement and weakening of the reduction reaction.The powder mass remains relatively stable from 1300 to 1400℃ and the TEC maintains at 9.59 to 9.56×10-6 K-1.Partial of Ce3+is oxidized to Ce4+from 1400 to 1500℃,and the TEC decreases from 9.56 to 9.29×10-6 K-1,as the oxidation reaction begins and intensifies.The percentage contents of Ce3+are 35.30%,38.13%,37.65%and 33.29%at 1200℃,1300℃,1400℃ and 1500℃,respectively.During cooling,the TEC increases from 9.28 to 9.46×10-6 K-1 at 1500℃ to 1400℃.The TEC maintains from 9.50 to 9.60×10-6 K-1 at 1400 to 1200℃.Before and after high-temperature heat treatment,the crystal structure and cell parameter of the powder change slightly and it can be ignored.