Fine grained Nd3+:Lu2O3 transparent ceramic with enhanced photoluminescence

Fine-grained Nd³⁺:Lu₂O₃ transparent ceramic was developed by a two-step sintering method in flowing H₂ atmosphere at T₁ = 1720 °C for 15 min and T₂ = 1620 °C for 10 h. The initial nanopowders were synthesized by a wet chemical processing with a uniform particle size of about 40 nm. The average grain size of the obtained 3 at.% Nd³⁺:Lu₂O₃ ceramic was 406 nm, which is ∼150 times smaller than the coarse-grained ceramic by normal H₂ sintering. The emission intensity of the fine-grained transparent ceramic is 3 times of its coarse-grained counterpart, indicating higher Nd concentration without serious quenching in fine-grained transparent ceramic is possible, which agreed well with the prediction of an atomistic modeling work with YAG. EXAFS research demonstrated that with decreasing grain size, higher degree of disorder factor of the local environment of doped Nd atoms was discovered.     

Gd3Al3Ga2O12:Ce,Mg2+ transparent ceramic phosphors for high-power white LEDs/LDs

Gd₃Al₃Ga₂O₁₂:1.5%Ce, xMg²⁺ (GAGG:1.5%Ce, xMg²⁺) transparent ceramic phosphors (TCPs) were prepared via a two-step sintering method. The effects of MgO on microstructures and luminescent properties of GAGG:Ce TCPs are investigated for the first time. For the optimized Mg²⁺ of x = 0.5%, the in-line transmittance of the obtained TCP reaches 78.6%. Performances of the titled TCPs in high-power light-emitting diodes (LEDs) and laser diodes (LDs) lighting are illustrated. The optimized TCP shows the luminous efficacy of 84.0 lm W^?1 in LD lighting. This work provides a strategy to modify TCPs for the next-generation LD lighting.     

The effect of Lu3+ doping upon YAG:Ce phosphor ceramics for high-power white LEDs

Intense green emission is extremely significant to the color rendering index (CRI) of white LEDs. Various green-emitting YLuAG:Ce phosphor ceramics were successfully prepared by vacuum sintering. The effects of Lu³⁺ doping on structure and luminescence property were investigated in detail. In comparison with YAG:Ce, YLuAG:Ce ceramics own smaller grain size, better luminescence performance and higher thermal stability. The photoluminescence (PL) intensity of YLuAG:Ce ceramics increases by 23.6 % due to the “light scattering enhanced effect”. Furthermore, the Ce³⁺ emission is obviously blue-shifting from 533 nm to 519 nm, and the intensity of YLuAG:Ce ceramics reduces only about 8.9 % at 250 °C, showing better thermal stability (vs 11.1 % of YAG:Ce). The LE of LED packaged by YLuAG:Ce ceramic is up to 148.88 lm/W when the doping Lu³⁺ y is 2.1. The above results show that tailored YLuAG:Ce phosphor ceramic is a potential green-emitting color converter for high-power LEDs (hp-LEDs).     

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.     

Highly transparent and color-adjustable Eu2+ doped SrO-SiO2-Al2O3 multilayered glass ceramic prepared by controlling crystallization from glass

Multilayered inorganic transparent materials have been widely used as laser materials, scintillators and phosphors due to their excellent combined properties or functions. However, owing to the limitation of the current preparation technology, only the ceramics with cubic crystal structure could be fabricated into multilayered transparent materials, which has greatly obstructed the diversity of multilayered transparent materials. Here we report a novel non-cubic multilayered transparent phosphor with a ceramic/glass/ceramic sandwich-like structure prepared by controlling crystallization from Eu₂O₃-SrO-Al₂O₃-SiO₂ bulk glass. The ceramic thicknesses, total transmittances, emission colors and the fluorescence quantum yields of the samples can be adjusted continuously within a certain range. The multilayered transparent phosphor could be used as a potential candidate for the white LEDs with high color rendering index. It can be anticipated that the controlled crystallization from bulk glass method is a simple, fast, cost-effective and promising synthesis approach to prepare non-cubic transparent materials with ceramic/glass/ceramic structures.     

Growth of YAG:Ce3+-Al2O3 eutectic ceramic by HDS method and its application for white LEDs

The resin-free YAG:Ce³⁺-Al₂O₃ eutectic ceramic phosphor for white light emitting diodes (WLEDs) was successfully grown in vacuum by Horizontal Directional Solidification method (HDS). X-ray diffraction and scanning electron microscopy indicate that this material has a typical eutectic structure of interpenetrating sapphire and garnet phases. The excitation spectra, emission spectra and temperature characteristics of the eutectic show that it is characterized by a wide excitation band and it has good stability in high temperature. In X-ray photoelectron spectroscopy, annealing in an air atmosphere could eliminate the oxygen vacancies and didn’t change the Ce³⁺ valence in the eutectic. The YAG:Ce³⁺-Al₂O₃ eutectic ceramic with different thickness was fixed in COB (chip on board) element for researching the performance of the WLEDs with the phosphor. The electroluminescence characterization of the WLEDs show that the WLEDs with the eutectic ceramic are more excellent than the common commercial WLEDs.     

Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

In this work, MgAl₂O₄: Cr³⁺ transparent ceramics have been synthesized by the hot press sintering techniques, and the effect of the sintering aid Gd₂O₃ and its content on the densification, microstructure, and optical, photoluminescence was studied and discussed. The relative density reached 99.29% with 0.8 wt% Gd₂O₃ as a sintering aid, and the optical transmittance at 686 nm and 1446 nm were approximately 76%. As Gd₂O₃ content continued to increase, the grain size of the ceramics became smaller and uniform, accompanied by some pores with the size of ～1 μm. The ceramics with 4.0 wt% Gd₂O₃ showed a higher transmittance, of 82% at 1446 nm. Additionally, Gd₂O₃ was helpful for Cr³⁺ in the sites of octahedral symmetry, which increased the quantum yield. The quantum yield of MgAl₂O₄: Cr³⁺ with 0.8 wt% Gd₂O₃ was about 0.175, which was 36% higher than that of ceramic without Gd₂O₃. In short, the sintering aid Gd₂O₃ not only contributed to improving the densification, homogenizing the grain size, and heightening the optical transmittance but also enhanced the quantum yield of Cr³⁺.     

Additive-free Y2O3:Eu3+ red-emitting transparent ceramic with superior thermal conductivity for high-power UV LEDs and UV LDs

To obtain red-emitting luminescent material for high-power UV LED and UV LD applications, an additive-free Y₂O₃:Eu³⁺ phosphor ceramic was successfully prepared in this work. The nitrate pyrogenation method is applied to obtain raw nanopowders with high reactivity, and a hybrid sintering method combining low-temperature presintering and subsequent hot isostatic pressing (HIP) is then applied to realize full densification of the final ceramic products. The effects of the presintering temperature on the density, microstructural, and optical properties are investigated in detail. The HIP-treated Y₂O₃:Eu³⁺ ceramic presintered at 1450 °C exhibits a high transmittance near 80 % at 600 nm. Due to the nonuse of sintering additives, the thermal conductivity of Y₂O₃:Eu³⁺ ceramic product reaches 10.9 Wm⁻¹ K⁻¹ at room temperature. The achieved Y₂O₃:Eu³⁺ ceramic also exhibits good applicability under the excitation of a UV LED chip and UV laser light, showing promise as a color converter for high-power UV LED and UV LD applications.     

Al2O3-Ce:GdYAG composite ceramic phosphors for high-power white light-emitting-diode applications

In order to meet the increasing demand of high-power light-emitting-diode (LED) lighting, state-of-the-art white light-emitting diode technology needs phosphors with high thermal conductivity and high luminous efficacy as color converters. In this work, translucent Al₂O₃-Ce:GdYAG composite phosphors were prepared by solid-state reactive sintering. The microstructure shows that the Al₂O₃ particles are uniformly dispersed in the Ce:GdYAG matrix. These particles can not only improve the thermal conductivity of the ceramics, but also promote the extraction efficacy. The luminous characteristics of the Ce:GdYAG and Al₂O₃-Ce:GdYAG ceramics were analyzed after being packaged with blue LED. When the molar ratio of Al₂O₃/Ce:GdYAG is 0.8, a high luminous efficacy value of 112.6 lm/W is achieved by the Al₂O₃-Ce:GdYAG composite ceramic phosphor with the thickness of 0.4 mm, as well as the highest CRI valve of 71.4. The appropriate photoelectric properties of this kind of ceramic phosphor make it a promising candidate for high-power LED device.     

Fabrication and optical characterizations of hot-pressed transparent SrF2/Nd:SrF2/SrF2 composite ceramic

A novel transparent SrF₂/Nd:SrF₂/SrF₂ composite ceramic with sandwich-like laminar configuration was designed and successfully fabricated by the hot-pressed sintering method. The composite ceramic possesses an apparent transition interfacial (about 200 μm in thickness) between SrF₂ and Nd:SrF₂ layers, formed by non-uniform distribution of raw powders and the diffusion of Nd ions during the high temperature sintering. The average grain sizes of SrF₂ and Nd:SrF₂ layers are about 262.1 μm and 28.6 μm, respectively. For a 2-mm thick transparent SrF₂/Nd:SrF₂/SrF₂ composite ceramic hot-pressed at 900 °C for 2 h, the transmittance at 500 nm and 1200 nm are about 49.6 % and 62.3 %, respectively. The microstructure, emission spectra and thermal conductivities of ceramics are also detected and studied.     

Effects of Ho3+ concentration on the fabrication and properties of Ho: Y2O3-MgO nanocomposite for Mid-infrared laser applications

Infrared transparent Ho: Y₂O₃-MgO nanocomposite ceramics with a volume ratio of 50:50 (RE₂O₃: MgO) were prepared by combining sol-gel powder synthesis and hot-pressing sintering techniques. In order to obtain Ho: Y₂O₃-MgO nanocomposite ceramics with fine grain size, dense microstructure and homogeneous phase domains, the effect of sintering temperature and Ho³⁺ doping concentration were studied. Transmittance and SEM measurement revealed that the grain size of 3 at.% Ho: Y₂O₃-MgO ceramic sintered at 1250 °C is 141 nm, and the transmission is up to 85.2% at 5 μm. The detailed spectroscopic investigation of x at.% Ho: Y₂O₃-MgO (x = 1, 3, 5, 7, 9, 15) ceramics was performed. The nanocomposites exhibited photoluminescence properties similar to that of Ho: Y₂O₃ crystals and ceramics. In addition, the thermal conductivity of 3 at.% Ho: Y₂O₃-MgO ceramic is 13.04 W/m·K, which is superior to that of Ho:Y₂O₃ ceramics. The high transmission, excellent thermal conductivity, and outstanding optical characteristics indicated that Ho: Y₂O₃-MgO ceramics is a promising material for efficient infrared solid-state laser.     

Fabrication and optical characterizations of Yb,Er codoped CaF2 transparent ceramic

Nanoparticles of Yb, Er codoped calcium fluoride were obtained by a co-precipitation method. Scanning electron microscope (SEM) and X-ray powder diffraction (XRD) analysis showed that the obtained nanoparticles were single fluorite phase with grains size around 30–50 nm. Yb, Er:CaF₂ transparent ceramics were fabricated by hot pressing (HP) the nanoparticles at a temperature of 800 °C in a vacuum environment. For a 2 mm thickness ceramic sample, the transmittance at 1200 nm reached about 83%. Microstructures were characterized using SEM analysis, and the average grain size was about 700 nm. Grain boundaries of the ceramic sample were clean and no impurities were detected. The absorption, upconversion and infrared emission spectra of transparent ceramic sample under 978 nm excitation were measured and discussed.     

Spark plasma sintering of transparent Y2O3 ceramic using hydrothermal synthesized nanopowders

Y₂O₃ nanopowders were synthesized by the hydrothermal treatment of Y(NO₃)₃·6H₂O and citric acid (CA) as Y⁺³ and the capping agent, respectively. The effect of different CA:Y⁺³ mol ratios, heat treatment time, and calcination temperature was investigated in order to determine their influence on the morphology, particle size and phase of Y₂O₃ nanopowders. The narrow size distribution of particles was obtained with CA:Y⁺³ mol ratio=1.6, heat treatment time of 6 h, and a calcination temperature at 900 °C for 90 min. Then, the synthesized Y₂O₃ nanopowder was consolidated by the spark plasma sintering technique at 1500 °C with a heating rate of 100 °C/min and held for 8 min before turning off the power. As a result, the ceramic prepared with 3 mm thickness got the highest transmission of 80% at 2.5–6 µm wavelength. The highest density and the grain size of yttria ceramic were 99.58% and 1–1.2 µm at 1500 °C, respectively.     

A single-structured LuAG:Ce,Mn phosphor ceramics with high CRI for high-power white LEDs

The realization of high color rendering index (CRI) is still a great challenge for high-power LEDs (hp-LEDs), which is hindered by the phosphor converter. In this work, based on the strategy of Ce³⁺ and Mn²⁺ multi-ion substitution, the single-structured LuAG:Ce,Mn ceramics with high CRI were prepared via regulating the ratio of tri-color (red, green, and blue) components. The effects of Mn²⁺-Si⁴⁺ pairs doping content on the crystal structure, morphologies, and luminescence properties were investigated in detail. The red emission centered at 590  and 750 nm were effectively compensated by regulating Mn²⁺ occupancy sites, resulting in a significant improvement of CRI. Pure white light with general CRI R_a up to 91.0, special CRI R₉ reaching 37.9 and LE as high as 85.07 lm/W was achieved, when the hp-LEDs were constructed from related phosphor ceramic Ce02Mn7. These results suggest that the LuAG:Ce,Mn phosphor ceramics are highly promising color converters for hp-LEDs application.     

The effect of SiC addition on photoluminescence of YAG:Ce phosphor for white LED

An effective way of improving photoluminescence (PL) of YAG:Ce by addition of small amount of SiC and sintering in air was described. The breakdown of SiC during sintering process in air was employed to provide the presence of SiO₂ and CO both of which are known to be beneficial in enhancing the PL of YAG:Ce phosphor. SiC in the form of a fine powder was added to YAG:Ce powder and sintered to densities of >99% of theoretical density. The highest luminescence was measured in sample containing 0.08?wt% SiC. The effect of the formed SiO₂ and CO was discussed and their contribution to the emission intensity was assessed. The enhancement of PL intensity is attributed to the formation of vacancies, both on Y sub-lattice and on oxygen sub-lattice and their ability to release the electrons for subsequent reduction of Ce⁴⁺ to Ce³⁺ which plays a role of luminescence activator.     

Energy transfer and controllable colors of upconversion emission in Er3+ and Dy3+ co-doped CaF2 transparent ceramics

x at. % Er³⁺, 3 at. % Dy³⁺: CaF₂ transparent ceramics (x=1-5) with good transparency were fabricated by hot-pressed sintering. The phase composition of nanoparticles and transparent ceramics, microstructure, in-line transmittance, upconversion spectra and lifetime of transparent ceramics, as well as energy transfer mechanism between Er³⁺ and Dy³⁺ were investigated. The mean grain sizes of nanoparticles decreased from 33.0 nm to 26.2 nm with the Er³⁺ doping concentration increasing from 1 to 5 at.%. The microstructure of ceramic samples presented nearly dense microstructure and EDS analysis indicated Er³⁺ and Dy³⁺ were uniformly incorporated into CaF₂ lattice. Under 900 nm excitation, the emission intensity for ⁴F_9/2→⁶H_15/2 transition of Dy³⁺ decreased and for ⁴S_3/2→⁴I_15/2 transition of Er³⁺ increased, the lifetime for the ⁴F_9/2 level of Dy³⁺ decreased while the ⁴F_7/2 level of Er³⁺ increased with the raise of Er³⁺ doping concentration. The energy transfer mechanism was proved to be the dipole-dipole interaction. The upconversion luminescence color was tuned from orange through yellow to green by changing the Er³⁺/Dy³⁺ ratio. In addition, the Vickers hardness, fracture toughness, and the thermal conductivity of Er³⁺, Dy³⁺: CaF₂ transparent ceramics were discussed. All the results showed the Dy³⁺ could be used as a sensitizer for Er³⁺: CaF₂ transparent ceramic in the upconversion field.     

Dual color tuning in Ce3+-doped oxyfluoride ceramic phosphor plate for white LED application

Ceramic phosphor plates of cerium (Ce³⁺)-doped oxyfluoride were fabricated by the solid-state reaction method. These phosphors exhibit efficient emission, with the novel feature of color tuning by varying both the doping concentration and excitation wavelength. As the Ce³⁺ concentration increases, the excitation spectrum broadens by a factor of 1.6, and the excitation peak wavelength shifts from 390 to 435 nm, and there is a variation in excitation energy of ~ 10%. Luminescence spectrum of low Ce³⁺ concentration samples is tuned from blue to green with the change of excitation wavelength. The emission peak exhibits a shift of 58 nm into the red spectral region, varying the Ce³⁺ concentration from 0.05 to 0.1 mol% ; whereas this shift is only 6 nm when Ce³⁺ content changes from 0.25 to 1 mol%. Photoluminescence (PL) quantum yield has achieved 76%. The crystal structure was examined using X-ray diffraction to explain its possible influence on the redshift luminescence. A proof of concept of white LED was constructed using a 450 nm blue LED chip with an oxyfluoride phosphor plate, showing a luminous efficacy (LE) of 64 lm/W with a color rendering index of 74.     

Scintillation and dosimeter properties of CaF2 transparent ceramics doped with Nd3+ produced by SPS

We have developed CaF₂:Nd transparent ceramics with varying doping concentrations of Nd by spark plasma sintering (SPS) and evaluated the optical, scintillation and dosimeter properties. The samples showed effective absorption peaks in the visible and near infrared regions due to the 4f-4f transitions of Nd³⁺. In scintillation properties, Nd³⁺ was also active and showed the 4f-4f radiative transitions of Nd³⁺ which appeared at 860 and 1064 nm under X-ray irradiation. In photoluminescence under 160 nm excitation, the samples showed emission peaks in the vacuum ultraviolet region due to the 5d-4f transitions of Nd³⁺. Furthermore, the samples showed thermally stimulated luminescence (TSL) exhibiting glow peaks at 100, 150 and 380 °C. Among the present samples the 5% Nd-doped sample showed the highest sensitivity which allowed to measure radiation dose from 0.1 to 1000 mGy with a good linear response.     

Vacuum sintering of highly transparent La1.28Yb1.28Zr2O7.84 ceramic using nanosized raw powders

Highly transparent La_1.28Yb_1.28Zr₂O_7.84 ceramic was prepared by vacuum sintering using nanosized raw powders, which were synthesized by a simple solution combustion method using rare earth nitrate as the raw materials. The as-burnt powders were calcined at 1200?℃ and then ball-milled for 24?h with resultant particle size of about 60?nm. The two phases, cubic pyrochlore and defective fluorite, are uniformly distributed in the ceramic. La_1.28Yb_1.28Zr₂O_7.84 transparent ceramic with the maximum in-line transmittance of 83.9% was successfully prepared at 1850?℃ for 6?h in a vacuum furnace.     

Preparation and properties of novel Tb3Sc2Al3O12 (TSAG) magneto-optical transparent ceramic

Novel Tb₃Sc₂Al₃O₁₂ (TSAG) magneto-optical transparent ceramic was fabricated by vacuum sintering. The phase composition, microstructure, optical quality, thermal properties, and magneto-optical properties of TSAG ceramic were measured. It is shown that the increase in holding time has an effect on the grain size of TSAG ceramic. It is noted that TSAG35 ceramic presents the highest transmittance, corresponding to 81.5 % at the wavelength of 1064 nm. The thermal properties of TSAG ceramic are close to or superior to that of the reported TSAG and TGG crystals. The Verdet constant of TSAG ceramic is comparable to that of reported TSAG crystal, and 1.2 times that of TGG crystal. The results indicate that the novel TSAG ceramic is comparable to TSAG crystal in terms of magneto-optical properties and superior to TGG crystal, making it a candidate material for magneto-optical materials to be used in high-power lasers.