Preparation,Structure And Properties Of Multivalent Cation High-Entropy Oxides

High entropy ceramics have great application potential in the fields of catalysis,high temperature heat insulation and new energy.The microstructure,valence state distribution and thermal stability of different component systems are of great significance to the development of high entropy ceramic oxides.In this paper,the microstructure,valence state and distribution,factors affecting the formation of high entropy single phase and thermal stability of two kinds of high entropy oxides containing multiple cations were studied particularly.In this paper,fluorite type high entropy oxides with Ce,Zr,Y and La as main elements were prepared,and their microstructure,element distribution,crystal structure and valence electron binding state were studied.The effect of the total content of trivalent elements Y and La on the formation of a single fluorite phase was investigated.The phase stability at different temperatures was determined by quenching experiment.The stability and grain growth behavior of thermal cycling experimental phase at 1500℃to room temperature were analyzed.In order to enrich the high entropy oxide system with multivalent elements,the spinel type high entropy oxides were successfully synthesized by introducing multivalent transition group elements Mg,Cu,Cr,Mn,Ni and Ti.The microstructure,element distribution and crystal structure of the high entropy oxides were characterized,and the distribution of the valence states and cations of each element were analyzed.The influence of any 5-member system on the formation of single spinel structure was investigated.The phase stability of high entropy spinel oxides under 1500℃and quenching conditions was determined.The main conclusions are summarized as follows:（1）(Ce_0.25Zr_0.25Y_0.25La_0.25)O_2-δis a fluorite type high entropy oxide with homogeneous distribution of elements at the micron and nanometer levels.Its cell shrinks to some extent because of the existence of smaller Zr⁴⁺and oxygen vacancy.Grains agglomerate and sintered obviously,and the average grain size is 1.58μm.Y and La are in+3 valence state,Zr is in+4valence state,Ce is in+3 and+4 mixed valence state.The nominal chemical formula is:Ce₀³.⁺067Ce₀⁴.⁺183Y₀³.⁺25La₀³.⁺25Zr₀⁴.⁺25)O_1.716.By exploring the influence of trivalent element content on single fluorite phase,it can be seen that the lower entropy value or the higher total trivalent element content,the lower formation ability of fluorite structure.（2）(Ce_0.25Zr_0.25Y_0.25La_0.25)O_2-δremains single-phase in the range of 1000℃to 1500℃;after 56 hours of thermal cycling at 1500℃to room temperature,phase separation does not occur,showing good phase stability.With the extension of thermal cycle time,the content of Ce³⁺and oxygen vacancy increased,which causing the cell parameters decreased.The grain size also increases with the extension of holding time,and enters the slow growth stage after16 hours of thermal cycling.The system with total trivalent element content of 20%,40%and60%also showed excellent phase stability after 56 hours of thermal cycling at 1500℃;For 80%system,the long time of heat made part of the element reconstruction and the impurity phase content decreased.（3）(Mg_0.5Cu_0.5Cr_0.5Mn_0.5Ni_0.5Ti_0.5)O₄ distribution of each element is a six-component spinel high entropy oxide with homogeneous distribution.Its grain grew up evidently and every single grain size is more than 20μm.Multiple cations contain a variety of valence states and the nominal chemical formula is(Mn₀².⁺19Mn₀⁴.⁺19Mg₀².₂₄⁺Cu₀¹.⁺18Cu₀².⁺17Cr₀⁶.⁺14Ti₀⁴.⁺21)[Mn₀³.⁺23Mg₀².₃₆⁺Cu₀¹.⁺18Cu₀².⁺25Ni₀².⁺32Ni₀³.⁺28Cr₀³.₄₆⁺Ti₀³.⁺39]O_4.0534,where the parentheses are cations distributed in tetrahedral interval,and the middle parentheses are cations distributed in octahedral interval.（4）In the six-component,none of the five-component systems without Ti or Cr elements forms a single spinel structure,and the morphologies of other five-member spinel systems are obviously different.The 6-component and 5-component spinel high entropy oxides did not undergo phase transformation at 1500℃for 2 hours.Phase separation does not occur in the range of 400℃to 1200℃for 6-component high entropy spinel oxides.The reversible phase evolution of the five-element without Cu system occurs at 1000℃-1300℃.