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Open Access Research Article Just Accepted
A high entropy rare-earth phosphate and its principle single component REPO4 for environmental barrier coatings
Journal of Advanced Ceramics
Available online: 23 January 2025
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Xenotime rare-earth (RE) phosphates are emerging as promising materials for environmental barrier coatings (EBCs) for SiC-based ceramic-matrix-composites (CMCs) due to their close coefficients of thermal expansion (CTE) and resistance to calcium–magnesium–alumina-silicate (CMAS) corrosion. In this work, a high-entropy (HE) (Sc0.2Lu0.2Yb0.2Er0.2Y0.2)PO4 and five single-component REPO4 (RE = Sc, Lu, Yb, Er, and Y) compounds were synthesized, and their stability, thermal properties, and CMAS corrosion resistance were investigated. The CTE values of four REPO4 compounds (RE = Lu, Yb, Er, Y~5.6-6×10⁻⁶ °C⁻¹) are close to that of SiC-CMC (4.5-5.5×10⁻⁶ °C⁻¹); while ScPO4 (6.98×10⁻⁶ °C⁻¹) and HE (5RE0.2)PO4 (6.39×10⁻⁶ °C⁻¹) show slightly higher values in the temperature range of 200 to 1300 °C.  The HE phosphate has the lowest thermal conductivity due to size and mass disorder. Systematic CMAS corrosion tests at 1300 °C for 5, 45, and 96 hours reveal that all RE phosphates form a continuous and dense reaction layer predominantly composed of Ca8MgRE(PO4)7, effectively impeding CMAS penetration. Additionally, REPO4 with smaller RE³⁺ cations displays a slower reaction rate and reduced corrosion kinetics, as evidenced by the smaller thickness of the reaction layer. A larger negative difference in optical basicity (OB) value between REPO4 and CMAS signifies a greater corrosion resistance. Mechanistic understanding of the CMAS corrosion and the elucidation of the effects of critical parameters such as ionic mass and ionic radius of RE elements on their thermal properties and the CMAS corrosion kinetics are useful for the development of novel xenotime-type phosphates as EBCs for SiC-CMCs.

Open Access Research Article Issue
A systematic study of thermomechanical properties and calcium–magnesium–aluminosilicate (CMAS) corrosion of multicomponent rare-earth phosphates
Journal of Advanced Ceramics 2024, 13(11): 1807-1822
Published: 21 November 2024
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Multicomponent rare earth phosphates hold immense potential as next-generation environmental barrier coatings (EBCs), offering enormous possibilities and flexibility by controlling and varying their components and fractions to tailor their performance. In this work, the key material parameters (e.g., ionic size and ionic size disorder) and the elements governing their thermal‒mechanical properties and resistance against calcium‒magnesium‒aluminosilicate (CMAS) corrosion were elucidated. The thermal conductivities of multicomponent rare-earth phosphates correlate well with cation size disorder, but no clear trend is identified for the coefficient of thermal expansion (CTE). Er-containing compositions display low CTEs and high fracture toughness. Rapid formation of a dense interfacial layer occurs for most CAMS corrosion-resistant compositions when tested at 1300 °C, e.g., (Lu0.2Yb0.2Er0.2Y0.2Gd0.2)PO4 and (Lu0.2Yb0.2Er0.2Dy0.2Gd0.2)PO4. These multicomponent phosphates also display the least recession upon molten CMAS attack at 1400 °C without significant volumetric swelling, which is superior to their single-component counterparts and state-of-the-art EBCs based on rare-earth disilicates. In contrast, Sc-containing multicomponent phosphates display inferior performance against CMAS corrosion and penetration. A mechanistic understanding and understanding of the kinetics of the interfacial interaction at higher temperatures, as well as the key parameters governing their thermomechanical properties and CMAS corrosion, are valuable for designing data-driven materials of multicomponent phosphates for EBC applications.

Open Access Research Article Issue
Cs3Bi2I9–hydroxyapatite composite waste forms for cesium and iodine immobilization
Journal of Advanced Ceramics 2022, 11(5): 712-728
Published: 02 April 2022
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Perovskite-based ceramic composites were developed as potential waste form materials for immobilizing cesium (Cs) and iodine (I) with high waste loadings and chemical durability. The perovskite Cs3Bi2I9 has high Cs (22 wt%) and I (58 wt%) content, and thus can be used as a potential host phase to immobilize these critical radionuclides. In this work, the perovskite Cs3Bi2I9 phase was synthesized by a cost effective solution-based approach, and was embedded into a highly durable hydroxyapatite matrix by spark plasma sintering to form dense ceramic composite waste forms. The chemical durabilities of the monolithic Cs3Bi2I9 and Cs3Bi2I9–hydroxyapatite composite pellets were investigated by static and semi-dynamic leaching tests, respectively. Cs and I are incongruently released from the matrix for both pure Cs3Bi2I9 and composite structures. The normalized Cs release rate is faster than that of I, which can be explained by the difference in the strengths between Cs–I and Bi–I bonds as well as the formation of insoluble micrometer-sized BiOI precipitates. The activation energies of elemental releases based on dissolution and diffusion-controlled mechanisms are determined with significantly higher energy barriers for dissolution from the composite versus that of the monolithic Cs3Bi2I9. The ceramic-based composite waste forms exhibit excellent chemical durabilities and waste loadings, commensurate with the state-of-the-art glass-bonded perovskite composites for I and Cs immobilization.

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