Translucent Y3Al5O12 ceramics

The sintering of Y₃Al₅O₁₂ to full density and translucency using either SiO₂ or MgO as a dopant is described. The resulting ceramics have a regular microstructures and low optical absorption coefficients.     

Energy transfer and enhanced 1532 nm emission in Er and Ce co-doped Y3Al5O12 nano-particles

A series of Y₃Al₅O₁₂:Ce³⁺, Er³⁺ (YAG:Ce, Er) nano-particles were synthesized by polymer-assisted sol–gel method. X-ray diffraction measurements reveal that a pure crystalline phase of YAG is achieved at temperature as low as 800 °C. The energy transfer from Ce³⁺ to Er³⁺ is studied based on photoluminescence spectroscopy and fluorescence decay patterns. It results that the emission intensity of Er³⁺ at near infrared (NIR) 1532 nm under indirect excitation of Ce³⁺ (460 nm) is 10 times stronger than that of direct excitation of Er³⁺ (275 or 380 nm). The energy transfer efficiency is estimated as 95.5% for YAG:Ce_0.03Er_0.09 sample. The very efficient energy transfer path and mechanism are also discussed.     

Effect of metal halide fluxes on the microstructure and luminescence of Y3Al5O12:Ce phosphors

Yellow-emitting Y_2.95Al₅O₁₂:0.05Ce (YAG:Ce) phosphor particles with high luminescence efficiency under 450 nm ultraviolet (UV) excitation were prepared by a heat treatment of submicrometer-sized oxide powder mixture in a reducing atmosphere at 1550 °C. Prior to the heat treatment, the oxide mixture was blended with 5 wt% of metal halides – BaF₂, BaCl₂, NaF, NaCl, and KF – as a flux. It was observed that YAG:Ce particles prepared with fluorine containing flux demonstrated spherical morphology, high photoluminescence properties, and diameters of 5–20 μm, whereas those prepared with chlorine were small-sized particles (≤5 μm) with relatively low intensity. The highest relative photoluminescence (PL) intensity (∼118%) was obtained for the samples prepared with 5% BaF₂. A set of rules was established for the formation of spherical YAG:Ce particles in the presence of this flux.     

Comparison of fluorescence spectra of Yb:Y3Al5O12 and Yb:YAlO3 single crystals

Yb:Y₃Al₅O₁₂ (Yb:YAG) single crystals with Yb doping concentration 0.5 at.%, 5 at.%, 15 at.%, 25 at.%, 50 at.%, 100 at.% and Yb:YAlO₃ (Yb:YAP) single crystals with Yb doping concentration 0.5 at.%, 5 at.%, 15 at.%, 30 at.% were grown by the Czochralski process. The fluorescence spectra of these crystals and the effects of self-absorption on the shape of the fluorescence spectra were studied. Through comparing the fluorescence spectra of Yb:YAG and Yb:YAP, all results indicate that the effects of self-absorption on the fluorescence spectra of Yb:YAP are remarkably stronger than that of Yb:YAG at the same Yb concentration.     

激光双面区熔Al2O3/Y3Al5O12共晶自生复合陶瓷的制备与表征

采用激光区熔凝固技术制备大尺寸Al2O3/Y3Al5O12(YAG)共晶自生复合陶瓷,考察双面区熔条件下大尺寸氧化物共晶陶瓷的熔化及凝固成形规律,采用扫描电镜、能谱和X射线衍射对其凝固组织特征进行了表征和分析.研究结果表明:在优化的凝固工艺下,激光双面区熔增加了熔凝层的厚度,获得了熔凝层厚度8.2 mm,长度65.0 mm,致密度达98.5%±1%的Al2O3/YAG共晶陶瓷;共晶熔凝层厚度随激光扫描速率的减小而增加,随激光功率的增大而增大,并且致密度随着激光功率的增大呈现先增大后减小的趋势;双面区熔后的Al2O3/YAG共晶陶瓷微观组织由均一分布、相互交织的Al2O3和YAG共晶相组成,共晶层片间距较小(1.0~3.5μm),且与凝固速度满足Jackson-Hunt公式;共晶间距随扫描速率的增大逐渐减小;双面区熔界面处共晶组织生长具有连续性,界面结合良好;共晶陶瓷的Vickers硬度为(18.6±1.0)GPa.     

Color tuning of Y3Al5O12:Ce phosphor and their blend for white LEDs

Gadolinium or lanthanum co-doped (0.5 mole) yttrium aluminum garnet doped with cerium phosphors were synthesized by a citric acid gel method and the effect of co-dopants on the structural and luminescent properties were studied. A significant peak shift in the photoluminescence spectra of yttrium aluminum garnet doped cerium was observed from 535 to 556 and 576 nm for gadolinium or lanthanum co-doped phosphors, respectively. The color tuned phosphor were blended with yttrium aluminum garnet doped cerium which showed a considerable improvement in the Commission International De Eclairage chromaticity co-ordinate values of gallium nitride based blue light emitting diode pumped white light. White light emitted from yttrium aluminum garnet doped cerium shows a Commission International De Eclairage value of (0.229, 0.182) whereas the yttrium aluminum garnet doped cerium phosphor blended with gadolinium or lanthanum co-doped phosphor shows (0.262, 0.243) and (0.295, 0.282), respectively. These results demonstrate the possibility to use these phosphor blends to enhance the white light generation in the field of white-light emitting diode solid-state lighting.     

Influence of fuels on the morphology of undoped Y3Al5O12 and photoluminescence of Y3Al5O12:Eu prepared by a combustion method

Undoped and Eu-doped yttrium aluminum garnet nano-powders were prepared by a facile combustion method with citric acid/ethylene diamine tetraacetic acid (EDTA) as fuels and nitrates as oxidizers. The precursors and powders calcined at 1030 °C were investigated using thermogravimetric (TG), differential thermal analysis (DTA), X-ray diffraction (XRD), scanning electron microscope (SEM), and Brunauer-Emmett-Teller (BET) surface area measurements. It was found that the powders could be indexed with a garnet structure. The grains were in shape of hemispherical with sizes between 60 nm and 100 nm. With decreasing the citric acid/EDTA ratio, the crystallite size decreased steadily and the specific surface area increased. Investigations of photoluminescence (PL) revealed that as-synthesized YAG:Eu³⁺ phosphor samples exhibited an orange emission band with a main peak at 591 nm under the excitation of 394 nm. As citric acid amounts increased, the quality of crystallinity became higher and the luminescent properties were monotonously enhanced.     

Low-temperature synthesis,phonon and luminescence properties of Eu doped Y3Al5O12 (YAG) nanopowders

This contribution presents two simple and cost-effective routes for the low-temperature and large-scale production of pure and Eu-doped Y₃Al₅O₁₂ (yttrium aluminum garnet YAG) nanopowders. The proposed methodologies combine a mechanically assisted metathesis reaction or coprecipitation from solution followed by crystallization of the obtained precursors from molten sodium nitrate/nitrite. Both procedures allow obtaining pure and/or doped YAG nanopowders at remarkably low temperatures, i.e. already at 350 °C although firing at 500 °C is needed in order to get single phase and fully crystalline materials. As-obtained samples were characterized by XRD, TEM, Raman, IR and luminescence methods. These methods showed that the mean crystallite size is near 23–31 and 51 nm, when crystallization is performed from the amorphous precursor obtained by a mechanically assisted metathesis reaction and coprecipitation, respectively. Raman and IR spectra indicated better crystallinity of the powders prepared at 500 °C. The emission study showed that the intensity ratio between hypersensitive ⁵D₀ → ⁷F₂ and magnetic-dipole ⁵D₀ → ⁷F₁ transitions of Eu³⁺ is significantly larger than expected for well-crystallized YAG. Origin of this behavior is discussed.     

Influence of additional Eu coactivator on the luminescence properties of Tb3Al5O12:Ce, Eu

The luminescence properties of doubly activated terbium aluminium garnet samples were investigated in the present study. Commercial Tb₃Al₅O₁₂:Ce³⁺ (TAG:Ce) shows the typical Ce³⁺ ion luminescence of the allowed Ce³⁺ d–f transition. Eu³⁺ co-doping, however, reveals interesting results. In TAG:Ce, Eu, both Ce³⁺ and Eu³⁺ luminescence was measured at different Ce and Eu activator concentrations. The Ce³⁺ ion can be used as a sensitizer in the TAG lattice that transfers its energy directly or via the Tb sublattice to the activator Eu³⁺. The energy-level diagram was proposed to explain the luminescence mechanism. Application of TAG:Ce, Eu with improved chromaticity coordinates CIE (Commission Internationale de l’Eclairage) and color rendering index (CRI) in light emitting diodes (LEDs) is demonstrated.     

Flame aerosol deposition and annealing of Y3Al5O12:Tb phosphor coatings

Sub-micrometer-sized powders of Y₃Al₅O₁₂:Tb phosphor (d_SEM = 320 nm) were prepared by flame-assisted spray pyrolysis of aqueous precursors in a premixed propane/air flame and in situ deposited onto quartz substrates. Phosphor screens with densities of up to 0.7 mg cm⁻² could be produced within 20 min. As-deposited coatings were amorphous and required a thermal post-treatment. After annealing in an oven for 2 h (T ≥ 900 °C), the yttrium aluminum garnet phase (YAG:Tb) was obtained. Alternatively, the phosphor coatings were treated by an impinging flame in the same setup used for the deposition. Quasi-amorphous Y₃Al₅O₁₂:Tb coatings demonstrated bright green photoluminescence upon flame annealing at T ≈ 1100 °C for just several minutes and could outperform YAG:Tb when excited in the wavelength ranges 205–220 nm and 230–260 nm. For example, brightness of emission from the quasi-amorphous coatings was up to five times higher than that of the fully crystalline YAG:Tb phosphor at a technically important wavelength of 254 nm.     

Neodymium substitution effects in sol–gel derived Y3−xNdxAl5O12

Yttrium aluminium garnet (YAG) powders substituted by neodymium Y_3−xNd_xAl₅O₁₂ (x = 0.1, 0.25, 0.35, 0.5, 0.6, 0.7, 0.8, 1.5, 2.0, 2.5, and 3.0) were prepared by a simple aqueous sol–gel method using aluminium nitrate nonahydrate, yttrium oxide, neodymium oxide as the starting materials and ethane-1,2-diol as complexing agent. The powders annealed at 1000 °C in air were characterized by X-ray diffraction (XRD) analysis, infrared (IR) spectroscopy and scanning electron microscopy (SEM). It was demonstrated, however, that the total substitution of yttrium by neodymium does not proceed in the YAG. Pure cubic garnet phase was formed only at low concentration of neodymium (x = 0.1, 0.25, 0.35, 0.5, 0.6, 0.7, 0.8 and 1.5). With further substitution, when the amount of neodymium was x = 2.0, 2.5 and 3.0 the main part of garnet phase transformed in to the perovskite neodymium aluminate (NdAlO₃) phase.     

Preparation of Y2O3:Ce phosphors by homogeneous precipitation inside bicontinuous cubic phase

Yttrium oxide doped with cerium (Y₂O₃:Ce³⁺) blue emitting phosphors was prepared by a new method called the bicontinuous cubic phase (BCP) method. The experimental results showed that the prepared precursors were amorphous yttrium hydroxide with a spherical shape and primary size 30-50 nm. After heat treatment, high crystallinity and luminescence efficiency phosphors were obtained. The obtained Y₂O₃:Ce³⁺ phosphors had a strong blue emitting at 400 nm. The optimum Ce³⁺ concentration was 1 mol% to obtain the highest PL intensity. This study indicated that the calcining temperature of 700 °C needed for high luminescence efficiency in this work is much lower than 1000 °C or above needed for the conventional solid-state method.     

Synthesis of Tb3Al5O12 (TAG) transparent ceramics for potential magneto-optical applications

Tb₃Al₅O₁₂ transparent ceramics have been prepared by solid state reaction and vacuum sintering. The optical quality and the microstructure of the samples were investigated. The sample sintered at 1650 °C possessed relatively good optical transparency from 400 nm to 1600 nm. The Verdet constant measured at 632.8 nm of the quasi-pore-free Tb₃Al₅O₁₂ transparent ceramic was −172.72 rad T⁻¹ m⁻¹, which was close to the counterpart of Tb₃Al₅O₁₂ single crystal. The thermal conductivity of the sample was also measured. To the best of our knowledge, this is the first time that Tb₃Al₅O₁₂ transparent ceramic with relatively good optical quality and magneto-optical property has been reported.     

A novel blue-emitting Sr3Al2O5Cl2:Ce,Li phosphor for near UV-excited white-light-emitting diodes

A novel blue-emitting Sr₃Al₂O₅Cl₂:Ce³⁺,Li⁺ phosphor has been synthesized by solid state reaction. The excitation spectrum shows a broad band extending from 300 to 400 nm, and the emission spectrum shows a broad blue band peaking at 450 nm with a half width of about 100 nm. The emission intensity at 250 °C remains at about 50% of that at room temperature. The decay curve at the emission peak consists of fast and slow components. The Sr₃Al₂O₅Cl₂:Ce³⁺,Li⁺ should be a promising blue phosphor for near ultraviolet-based white-light-emitting diodes.     

Enhancement of Er I13/2 population in Y2O3 by energy transfer to Ce

We report measurements of the energy transfer between Er³⁺ and Ce³⁺ in Y₂O₃. The transition between the Er^{3+ 4}I_11/2 and ⁴I_13/2 excited states can be stimulated by energy transfer to Ce³⁺, augmenting the population in the ⁴I_13/2 state at the expense of that in the ⁴I_11/2 state. Experiments were performed on Y₂O₃ planar waveguides doped with 0.2 at.% erbium and 0–0.42 at.% cerium by ion implantation. From measurements of Er³⁺ decay rates as a function of cerium concentration we derive an energy transfer rate constant of 1.3×10⁻¹⁸ cm³/s. The efficiency of the energy transfer amounts to 0.47 at 0.42 at.% cerium. The energy transfer rate constant measured in Y₂O₃ is two times smaller for Er³⁺→Ce³⁺ than that for Er³⁺→Eu³⁺ in the same material.     

Preparation and characterization of Y3Al5O12 (YAG) nano-powder by co-precipitation method

YAG precursor was synthesized by a co-precipitation method from a mixed solution of aluminum and yttrium nitrates with aqueous ammonia as the precipitator. The structure, phase evolution and morphology of YAG precursor and the sintered powders were studied by means of IR, TG/DTA, XRD, TEM methods. It was found the precursor with approximate composition of Al(OH)₃·0.3[Y₂(OH)₅·(NO₃)₂·2H₂O] directly transformed to pure-YAG phase at 800 °C and no intermediate phases were detected. YAG nanocrystalline powders from sintering the precursor at different temperatures were less-aggregated and the diameters of the grains were about 40-100 nm. BET surface area of the particles decreased with increase of calcination temperature and the powder sintered at 800 °C can be used for fabrication of transparent YAG ceramics.     

Synthesis of submicron-sized spherical Y2O3 powder for transparent YAG ceramics

Spherical monodispersed, submicron-sized Y₂O₃ powder was prepared via a homogeneous precipitation method using nitrate and urea as raw materials. The structure, phase evolution and morphology of Y₂O₃ precursor and the calcined powder were studied by FTIR, TG/DTA, XRD and SEM methods. The sphere size of the precursor was about 250 nm and that of Y₂O₃ powder calcined at 800 °C for 2 h was about 200-210 nm. With the spherical Y₂O₃ powder and a commercial Al₂O₃ ultrafine powder, high transparent YAG ceramics was fabricated by vacuum sintering at 1780 °C for 6 h through a solid-state reaction method. The in-line transmittances of the as-fabricated YAG ceramics at the wavelength of 1064 nm and 400 nm were 82.8% and 79.5%, respectively, which were much higher than that of the YAG ceramics with a commercial Y₂O₃ powder and a commercial Al₂O₃ ultrafine powder directly. The superior properties are attributed to the good morphology, dispersibility and uniform grain size of the as-prepared spherical Y₂O₃ powder, which matches that of the commercial Al₂O₃ powder.     

Low-temperature synthesis and characterization of yttrium-gallium garnet Y3Ga5O12 (YGG)

To obtain yttrium-gallium garnet (Y₃Ga₅O₁₂, YGG) a simple “chimie douce” method has been developed. This sol-gel method yielded excellent starting gel precursor for the fabrication of YGG phase, which could be used as host material for optical applications. The pattern of X-ray diffraction analysis of the ceramic sample sintered for 10 h at 1000 °C showed the formation of monophasic Y₃Ga₅O₁₂ phase. The phase transformations, composition and micro-structural features in the gels and polycrystalline sample were studied by thermoanalytical methods (TGA/DTA), powder X-ray diffraction analysis (XRD), infrared spectroscopy (IR) and scanning electron microscopy (SEM). The quality of the resulting products (homogeneity, crystallisation temperature, grain size, grain size distribution, etc.) is discussed.     

Upconversion and infrared luminescences in Er/Yb codoped Y2O3 and (Y0.9La0.1)2O3 transparent ceramics

Er³⁺ and Yb³⁺ codoped Y₂O₃ and (Y_0.9La_0.1)₂O₃ transparent ceramics were fabricated by the conventional ceramics processing with nanopowders. Compared to Er/Yb:Y₂O₃, Er/Yb:(Y_0.9La_0.1)₂O₃ ceramics had higher transmittance. Intense upconversion (UC) and infrared emission (1543 nm) were observed under excitation of 980 nm. According to three intensity parameters Ω₂, Ω₄, and Ω₆ fitted by the Judd-Ofelt theory, the spectroscopic quality parameters (X), radiative lifetimes (τ_rad), and emission cross-sections (α_em) were determined. Er/Yb:(Y_0.9La_0.1)₂O₃ ceramics owned broader peaks and longer lifetime (12.3 ms) at 1548 nm due to the glass-like structure of (Y_0.9La_0.1)₂O₃ ceramics. The results showed Y₂O₃ and Y_1.8La_0.2O₃ transparent ceramics are promising gain media for developing the solid-state 1.5 μm optical amplifiers and tunable UC lasers.     

Morphology control and luminescent property of Y3Al5O12:Tb particles prepared by spray pyrolysis

A spray pyrolysis process was used to prepare spherical yttrium aluminum garnet (Y₃Al₅O₁₂:Tb) phosphor particles with enhanced luminescence properties. The aim of the process was to improve the morphology and luminescent intensity of the Y₃Al₅O₁₂:Tb phosphor particles by modifying the precursor solution. The particles produced from a nitrate aqueous solution were spherical with a hollow structure that was deformed by the post treatment at 1400 °C. To avoid the hollowness, the nitrate solution was modified by the addition of an NH₄OH solution so that a polycation solution could be obtained. Compared with the hollow particles prepared from the nitrate aqueous solution, the Y₃Al₅O₁₂:Tb particles with the spherical morphology and nonaggregated structure, even after the post treatment, were successfully prepared and found to have an improved photoluminescence and cathodoluminescence intensity.