High-entropy(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))2Hf_(2)O_(7) ceramic: A promising thermal barrier coating material

Thermal barrier coating(TBC)materials perform an increasingly important role in the thermal or chemical protection of hot components in a gas turbine.In this study,a novel high entropy hafnate(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7) was synthesized by solution combustion method and investigated as a potential TBC layer.The as-synthesized(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7) possesses a pure single disordered fluorite phase with a highly homogeneous distribution of rare earth(RE)cations,exhibiting prominent phase stability and excellent chemical compatibility with Al_(2)O_(3) even at 1300°C.Moreover,(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7) demonstrates a more sluggish grain growth rate than Y_(2)Hf_(2)O_(7).The thermal conductivity of(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7)(0.73-0.93 W m^(-1)K^(-1))is smaller than those of components RE_(2)Hf_(2)O_(7) and many high entropy TBC materials.Beside,the calculated thermal expansion coefficient(TEC)of(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7)(10.68×10^(-6)/K,1100°C)is smaller than that of yttriastabilized zirconia(YSZ).Based on the results of this work,(Y_(0.2)Gd_(0.2)Dy_(0.2)Er_(0.2)Yb_(0.2))_(2)Hf_(2)O_(7) is suitable for the next generation TBC materials with outstanding properties.

Calcium-magnesium-alumina-silicate(CMAS)resistant high entropy ceramic(Y_(0.2)Gd_(0.2)Er_(0.2)Yb_(0.2)Lu_(0.2))_(2)Zr_(2)O_(7) for thermal barrier coatings

A novel high-entropy material,(Y_(0.2)Gd_(0.2)Er_(0.2)Yb_(0.2)Lu_(0.2))_(2)Zr_(2)O_(7)was successfully synthesized by the solid state reaction method and spark plasma sintering,and investigated as a promising thermal barrier coating material.Rare-earth elements were distributed homogeneously in the pyrochlore structure.It was found that the prepared high-entropy ceramic maintains pyrochlore structure at the temperature up to 1600℃,and it possesses a similar thermal expansion coefficient(10.2×10^(−6)K^(−1) at 25-900℃)to that of YSZ,low thermal conductivity(<0.9 W m^(-1)K^(−1) at 100-1000℃)and good CMAS resistance(infiltration depth is 22μm after annealed at 1300℃for 24 h).The corrosion process was investigated,and RE elements distributing homogeneously in(Y_(0.2)Gd_(0.2)Er_(0.2)Yb_(0.2)Lu_(0.2))_(2)Zr_(2)O_(7)show different diffusion rates in CMAS.RE^(3+) with a larger radius(closer to Ca^(2+))is easier to react with CMAS to form an apatite phase.

High entropy defective fluorite structured rare-earth niobates and tantalates for thermal barrier applications

Rare-earth tantalates and niobates(REjTaO7 and REjNbO7)have been considered as promising candidate thermal barrier coating(TBC)materials in next generation gas-turbine engines due to their ultra-low thermal conductivity and better thermal stability than yttria-stabilized zirconia(YSZ).However,the low Vickers hardness and toughness are the main shortcomings of RE;TaO-and REjNbOr that limit their applications as TBC materials.To increase the hardness,high entropy(Yu3Ybu3Er/3)sTaOr,(Y13YbnErns)NbO,and(Sm1/6Eu1/6Y 1/6Yb1/6Lu1/6Er1/6)3(Nb1/2Ta1/2)O7 are designed and synthesized in this study.These high entropy ceramics exhibit high Vickers hardness(10.912.0 GPa),close thermal expansion coefficients to that of single-principal-component RE3TaO,and RE;NbO,(7.9×10^-6-10.8×10-6 C-1 at room temperature),good phase stability,and good chemical compatibility with thermally grown Al2O3,which make them promising for applications as candidate TBC materials.

High-entropy Sm_(2)B_(2)O_(7)(B=Ti,Zr,Sn,Hf,Y,Yb,Nb,and Ta) oxides with highly disordered B-site cations for ultralow thermal conductivity

Materials with ultralow thermal conductivity and good thermal stability are of great interest in numerous applications such as energy storage and conversion devices,and thermal insulation components.In this work,a family of high-entropy Sm_(2)B_(2)O_(7)(B=Ti,Zr,Sn,Hf,Y,Yb,Nb,and Ta) oxides with highly disordered cations on the B-site has been synthesized by introducing large atomic-size mismatch,mass and charge disorder.Through tuning the composition,the high-entropy Sm_(2)B_(2)O_(7) oxides can be engineered from pyrochlore to fluorite structure,accompanied with an order-disorder transition.The pyrochlore Sm_(2)(Nb_(0.2)Sn_(0.2)Ti_(0.2)Y_(0.2)Zr_(0.2))_(2)O_(7) and fluorite Sm_(2)(Nb_(0.2)Ta_(0.2)Y_(0.2)Yb_(0.2)Zr_(0.2))_(2)O_(7) exhibit low thermal conductivities of 1.35 W·m^(-1)·K^(-1) and 1.23 W·m^(-1)·K^(-1),respectively,indicating their good thermal insulation.In addition,the high-entropy fluorite Sm_(2)(Nb_(0.2)Ta_(0.2)Y_(0.2)Yb_(0.2)Zr_(0.2))_(2)O_(7) also shows average thermal expansion coefficient of 10.2 × 10^(-6)℃^(-1) and high-temperature stability even after thermal exposure at 1600 °C in air for 30 h.These results indicate that the high-entropy Sm_(2)B_(2)O_(7)(B=Ti,Zr,Sn,Hf,Y,Yb,Nb,and Ta) can be promising candidates for thermal barrier coatings and thermally insulators.

High-entropy ferroelastic(10RE_(0.1))TaO_(4) ceramics with oxygen vacancies and improved thermophysical properties

The primary purpose of this work is to optimize the thermophysical properties of rare-earth tan-talate ceramics using the high-entropy effect.Here,the high-entropy rare-earth tantalate ceramic(Y_(0.1)Nd_(0.1)Sm_(0.1)Gd_(0.1)Dy_(0.1)Ho_(0.1)Er_(0.1)Tm_(0.1)Yb_(0.1)Lu_(0.1))TaO_(4)((10RE_(0.1))TaO_(4))is synthesized successfully.The lat-tice distortion and oxygen vacancy concentration are characterized firstly in the rare-earth tantalates.Notably,compared with single rare-earth tantalates,the thermal conductivity of(10RE_(0.1))TaO_(4) is reduced by 16%-45%at 100℃ and 22%-45%at 800℃,and it also presents lower phonon thermal conductivity in the entire temperature range from 100 to 1200℃.The phonon thermal conductivity(1.0-2.2 W m^(-1) K^(-1),100-1200℃)of(10RE_(0.1))TaO_(4) is lower than that of the currently reported high-entropy four-,five-and six-component rare-earth tantalates.This is the result of scattering by the ferroelastic domain,lattice distortion associated with size and mass disorder,and point defects,which target low-,mid-and high-frequency phonons.Furthermore,(10RE_(0.1))TaO_(4),as an improved candidate for thermal barrier coatings materials(TBCs),has a higher thermal expansion coefficient(10.5×10^(-6)K^(-1) at 1400℃),lower Young’s modulus(123 GPa)and better high-temperature phase stability than that of single rare-earth tantalates.