Ce:YScAG phosphor-converted transparent ceramics with high thermal saturation and weak concentration quenching for LED and LD white lighting

In the high-power white light LEDs/LDs area, obtaining phosphor-converted materials with high thermal stability and high luminous emittance with proper blue/yellow light ratio has been the main challenge in recent years. In this study, a group of (Ce_xY_1-x)₃(Sc_yAl_1-y)₅O₁₂ transparent ceramics with high optical quality were proposed to rise to that challenge. Their spectra were regulated by incorporating Sc³⁺, showing blue shifted emission bands (peak position from 554 nm–538 nm), blue shifted excitation bands (462–445 nm) and narrowed full width at half maxima (120–112 nm). Significantly, the prepared Ce:YScAG transparent ceramics (TC) exhibited decent thermal quenching performance with the photoluminescence intensity at 150 °C maintaining 88.7% of its original value at room temperature. The Sc incorporation impacted the atoms’ occupation and distance, crystal field splitting and energy band structure. Under remote LD excitation mode, the luminous efficiency of the prepared Ce:YScAG TC can achieve 164.8 lm/W. And even if the Ce³⁺ doping reaches 2.0 at%, the LE can still maintain 117.8 lm/W, exhibiting decent concentration quenching characteristic. Consequently, Ce:YScAG TCs have great potential as promising phosphor-converted materials in future high-power LED and LD white lighting.     

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.     

Layered Y3Al5O12:Pr/Gd3(Ga,Al)5O12:Ce optical ceramics: Synthesis and photo-physical properties

We report a study of composite scintillating ceramics based on coupled layers of two different garnets, namely Ce-doped gadolinium gallium aluminium (GGAG:Ce) and Pr-doped yttrium aluminium (YAG:Pr), fabricated by hot isostatic pressing. Two samples were prepared, with different GGAG:Ce layer thickness, 120 µm and 690 µm respectively, but with a comparable overall thickness of 1.4 mm. The key finding is that the material architecture strongly determines the scintillation response. The radioluminescence is that expected from the irradiated material when a thick layer of ceramic is exposed to X-rays. Conversely, exposing a thin layer allows a non-null probability —about 0.3% for 120 µm of GGAG— of finding an X-ray photon in the underlying layer, and thus radioluminescence from both materials is recorded. We believe these results can extend the potential of layered optical ceramics for advanced devices, such as energy- and direction-sensitive X-ray detectors.     

Fabrication of Ce:YAG,Ce,Cr:YAG and Ce:YAG/Ce,Cr:YAG dual-layered composite phosphor ceramics for the application of white LEDs

(Ce_0.001Y_0.999)₃Al₅O₁₂ and (Ce_0.001Y_0.999)₃(Cr_xAl_1−x)₅O₁₂ (x=0.001−0.005) transparent ceramics were synthesized by the solid state reaction and vacuum sintering and their optical properties were measured. High quality white light was obtained when the Ce:YAG/Ce,Cr:YAG dual-layered composite ceramic was directly combined with commercial blue LED chip. A maximum luminous efficacy exceeding 76 lm/W at a low correlated color temperature of 4905 K was obtained. The color temperature can be controlled by variations of Cr³⁺ concentration and the ceramic thickness. Hence, the Ce:YAG/Ce,Cr:YAG dual-layered composite phosphor ceramic may be a promising candidate for white LEDs.     

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.     

Dual effect synergistically triggered Ce:(Y,Tb)3(Al,Mn)5O12 transparent ceramics enabling a high color-rendering index and excellent thermal stability for white LEDs

Ce:Y₃Al₅O₁₂ transparent ceramics (TCs) with appropriate emission light proportion and high thermal stability are significant to construct white light emitting diode devices with excellent chromaticity parameters. In this work, strategies of controlling crystal-field splitting around Ce³⁺ ion and doping orange-red emitting ion, were adopted to fabricate Ce:(Y,Tb)₃(Al,Mn)₅O₁₂ TCs via vacuum sintering technique. Notably, 85.4 % of the room-temperature luminescence intensity of the TC was retained at 150 °C, and the color rendering index was as high as 79.8. Furthermore, a 12 nm red shift and a 16.2 % increase of full width at half maximum were achieved owing to the synergistic effects of Tb³⁺ and Mn²⁺ ions. By combining TCs with a 460 nm blue chip, a warm white light with a low correlated color temperature of 4155 K was acquired. Meanwhile, the action mechanism of Tb³⁺ ion and the energy transfer between Ce³⁺ and Mn²⁺ ions were verified in prepared TCs.     

Porous Ce:YAG ceramics with controllable microstructure for high-brightness laser lighting

In order to meet the increasing demand for high-power laser diode lighting and displays, phosphor converters with high-brightness and high-directionality ought to be constructed to enhance the luminance and luminous efficacy. However, the pores formed during the sintering of phosphor ceramics affect the scattering effect and directionality of light. Therefore, porosity optimization and pore size regulation need to be explored. In this work, a series of Ce:YAG ceramics with various porosities and pore sizes were prepared. The influences of porosity and pore size on the microstructure, light confinement ability, and optical properties of Ce:YAG ceramics were studied. The ceramic phosphor with a porosity of 10 vol.% and a pore size of 3 μm exhibits a good spot confinement ability and shows a high luminous flux value of 3430 lm and a central luminance (1669 592 cd/m²) under blue laser excitation. The 10 vol.% Ce:YAG ceramic phosphor with a pore size of 5 μm has the highest emission intensity and gives a maximum luminous efficacy of 268 lm/W and a luminous flux of 4020 lm under 30 W/mm² blue laser excitation. Thus, the porous Ce:YAG ceramics are expected to be a promising candidate for high-brightness laser lighting and projection applications.     

LED白光照明用(Y0.98-xSrx)3Al5O12:Ce0.06和(Y0.98-xBax)3Al5O12:Ce0.06系列荧光粉的晶相与荧光光谱

本研究采用较温和的高温固相法,用碱土金属离子Sr2+和Ba2+取代Y3+离子进行基质取代,合成了一系列(Y0.98-xSr)3Al5O12:Ce0.06和(Y0.98-xBax)3Al5O12:Ce0.06荧光粉.运用XRD对荧光粉进行了表征,试验结果表明:在一定的掺杂取代范围内,这些体系具有立方石榴石结构.运用荧光光...     

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.     

CO2 laser post-treatment of sol-gel-derived Y3Al5O12:Ce phosphor ceramic powders

Cerium-doped yttrium aluminum garnet (Y₃Al₅O₁₂:Ce, YAG:Ce) was prepared using a sol-gel method and then fired for CO₂ laser post-treatments. Phase transformations and formation of impurities were not observed in YAG:Ce after CO₂ laser sintering. The shift of the diffraction peak and the appearance of another Raman peak indicate a more homogeneous distribution of Ce activators and enhanced crystallinity in laser-sintered YAG hosts. Larger spheres (100–200 μm) with tiny crystallites (<10 μm) were observed on the smoother surface in the laser-sintered YAG:Ce, unlike the irregular, porous, and layered powders in the sol-gel-derived YAG:Ce (1–100 μm). Photoluminescence (PL) measurements revealed an emission increase of 180% and a red shift of the emission peak for the laser-sintered YAG:Ce powders compared with the sol-gel-derived powders. Both have comparable thermal PL quenching behavior; however, the YAG:Ce powders with CO₂ laser treatment exhibited a PL efficiency improvement of approximately 4%.     

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.     

Transparent YAG:Ce ceramics for WLEDs with high CRI: Ce3+ concentration and sample thickness effects

Transparent YAG ceramics with different Ce³⁺ doping concentrations and various sample thickness have been fabricated via solid-state sintering under vacuum, for the purpose of high power white light emitting diodes (WLEDs). Their phase compositions were checked by X-ray diffraction (XRD). Optical and luminescence characteristics were investigated by transmittance, absorption spectra and photoluminescence examinations. It is found that by altering the Ce³⁺ concentration and sample thickness, the CIE color coordinates of the assembled LED devices can be tailored to white light region. More importantly, the color rendering index (CRI) of the LED devices got higher with decreased Ce³⁺ doping concentration and sample thickness. Meanwhile, the effect of Ce³⁺ concentration on the CRI was found more significant compared to that of the sample thickness. This study provides an efficient approach to tailor the luminescence properties, especially to improve the CRI of the WLEDs.     

Facile preparation and optical properties of Te/Pb-free Y3Al5O12:Ce3+ phosphor-in-glass via a screen-printing route for high-power WLEDs

The applications in high-power fields of white light-emitting diodes (WLEDs) are restricted due to their poor thermal stability. Transparent Y₃Al₅O₁₂:Ce³⁺ phosphor-in-glass (YAG-PiG) provides an efficient way to resolve this problem. In this work, novel transparent Te/Pb-free YAG-PiGs are successfully prepared via a facile screen-printing route. It is revealed that 5 mol% Eu₂O₃ optimizes the concentration in substrate glass. Slight erosion of YAG particles occurs in the as-prepared samples. With increasing YAG concentration and film thickness, the intensities of photoluminescence spectra of YAG-PiGs increase. The energy transfer from Eu³⁺ to Ce³⁺ is greatly suppressed. Superior thermal stability and high quantum efficiency are obtained. The optimal optical performance values in terms of a luminous efficiency of 117.2 lm/W, a correlated color temperature of 5859 K and a color rendering index of 77.2 are achieved in YAG-PiGs. All of the results reveal that the as-prepared YAG-PiGs are promising candidates for high-power WLEDs.     

Mn ions-activated Gd3(Al,Ga)5O12 garnet solid-solution ceramics: Cation substitution for dual wavelength red-emission

Red-emitting color-convertors have attracted considerable attention for promising applications in solid-state lighting (SSL) to improve color rendition. However, the current nitride and fluoride phosphor powders have encountered several challenges, such as high cost, narrow emission bands, and insufficient stability during operation, which limit the development of high-power full-spectrum SSL. In this study, thermally robust Gd₃(Al,Ga)₅O₁₂:Mn (GAGG:Mn) solid-solution ceramics (SSCs) with dual wavelength red-emission bands were prepared via an oxygen solid-state sintering reaction. The doped Mn ions occupied octahedral Al³⁺ and Ga³⁺ sites to generate Mn⁴⁺ luminescent centers with pronounced deep-red emissions peaking at 698 nm (²E → ⁴A₂), and Mn²⁺ luminescent centers with broad red emissions at 628 nm (⁴T₁ → ⁶A₁). Because the cationic radius matching effect induced the regulation of valence state of Mn, the photoluminescence of the GAGG:Mn SSCs can be tailored by the substitution of Al³⁺ with Ga³⁺. Moreover, the Mn³⁺ also existed in the GAGG lattice host, and their concentration decreased with increasing Ga³⁺ contents owing to the mismatch of ionic radius between Mn³⁺ and Ga³⁺ ions. With the optimization of Al/Ga ratio and concentration of Mn ions, a broad emission band ranging from 550 to 800 nm (bandwidth = 250 nm) was achieved from Gd₃Al₃Ga₂O₁₂:0.3%Mn SSCs upon 465-nm excitation. Moreover, the GAGG:Mn SSC has over 17-fold enhanced thermal conductivity compared with the corresponding phosphor powder. This paper opens a door of regulating the valence state of luminescence centers with cation substitution and the application of oxide red-emitting color-convertors.     

Microstructure tailoring of red-emitting AlN-CaAlSiN3:Eu2+ composite phosphor ceramics with higher optical properties for laser lighting

Laser lighting is superior to light-emitting diodes (LEDs) in brightness, beam aperture and efficiency, which largely depends on the light extraction efficiency and thermal properties of color converters. In this paper, we proposed to improve the light extraction efficiency of red-emitting AlN-CaAlSiN₃:Eu composite phosphor ceramics by controlling the light scattering, and to improve the thermal conductivity of the phosphor ceramics by increasing grain size. The composite phosphor ceramics with moderate light scattering and high thermal conductivity can be obtained by gas pressure sintering (GPS) and the followed hot isostatic pressing (HIP). The saturation threshold of the sample is increased from 7.0 to 10.8 W, and the luminous flux is enhanced from 32.3 to 51.0 lm after the HIP post-treatment, which is 55.5% higher than the previously reported dense AlN-CaAlSiN₃:Eu composite phosphor ceramics (32.8 lm). This work emphasizes the role of pore size distribution in light extraction efficiency of phosphor ceramics.     

CaAlSiN3:Eu/glass composite film in reflective configuration: A thermally robust and efficient red-emitting color converter with high saturation threshold for high-power high color rendering laser lighting

To achieve high color rendering and proper color temperature, a red color converter is essential for phosphor-converted white lighting devices. CaAlSiN₃:Eu²⁺ (CASN) is a highly suitable red phosphor for white light-emitting diodes. However, it can be hardly used in high-power laser lighting due to poor thermal/chemical performance of the phosphor/silicone resin mixture. A series of all-inorganic CASN-based phosphors (e.g., composite ceramic and phosphor-in-glass) were developed to avoid the use of resin. However, new challenges emerged: none of them showed sufficient luminous efficacy (i.e., >50 lm/W) and adequate saturation-threshold (i.e., >30 W or 10 W/mm²). Here, we report a facile fabrication of CASN/glass composite films using a simple and efficient blade-coating method. Upon 450 nm excitation, the resultant composite film presents a high internal quantum efficiency of ~83%, comparable to that of pristine CASN powder (~90%). When irradiated with a blue laser, the composite film shows a record high luminous efficacy of 82 lm/W. Furthermore, its saturation threshold was investigated in high power and high power density mode, respectively. When measured in high power mode, it shows a high saturation threshold over 29.7 W (1.75 W/mm²), thus achieving a high luminous flux of 1576 lm; when measured in high power density mode, it shows a saturation threshold of ~10.2 W/mm² (1.13 W). With abovementioned excellent properties, the CASN/glass composite film has great potential for use in high-power and high color rendering laser lighting.     

Modeling and Monte Carlo simulation on photothermal effect in Gd3Al3Ga2O12:Ce3+/Y3Al5O12:Cr3+ layered composite ceramic

Layered composite ceramics have wide application in solid-state lasers. However, the photothermal effect in the layered ceramics has not been clarified, due to the interface effect between layers. In this work, the model of photon propagation and thermal distribution in the Gd₃Al₃Ga₂O₁₂:Ce³⁺/Y₃Al₅O₁₂:Cr³⁺ layered ceramics are established. The property of photon absorption, reflection, transmission, and thermal distribution are studied by combining the Monte Carlo method and the convolution method. It is found that the photon absorption distribution and thermal distribution of this layered ceramics show the gradient change. Furthermore, this change is strongly dependent on the type, beam width, and power of laser. The temperature of layered ceramics induced by Gaussian beam is higher than that induced by flat circular beam. This work provides a useful research method for the design of layered ceramic materials with excellent scintillation performance.     

Processing thin,dense, transparent Ce:Y3Al5O12 films from flame made nanopowders for white light applications

Flame made metal oxide nanopowders enable processing of dense, transparent thin (< 50 μm) films of Ce³⁺ doped Y₃Al₅O₁₂ for white light applications. The addition of very small amounts of SiO₂ (0.14 wt. %) and the use of a final 95:5 N₂:H₂ atmosphere sintering step permits nearly complete removal of pores from films originally sintered in O₂. Furthermore, the introduction of this final step allows reduction in processing temperatures needed to effect Ce⁴⁺ reduction to Ce³⁺ by several hundred degrees below typical temperatures of >1600 °C. At 20–50 μm, the reported films are also much thinner than previously reported for the same materials normally produced by solid state reactions of micron size powders. Spectrofluorometric measurements of the dense transparent films exhibit excitation spectra centered around 450 nm and broad emission spectra in the 470–750 nm range with two peaks centered at 537 and 570 nm, confirming their applicability as a phosphor for white light emitting diodes.     

Comparative study of Yb:Lu3Al5O12 and Yb:Lu2O3 laser ceramics produced from laser-ablated nanopowders

We present a comparative study of two Lu-based oxide ceramics doped with Yb³⁺ ions, namely Yb:Lu₃Al₅O₁₂ (garnet) and Yb:Lu₂O₃ (sesquioxide), promising for thin-disk lasers. The ceramics are fabricated using nanopowders of 3.6 at.% Yb:Lu₂O₃ and Al₂O₃ produced by laser ablation: Yb:Lu₃Al₅O₁₂ – by vacuum sintering at 1800 °C for 5 h with the addition of 1 wt% TEOS as a sintering aid, and Yb:Lu₂O₃ – by vacuum pre-sintering at 1250 °C for 2 h followed by Hot Isostatic Pressing at 1400 °C for 2 h under Ar gas pressure of 207 MPa. The comparison includes the structure, Raman spectra, transmission, optical spectroscopy and laser operation. The crystal-field splitting of Yb³⁺ multiplets is revealed for Lu₃Al₅O₁₂. A continuous-wave (CW) Yb:Lu₃Al₅O₁₂ ceramic microchip laser generates 5.65 W at 1031.1 nm with a slope efficiency of 67.2%. In the quasi-CW regime, the peak power is scaled up to 8.83 W. The power scaling for the Yb:Lu₂O₃ ceramic laser is limited by losses originating from residual coloration and inferior thermal behavior.     

Dielectric resonator antenna with Y3Al5O12 transparent dielectric ceramics for 5G millimeter-wave applications

Microwave dielectric ceramics are considered to be one of the key materials for dielectric resonators (DR) and have very broad application prospects in the fifth generation (5G) mobile communication system. Here we have prepared high-quality factor Y₃Al₅O₁₂ (YAG) transparent dielectric ceramics using high-purity α-Al₂O₃ and Y₂O₃ powders by cold isostatic pressing of the vacuum sintered with tungsten meshes as the heating elements. Optimum relative permittivity () ~10.53, quality factor Q × f (Q = 1/dielectric loss, f = resonant frequency) ~95, 270 GHz (at =7.37 GHz), and temperature coefficient of resonant frequency (TCF) ~ −51.7 ppm °C⁻¹ were obtained at a sintering temperature of 1780°C for 12 h. For the first time, YAG transparent ceramic dielectric resonator antenna (DRA) is designed as a dominant mode and a higher-order mode using the aperture coupling feeding configuration excitation. The proposed transparent dielectric ceramic DRA can provide a broad impedance bandwidth of 4.193 GHz (ranging from 21.90 to 26.09 GHz) for S₁₁ < −10 dB, radiation efficiency of 92.1%, and compact DR unit. The proposed DRA can be used potentially as a 5G millimeter (mm)-wave multiple-input-multiple-output (MIMO) antenna unit.