(La_(0.2)Ce_(0.2)Nd_(0.2)Sm_(0.2)Eu_(0.2))PO_4: A high-entropy rare-earth phosphate monazite ceramic with low thermal conductivity and good compatibility with Al_2O_3

Low thermal conductivity, matched thermal expansion coefficient and good compatibility are general requirements for the environmental/thermal barrier coatings(EBCs/TBCs) and interphases for Al_2O_3 f/Al_2O_3 composites. In this work, a novel high-entropy(HE) rare-earth phosphate monazite ceramic (La_(0.2)Ce_(0.2)Nd_(0.2)Sm_(0.2)Eu_(0.2))PO_4 is designed and successfully synthesized. This new type of HE rare-earth phosphate monazite exhibits good chemical compatibility with Al_2O_3, without reaction with Al_2O_3 as high as 1600?C in air. Moreover, the thermal expansion coefficient(TEC) of HE (La_(0.2)Ce_(0.2)Nd_(0.2)Sm_(0.2)Eu_(0.2))PO_4(8.9 × 10-6/?C at 300–1000?C) is close to that of Al_2O_3. The thermal conductivity of HE (La_(0.2)Ce_(0.2)Nd_(0.2)Sm_(0.2)Eu_(0.2))PO_4 at room temperature is as low as 2.08 W·m-1·K-1, which is about 42% lower than that of La PO4. Good chemical compatibility, close TEC to that of Al_2O_3, and low thermal conductivity indicate that HE (La_(0.2)Ce_(0.2)Nd_(0.2)Sm_(0.2)Eu_(0.2))PO_4 is suitable as a candidate EBC/TBC material and an interphase for Al_2O_3 f/Al_2O_3 composites.     

(TiZrHf)P2O7: An equimolar multicomponent or high entropy ceramic with good thermal stability and low thermal conductivity

ZrP₂O₇ is a promising material for high temperature insulating applications. However, decomposition above 1400 °C is the bottleneck that limiting its application at high temperatures. To improve the thermal stability, a novel multicomponent equimolar solid solution (TiZrHf)P₂O₇ was designed and successfully synthesized in this work inspired by high-entropy ceramic (HEC) concept. The as-synthesized (TiZrHf)P₂O₇ exhibits good thermal stability, which is not decomposed after heating at 1550 °C for 3 h. It also shows lower thermal conductivity (0.78 W m⁻¹ K⁻¹) compared to the constituting metal pyrophosphates TiP₂O₇, ZrP₂O₇ and HfP₂O₇. The combination of high thermal stability and low thermal conductivity renders (TiZrHf)P₂O₇ promising for high temperature thermal insulating applications.     

(Y_(0.25)Yb_(0.25)Er_(0.25)Lu_(0.25))_2(Zr_(0.5)Hf_(0.5))_2O_7: A defective fluorite structured high entropy ceramic with low thermal conductivity and close thermal expansion coefficient to Al_2O_3

正Al_2O_3f/Al_2O_3 ceramic matrix composites (CMC) are promising candidate materials of blades and combustor liners of future gas turbines in light of their higher temperature capability, higher environmental stability and oxidizing-free capacity [1–3]. Nevertheless,grain growth, sintering and creep deformation at high operation     

Porous high entropy (Zr0.2Hf0.2Ti0.2Nb0.2Ta0.2)B2: A novel strategy towards making ultrahigh temperature ceramics thermal insulating

Transition metal diborides based ultrahigh temperature ceramics (UHTCs) are characterized by high melting point, high strength and hardness, and high electrical and thermal conductivity. The high thermal conductivity arises from both electronic and phonon contributions. Thus electronic and phonon contributions must be controlled simultaneously in reducing the thermal conductivity of transition metal diborides. In high entropy (HE) materials, both electrons and phonons are scattered such that the thermal conductivity can significantly be reduced, which opens a new window to design novel insulating materials. Inspired by the high entropy effect, porous HE (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ is designed in this work as a new thermal insulting ultrahigh temperature material and is synthesized by an in-situ thermal borocarbon reduction/partial sintering process. The porous HE (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ possesses high porosity of 75.67%, pore size of 0.3–1.2 μm, homogeneous microstructure with small grain size of 400–800 nm, which results in low room temperature thermal diffusivity and thermal conductivity of 0.74 mm² s⁻¹ and 0.51 W m⁻¹ K⁻¹, respectively. In addition, it exhibits high compressive strength of 3.93 MPa. The combination of these properties indicates that exploring porous high entropy ceramics such as porous HE (Zr_0.2Hf_0.2Nb_0.2Ta_0.2Ti_0.2)B₂ is a novel strategy in making UHTCs thermal insulating.