Hot corrosion behaviors of SrZrO3 ceramic co-doped with Y2O3 and Yb2O3 in molten Na2SO4, V2O5, and Na2SO4 + V2O5 salts mixture

The hot corrosion behaviors of Sr(Y_0.05Yb_0.05Zr_0.9)O_2.95 (SYYZ) ceramic were investigated in Na₂SO₄, V₂O₅, and Na₂SO₄ + V₂O₅ salts mixture, respectively. Na₂SO₄ did not react with SYYZ ceramic at 900, 950 and 1000 °C. m-ZrO₂, YVO₄ and YbVO₄ were the main corrosion products on the SYYZ ceramic surface in V₂O₅ at 800 and 900 °C, whereas Sr₃V₂O₈ and t-ZrO₂ appeared at 1000 °C. In Na₂SO₄ + V₂O₅ salts mixture, the corrosion products were Sr₃V₂O₈ and t-ZrO₂ at 800 and 900 °C on the SYYZ ceramic surface, however, a new phase of SrZrO₃ developed at 1000 °C. The phase transformation and chemical interaction are the primary corrosion mechanisms for degradation of SYYZ ceramic.     

Thermochemical stability of Y2Si2O7 in high-temperature water vapor

The thermochemical stability of Y₂Si₂O₇ was assessed in a high-temperature high-velocity water vapor environment to improve the understanding of the mechanisms that lead to SiO₂ depletion. Spark plasma sintered Y₂Si₂O₇ specimens were exposed in a steam-jet furnace at 1000°C and 1200°C for 3-250 hours, steam velocities of 131-174 m/s and at 1 atm H₂O pressure. These exposures resulted in the selective volatilization of SiO₂ to form volatile Si(OH)₄ and porous Y₂SiO₅. Microstructural evolution from fine rectangular pores at short times to larger rounded pores at longer times was observed. Mechanisms contributing to the overall depletion reaction kinetics were evaluated and include the interface reaction to form Y₂SiO₅ and Si(OH)₄ (g), Y₂SiO₅ coarsening, development of tortuosity in the pore network and diffusion of H₂O (g) and Si(OH)₄ (g) through pores by molecular diffusion and/or Knudsen diffusion. SiO₂ depletion was found to follow parabolic volatilization kinetics (k_p = 0.38 µm²/h) at 1200°C indicating the reaction is limited by a diffusion process, most likely the outward diffusion of Si(OH)₄ (g) through pores. Results are utilized to assess the viability of Y₂Si₂O₇ and other rare-earth silicates as environmental barrier coating (EBC) materials for SiC ceramic matrix composites (CMCs).     

不锈钢表面SiO2-ZrO2-Al2O3-Cr2O3陶瓷涂层性能的研究

将氧氯化锆、正硅酸乙脂、无水乙醇3种物质按物质的量比135放进烧杯搅拌均匀后,再放进微波炉加热10s取出,得到SiO2-ZrO2溶胶。然后,将100mL氨水、50mL硝酸铝溶液和50mL硝酸铬溶液混合均匀后,放进微波炉加热1min后取出,即制得Al2O3-Cr2O3溶胶。取SiO2-ZrO2溶胶和Al2O3-Cr2O3溶胶按n(SiO2)n(ZrO2)n(Al2O3)n(Cr2O3)=6211混合,搅拌2h,得到SiO2-ZrO2-Al2O3-Cr2O3复合溶胶。将打磨、除锈、除油处理后的不锈钢基体浸入SiO2-ZrO2-Al2O3-Cr2O3溶胶一定时间后,以浸渍提拉法得到均匀的溶胶涂层,真空干燥48h,经700℃热处理1h后便可得到SiO2-ZrO2-Al2O3-Cr2O3复合陶瓷涂层。采用XRD、IR和SEM对不同条件热处理的复合陶瓷涂层的物相组成、表面形貌进行分析,并对复合陶瓷涂层的性能进行了研究。结果表明(1)SiO2-ZrO2-Al2O3-Cr2O3复合溶胶热处理后为非晶态材料,且在凝胶中形成了三维的硅氧四面体网络骨架;(2)涂层试样不出现龟裂或脱落的循环次数(900℃,空冷)在15~30次范围内,涂层的抗热震性较好;(3)在700℃热处理1h条件下,涂层单位面积的质量损失最小,具有较好的抗腐蚀性;(4)有涂层试样较无涂层试样的氧化速率低,且以3次涂膜的氧化速率最低;涂层由粒径为2~3μm左右的微粒组成,涂层较致密,抗氧化性较好;(5)有涂层试样的耐磨性均优于无涂层试样,有涂层试样的以3次涂膜的耐磨性最好。     

Synthesis and thermophysical performances of complex Ca3Ln3Ti7Ta2O26.5 (Ln=Dy and Er) oxides

Ca₃Dy₃Ti₇Ta₂O_26.5 and Ca₃Er₃Ti₇Ta₂O_26.5 oxides were synthesized using a high-temperature solid-state fritting technique, and the thermophysical performances of these two oxides were investigated. Both Ca₃Dy₃Ti₇Ta₂O_26.5 and Ca₃Er₃Ti₇Ta₂O_26.5 show a monophasic pyrochlore-type lattice. The thermal conductivity of Ca₃Er₃Ti₇Ta₂O_26.5 is lower than that of Ca₃Dy₃Ti₇Ta₂O_26.5. The oxides exhibit lower thermal conductivities than YSZ owing to their complex elemental compositions, large number of ions, and high oxygen vacancy concentrations. The thermal expansion coefficients of the obtained oxides are similar to that of YSZ.     

Study on thermal shock resistance of Sc2O3 and Y2O3 co-stabilized ZrO2 thermal barrier coatings

In order to reveal the effect of Sc₂O₃ and Y₂O₃ co-doping system on the thermal shock resistance of ZrO₂ thermal barrier coatings, Y₂O₃ stabilized ZrO₂ thermal barrier coatings (YSZ TBCs) and Sc₂O₃–Y₂O₃ co-stabilized ZrO₂ thermal barrier coatings (ScYSZ TBCs) were prepared by atmospheric plasma spraying technology. The surface and cross-section micromorphologies of YSZ ceramic coating and ScYSZ ceramic coatings were compared, and their phase composition before and after heat treatment at 1200 °C was analyzed. Whereupon, the thermal shock experiment of the two TBCs at 1100 °C was carried out. The results show that the micromorphologies of YSZ ceramic coating and ScYSZ ceramic coating were not much different, but the porosity of the latter was slightly higher. Before heat treatment, the phase composition of both YSZ ceramic coating and ScYSZ ceramic coating was a single T′ phase. After heat treatment, the phase composition of YSZ ceramic coating was a mixture of M phase, T phase, and C phase, while that of ScYSZ ceramic coating was still a single T′ phase, indicating ScYSZ ceramic coating had better T′ phase stability, which could be attributed to the co-doping system of Sc₂O₃ and Y₂O₃ facilitated the formation of defect clusters. In the thermal shock experiment, the thermal shock life of YSZ TBCs was 310 times, while that of ScYSZ TBCs was 370 times, indicating the latter had better thermal shock resistance. The difference in thermal shock resistance could be attributed to the different sintering resistance of ceramic coatings and the different growth rates of thermally grown oxide in the two TBCs. Furthermore, the thermal shock failure modes of YSZ TBCs and ScYSZ TBCs were different, the former was delamination, while the latter was delamination and shallow spallation.     

Thermophysical performances of Sm2O3-doped Gd3TaO7 oxides for thermal barrier coatings

To investigate the effect of Sm₃O₃ addition on the thermophysical performances of Gd₃TaO₇, (Gd_1−xSm_x)₃TaO₇ oxides were synthesised using sol-gel and sintering with high-temperature technologies, and their thermophysical properties were researched. The investigations exhibit that the obtained powders comprise well-distributed particles, and the bulk specimens have densified microstructures. The obtained ceramics have single pyrochlore-lattice. Owing to varied scattering strength coefficient of phonon caused by the differences in ionic radius and mass between the substituting and substituted elements, the value of thermal conductivity of (Gd_1−xSm_x)₃TaO₇ decreases firstly and further increases with the increase fraction of Sm₂O₃. The coefficient of thermal expansion of (Gd_1−xSm_x)₃TaO₇ is ameliorated owing to the higher ionic radius of Sm³⁺ than Gd³⁺. Except for Sm₃TaO₇, the synthesised ceramics display outstanding lattice steadiness up to 1400 °C.     

The toughening of pyrochlore La2Zr2O7 by a ferroelastic NdAlO3 second phase for potential thermal barrier coating applications

The poor fracture toughness of La₂Zr₂O₇ severely limits its application as a high temperature thermal barrier coating topcoat material. To toughen it, a ferroelastic second phase, NdAlO₃, with a Curie temperature of 1367°C has been introduced to form x NdAlO₃/(1-x) La₂Zr₂O₇ composite ceramics by a spark plasma sintering technique, where x = 10, 20, 30, 40, and 50 mol%. The fracture toughness of sintered composite ceramic compacts is measured at both room temperature and 1200°C, respectively, by a single-edge-notch beam test method. The results show that, at room temperature, the residual compressive stress in the La₂Zr₂O₇ matrix plays an important role in the toughening of composite ceramics. By eliminating this factor, the remaining toughening effects agree with the measured fracture toughness at 1200°C, suggesting that the other toughening is probably ferroelastic domain switching toughening and that it is still valid at high temperature. Furthermore, the toughening effect arising from ferroelastic domain switching is governed by the overall domain switching zone, which is determined by both individual domain switching zone width and the “concentration” of ferroelastic phases. A relatively high coercive stress of NdAlO₃ and relatively low residual tensile stress in NdAlO₃ second phases contribute to negligible influence of residual tensile stress on domain switching zone width, leading to the continuous increase of fracture toughness of composite ceramics with more NdAlO₃ added at room temperature.     

含Y2O3的ZnO-Bi2O3-B2O3系统玻璃热学与电学性能研究

用红外光谱仪、拉曼光谱仪和差热分析仪研究了用“熔化-急冷”制得的用于SOFC封装的含Y2O3的ZnO-Bi2O3-B2O3系统玻璃的结构和转变温度(Tg);用X射线衍射仪、热膨胀系数仪和高阻计研究了由“模压成型-热处理”制得的该玻璃制品的微晶化、热膨胀系数(α)和体积电阻率(p)情况.结果表明:Y2O3≤0.5～1.0wt.％时,Y2O3能促进[BO3]向[BO4]转变,使α降低;Y2O3≥0.5～1.0wt.％时,玻璃中Bi-O键增多,又使α降低趋势趋缓;随Y2O3增加,其作用可能由破坏网络结构逐渐向增强网络结构转变,致T8先降后升;添加Y2O3延缓了玻璃的析晶;微晶化能提高α和ρ;添加Y2O3虽致α和ρ下降,但其值仍在SOFC封接玻璃的要求范围内.     

Phase evolution and thermophysical properties of high-entropy RE2(Y0.2Yb0.2Nb0.2Ta0.2Ce0.2)2O7 oxides

Pursuing novel thermal barrier–coating materials with lower thermal conductivity and high-temperature stability can simultaneously improve the working efficiency and service temperature of a gas turbine. In this study, a series of high-entropy RE₂(Y_0.2Yb_0.2Nb_0.2Ta_0.2Ce_0.2)₂O₇ (RE = La, Nd, Sm, Gd, Dy, and Er) oxides were prepared though solid-state reaction. Through tuning the rare-earth cations, an order–disorder transition occurs from certain partially ordered weberite structure (C222₁) to disordered defective fluorite structure (Fm$\overline{3}$m). All the high-entropy RE₂(Y_0.2Yb_0.2Nb_0.2Ta_0.2Ce_0.2)₂O₇ oxides possess low thermal conductivity in the range of 0.91–1.34 W m⁻¹ K⁻¹ at room temperature, which can be attributed to increased lattice anharmonicity and disorder, resulting in additional phonon scattering. Herein, we proved that the incorporation of heterovalent cations at B-sites in high-entropy A₂B₂O₇ crystals is an effective strategy to reduce the thermal conductivity without compromising the decrease of oxygen vacancy. Moreover, the high-entropy RE₂(Y_0.2Yb_0.2Nb_0.2Ta_0.2Ce_0.2)₂O₇ oxides show the relatively higher thermal expansion coefficients of 10.3–10.7 × 10⁻⁶°C⁻¹ and excellent phase stability at elevated temperatures.     

Protecting the MoSi2 healing particles for thermal barrier coatings using a sol-gel produced Al2O3 coating

A successful sol-gel process to encapsulate molybdenum di-silicide MoSi₂ particles with a closed and thermally stable Al₂O₃ layer using aluminium tri-sec-butoxide as a precursor is presented. The processing conditions such as precursor selection and temperature were optimized through analysing the interaction of the MoSi₂ particles with the sol. The application of the sol-gel based coating was followed by calcining the coated particles at temperatures between 900 and 1200?°C in Ar. The shell composition and the mechanical stability of the microcapsule were analysed by means of X-ray diffraction, scanning electron microscopy and thermogravimetric analysis. Upon calcining at 1200?°C in Ar, the MoSi₂ core remains intact as it is, covered by an α - alumina shell with a thickness of about 0.6?μm. The stability tests proved that the encapsulate particles are about five times more oxidation resistant than the uncoated MoSi₂ particles.     

Marked reduction in the thermal conductivity of (La0.2Gd0.2Y0.2Yb0.2Er0.2)2Zr2O7 high-entropy ceramics by substituting Zr4+ with Ti4+

The (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ (x = 0–0.5) high-entropy ceramics were successfully prepared by a solid state reaction method and their structures and thermo-physical properties were investigated. It was found that the high-entropy ceramics demonstrate pure pyrochlore phase with the composition of x = 0.1–0.5, while (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂Zr₂O₇ shows the defective fluorite structure. The sintered high-entropy ceramics are dense and the grain boundaries are clean. The grain size of high-entropy ceramics increases with the Ti⁴⁺ content. The average thermal expansion coefficients of the (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ high-entropy ceramics range from 10.65 × 10^?6 K^?1 to 10.84 × 10^?6 K^?1. Importantly, the substitution of Zr⁴⁺ with Ti⁴⁺ resulted in a remarkable decrease in thermal conductivity of (La_0.2Gd_0.2Y_0.2Yb_0.2Er_0.2)₂(Zr_1-xTi_x)₂O₇ high-entropy ceramics. It reduced from 1.66 W m^?1 K^?1 to 1.20 W m^?1 K^?1, which should be ascribed to the synergistic effects of mass disorder, size disorder, mixed configuration entropy value and rattlers.     

Preparation and properties of LaMgAl11O19 thermal barrier coatings doped with Gd2O3

As a rare earth hexaaluminate, LaMgAl₁₁O₁₉ (LMA) has been one of the most promising materials used as thermal barrier coatings (TBCs). A large amount of amorphous phase, however, often exists in the plasma-sprayed LMA coating and significantly reduces the service lifetime of TBCs. In this study, La_1-xGd_xMgAl₁₁O₁₉ (x = 0, 0.2, 0.4, 0.6, and 0.8) ceramic powders are synthesised by solid-state reaction, and all of these powders are employed to prepare the corresponding coatings. The phase compositions and microstructures of samples are examined by X-ray diffraction and scanning electron microscopy, respectively. The linear thermal expansion behaviour and thermal cycling behaviour of the coatings are also analysed. The results show that the amorphous phase content is decreased and the thermal expansion behaviour is improved by doping the coatings with Gd₂O₃. The thermal cycling lifetime of the coating, however, basically remains unchanged.     

Preparation of Pd-doped Y3Al5O12 thermal barrier coatings using cathode plasma electrolytic deposition

Here, noble metal Pd-doped Y₃Al₅O₁₂ thermal barrier coatings (TBCs) were efficaciously prepared by means of cathode plasma electrolytic deposition (CPED). The formation mechanism of the Y₃Al₅O₁₂ coatings and the difference in coating performance before and after doping with Pd were analyzed. The results indicated that the preparation of the Y₃Al₅O₁₂ TBCs by using the CPED method could be divided into three stages, and the phase compositions of the coatings obtained with different deposition times were different. A single-phase Y₃Al₅O₁₂ TBCs with a 115-μm thickness was obtained after a deposition time of 20 min. After Pd doping, the average surface roughness of the TBCs decreased from 27.72 to 13.84 μm, and the high-temperature oxidation resistance and thermal shock resistance at 1050 °C improved significantly.     

Effect of Ta5+ doping on the thermal physical properties of defective fluorite Y3NbO7 ceramics

The effects of substituting the B cation in A₃BO₇ ceramics on their thermal physical properties were investigated by applying a large mass difference. Y₃(Nb_1-xTa_x)O₇ (x = 0, 0.1, 0.2, 0.3, 0.4, and 0.5) ceramics were synthesized, and their structural characteristics were determined. All the fabricated Y₃(Nb_1-xTa_x)O₇ ceramics showed defective fluorite structures and glass-like low thermal conductivity (1.18−2.04 W/m∙K at 25°C) because of the highly distorted crystal structure and significant mass difference. Substitution with Ta⁵⁺ enhanced the sintering resistance, leading to superior thermal-insulating performance via grain boundary scattering. Furthermore, the ceramics exhibited excellent coefficients of thermal expansion, implying the promising applicability of Y₃(Nb_1-xTa_x)O₇ as new thermal barrier materials. The effect of mass difference on the thermomechanical properties of the ceramics was examined, suggesting a simple strategy for engineering the chemical composition of new thermal barrier materials.     

Phase stability and thermal conductivity of RE2O3 (RE=La,Nd, Gd,Yb) and Yb2O3 co-doped Y2O3 stabilized ZrO2 ceramics

Y₂O₃ stabilized ZrO₂ (YSZ) has been considered as the material of choice for thermal barrier coatings (TBCs), but it becomes unstable at high temperatures and its thermal conductivity needs to be further reduced. In this study, 1 mol% RE₂O₃ (RE=La, Nd, Gd, Yb) and 1 mol% Yb₂O₃ co-doped YSZ (1RE1Yb–YSZ) were fabricated to obtain improved phase stability and reduced thermal conductivity. For 1RE1Yb–YSZ ceramics, the phase stability of metastable tetragonal (t′) phase increased with decreasing RE³⁺ size, mainly attributable to the reduced driving force for t′ phase partitioning. The thermal conductivity of 1RE1Yb–YSZ was lower than that of YSZ, with the value decreasing with the increase of the RE³⁺ size mainly due to the increased elastic field in the lattice, but 1La1Yb–YSZ exhibited undesirably high thermal conductivity. By considering the comprehensive properties, 1Gd1Yb–YSZ ceramic could be a good potential material for TBC applications.     

Microstructure and luminescence properties of YSZ-based thermal barrier coatings modified by Eu2O3

In order to study the variation of rare earth oxides during thermal failure of thermal barrier coatings, Eu2O3-doped YSZ coatings with 0.5 mol%, 1.0 mol% and 1.5 mol% were prepared by explosive spraying. SEM, XRD, EDS and microhardness tester were used to analyze the effect of different rare earth oxide doping content on the morphology, composition and mechanical properties of the coatings. The results showed that with the increase of rare earth oxide doping content, the porosity of the coatings decreased, and the microhardness and fracture toughness increased. When the doping amount of rare earth oxide is 1.0 mol%, the bonding strength and thermal cycle times of the coating are the highest, 33.4 Mpa and 185 times respectively. With the increase of the doping amount of rare earth, the luminous intensity of the sprayed coating increases. After thermal shock test, the luminous intensity of Eu2O3-doped YSZ coatings at 592 and 608 nm decreased to a certain extent.     

Resistance of 2ZrO2·Y2O3 top coat in thermal/environmental barrier coatings to calcia‐magnesia‐aluminosilicate attack at 1500°C

Internally cooled, hollow SiC‐based ceramic matrix composites (CMCs) components that may replace metallic components in the hot section of future high‐efficiency gas‐turbine engines will require multilayered thermal/environmental barrier coatings (T/EBCs) for insulation and protection. In the T/EBC system, the thermally insulating outermost (top coat) ceramic layer must also provide resistance to attack by molten calcia‐magnesia‐aluminosilicate (CMAS) deposits. The interactions between a potential candidate for top coat made of air‐plasma‐sprayed (APS) 2ZrO₂·Y₂O₃ solid‐solution (ss) ceramic and two different CMASs (sand and fly ash) are investigated at a relevant high temperature of 1500°C. APS 2ZrO₂·Y₂O₃(ss) top coat was found to resist CMAS penetration at 1500°C for 24 hours via reaction products that block CMAS penetration pathways. In situ X‐ray diffraction (XRD) studies have identified the main reaction product to be an Ca‐Y‐Si apatite, and have helped elucidate the proposed mechanism for CMAS attack mitigation. Ex situ electron microscopy and analytical spectroscopy studies have identified the advantageous characteristics of the reaction products in helping the CMAS attack mitigation in the APS 2ZrO₂·Y₂O₃(ss) coating at 1500°C. Finally, the Y³⁺ solubility limit and transport behavior are identified as potential comparative tools for assessing the CMAS resistance ability of top‐coat ceramics.     

Y2O3掺杂CaO-Al2O3-SiO2系微晶玻璃的研究

在CaO-Al2O3-SiO2(CAS)系统微晶玻璃中引入稀土氧化物Y2O3,结合XRD、SEM等测试手段,研究了Y2O3的引入对微晶玻璃烧结过程、微观结构以及性能的影响。实验结果表明:随着Y2O3的引入,微晶玻璃烧结温度降低,烧结时间变短。当Y2O3质量分数为3.25%时,CAS微晶玻璃具有最佳的制备工艺和最好的力学性能。     

New class of high-entropy defect fluorite oxides RE2(Ce0.2Zr0.2Hf0.2Sn0.2Ti0.2)2O7 (RE = Y,Ho, Er,or Yb) as promising thermal barrier coatings

High-entropy ceramics exhibit great application potential as thermal barrier coating (TBC) materials. Herein, a series of novel high-entropy ceramics with RE₂(Ce_0.2Zr_0.2Hf_0.2Sn_0.2Ti_0.2)₂O₇ (RE₂HE₂O₇, RE = Y, Ho, Er, or Yb) compositions were fabricated via a solid-state reaction. X-ray diffraction (XRD) and energy dispersive spectrometry (EDS) mapping analyses confirmed that RE₂HE₂O₇ formed a single defect fluorite structure with uniform elemental distribution. The thermophysical properties of the RE₂HE₂O₇ ceramics were investigated systematically. The results show that RE₂HE₂O₇ ceramics have excellent high-temperature phase stability, high thermal expansion coefficients (10.3–11.7 × 10^?6 K^-1, 1200 ℃), and low thermal conductivities (1.10-1.37 W m^-1 K^-1, 25 ℃). In addition, RE₂HE₂O₇ ceramics have a high Vickers hardness (13.7–15.0 GPa) and relatively low fracture toughness (1.14-1.27 MPa m^0.5). The outstanding properties of the RE₂HE₂O₇ ceramics indicate that they could be candidates for the next generation of TBC materials.