Fabrication,Microstructure, and Luminescent Properties of Ce3+ ‐Doped Lu3Al5O12 (Ce:LuAG) Transparent Ceramics by Low‐Temperature Vacuum Sintering

The fabrication of 0.5 mol% Ce:LuAG transparent ceramics starting from synthetic nanosized Ce:LuAG powders was investigated by low temperature vacuum sintering. It was found that high quality optical Ce:LuAG ceramics could be densified successfully by vacuum sintering (<10^–3 pa) at 1750°C for 10 h. The in‐line optical transmittance of as‐sintered Ce:LuAG ceramics with thickness of 0.7 mm could reach 73.48% at the wavelength of 550 nm. The microstructure observations revealed that transparent Ce:LuAG ceramics were composed of uniform LuAG grains with average size of 9 μm and HRTEM morphology indicated that no impurity segregation existed at grain boundaries or within Ce:LuAG grains. It was also demonstrated that the annealing treatment (at 1450°C for 20 h in air) could greatly enhance the luminescent intensity of as‐sintered Ce:LuAG ceramics under excitation of X‐ray radiation (75 kV, 25 mA), which makes it a potential candidate to be applied in radiation detector.     

Residual porosity and optical properties of spark plasma sintered transparent polycrystalline cerium-doped YAG

Transparent cerium-doped yttrium aluminum garnet (Ce:YAG) phosphors are promising candidates for high-power white light emitting diode applications. In the present study, Ce:YAG powder was synthesized by a co-precipitation method and highly transparent ceramics were fabricated by spark plasma sintering. The effects of temperature and pressure, as well as post-sintering treatments (annealing or hot isostatic pressing), on residual porosity were studied by electron and confocal laser microscopy. Correlation between residual porosity characteristics (pore size and volume fraction) and optical properties (in-line transmittance and photoluminescence intensity) of the luminescent transparent ceramics was established.     

A kind of bilayer structure ceramic scintillators designed for phoswich detectors

The phoswich detectors can determine the location of nuclear interactions with the matrix and it is an effective approach to improve the spatial resolution at the edge of the field of view in positron emission tomography. In this study, we proposed a kind of bilayer structure garnet ceramic scintillators for phoswich detectors. Bilayer Pr:LuAG/Ce:LuAG ceramic scintillations with different thickness ratio were fabricated through solid‐state reaction and vacuum sintering without further cutting or stacking process. The phase, microstructure, fluorescence, and scintillation properties were characterized and studied. The decay time of the Pr:LuAG layer was about 21.8 ns while that of the Ce:LuAG layer was 58.6 ns. Different 5d‐4f emission bands of 290‐450 and 475‐700 nm were observed in the Pr:LuAG layer and Ce:LuAG layer, respectively. These two features meet the basic requirements of phoswich detectors. Separation of interactions with γ‐ray in different layers has achieved in the pulse height spectra. These properties reveal that this kind of multilayer structure garnet ceramic scintillators have potential application value in the field of phoswich detectors.     

Fabrication of Transparent Cerium-Doped Lutetium Aluminum Garnet Ceramics by Co-Precipitation Routes

Nanosized, highly sinterable Ce-doped lutetium aluminum garnet (LuAG:Ce) powders were synthesized by the urea homogeneous co-precipitation routes. LuAG:Ce transparent ceramics with an average grain size of about 6 μm were successfully fabricated at 1800°C for 10 h by vacuum sintering without sintering aids using the powders calcined at 1000°C for 2 h. The transmittance in the visible light region reaches 65%. The radioluminescence spectrum displays a broad band located at 480–650 nm consisting of two emissions due to the transitions from the lowest 5 d excited state to the 4 f ground state of Ce³⁺, which is consistent with that of LuAG:Ce single crystal and well coupled with the silicon photodiodes.     

Dy3+-doped Y3Al5O12 transparent ceramic for high efficiency ultraviolet excited single-phase white-emitting phosphor

A kind of Dy-doped yttrium aluminum garnet (YAG) transparent ceramic for ultraviolet excited single-phase white light-emitting phosphor was investigated, which has high-quantum efficiency (45%). The temperature field of Dy:YAG transparent ceramic was calculated by steady-state thermal simulation. Moreover, by combining with 365 nm light-emitting diodes (LED) chip directly, the Commission Internationale de l’Éclairage coordinate (x = 0.33, y = 0.35) is close to the standard equal energy white light illumination. The Dy:YAG transparent ceramic, possessing of good optical and thermal properties, is promising for applications in high-power LEDs devices.     

Transparent YAG:Ce ceramic with designed low light scattering for high-power blue LED and LD applications

Yellow-emitting YAG:Ce transparent ceramic is recognized as an ideal color converter in high-power blue LEDs and LDs, but the absence of scattering centers in its microstructure leads to a low light extraction efficiency and poor light uniformity. Here, taking advantage of the scattering effect and the transparency of YAG:Ce ceramics, Ce-free YAG phase was used as a second component to form a composite with YAG:Ce phosphor. The sintered YAG:Ce-YAG ceramic possessed a high transparency of ～63 % and a thermal conductivity of 8.9 Wm^?1 K^?1. Due to its beneficial thermal properties and high external quantum efficiency of 70.2 %, the YAG:Ce-YAG ceramic could be excited under a high blue-laser flux density of up to 9.60 W/mm² and showed a luminous emittance of 1220 lm/mm². Due to light scattering arising from the slightly different refractive indices of the two phases, the designed YAG:Ce-YAG ceramic showed better lighting effects than a single-phase transparent YAG:Ce ceramic.     

Optical properties of YMASG:Ce fluorescent ceramics prepared by hot-pressure sintering

Cerium-doped yttrium aluminum garnet (YAG:Ce) based transparent ceramics have been widely used in fluorescent lighting as high-quality inorganic fluorescent conversion materials. This paper further explores the Mg²⁺-Si⁴⁺ ions doped YAG:Ce transparent ceramics by combining the solid-phase reaction method with vacuum hot-pressure sintering and implementing protection measures against hot-pressure mold contamination, and also investigates the effect of different Mg²⁺-Si⁴⁺ doping contents on the structure, transmittance and luminescence properties of the ceramics under hot-pressure sintering. In this work, pure-phase YMASG:Ce transparent fluorescent ceramics with a grain size of about 3-6 μm and clear and clean grain boundaries were obtained with an In-line transmittance of 67% at 800 nm. Under the excitation at 460 nm, the emission peak was red-shifted by 26 nm and the full width at half maxima (FWHM) was broadened with the increase of Mg²⁺-Si⁴⁺ content, which shows that the Mg²⁺-Si⁴⁺ ion pair effectively complements the absence of the red light component in the YAG:Ce emission spectrum. The optimized YMASG:Ce ceramics obtained high-quality warm white light with a low correlated color temperature (CCT) and a high color rendering index (CRI) under the excitation of the blue LED chip. This work proved the feasibility of vacuum hot-pressure sintering to prepare YMASG:Ce transparent fluorescent ceramics, and provided a new approach for studying YMASG:Ce-based ceramics, which was significant for the application of high-power visible laser illumination.     

Mixed precipitants derived nanocrystalline powders and RE doped LuAG transparent ceramics

Lu₃Al₅O₁₂ (LuAG) nanocrystalline powders were synthesized by using ammonium hydroxide (NH₄OH, AH) and ammonium hydrogen carbonate (NH₄HCO₃, AHC) as mixed precipitant. In the absence of sintering aids such as TEOS, MgO or ZrO₂, the obtained LuAG powders showed good sinterability in H₂ atmosphere (PLSH) at low temperature. The in-line transmittance of LuAG ceramic reached 81% in the whole visible light band from 400 nm to 800 nm. The average grain size of obtained transparent ceramics was ranged in 1–6 μm at different sintering temperatures by PLSH. Various kinds of rare earth ions, such as Nd, Yb, Ce, Pr, and Tm doped RE:LuAG transparent ceramics could be prepared by PLSH technology without sintering aids and HIP post-treatment. Through PLSH technology, RE:LuAG transparent ceramics show high optical quality and large aperture size.     

Microstructure and photoluminescence properties of laser sintered YAG:Ce phosphor ceramics

Cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor ceramics were fabricated by CO₂ laser sintering under various powers. X-ray diffraction results indicated that all samples crystallized as a YAG phase without any impurity phase in the power range of 15–40 W. The reaction profile caused by laser ablation increased from 0.28 mm (15 W) to 2.68 mm (40 W), and it appeared deeper in the scanning sequence because of the low rigidity of YAG melts compared with solid precursors. Photoluminescence (PL) measurements revealed that the YAG:Ce sample by 15 W laser sintering upon blue light excitation presented more intense yellow emission (enhancement of 54%) as compared with YAG:Ce by solid-state reaction (SSR, 1500 °C). The laser beams were believed to distribute activators (Ce) more homogeneously compared with SSR. Additionally, a rougher surface of YAG:Ce phosphor ceramics with periodic holes caused by laser ablation resulted in more excitation and emissivity of PL.     

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...     

Fabrication and Scintillation Performance of Nonstoichiometric LuAG:Ce Ceramics

Nonstoichiometric LuAG:Ce Ceramics ([Lu_(1–x)Ce_x]₃Al₅O₁₂, x = 0.005) with different excess of Lu³⁺ were designed on the basis of Lu₂O₃‐Al₂O₃ phase diagram and fabricated by a solid‐state reaction method. Without using any traditional sintering aids, pure phase and good optical performance were obtained in such a Lu‐rich LuAG:Ce ceramics. In addition, scintillation efficiency and light yield of 1% excess of Lu³⁺ ceramic sample were found 16 times and 1.82 times higher than that of commercial Bi₄Ge₃O₁₂ (BGO) single crystals, respectively. Such values are comparable or even better than those in most of LuAG:Ce single crystals. However, antisite defects were also induced by excess of Lu doping, whose luminescence was found at 350–410 nm in Lu‐rich LuAG:Ce ceramics. The relationship of excess content of Lu and the microstructure, optical quality, and scintillation performance were clarified and discussed. Furthermore, by utilizing X‐ray absorption near edge spectroscopy technique, the charge state stability of cerium in Lu‐rich LuAG:Ce ceramics was examined. It appears that the excess of isovalence Lu³⁺ doping has a negligible effect on the cerium valence instability and creation of stable Ce⁴⁺ center.     

One‐Pot Synthesis of Submicrometer‐Sized Ce:YAG Spherical Particles by Solvothermal Process Using Alcohol Solvents

A rapid synthesis method for preparing homogeneous and submicrometer‐sized cerium‐doped yttrium aluminum garnet (Ce:YAG) particles was proposed in this work. Instead of precipitating the YAG amorphous precursor prior to the solvothermal process, cerium, yttrium, and aluminum nitrates were dissolved in alcoholic solvents to form transparent and clear solutions that were directly transferred into the stainless autoclave to undergo the solvothermal reaction. Four different alcohol solvents, that is, ethanol, 2‐propanol, 2‐butanol, and tert‐butanol, were used as the reaction medium. The crystallization mechanisms in correlation with the alcohol solvent properties, such as the supercritical point and the dielectric constant, were discussed. The interaction between the alcohol solvent and the precursor salts were also investigated. Well‐dispersed and spherical Ce:YAG particles in size ranging from 200 to 350 nm were obtained in one pot via solvothermal reaction at 285°C from 5 to 8 h.     

钇铝石榴石透明激光陶瓷的研究进展

透明钇铝石榴石(aluminum-yttrium garnet,YAG)陶瓷具有良好的化学稳定性和光学性能,是一种很有前途的单晶激光材料的替代物。同单晶相比,多晶YAG陶瓷具有许多优点,如：大尺寸材料易于制备,成本低适合大规模生产等。此外,因掺杂浓度高可得到较大的输出功率。对透明YAG激光陶瓷的光学特性以及制备工艺做了重点介绍,并对研究进展进行综合评述。最后,展望该领域的发展前景及今后的研究趋势。     

微乳液法制备纳米球形YAG:Ce3+荧光粉及其发光性能

采用反相微乳液法,以水/曲拉通X-100/正己醇/(环己烷 正己烷)为微乳体系,铝(Al)、钇(Y)和(Ce)的硝酸盐和氯化物作为起始反应物,氨水作为沉淀剂,成功制备了纳米球形铈掺杂钇铝石榴石(cerium doped yttrium aluminum garnet,YAG:Ce3 )荧光粉.实验表明:单相YAG可以在800℃合成,比周相反应法合成温度(1 500℃)大幅度降低.用失重-差热分析仪、Fourier变换红外光谱仪、X射线衍射仪、透射电镜、荧光分光光度计等对粉体特性进行了表征.结果表明:粉体颗粒粒径约为50nm,粒度分布均匀,球形度好,分散性好,粉体的激发光谱为双峰结构,主激发波长为469nm,发射波长为532nm,适用于白光发光二极管.     

Microwave Synthesis of Homogeneous YAG Nanopowder Leading to a Transparent Ceramic

A homogeneous and extremely fine yttrium aluminum garnet (YAG) precursor powder was synthesized from solutions with a low [urea]/[metal ions] ratio under microwave irradiation, and pure phase YAG was directly crystallized from it at 1173 K. In the presence of sulfate ions, a fibrillar precursor that takes on a dendritic skeleton was generated, and easily dispersible YAG nanopowder with a particle size of 20–30 nm was obtained at 1373 K. A transparent YAG ceramic with fine grains was sintered from this YAG nanopowder at 1973 K for 10 min in a graphite furnace. At the wavelength of 1060 nm, the in-line light transmittance of the YAG ceramic is up to 67%. The mechanism behind the influence of the microwave irradiation and sulfate ions on the characteristics of YAG is thoroughly discussed.     

Spectrum regulation of YAG:Ce transparent ceramics with Pr,Cr doping for white light emitting diodes application

YAG:Ce transparent ceramics with high luminous efficiency and color render index were prepared via a solid state reaction-vacuum sintering method. Cr³⁺and Pr³⁺ were applied to expand the spectrum of YAG:Ce transparent ceramics. As prepared ceramics exhibit luminescence spectrum ranging from 500 nm to 750 nm, which almost covers full range of visible light. After the concentration optimization of Ce³⁺, Pr³⁺ and Cr³⁺, high quality white light was obtained by coupling the YAG:Ce,Pr,Cr ceramics with commercial blue LED chips. Color coordinates of the YAG:Ce,Pr,Cr ceramics under 450 nm LED excitation vary from cold white light to warm white light region. The highest luminous efficiency of WLEDs encapsulated by transparent YAG:Ce,Pr,Cr ceramic was 89.3 lm/W, while its color render index can reach nearly 80. Energy transfers between Ce³⁺ → Pr³⁺ and Ce³⁺ → Cr³⁺ were proved in co-doped ceramic system. Transparent luminescence ceramics accomplished in this work can be quite prospective for high power WLEDs application.     

Composition and properties tailoring in Mg2+ codoped non-stoichiometric LuAG:Ce,Mg scintillation ceramics

A comprehensive study of the optical, radioluminescence and scintillation properties of both the Lu³⁺ rich and Lu³⁺ deficient non-stoichiometric Lu_3+xAG:Ce,Mg (Lu_3+xAl₅O₁₂:Ce,Mg, x = −4, −1, +1 and +4 at.%) ceramics are performed, completed further by the microstructure and defects characterization. Small deviation from the stoichiometric composition as well as Mg²⁺ codoping plays a crucial role in ceramic transparency, radioluminescence intensity and the timing characteristics of scintillation response. The Lu_Al antisite defects could be suppressed efficiently by controlling Lu³⁺ content below stoichiometry of LuAG host. MgO (Mg²⁺ ions) as effective sintering aids, can improve both the optical quality and scintillation performance (light yield, scintillation decay times and the ratio of fast decay components). We generally discuss the composition dependence of defects and properties tailoring. We also performed the systematic comparative study with the stoichiometric LuAG:Ce,Mg ceramic and the commercial BGO and LuAG:Ce single crystals.     

Agglomeration Control of Nd:YAG Nanoparticles Via Freeze Drying for Transparent Nd:YAG Ceramics

Neodymium-doped yttrium aluminum garnet (Nd:YAG) nanopowders were synthesized by the carbonate coprecipitation method. The effects of freeze drying and conventional oven drying of the precursor on the agglomeration of the Nd:YAG nanopowders were compared. The optical properties of the Nd:YAG nanopowders and the corresponding sintered Nd:YAG transparent ceramics were also investigated. The Nd:YAG nanopowders synthesized from freeze-dried precursor showed better dispersion and narrower particle size distribution compared with the powders synthesized from conventional oven drying. As a result, the Nd:YAG nanopowders synthesized from freeze-dried precursor have good sinterability, and Nd:YAG transparent ceramics were fabricated by vacuum sintering at 1750°C for 5 h.     

Layered Ceramic Phosphors Based on CaAlSiN3:Eu and YAG:Ce for White Light‐Emitting Diodes

To develop warm‐white light‐emitting diodes via conversion phosphors, blue light‐emitting diodes are generally combined with mixtures of green and red‐emitting phosphor powders. Generally, the phosphors are provided by resin embedded particle dispersions. Such resin‐based solutions cause several drawbacks with respect to LED lifetime and quality. Therefore, it has been investigated whether the red‐emitting nitride phosphor CaAlSiN₃:Eu and the green‐emitting oxidic phosphor YAG:Ce can be cofired to layered ceramic composites. The shrinkage behavior and the composition of the interface in dependence of sintering temperature and the effect of interdiffusion processes at the interface on the luminescence properties were investigated. The formation of secondary phases at the interface in the cofired structures was found to limit the phosphor functionality for the nitride‐based CaAlSiN₃:Eu in such composite ceramics. To counteract this, sacrificial interlayers were introduced to produce multilayered ceramics comprising CaAlSiN₃:Eu and YAG:Ce for LED lighting applications. It is shown for the first time, that it is possible to sinter layered CaAlSiN₃:Eu and YAG:Ce composite ceramics in a pressureless process at moderate sintering temperatures if one uses thin‐film passivated interfaces to reduce luminescence‐disturbing diffusion phenomena. These results demonstrate that diffusion barriers can be suitable means to obtain layered ceramic composites comprising CaAlSiN₃:Eu and YAG:Ce in a pressureless sintering process with good optical properties.     

On fast LuAG:Ce scintillation ceramics with Ca2+ co-dopants

“Defect engineering” was a valid strategy to modify the performance of LuAG:Ce scintillator, usually realized by Me²⁺/Me⁺ co-doping. To investigate the effects of Ca²⁺ co-doping on the scintillation properties of LuAG:Ce, a set of LuAG:Ce ceramics with Ca²⁺ concentrations ranging from 0 to 0.5 at.% were manufactured. The absorption spectra, radioluminescence spectra (RL spectra), light yield, RL spectra as a function of temperature, decay time, and TSL curves of the ceramic products were carefully measured. With Ca²⁺ co-doping, the scintillation performance of LuAG:Ce ceramics was greatly improved. Especially for the 0.2 at.% Ca²⁺ co-doped one, it has a high light yield value of 24, 400 ph/MeV, a fast scintillation decay time of 48 ns, and a small slow component contamination. And the role of Ca²⁺ in the scintillation mechanism of LuAG:Ce ceramics was also discussed in this paper.