Cerium-doped Yttrium Aluminum Garnet Research Articles

(Invited) Unraveling the Impact of Different Thermal Quenching Routes on Luminescence Efficiency of Y3Al5O12:Ce3+ Phosphor: Insights from Mode-Selective Vibrational Excitation Experiments

Cerium doped yttrium aluminium garnet, Y3Al5O12:Ce3+, is the prototype material for solid-state white lighting and it still is an important white LED phosphor. However, fundamental understanding of the thermal quenching of luminescence, which leads to a pronounced reduction of the emission intensity under high-power light-emitting diode operation, remains to be obtained. Here we show, through a multi-technique approach based on photoluminescence, thermoluminescence and mode-selective vibrational excitation experiments that thermal quenching of luminescence in Y3Al5O12:Ce3+ is caused by a combined effect of thermal ionization, thermally activated concentration quenching, and thermally activated 5d - 4f crossover relaxation via electron-phonon coupling, and establish the general trends upon variation of the Ce3+ concentration and temperature. Thermal quenching below 600 K is primarily the result of concentration quenching and crossover relaxation, which can be suppressed by keeping the Ce3+ dopant concentration far below 0.7 mol%, whereas for temperatures above 600 K thermal ionization is the predominating quenching process. This new insight into the interplay between different thermal quenching processes provides design principles for optimizing the light emittance and colour stability of new phosphor materials used in white lighting devices characterized by certain operating temperatures.

ENHANCING PHOTOMETRIC PERFORMANCE OF YAG:CeCERAMICS: INVESTIGATING THE ROLE OF ANNEALING IN RADIATION-ASSISTED SYNTHESIS

Ceramic samples of cerium-doped yttrium aluminum garnet (YAG:Ce) were successfully synthesized utilizing a high-powered electron flux field with a considerable energy level of 1.4 MeV and a power density of 23 kW/cm2. The ceramics were formed in a remarkable time span of just one second from a specifically prepared mix of yttrium, aluminum, and cerium oxides. The process of radiation-assisted synthesis of ceramics within radiation flux fields fundamentally deviates from the methodologies commonly employed today. Analyzed diffraction patterns closely align with those documented for YAG:Ce crystals, both in peak position and proportion. Furthermore, every sample consistently demonstrated a space group symmetry of Ia-3d. The luminescence and excitation spectra of ceramics synthesized in this study closely resemble those of YAG:Ce ceramics produced by other methods and YAG:Ce -based phosphors. The luminescence bands exhibit high efficiency, and the intensity ratios of the UV bands vary among the studied phosphors. The ceramics' radiation-to-luminescence conversion efficiency was found to be impressive, achieving scores of 0.57 and 0.48 in the industrial phosphors SDL 4000 and YAG-02, respectively. It was also observed that an increase in quantum efficiency of the samples could be achieved via high-temperature annealing. High conversion efficiency underscores the potential of the outlined luminescent ceramics synthesis method.

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.

Sintering YAG:Ce phosphor ceramic layers onto glass substrates by CO2 laser for WLED applications

Cerium-doped yttrium aluminum garnet (YAG:Ce) phosphor layers were sintered onto glass using CO2 laser. The layer adhesion was found to depend on the spin coating thickness. The x-ray diffraction indicated the formation of YAG phase, with minor impurities of Y2SiO5 and Y4Al2O9. When the coating thickness was 65 μm, the laser beam reacted thoroughly with glass and YAG precursors, and a liquid phase was formed, producing a thin amorphous layer (∼5 μm) on the surface grooves. However, when the thickness was increased to 170 μm, the laser beam could not penetrate to react entirely with YAG precursors, resulting in more raw residues and poor adhesion. The sintered YAG:Ce phosphor layers on glass were characterized via photoluminescence, presenting yellow emission with a thermal quench to 75% at 125 °C, which is comparable to that of the solid-state reacted YAG:Ce ceramics.

Influence of down-shifting particle’s size on monocrystalline silicon solar cells

The potential cerium doped yttrium aluminum garnet (YAG:Ce3+) phosphor particles of different sizes are mixed with ethylene-vinyl acetate (EVA) to make luminescent downshifting films, which are then utilized for packaging monocrystalline silicon solar cells. The luminescence of YAG:Ce particles with the different diameters (3–5 µm, 5–8 µm, 8–10 µm, 10–15 µm, 15–20 µm, and 20–30 µm), the transmission of the down-shifting EVA films with the phosphor particles and performance of the packaged solar cells have been investigated. Experimental results show that the particles with 15–20 µm have the highest luminescent intensity. When the mass ratio of phosphors to EVA is fixed, the films with 20–30 µm have the highest transmission. The most significant increase in solar cell conversion efficiency after packaging is due to the downshifting EVA films with 15–20 µm particles. This research could be useful in the selection of downshifting phosphors for solar cells in the future.

Investigation on the effect of vacuum annealing time on structural and optical properties of YAG:Ce nanoparticles prepared by mixed-fuel microwave solution combustion synthesis

Cerium doped yttrium aluminum garnet (YAG:Ce) nanoparticles were synthesized via mixed-fuel microwave solution combustion synthesis and further optimized by post annealing treatment. The effects of vacuum annealing time and ambient on the phosphor structural, morphological, compositional, and optical properties were investigated in detail. The structural study performed with X-ray diffraction analysis and Rietveld refinement revealed a direct crystallization from amorphous into single phase YAG:Ce was achieved at 1050 °C for 1 h. The increase in annealing time enhances the powder crystallinity and promotes crystal growth to form larger particles (50–85 nm). However, prolonged annealing would cause the partial oxidation of Ce3+ into optically inert Ce4+ ions, which was likely due to the increased Ce segregation. X-ray photoelectron spectroscopy evidenced that vacuum annealing has a higher tendency to suppress Ce3+ oxidation in comparison to air annealing, resulting in a significant enhancement of photoluminescence emission intensity by 20%. The optimum emission intensity was obtained for YAG:Ce nanoparticles after 5 h of vacuum annealing treatment with color coordinates of (0.391, 0.569). The white light emitting diode (WLED) fabricated with the optimized YAG:Ce nanoparticles showed a cool-white light with a maximum luminous efficacy of radiation of 285 lm/W and a corresponding color rendering index of 83.

Luminescence efficiency of cerium-doped yttrium aluminum garnet ceramics formed by radiation assisted synthesis

The variety of applications of yttrium-aluminum garnet (YAG)-based luminescent materials and the morphology necessary for these purposes required the development of many technologies for their synthesis. All synthesis technologies used are complex. The structural phase of yttrium-aluminum garnet is formed with any technology, at temperatures exceeding 1,500 °C. The starting materials for the synthesis are metal oxides of aluminum, yttrium and other oxides for activation and modification. It seems possible to use hard radiation to form a new phase. Radiation synthesis of ceramics is realized in less than 1 s, without the use of any additives and influences. The synthesis was carried out at the electron accelerator of the Institute of Nuclear Physics (Novosibirsk). In this work, we studied the spectral-kinetic and quantitative characteristics of luminescence for the first time obtained by the method of radiation synthesis of ceramic samples of yttrium-aluminum garnet doped with cerium with statistical processing of their values. The dependences of the reproducibility of the spectral characteristics of the luminescence of the samples on the preliminary preparation of the charge for synthesis have been investigated. Several cycles of luminophore brightness studies have been performed. It is shown that the obtained ceramics based on yttrium-aluminum garnet doped with cerium possesses the required spectral-kinetic properties, and the efficiency of conversion of the chip radiation into luminescence is achieved, which is comparable to that available in commercial phosphors. The maximum measured values of the position of the bands are from 553.5 to 559.6 nm. Brightness values range from 4,720 to 1,960 cd/m2. It was found that the main reason for the scatter in the characteristics of the luminescent properties of ceramics of yttrium-aluminum garnet, activated by cerium obtained by radiation assisted synthesis is the high rate of synthesis and, especially, the high rate of cooling of the samples.

Dosimeter Based on YAG: Ce Phosphor via Sol-Gel Method for Online X-ray Radiation Monitoring

This paper focuses on the preparation of cerium-doped yttrium aluminum garnet (YAG: Ce) powder with several concentration gradients via the sol-gel method by detecting its structural characteristics via X-ray diffraction (XRD) patterns and scanning electron microscope (SEM) to verify the generation of a complete crystal phase and evenly distributed nanopowder. On this basis, the luminescence characteristics of Ce3+ are explored, the mechanism and model are discussed based on the spectra, and the ideal doping concentration was obtained by comparing the luminescence intensity along with the fluorescence quenching theory and fluorescence decay spectra of samples with different doping concentrations. Several radiation dosimeters based on YAG: Ce phosphors were made; the online radiation monitoring function was realized under the exposure of a standard X-ray source; the repeatability, accuracy, and sensitivity of the system were verified by experiments; and the factors affecting dosimeter response are discussed. This paper verifies the possibility of adhibiting YAG: Ce fluorescent powder for online X-ray monitoring, and lays the foundation for further research.

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%.

Luminescence characterizations of YAG:Ce crystal via sol-gel method for radiotherapy

Abstract Cerium doped yttrium aluminum garnet (YAG:Ce) crystal powders were synthesized using the sol-gel method. The effects of doping concentration and sintering atmosphere were investigated via X-ray diffraction (XRD), photoluminescence (PL), and cathodoluminescence (CL). The XRD measurement indicates that doping ions do not change the crystal lattice structure of YAG crystal. PL results show that YAG:Ce samples sintered in inert atmosphere have better luminescence property around 550 nm and the optimum Ce doping concentration is 0.5 mol% as an optimal YAG:Ce powder to fabricate a dosimeter probe. Radioluminescence (RL) properties of crystal dosimeter also has been studied using clinical linear accelerator at the ultra-low dose from 3.6 to 27 cGy with dose rate from 80 to 600 cGy/min. RL measurements for dose rate and total dose show a good linear response of intensity, indicating potential applications in remote radiotherapy and radiation monitoring.

Investigation of YAG:Ce-Based Optical Fibre Sensor for Use in Ultra-Fast External Beam Radiotherapy Dosimetry

Radiation dosimetry plays a crucial role in the quality assurance in radiotherapy. To date, this is limited to monitoring the overall dose over the complete beam time, and so, there is limited information on the actual beam quality. An optical fibre sensor is developed based on cerium-doped yttrium aluminum garnet (YAG:Ce) scintillator. The temporal resolution and the stability of this scintillator-based optical fibre sensor have been experimentally evaluated and directly compared with similarly fabricated sensors based on terbium activated gadolinium oxysulfide and commercial plastic scintillating fibre, BCF-12. The photon yield and repeatability of the YAG-based sensor are compared in the clinical setting with the BCF-12 sensor and both show a high stability. The coefficient of variation of the YAG sensor is 0.7%, which is lower than the BCF sensor with a coefficient of variation of 1.4%.

Luminescent and magnetic properties of cerium-doped yttrium aluminum garnet and yttrium iron garnet composites

Functional materials exhibiting magnetic and luminescent properties have been recognized as an emerging class of materials with great potential in advanced applications. Herein, properties of multifunctional ceramic composites consisting of two garnets, luminescent cerium-doped Y3Al5O12 (Ce:YAG) and magnetic Y3Fe5O12 (YIG), are reported. On increasing the sintering temperature, both the photoluminescence and saturation magnetization of the Ce:YAG-YIG composites decreased gradually because of the interdiffusion of trivalent ions such as Al3+ and Fe3+. At a constant sintering temperature of 1100 °C, the YIG contents in the composites increased, thereby causing their luminescent properties to degrade and the saturation magnetizations to increase. For application to electronics, Ce:YAG-YIG composite thin films were integrated on quartz substrates by sputtering the ceramic target. The composite thin films exhibited both magnetic and luminescent properties after annealing. These techniques facilitate the incorporation of multifunctional nanocomposites into various devices.

Ce-doped YAG phosphor powder synthesized via microwave combustion and its application for white LED

Cerium-doped yttrium aluminum garnet (YAG : Ce3 + ) powder phosphor have been extensively studied as phosphors for blue to white light conversion. Y(3 − x)Al5O12 : x Ce3 + (x = 2.0 at. %) were successfully synthesized by microwave-assisted combustion (MW) using the organic fuel urea. A direct conversion from the amorphous phase to the cubic one was obtained at 1050°C for 5 h, together with an increase in the particle size into the range of 50 to 60 nm and decrease in the specific surface area. The as-prepared precursors and powder sintered at 1050°C were characterized for their structure, particle size, morphology, electroluminescence properties, and chromaticity by x-ray diffraction, Brunauer–Emmett–Teller method, Fourier transform infrared spectroscopy, field emission-scanning electron microscopy, transmission electron microscopy, electroluminescence, and standard CIE 1931 chromaticity analysis (CIE) chromaticity diagram, respectively. The results show that the obtained sintered YAG : Ce3 + phosphor powder has spherical-shaped particles and strong yellow emission compared to as-prepared phosphor powder. White light emitting diodes with proper color rendering index, and tunable correlated color temperature properties can be produced by controlling the injection currents and coating thickness of the sample, offering daylight white and neutral white LEDs.

Enhanced optical properties of YAG:Ce yellow phosphor by modification with gold nanoparticles

In this paper methods of synthesis and characterization of cerium doped yttrium aluminum garnet (YAG:Ce) and the improvement of properties by in situ modification using gold nanoparticles are presented. YAG:Ce phosphor was obtained by cations co-precipitation followed by thermal treatment at 1000 °C, while for the development of the Au-YAG:Ce nanocomposites, the Turkevich method was used. The quality of the phosphor particles and composite has been structurally, morphologically and optically studied. It has been emphasized the obtaining of high purity phosphor and maintenance of the YAG:Ce garnet structure after modifying the phosphor surface with gold nanoparticles. Also, the use of gold nanoparticles has led to improvements in optical and morphological properties as a result of lowering the agglomeration trend. Improvement of optical properties makes possible the use of Au-YAG:Ce composite thus developing low energy light sources and with high performance.

Titania-/cerium-doped yttrium aluminum garnet protective/downconversion layer for photovoltaics in harsh environments

Solar cells operating in harsh conditions such as extraterrestrial environment suffer from ionizing UV irradiation from the sun and radiation damages from high-energy particles. Titania (TiO2) can achieve high transmittance in the visible and near-infrared while efficiently blocking UV irradiations. Cerium-doped yttrium aluminum garnet (YAG:Ce) is commercially used as an efficient spectral downconverter in white-light LEDs, and it has also been studied for its potential as a radiation-withstanding scintillator. We propose TiO2 / YAG : Ce thin films utilizing TiO2’s UV cut-off properties as well as YAG:Ce’s downshifting properties as both multipurpose protector and performance enhancer for solar cells.

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.

Multi-point gamma-ray monitoring at radioprotection levels with image devices

A multi-point system for the simultaneous interrogation of several extrinsic polymer optical fiber-based gamma-ray sensors is demonstrated. As a proof of concept, four different sensing heads have been measured, all of them based on a chemically etched fiber tip plus an inorganic scintillator powder. Two inorganic scintillators have been tested, namely Terbium doped Gadolinium Oxysulfide (Tb:GADOX) and Cerium doped Yttrium-Aluminum garnet (YAG:Ce). These sensing heads are interrogated with a commercial image device (CMOS camera). Such device does not need any cooling stage, optical filters or image intensifier elements. The response for each individual transducer is inside a 2D grid-shape array image, which provides a multiplexing method scalable with the number of the desired interrogation points. The interrogation procedure takes advantage of well-stablished digital image processing techniques, which include the selection of the regions of interest (ROIs) associated to the response of each individual sensing-head, plus the suitable RGB channel selection to improve the signal to noise ratio in the final measurement. Thus, only the Green channel has been found to render useful signal for gamma ray monitoring. Furthermore, the selection of the exposure time in the CMOS camera also permits to adjust the radiation levels to be detected.All tests were run under gamma irradiation from a calibrated 137Cs source, the reference isotope for environmental protection. The obtained results show the feasibility of simultaneously interrogating several gamma ray sensors, which can provide reliable measurements to quickly detect the presence of hazardous radioactive materials in large accidental leakage scenarios.

White light color tuning ability of hybrid Dibenzoylmethane/YAG:Ce nanophosphor

In this study, we have considered the synthesis of a novel hybrid material consisting of cerium-doped yttrium aluminium garnet (YAG:Ce) nanophosphor surface decorated by UV/blue sensitizing Dibenzoylmethane (DBM) organic molecule. A comparative evaluation of synthesized hybrid material YAG:Ce/DBM nanophosphor and YAG:Ce nanophosphors was made using a combination of different analytical techniques like X-ray diffraction, Scanning electron microscopy, UV–Visible-Infrared absorption, Photoluminescence techniques etc. Due to the presence of organic ligand, the hybrid nanophosphor has extended excitation.We have monitored the emission spectra at excitation with 355 nm, 375 nm, 405 nm, and 465 nm. The synthesized luminescent hybrid material was characterized by various spectroscopic techniques and its photophysical properties were thoroughly investigated. The color perception of hybrid YAG:Ce/DBM nanophosphor is significantly modified due to the mixing of blue color in the green-yellow emission of YAG:Ce nanophosphor at different excitations which yield CIE coordinates (0.37, 0.59).

Design of binder-free phosphor paste for warm white LEDs

Cerium-doped yttrium aluminum garnet (YAG: Ce) is used to convert blue light which is emitted from InGaN-based LED chip for white light. In this study, the phosphor paste was prepared using monodispersed spherical YAG: Ce phosphor synthesized by co-precipitation based on one-pot synthesis. Also, orange-red emitting Sr2Si5N8:Eu2+ (SSN: Eu) phosphor was added to confirm the change in optical characteristics. Alpha terpineol was used as a paste solvent because it enabled easy screen-printing. Phosphor paste films for optoelectronic devices were prepared by coating on a glass substrate using a doctor blade method. We compared the conversion efficacy, color rendering index (CRI), and correlated color temperature (CCT) of YAG: Ce which is added to various amounts of SSN: Eu. We measured scanning electron microscopy (SEM) images to confirm the difference of porosity between, depending on binder addition.

Microstructure development and photoluminescence of annealed nanosized Ce:YAG/Al2O3 and Ce:YAG/Cr:Al2O3 powder composites

In an effort to control aggregation and sintering of phosphor nanoparticles at elevated annealing temperatures, glycothermally synthesized cerium-doped yttrium aluminum garnet (Ce:YAG) nanoparticles were annealed in a matrix of aluminum oxide between 1000 °C and 1200 °C. Scanning electron microscopy images showed that glycothermal synthesis yields ∼100 nm particles, and that the alumina matrix was able to control grain growth of Ce:YAG at annealing temperatures up to 1200 °C. Analysis by x-ray diffraction and Fourier transform infrared spectroscopy showed an increase in the degree of crystallinity at increasing temperatures as well as the evolution of alumina phases. Photoluminescence of the composite product showed the expected broad Ce:YAG spectrum, with characteristic chromium R lines present due to the formation of corundum at 1200 °C with trace chromium content. The same procedure was performed to synthesize a Ce:YAG/Cr:Al2O3 nanocomposite, yielding photoluminescence of both the expected Ce:YAG and Cr:Al2O3 peaks as well as clear evidence of energy transfer between Ce and Cr centers in YAG. The luminescence of these composites was used to determine their CIE colour co-ordinates. It was found that the colour profile of the resulting emission may be tuned by adjusting the Cr content and annealing conditions of the composite materials.