Rare Earth Silicates Research Articles

Understanding Environmental Barrier Coating Lifetimes and Performance for Industrial Gas Turbines

Abstract Hydrogen or hydrogen blend fuels are expected to replace natural gas in land-based industrial gas turbines (IGTs) to support a greener power economy. Silicon carbide (SiC) base ceramic matrix composites (CMCs) are considered for replacement of Ni-based superalloys to facilitate future efficiency improvements. SiC CMCs require environmental barrier coatings (EBCs) to mitigate volatilization from high-temperature steam, thus making the EBC lifetime critical information for identifying CMC component lifetimes. The goal of this project is to determine the maximum bond coating temperature underneath the EBC for achieving an IGT component lifetime goal of 25,000 h, which is far greater than current CMC component lifetime requirements for aeroturbine applications. To provide data for the lifetime model, laboratory testing used atmospheric plasma-sprayed rare-earth silicate EBCs on monolithic SiC substrates with an intermediate Si bond coating. Specimens exposed to 1-h thermal cycles in flowing air–steam environments and reaction kinetics were assessed from 700 °C to 1350 °C by measuring the thickness of the thermally grown silica scales. The silica growth and phase transformation appear critical in predicting EBC lifetime and several strategies have been explored to reduce the oxide growth rate and improve EBC durability at elevated temperatures. Advanced characterization using Raman spectroscopy has helped clarify this system.

Exploring Sodium Gadolinium Silicate as a Solid Electrolyte for Next-Generation Sodium Batteries

Sodium-ion batteries (SIBs) represent a promising alternative to commercially prevalent Lithium-ion batteries (LIBs), primarily attributed to the economic feasibility and widespread availability of sodium resources. Solid-State Sodium Batteries (SSSBs) have garnered significant attention owing to their capacity to mitigate safety risks associated with organic liquid electrolytes by utilizing solid electrolytes. Additionally, they offer higher energy density and wide operational temperature for next-generation, cost-effective battery technology.[ 1 ] Sodium rare-earth silicates are emerging as a solid electrolyte with enhanced ionic conductivity of 10-3 S cm-1 at room temperature (RT), alongside notable mechanical stability and a wide electrochemical window, attributed to their three-dimensional framework structure.[ 2 ] In this study, sodium gadolinium silicates were synthesized through a conventional solid-state method followed by high-temperature sintering. The highest ionic conductivity of 7.25 x 10-4 S cm-1 at room temperature was achieved through sintering at 1075 °C. Galvanostatic cycling experiments were conducted to assess the compatibility of Na metal with the silicate solid electrolyte, achieving a critical current density (CCD) of 0.4 mA cm-2 and long-term stability for 6000 h at 0.1 mA cm-2. Charge-discharge studies employed a battery configuration comprising Na metal anode, silicate solid electrolyte, and Na3V2(PO4)3 cathode, demonstrating excellent capacity retention of 98% after 100 charge-discharge cycles at 0.1 C.[ 3 ] These findings highlight the potential of sodium gadolinium silicate as a promising solid electrolyte for next-generation sodium batteries, characterized by its high ion conductivity and stable interface compatibility.

In-situ observation and directional growth of Ca2Lu8(SiO4)6O2 in Lu2SiO5 attacked by CMAS at 1500 °C

Excellent resistance to CMAS is one of the key parameters for environmental barrier coating (EBC) operated in gas turbine engines. The interaction between CMAS and EBC was usually investigated by the characterization of reaction products after corrosion. The lack of direct observation of the high-temperature CMAS corrosion processes hinders the understanding of CMAS-induced degradation of EBC. In this work, the interaction between CMAS and an EBC candidate (Lu2SiO5) was investigated by an in-situ observation method, where the melting of CMAS and the growth of reaction products were first observed. It was found that the reaction product Ca2Lu8(SiO4)6O2 tends to precipitate vertically to form a textured structure. In addition, Lu2SiO5 tends to decompose into Lu2Si2O7 and Lu2O3 at 1500 °C, which induces an intergranular infiltration of CMAS and leads to the formation of Ca2Lu8(SiO4)6O2 fringes. Compared with the corrosion behavior at 1300 °C, Lu2SiO5 exhibits fast interaction with CMAS and poor CMAS resistance at 1500 °C. The results provide important guidelines on the design of rare earth silicate EBC.

Exploring high-performance environmental barrier coatings for rare earth silicates: A combined approach of first principles calculations and machine learning

RE2Si2O7 is promising materials for environmental barrier coating (EBC), but the vast phase space poses challenges for the screening of RE2Si2O7. It follows that a combined approach of first principles calculations and machine learning is proposed for this problem, with establishing a comprehensive database comprising β-, γ- and δ-RE2Si2O7 (RE=La–Lu, Y, Sc) and correlating their mechanical/thermal properties on structural characteristics. It is revealed the [O3Si–O–SiO3] structure and polyhedron distortion affect mechanical properties of RE2Si2O7, while criteria for selecting RE2Si2O7 with low thermal conductivity are identified, including complex crystal structures, chemical bond inhomogeneity, and strong non-harmonic lattice vibrations. Also, the machine learning model accurately predicts the coefficient of thermal expansion (CTE) and minimum thermal conductivity (λmin) of RE2Si2O7, with volume and mass variations identified as critical factors, respectively. This integrated approach efficiently screens RE2Si2O7 for EBC application and enables rapid assessments of their thermal properties.

High-throughput composition screening of high-entropy rare-earth monosilicates for superior CMAS corrosion resistance up to 1873 K

Superior calcium-magnesium-aluminum-silicate (CMAS) corrosion resistance is the key to the applications of high-entropy rare-earth monosilicates (HEREMs) as environmental barrier coatings (EBCs) for next-generation gas turbine engines. Here, we report a high-throughput composition screening strategy to achieve superior CMAS corrosion resistance in HEREMs for the first time. Specifically, we first fabricate ten kinds of (Ho0.25Yb0.25Lu0.25X0.25)2SiO5 (X = Sc, Tm, Er, Y, Dy, Gd, Eu, Sm, Nd, and La) HEREM samples using a high-throughput pressure-less sintering technique, and subsequently screen their CMAS corrosion resistance at 1673 K via a high-throughput testing apparatus. Among all the as-fabricated samples, the HEREM-Er samples are found to have the best CMAS corrosion resistance, even up to 1873 K, which outperforms the EBC materials that have been reported. Such superior CMAS corrosion resistance of the HEREM-Er samples is primarily attributed to the formation of a dense apatite barrier layer, which results from its decreased melting point, as confirmed by first-principles calculations and molecular dynamic simulations. Our work provides a guide for designing exceptional anti-CMAS corrosion high-entropy rare-earth silicates.

Structural, optical and photoluminescence characteristics of apatite type lanthanide silicates

This article reports the solid state synthesis of apatite type rare earth-transition metal silicates Nd4MnSi3O13 and Sm4MnSi3O13. These oxyapatite type lanthanide silicates were characterized through structural, micro structural, optical and photoluminescence properties. The indexed X-ray diffraction patterns and Raman analysis confirm that both the samples crystallize in single phase hexagonal symmetry with space group P63/m. Transmission electron microscopy images along with electron diffraction pattern indicate the good crystalline nature of both the samples which further support the results obtained from X-ray diffraction study. The optical band gap energies are found to be 4.1 eV and 3.9 eV for Nd4MnSi3O13 and Sm4MnSi3O13 respectively. The wide optical band gap values of these semiconductor silicates make enable them for fundamental and technological applications. The luminescent properties and CIE coordinates showed the green-yellow emission of these synthesized silicates which further indicate that these compounds may have uses in solid state lighting because of the tunable luminescence.

Rapid combustion synthesis of high-entropy rare-earth silicate nanopowders via SiO2 templates

The synthesis of high-quality nanopowders holds immense significance in the advancement of high-performance high-entropy rare-earth silicates (HERESs). Here, for the first time, we present the synthesis of HERES nanopowders, specifically high-entropy rare-earth monosilicates (HEREMs) and disilicates (HEREDs), using a rapid combustion synthesis method. Benefiting from the shortened synthesis time and the utilization of ultrafine nano SiO2 templates, grain coarsening can be effectively hindered. As a result, the single-phase HEREM nanopowders (∼60 nm in size) with homogeneously distributed elements can be successfully synthesized. Additionally, the feasibility of this method for the HERED synthesis is also investigated. This research provides an efficient approach to synthesizing high-quality HERES nanopowders.

Investigation on improving the comprehensive performance of environmental barrier coating materials by high-entropy multiphase design

It is difficult to obtain a single-phase environmental barrier coating material that simultaneously offers the advantages of low thermal conductivity, a suitable coefficient of thermal expansion, and excellent corrosion resistance. Herein, to synthesize the advantages of single-phase materials, we have developed an effective approach for the design of high-entropy multiphase ceramics of rare earth oxides and silicates. Such a specific design approach is capable of making high-entropy RE2SiO5/RE2O3 and RE2SiO5/RE2Si2O7 (RE = Lu, Yb, Tm, Er, Ho, and Y) multiphase ceramics as two types of potential environmental barrier coating materials for Al2O3f/Al2O3 and SiCf/SiC ceramic matrix composites.

Mechanical and thermal properties for corrosion products of lutetium silicates against CMAS

AbstractRare earth silicates are promising environmental/thermal barrier coating (E/TBC) materials facing severe CMAS (CaO‐MgO‐Al2O3‐SiO2) corrosion. Previous studies mainly focused on the intrinsic properties of precorrosion coatings, but there were few studies on their CMAS corrosion products that play a crucial role in the performance of coatings in postservice stage. In this work, the mechanical and thermal properties of nine corrosion products between lutetium silicates and CMAS are studied using first‐principles calculations. Their differences of elastic stiffness are attributed to the different crystal structures and bonding strength. The T:O ratio is identified as a factor of the crystal structure for silicate products, and it has a good correlation with their elastic stiffness. Moreover, the divergences of thermal conductivity are dominated by three essential factors, that is, atomic vibration intensity, lattice vibrational anharmonicity, and complexity of crystal structure. Compared with rare earth silicates, six products, that is, the α‐CaSiO3, β‐CaSiO3, Ca2MgSi2O7, Ca2Al2SiO7, CaAl2Si2O8, and Ca2Lu8(SiO4)6O2, showing good damage tolerance and low thermal conductivities, are predicted to be advantageous to E/TBCs. These discoveries reveal the mechanical/thermal properties of corrosion products between lutetium silicates and CMAS and are expected to support the future researches on the performance of E/TBC in the postservice stage.

Phase and properties evolution of plasma sprayed rare earth silicate coatings

Rare-earth silicates have been pursued for many years as candidates for Environmental Barrier Coatings for the protection of silicon carbide (SiC) based components in the new generation of aero and power production turbine engines. The properties and behavior of these materials are usually reported based on theoretical studies or measurements done on bulk specimens, but their prospective use is in the form of coatings. One of the most extended ways of depositing these coatings is plasma spray which produces partially or totally amorphous deposit and changes towards SiO2 lean chemical composition. In this work is comparatively discussed the effect of plasma spray deposition on Yb2Si2O7, Y2Si2O7 and the mixed cation silicate (Y0.5Yb0.5)2Si2O7 coatings. The phase evolution with temperature and its impact on the coefficient of thermal expansion, microstructure and thermal conductivity are described. The stability of crystalline phases, properties and absence of cracks in its microstructure makes (Y0.5Yb0.5)2Si2O7 a more optimum candidate among the studied rare-earth silicates for environmental barrier applications.

Influence of chemistry and surface roughness of various thermal barrier coatings on the wettability of molten sand

High temperature wettability experiments were carried out to investigate how chemistry and surface roughness affect wetting characteristics of molten sand several promising thermal/environmental barrier coatings. Contact angle experiments were performed on varying as-sprayed and polished yttria-stabilized zirconia (YSZ), rare-earth oxide-YSZ (REO-YSZ), and rare-earth silicate coatings subjected to molten synthetic sand at 1260 °C. The results showed that the spreading rate of molten sand was higher on silicate-based environmental barrier coatings (~60°/min) than on Zr-based thermal barrier coatings (~25°/min). Increased surface roughness had opposite effects on the wettability depending on the reactivity of the coatings. Wettability decreased on coatings such as YSZ (which did not react with sand) but increased for REO-YSZ coatings (which formed reaction products with sand). It was found that while the REO-YSZ coatings were able to promote formation of the apatite that performed as a sealing layer to reduce further infiltration, molten sand wetting increased. YSZ was found to have greater wetting resistance in the roughened state while the REO-YSZ coatings had greater wetting resistance in the polished state. Surface energy, microstructure, and molten sand penetration also play a key role in the wetting kinetics and are discussed. This work reveals the dependency of chemistry and surface roughness on influencing the “sandphobicity” (analogous to hydrophobicity) of a material and provides insights for optimizing these variables to realize more “sandphobic” coatings.

A thermodynamic study of the influence of the Al2O3 content on the CaO-SiO2-YO1.5 system

In aircraft engines applications, the interactions between rare earth silicates (mainly yttrium silicates) used as Environmental Barrier Coatings (EBCs) on turbine blades and liquid calcium-magnesium-aluminosilicates (CMAS) due to the particles ingested by the engines may lead to severe damages at high temperature. Due to the large range of composition of CMAS, simplified model systems must be used to have a good comprehension of the thermochemical phenomena leading to degradation. The interaction between ceramics and CMAS can lead to the precipitation of phases as apatite Ca2Y8(SiO4)6O2 and cyclosilicate Ca3Y2(Si3O9)2 which might provide protection to the EBC. However, their thermodynamic stability depends on many parameters. In the present work, the influence of alumina Al2O3 addition to the ternary system CaO-SiO2-YO1.5 has been studied in order to verify the equilibrium with the liquid field at 1300 °C.

A review on multicomponent rare earth silicate environmental barrier coatings

Rare earth (RE) silicate environmental barrier coatings (EBCs), with superior physicochemical properties and resistance to hot water-oxygen and calcium-magnesium-aluminosilicate (CMAS) corrosion, are ideal candidates for safeguarding the next-generation aviation engines made of SiC ceramic matrix composites (CMCs). However, improving the corrosion resistance of RE silicates remains a critical challenge due to severe environmental degradation at 1500 °C. Herein, by doping multiple RE elements and designing high entropy formulations, researchers found that multicomponent RE silicates could establish stable lattice structures, phase compositions, and reaction products while impeding the infiltration of corrosive agents. This paper reviews the modulation of their phase stability, physicochemical properties, wettability, and corrosion behaviors. It looks at how state-of-the-art optimization solutions of multicomponent and high entropy doping strategies affected corrosion resistant behaviors. In conclusion, the authors suggest further studies to optimize the corrosion resistance of multicomponent RE silicates and highlight the importance of integrating multicomponent doping with high-throughput and other optimization methods, thereby unlocking an array of possibilities for advancing RE silicate EBCs.

X-ray excited optical luminescence from lanthanide ions in double phosphate and double silicate crystalline hosts

X-ray Excited Optical Luminescence (XEOL) from rare earth doped whitlockite phosphates and silicate garnets [Ca9La(PO4)7 and Ca9Lu(PO4)7, doped with 1 mol% Ce3+; Ca3Sc2Si3O12, doped with 0.5 mol% Pr3+, 1 mol% Eu3+ and 1 mol% Tb3+], was obtained upon X-ray excitation with a tunable synchrotron light source. Excitation at photon energies across the M5,4-edge of the dopants reveals additional information not normally obtainable with conventional UV/visible or an X-ray line source (Al Kα). It is found that despite the low concentration, photoluminescence yields excited with photon energy at the M5,4-edge of the rare-earth dopant can positively and negatively affect the overall energy transfer to the optical center. The implication of these observations is discussed. The application of XEOL in the investigation of X-ray scintillators is illustrated.

Nanomechanical characterization of nanostructured Lu2SiO5 environmental barrier coatings by nano-indentation

Nanostructured rare-earth silicates are widely considered as environmental barrier coatings for the new generation of silicon-based composites. In current work, the nanostructured Lu2SiO5 coatings were deposited successfully using nanostructured Lu2SiO5 feedstocks by atmospheric plasma spraying (APS). Moreover, the nanomechanical properties of Lu2SiO5 coatings were characterized systematically by nano-indentation. Results indicate that the grain growth of Lu2SiO5 coatings is not obvious compared to Lu2SiO5 feedstocks. The nanostructured Lu2SiO5 coatings exhibit bi-modal nanomechanical properties. The elastic modulus and nanohardness of Lu2SiO5 coatings are 148.84 ± 28.00 GPa and 8.51 ± 4.12 GPa, respectively. In addition, the elastic work and plastic work of Lu2SiO5 coatings are 10.387 ± 1.283 nJ and 18.437 ± 6.823 nJ, respectively. The proportion of consumption of energy of Lu2SiO5 coatings is 0.636 ± 0.099 during indentation, which means that nanostructured Lu2SiO5 coatings process an excellent wear resistance.

Crystal Growth of the R2SiO5 Compounds (R = Dy, Ho, and Er) by the Floating Zone Method Using a Laser-Diode-Heated Furnace

In recent years, rare earth silicate compounds have attracted the extensive attention of researchers owing to their potential for applications in scintillation crystals in gamma ray or X-ray detectors, as well as in thermal or environmental barrier coatings. Large high quality crystals of three members of the rare earth monosilicates family of compounds, R2SiO5 (with R = Dy, Ho, and Er), have been grown by the floating zone method, using a laser-diode-heated floating zone furnace. Crystal growths attempts were carried out using different parameters in order to determine the optimum conditions for the growth of these materials. The phase purity and the crystalline quality of the crystal boules were analysed using powder and Laue X-ray diffraction. Single crystal X-ray diffraction experiments were carried out to determine the crystal structures of the boules. The optimum conditions used for the crystal growth of R2SiO5 materials are reported. The phase purity and high crystalline quality of the crystals produced makes them ideal for detailed investigations of the intrinsic physical and chemical properties of these materials.

Integrating State of the Art Zirconia Thermal Barriers with Ytterbium Silicate Environmental Barriers for Silicon-Based Ceramic Turbine Components

AbstractAs gas turbine firing temperatures continue to increase for the sake of improved operating efficiency, the material's transition from Ni-based superalloy components toward ceramic matrix composites (CMCs) is concurrently in progress. Due to the complex nature of the turbine operating environment (envisaged ultrahigh temperatures, presence of water vapor, etc.), coating solutions for these CMCs are still on the forefront of design optimizations. Typically, rare-earth (RE) silicate environmental barrier coatings (EBCs) have been utilized to protect the CMCs from impinging water vapor; however, they lack the thermal insulation properties to enable continued use of simple and/or easily accessible bond coat materials (i.e., silicon). Combined thermal-environmental barrier coatings (T-EBCs) are such a multifaceted surface solution. T-EBCs have been considered in the past, but to this point have not been demonstrated to be technologically robust either due to high implementation costs or complex processing. This study utilizes and combines straightforward and well-established processes—such as plasma-sprayed 7 wt.% yttria-stabilized zirconia—to demonstrate the feasibility of MultiLayered T-EBCs comprised of zirconia-based oxides and RE silicate EBCs in a single coating. The results show that despite high thermal mismatch strains, the structures cannot only be deposited, but also in certain circumstances sustain cyclic thermomechanical loading.

Effect of lattice distortion in high-entropy RE2Si2O7 and RE2SiO5 (RE=Ho, Er, Y, Yb, and Sc) on their thermal conductivity: Experimental and molecular dynamic simulation study

High-entropy materials are considered to be born with lattice distortion, which is still lack of comprehensive investigation in rare earth silicates to date. In this paper, we confirmed the existence of lattice distortions in high-entropy rare-earth silicates via experiments and molecular dynamic simulations. The effects of the lattice distortion on the thermophysical properties are elucidated. Simulation results indicate the lattice distortion is present in both cation and anion sublattice, leading to the compressing and stretching of atomic bond as well as fluctuation of atomic bond strength. Accordingly, lattice distortion in the Si and O sublattice is also verified by the Raman spectra. Furthermore, the thermal conductivity of high-entropy rare earth silicates remarkably reduces and exhibits glass-like behavior, which is confirmed by experiments in cooperation with molecular dynamic simulations. Simulation also reveals that the lifetimes and group velocities of vibrational modes are significantly reduced by lattice distortion, which result in the reduction of overall thermal conductivity of high-entropy rare-earth silicates.

Green preparation of high entropy ceramics (Y0.2Sm0.2Eu0.2Er0.2Yb0.2)2SiO5 with low thermal conductivity by molten salt synthesis

High entropy rare earth silicate ceramics have been widely studied as potential material for environmental barrier coatings (EBCs). At present, the synthesis method of high entropy rare earth silicate ceramics is generally solid state reaction method, which has some problems such as high energy consumption, low efficiency and low purity of synthesized products. In order to solve these problems, high entropy (Y0.2Sm0.2Eu0.2Er0.2Yb0.2)2SiO5 was prepared by molten salt synthesis (MSS). By adjusting the reaction temperature and the amount of molten salt, it was found that the pure high entropy (Y0.2Sm0.2Eu0.2Er0.2Yb0.2)2SiO5 ceramic powders could be synthesized at 1400 °C when Na2SO4 and K2SO4 are selected as molten salts(Na+:K+ = 1:1), and the mass ratio of molten salt to reactant is 1: 1. The molten salt synthesis significantly reduced the reaction temperature and accelerated the reaction rate. The prepared high entropy (Y0.2Sm0.2Eu0.2Er0.2Yb0.2)2SiO5 has a monoclinic X2-type Re2SiO5 structure and a low thermal conductivity of 0.96 ± 0.03 W m-1 K-1 due to the high entropy effect. This can be explained by the existence of high concentration oxygen vacancies and the highly disordered arrangement of multicomponent cations in a unique high entropy configuration. This material also shows good infrared reflectance, which makes (Y0.2Sm0.2Eu0.2Er0.2Yb0.2)2SiO5 a potential environmental barrier coating material.

Constructing flake-like ternary rare earth Pr3Si2C2 ceramic on SiC whiskers to enhance electromagnetic wave absorption properties

In this work, flaky-like ternary Pr3Si2C2 was successfully coated in SiC whiskers (SiCw) via a molten salt process to augment the electromagnetic wave absorption (EWA) efficiency and expand the application potential of ternary rare earth silicate (RE3Si2C2) materials. Comprehensive investigations into the phase composition, morphological characteristics, EWA capacity, and corresponding mechanisms of the material were conducted. Furthermore, the electric field distribution is simulated and calculated for both vertical and horizontal incidence of the electromagnetic wave (EMW) on the specimens. Results showed that the incorporation of Pr3Si2C2 coating on SiC whiskers induces the development of heterogeneous interfaces and conductive networks, thereby augmenting polarization loss and conductive polarization, correspondingly enhancing the EWA performance of the material. In synergy with excellent attenuation ability and enhanced impedance matching, the Pr3Si2C2/SiCw composite demonstrates superior EWA performance in comparison to pristine SiCw. The minimum reflection loss (RLmin) of Pr3Si2C2/SiCw composites was achieved −48.117 dB accompanied by the effective absorption bandwidth (EAB) of 5.04 GHz at a thin thickness of merely 2.71 mm in the frequency range of 2–18 GHz. This research will serve as a valuable reference for the investigation of ternary rare earth ceramic-based EWA material.