Vanadate Hot Corrosion Behavior of India, Yttria-Stabilized Zirconia

Powders of In₂O₃–Y₂O₃"dual stabilized"ZrO₂ (IYSZ) were prepared using sol-gel procedures and tested for resistance to destabilization by molten NaVO₃ at 700° and 900°C. IYSZ powders with stabilizer oxide compositions ranging from 100% down to about 50% In₂O₃ were superior to 100%-Y₂O₃-stabilized ZrO₂ (YSZ) in resistance to destabilization, especially at 700°C. Small additions of Y₂O₃ were speculated to reduce the acidity of the ZrO₂ oxide anion lattice, and, therefore, improve bonding of In₂O₃ (more acidic than Y₂O₃) into the ZrO₂ lattice.     

Anisotropic Ionic Conductivity Observed in Superplastically Deformed Yttria-Stabilized Zirconia/Alumina Composite

Ionic conductivity measurements on a yttria-stabilized tetragonal zirconia polycry stall alumina composite subjected to superplastic deformation demonstrate anisotropic character. Parallel to the pressing direction, the grain-boundary resistance to oxygen ion mobility is 25% to 30% higher than that measured perpendicular to the pressing direction. The same directional dependency on the volume conductivity is observed but is less pronounced, showing approximately a 9% difference. Microstructural evidence reveals an agglomeration and elongation of alumina particles perpendicular to the pressing direction, and it is suggested that this phenomenon restricts the passage of ions parallel to the compression direction, giving rise to the anisotropic nature of the conductivity measurements.     

In Situ Raman Spectroscopic Study of Yttria-Stabilized Zirconia Attack by Molten Sodium Vanadate

Raman spectroscopy has been used to study the molten salt attack of ceramics used as thermal barrier coatings. Zirconia stabilized with 8 wt% yttria was immersed in sodium metavanadate melts and in sodium metavanadate/sodium sulfate melts. In situ Raman measurements allowed simultaneous observation of the ceramic phases and salt chemistry during the attack process. The ceramic was seen to transform from the cubic/tetragonal to the monoclinic phase, concurrent with chemical changes in the molten salt layer in contact with the ceramic. These in situ measurements were complemented by conventional postexposure examination and by postexposure Raman measurements. The rate of attack was found to be quite sensitive to the mole fraction of vanadate in the molten salt solution.     

Effects of Yttria-Stabilized Zirconia on Plasma-Sprayed Hydroxyapatite/Yttria-Stabilized Zirconia Composite Coatings

The low bonding strength between hydroxyapatite (HA) and the metal substrate interface of plasma-sprayed HA coating has been a point of potential weakness in its application as a biomedical prosthesis. In the present study, yttria-stabilized (8 wt%) zirconia (YSZ) has been used to enhance the mechanical properties of HA coatings. The effects of YSZ additions (in the range 10–50 wt%) on the phase composition, microstructure, bond strength, elastic modulus, and fracture toughness of plasma-sprayed HA/YSZ composite coatings have been studied. The results indicated that decomposition of HA during plasma spraying was reduced significantly with the addition of zirconia. The higher the zirconia content, the lower the amount of calcium oxide, tricalcium phosphate, and tetracalcium phosphate formed in the coatings. In addition, there was a trace of calcium zirconate formed when less than 30 wt% zirconia was present. A solid solution of HA mixed with YSZ formed during plasma spraying; however, the amount of unmelted particles increased as the zirconia increased. The mechanical properties of the HA/YSZ composite coatings, such as bond strength, elastic modulus, and fracture toughness, increased significantly as the contents of zirconia increased.     

Microstructure and High-Temperature Mechanical Behavior of Alumina/Alumina–Yttria-Stabilized Tetragonal Zirconia Multilayer Composites

Layered composites of alternate layers of pure Al₂O₃(thickness of 125 μ m) and 85 vol% Al₂O₃-15 vol% ZrO₂ that was stabilized with 3 mol% Y₂O₃(thickness of 400 μ m) were obtained by sequential slip casting and then fired at either 1550° or 1700°C. Constant-strain-rate tests were conducted on these materials in air at 1400°C at an initial strain rate of 2 × 10^-5 s^-1. The load axis was applied both parallel and perpendicular to the layer interfaces. Catastrophic failure occurred for the composite that was fired at 1700°C, because of the coalescence of cavities that had developed in grain boundaries of the Al₂O₃ layers. In comparison, the composite that was fired at 1550°C demonstrated the ductility of the Al₂O₃+YTZP layer, but at a flow stress level that was determined by the Al₂O₃ layer.     

Space Charge-Mediated Ionic Transport in Yttria-Stabilized Zirconia–Alumina Composite Membranes

This paper reports ionic conductivity of yttria-stabilized zirconia (YSZ)–Al₂O₃ composite membranes. The tape cast specimens were subjected to binder burnout (500°C) and sintering (1550°C) processes to obtain 200–300 μm thick membranes. The ionic conductivity and microstructure of the membranes were characterized and are discussed in this paper. The ionic conductivity of the composite specimens was enhanced and was correlated with the number of charge carrier and their mobility. The solubility of Al₂O₃ in YSZ was minimal and nanosize Al₂O₃ of the batch sintered into microsize and existed as a distinct phase. The scanning electron microscopy micrographs revealed that YSZ and Al₂O₃ grains were strained.     

Hot Isostatic Pressing of a Presintered Yttria-Stabilized Zirconia Ceramic

The effect of hot isostatic pressing on the bend strength of ZrO₂–3 mol% Y₂O₃ was critically dependent on the presintering process. Optimally sintered bodies contained no open porosity and exhibited large increases in strength following hot isostatic pressing. When open porosity of as little as 0.3% persisted after sintering, hot isostatic pressing increased the bulk density, but little or no increase in strength was realized. Two-parameter Weibull analysis of the strength data was used to quantify the strength improvement obtained following hot isostatic pressing. Typical fracture-initiating flaws were identified through optical and electron microscopy.     

Mechanism of Thermal Transport in Zirconia and Yttria-Stabilized Zirconia by Molecular-Dynamics Simulation

We present results of molecular-dynamics simulations of the thermal conductivity, κ, of ZrO₂ and Y₂O₃-stabilized ZrO₂ (YSZ). For both pure ZrO₂ and YSZ with low concentrations of Y₂O₃, we find that the high-temperature κ is typical of a crystalline solid, with the dominant mechanism being phonon-phonon scattering. With increasing Y₂O₃ concentration, however, the mechanism changes to one more typical of an amorphous system. In particular, phononlike vibrational modes with well-defined wave vectors appear only at very low frequencies. As in amorphous materials, the vast majority of vibrational modes, while delocalized, do not propagate like ordinary phonon modes but transport energy in a diffusive manner. We also find that the few highest frequency modes are localized and do not contribute to κ.     

硝酸熔盐对不锈钢材料腐蚀行为的研究

通过熔盐热腐蚀浸泡实验,研究了301奥氏体不锈钢在不同温度硝酸熔盐中的腐蚀性能,考察了熔盐温度、浸泡时间对不锈钢材料表面结构和各主要组成元素含量的影响。结果表明,提高熔盐温度和延长浸泡时间加速不锈钢材料的腐蚀,致使不锈钢材料表面铁元素含量降低,氧元素含量提高。这为高温熔盐关键部件基体材料的选择提供重要依据。     

Ceria–Yttria-Stabilized Zirconia Composite Ceramic Systems for Applications as Low-Temperature Electrolytes

Ceramic samples of Ce_{1− x} Gd _x O_{2− y} and yttria-stabilized zirconia (YSZ) were prepared using solid-state methods. Polished faces of disks of these materials were held in intimate contact in a reaction cell at temperatures ranging from 1000° to 1300°C for durations up to 72 h. XRD, SEM, and microprobe Raman techniques were used to analyze the resulting reactions and ion diffusion. No reaction was observed at 1000°C after 72 h between the 10-mol%- and 20-mol%-Gd-doped CeO₂ and the YSZ. However, at 1300°C, a mixing region 25 μ m wide occurred, resulting in a cubic phase, where Zr⁴⁺ ions diffused into the CeO₂.     

Influence of Zirconia Interfacial Coating on Alumina Fiber‐reinforced Alumina Matrix Composites

The present study demonstrates an approach for fabricating fiber‐reinforced ceramic matrix composites (CMCs) involving the coating of 2‐dimensional woven alumina fibers with zirconia layer by sol gel, followed by impregnation of these coated fibers with alumina matrix and pressureless sintering. To emphasize the benefits of the zirconia coating on these CMCs, a reference sample without interfacial coating layer was prepared. The zirconia‐coated CMCs showed superior flexural strength and thermal shock resistance compared with their uncoated counterparts. Foreign object damage tests carried out on the ZrO₂ coated CMCs at high impact speed showed localized damage without any shattering.     

Mechanism of Reaction between Lanthanum Manganite and Yttria-Stabilized Zirconia

The reaction of La_{1- x} Ca _x MnO₃ ( x = 0, 0.1, 0.2) with ZrO₂-8 mol% Y₂O₃ (YSZ) has been investigated at temperatures ranging from 1300° to 1425°C in air. Substitution of Ca for La in LaMnO₃ depresses the reactivity with YSZ. A layer of La₂Zr₂O₇ is formed at the La_{1- x} Ca _x MnO₃/YSZ interface after an induction period, and its formation is accelerated when the La_{1- x} Ca _x MnO₃ phase is porous. The reaction proceeds by unidirectional diffusion of La, Mn, and/or Ca ions, mainly Mn ions, into YSZ. The diffusion coefficients of La and Mn ions in YSZ, which are estimated using a LaMnO₃/single-crystal YSZ couple, are much lower than that of oxygen ion. From the experimental data, a reaction mechanism is proposed.     

Effect of Alumina on Sintering Behavior and Electrical Conductivity of High-Purity Yttria-Stabilized Zirconia

The sintering behavior and electrical conductivity of high-purity 8-mol% Y₂O₃-stabilized ZrO₂ (8YSZ) with Al₂O₃ additions were investigated. The addition of 1 wt% AI₂O₃ to 8YSZ provided dense, sintered samples with 9.1% relative density at 1400°C without a holding time. Addition of 1 wt% SiO₂ enhanced the sinterability of 8YSZ. Na₂O addition of 0.1 wt% remarkably lowered it. Electrical conductivity at 1000°C in air increased slightly with increased Ai₂O₃ content up to 1 wt% and then monotonously decreased. 8YSZ with 1 wt% AI₂O₃ showed the maximum conductivity of 0.16 S/cm at 1000°C.     

Thermal Conductivity of Plasma-Sprayed Monolithic and Multilayer Coatings of Alumina and Yttria-Stabilized Zirconia

The temperature dependence of the thermal conductivity of plasma-spray-deposited monolithic coatings, as well as multilayer coatings that consisted of Al₂O₃ and ZrO₂ that was stabilized by 8% Y₂O₃ (YSZ), was investigated. The coatings exhibited a large reduction in thermal conductivity at all temperatures, when compared to the bulk monolithic Al₂O₃ and YSZ. This reduction was due to porosity as well as thermal resistance that was caused by interfaces in the coatings. The largest decrease in the thermal conductivity of the coatings, relative to monolithic fully dense materials, was due to splat interfaces within each layer, as well as the coating/substrate interface. On the other hand, the multilayer coatings showed little variation in the thermal conductivity, relative to the number of layers, which suggests that the influence of interlayer interfaces on heat transfer is relatively small. A one-dimensional analysis of steady-state heat transfer has been presented to illustrate the significance of porosity, splat interfaces, and interlayer interfaces, with respect to the overall thermal conductivity of multilayer coatings.     

Tape Casting of Fine Alumina/Zirconia Powders for Composite Fabrication

Ceramic films, containing AI₂O₃, with up to 40 vol% ZrO₂, have been fabricated using the tape casting process. Finer powders (average mean diameter of 250–300 nm) than have generally been reported for tape casting were used in this study. The optimum formulation for tape casting is affected substantially by decreasing particle size. For example, the amount of dispersant needed is increased. Moreover, the amount of plasticizer/binder must be increased so as to maintain the solids content in the dried tapes below a critical level (about 55 vol% in this case), which decreases with particle size. Rheological studies on the effectiveness of menhaden fish oil and phosphate ester as dispersants show that phosphate ester can be used in lower concentrations, for the preparation of higher solids loading slurries, and was therefore selected for further study. The amount of dispersant required to obtain minimum slurry viscosity was found to be primarily dependent upon the effective particle surface area, defined as that available to the dispersant molecules. In the case of particles composed of agglomerated crystallites (such as the ZrO₂, powder used here), this may be considerably less than that measured by nitrogen absorption. Moreover, the porous internal structure of such powders is filled with solvent, which increases the effective solids loading of the slurry, and thus its viscosity. Particle morphology also influences the packing efficiency; i.e., the green density decreases as ZrO₂, is added.     

氟碳/石墨烯复合涂层的耐蚀性能研究

采用超声分散技术制备了氟碳树脂/石墨烯复合涂层,通过结合强度测试、接触角测试、电化学测试和盐雾试验等研究了石墨烯对氟碳涂层性能的影响。结果表明,涂层中加入石墨烯后其结合强度、接触角和耐盐雾性能均有一定程度的提高。石墨烯含量为0.05wt%时,复合涂层的涂层电阻和电荷转移电阻比其他涂层高2~3个数量级,具有最好的腐蚀防护性能。     

Formation of a Zinc Oxide/Yttria-Stabilized Zirconia Interface

Zinc oxide was deposited on the (111) face of an yttria-stabilized zirconia single crystal using the chemical vapor phase method. Zinc vapor was obtained by reducing polycrystalline ZnO in hydrogen at 1200°C. The deposition of the ZnO was accomplished by the reoxidation of zinc vapor at temperatures of 1300° to 1345°C.     

Postsintering Hot Isostatic Pressing of Ceria-Doped Tetragonal Zirconia/Alumina Composites in an Argon–Oxygen Gas Atmosphere

Ceria-doped tetragonal zirconia (Ce-TZP)/alumina (Al₂O₃) composites were fabricated by sintering at 1450° to 1600°C in air, followed by hot isostatic pressing (postsintering hot isostatic pressing) at 1450°C and 100 MPa in an 80 vol% Ar–20 vol% O₂ gas atmosphere. Dispersion of Al₂O₃ particles into Ce-TZP was useful in increasing the relative density and suppressing the grain growth of Ce-TZP before hot isostatic pressing, but improvement of the fracture strength and fracture toughness was limited. Postsintering hot isostatic pressing was useful to densify Ce-TZP/Al₂O₃ composites without grain growth and to improve the fracture strength and thermal shock resistance.     

氮化硅陶瓷的熔盐腐蚀研究进展

氮化硅陶瓷具有优良的高温性能,广泛应用于各种高温部件中,但由于在高温时氮化硅在热力学上不稳定,易和环境介质反应产生腐蚀失效,因此氮化硅陶瓷抗腐蚀性能研究近年越来越得到重视。本文简述了氮化硅陶瓷熔盐腐蚀的背景及腐蚀性能测试方法,对氮化硅陶瓷在多种钠盐中的熔盐腐蚀机理、动力学研究等进行归纳总结,并对今后提高氮化硅陶瓷抗腐蚀性能研究提出了建议。