Preparation And Thermophysical Properties Of The High-entropy Sr(Zr_0.2Hf_0.2Ce_0.2Yb_0.2Me_0.2)O_3-x Ceramics

In view of the problem that traditional YSZ materials will undergo sintering,phase transformation and peeling on the coating surface when they are in service at higher than 1200℃ for a long time,seeking a new type of ceramic with good thermophysical properties as candidate material for thermal barrier coating is one of the hot spots in the research field.The research is on the basis of traditional SrZrO3 material,the perovskite structure.The high entropy perovskite ceramics consist of six kinds of equal-mole ratio of five elements at B position which were successfully prepared by the method of solid-state reaction.The chemical formula for which is the Sr(Zr0.2Hf0.2Ce0.2Yb0.2Me0.2)O3-x(Me=Y,Gd,Ti,Sn,Nb,Ta).The phase composition of ceramic bulks was analyzed by XRD,the microstructure and composition of ceramic were studied by SEM and EDS.The thermal diffusivity of the ceramic was measured by a laser thermal conductivity meter,and the thermal conductivity of the ceramic from room temperature to 1400℃ was calculated.The thermal expansion coefficient of ceramic was measured by high-temperature thermal expansion instrument to investigate whether the thermal expansion coefficient matched well with the coating substrate material.TG-DSC analysis of ceramics was carried out by using synchronous thermal analysis technology to study the phase stability of ceramics at high temperature.The effects of different elements,valence states and ionic radius on the formation of the entropy-stable structure,and the effects of the appearance of the second phase on the mechanical and thermophysical properties of the ceramics were analyzed.The results showed that the structure of high-entropy ceramics synthesized by solid-phase method are mainly cubic phase,and contains a small amount of second phase rare earth oxide solid solution,which is mainly consisted of fluorite structure.Because the saturated vapor pressure of SrO is higher than that of ZrO2 and other rare earth oxides,the volatile amount of SrO is greater than that of those oxides in the process of ceramic powder preparation,which will cause the appearance of the second phase and composition segregation in the as-prepared ceramics.It was found by XRD and EDS that there was a small amount of second phase in most of the ceramics.The second phases of HE Sr(Zr0.2Hf0.2Ce0.2Yb0.2Me0.2)O3-x(Me=Y,Gd,Ti,Nb)are accordingly Sr(Y0.55Yb0.45)O2.5,Sr(Gd0.55Yb0.45)O2.5,Yb0.4Ce0.6O1.8,Yb0.Ce0.9O1.95,while there were no obvious secondary phase in HE Sr(Zr0.2Hf0.2Ce0.2Yb0.2Me0.2)O3-x(Me=Sn,Ta)ceramic.The thermal conductivities of these ceramic have a similar change rule with temperature,gradually reduced with the increase of temperature(from room temperature to 800℃),as the temperature continues rising,the thermal conductivity goes up.The thermal conductivity of HE Sr(Zr0.2Hf0.2Ce0.2Yb0.2Me0.2)O3-x(Me=Y,Gd,Ti,Sn,Nb,Ta)ceramic is 1.30 W·m-1·K-1,1.28 W·m-1·K-1,1.30 W·m-1·K-1,1.37 W·m-1·K-1,1.16 W·m-1·K-1 and 1.30 W·m-1·K-1,respectively at 1000℃,which is much lower than that of SrZrCO3(1.96 W·m-1·K-1,1000℃)and YSZ(2.12 W·m-1·K-1,1000℃).Meanwhile,the theraml coefficient of the HE ceramic is correspondingly 14.0×10-6·K-1,15.9×10-6·K-1,12.8×10-6·K-1,13.1×10-6·K-1,14.6×10-6·K-1 and 14.4×10-6·K-1,it is significantly improved compared with that of SrZrO3(10.9×10·6·K-1,1000℃).In addition,TG-DSC showed that the HE ceramics did not undergo phase transformation(600-1400℃),and there was no significant change in the phase structure compared with the as-prepared ceramics after the cyclic heat-treatment at 1400℃ for 100 h,200 h,300 h respectively.With the increase of aging time,the phase composition and content of ceramics remained unchanged.After analyzing the mechanical properties of the ceramics,it was found that the maximum density of the ceramics was 98.3%(Me=Ta).The minimum Vickers hardness was(4.14±0.11)GPa(Me=Nb)and the maximum was 7.64±0.26 GPa(Me=Ti).The fracture toughness was 1.12±0.14 MPa·m1/2(Me=Gd)and 2.01±0.04 MPa·m1/2(Me=Ti),respectively.Compared with SrZrO3 ceramics(87.7%,3.04±0.11 GPa,1.27±0.14 MPa·m1/2),the ceramics showed a different degree of improvement.Considering the thermal conductivity,thermal expansion coefficient,mechanical properties,second phase and the thermal stability at high temperature,it is not hard to find that HE Sr(Zr0.2Hf0.2Ce0.2Yb0.2Me0.2)O3-x(Me=Y,Gd,Sn,Nb,Ta)ceramics are promising for applications as candidate TBC materials.