Preparation,thermal Properties And CMAS Corrosion Behavior Of La₂B₂O₇ High Entropy Ceramics

Thermal barrier coating(TBC)of low thermal conductivity is critically important to improve the thermal efficiency of engines.In the past few decades,series of oxide ceramics have been developed as innovated TBC materials.Re₂B₂O₇rare earth oxides have the characteristics of low thermal conductivity,excellent thermal stability,good ionic conductivity and so on,which can improve the thermal cycle life durability of coating materials significantly if were incorporated into the coating.However,there is space to improve of the thermal conductivity and reliability of Re₂B₂O₇ reinforced coating.High entropy ceramics is a new kind of high performance ceramics developed recently,with high entropy effects,Hysteric diffusion effect,lattice distortion effect and cocktail effect,which could improve the property of material remarkably.In this thesis,a new criterion of unequal valence principle is proposed to predict the formation of single-phase high entropy oxides based on the structure of Re₂B₂O₇.According to this criterion,La₂(ZnMgCeWMo)₂O₇ fluorite structure high entropy oxides and La₂(Zn Mg Zr WMo)₂O₇,La₂(ZnMgHfWMo)₂O₇,La₂(ZnMgSnWMo)₂O₇ pyrochlore structure high entropy oxides were successfully prepared via a solid-phase sintering method with raw materials of La₂O₃,ZnO,MgO,CeO₂,HfO₂,ZrO₂,SnO₂,WO₃,MoO₃.The phase composition,microstructure and distribution of elements of the high entropy ceramics were analyzed by X-ray diffraction(XRD),scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).At the same time,the possible formation mechanism of high entropy oxides is proposed.The thermal physical properties,such as thermal conductivity,thermal expansion coefficient and thermal stability,as well as the high temperature CMAS(Ca O,MgO,Al₂O₃,SiO₂)corrosion property were investigated systematically,which benefits a lot to the design of novel TBC materials.Main results are focusing on the following three parts:The formation criterion of high entropy Re₂B₂O₇-structure oxide:an unequal valence principle is proposed(by considering size effect and Pauling first principle).five elements of different valences were designed to replace the B site of Re₂B₂O₇oxide,which make a breakthrough of the traditional criterion of equal valence principle.It is found that La₂(Zn Mg Zr WMo)₂O₇,La₂(ZnMgHfWMo)₂O₇ and La₂(ZnMgSnWMo)₂O₇ fluorite type high entropy oxides can be obtained successfully according to the conclusion of unequal valence principle.Meanwhile,these three compounds are all octahedral,and their B-site elements randomly occupy the vacancies that distribute uniformly in the crystals.La₂(ZnMgCeWMo)₂O₇ fluorite high entropy oxides undergo a reverse reaction at 1500? to decompose to MgO and CeO₂ due to the large B-ion radius,indicating that the crystal structure of La₂B₂O₇rare earth oxides appear an order-disordered transition when r(A³⁺)/r(B⁴⁺)<1.46.In the control groups including La₂(Zn Mg Ti WMo)₂O₇ and La₂(Zn Ni Ti WMo)₂O₇,the ion radius exceeds the limit range of the criterion and thereby the single phase high entropy oxide cannot be produced.Instead,monoclinic La₂(Zn Mg Ti WMo)₂O₇,MgO,La₂WMoO₉,La TiO₃ and other polyphase compounds appear after reaction.These results testify that the unequal valence criterion proposed here is feasible.Thermophysical property characterization:thermal conductivity,thermal expansion coefficient and thermal stability of as obtained La₂B₂O₇ high entropy oxides were studied systematically.It is found that four kinds of La₂B₂O₇ high entropy oxides all have high thermal expansion coefficient and low thermal conductivity.With the increasement of average ionic radius of B site,thermal expansion coefficient and thermal conductivity of these four oxides both increased.And thermal expansion coefficients increase with the augment of temperature.At1200?,the thermal expansion coefficients of La₂(ZnMgCeWMo)₂O₇,La₂(Zn Mg Zr WMo)₂O₇,La₂(ZnMgHfWMo)₂O₇ and La₂(ZnMgSnWMo)₂O₇ all reach a maximum value of 14.3×10^-6K^-1,13.9×10^-6K^-1,13.4×10^-6K^-1 and 12.6×10^-6K^-1,respectively.With the increasement of temperature,the thermal conductivity of these four samples all decrease initially and then increase.Fluorite La₂(Zn Mg Cew Mo)₂O₇reaches the lowest thermal conductivity(1.4 W/m·K)at 800?,while the other three materials all reached the lowest thermal conductivity at 600? with values of 0.8W/m·K,1.1 W/m·K and 1.1 W/m·K,respectively.Therefore,La₂(Zn Mg Zr WMo)₂O₇,La₂(ZnMgHfWMo)₂O₇ and La₂(ZnMgSnWMo)₂O₇ show a quite good thermal stability.High temperature CMAS corrosion performance:CMAS corrosion test were conductedforLa₂(ZnMgCeWMo)₂O₇,La₂(Zn Mg Zr WMo)₂O₇,La₂(ZnMgHfWMo)₂O₇ and La₂(ZnMgSnWMo)₂O₇.It is found that after CMAS corrosion,apatite(Ca La₄(SiO₄)₃O),magnesia-alumina spinel(Mg Al₂O₄)and cal-alumina xanthite(Ca₂Al₂O₇Si)were formed.And the corrosion cross section shows three layers of CMAS residual layer,bonding layer and high entropy ceramic matrix.According to the depth of bonding layer,La₂(ZnMgCeWMo)₂O₇,La₂(Zn Mg Zr WMo)₂O₇ and La₂(ZnMgHfWMo)₂O₇ all have excellent corrosion resistance to CMAS since the bonding layer depth is only 8.3?m,13.5?m,12.57?m at the surface of each sample after 20 h of CMAS corrosion,Obviously,La₂(ZnMgCeWMo)₂O₇ fluorite high entropy oxides owe the best corrosion resistance to CMAS.