High-temperature Na2SO4 interaction with air plasma sprayed Yb2Si2O7 + Si EBC system: Topcoat behavior

The high-temperature interaction between ~2.5 mg/cm² of Na₂SO₄ and an atmospheric plasma sprayed (APS) Yb₂Si₂O₇ topcoat–Si bond coat system on SiC CMC substrates was studied for times up to 240 h at 1000°C–1316°C in a 0.1% SO₂–O₂ gaseous environment. Yb₂Si₂O₇ reacted with Na₂SO₄ to form Yb₂SiO₅ and an intergranular amorphous Na-silicate phase. Below 1200°C, the reaction was sluggish, needing days to cause morphological changes to the “splat microstructure” associated with APS coatings. The reaction was rapid at 1200°C and above, needing only a few hours for the entire topcoat to transform into a granulated microstructure consisting of Yb₂SiO₅ and Yb₂Si₂O₇ phases. Na₂SO₄ deposits infiltrated the Yb₂Si₂O₇ topcoat and transformed into an amorphous Na-silicate in less than 1 h at all exposure temperatures. Quantitative assessment of the Yb₂SiO₅ area fraction in the topcoat showed a linear decrease over time at 1316°C, attributed to reaction with the SiO₂ thermally grown oxide (TGO) formed on the Si bond coat and rapid transport through the interpenetrating amorphous Na-silicate grain boundary phase. It was predicted that nearly 2 weeks is needed for complete removal of Yb₂SiO₅ from the topcoat at 1316°C for a single applied loading of Na₂SO₄.     

Mechanical properties and microstructure of Yb2SiO5 environmental barrier coatings under isothermal heat treatment

Yb₂SiO₅ (ytterbium monosilicate) top coatings and Si bond coat layer were deposited by air plasma spray method as a protection layer on SiC substrates for environmental barrier coatings (EBCs) application. The Yb₂SiO₅-coated specimens were subjected to isothermal heat treatment at 1400 °C on air for 0, 1, 10, and 50 h. The Yb₂SiO₅ phase of the top coat layer reacted with Si from the bonding layer and O₂ from atmosphere formed to the Yb₂Si₂O₇ phase upon heat treatment at 1400 °C. The oxygen penetrated into the cracks to form SiO₂ phase of thermally grown oxide (TGO) in the bond coat and the interface of specimens during heat treatment. Horizontal cracks were also observed, due to a mismatch of the coefficient of thermal expansion (CTE) between the top coat and bond coat. The isothermal heat treatment improves the hardness and elastic modulus of Yb₂SiO₅ coatings; however, these properties in the Si bond coat were a little bit decreased.     

Investigation of oxygen ion transport and surface exchange properties of PrBaFe2O5+δ

The oxygen transport properties and chemical stability of PrBaFe₂O_5+δ (PBF) double-perovskite oxide were systematically investigated as a chemically stable, highly oxygen-permeable membrane and solid oxide fuel cell (SOFC) electrode under a CO₂-containing or reducing atmospheres. The oxygen permeation flux of 0.7 mm-thick samples and the oxygen ion conductivity were 4.7 × 10^?1 mL?cm^?2 min^?1 and 0.12 S?cm^?1 at 900 °C, respectively, which are comparable to those of PrBaCo₂O_5+δ, exhibiting the most superior performance among oxides with a double-perovskite structure. Moreover, the bulk diffusion and surface exchange coefficients estimated from the electrical conductivity relaxation analysis were generally comparable to those of PrBaCo₂O_5+δ. The characteristic thickness estimated from the membrane and conductivity relaxation tests was ～0.6 mm at 900 °C. The results indicate the significant influence of the surface exchange reaction on the permeability within a thickness of 0.5–1.7 mm. The PBF double-perovskite oxide exhibited superior chemical stability, compared to typical oxides such as PrBaCo₂O_5+δ under a CO₂-containing atmosphere. All results suggest that PrBaFe₂O_5+δ exhibits high oxygen diffusivity with high chemical stability under CO₂-containing or reducing atmospheres.     

ZnO – Yb2O3 composite optical ceramics: Synthesis,structure and spectral-luminescent properties

Zinc oxide optical ceramics containing 0–2 wt% ytterbium are prepared by uniaxial hot pressing of commercial oxides at 1150 and 1180 °C. The ceramics have the main crystalline phase of hexagonal wurtzite-type ZnO. Ytterbium ions do not enter the ZnO crystals but form a cubic sesquioxide phase of Yb₂O₃ located at the ZnO grain boundaries. Yb acts as an inhibitor for the ZnO grain growth. The ceramics exhibit transmittance up to 60 % in the visible. Their transmission in the infrared is determined by the free charge carrier absorption. The Yb³⁺ ions are found in C₂ and C_3i sites in Yb₂O₃ crystals. Under X-ray excitation, the ceramics exhibit intense luminescence bands in the blue (near-band-edge emission) and green (defect emission) whose positions, intensities and decay times depend on the Yb content. Yb₂O₃ causes a redistribution of luminescence intensity in favor of the near-band-edge emission and fastens the emission decay.     

Structural and electrical characterisation of silica-containing yttria-stabilised zirconia

Zirconia stabilised by yttria has a high oxide ion conductivity at high temperature and, therefore, is currently used as electrolyte in Solid Oxide Fuel Cells. Silica is normally avoided in this material because formation of amorphous silica phases along the grain boundaries causes an increased grain boundary impedance. The present study examines the effect of SiO₂ and SiO₂ with Mn-oxide on the structure and resistivity of yttria stabilised zirconia electrolyte materials. During fabrication of Solid Oxide Fuel Cells, Mn readily diffuses from the manganite-based cathode into the electrolyte. It is shown that a grain boundary phase which causes an insulating layer in the grain boundaries is formed when both SiO₂ and Mn-oxide coexist in the samples, whereas such effects are much less pronounced when only SiO₂ is present.     

Influence of oxygen partial pressure on the oxygen diffusion and surface exchange coefficients in mixed conductors

The performances of mixed ionic and electronic conductors is used in many applications, such as oxygen transport membranes or electrodes in solid oxide fuel cells. The performances of these systems depend mainly on two fundamental parameters including oxygen diffusion (D_O) and the oxygen exchange coefficient (k). This work focuses on the impact of the oxygen partial pressure on oxygen diffusion and surface exchange coefficients of mixed conducting materials, as reported studies are scarce in the literature. In this way, two different mixed conducting materials are studied, namely, La_0.6Sr_0.4Fe_0.6Ga_0.4O_3-δ, a perovskite-type material, and the nickelate La₂NiO_4+δ. The D_O and k coefficients are determined by a specific oxygen permeation measurement and by the isotopic exchange depth profile method.     

Microstructure and nanomechanical properties of plasma-sprayed nanostructured Yb2SiO5 environmental barrier coatings

In this study, nanostructured and conventional Yb₂SiO₅ coatings were prepared by atmospheric plasma. The microstructure and nanomechanical properties of these coatings were compared before and after heat treatment. The results show that the nanostructured Yb₂SiO₅ coatings have a mono-modal distribution, and the conventional Yb₂SiO₅ coatings have a bimodal distribution. Both types of coatings had improved nanomechanical properties after heat treatment. However, the increased elastic modulus and nanohardness of the nanostructured Yb₂SiO₅ coating were more apparent than those of the conventional Yb₂SiO₅ coatings. The nanostructured Yb₂SiO₅ coating had a higher elastic modulus than the conventional Yb₂SiO₅ coating, reflecting its high density. Subsequently, the microscopic morphology and micromechanical properties of the coatings were analyzed after heat treatment. Defects in the coatings, including pores, and microcracks, were significantly reduced with grain growth after thermal treatment, and the nanostructured Yb₂SiO₅ coatings had improved healing ability and micro-mechanical properties.     

Volume expansion during reaction sintering of γ-Bi12SiO20

Preparation of γ-Bi₁₂SiO₂₀ from a compacted mixture of α-Bi₂O₃ and SiO₂ powders is described. A very large volume expansion, related to the phase formation, is observed during heat treatment at 600°C. It is shown that the dedensification results from a preferential diffusion of bismuth and oxygen ions towards SiO₂, through the layer of γ-Bi₁₂SiO₂₀ which forms around a silica grain. The expansion begins when γ-Bi₁₂SiO₂₀ grains form a continuous skeleton. When bismuth oxide grains are isolated in the skeleton, expansion and reaction rates are proportional. A quantitative model is proposed to describe this situation assuming an isotropic matter transfer and no coalescence between the γ-Bi₁₂SiO₂₀ grains.     

Influence of phase composition on microstructure and thermal properties of ytterbium silicate coatings deposited by atmospheric plasma spray

In this study, ytterbium silicate coatings with different compositions were designed by controlling the Yb₂O₃/ SiO₂ ratio and fabricated by atmospheric plasma spray. The microstructure and thermal properties of these coatings were characterized. Results showed that the Yb₂O₃-rich coatings contained Yb₂O₃ and Yb₂SiO₅ phases, which were characterized by Yb₂O₃ columnar grains, obvious interfaces between splats and many microcracks. The SiO₂-rich coatings were composed of Yb₂SiO₅ and Yb₂Si₂O₇ phases, which were composed of well bonded splats with many spherical pores. The Yb₂O₃-rich coatings had higher coefficient of thermal expansion values and lower thermal conductivities than the SiO₂-rich coatings. The SiO₂-rich coatings presented much better thermal cycling resistance than the Yb₂O₃-rich coatings. The relationship among phase composition, microstructure and thermal properties of ytterbium silicate coatings was analyzed. The results of this study may provide some clues for designs and applications of rare-earth silicates as environmental barrier coatings.     

Grain boundary blocking effects in Sm/Yb-doped AlN ceramics

The effect of microstructural changes on the electrical and thermal properties of AlN ceramics is studied in terms of cation size and nature of sintering aids (i.e. Sm₂O₃ and Yb₂O₃) in AlN ceramics. It is revealed that the addition of Yb₂O₃ to Sm-bearing AlN ceramics results in 80 % reduction of thermal conductivity with an increase of the grain boundary resistivity that is one order of magnitude larger than for the sample without Yb₂O₃. Additionally, the grain boundary/grain resistivity ratio is significantly increased, when the Sm₂O₃ sintering aid is employed instead of Yb₂O₃, for which the secondary phases at the grain boundaries and the triple junctions are responsible for the increase in the electrical resistivity. The microstructural investigations confirm the tendency of the secondary phase to segregate at the triple junctions in Sm-containing AlN ceramics while it is grain boundaries that are favored as segregation site in the case of Yb.     

Influence of rare-earth oxide additives on the oxidation resistance of Si3N4–SiC nanocomposites

The influence of different rare earth oxide additives (La₂O₃, Nd₂O₃, Sm₂O₃, Y₂O₃, Yb₂O₃ and Lu₂O₃) on the oxidation behaviour of carbon derived Si₃N₄–SiC micro-nanocomposites has been investigated. All investigated composites exhibited predominately parabolic oxidation behaviour indicated diffusion as the rate limiting mechanism. Except the Si₃N₄–SiC composite sintered with Lu₂O₃ the rate-limiting oxidation mechanism for all other materials was an outward diffusion of the additive cations along the grain boundary towards the surface. Such diffusion of cation has been strongly suppressed in the Lu-doped composite because of the beneficial effect of stable grain boundary phase and the presence of the SiC particles predominately located at the grain boundaries of Si₃N₄. Nanoparticles at the grain boundaries act as the obstacles for migration of cations of the additives resulting in superior oxidation resistance of Si₃N₄–SiC–Lu₂O₃ where the rate-limiting step is inward diffusion of oxygen through the oxide layer to the bulk ceramics.     

Autonomous crack healing ability of SiC dispersed Yb2Si2O7 by oxidations in air and water vapor

Yb₂Si₂O₇ is a popular environmental barrier coating; however, it decomposes into Yb₂SiO₅ in high-temperature steam environments. The thermal mismatch between Yb₂Si₂O₇ and Yb₂SiO₅ leads to the cracking and failure of the disilicate coating via oxidation. Dispersing SiC nanofillers into the Yb₂Si₂O₇ matrix is suggested to maintain the Yb₂Si₂O₇ matrix and promote crack self-healing. This study is aimed at clarifying the effect of water vapor on the self-healing ability of such composites. X-ray diffraction analysis and scanning electron microscopy were used to monitor the surface composition and the crack formation, respectively, in 10 vol% SiC-dispersed Yb₂Si₂O₇ composites. Annealing at temperatures higher than 750 °C in air or in a water vapor rich atmosphere led to strength recovery and the self-healing of indentation-induced surface cracks owing to volume expansion during the oxidation of SiC. The self-healing effect was influenced by the oxidation time and temperature. Rapid diffusion of H₂O as an oxidizer into the SiO₂ layer promoted self-healing in a water vapor rich atmosphere. However, accelerated oxidation at temperatures higher than 1150 °C formed bubbles on the surface. Fabricating composites with a small amount of Yb₂SiO₅ will be a solution to these problems.     

Microstructure evolution and thermomechanical properties of plasma‐sprayed Yb2SiO5 coating during thermal aging

Rare‐earth monosilicates (RE₂SiO₅, RE: rare‐earth elements), such as Yb₂SiO₅, have been developed for potential application as environmental barrier coating (EBC) materials. Yb₂SiO₅ coating would experience microstructure evolution under high‐temperature environment and accordingly its thermomechanical properties would be altered. In this study, Yb₂SiO₅ coating was fabricated by atmospheric plasma spray technique. The phase stability and microstructure change before and after thermal aging at 1300°C, 1400°C, and 1500°C were investigated. The changes in mechanical and thermal properties were characterized. The results showed that the as‐sprayed coating was mainly composed of Yb₂SiO₅ with a small amount of Yb₂O₃ and amorphous phase. Defects in the coating, including interfaces, pores, and microcracks, were greatly reduced with grain growth after thermal treatment. Thermal aging significantly modified the thermal and mechanical properties of the coating. The average CTE was increased by 13.1%, and the hardness and elastic modulus was increased by 42.4% and 49.4%, respectively, after thermal aging at 1500°C for 50 hour. The thermal conductivity of thermal‐aged coating was much higher than that of the as‐sprayed coating, which was still less than 2 W/(m·K). The influence of coating microstructure on the properties was analyzed and related to the failure mechanism of EBCs.     

High temperature performances of tri-layer environmental barrier coating with Si bond layer modified by Yb2O3

Environmental barrier coatings (EBCs) have been expected to be applied on the surface of ceramic matrix composites (CMCs). However, the oxidation and propagation cracking of the silicon bond layer are the most direct causes to induce the failure of EBCs under high temperature service environment. The modification of silicon bond layer has become an important method to prolong the service life of EBCs. In this work, the Yb₂O₃ have been introduced to the silicon bond layer, and three kinds of tri-layer Yb₂SiO₅/Yb₂Si₂O₇/(Si-xYb₂O₃) EBCs with modified Si bond layer by different contents of Yb₂O₃ (x = 0, 10 vol%, 15 vol%) were prepared by vacuum plasma spray technique. The thermal shock performance and long-term oxidation resistance of the EBCs at 1350 °C were investigated. The results showed that the addition of appropriate amount of Yb₂O₃ (10 vol%) can improve the structural stability and reduce the cracks of the mixed thermal growth oxide (mTGO) layer by forming the oxidation product of Yb₂Si₂O₇ during long-term oxidation. The excessive addition of Yb₂O₃ increased the stress during thermal shock as well as accelerated the oxygen diffusion during long-term oxidation, leading to the failure of EBCs. Moreover, the distribution uniformity of Yb₂O₃ deserves further consideration and improvement.     

Thermal shock resistance of tri‐layer Yb2SiO5/Yb2Si2O7/Si coating for SiC and SiC‐matrix composites

A new tri‐layer Yb₂SiO₅/Yb₂Si₂O₇/Si coating was fabricated on SiC, C/SiC, and SiC/SiC substrates, respectively, using atmospheric plasma spray (APS) technique. All coated samples were subjected to thermal shock test at 1350°C. The evolution of phase composition and microstructure and thermo‐mechanical properties of those samples before and after thermal shock test were characterized. Results showed that adhesion between all the 3 layers and substrates appeared good. After thermal shock tests, through microcracks which penetrated the Yb₂SiO₅ top layer were mostly halted at the Yb₂SiO₅‐Yb₂Si₂O₇ interface and no thermal growth oxide (TGO) was formed after 40‐50 quenching cycles, implying the excellent crack propagation resistance of the environmental barrier coating (EBC) system. Transmission electron microscopy analysis confirmed that twinnings and dislocations were the main mechanisms of plastic deformation of the Yb₂Si₂O₇ coating, which might have positive effects on crack propagation resistance. The thermal shock behaviors were clarified based on thermal stresses combined with thermal expansion behaviors and elastic modulus analysis. This study provides a strategy for designing EBC systems with excellent crack propagation resistance.     

Structure and electronic properties of δ-Bi2O3 tuned by vacancy and doping: A first-principles study

Pure Bi₂O₃ with high ionic conductivities is considered as a candidate material for an electrolyte in solid oxide fuel cells and oxygen separation membranes. However, its lower structural and thermal stability prevent it application in ion conductivity and photocatalysis at suitable temperatures. Metal oxides are usually used to stabilize its structure to lower temperatures and the underlying mechanism is still unclear. To shed light on the issue, vacancy ordered structures of pure and doped δ-Bi₂O₃ have been studied by first-principles calculations. It have been shown that the structure with combined <110> and <111> vacancy arrangements is energetically favorable compared to either <100>, <110> or <111> vacancy ordered structures. Electronic structure analyses have further verified that δ-Bi₂O₃ has a semiconductor character with an energy gap of 2.0 eV, consistent with the experiment results. The site occupation of doping ions is further analyzed by formation energy, geometry and electronic structures. It is evident that the substitution sites of doping ions depend on the type of the doping ions. The ions with large ion sizes tend to occupy the Bi(2) sites while the ions with small ion sizes tend to occupy the Bi(1) sites. At the same time, the probability of the Y ions occupying the oxygen vacancy sites and the optical properties of the Y-doped Bi₂O₃ are explored. Our investigations reveal that the electronic structure of oxides could be tuned by vacancy and interstitial defects for better conductivity, photocatalytic properties.     

Modeling oxygen permeability through topcoat and thermally grown oxide in dense Yb2Si2O7 environmental barrier coatings

The oxygen permeability of ytterbium disilicate (YbDS) topcoat (TC) and silicon dioxide (SiO₂) thermally grown oxide (TGO) is evaluated. The primary goal is to elucidate the oxidation mechanisms in environmental barrier coatings (EBCs). For this purpose, oxidant diffusion is investigated using physics-based and numerical modeling. The oxygen permeability constants are systematically evaluated and quantified in terms of thermodynamics using defect reactions and the parabolic rate constant (kp), respectively. Dry oxygen and wet oxygen conditions as well as different temperatures, partial pressures, and topcoat modifiers are investigated. The results offer evidence that the oxygen permeability constant for the YbDS topcoat is an order of magnitude higher than for the TGO. As such, the TGO hinders the oxidant diffusion stronger, proving to be the diffusion rate-controlling layer. Moreover, water vapor strongly increases the oxidant permeation with defect reactions playing a key role. It is suggested that the mass transfer through the topcoat is primarily by outward ytterbium ion diffusion and inward oxygen ion movement, with the latter being dominant, particularly in wet environments. The effect of topcoat modifiers on oxidant permeation is composition sensitive and seems to be related to their interaction with oxygen ions and their mobility.     

Strength improvement and purification of Yb2Si2O7‐SiC nanocomposites by surface oxidation treatment

A two‐step processing was developed to prepare Yb₂Si₂O₇‐SiC nanocomposites. Yb₂Si₂O₇‐Yb₂SiO₅‐SiC composites were first fabricated by a solid‐state reaction/hot‐pressing method. The composites were then annealed at 1250°C in air for 2 hours to activate the oxidation of SiC, which effectively transformed the Yb₂SiO₅ into Yb₂Si₂O₇. The surface cracks purposely induced can be fully healed during the oxidation treatment. The treated composites have improved flexural strength compared to their pristine composites. The mechanism for crack healing and silicate transformation have been proposed and discussed in detail.     

Scavenging effect on ionic conduction behaviour at the grain boundary of MgO partially stabilised zirconia

The scavenging effect of magnesium oxide (MgO) addition on electrical property of 9 mol-% MgO partially stabilised zirconia (Mg-PSZ) was investigated in terms of phase transformation and intergranular phase formation. The addition of MgO up to 5 mol-% caused a stabilisation of Mg-PSZ, which led to an increase in the cubic phase and a decrease in the monoclinic and tetragonal phases in Mg-PSZ. The Mg-PSZ with the addition of 5 mol-% of MgO also exhibited the maximum ionic conductivity (0.3915?S?cm^?1 at 1500°C) and forsterite (Mg₂SiO₄) was observed on the grain boundaries of Mg-PSZ. The intergranular phases, formed by reactions between the silicon in Mg-PSZ and MgO addition, reduced the grain boundary resistance, because the siliceous phase which is a hindrance for oxygen ion conduction was scavenged by the formation of Mg₂SiO₄.     

Fabrication of lanthanum-based perovskites membranes on porous alumina hollow fibre (AHF) substrates for oxygen enrichment

In this work, two types of lanthanum-based MIEC perovskite oxides, namely La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF) and La_0.6Sr_0.4Co_0.2Ni_0.8O_3-δ (LSCNi), were deposited onto porous alumina hollow fibre (AHF) substrates and used for oxygen enrichment. Such structure was developed to shorten oxygen ion diffusion distances in dense membranes and simultaneously leading to higher oxygen flux. The perovskite oxides were prepared using Pechini sol-gel method and deposited via a vacuum-assisted technique. The deposition of lanthanum-based membranes onto the outer and inner sides of the porous AHF has been facilitated through numerous microchannels in the AHF substrates. The effects of operating temperature and argon sweep gas flowrate on oxygen permeation flux of lanthanum-based AHF membrane were investigated. The results revealed that the oxygen permeation flux of LSCF-AHF and LSCNi-AHF increased with operating temperatures due to the improvement of bulk diffusion and surface exchange properties after the lanthanum-based perovskite deposition. Higher oxygen flux was observed for LSCNi-AHF as LSCNi possessed balanced oxygen ionic and electronic conductivities as compared to LSCF membranes. Benefitting from improved oxygen activation and vacancy generation properties after Ni substitution into the B-site ion of LSC perovskite, a dramatic increased oxygen fluxes up to 4.5 mL/min·cm² was observed at 950 °C. The present work demonstrated a feasible method for fabricating oxygen transport membrane (OTM) using porous AHF substrates