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In this study, a secondary sintering process was adopted to optimize the preparation process of single-phase high entropy A 2 Zr 2 O 7 pyrochlore-based waste forms by spark plasma sintering (SPS). This method eliminates the significant internal stress when preparing samples directly by SPS, and thus well preserves the integrity of the samples after annealing. The phase composition, microstructure, and leaching properties of high entropy A 2 Zr 2 O 7 pyrochlore were systematically investigated. Owning to the low sintering temperature (1450 °C) and short held time (3 min), the high entropy A 2 Zr 2 O 7 pyrochlore exhibited a fine average grain size of 0.8 μm. The high entropy A 2 Zr 2 O 7 pyrochlore oxides exhibit a reliable leaching resistance with low normalized release rates around 10−5 g/(m2∙d) for all elements. For the high entropy systems, the aqueous durability is mainly determined by the average grain size of ceramics and the temperature of the leaching test as well as traditional pyrochlore ceramics solidification. [ABSTRACT FROM AUTHOR]

Spark plasma sintering (SPS) is a fast and efficient method for preparing functional ceramics. This article investigates the phase evolution, thermal expansion and chemical stability of traditional (Y 1.2 Nd 1.8 Fe 5 O 12) and high-entropy ((Y, Gd, Sm, Eu, Dy) 3 Fe 5 O 12) garnet ceramics. Prepared under conventional SPS (50 MPa) conditions at 900–1100 °C. The results indicate that using SPS sintering method can significantly reduce the sintering temperature (1100 °C) and time (3 min) of garnet ceramics. (Y, Gd, Sm, Eu, Dy) 3 Fe 5 O 12 (1.6 × 10−6 K−1) exhibits superior thermal expansion stability compared to Y 1.2 Nd 1.8 Fe 5 O 12 (5.4 × 10−6 K−1). The strong REE-O bond energy and severe lattice distortion of high entropy garnet ceramics hinder the migration and diffusion of cations, resulting in higher leaching resistance of high-entropy garnet ceramics (∼10−7 g m−2 d−1-10−8 g m−2 d−1) compared to traditional garnet ceramics (∼10−5 g m−2 d−1-10−6 g m−2 d−1). Therefore, SPS has broad application prospects in the low-temperature preparation of high-entropy garnet ceramics for immobilization of actinide elements. [ABSTRACT FROM AUTHOR]

Two high‐entropy carbides, including stoichiometric (Zr,Ti,Nb,Ta,Hf)C and nonstoichiometric (Zr,Ti,Nb,Ta,Hf)C0.8, were prepared from monocarbides and ZrH2. Their sinterability, microstructures, mechanical properties, thermophysical properties, and oxidation behaviors were systematically compared. With the introduction of carbon vacancy, the sintering temperature was lowered up to 300°C, Vickers hardness was almost unaffected, whereas the strength decreased significantly generally due to the decrease of covalent bonds. The thermal conductivity shows a 50% decrease for nonstoichiometry high‐entropy carbide, which is a major consequence of the lower electrical conductivity. The oxidation resistance in high temperature water vapor was not sensitive to carbon stoichiometry. [ABSTRACT FROM AUTHOR]

Equiatomic multicomponent transition metal carbides containing different metal element numbers and carbide components were prepared from monocarbides by spark plasma sintering. The compositional effect on their mechanical properties were investigated in detail. Results indicate that all solid solutions exhibit higher hardness, strength, and modulus values than those obtained from rule of mixture, and it is prone to obtain higher mechanical properties in high-entropy compositions. Nevertheless, the variations of hardness, strength, toughness, and modulus show diverse trends upon the change of composition in carbide solid solutions. By comparison and analysis, it was concluded that entropy-related solid solution effect is a crucial parameter influencing the mechanical properties of metal carbides, however, other parameters such as microstructure, type of base monocarbides, phase purity are also negligible parameters which can have big influence on the overall mechanical properties. [ABSTRACT FROM AUTHOR]

Carbide solid solutions show diverse oxidation behaviors at 1000–1400 °C in water vapor. (Ti,Nb,Ta,Hf,Zr)C and (Ti,Nb,Ta,Hf)C exhibit parabolic or near-parabolic oxidation kinetics, while the oxidations of (Ti,Nb,Ta) and (Ti,Nb)C are mainly controlled by linear oxidation kinetics. The oxidation kinetics of (Ti,Nb,Ta,Hf,Zr)C shows a tendency of transition from parabolic to linear oxidation rate law with increasing temperature. On the contrary, a tendency of transition from linear to parabolic rate law with increasing temperature was observed in (Ti,Nb)C, which exhibits the highest oxidation resistance at investigated temperatures. The presence of dense Nb-doped TiO 2 oxidation layer is responsible for the high oxidation resistance. • Compositional influence on oxidation behaviors of metal carbide solid solutions in 1000–1400 °C water vapor was investigated. • Diverse oxidation behaviors were observed for different carbide solutions. • Binary (Ti,Nb)C shows optimized oxidation resistance among all carbides solid solutions investigated. [ABSTRACT FROM AUTHOR]

(Ti 0.8 Nb 0.2)C exhibits high oxidation resistance in water vapor up to 1300 ºC, with the addition of 10% wt. SiC, even higher oxidation resistance could maintain up to 1500 ºC. The oxidation scales of both (Ti 0.8 Nb 0.2)C and (Ti 0.8 Nb 0.2)C-SiC composite show preferential oxidation and steep, opposite chemical gradients of Ti and Nb, implying that the predominant controlling factor is the outward diffusion of the metal elements by the interstitialcy mechanism. Dense and oriented (Ti 1−x Nb x)O 2 oxidation scale accounts for the high oxidation resistance. A continuous and dense SiO 2 layer further improved the oxidation resistance in (Ti 0.8 Nb 0.2)C-SiC composite. • (Ti 0.8 Nb 0.2)C and (Ti 0.8 Nb 0.2)C-SiC with high oxidation resistance in water vapor up to 1500 °C were reported. • The outward diffusion of metal elements through interstitialcy mechanism plays a dominating role. • (Ti 1−x Nb x)O 2 oxidation scale and the contentious SiO 2 layer are responsible for the high oxidation resistance. [ABSTRACT FROM AUTHOR]