Rare Earth Silicates Research Articles

Ultra-low thermal conductivity and hydrophobic properties of high entropy β-type quaternary pyrosilicate

To raise the operating temperature of Si-based ceramic matrix composites (CMCs), thermal/environmental barrier coating (T/EBCs) materials, typically rare earth silicates are applied. However, the high thermal conductivity of each single silicate possible coating option and the mismatch of thermal expansion present a significant barrier in the development of novel materials to offer thermal insulation and environmental protection to CMCs. The entropy engineering is used to create a multicomponent equi-atomic single-phase pyro silicate with extremely low thermal conductivity and enhanced water vapor corrosion resistance at 1300 °C in 90% H2O and 10% O2 environment, specifically (Yb0.25Er0.25Tm0.25Sc0.25)2Si2O7. X-ray diffraction patterns confirms monophase formation and the thermal parameters such as specific heat capacity, thermal expansion and thermal diffusivity were determined and microstructures were described using scanning electron microscopy before and after water vapor corrosion. The ultra-low thermal conductivity of high entropy disilicates 0.90 W/m.°C at 1000 °C and very low value of weight loss 0.0014 g/cm2 is observed after 150 h corrosion time which is not reported before for even single or multicomponent high entropy rare earth disilicates.

Corrosion behavior of Al2O3-modified Yb2SiO5 environmental barrier coating under water vapor conditions at 1500 °C

Al2O3-modified Yb2SiO5 was prepared as environmental barrier coating (EBC), and the water vapor corrosion behavior at 1500 °C was investigated. The residual phase, Yb2O3, was observed in the as-prepared coating besides Yb2SiO5 and Al2O3, which preferred to react with Al2O3 to form toughened Yb3Al5O12, meanwhile gave rise to the growth of diffusion layer (Yb2Si2O7) and prevented the generation of the porous region under EBC layer after exposure. The growth stress of the TGO layer and the thermal stress during cooling were the driving sources of delamination, which limited the service life of EBCs. Conclusions on phase transformation and interface evolution in this work are expected to guide the design of rare earth silicate EBCs.

Y/YbB4-modified SiCf/SiC composites with enhanced water-oxygen corrosion resistance

Silicon carbide fibers-reinforced silicon carbide matrix (SiCf/SiC) composites, which are commonly used as thermal structural materials in different components, e.g., combustion chamber liners, are susceptible to water-oxygen corrosion, resulting in rapid performance degradation and structural collapse. To enhance their resistance to water-oxygen corrosion, the incorporation of rare-earth silicates (RE silicates) has been explored. In this study, novel Y/YbB4-modified SiCf/SiC composites, with superior resistance to water-oxygen corrosion, are fabricated using a combination of slurry infiltration (SI) and reactive melt infiltration (RMI) process. Results demonstrate that Y/YbB4-modified SiCf/SiC composites exhibit open porosity of 3.2 ± 0.4%, corresponding to remarkable reduction of 69.8% compared to CVI-SiCf/SiC composites. Furthermore, after being exposed to water-oxygen environment at 1200 °C for 100 h, the surface of Y/YbB4-modified SiCf/SiC composites displays various yttrium silicates and ytterbium silicates. The material exhibits strength retention of 76.84%, indicating a notable improvement of 15.71% over the CVI-SiCf/SiC composites.

Petrochemistry, rare metals-bearing minerals and spectrometric exploration in Khour Abalea, Abu Rusheid area, Eastern Desert, Egypt

On the Red Sea coast, Abu Rusheid area is located south Marsa Alam City (90 km) in the south Eastern Desert. Khour Abalea as a part of the Abu Rusheid area extends for about 1.0 km in E-W direction. It is subdivided into two main branches; the first one (the longest) extends about 0.7 km in E-W direction with 10–30 m in passing through mylonites, whereas the second branch extends about 0.5 km in ENE-WSW with about 5–20 m in width and passing through mylonites and ultra-mylonites. Microscopically, this rock is composed mainly of k-feldspars, plagioclases, biotite, and quartz as the main minerals, while the accessories include fluorite, allanite, secondary uranium (kasolite), and zircon. The hydrothermal alteration that occurs during the interaction of hydrothermal fluid and rock results in an M-W type tetrad effect range of 1.72–3.66 in the normalized REE chondrite patterns of the Khour Abalea mylonite rocks. The identified heavy minerals associated with Khour Abalea mylonite rocks can be categorized as a) radioactive minerals (soddyite, kasolite, thorite and uranothorite), b) rare earth minerals (monazite, xenotime and rare earth silicate), c) Nb–Ta minerals (columbite, fergusonite, ishikawaite and aeschynite), d) precious metals (gold, silver minerals (argentite, arcubisite and matildit), e) base metals (cassiterite, wolframite, chalcocite, galena, sphalerite pyrite and Bi minerals (bismite, bismuthinite and perite) and f) accessory mineral (zircon). According to the uranium mobilization equations, there are uranium enrichment in Khour Abalea mylonite.

Synthesis of Eu3+-Doped NaGd9Si6O26 Sub-Microcrystals from a NaGdF4@SiO2 Structure.

Rare earth silicate phosphors of high quantum efficiency with a stable performance are promising materials in the fields of display and illumination. The grain sizes of products synthesized via the conventional solid-state reaction method are usually too large to satisfy the requirements of color cast and extraction efficiency in high-resolution light-emitting devices (LEDs). We designed a synthetic route and successfully fabricated rare earth silicate NaGd9Si6O26 (NGSO) sub-microcrystals with a size ranging from 550 to 1200 nm. The reaction mechanism and optical properties were systematically investigated. The quantum efficiency of Eu3+-activated NGSO sub-microcrystals was about 36.6%. The LED encapsulated with these sub-microcrystals showed lower color deviation and higher lumen efficiency and lumen flux compared to that with NGSO fabricated using the conventional solid state reaction method.

A systematic analysis of the calcium-magnesium-aluminosilicate corrosion behavior of high-entropy (5Re0.2)2Si2O7 materials

The CaO-MgO-AlO1.5-SiO2 (CMAS) corrosion resistance of disilicates is an urgent task in environmental barrier coatings. Based on this, four high-entropy rare earth disilicates (Yb0.2Y0.2Lu0.2Sc0.2Gd0.2)2Si2O7, (Yb0.2Tm0.2Lu0.2Sc0.2Gd0.2)2Si2O7, (Yb0.2Tm0.2Lu0.2Sc0.2Er0.2)2Si2O7, and (Yb0.2Y0.2Lu0.2Ho0.2Er0.2)2Si2O7 and single-component Yb2Si2O7 for comparison were investigated. Results showed that cyclosilicate Ca3RE2(Si3O9)2 and apatite Ca2RE8(SiO4)6O2 are formed sequentially at 1300 ℃ for 48 h, which can block the CMAS from infiltrating the ceramics. The corrosion resistance of the four high-entropy rare earth silicates varied depending on the average RE3+ radius, but all performed better than Yb2Si2O7.

Design rules for the thermal and elastic properties of rare-earth disilicates

Rare-earth silicates are the current standard material for use as environmental barrier coatings for SiC-based ceramic matrix composites as hot-section components in gas-turbine engines. Expanding the design space to all available rare-earth elements to facilitate optimizing functionality requires an understanding of systematic trends in RE2Si2O7 properties. In this work, we combine first-principles calculations with experimental measurements of Young’s modulus, coefficient of thermal expansion, and thermal conductivity for a range of different RE2Si2O7 compositions and phases. Clear trends are observed in these properties as a function of the radius of the rare-earth cation. In the case of Young’s modulus and thermal expansion, these trends also hold for multi-component systems; while the thermal conductivity of multi-component systems is noticeably lower, indicating the potential of such materials to also act as thermal barriers. These results provide design rules for developing new thermal and environmental barrier coatings with stiffness and thermal expansion engineered to match that of the substrate, while simultaneously having reduced thermal conductivity.

Estimates of permeability of water vapor in rare earth silicate environmental barrier coatings using the modified theory of deal and grove

The salient elements of the theory of oxide growth under an environmental barrier coating are reviewed. The results from steam oxidation tests recently conducted on environmental barrier coated specimens are reported. The oxide thickness between the ceramic topcoat and Si substrate are reported as a function of time. Also, the results from previously conducted steam oxidation tests are reviewed. An approach to estimate the permeability of water vapor through the various coating systems using the oxide thickness versus time measurements and the theory of oxide growth under an environmental barrier coating is demonstrated. The approach allows one to rank the protection efficiency of the various coating systems against steam oxidation.

Accelerating the design of multicomponent rare earth silicates for SiCf/SiC CMC by combinatorial material chip design and high throughput screening

High throughput experimentation is employed to establish a ternary system with the compositional range of 30.8 mol.%–75.7 mol.% SiO2, 16.6 mol.%–61.7 mol.% Yb2O3, and 6.3 mol.%–14.1 mol.% Ho2O3 through co-sputtering deposition on one combinatorial material chip. Considering their application in advanced SiCf/SiC CMC, the phase composition and mechanical properties of samples with various RE/Si ratios and Yb/Ho ratios are comprehensively investigated. Chemical stability and thermal expansion compatibility between SiC and RE silicates with different compositions are also validated. Optimized materials for the application of environmental barrier coating and interphase for SiCf/SiC CMC are screened respectively according to the above trends and data. This work is a case study to establish a composition-property library for RE2O3-SiO2 compounds. It is inspired more complicated multicomponent RE silicates could be prepared and characterized by high throughput experimentation, accelerating the design and screening of promising optimal candidates.

YbSiOC ceramics with a multidimensional nanostructure for high-efficiency electromagnetic wave absorption

The controllable microstructure of a material plays a vital role in its electromagnetic (EM) wave absorption performance. Herein, a multidimensional nanostructure including a matchstick-like SiCnws network structure with a carbon nanocluster was grown in-situ in a porous Yb2Si2O7 ceramic by precursor infiltration and pyrolysis (PIP) to prepare YbSiOC ceramics. As the number of PIPs increased, the absorber (SiCnws and carbon) content increased to construct a multidimensional nanostructure, which also promoted conductivity loss, interfacial polarization loss and dipole polarization loss. The results showed that the YbSiOC ceramics (with an absorber content of 28.2 wt%) achieved a minimum reflection loss (RLmin) of − 50.0 dB at 9.39 GHz with 2.7 mm, and the effective absorption bandwidth (EAB) reached 4.2 GHz (in the X band) as the thickness increased from 2.1 to 3.5 mm. To conclude, the matchstick-like SiCnws and carbon nanoclusters build a multidimensional network structure that is capable of multiple reflections and scattering of EM waves in YbSiOC ceramics. This work proposes a feasible method to design rare earth silicates with EM absorbing performance, and it is expected to have durable applications in water-vapor environments.

Single-Crystal Terbium Silicate Chloride Core–Shell Nanowires and Nanotubes for Monolithically Integrated Optoelectronics

The density of doped rare-earth (RE) ions in most host materials is often too low to support sufficient optical gain for monolithically integrated optoelectronics, limited by the inevitable precipitation of RE ions at high density. This challenge can potentially be overcome with RE-containing single crystals. In this work, a monocrystalline terbium silicate chloride in the form of core–shell and tubular nanostructures is synthesized by one-step chemical vapor deposition. The Rietveld refinement analysis of X-ray diffraction shows that this single crystal has the formula Tb3(SiO4)2Cl and belongs to the orthorhombic Pnma space group with unit cell parameters a = 6.858 Å, b = 17.737 Å, c = 6.179 Å, and α = β = γ = 90° and a unit cell volume 7.54 nm3. To the best of our knowledge, single-crystal Tb3(SiO4)2Cl is a RE silicate that has never been reported before. Compared to Tb-doped materials, Tb3(SiO4)2Cl has the advantages of single crystal, higher Tb3+ density (1.6 × 1022 cm–3), and chemical/thermal stability (950 °C). Upon illumination, Tb3(SiO4)2Cl emits a strong characteristic green light with crystal-field-induced splitting and no concentration- and temperature-induced quenching. The conversion from core–shell nanowires to nanotubes is interpreted by the synergistic influence of surface chemical reactions and lateral atomic diffusion driven by the Kirkendall effect. Given the high terbium density and thermal stability of these nanostructures and the chemical involvement of silicon during their formation, they are promising candidates for the implementation of compact Si-compatible green light (e.g., giant optical gain, low-threshold waveguide amplifiers, and monolithic lasers) desirable for next-generation monolithically integrated optoelectronics.

Significantly improved corrosion resistance of high‐entropy rare‐earth silicate multiphase ceramics against molten CMAS

AbstractTo improve the ability of rare‐earth (RE) silicates to resist molten calcium–magnesium–aluminosilicate (CMAS) at high temperature, a novel high‐entropy (4RE0.25)2Si2O7/(4RE0.25)2SiO5 (RE = Y, Yb, Er, and Sc) multiphase ceramic was prepared by a two‐step process. During sintering, (4RE0.25)2SiO5 can react with SiO2 at the grain boundaries of (4RE0.25)2Si2O7, which can not only purify the grain boundary but also promote the growth of the original (4RE0.25)2Si2O7 grains, thereby significantly improving the ability to resist molten CMAS corrosion at high temperature. After corroding at 1500°C for 48 h, the reaction layer of the multiphase ceramic was only 55 μm thick. Our results confirm that the high‐entropy RE silicate multiphase ceramics represent an effective way to improve the ability to resist molten CMAS corrosion at high temperature.

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.

High ionic conducting rare-earth silicate electrolytes for sodium metal batteries

Sodium gadolinium silicate solid electrolyte showed an outstanding sodium plating/stripping performance for 1000 cycles that proves excellent interfacial contact between the sodium anode and solid electrolyte.

Corrosion Behavior and Mechanism of Aluminum-rich CMAS on Rare-earth Silicate Environmental Barrier Coatings:

Corrosion Behavior and Mechanism of Aluminum-rich CMAS on Rare-earth Silicate Environmental Barrier Coatings:

Steam oxidation and microstructural evolution of rare earth silicate environmental barrier coatings

AbstractA primary failure mode for environmental barrier coatings (EBCs) on SiC ceramic matrix composites (CMCs) is the oxidation of the intermediate Si‐bond coating, where the formation of SiO2 at the bond coating–EBC interface results in debonding and spallation. This work compares the microstructure evolution and steam oxidation kinetics of the Si‐bond coating beneath yttrium/ytterbium disilicate ((Y/Yb)DS) and ytterbium disilicate/monosilicate (YbDS/YbMS) EBCs to better understand the impact of EBC composition on oxidation kinetics. After 500 1‐h cycles at 1350°C, (Y/Yb)DS displayed a decreasing concentration of the monosilicate minor phase and increasing concentration of porosity as furnace cycling time increased, whereas the YbDS/YbMS EBC displayed negligible microstructural evolution. For both EBC systems, thermally grown oxide growth rates in steam were found to increase by approximately an order magnitude compared to dry air oxidation. The (Y/Yb)DS EBC displayed a reduced steam oxidation rate compared to YbDS/YbMS.

Current research art of rare earth compound modified SiC-CMCs for enhanced wet-oxygen corrosion resistance

Silicon carbide fiber reinforced silicon carbide ceramic matrix composites (SiC-CMCs) currently present wide application potential as thermal structure component in aero-engine. To solve the wet-oxygen corrosion induced degradation of SiC-CMCs attract growing global attention. In this work, the main preparation methods via introduction of rare earth compound like rare earth phosphates, rare earth oxides, rare earth silicates and rare earth silicide carbides into SiC-CMCs matrix and interphase and their microstructures, mechanical properties and wet-oxygen corrosion resistance are summarized in details. Finally, the challenges and prospects of rare earth compound modified SiC-CMCs are proposed.

Tailoring thermal and chemical properties of a multi-component environmental barrier coating candidate (Sc0.2Nd0.2Er0.2Yb0.2Lu0.2)2Si2O7

Environmental barrier coatings, typically rare earth silicates, have been successfully employed on ceramic matrix composites in the hot zone of gas turbine engines, allowing for high fuel burn temperatures and increased flight efficiency. Yet, environmental barrier coatings face challenges as turbine temperatures continue to increase, such as increased oxidation of the underlying substrate, environmental degradation, and coating spallation. Here, we show how multi-component rare earth silicates offer a unique solution for simultaneously co-optimizing phase stability, thermo-chemical, and thermo-mechanical properties through variation of the rare earth elements implemented in the coating material. Each rare earth element added in solution was chosen to enhance specific material properties. We found that chemical stability with the turbine gaseous environment or with molten deposits could be retained, and in some cases enhanced, without detrimental effects on the phase stability or thermo-mechanical behavior. Our results demonstrate how rare earth cation mixing can lead to a 50% reduction in thermal conductivity for the given rare earth silicate mixtures. Such decreases in thermal conductivity show promise for reducing the substrate operating temperature during use, thus making multi-component rare earth silicates a novel class of dual-purpose thermal/environmental barrier coating materials.

Interaction of high-entropy rare-earth monosilicate environmental barrier coatings subjected to corrosion by calcium-magnesium-alumino-silicate melts

The high-entropy design of rare-earth silicates has been expected to improve CMAS corrosion resistance, which is essential to the development of EBCs for aero-engines. In this work, CMAS corrosion behaviors of plasma-sprayed high-entropy monoslicates ((Lu0.25Yb0.25Er0.25Y0.25)2SiO5 and (Lu0.2Yb0.2Er0.2Ho0.2Y0.2)2SiO5) and Y2SiO5 coatings at 1350 °C for 25–50 h were investigated and compared. The corrosion products included apatite and garnet based on the dissolution-reprecipitation mechanism. The high-entropy coatings possessed improved corrosion resistance compared with the Y2SiO5 coating, which was mainly due to the doped RE elements with smaller ionic radius could improve the stability of crystal structure and form garnet phase more easily.

Design of multilayer cauliflower-like structure SiO2/SiC–Y2Si2O7 composite ceramics as high-efficiency electromagnetic wave absorbers

A reasonable design of the microstructure and phase composition of rare earth silicate composites can enhance their corrosion resistance and their electromagnetic (EM) properties. However, individual rare earth silicates cannot fulfill the requisites for novel EM absorption materials. Herein, we introduce SiC and SiO2 to establish novel SiO2/SiC–Y2Si2O7 composite ceramics with a multilayer cauliflower-like structure using sol-gel, pressure-free sintering, chemical vapor infiltration (CVI) and oxidation at 1100 °C (1.5 h). SiC increases the relative complex permittivity and attenuation capabilities and the SiO2 layer impedance matching mediator effectively balances the impedance matching properties. The SiO2/SiC–Y2Si2O7 composite ceramics, therefore, exhibit superior EM wave absorption properties: the reflection coefficient minimum (RCmin) is −21.5 dB, with a 3.65 mm thick covering, in the whole X band (4.2 GHz). This is due to the impedance matching, polarization loss, conductivity loss, and internal multiple reflections loss. In brief, this work provides a strategy for achieving the EM stealth characteristics of rare earth silicate materials.