Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composites

A porous oxide fiber coating was investigated for Nextel^™ 610 fibers in an alumina matrix. Polymeric-solution-derived yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) with a fugitive carbon phase was used to develop the porous fiber coating. Ultimate tensile strengths of tows and minicomposites following heat treatments in argon and/or air were used to evaluate the effect of the porous fiber coating. The porous YAG fiber coatings did not reduce the strength of the tows when heated in argon, and they degraded tow strength by only ∼20% after heating in air at 1200°C for 100 h. Minicomposites containing porous YAG-coated fibers were nearly twice as strong as those containing uncoated fibers. However, after heating at 1200°C for 100 h, the porous YAG coatings densified to >90%, at which point they were ineffective at protecting the fibers, resulting in identical strengths for minicomposites with and without a fiber coating.     

Low-Temperature Synthesis of Nanocrystalline Yttrium Aluminum Garnet Powder Using Triethanolamine

Nanocrystalline yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) was synthesized by pyrolysis of complex compounds of aluminum and yttrium with triethanolamine [(HOCH₂CH₂)₃N, (TEA)]. Loose and porous precursor was obtained on complete dehydration of the metal ion–triethanolamine complexes. Pure YAG powder was obtained by calcination of the precursor at 950°C. The precursor was characterized by simultaneous thermogravimetry, differential scanning calorimetry, and mass spectra analyses (TG–DSC–MS). The heat-treated powders were characterized by X-ray diffractometry (XRD), specific surface area measurements, and transmission electron microscopy (TEM). The average crystallite size as determined from X-ray line broadening and transmission electron microscopy studies was ∼40 nm. The effects of the calcination temperature and the ratio of triethanolamine to mixed metal ions were also studied.     

Synthesis of Yttrium Aluminum Garnet from a Mixed-Metal Citrate Precursor

Yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) was synthesized using a polymeric precursor derived from a mixed-metal citric acid/ethylene glycol/ethanol solution. YAG was found to crystallize directly from an amorphous precursor beginning at temperatures as low as 600°C within 1 h in air. The polymer resin concentration was found to have an effect on the temperature of crystallization initiation. However, all precursors produced a well-crystallized YAG powder within 1 h at 800°C in air. Formation of phase-pure YAG in an argon atmosphere did not occur until heating for 1 h at 1000°C. An optimum cation-to-resin ratio to maximize reactivity provides a polymeric network to ensure a homogeneous dispersion of cations, yet minimizes cation diffusion distances within the char by limiting excess free carbon after polymer pyrolysis.     

Influence of Moisture on Ultra-High-Temperature Tensile Creep Behavior of in Situ Single-Crystal Oxide Ceramic Alumina/Yttrium Aluminum Garnet Eutectic Composite

Tensile creep tests were conducted for an in situ single-crystal alumina/yttrium aluminum garnet (Al₂O₃/Y₃Al₅O₁₂ (YAG)) binary system eutectic composite at temperatures between 1773 and 1873 K in air and in a moist environment having a water-vapor pressure range of 0.06–0.6 MPa, under a constant tensile stress range of 100–160 MPa. The Al₂O₃/YAG eutectic composite exhibited a stress exponent of 8–13, indicative of tensile creep behavior characterized by a dislocation back-stress mechanism. Water-vapor pressures ≤0.4 MPa led to a significant acceleration of creep rates as a result of enhanced dislocation mobility in the Al₂O₃ and YAG phases.     

Creep Mechanism of Polycrystalline Yttrium Aluminum Garnet

The high-temperature deformation behavior of a fine-grained polycrystalline yttrium aluminum garnet (YAG) was studied in the temperature range of 1400° to 1610°C using constant strain rate compression tests under strain rates ranging from 10⁻⁵/s to 10⁻³/s. The stress exponent of the creep rate, the activation energy in comparison with that for single-crystal YAG, and the grain size dependence suggest that Nabarro–Herring creep rate limited by the bulk diffusion of one of the cations (Y or Al) is the operative mechanism.     

热处理温度对钇铝石榴石(Y3Al5O12)析晶的影响

透明YAG多晶陶瓷具有优良的光学、机械与化学性能,逐渐成为新一代固体激光基质材料.分散均匀、团聚轻、超细颗粒的纳米前驱体粉末有利于制备出高度透明的激光陶瓷.以Y2O3,Al(NO3)3·9H2O和柠檬酸为原料,采用柠檬酸-凝胶法和低温自蔓延燃烧反应相结合制备出黑色粉末,经1100℃烧结出YAG纳米前驱体粉末.采用TG-DTA,XRD,FT-IR和TEM测试手段对YAG前驱体粉末进行表征,采用谢莱公式计算出不同烧结温度下晶粒尺寸.研究结果表明:随热处理温度升高,晶粒平均尺寸增加,标准偏差以小幅度增加,晶粒尺寸分布曲线保持一致,晶格参数减小.随热处理时间增加,晶粒主要以晶界扩散形式线性长大.纳米晶粒表面原子呈不规则分布,导致晶格参数增加,前驱体颗粒由单个晶粒所构成.     

Spherical Yttrium Aluminum Garnet Embedded in a Glass Matrix

Containerless processing was used to investigate the glass-forming behavior of Al₂O₃–Y₂O₃ glass. The amorphous bulk samples were obtained at compositions with 25–37.5 mol% yttria when the melt was cooled at a cooling rate of ∼250 K/s. Although small spherical particles (∼10 μm) with the same composition of the matrix were detected in the amorphous samples with 32.5–37.5 mol% yttria, the microfocus X-ray diffraction result indicated that the small spherical particles were crystalline Y₃Al₅O₁₂ garnet (YAG), rather than being amorphous. This observation suggested that small YAG particles could not grow larger after their nucleation, because of the high viscosity at high undercooling and the high cooling rate, which would graze the nose of the continuous cooling temperature diagram of YAG.     

Low-Temperature Fabrication of Transparent Yttrium Aluminum Garnet (YAG) Ceramics without Additives

A carbonate precursor of yttrium aluminum garnet (YAG) with an approximate composition of NH₄AlY_0.6(CO₃)_1.9(OH)₂·0.9H₂O was synthesized via a coprecipitation method from a mixed solution of ammonium aluminum sulfate and yttrium nitrate, using ammonium hydrogen carbonate as the precipitant. The precursor precipitate was characterized using chemical analysis, differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. The sinterability of the YAG powders was evaluated by sintering at a constant rate of heating in air and vacuum sintering. The results showed that the precursor completely transforms to YAG at ∼1000°C via the formation of a yttrium aluminate perovskite (YAP) phase. YAG powders obtained by calcining the precursor at temperatures of ≤1200°C were highly sinterable and could be densified to transparency under vacuum at 1700°C in 1 h without additives.     

钇铝石榴石纤维的制备和应用研究进展

钇铝石榴石纤维具有耐高温、抗氧化、低导热率、优异的抗高温蠕变性和良好的光学性能,是一种理想的结构增强材料、绝热耐火材料和光学材料.本文重点评述了近年来钇铝石榴石纤维制备和应用的研究进展,并展望了钇铝石榴石纤维制备和应用的发展趋势.     

Synthesis of Yttrium Aluminum Garnet by the Glycothermal Method

The reaction of a stoichiometric mixture of aluminum isopropoxide and yttrium acetate in 1,4-butanediol at 300°C yielded crystalline yttrium aluminum garnet having an approximate particle size of 30 nm. No other phases were detected. The use of ethylene glycol in place of 1,4-butanediol afforded an amorphous product.     

Yttrium Aluminum Garnet (YAG) Films through a Precursor Plasma Spraying Technique

Coatings of yttrium aluminum garnet (Y₃Al₅O₁₂, YAG), which is a promising high-temperature ceramic, were developed for the first time using a novel precursor plasma spraying (PPS) technique. The precursor sol was sprayed using a radio-frequency induction plasma technique. X-ray diffraction analysis of the as-sprayed coatings confirmed that a metastable hexagonal yttrium aluminate (H-YAlO₃) was the major phase. The above-described specimen, on further treatment with plasma, was converted to cubic garnet (YAG) as the major phase, with a minor amount of orthorhombic YAlO₃ (O-YAP) phase. ²⁷Al magic-angle spinning nuclear magnetic resonance of the YAG coating corroborated the X-ray results and confirmed the presence of YAG and O-YAP phases. Formation of the garnet phase through the PPS technique is proof that the chemistry can be controlled in the plasma. This finding opens up new avenues for developing complex functional oxide deposits.     

Grain Boundary Grooving Studies of Yttrium Aluminum Garnet (YAG) Bicrystals

Grain boundary grooving experiments were conducted with Σ5 (210) twist boundaries in Y₃Al₅O₁₂ (YAG) with the goal of extracting information on diffusion in YAG. Planar boundaries oriented 90° to the surface were annealed in air at various times and temperatures. Atomic force microscopy was used to characterize the subsequent grooves. The Mullins approach leads to the following expression for the diffusion coefficient: D (m²/s) = 3.9 × 10⁻¹⁰ exp[−330 ± 75 (kJ/mol)/ RT ]. The relatively low activation energy agrees well with earlier oxygen tracer diffusion measurements on YAG, suggesting that oxygen is the limiting diffusing species in boundary grooving of YAG.     

Ytterbium Cation Diffusion in Yttrium Aluminum Garnet (YAG)—Implications for Creep Mechanisms

The lattice and grain-boundary diffusion coefficients of ytterbium, which substitutes for yttrium, have been determined in high-purity, stoichiometric yttrium aluminum garnet (YAG) polycrystals in the temperature range 1400°–1550°C, in air. Ytterbium oxide thin films were produced on the YAG surfaces by a dipping method. After diffusion treatments, the penetration profiles were established by secondary ion mass spectroscopy, and the diffusion coefficients were calculated from the thin-film solution of Fick's equation. The difference between the volume and grain-boundary diffusion coefficients is ∼5 orders of magnitude in the temperature range studied. The cation activation energies (∼550 kJ/mol) are much larger than those for oxygen (∼300–350 kJ/mol). The effective diffusion coefficient deduced from high-temperature deformation data reported in the literature for YAG polycrystals, assuming grain-boundary sliding accommodated by volume diffusion, is in excellent agreement, both in magnitude and activation energy, with the cation diffusion data.     

Bulk, Dense, Nanocrystalline Yttrium Aluminum Garnet by Consolidation of Amorphous Powders at Low Temperatures and High Pressures

Amorphous Al₂O₃–37.5% Y₂O₃ powders, prepared using spray pyrolysis followed by partial or complete thermal decomposition, were hot-pressed at 315°–640°C and 500 or 750 MPa uniaxial pressure. Hot pressing of fully decomposed amorphous powder at 450°–640°C at pressures up to 750 MPa led to densification (up to 96%) as well as nanocrystallization of yttrium aluminum garnet (YAG). When the pressure was applied during heating, instead of after reaching the final temperature, higher relative densities resulted. Fully crystalline starting powder did not densify. The low true density of the amorphous phase (3.1 g·cm⁻³) was believed to be responsible for the densification through enhanced ionic mobilities.     

Effect of Yttria and Yttrium-Aluminum Garnet on Densification and Grain Growth of Alumina At 1200°–1300°C

Densification and grain growth of alumina were studied with yttria or yttrium-aluminum garnet (YAG) additives at the relatively low temperatures of 1200°–1300°C. Yttria doping was found to inhibit densification and grain growth of alumina at 1200°C and, depending on dopant level, had a lesser effect at 1300°C. At 1200°C, yttria inhibits densification more than it hinders grain growth. The rate of grain growth increases faster with temperature than the rate of densification. Alumina-YAG particulate composites were difficult to sinter, yielding relative densities of only 65% and 72% after 100 h at 1200° and 1300°C, respectively. Pure YAG compacts exhibited essentially no densification for times up to 100 h at 1300°C.     

钇铝石榴石(YAG)激光透明陶瓷研究进展

钇铝石榴石(YAG)激光透明陶瓷由于具有单晶、玻璃激光材料无可比拟的优势而成为研究热点,并得到迅速发展,高性能的稀土元素掺杂YAG透明激光陶瓷被相继报导.本文综述了近年来国内外关于YAG激光透明陶瓷的最新研究成果.主要包括YAG微细粉体合成、烧结添加刺及多晶YAG透明陶瓷的致密化烧结技术,并对比了YAG透明陶瓷相对于Y...     

A Novel Way to Synthesize Yttrium Aluminum Garnet from Metal–Inorganic Precursors

Yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) was synthesized by sol–gel processing from the stoichiometric amounts of aluminum pellets, Y(NO₃)₃·6H₂O, and Al(NO₃)₃·9H₂O or AlCl₃·6H₂O, with suitable kinds of acid (citric acid, acetic acid, etc.) as catalysts. Polycrystalline YAG powder was obtained by drying the YAG precursor followed by calcination at temperatures above 900°C. Thermogravimetry/differential thermal analysis and Fourier transform infrared specotrscopic analyses in air showed an exothermic peak at ∼900°C, attributed to the formation of a polycrystalline YAG phase and weight loss of 60% at 1000°C, caused by the decomposition of hydroxyl and NO₃⁻, etc. X-ray diffraction analysis showed that YAG can be formed at 900°C, and no other intermediate was observed. In particular, the YAG sol can be used for dry-spinning fibers with the aid of some organic polymer.     

Structural Evolution in Sol–Gel-Derived Yttrium Aluminum Garnet–Alumina Precursor Fibers

A sol leading to a eutectic Al₂O₃–Y₃Al₅O₁₂ composition was spun into fibers. These fibers were dried and pyrolyzed between 200° and 850°C in a nitrogen or water-vapor atmosphere. Pyrolysis in nitrogen resulted in dense, amorphous fibers with considerable residual carbon content. In water vapor, fibers also remained amorphous, but organics were almost completely removed. The loss of organics created micropores that grew as the pyrolysis temperature increased. The amorphous fiber structures were examined by nitrogen adsorption, helium pycnometry, and transmission electron microscopy. Young's moduli of the pyrolyzed fibers were measured, and porosities of the fibers were calculated from both nitrogen-adsorption data and elastic behavior. Sintering tests showed the best sinterability for fibers pyrolyzed in water vapor to 385°–500°C.     

共沉淀法合成掺钕钇铝石榴石纳米粉体的研究

掺钕钇铝石榴石(Nd:YAG)多晶透明陶瓷具有良好的化学稳定性、光学性能和耐高温性能,是一种很有前途的激光工作物质.以Al(NO3)·9H2O,Y2O3,Nd2O3,(NH4)2SO4和NH4HCO3为原料,正硅酸乙酯为添加剂,采用共沉淀法制备出分散均匀、团聚程度轻、YAG立方晶相的Nd:YAG纳米前驱体粉末,采用TG/DTA,XRD,FT-IR和TEM等测试手段对Nd:YAG陶瓷材料进行表征.研究结果表明:前驱体粉末在800℃时为无定型态,当温度达到900℃时析出大量的晶体YAlO3(YAP)和少量的YAlO3(YAP),当温度达到1100℃时就全部转化为立方晶相;前驱体纳米粉末中存在轻微的团聚,主要是在1100℃高温时晶粒发生了生长,连接在一起,但是作为团聚整体而言,颗粒分布比较均匀.     

Effect of Moisture on the High-Temperature Stability of Unidirectionally Solidified Al2O3/YAG Eutectic Composites

The corrosion resistance of a unidirectionally solidified alumina/yttrium aluminum garnet (Al₂O₃/YAG) eutectic mixture was investigated at high temperature. Samples were exposed to high temperature (1200°–1800°C) in different atmospheres, which included argon, argon/water vapor, air, and air/water vapor. The most important microstructural changes occurred at the interface between the YAG and the Al₂O₃. Those changes consisted of localized thermal grooving, especially when the corrosive atmosphere contained water vapor. The samples exhibited significant weight loss at high temperature (1800°C) after 20 h of exposure. The calculated volume gain that was induced by the increased surface relief was low and limited, except when the corrosive atmosphere contained air, which indicated that the presence of air (particularly oxygen) induced a more-active corrosion process. On the other hand, no change in the flexural strength was observed, even after 100 h at 1800°C in a humid atmosphere, because of the cross-linked structure of the composite, which limited propagation of the groove.