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Plasma electrolytic oxidation (PEO) has become an effective surface modification method to form ceramic coatings on orthorhombic Ti 2 AlNb alloy due to its high productivity, economic efficiency and ecological friendliness. In the present work, different contents of Al 2 O 3 additive were introduced into a NaAlO 2 basic electrolyte with the aim of improving the high-temperature properties of PEO ceramic coatings on Ti 2 AlNb alloy. The influence of Al 2 O 3 addition in NaAlO 2 electrolyte on microstructure, adhesive strength, isothermal oxidation behavior and tribological properties of PEO coatings at elevated temperatures was investigated by means of X-ray diffraction, scanning electron microscopy, the pull-off method, isothermal oxidation tests as well as ball-on-disc friction and wear tests. The introduction of Al 2 O 3 into the basic electrolyte promotes the formation of Al 2 TiO 5 phase in the PEO coatings. Isothermal oxidation tests at 800 °C up to 150 h showed that PEO coatings with Al 2 O 3 addition exhibit better oxidation resistance than those formed in the basic electrolyte. The PEO coating formed in the electrolyte with 4 g L−1 Al 2 O 3 additive exhibits the least mass gain of only 1.33 mg cm−2. The micro-hardness of the PEO coatings prepared with Al 2 O 3 additive is improved due to the presence of Al 2 TiO 5 phase, however, the adhesive strength of PEO coating is slightly weakened due to the formation of defects inside ceramic coating. The wear resistance of Ti 2 AlNb alloy is improved significantly by PEO coatings at room temperature due to high hardness and porous surface morphology. The friction coefficient and wear rate of the PEO coatings prepared with Al 2 O 3 addition is comparable to those formed in the basic electrolyte at both room temperature and 600 °C. • Al 2 O 3 additive was used to prepare PEO coatings and the influences were discussed. • Coatings with Al 2 O 3 additive exhibit better high temperature oxidation resistance. • Wear resistance of Ti 2 AlNb alloy is improved by PEO coatings at room temperature. [ABSTRACT FROM AUTHOR]

Plasma electrolytic oxidation (PEO) ceramic coatings were formed on Ti2AlNb alloy in various NaAlO2 electrolytes containing 2 g L−1, 4 g L−1, and 6 g −1 Na2CrO4 additive, respectively. The influence of Na2CrO4 additive in NaAlO2 electrolyte on structure and high-temperature oxidation behavior at 800 °C was investigated. The Na2CrO4 additive in the NaAlO2 electrolyte not only promotes the formation of γ-Al2O3 phase and densification of ceramic coatings, but also participates directly in the growth of ceramic coating to form new Cr3O and (Al0.948Cr0.052)2O3 phases. The PEO ceramic coatings formed in NaAlO2 electrolytes with 2 g L−1 and 4 g L−1 Na2CrO4 additive show better oxidation resistance than those PEO coatings formed in a NaAlO2 basic electrolyte based on isothermal oxidation tests at 800 °C up to 150 h. A thin and uniform isothermally oxidized layer is formed in the interlayer adjacent the substrate, which protects the substrate from the inward diffusion of oxygen and the outward diffusion of metallic elements. The PEO ceramic coatings formed in NaAlO2 electrolyte with 4 g L−1 Na2CrO4 additive exhibit the least mass gain among all the specimens, which is only a half of the ceramic coating formed in a NaAlO2 basic electrolyte without any Na2CrO4 additive. [ABSTRACT FROM AUTHOR]

Porous GdSmZr 2 O 7 (GSZ) with different porosities has been prepared by the solid state reaction, and GSZ-carbonates composites have been prepared by infiltrating (Li 0·52 Na 0.48) 2 CO 3 (LNC) molten carbonates. Phase structure, microstructure and electrical property have been analyzed by X-ray diffractometer (XRD), scanning electron microscope (SEM) and electrochemical impedance spectroscopy (EIS). SEM results show that the voids in porous GSZ are uniformly distributed. The relative density of GSZ-LNC composites obtained by molten salt infiltration is above 93%. The electrical conductivity of the GSZ-LNC composites obtained by molten salt infiltration reaches the highest value of 0.50 S cm−1 at 600 °C, which is much higher than that of GSZ-LNC composites prepared by traditional mechanical mixing method. This excellent electrical property strongly indicates that the GSZ-LNC composite obtained by molten salt infiltration is a promising ionic conductor. [ABSTRACT FROM AUTHOR]

A novel GdSmZr 2 O 7 –(Li 0.52 Na 0.48 ) 2 CO 3 composite electrolyte material is successfully developed by pressureless-sintering for intermediate temperature solid oxide fuel cells. GdSmZr 2 O 7 nanopowders are synthesized by a low cost chemical coprecipitation and calcination method. The phase structure and morphology as a function of the calcination temperature are investigated by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The resulting GdSmZr 2 O 7 nanopowders after calcination at 800 °C for 5 h exhibit good crystallization and uniform particle morphology with an average size of 50 nm. The GdSmZr 2 O 7 –40 wt.% (Li 0.52 Na 0.48 ) 2 CO 3 composite sintered at 600 °C for 1 h has a high conductivity of 0.54 S cm −1 at a temperature of 600 °C. From the Arrhenius curve of the composite, the activation energy has different temperature behaviors around the knee region of 480 °C, where the slope changes and separates the intrinsic oxygen vacancy conduction dominated region from the ‘superionic highway’ region. For GdSmZr 2 O 7 –40 wt.% (Li 0.52 Na 0.48 ) 2 CO 3 composite at temperatures above 480 °C, there is good correlation between ionic diffusivity and conductivity. The introduction of (Li 0.52 Na 0.48 ) 2 CO 3 carbonate into GdSmZr 2 O 7 ceramics significantly promotes the enhancement of its conductivity by one or two orders of magnitude at the intermediate temperatures, especially above 480 °C. A maximum power density of 221 mW cm −2 at 650 °C was achieved for excellent performance of fuel cell based on GSZ–40LN composite electrolyte. [ABSTRACT FROM AUTHOR]

Calcia-magnesia-aluminosilicate (CMAS) corrosion behaviors of hot-pressed (Y 1- x Yb x) 2 SiO 5 ceramics at 1673 K and 1773 K have been investigated. The main corrosion product of nonstoichiometric Ca-RE-Si-O apatite phase evolves with increasing temperature or prolonging corrosion time. The growth of apatite layer at 1673 K follows approximately a linear rate law, however it follows a proximately parabolic rate law at 1773 K. The growth rate of the apatite layer at 1773 K is lower than that at 1673 K in the process of interaction between CMAS and rare-earth monosilicates. Yb-doping mediates the CMAS corrosion resistance of Y 2 SiO 5 effectively by changing the growth rate of apatite layer and the composition of formed apatite phase. This work provides new insights on compositional regulation of (Y 1- x Yb x) 2 SiO 5 EBCs materials to resist CMAS corrosion. [ABSTRACT FROM AUTHOR]

[Display omitted] • The mechanism of phase transition from X2-RE 2 SiO 5 to apatite is revealed. • The formation of apatite in corroded Y 2 SiO 5 is inhibited by introducing Yb. • Silica-rich grain boundary is considered to reduce the corrosion degree. Environmental barrier coatings (EBCs) are usually coated on SiC ceramic or ceramic matrix composites (CMCs), in order to prevent the attack from corrosive media like water vapor and molten oxides. Solid-solutionizing strategy containing high-entropy engineering is considered as an effective way to mediate corrosion resistances of rare-earth silicates, which are promising candidates for the topcoats of EBCs. In this work, high-temperature corrosion behaviors of hot-pressed (Y 1− x Yb x) 2 SiO 5 ceramics have been investigated in a 50 %H 2 O–50 %O 2 environment at temperatures of 1573/1673/1773 K for 100 h. The main corrosion product of Y 2 SiO 5 is apatite phase, whose formation can be distinctly inhibited by introducing Yb. Consequently, introducing Yb avoids the tensile stress and cracks effectively, which are induced by the volume reduction due to the phase transition to apatite at elevated temperatures. The inhibition of apatite formation by introducing Yb is attributed to the relatively lower volatilization pressure of Yb 2 O 3 than Y 2 O 3 and the smaller cationic radius of Yb3+ than Y3+. Meanwhile, the composition deviations between grain boundary and grain surface results in the grain surface corrosion and ridge-like corrosion characteristics. [ABSTRACT FROM AUTHOR]

In the present work, microstructural refinement and mechanical response of Al 2 O 3 –ZrO 2 eutectics fabricated by a pulse discharge plasma assisted melting (PDPAM) method were investigated. The solidified microstructure evolves from polygonal eutectic colonies into irregular cellular colonies with increasing the superheating temperature of the melt from 1820 °C to 1900 °C. The average eutectic spacing inside the colonies decreases from 1.80 ± 0.10 μm to 0.25 ± 0.06 μm, and the coarse inter-colonial structure is refined, which is attributed to the increase in undercooling temperature. High-temperature microstructural stability of Al 2 O 3 –ZrO 2 eutectics is improved significantly as contrasted with the as-sintered ceramics. Besides, the load dependence of Vickers hardness for Al 2 O 3 –ZrO 2 eutectics is investigated. [ABSTRACT FROM AUTHOR]

In this work, CMAS corrosion behavior of hot-pressed (Y 1- x Yb x) 2 SiO 5 ceramics at 1573 K within 32 h has been investigated. The corrosion products contain non-stoichiometric apatite and garnet, which form according to the dissolution-reprecipitation mechanism. With abundant calcia-magnesia-aluminosilicate, the growth rate of apatite layer increases rapidly with prolonging corrosion time, which results in the rapid corrosion especially after 4 h. Furthermore, the measured thickness of apatite layer in (Y 1- x Yb x) 2 SiO 5 is much lower than the predicted value based on the optical basicity (OB) theory. This work provides a new insight of solid-solutionizing strategy in the resistance of rare-earth monosilicate to CMAS corrosion. • The producing conditions of two different CMAS corrosion products are revealed. • The variation on the growth rate of apatite layer over corrosion time is discussed. • The apatite layer thickness is smaller than the predicted value based on OB theory. [ABSTRACT FROM AUTHOR]

• Five kinds of pyrochlore high-entropy rare-earth zirconates have been prepared. • High-entropy strategy improves the electrical conductivity of rare-earth zirconate. • (Nd 0.2 Eu 0.2 Sm 0.2 Gd 0.2 Dy 0.2) 2 Zr 2 O 7 has a conductivity of 7.1 × 10–3 S·cm–1 at 1073 K. • Higher mobile vacancy concentrations, lower activation energy and the preferential O-ion migration pathway result the enhanced ion conductivity. In the present work, a series of new high-entropy rare-earth zirconates were prepared by a solid state reaction method. The crystal structure, microstructure and electrical properties of high-entropy rare-earth zirconates were characterized by means of X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and electrochemical impedance spectroscopy. All the high-entropy rare-earth zirconates have a single-phase pyrochlore structure with a relative density of more than 95.7%. The grain sizes of high-entropy rare-earth zirconates are 1.3–2.4 µm, which are finer than low-entropy rare-earth zirconates prepared by the same process. (Nd 0.2 Sm 0.2 Eu 0.2 Gd 0.2 Dy 0.2) 2 Zr 2 O 7 has the excellent electrical conductivity of 7.1 × 10–3 S·cm–1 at 1073 K. [ABSTRACT FROM AUTHOR]

Four kinds of quinary high-entropy rare-earth (RE) monosilicates and (Y 1- x Yb x) 2 SiO 5 have been synthesized by solid-state reaction method. Selected rare-earth oxides of Lu 2 O 3 to Gd 2 O 3 react with silica to form homogeneous monosilicate solid solutions at a sintering temperature of 1773 K for 4 h. Wherein Gd-Si-O transforms from oxy-apatite type to X2-RE 2 SiO 5 with increasing the synthesis temperature from 1573 K to 1773 K. Increasing the rare-earth ion species changes the distortion degrees of [REO m ] a little bit. However, it's more effective to improve the distortion degrees with only introducing Yb3+ into Y3+ sites of Y 2 SiO 5. Stronger distortion of [REO m ] improves the high-temperature resistance to thermal deformation, resulting in a lower coefficient of thermal expansion (CTE). Due to the highest distortion degree of [REO m ], (Y 0.2 Yb 0.8) 2 SiO 5 exhibits the lowest CTE of 6.78 × 10−6 K−1 at 1473 K. Meanwhile, RE-doping enhances the distortion and phonon scattering, contributing to a lower thermal conductivity. [ABSTRACT FROM AUTHOR]

Perovskite-structured Sn- or Ta-doped Li 0.24–(q –4) x La 0.587 Ti 1– x M x O 3 (LLTM x O, M = Sn or Ta, q is equal to the valence of M, x = 0, 0.02, 0.04, 0.06, 0.08) solid electrolytes are prepared by the pressureless sintering method for Li ion batteries. The phase composition, microstructure and electrical performance as a function of the type of doping elements and the amounts corresponding are studied by XRD, Raman spectrometer, SEM and electrochemical impedance spectroscopy. The LLTM x O pellets sintered at 1350 °C for 12 h have high relative densities (96.2%–97.9%) and high conductivities, where the total conductivity of LLTTa 0.04 O and LLTSn 0.02 O reach 4.09 × 10−4 S cm−1 and 2.98 × 10−4 S cm−1 at room temperature (RT) respectively. Compared with the pure specimen (1.52 × 10−4 S cm−1), the total conductivities of the doped specimens are more than doubled. In addition, Sn or Ta doping can significantly reduce the activation energy, with LLTTa 0.04 O exhibiting the low activation energy of 0.31 eV. The improvement in the total conductivity is attributed to the expansion of the Li ion transmission channels, the improvement of the relative density and the rise in the Li ion vacancy concentration, which are all resulted from the effect of dopant. • LLTM x O with high relative density were synthesized by pressureless stinering. • The LLTTa 0.04 O has a high conductivity of 4.09 × 10−4 S cm−1 at RT. • The LLTTa 0.04 O has a low activation energy of 0.31 eV. • Ta or Sn doping can more than double the total conductivety. • The effects of Sn or Ta doping on the total conductivity were investigated. [ABSTRACT FROM AUTHOR]

The single-phase (Ti 0.2 V 0.2 Nb 0.2 Ta 0.2 Mo 0.2)(C 0.9 N 0.1) high-entropy carbonitride powder with an average particle size of 203 nm and a low oxygen content of 0.2915 wt% was synthesized by the carbothermal reduction-nitridation method at 1600 °C for 4 h. Subsequently, (Ti,V,Nb,Ta,Mo)(C,N) high-entropy carbonitride ceramics were prepared by hot pressing at temperatures of 1600 °C to 1900 °C. Increasing the sintering temperature is conducive to the uniform distribution of metallic elements, however, the hardness of high-entropy carbonitride ceramics is also reduced. (Ti,V,Nb,Ta,Mo)(C,N) ceramic hot-pressed at 1600 °C for 1 h has a relative density of 99.8%, an average grain size of 0.37 ± 0.08 µm, nanoindentation hardness of 32.4 ± 0.9 GPa, Vickers' hardness of 24.0 ± 0.7 GPa at 9.8 N and fracture toughness of 4.87 ± 0.25 MPa·m1/2, which is comparable to those of high-entropy carbides and nitrides. In this study, a novel and convenient method for the preparation of (Ti,V,Nb,Ta,Mo)(C,N) high-entropy carbonitride ceramics was established. • High-entropy carbonitride powders with fine particle size can be synthesized by carbothermal reduction-nitridation method. • The carbon content in the raw powder mixture will affect the single-phase formation ability. • High-performance high-entropy carbonitride ceramics can be prepared from high-entropy carbonitride powders. [ABSTRACT FROM AUTHOR]

Eighteen different 5-component equimolar rare earth zirconates were successfully synthesized by solid state reaction method. Among them, 5 of them exhibit a high-entropy fluorite structure, 8 have a high-entropy pyrochlore structure, and the remaining 5 have dual-phase structures of both pyrochlore and fluorite. For high-entropy rare earth zirconates with single-phase structure, the lattice parameter is almost linearly correlated with the average cation radius ratio. In combination with a large number of previous reports, a summary is made to study the synergistic effects of the average cation radius ratio, configuration entropy and size disorder on the single-phase formation ability of multi-component rare earth zirconates. The conclusion verifies that the size disorder of about 5% is the boundary for the multi-component rare earth zirconates to form a single-phase or dual-phase region, while the average cation radius ratio of 1.467 is the demarcation between a pyrochlore and a defective fluorite structure in the single-phase zone, and the configuration entropy had less effect. Excessive size disorder leads to the enrichment or barrenness of certain components, which is the main reason for the formation of dual phase ceramics. • 18 Different equimolar 5-component rare earth zirconates have been prepared. • The size disorder (δ *) can predict the single-phase formation ability of zirconates. • δ * = 5% is the boundary between the single- and dual-phase rare earth zirconates. • The single-phase forming ability of rare-earth zirconates is independent of entropy. [ABSTRACT FROM AUTHOR]