Opaque Gd_(2)Zr_(2)O_(7)/GdMnO_(3) thermal barrier materials for thermal radiation shielding:The effect of polaron excitation

Opaque thermal barrier materials play a pivotal role in thermal radiation shielding of turbine blades,since the intensity of thermal radiation rapidly increases with the increase of operating temperature of gas turbines and has become a new and major concern for the durability of metallic blades.The conventional thermal barrier coating(TBC)materials such as YSZ and Gd_(2)Zr_(2)O_(7),however,are almost translucent to thermal radiation and are unable to protect the blades at such harsh environment.Although searching for new thermal barrier materials is significant,it is still a challenge to make the current TBC materials opaque without significantly modifying the composition or other physical properties.To cope with this challenge,GdMnO_(3) is incorporated as an absorptive second phase into Gd_(2)Zr_(2)O_(7) in this work,which is originally translucent(absorption coefficient 10^(1)-10^(2) m^(-1))in the near-infrared wavelengths.Intriguingly,with less than 5 wt.%GdMnO_(3),the Gd_(2)Zr_(2)O_(7)/GdMnO_(3) becomes opaque to thermal radiation and successfully refrains the rise of thermal conductivity at high temperatures.Meanwhile,the lattice thermal conductivity and mechanical properties are almost unchanged.The small polaron mechanism is confirmed for GdMnO_(3),leading to a high absorption coefficient(>10^(6) m^(-1))for near-infrared radiation.To understand the underling mechanism,a theoretical model is built to estimate the absorption coefficient of the Gd_(2)Zr_(2)O_(7)/GdMnO_(3) composites(>10^(4) m^(-1)).This paper proposes a powerful strategy to design thermal-radiation-shielding TBCs through incorporating minor second-phase particles with high-absorption mechanism,such as polaron excitation.

In-situ synthesis of gadolinium niobate quasi-binary composites with balanced mechanical and thermal properties for thermal barrier coatings

Yttria-stabilized zirconia(YSZ)has been used as a thermal barrier coating(TBC)material in gas turbines for several decades.Although continuous efforts have been made to develop novel TBC materials that can work at a higher temperature,no single material other than YSZ has all the desired attributes for the TBCs.In this paper,we report the in-situ synthesis of quasi-binary GdNbO_(4)/Gd_(3)NbO_(7)composites based on the simple Gd_(2)O_(3)-Nb_(2)O_(5)binary phase diagram.The fracture toughness of these quasi-binary composites is remarkably enhanced compared with the value predicted by the rule of mixtures because the ferroelastic domain switching is more activated due to the residual stress in the quasi-binary composites,which triggers more crack defections due to the enlarged process zone.Additionally,the Gd_(3)NbO_(7)phase provides a low thermal conductivity due to the substantial chemical inhomogeneity,which diffuses phonons.Gd_(3)NbO_(7)/GdNbO_(4)exhibits a balanced thermal conductivity of 1.6 W/(m·K)at 1073 K and a toughness value of 2.76 MPa·m0.5,and these values are among the best comprehensive properties that have been obtained for new TBC materials.The work demonstrates a feasible approach of designing a new TBC material with balanced properties and can be easily fabricated.

Medium-entropy(Me,Ti)_(0.1)(Zr,Hf,Ce)_(0.9)O_(2)(Me=Y and Ta):Promising thermal barrier materials for high-temperature thermal radiation shielding and CMAS blocking

With continuous enhancement of gas-turbine inlet temperature and rapid increase of radiant heat transfer,thermal barrier coating(TBC)materials with a combination of low thermal conductivity and good high-temperature thermal radiation shielding performance play vital roles in ensuring the durability of metallic blades.However,yttria-stabilized zirconia(YSZ),as the state-of-the-art TBC and current industry standard,is unable to meet such demands since it is almost translucent to high-temperature thermal radiation.Besides,poor corrosion resistance of YSZ to molten calcia-magnesia-alumina-silicates(CMAS)also impedes its application in sand,dust,or volcanic ash laden environments.In order to improve the hightemperature thermal radiation shielding performance and CMAS resistance of YSZ and further reduce its thermal conductivity,two medium-entropy(ME)oxide ceramics,ME(Y,Ti)_(0.1)(Zr,Hf,Ce)_(0.9)O_(2)and ME(Ta,Ti)_(0.1)(Zr,Hf,Ce)_(0.9)O_(2),were designed and prepared by pressureless sintering of binary powder compacts in this work.ME(Y,Ti)_(0.1)(Zr,Hf,Ce)_(0.9)O_(2)presents cubic structure but a trace amount of secondary phase,while ME(Ta,Ti)_(0.1)(Zr,Hf,Ce)_(0.9)O_(2)displays a combination of tetragonal phase(81.6 wt.%)and cubic phase(18.4 wt.%).Both ME(Y,Ti)_(0.1)(Zr,Hf,Ce)_(0.9)O_(2)and ME(Ta,Ti)_(0.1)(Zr,Hf,Ce)_(0.9)O_(2)possess better high-temperature thermal radiation shielding performance than YSZ.Especially,the high-temperature thermal radiation shielding performance of ME(Ta,Ti)_(0.1)(Zr,Hf,Ce)_(0.9)O_(2)is superior to that of ME(Y,Ti)_(0.1)(Zr,Hf,Ce)_(0.9)O_(2)due to its narrower band gap and correspondingly higher infrared absorbance(above 0.7)at the waveband of 1 to 5μm.The two ME oxides also display significantly lower thermal conductivity than YSZ and close thermal expansion coefficients(TECs)to YSZ and Ni-based superalloys.In addition,the two ME oxides possess excellent CMAS resistance.After attack by molten CMAS at 1250℃for 4 h,merely~2μm thick penetration layer has been formed and the structure below the penetration layer is still intact.These results demonstrate that ME(Me,Ti)_(0.1)(Zr,Hf,Ce)_(0.9)O_(2)(Me=Y and Ta),especially ME(Ta,Ti)_(0.1)(Zr,Hf,Ce)_(0.9)O_(2),are promising thermal barrier materials for high-temperature thermal radiation shielding and CMAS blocking.