Preparation and characterization of a novel monazite-type high-entropy (La1/7Ce1/7Pr1/7Nd1/7Sm1/7Eu1/7Gd1/7)PO4 for thermal/environmental barrier coatings

A novel monazite-type high entropy (La 1/7 Ce 1/7 Pr 1/7 Nd 1/7 Sm 1/7 Eu 1/7 Gd 1/7 )PO 4 (HE (7RE 1/7 )PO 4 ) designed for the thermal/environmental barrier coatings (T/EBCs) material of Al 2 O 3 f /Al 2 O 3 ceramic matrix composites (CMCs) has been firstly prepared and comprehensively characterized in this work. HE (7RE 1/7 )PO 4 with a homogeneous rare-earth element distribution exhibits good high-temperature stability up to 1750 °C and good chemical compatibility with Al 2 O 3 up to 1600 °C in air. The thermal expansion coefficient (TEC) of HE (7RE 1/7 )PO 4 (9.48 ×10 −6 /°C) is close to that of Al 2 O 3 f /Al 2 O 3 CMCs (9.0 ×10 −6 /°C) at room temperature to 900 °C. Moreover, HE strategy effectively reduces the thermal conductivity of HE (7RE 1/7 )PO 4 (2.42 W·m −1 ·K −1 to 1.14 W·m −1 ·K −1 from room temperature to 1000 °C) in comparison with those of single-component REPO 4 with the average values of 3.15 W·m −1 ·K −1 and 1.56 W·m −1 ·K −1 . These good comprehensive properties allow HE (7RE 1/7 )PO 4 to become a promising T/EBCs material for Al 2 O 3 f /Al 2 O 3 CMCs in the future.