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...     

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.     

Oxygen Ion Diffusion in Single-Crystal and PoIycrystaIIine Yttrium Iron Garnet

The oxygen ion self-diffusion coefficient was determined for single-crystal and polycrystalline yttrium iron garnet (Y₃Fe₅O₁₂). The rate of exchange between oxygen gas enriched with the stable isotope ¹⁸O and solid yttrium iron garnet was measured. Oxygen ion diffusion rates were found to be the same in single-crystal and 8μ polycrystalline Y₃Fe₅O₁₂ between 1100° and 1400°C. This is in contrast to previous measurements of anion diffusion in several alkali halides and in AI₂O₃ where a strong dependence of diffusion rates on the presence of grain boundaries was found. Enhancing oxidation rates in dense, reduced yttrium iron garnet at low temperature by minimizing the final fired grain size in the sintering process does not appear to be possible on the basis of the results obtained in this investigation. The temperature dependence of the diffusion coefficient of oxygen measured at 100 torr can be represented by D = 0.40 exp (-65.4/RT) cm² per second.     

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.     

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.     

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.     

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.     

Phase Development and Luminescence in Chromium-Doped Yttrium Aluminum Garnet (YAG:Cr) Phosphors

The phase development and luminescence of chromium-doped yttrium aluminum garnet (Y₃Al₅O₁₂:Cr or YAG:Cr) phosphors, prepared by both a chemical precipitation technique and a solid-state reaction, were studied. The YAG structure formed at a much lower temperature and by a different phase development sequence when the chemical method was employed. The light output of the chemically synthesized powders, measured by laser excitation, was discovered to increase with increasing heat treatment temperature and was found to be brightest when the YAG:Cr phosphor had excess aluminum.     

The Influence of Cation Additives on Grain‐Boundary Mobility in Yttrium Aluminum Garnet (YAG)

Pure yttrium aluminum garnet (YAG) nano‐powders, doped with 1.4 at.% of La₂O₃, ZrO₂, MgO, Nb₂O₅, and SiO_2, were vacuum sintered to full density and subjected to grain growth at 1700°C, for up to 15 h. The YAG intrinsic grain‐boundary (GB) mobility, determined from the pure fully dense YAG specimens, was 2.9 × 10^?16 [m³·N^?1·s^?1]. All dopants, except for La₂O₃, increased the GB mobility compared to pure YAG (La₂O₃ didn't cause any significant change in YAG's GB mobility). All GBs were found to be free of secondary phases or intergranular films. These findings differ from numerous publications where dopants were found to either inhibit grain growth by solute drag, or to enhance grain growth due to liquid phase sintering. It was found that the GB mobility systematically increased with the decrease in the dopant cation radius. Moreover, it seems that vacancy population plays an important role in determining the GB mobility of YAG.     

Microstructural Evolution During Vacuum Sintering of Yttrium Aluminum Garnet Transparent Ceramics: Toward the Origin of Residual Porosity Affecting the Transparency

Controlling residual amount of defects in transparent ceramics is a major challenge for laser applications. This study was focused on microstructural evolution of Nd:YAG ceramics during their reactive solid‐state sintering which was correlated to their optical transmittance. From microstructural observations, the microstructural maps and grain size‐density and grain size‐pore size sintering trajectories of Nd:YAG ceramics were established as a function of silica content. For densities higher than 99.7%, the occurrence of intragranular porosity was correlated to a critical pore radius of 0.16 μm. Silica appears to favor the formation of intragranular porosity which was attributed to the increasing of the grain growth rate compared with the densification one. An analytical model was established by coupling the analytical laws derived from sintering trajectories and the classical theory of light diffusion, allowing to correlate the microstructural features of transparent Nd:YAG ceramics to their optical properties.     

Fabrication of Transparent Yttria Ceramics by the Low-Temperature Synthesis of Yttrium Hydroxide

Thin flakes of yttrium hydroxide agglomerated in a manner resembling houses of cards with aging at 10°C. The agglomerate then dissociated into fine yttria particles with calcination at >800°C. The particle size of the calcined powder increased appreciably as the calcination temperature increased. The shrinkage curve indicated similar densification behavior among undoped yttria powders calcined at 800°–1000°C, despite considerable particle growth as the calcination temperature increased. Increasing the calcination temperature to >1000°C shifted the shrinkage curve appreciably to the high-temperature region. Sulfate-ion-doped yttria particles had round edges, irrespective of calcination temperature, in contrast to the sharp edges of the undoped yttria particles. A calcination temperature of <1000°C resulted in skeleton yttria particles, which exhibited poor sinterability. At a calcination temperature >1000°C, the skeleton particles dissociated into monodispersed particles that densified easily. When the calcination temperature was >1000°C and the average particle sizes were similar, the undoped and sulfate-ion-doped yttria showed similar densification rates. The transparency of the sintered yttria ceramics was dependent on both the calcination temperature and sulfate-ion doping: that is, sulfate-ion doping and calcining at 1100°C were both necessary conditions for the fabrication of a transparent body.     

Fabrication of Transparent Cerium-Doped Lutetium Aluminum Garnet (LuAG:Ce) Ceramics by a Solid-State Reaction Method

Polycrystalline, transparent LuAG:Ce ceramics were fabricated by a solid-state reaction method using high-purity Lu₂O₃, Al₂O₃ and CeO₂ powders. The mixed powder compacts were sintered at 1770°C for 10 h under vacuum and annealed at 1450°C for 20 h in air. The transmittance in the visible light region reaches 70%. The emission spectra at 470–650 nm wavelengths are consistent with that of LuAG:Ce single crystals, and satisfy the property requirements of a scintillator.     

Effect of Yttrium Aluminum Garnet Additions on Alumina-Fiber-Reinforced Porous-Alumina-Matrix Composites

The effects of incorporating yttrium aluminum garnet (YAG) into a porous alumina matrix reinforced with Nextel 610 alumina fibers were investigated. Composites with various amounts of YAG added to the matrix were prepared to determine its effect on retained tensile strengths after heating to 1100° and 1200°C. Strengths of YAG-containing composites were slightly lower than those of an all-alumina-matrix composite after heating for 5 h to 1100°C. However, after heating for 5 or 100 h at 1200°C, all the YAG-containing composites displayed greater strengths and greater strains to failure than the all-alumina composite. At the higher temperature, the presence of YAG is believed to inhibit the densification of the matrix, which helps to maintain higher levels of porosity and weaker interparticle bonding that allows for crack-energy dissipation within the matrix. A reduction in grain growth of the fibers by the presence of segregated Y was also observed, which may also contribute to higher fiber strength, thereby increasing the retained strengths of the YAG-containing composites.     

A comparative study of the synthesis of nanocrystalline Yttrium Aluminium Garnet using sol-gel and co-precipitation methods

Nanocrystalline yttrium aluminium garnet (nYAG) powder has been synthesized via sol-gel and co-precipitation methods using nitrate precursors. Thermal evolution and crystallisation kinetics of both the methods were investigated. The optimised calcination condition for the formation of nYAG was also examined. It was found that a complete transformation to nYAG was observed at 925 °C/2 h and 1000 °C/1 h for the coprecipitation and sol-gel samples respectively. An intermediate YAlO₃ phase was formed at 900 °C in all powders regardless of the synthesis methods. The powder morphologies obtained from TEM revealed very similar particle sizes for the two routes (20–30 nm); whilst the extent of agglomeration was higher for the sol-gel method. It was also observed that by controlling the pH in a narrow range, maintaining the precipitate processing temperature and dehydrating excess OH- ions in the precipitates using n-butanol treatment, the extent of agglomeration was further reduced in the co-precipitated nYAG powder.     

Grain Boundary Diffusion of Oxygen in Alumina Ceramics

The volume and grain boundary diffusion coefficients (D_v and δD_gb) of oxygen at 1673 K in pure and Mg-doped alumina were studied by using a resistive anode encoder of SIMS. The effect of Mg-doping on D_v, was observed clearly. The differences on δD_gb in pure and Mg-doped alumina were about 3 orders and 1 order of magnitude, respectively. The Mg-doping in alumina is useful for homogenizing grain boundary diffusion.     

Influence of Oxygen Content on Formation of Yttrium α-SiAlON Ceramics

Low porosity α- and β-SiAION composite material was prepared when the powder mixture intended for preparation of yttrium α-SiAlON, with the formula Y_0.4Si_12–m+n Al_m+nO_nN_16–n, was attritor milled in isopropyl alcohol or contained excess oxygen (n > 0.6). The region of stability of single-phase yttrium α-SiAlON was smaller at lower temperatures. Wet milling (in isopropyl alcohol) of AIN powder was found to introduce excess oxygen into the milled powder.     

Eutectic Precipitation of the Spinel Solid Solution-Yttrium Aluminum Garnet (YAG) System

Isothermally solidified microstructures of the alumina-rich spinel solid solution—yttrium aluminum garnet (YAG) composite was investigated. It was identified that the eutectic phase relation of the YAG—spinel system could be maintained with the formation of an alumina-rich spinel solid solution, despite the significant change in eutectic compositions. In addition, divorced eutectic precipitates that have a spinel-rich composition were observed surrounding the YAG dendrites.