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.     

Optical and Scintillation Properties of Ce3+‐Doped LuAG and YAG Transparent Ceramics: A Comparative Study

Cerium‐doped lutetium aluminum garnet (LuAG:Ce) and yttrium aluminum garnet (YAG:Ce) transparent ceramics of same dimension were fabricated and their optical and scintillation properties were studied. LuAG:Ce transparent ceramic showed higher light yield under UV and X‐ray excitation with respect to YAG:Ce transparent ceramic. YAG:Ce transparent ceramic showed higher light yield under gamma excitation and better energy resolution, which could be due to the considerable amount of slower emission (38.5%) in LuAG:Ce as well as lower optical transparency with respect to YAG:Ce ceramic.     

High lumen density of Al2O3-LuAG: Ce composite ceramic for high-brightness display

Thermally robust and highly efficient green-emitting luminescent ceramics are gradually attracting great attention as promising phosphors using in high-brightness laser phosphor display to reduce serious speckle noise as well as high cost. However, lumen density is still seriously restricting their potential applications especially under high-power density laser due to insufficient absorption of blue laser and significant thermal quenching. Here, we report an Al₂O₃-LuAG: Ce composite ceramic phosphor (CCP) for high-brightness laser phosphor display. Owing to good optical properties and high thermal conductivity of Al₂O₃, the Al₂O₃-LuAG: Ce CCP shows high photoluminescence quantum yield (79.6%), low thermal quenching (only 3.2% loss in luminescence at 200°C), and high thermal conductivity (18.9 W·m⁻¹·K⁻¹). Moreover, the Al₂O₃, as scattering centers, enhances the Rayleigh–Mie scattering of the blue laser, and hence the absorption of the Al₂O₃-LuAG: Ce CCP exhibits a remarkable improvement (~2.3 times) at 450 nm. Finally, with optimized thickness (0.3 mm) of Al₂O₃-LuAG: Ce CCP, an excellent luminous efficiency (216 lm·W⁻¹) and outstanding lumen density (6129 lm·mm⁻²) of the green-emitting light source was obtained by driving under a high-power density (28.33 W·mm⁻²) blue laser. All of those validate the suitability of the Al₂O₃-LuAG: Ce CCP for high-brightness display.     

Layered array Al2O3-LuAG: Ce composite ceramic phosphors for high-brightness display

Thermally robust and efficient composite ceramic phosphors (CCPs) combined both the merits of matrix and phosphor have received growing interests. However, high matrix content (e.g., Al₂O₃ >40 wt%) brings diluted activated ion concentration and dropped photoluminescence (PL) quantum yield (QY). Here, a novel layered array Al₂O₃-LuAG: Ce CCP, where Al₂O₃ and LuAG: Ce thin layers (10–250 µm) are alternately arranged, was presented. Owing to the special structure, thermal phonons and photons are respectively routed into Al₂O₃ layers and LuAG: Ce layers, which weakens the influence of Al₂O₃ and heat accumulation on PL properties. Consequently, it exhibits high PLQY (84.1%) and good thermal conductivity (17.1 W·m^?1·K^?1). When it is irradiated under high-power density (27.2 W·mm^?2) blue laser, the luminous efficiency and lumen-density are promoted to 220 lm·W^?1 and 5994 lm·mm^?2, respectively. This work provides a promising new microstructure in developing novel phosphor converters for high-brightness laser phosphor display.     

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).     

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.     

The Cr‐doping effect on white light emitting properties of Ce:YAG phosphor ceramics

The Cr/Ce‐doped YAG transparent ceramic was fabricated by the solid‐state reaction in vacuum. The Cr/Ce‐doped YAG ceramic phosphor effectively complement the red spectral component and improve the color rendering performance when excited by blue light that is due to the effective energy transfer between Cr³⁺ ion and Ce³⁺ ion. However, the energy transfer from Ce³⁺ to Cr³⁺ ion leads to energy loss and therefore the luminous efficacy of the WLED which is composed of blue LED chip and the Cr/Ce‐doped YAG ceramic phosphor decreases. The composite phase structure of ceramic phosphor is designed for improving the extraction efficacy and increasing the luminous efficacy by breaking the total internal reflection (TIR) at the interface between air and ceramic.     

Assessment of conversion efficiency of Cr4+ ions by aliovalent cation additives in Cr:YAG ceramic for edge cladding

0.25at.% Cr:YAG ceramics were successfully fabricated as the edge cladding of Yb:YAG transparent ceramic slabs through vacuum sintering of co‐precipitated powders, using oxide additives to introduce different cations. The effects of various cation additives (Si⁴⁺, Ca²⁺, and Si⁴⁺ + Ca²⁺) on the conversion efficiency of Cr⁴⁺ ions and optical characteristics of the Cr:YAG edge cladding were investigated. Measurements of the absorption spectra of the Cr:YAG ceramics without any additives revealed 2 absorption bands centered at 430 nm and 600 nm, which imparted the sample with a green color. The introduction of only Si⁴⁺‐bearing additive did not promote the transition of Cr ions from the 3+ to 4+ state. Theoretical analysis and experimentation revealed that the addition of CaO not only enhanced the microstructure and improved the transmittance of the Cr:YAG ceramic, but also introduced vacancies that assisted in the formation of Cr⁴⁺ ions. It was determined that CaO has the same effect on the conversion efficiency of Cr⁴⁺ ions whether it is added as a single additive or in combination with SiO₂. The underlying mechanisms by which these aliovalent cation additives influence the formation of Cr⁴⁺ ions and affect optical properties are discussed in detail. High quality composite ceramics with Yb:YAG transparent ceramic slabs and dark brown‐colored Cr⁴⁺: YAG ceramic edge cladding were achieved through the addition of 0.05 wt.% CaO to the edge cladding, with no interfacial effects between the 2 regions being observed.     

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.     

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.     

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.     

Mn2+ activated MgAlON transparent ceramic: A new green-emitting transparent ceramic phosphor for high-power white LED

A novel Mn²⁺ activated green-emitting MgAlON transparent ceramic phosphor was synthesized from Mg_0.21Al_2.57O_3.80N_0.20:0.03Mn²⁺ (MgAlON:Mn) phosphor powder by pressureless sintering combining with hot isostatic pressing. By crystalline structure refinement and cathodoluminescence (CL) characterization, it is demonstrated that Mn²⁺ was dissolved in the spinel lattice and occupied the tetrahedral site. The ceramic, retaining high transmittance in UV–vis region (up to 82% at 800 nm) and excellent thermal-mechanical properties of MgAlON transparent ceramic-matrix, shows a strong green emission at 513 nm under 445 nm light excitation. Compared with its powder counterpart, the ceramic phosphor exhibits higher green color purity, higher internal quantum efficiency (47%) and lower thermal quenching. It is suggested that this novel green solid phosphor could be applied in high color rendering and high-power white light-emitting diodes when combined with a red solid phosphor and a blue LED chip.     

Phase composition,microstructure and luminescent property evolutions in “light-scattering enhanced” Al2O3-Y3Al5O12: Ce3+ ceramic phosphors

Classic transparent ceramics for laser gain medium and window materials may benefit from their distinctive features of structural homogeneity and high transparency at large scales. However, this has restricted the ability of the ceramics for local light management. Herein, a strategy for ceramic-phosphor design was conducted in the present work via introducing light-scattering centers into single phased YAG: Ce³⁺ transparent ceramics, and an enhancement of light extraction was experimentally realized through the control of Al₂O₃ grain size. UV–vis-NIR diffuse reflectance spectra further confirmed the important roles of the excrescent Al₂O₃ grains. More importantly, PL characterization shows orange-white light emission with high brightness at high temperature. The results highlight that the unique configuration enables simultaneous control of light propagation, luminous efficiency and thermal stability of luminescence, and the design strategy may create great opportunities for laser lighting and displays with high laser power density.     

High-performance Al2O3‒YAG:Ce composite ceramic phosphors for miniaturization of high-brightness white light-emitting diodes

The miniaturization of high-brightness white light-emitting diodes (WLEDs) is limited by the low thermal performance of phosphors. In this study, the microstructure, optical properties, and thermal performance of Al₂O₃–Y₃Al₅O₁₂:Ce³⁺ (Al₂O₃–YAG:Ce) composite ceramics fabricated by hot pressing were investigated. By promoting the growth of Al₂O₃ grains while maintaining a high composite density, thermal performance of the composite ceramics was significantly increased. The thermal conductivity of a Al₂O₃–40-vol% YAG:Ce ceramic reached 21.8 W/m/K, which is close to the theoretical value. In addition, this composite ceramic exhibited the highest energy efficiency. After packaging with a high-power LED chip with dimensions of 1 mm × 1 mm, a high luminous flux of 639 lm was generated, while the reduction in output power at 250 °C was as low as 6%. This indicated excellent high-temperature stability and potential for applications in solid-state lighting.     

Preparation and optical properties of MgAl2O4-Ce:GdYAG composite ceramic phosphors for white LEDs

High-power light-emitting-diode (LED) lighting has attracted great attention in high-luminance applications. However, the color converters with high stability, high thermal conductivity and high efficiency are scarce. In this work, we developed promising MgAl₂O₄-Ce:GdYAG composite ceramic color converters in which a broadband yellow-emitting phosphor Ce:GdYAG is combined with a thermally stable spinel phase MgAl₂O₄. The microstructure, phase composition, photoluminescence and thermal stability were investigated in detail. After being packaged with blue LED, a high luminous efficacy value of 108.4 lm/W is achieved by the composite ceramic phosphor with the molar ratio (MgAl₂O₄ to Ce:GdYAG) of 1.0 and the thickness of 0.2 mm, as well as high CRI value of 70 and low CCT value of 4543. The light scattering nature of composite ceramic phosphors makes them an ideal and promising candidate for high-brightness white LEDs.     

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.     

Structural and Thermo‐Mechanical Properties of Nd: Y2O3 Transparent Ceramics

Various content of neodymia Nd: Y₂O₃ (Nd: 0.5–5.0 at.%) transparent ceramics were fabricated by vacuum sintering. The prepared Nd: Y₂O₃ ceramics exhibit high transmittance (~80%) at the wavelength of 1100 nm. It is found that the increase in Nd concentration enhances the grain size growth, while decreases the phonon energy, which is benefit for improving both the luminescence quantum and up‐conversion efficiency. The thermal conductivity and thermal expansion coefficient of the transparent 1.0 at.% Nd: Y₂O₃ ceramic is 5.51 W·(m·K)^?1 and 8.11 × 10^?6 K^?1, respectively. The hardness and the fracture toughness of the transparent ceramic is 9.18 GPa and 1.03 Mpa·m^1/2, respectively. The results indicate that the Nd: Y₂O₃ transparent ceramic is a potential candidate material for laser.     

Optoelectronic multifunctionality of combustion-activated fluorine-doped tin oxide films with high optical transparency

As transparent conducting oxides (TCOs) have been widely used as a common component of many optoelectronic applications, ensuring high conductivity and transparency TCOs has become a pivotal concern. In the present study, we report developing the combustion-activated pyrolysis route of horizontal ultrasonic spray pyrolysis deposition (HUSPD) as a novel strategy to form highly transparent conducting fluorine-doped tin oxide (FTO) films. Compared to the basic route, the combustion-activated FTO films showed an attractive transparent conducting performance (figure of merit of 5.34?×?10^?2?Ω^?1) with a highly improved optical transparency (90.1%) due to the formation of a smooth and dense film structure to reduce light scattering on the surface, and a decrease of oxygen vacancies to broaden the optical bandgap, all of which yielded an excellent performance as compared to the previously reported studies on the FTO films. Moreover, when the combustion-activated FTO films were used as TCOs of electrochromic devices and dye-sensitized solar cells, they acquired multifunctional effects of (a) an efficient electron transfer by (200) preferred orientations of the FTO; (b) a relaxed light scattering on the interface due to smooth and dense surface morphology of the FTO films; and (c) a broad optical bandgap by decreased oxygen vacancies, resulting in an impressive improvement of both electrochromic and photovoltaic performances. Taken together, our results demonstrate that combustion-activated FTO films are an attractive technique for forming high-performance TCOs that can further be used in multifunctional optoelectronic devices.     

Fabrication and kW-level MOPA laser output of planar waveguide YAG/Yb:YAG/YAG ceramic slab

Using the tape casting method combined with vacuum sintering and hot isostatic pressing, high-quality planar waveguide YAG/10 at.%Yb:YAG/YAG ceramics were successfully prepared. For the sample presintered at 1750°C for 30 hours and then HIPed at 1700°C for 3 hours in 200 MPa argon, the in-line transmittance reached 82.5% at 400 nm and the average grain size was ~17.1 μm. The diffusion behaviors of Yb ions across the contact boundary between the cladding YAG layer and the core Yb:YAG layer were determined by Fick's second law. Then, a 1030 nm continuous-wave (CW) Yb:YAG planar waveguide ceramic laser based on the structure of master oscillator power amplification (MOPA) was realized. After a single-pass amplification, the maximum output of the ceramic slab (60 × 10 × 1 mm³) reached 1251 W and the corresponding optical-to-optical efficiency was 30.0%, which is the highest output power of a Yb:YAG planar waveguide ceramic laser to the best of our knowledge.     

Ion-exchanged LTA zeolite derived nepheline phase NaAlSiO4:Eu2+ ceramic phosphor for laser illumination

A reliable yellow phosphor converter that can be efficiently excited by a 405 nm bluish violet laser is in high demand for laser illumination applications. A NaAlSiO₄:Eu²⁺ phosphor with a quantum efficiency reaching 92% was obtained using LTA zeolite as the raw material. NaAlSiO₄:Eu²⁺ ceramics with suitable porosities for laser illumination were prepared from the phosphor powders via spark plasma sintering. The ceramics lost only 2% of the quantum efficiency compared to the powders, maintained good thermal quenching properties (30% drop at 150 °C), and showed good thermal conductivity (2.02 W‧m⁻¹‧K⁻¹). The NaAlSiO₄:Eu²⁺ ceramic with 405 nm bluish violet lasers, with the increase in laser power density to 9.15 W/mm², exhibited an increasing luminous flux (23.83–70.26 lm) and maintained a stable luminous efficacy (47.7–46.8 lm/W), and the temperature distribution of the ceramic remained uniform and stable under long-time laser irradiation. This indicates that the nepheline-phase NaAlSiO₄:Eu²⁺ ceramic is a promising material for laser illumination.