Luminescent properties of MgAl2O4 ceramics doped with rare earth ions fabricated by spark plasma sintering technique

MgAl₂O₄ ceramics doped with rare earth ions (Eu²⁺ and Ce³⁺ ions) were fabricated by spark plasma sintering technique. A complex characterization of the crystalline and defect structure of the ceramic by XRD was carried out. Absorption, excitation, photo- and cathodoluminescence spectra were studied. The photoluminescence spectrum shifts to the blue region with a maximum at λ_em =?475?nm for the MAS:0.1Ce ceramics. The nature of this luminescence can be caused by the radiative transitions in the cerium ion 5d–4f. The emission spectrum of MAS:0.1Eu has a “green” band emission in range of 400–700?nm centered around 500?nm, which can be ascribed to the allowed 4f⁶5d¹→4f⁷ (5d–4f) transition of Eu²⁺. In the millisecond time range, simultaneously with the emission of the complex host centers, the impurity luminescence bands of the chromium ion are recorded. It was shown that cathodoluminescence spectra in nanosecond time range can be decomposed into several emission bands at 2.72, 3.01, 3.37, 3.63–3.82?eV caused by F-type centers. It was demonstrated that the Eu²⁺ and Ce³⁺ ions lead to change the intensity ratio of the luminescence bands. The luminescence decay kinetics of synthesized spinel ceramics in nano- and millisecond time range were investigated in detail.     

Highly thermal stable and color tunable composite fluorescent ceramics for high-power white LEDs

All-inorganic fluorescent materials with high luminescence efficiency, high thermal stability and adjustable spectrum are urgently needed, especially for high-power white LEDs. In this work, Y_2.84Lu_0.1Al₅O₁₂: 0.06Ce³⁺ fluorescent ceramics were prepared firstly by vacuum sintering technology, and then Y_2.84Lu_0.1Al₅O₁₂: 0.06Ce³⁺/SrAlSiN₃: Eu²⁺ composite fluorescent ceramics were synthesized by technology screen-printing and laser ablation. Under 460 nm excitation, the composite fluorescent ceramic exhibits a broad emission band from 500 nm to 675 nm, which is attributed to the 5d → 4f transitions of Ce³⁺ and Eu²⁺ ions, respectively. By controlling the screen-printed times, the color coordinates of the composite fluorescent ceramics could be tuned from (0.3125, 0.2437) to (0.4106, 0.3824), and the correlated color temperature can vary from 3296 to 9689 K. In particular, the thermal stability of composite fluorescent ceramics is improved obviously after laser ablation. At 423 K, the luminescence intensity at 535 nm and 620 remains 91% and 94% of that at room temperature, respectively. Combining a 460 nm blue chip and the composite fluorescent ceramic, a white LED with CRI = 90, and the maximum luminous efficiency can be up to 148 lm/W. Our results indicate that Y_2.84Lu_0.1Al₅O₁₂: 0.06Ce³⁺/SrAlSiN₃: Eu²⁺ composite fluorescent ceramics could be used in high-power 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.     

3D printed ceramic phosphor and the photoluminescence property under blue laser excitation

An Al₂O₃/YAG: Ce³⁺ ceramic phosphor was fabricated for high-flux laser lighting using the digital lighting process (DLP)-based 3D printing method for the first time. The photocurable ceramic suspension for 3D printing was prepared by blending well-treated Al₂O₃/YAG: Ce³⁺ composite powders with photosensitive resin monomers and photo-initiators. The printing parameters, debinding and sintering processes were designed delicately to fabricated the dense sub-millimeter-sized cylinder ceramic with high dimensional accuracy. The ceramic showed excellent luminescence property under blue laser excitation with a threshold of 20.7 W/mm², higher than that prepared via dry-pressing followed by vacuum sintering. The luminescence properties and the microstructures of both ceramics were further comparatively investigated to find the possible interpretations for improvement of laser flux for the 3D-printed ceramic. The present work indicated that the new developed 3D printing method was promising for preparing luminescent ceramics for high-flux laser lighting in a rapid, effective, low-cost and precision-controlled manner.     

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.     

Fabrication and Scintillation Performance of Nonstoichiometric LuAG:Ce Ceramics

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

Efficient energy transfer of Ce to Nd in SrF2 transparent ceramics for enhancing NIR emission

High optical quality Nd³⁺ and Ce³⁺ co-doped SrF₂ (Nd³⁺, Ce³⁺: SrF₂) transparent ceramics were fabricated successfully by a simple hot-pressing (HP) method. The phase composition, in-line transmittance, absorption and emission spectra, as well as the detailed energy transfer of Nd³⁺ and Ce³⁺ were investigated. In addition, the Judd- Ofelt (J-O) theory was adopted to evaluate the luminescence property. The SrF₂ transparent ceramic samples exhibited excellent optical properties, up to 82 % at 400 nm and 92.5 % at 1054 nm. The fracture surface of SrF₂ transparent ceramic proved nearly dense microstructure and EDS results demonstrated uniform doping. The addition of cerium ions changed the crystal field environment of neodymium ions and shifted the emission peak to higher wavelengths at 796 nm excitation. Moreover, through the energy transfer process of Ce³⁺ to Nd³⁺, the occurrence of concentration quenching phenomenon was avoided under 298 nm excitation, and the emission cross-section of ⁴F_3/2→⁴I_11/2 increased to 3.1 × 10⁻²⁰ cm².     

Ce/Mn/Cr: Y3Al5O12 phosphor ceramics for white LED and LD lighting with a high color rendering index

Ce/Mn/Cr: Y₃Al₅O₁₂ transparent ceramics with a pure garnet structure and a high color rendering index were prepared by a solid-state reaction method. Mn²⁺ and Cr³⁺ enhance the emission between 500 and 700 nm and expand the conventional Ce: YAG phosphors spectrum. The Ce³⁺ can work both, as activators and sensitizers, and the intense energy transfer from Ce³⁺ to Mn²⁺/Cr³⁺ is realized through the non-radiative and radiative processes. In the sample with the optimized doping concentration the high color rendering index (CRI) value of 75.3 can be achieved under a 450 nm laser diode excitation. The chromaticity coordinates can be tuned from (0.3125, 0.3232) to (0.2917,0.2851) by varying the doping concentration. With the increasing Mn²⁺/Cr³⁺ doping concentration, the lifetime of Ce³⁺, quantum efficiency and luminous efficiency are all gradually decreased. This work effectively offers a scheme for realizing the high color rendering performance of phosphor-converted transparent ceramics in white LEDs/LDs.     

Effect of air annealing on the optical properties and luminescence performance of Ce:YAG ceramics for light-emitting diodes at different Ce concentrations

(Y_1-x%Ce_x%)₃Al₅O₁₂ (x = 0.2,0.4,0.6,0.8,1.0) transparent ceramics were fabricated by vacuum sintering technology, followed by air annealing at different temperatures. Transmittance of ceramics, valence of cerium, and luminescent properties with varying annealing temperatures are studied in detail. The negative effect of Ce³⁺ oxidation induced by annealing gets increasingly evident when Ce concentration increases. Collaborating Ce:YAG ceramics with InGaN blue chips, light-emitting diodes (LEDs) with superior performance were constructed. The relationships between Ce concentration, annealing temperature, and luminous flux of LEDs are elucidated, showing that the optimized annealing temperature of Ce:YAG ceramics decreases from 1200 °C to 900 °C as Ce concentration increases from 0.2 at% to 1.0 at%. The luminous fluxes of optimized LEDs increase by ～10 % compared with that of unannealed LEDs.     

Highly transparent cerium-doped yttria ceramics for full-band UV-shielding window applications

Highly transparent cerium-doped yttria ceramics were fabricated via a pressureless sintering method with an addition of 10 at.% La₂O₃ and 1 at.% ZrO₂ as a binary sintering additive. With an increase in the Ce doping concentration from 0 to 3 at.%, the optical absorption edge of yttria ceramics exhibited a significant red shift from 290 to 380 nm as a result of the 4f–5d transition of Ce³⁺ and the charge transfer of Ce⁴⁺ (i.e., Ce⁴⁺+e^–→Ce³⁺). For a 2-mm thick sample doped with 3 at.% Ce, the total ultraviolet radiation (UV) transmittance (220–400 nm) is only 1.7%, indicating a nearly complete UV absorption. Meanwhile, the specimens possess high in-line transmittance levels in both the visible and the infrared regions (i.e., >70% at 450 nm and ∼80% at 1100 nm). Additionally, the present specimens were confirmed to have good enough mechanical strength levels (∼165 MPa) and thermal conductivity (∼5 W/m·K), which are comparable or even better compared to those of previously reported transparent yttria ceramics. The results of this work indicate that cerium-doped transparent yttria ceramics are promising candidate materials for full-band UV-shielding window applications.     

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.     

Application of Judd–Ofelt theory in analyzing Nd3+ doped SrF2 and CaF2 transparent ceramics

Nd³⁺ doped SrF₂ and CaF₂ transparent ceramics were fabricated by vacuum hot-press sintering and the absorption spectra, emission spectra as well as luminescence decays of the samples were measured. Judd-Ofelt (J–O) theory was used to analyze the optical performance of Nd³⁺ in these two isostructural hosts. The Nd: SrF₂ transparent ceramic was found to have smaller line strength, larger radiative lifetime and smaller Ω₂ value (corresponding to more ionic Nd³⁺-ligand bonding and more symmetry of Nd³⁺ environment). These features made it easier for Nd: SrF₂ to realize population inversion and strong emission, thus doing good to laser performance. The strong emission of ⁴F_3/2 → ⁴I_9/2 transition in Nd: SrF₂, which was predicted by J–O theory and demonstrated by luminescence spectrum, made it possible to achieve effective laser output around 900 nm. The intensity parameters and radiative lifetimes of ceramics were found comparable with their corresponding single crystals.     

Processing of Transparent Polycrystalline AlON:Ce3+ Scintillators

A new polycrystalline ceramic scintillator is reported for potential use in radiation detection and medical imaging applications. The goal was to develop cerium‐activated aluminum oxynitride (AlON:Ce³⁺) ceramics, which can be produced using ceramic processes in comparison to the high‐cost, low‐yield single‐crystal growth technique. A phase pure AlON:Ce³⁺ powder with cubic symmetry was successfully synthesized at high temperature under a reducing atmosphere to convert Ce⁴⁺ to Ce³⁺ in the solid solution. Two different activator concentrations (0.5 and 1.0 mol%) were explored. Fully dense and transparent AlON:Ce³⁺ ceramics were produced by a liquid‐phase‐assisted pressureless sintering. The crystal field splitting around the Ce³⁺ activator in the AlON was comparable to the splitting induced by Br^? and the Cl^? ligands, which produced an emission spectrum perfectly matching the maximum quantum efficiency range of the photomultiplier tube for radiation detection. Both optical excitation and radiation ionizations in AlON:Ce³⁺ were demonstrated. Challenges and mechanisms related to the radioluminescence efficiency are discussed.     

Theoretical investigation of thermodynamic and optoelectronic properties of Ce4+ doped SrZrO3 ceramics: A DFT study

Theoretical insight into the thermodynamic, structural, electronic and optical properties of SrZr_1-xCe_xO₃ (x = 0, 0.037 and 0.125) ceramics is presented using DFT calculations for predicting their potential applications as photocatalysts. The stable incorporation of Ce⁴⁺ dopant at Zr site of SrZrO₃ is examined in terms of enthalpies of formation and defect formation energies using valid limits of atomic chemical potentials of the species involved in substitutional doping. Our results indicate expansion in the SrZrO₃ lattice and decrease of band gap with increasing cerium doping concentration. Significant differences are observed in the electronic and optical properties of SrZr_1-xCe_xO₃ ceramics due to different nature of unoccupied Ce-4f states above the Femi level when cerium doping concentration increases from x = 0.037 to x = 0.125. Inclusion of the spin-orbit coupling in our DFT calculations are found to cause splitting of the unoccupied Ce-4f states above Fermi level for high concentration of doping. The trends observed in the structural and electronic properties of SrZr_1-xCe_xO₃ ceramics with increasing cerium doping concentration are found to be qualitatively similar for calculations performed without and with the inclusion of spin-orbit coupling. We show that the near UV absorption of SrZrO₃ can be considerably enhanced by increasing cerium doping concentration beyond x = 0.037; making Ce⁴⁺ doped SrZrO₃ potential heterogeneous photocatalysts.     

Laser sintering and influence of the Dy concentration on BaAl2O4:Eu2+, Dy3+ persistent luminescence ceramics

This work reports the effect of the Dy concentration on the persistence luminescence properties of Eu doped barium aluminate (BaAl₂O₄) laser-sintered ceramics. For this study, the ceramics were first sintered using the laser sintering technique, based on a CO₂ laser as the heating source, in an ambient atmosphere. The structural and morphology characteristics of the samples were investigated by x-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques, respectively. The laser-sintered ceramics presented are shown to be phase pure (single phased), and, for highest Dy concentration sample, a spurious Dy-rich phase at the grain boundary was observed. All samples exhibit the characteristic blue-green emission from the Eu²⁺ ion, due to the 4f⁶5d¹–4f⁷ transition (495 nm), even when they have been sintered in air. Finally, a clear dependence of the persistent luminescence intensity and decay time with the Dy concentration was verified.     

Impact of Eu3+ Dopants on Optical Spectroscopy of Ce3+: Y3Al5O12‐Embedded Transparent Glass‐Ceramics

Transparent glass‐ceramics containing Ce³⁺: Y₃Al₅O₁₂ phosphors and Eu³⁺ ions were successfully fabricated by a low‐temperature co‐sintering technique to explore their potential application in white light‐emitting diodes (WLEDs). Microstructure of the sample was studied using a scanning electron microscope equipped with an energy dispersive X‐ray spectroscopy. The impact of co‐sintering temperature, Ce³⁺: Y₃Al₅O₁₂ crystal content and Eu³⁺ doping content on optical properties of glass‐ceramics were systematically studied by emission, excitation spectra, and decay curves. Notably, the spatial separation of these two different activators in the present glass‐ceramics, where Ce³⁺ ions located in YAG crystalline phase while the Eu³⁺ ones stayed in glass matrix, is advantageous to the realization of both intense yellow emission assigned to Ce³⁺: 5d→4f transition and red luminescence originating from Eu³⁺: 4f→4f transitions. As a result, the quantum yield of the glass‐ceramic reached as high as 93%, and the constructed WLEDs exhibited an optimal luminous efficacy of 122 lm/W, correlated color temperature of 6532 K and color rendering index of 75.     

Structures and electrical conductivities of Gd3+ and Fe3+ co-doped cerium oxide electrolytes sintered at low temperature for ILT-SOFCs

Gd³⁺ and Fe³⁺ co-doped cerium oxide electrolytes, Ce_0.9Gd_0.1‐xFe_xO_2-δ (x?=?0.00, 0.01, 0.03, 0.05, 0.07, 0.10), were prepared by co-precipitation for ultrafine precursor powders and sintering for densified ceramic pellets. The crystal and microscopic structures were characterized by XRD, FESEM and Raman spectroscopy and their electrical properties were studied by AC impedance spectroscopy and the measurement of single cell's outputs. In comparison with Ce_0.9Gd_0.1O_1.95, the ceramic pellets of Ce_0.9Gd_0.1‐xFe_xO_2-δ with a relative density of 95% can be obtained after sintered at 1000?°C for 5?h, showing a remarkably enhanced sintering performance with a sintering temperature reduction of 500?°C, which might be ascribed to the highly activated migration of constituent species in the cerium oxide lattice doped with Gd³⁺ and Fe³⁺ions. Moreover, the electrical conductivity of Ce_0.9Gd_0.1‐xFe_xO_2-δ can be significantly enhanced depending on the mole fraction x, with Ce_0.9Gd_0.07Fe_0.03O_1.95 exhibiting the highest electrical conductivity of 38 mS/cm at 800?°C, about 36% higher than that of Ce_0.9Gd_0.1O_1.95 electrolyte sintered at 1500?°C for 5?h. So, The Gd³⁺ and Fe³⁺ co-doped cerium oxide would be an excellent candidate electrolyte for ILT SOFCs due to its prominent sintering performance and enhanced electrical conductivity.     

The luminescence properties,energy transfer mechanism of the color tunable and high quantum efficiency LaAl2.03B4O10.54: Ce3+, Tb3+ phosphors

Tb³⁺ is a typical green emitting luminescence center in inorganic compounds. However, the absorption of Tb³⁺ is very weak in ultraviolet spectral region (from 240 to 400 nm). Ce³⁺ is often used as a sensitizer to transfer energy to Tb³⁺. In this paper, Ce³⁺ and Tb³⁺ were co-doped into a novel aluminates-borates LaAl_2.03B₄O_10.54. Ce³⁺ can absorb UV light (from 240 to 340 nm) and transfer absorbed energy to co-doped Tb³⁺ effectively and bring bright green emission of Tb³⁺. The crystal structure and fluorescence spectra of phosphors, the efficiency of energy transfer between Ce³⁺ and Tb³⁺, decay dynamics, the thermal stability and internal quantum efficiency of luminescence have been investigated in detail. These results indicate that the color tunable LaAl_2.03B₄O_10.54: Ce³⁺, Tb³⁺ phosphor is a potential green-emitting material. Especially, analysis about the relationship of the doping concentration of luminescence centers and thermal stability of luminescence points out a feasible way to enhance the thermal stability of luminescence in the future.     

Intense red emission via energy transfer from (Ce3+/Eu3+):P2O5+NaF+CaF2+AlF3 glasses for warm light sources

Europium (Eu³⁺)-doped fluorophosphate (PNCA:P₂O₅+NaF + CaF₂+AlF₃) glasses with the addition of cerium (Ce³⁺) ions were fabricated by the melt-quenching technique to know their ability for the bright red (615 nm) luminescence. The emission (PL) and excitation (PLE) spectra, decay curve measurements as well as energy transfer (ET) process of Ce³⁺→ Eu³⁺ were studied in detail. An excitation spectrum related to the ⁷F₀→ ⁵D₂ level of Eu³⁺ is used to estimate the phonon energy (1121 cm^?1) of the title glass host. Under ultraviolet (UV) irradiation of 299 nm, the PL spectra of (Ce³⁺/Eu³⁺):PNCA glasses show intense red emission at 615 nm whereas the lifetime decrease with respect to increase of Eu³⁺ that could support the observed efficient ET from Ce³⁺ to Eu³⁺. The ET:Ce³⁺ →Eu³⁺ via quadrupole-quadrupole process was confirmed by Reisfeld's approximation and Dexter's ET formula. The ET efficiency (η_ET) and critical distance (R_c) were also calculated. Interestingly, the (Ce³⁺/Eu³⁺):PNCA glasses showed intense red light emission with low correlated color temperatures and the corresponding color purity reached as great as 99%, indicating its potentiality as a red component for warm light sources.     

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).