Rare-earth Disilicates Research Articles

Microstructure and water corrosion behavior of (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7 high-entropy rare-earth disilicate coating for SiC coated C/C composites

In order to make carbon/carbon composites suitable for application in gas turbine engine, it is necessary to develop environmental barrier coatings (EBCs) to protect them from reacting with water vapor. In our previous work, a novel high-entropy rare-earth disilicate (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7 ((5RE0.2)2Si2O7) has been developed and verified as a promising candidate for EBCs. In this work, the (5RE0.2)2Si2O7 coating was synthesized on the surface of SiC coated C/C composites by supersonic atmospheric plasma spraying method. The protective performance and mechanism of this coating under high temperature water vapor environment was explored in detail. Results showed that the weight change of the sample coated with (5RE0.2)2Si2O7 was only 0.2% after corrosion for 100 h at 1500 ºC, which proved that (5RE0.2)2Si2O7 coating could significantly improve the resistance of C/C composites against water vapor corrosion. This work may provide theoretical basis for the design and application of high-entropy rare-earth silicates as EBCs.

Crystallization and phase evolution in novel (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 environmental barrier coating

AbstractDesign of multiple rare‐earth (RE) principal elements is a practical strategy to optimize key property of RE disilicate environmental barrier coating (EBC). However, multi‐RE component in the feedstocks may also introduce critical fabrication problems due to the different evaporating and crystallizing behaviors of the EBC containing various RE composition. We herein report the preparation of a novel multi‐RE disilicate (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7 EBC by atmospheric plasma spraying (APS). The crystallization behavior of the as‐deposited coating annealed at 1100°C–1300°C with different durations (5 and 20 h) in air and argon atmospheres were investigated through comprehensive characterization techniques. RE disilicate, RE mono‐silicate, and RE apatite were detected in the as‐sprayed coatings, and mono‐silicate and apatite transform to RE2Si2O7 with the increment of temperature and extension of annealing duration. Furthermore, the contribution of multiple RE to the formation of apatite phase is discussed. The results provide a fundamental understanding and may guide the controllable preparation of multi‐RE disilicate EBC via APS method.

(Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2Si2O7: A novel high-entropy rare earth disilicate porous ceramics with high porosity and low thermal conductivity

Herein, a novel quinary high-entropy rare earth disilicate (Sm0.2Eu0.2Tb0.2Dy0.2Lu0.2)2Si2O7 ((5RE1/5)2Si2O7) porous ceramics were synthesized through a sol-gel/vacuum freeze-drying method with high-temperature calcination process. The microstructure, phase composition, thermal conductivity, compressive strength, and thermal stability of the as-prepared (5RE1/5)2Si2O7 porous ceramics were systematically investigated. The results indicated that the samples possess high porosity of 76.83%, pore size of 0.3–1.2 μm, homogeneous microstructure with small grain size of 400–800 nm, which results in low thermal conductivity of 0.94 W/(m·K) at 1273 K and high compressive strength of 6.55 MPa. These excellent properties indicated that as-prepared (5RE1/5)2Si2O7 porous ceramics has a great potential application value in the field of high-temperature thermal insulation materials.

High-entropy rare-earth disilicate (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7: A potential environmental barrier coating material

A new high-entropy ceramic (Lu0.2Yb0.2Er0.2Tm0.2Sc0.2)2Si2O7 ((5RE0.2)2Si2O7) was proposed as a potential environmental barrier coating (EBC) material for ceramics matrix composites in this work. Experimental results showed that the (5RE0.2)2Si2O7 synthesized by solid-phase sintering was a monoclinic solid solution and had good phase stability proved by no obvious absorption/exothermic peak in the DSC curve from room temperature to 1400 °C. It performed a lower coefficient of thermal expansion (2.08 ×10−6-4.03 ×10−6 °C−1) and thermal conductivity (1.76–2.99 W•m−1•°C−1) compared with the five single principal RE2Si2O7. In water vapor corrosion tests, (5RE0.2)2Si2O7 also exhibited better water vapor corrosion resistance attributed to the multiple doping effects. The weight loss was only 3.1831 × 10−5 g•cm−2 after 200 h corrosion at 1500 °C, which was lower than that of each single principal RE2Si2O7. Therefore, (5RE0.2)2Si2O7 could be regarded as a remarkable candidate for EBCs.

Interaction of multicomponent disilicate (Yb0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7 with molten calcia-magnesia-aluminosilicate

Environmental barrier coating (EBC) materials that are resistant against molten calcia-magnesia-aluminosilicate (CMAS) corrosion are urgently required. Herein, multicomponent rare-earth (RE) disilicate ((Yb0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7, (5RE)2Si2O7) was investigated with regard to its CMAS interaction behavior at 1400 °C. Compared with the individual RE disilicates, the (5RE)2Si2O7 material exhibited improved resistance against CMAS attack. The dominant process involved in the interaction of (5RE)2Si2O7 with CMAS was reaction-recrystallization. A dense and continuous reaction layer protected the substrate from rapid corrosion at high temperatures. The results demonstrated that multicomponent strategy of RE species in disilicate can provide a new perspective in the development of promising EBC materials with improved corrosion resistance.

Recession of Environmental Barrier Coatings under High-Temperature Water Vapour Conditions: A Theoretical Model.

Rare-earth disilicates are the major material used on the top layer of environmental barrier coating (EBC) systems. Although rare-earth disilicates are highly resistant to water vapour, corrosion due to water vapour at high temperature is still one of the main reasons of failure of EBC systems. In this study, a corrosion model of ytterbium disilicates in water vapour at high temperature was derived, based on the gas diffusion theory. Using this theoretical model, we studied the evolution rule of the corroded area on the top layer of the EBC under gas flow at high temperature. The influence of the various parameters of the external gas on the corrosion process and the corrosion kinetics curve were also discussed. The theoretical model shows that the increase in gas temperature, gas flow velocity, water partial pressure, and total gas pressure accelerate coating corrosion. Among these factors, the influence of total gas pressure on the corrosion process is relatively weak, and the effect of the continuous increase of the gas velocity on the corrosion process is limited. The shape of the corrosion kinetics curve is either a straight or parabolic, and it was determined by a combination of external gas parameters.

Optical Floating Zone Crystal Growth of Rare-Earth Disilicates, R2Si2O7 (R = Er, Ho, and Tm)

The wealth of structural phases seen in the rare-earth disilicate compounds promises an equally rich range of interesting magnetic properties. We report on the crystal growth by the optical floatin...

High temperature corrosion of (Er0.25Tm0.25Yb0.25Lu0.25)2Si2O7 environmental barrier coating material subjected to water vapor and molten calcium–magnesium–aluminosilicate (CMAS)

Environmental barrier coatings (EBCs) are being developed to improve the durability of SiCf/SiC CMC components against harsh combustion environment. Among the most promising EBC candidates, rare-earth (RE) disilicates attract attentions for the good thermal and chemical compatibility with SiC CMCs. We herein investigate the high temperature corrosion resistances of a new multicomponent β-(Er0.25Tm0.25Yb0.25Lu0.25)2Si2O7 disilicate subjected to hot water vapor and molten CMAS. Multicomponent (Er0.25Tm0.25Yb0.25Lu0.25)2Si2O7 shows superior water vapor corrosion resistance at 1400 °C and significantly enhanced CMAS resistance at 1500 °C compared with single principal RE disilicates. The present results highlight the merit of multiple RE engineering in advanced EBCs for the improvement of crucial high temperature corrosion resistances in engine applications.

Transition from penetration cracking to spallation in environmental barrier coatings on ceramic composites

The article addresses the competition between driving forces for penetration cracking and for spallation in bilayer environmental barrier coatings on SiC-based composites. Coatings of interest consist of an outer rare-earth monosilicate (MS) and an inner rare-earth disilicate (DS). Finite element analysis is used to compute energy release rates (ERR) for cracking as functions of misfit strains, layer thicknesses and size of putative cracks. The results indicate that, for certain property combinations, ERRs for spallation in the DS layer exceed those for continued penetration. The implication is that penetration cracks could branch and cause coating spallation. A case study comparing bilayers of Yb- and Y-silicates reveals domains in which (i) the tolerable thickness of MS is high, but eventual spallation could potentially remove the entire DS coating along with the adjoining MS, and (ii) the tolerable thickness of MS is low, but, if it were to occur, spallation would follow a path near the MS/DS interface, leaving most of the DS intact. The behaviors prove to be sensitive to the thermal expansion coefficient of the underlying composite, even over the rather narrow range of values typical of SiC-based composites.

High-Temperature thermochemical interactions of molten silicates with Yb2Si2O7 and Y2Si2O7 environmental barrier coating materials

The thermochemical behavior of EBC candidate materials yttrium disilicate (Y2Si2O7) and ytterbium disilicate (Yb2Si2O7) was evaluated with three calcium-magnesium-aluminosilicate (CMAS) glasses possessing CaO:SiO2 ratios relevant to gas turbine systems. Pellet mixtures of 50 mol% EBC powder to 50 mol% CMAS glass powder were heat treated at 1200°C, 1300°C, and 1400°C. The products of these interactions were evaluated using X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy. Above glass melting temperatures, exposure of the disilicates primarily resulted in dissolution into the molten glass followed by precipitation of a Ca2RE8(SiO4)6O2 (RE = Yb3+, Y3+) apatite-type silicate and/or rare earth disilicate. In glasses with high CaO concentrations, apatite readily forms while the disilicate material is consumed by the reaction. As CaO content decreases, the disilicate phase becomes the main reaction product. Overall, reactions with yttrium disilicate favored more apatite crystallization than ytterbium disilicate. The viability of using these disilicates in various operating environments is discussed.

Optimal rare-earth disilicates as top coat in multilayer environmental barrier coatings

In last decade, rare-earth disilicates have been considered as candidate materials for top coat in multilayer environmental barrier coatings (EBCs). However, optimal coating material has not yet been found. Previous studies revealed that parabolic rate constant of thermally grown silica scale (silica-TGO) beneath EBCs played a key role in the durability of SiC matrix ceramic composites coated with EBCs. To determine optimal coating material, in this study, Lu2Si2O7 and Sc2Si2O7 coatings were first corroded under quasi-static water-vapor corrosion conditions. The evolution of silica-TGO beneath coatings was then examined in detail. Accordingly, parabolic rate constants of silica-TGO growth were calculated. Compared with parabolic rate constants of silica-TGO growth beneath barium-strontium aluminosilicate (BSAS), Yb2Si2O7, and Y2Si2O7-BSAS coatings in previous studies, Y2Si2O7-BSAS coating shows the lowest parabolic rate constant and growth rate of silica-TGO. Crystal structure of rare-earth disilicates was also studied. Results showed that bond lengths of Y–O and Si–O in Y2Si2O7 were short, indicating that it possessed low ionic oxygen permeability. This resulted in its low parabolic rate constant of silica-TGO growth. Therefore, Y2Si2O7 is the optimal material as top coat in multilayer EBCs.

Formation and growth of silica layer beneath environmental barrier coatings under water-vapor environment

Environmental barrier coatings (EBCs) have widely been employed as protective coatings for SiC-based ceramic matrix composites (CMC-SiCs) against combustion corrosion in advanced gas turbines. During service, a layer of oxides forms beneath EBCs, which plays key role in lifetime of coated CMC-SiC parts. In this study, the kinetics of thermally grown silica oxide (silica-TGO) were examined through rare earth disilicates coated CMC-SiC composites subjected to corrosion under 50%H2O-50%O2 gas flow environment. Results revealed that growth of silica-TGO followed parabolic behavior, indicating diffusion controlled process. Calculated growth rates and activation energies of silica-TGO suggested that oxygen ion was the main active oxidant involved at relatively low partial pressure of water vapor. Corrosion environments containing different H2O/O2 ratios were also investigated and the data indicated that water vapor was also involved in growth of silica-TGO. The role of EBCs in corrosion process was also analyzed and discussed.

Thermal Cycling and High-Temperature Corrosion Tests of Rare Earth Silicate Environmental Barrier Coatings

Lutetium and yttrium silicates, enriched with an additional secondary zirconia phase, environmental barrier coatings were synthesized by the solution precursor plasma spraying process on silicon carbide substrates. A custom-made oven was designed for thermal cycling and water vapor corrosion testing. The oven can test four specimens simultaneously and allows to evaluate environmental barrier performances under similar corrosion kinetics compared to turbine engines. Coatings structural evolution has been observed by SEM on the polished cross sections, and phase composition has been analyzed by XRD. All coatings have been thermally cycled between 1300 °C and the ambient temperature, without spallation, due to their porosity and the presence of additional secondary phase which increases the thermal cycling resistance. During water vapor exposure at 1200 °C, rare earth disilicates showed a good stability, which is contradictory with the literature, due to impurities—such as Si- and Al-hydroxides—in the water vapor jets. The presence of vertical cracks allowed the water vapor to reach the substrate and then to corrode it. It has been observed that thin vertical cracks induced some spallation after 24 h of corrosion.

Exploration of the low thermal conductivities of γ-Y2Si2O7, β-Y2Si2O7, β-Yb2Si2O7, and β-Lu2Si2O7 as novel environmental barrier coating candidates

Thermal conductivities of γ-Y2Si2O7, β-Y2Si2O7, β-Yb2Si2O7, and β-Lu2Si2O7 were investigated by combined first-principles calculations and experimental evaluation. Theoretical calculation was used to predict the elastic properties, anisotropic minimum thermal conductivities, and temperature dependent lattice thermal conductivities. Experimentally, thermal conductivities of these disilicates were measured from room temperature to 1273K. In addition, their experimental intrinsic lattice thermal conductivities were determined from the corrected thermal diffusivity data after removing the extrinsic contributions from phonon scattering by defects and thermal radiation. The experimental lattice thermal conductivities match well with the theoretical predictions. Furthermore, Raman spectra of the disilicates was measured and used to estimate the optical phonon relaxation time. The present results clearly disclose the specific material parameters that determine the low thermal conductivity of RE2Si2O7 and may provide guidelines for the optimal thermal conductivity of rare earth disilicates.

White light emission of Eu3+/Ag co-doped Y2Si2O7

The Eu3+/Ag co-doped rare earth disilicate Y2Si2O7 microcrystal was synthesized by sol-gel method. Through controlling the thermal treatment process of Y2Si2O7:Eu3+/Ag precursor, various phases (amorphous, α, β, γ, δ) were prepared. White light emission was observed under UV light excitation in the samples heavily doped with Ag. The white light was realized by combining the intense red emission of Eu3+, the green emission attributed to the very small molecule-like, non-plasmonic Ag particles (ML-Ag-particles), and the blue emission due to Ag ions. Results demonstrated that Eu3+/Ag co-doped Y2Si2O7 microcrystal could be potentially applied as white light emission phosphors for UV LED chips.

Calcium–magnesium–aluminosilicate corrosion behaviors of rare-earth disilicates at 1400°C

Abstract Environmental barrier coatings (EBCs) are used to prevent oxidation of underlying ceramic matrix composite (CMC) structural components in gas turbines. When the siliceous minerals deposit on the surface of EBCs, a glassy melt of calcium–magnesium–aluminosilicate (CMAS) will be formed, leading to the EBCs degradation. In this study, seven rare-earth disilicates (RE2Si2O7, RE = Yb, Lu, La, Gd, Eu, Sc, and Y) were fabricated to analyze their CMAS corrosion behaviors. The results indicated that the RE2Si2O7 could react with the CMAS in the temperature range of 1250–1350 °C. Reaction zones formed at the interfaces. For the Yb2Si2O7, Lu2Si2O7, La2Si2O7, Eu2Si2O7 and Gd2Si2O7, the reaction zones dissolved into the molten CMAS and separated from the RE2Si2O7. As for the Sc2Si2O7 and Y2Si2O7, the reaction zones could stay at the interface. They could effectively block the molten CMAS to penetrate into the RE2Si2O7 and protect them from CMAS corrosion.

Powder synthesis and electrical, dielectric spectroscopic characterization of rare earth disilicates (D-Er2Si2O7): Transistor scaling-22 nm or beyond

Rare earth disilicates are now a day's being analyzed as a dielectric layer for transistor scaling for the advanced 22nm regime or beyond. So to explore these materials, the polymorphic powdered Er2Si2O7 (D phase) is synthesized by solid state double sintering method to study its characteristics. Structural characterization has been performed by X-ray diffraction. SEM and EDX results shows the rods like morphology of particles and composition. The dc electrical properties are evaluated by two probe method as a function of temperature. The dielectric spectroscopic measurements of D-Er2Si2O7 are performed in the temperature range 300–420K and frequency range 1kHz to 1MHz. The dc electrical transport phenomenon is analyzed using Mott’s variable-range hopping approach. The ac conductivity σac(ω) is obtained through the dielectric spectroscopic measurements.

The effect of polymorphic structure on the structural and chemical stability of yttrium disilicates

Under pressure and temperature conditions like those found in deep geological repositories (DGRs), rare-earth cations may react with silicates to form rare-earth disilicates. This study establishes the stability range of yttrium disilicates in response to changes in pH. The α, β, γ, and δ polymorphs of Y 2 Si 2 O 7 were synthesized by the sol-gel process at temperatures between 1100 and 1650 °C and subjected to pH stat leaching tests. By measuring the Y and Si leaching rates and monitoring the transformation of the crystalline and amorphous phases, we showed that yttrium disilicates were stable at pH > 5. At pH 2 Si 2 O 7 phases, with the δ polymorph showing the lowest leaching rates. Because rare-earth compounds can be used as a proxy for analogous actinide hosts, the results of this study can be used to predict the performance of engineered barriers in DGR.

Rare-Earth Disilicates As Oxidation-Resistant Fiber Coatings for Silicon Carbide Ceramic-Matrix Composites

Current SiC-based ceramic–matrix composites (SiC–SiC CMCs) rely on carbon or boron nitride fiber–matrix interphases for toughness and flaw tolerance. However, oxidation of these interphases can be performance limiting in many CMC applications. The γ-polymorph of the rare-earth disilicates (RE2Si2O7) is a potential oxidation-resistant alternative to carbon or BN. The formation of γ-Y2Si2O7 and γ-Ho2Si2O7 at different temperatures and processing environments was investigated. Silica–yttrium hydroxide and silica–holmium hydroxide dispersions were made and heat treated at 1200°–1400°C for 8 h in air and argon. LiNO3 was added to the dispersions to enhance the formation of γ-Y2Si2O7 and γ-Ho2Si2O7. The effects of excess silica and LiNO3 dopant on the formation of γ-Y2Si2O7 were investigated. Coatings of Y2Si2O7 and Ho2Si2O7 were made on α-SiC plate and SCS–0 SiC fiber using these dispersions. These were heat treated in argon and argon—500 ppm oxygen mixtures at 1400°C/8 h. For coatings heat treated in argon—500 ppm oxygen mixtures, X-ray diffraction showed the formation of single phase γ-Ho2Si2O7 and a mixture of γ and β-Y2Si2O7 at 1400°C. Scanning electron microscopic image analysis gave an estimate of 18 vol% of excess silica for γ-Y2Si2O7 formed with high Si:Y ratio and ∼5 vol% excess silica for material formed with lower Si:Y ratio. Transmission electron microscopy of samples directly beneath indentations showed both extensive dislocation slip and fracture.

Stability of Rare-Earth Disilicates: Ionic Radius Effect

Rare-earth (RE) disilicates, of general formula RE2Si2O7, are one of the products of the chemical reaction between RE (elements), which are actinide simulators, and the silicates used in the engineered barrier systems of deep geological repositories (DGPs). The aim of this paper is to establish the stability range of the disilicate phase as function of the nature of the RE (RE=Sc, Lu, or Y) and examine whether this phase would permit RE leaching under experimental conditions simulating those of the DGP. The β-polymorphs of the RE disilicates were synthesized by the sol–gel method and subsequently submitted to a pHstat leaching test. The rates of RE and Si leaching were measured and the transformation of the crystalline and amorphous phases was examined by X-ray powder diffraction and nuclear magnetic resonance techniques. The results indicate that the disilicate phases were stable within a wide range of pH, their stability being related to the hydrated ratio of the RE. Disilicate stability increased with the ionic radius of the RE. As a result, the Sc disilicate was stable throughout the pH range tested, whereas Y and Lu disilicate leaching was only observed at pH<4. Thus, it was confirmed that the formation of the disilicate phases could contribute to the confinement of radioactive wastes in engineered barriers.