Cooperative cation substitution facilitated construction of garnet oxynitride MgY2Al3Si2O11N:Ce3+ for white LEDs

Structural modification is an important means to induce redshift of Ce³⁺ emission in garnet phosphor. We intend to design and synthesize garnet oxynitride compounds which combine attributes of rigidity inherited from garnet structure and of high covalence characteristic of oxynitride compounds. However, impurity phase usually occurs in the nitridation of garnet phosphor, due to the low solubility of nitrogen in oxides. We herein exploit the cooperative cation substitution strategy to facilitate the incorporation of nitrogen in Y₃Al₅O₁₂. It is found that partial substitution of Y³⁺‐Al_tet³⁺ pairs by Mg²⁺‐Si⁴⁺ pairs can diminish the phase instability caused by the replacement of Al_tet³⁺‐O^2? by Si⁴⁺‐N^3?. A novel pure garnet phase oxynitride phosphor MgY₂Al₃Si₂O₁₁N:Ce³⁺ with a higher substitution content of N has been obtained and the successful incorporation of N in the garnet phosphor is confirmed by the Rietveld refinements of XRD, XPS, and TEM. The emission and excitation spectra indicate that the blue‐light‐excitable MgY₂Al₃Si₂O₁₁N:Ce³⁺ phosphor exhibited a bright yellow‐orange emission peaking at 570 nm, which is redshifted by 28 nm when compared to YAG:Ce³⁺. The garnet oxynitride phosphor exhibit excellent thermal stability with high quantum efficiency and is a promising candidate for warm white LED.     

Phase formation and spectral evolution of (Sr,Ba)2Si(O,N)4:Eu2+ phosphors

Sr_2‐xBa_xSi(O,N)₄:Eu²⁺ (SB_xSON:Eu²⁺) oxynitridosilicate phosphors were prepared via incorporation of N^3?, Eu²⁺, and Ba²⁺ ions into Sr₂SiO₄ (SSO) lattices. X‐ray diffraction patterns of the prepared powders revealed that SB_xSON:Eu²⁺ was a solid‐solution form of SSO. An increase in x values caused a phase transition and an expansion of the unit cell. The photoluminescence excitation (PLE) spectra of SB_xSON:Eu²⁺ were broad, covering the ultraviolet range to the visible range. Corresponding PL emission spectra strongly depended on the excitation wavelengths and consisted of two emission bands, one in the green‐blue region (A‐band) and the other in the red region (B‐band), which were assigned to Eu(I) and Eu(II), respectively. The B‐band resulted from a dramatic red‐shift of the green emission band assigned to Eu(II) of SSO:Eu²⁺, revealing that the nitridation process preferentially affected the Eu(II) sites. This behavior was explained by crystal field splitting, the fluorescence decay time, and thermal quenching. The Ba²⁺ substitution caused evolution of the PL spectra, and its effects on the spectra were discussed under consideration of ionic size and covalence.     

Novel cyan-emitting KBaScSi2O7:Eu2+ phosphors with ultrahigh quantum efficiency and excellent thermal stability for WLEDs

Owing to the conventional phosphor-converted white LEDs (pc-WLEDs) generally suffer from blue-green cavity, thus, developing an appropriate phosphors covering both the blue and green regions in their emission spectra are very urgent. Herein, a novel Sc silicate phosphor, KBaScSi₂O₇:Eu²⁺ (KBSS:Eu²⁺), has been successfully designed and prepared via a solid-state reaction. The crystal structure, luminescent properties, thermal quenching, quantum efficiency as well as its application in UV-pumped WLEDs have been investigated systematically. The KBSS:Eu²⁺ phosphor exhibits a strong and broad excitation band ranging from 290 to 450 nm, and gives a sufficient cyan emission of 488 nm with a full-width half-maximum (FWHM) of 70 nm, which filled the blue-green cavity. Importantly, the optimized KBSS:Eu²⁺ phosphor possesses an ultrahigh quantum efficiency (QE) up to 91.3% and an excellent thermal stability retaining 90% at 423 K with respect to that measured at room temperature. Finally, the as-fabricated UV-based WLEDs device, with only coupled the mixture of KBSS:Eu²⁺ cyan phosphor and CaAlSiN₃:Eu²⁺ red ones to a commercial 365nm UV chip, exhibits a satisfactory color-rendering index (Ra = 88.6), correlated color temperature (CCT = 3770K), and luminous efficiency (LE = 21 lm/W).     

Apatite oxynitride phosphor (Mg,Y)5Si3(O,N)13:Ce3+,Mn2+: A single-phased host with solar-like and efficient emission

During pursuing high color rendering index for full-color-emitting phosphor, low quantum efficiency (QE) is usually accompanying. We intend to elevate the luminescence efficiency when realizing a solar-like spectra distribution, by constructing apatite structure oxynitride, inheriting high covalence and rigidity from oxynitride, and suitable multiple cation sites from oxyapatite compounds. Full-color-emitting apatite structure oxynitride phosphor (Mg,Y)₅Si₃(O,N)₁₃:Ce³⁺,Mn²⁺ has been prepared, and the crystal sites’ occupancies of activators in this host were favorable for white emission. (Mg,Y)₅Si₃(O,N)₁₃:Ce³⁺,Mn²⁺ phosphor shows whole visible light with emission wavelength ranging from 370 to 750 nm, matching the spectra of sunlight quite well. The fabricated white light-emitting diode lamp demonstrated the distinctive overall performance of QE and chromaticity properties (R_a and R₉). Furthermore, correlated color temperature is tunable from cool nature to warm white. The obtained lamp possesses the feature of less blue light hazard and high saturation of red degree, compared with the commercial YAG-based lamp.     

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.     

Formation of β-Silicon Nitride Crystals from (Si,Al,Mg,Y)(O,N) Liquid: II, Population Dynamics and Coarsening Kinetics

Precipitation, growth, and coarsening of Si₃N₄ crystals in (Si,Al,Mg,Y)(O,N) liquids at 1680°C has been studied. Contrary to the common observation in kinetics, coarsening rates of crystals in length and width are found to accelerate when the total volume of crystals remains little changed. This is attributed to the concomitant β-Si₃N₄ to β'-SiAlON transformation, which introduces an additional driving force for crystal dissolution and reprecipitation. As a result of the additional driving force, which has a nonmonotonic size dependence, the normalized size distribution is expected to evolve with time, initially broadening, then shifting skewing as the transformation passes the midpoint, and finally converging to a sharp distribution as the transformation completes. These evolutions have been observed in all the compositions studied.     

Broadband near-infrared emission enhancement in K2Ga2Sn6O16:Cr3+ phosphor by electron-lattice coupling regulation

Cr³⁺-doped phosphors have recently gained attention for their application in broadband near-infrared phosphor-converted light-emitting diodes (pc-LEDs), but generally exhibit low efficiency. In this work, K₂Ga₂Sn₆O₁₆:Cr³⁺ (KGSO:Cr) phosphor was designed and synthesized. The experimental results show that the Cr³⁺-doped phosphor exhibited broadband emissivity in the range 650-1300 nm, with a full width at half maximum (FWHM) of approximately 220-230 nm excited by a wavelength of 450 nm. With the co-doping of Gd³⁺ ions, the internal quantum efficiency (IQE) of the KGSO:Cr phosphor increased from 34% to 48%. The Gd³⁺ ions acted neither as activators nor sensitizers, but to justify the crystal field environment for efficient Cr³⁺ ions broad emission. The Huang-Rhys factor decreased as the co-doping of Gd³⁺ ions increased, demonstrating that the nonradiative transitions were suppressed. An efficient strategy for enhancing the luminescence properties of Cr³⁺ ions is proposed for the first time. The Gd³⁺–co-doped KGSO:Cr phosphor is a promising candidate for broadband NIR pc-LEDs.     

Highly efficient yellow-orange emission and superior thermal stability of Ba2YAl3Si2O12:Ce3+ for high-power solid lighting

With great economic benefits, white LEDs (w-LEDs) have aroused worldwide attention. For phosphor-converted w-LED, highly efficient emission and good thermal stability of phosphor are significant parameters in practical application. Here, a yellow-orange garnet-structural phosphor, Ba₂YAl₃Si₂O₁₂:xCe³⁺ (x = 0-0.1) (BYAS:xCe³⁺) was developed by solid solution design. The broad emission spectrum of the as-synthesized phosphor could guarantee the effective increase of the color rendering index when it is combined with the InGaN blue chip. Benefiting from the garnet-type highly rigid framework, BYAS:xCe³⁺ exhibits an excellent thermal stability (50%@673K of the initial integrated intensity at 280 K) as well as high absolute quantum efficiency (80.4%@460 nm excitation light). Utilizing the approach of “phosphor-in-glass” (PiG), a high-power warm w-LED is achieved based on a blue LED plus PiG and the illuminance of this w-LED device can be as high as 4227 lx.     

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.     

A broadband yellow-emitting nitridoalumosilicate Ca4SiAl3N7:Ce3+ phosphor for solid-state lighting

Developing an efficient broadband yellow phosphor with more red-light components and small thermal quenching is of great significance for solid-state lighting. In this study, a broadband yellow-emitting nitridoalumosilicate Ca₄SiAl₃N₇:Ce³⁺ phosphor was successfully synthesized by a solid-phase method at comparatively low temperature (1350 °C) and normal pressure. The crystal structure and electronic structure of Ca₄SiAl₃N₇ were studied using Rietveld refinement and density functional theory. The photoluminescence properties of the Ca₄SiAl₃N₇:Ce³⁺ phosphor were studied, including excitation and emission spectra, time-resolved photoluminescence spectra and temperature-dependent emission spectra. The results show that the Ca₄SiAl₃N₇:Ce³⁺ phosphor can be effectively excited by the blue chip and emit a strong broadband yellow light with maximum at 568 nm and the half width of 142 nm. Moreover, the Ca₄SiAl₃N₇:Ce³⁺ phosphor exhibits good thermal stability, which can still maintain 75% and 68% of the strength at room temperature when at 150 °C and 200 °C, respectively, and without spectral shift. A warm WLED can be realized by combining Ca₄SiAl₃N₇:Ce³⁺ yellow phosphor and blue LED chip. This study provides insights into developing novel broadband yellow nitridoalumosilicate phosphor with more red-light components, small thermal quenching and simple synthesis conditions.     

Thermoluminescence response and trap features of gamma-irradiated Sr2Al2SiO7:Dy3+ phosphors

This study mainly focuses on the thermoluminescence features and trap parameter analysis of Sr₂Al₂SiO₇:Dy³⁺ phosphors prepared via the low-temperature combustion method. The phase confirmation, structural and morphological studies are performed using XRD, FTIR, SEM and EDX characterizations. The energy gap of pure and Dy³⁺ doped phosphors is calculated. And the band gap decrease with doping. The optimum concentration was 3 mol% from the emission spectra; hence, this sample was irradiated with 3 Gy–5 kGy gamma doses. The samples showed high sensitivity and the MDD value is 0.21 mGy. The trap parameters were determined following deconvolution of the glow curves. Four traps were found in the samples, and the trap lifetime increased with increasing gamma dose. The trap depth increased The linear response from 3 Gy to 250 Gy implies that the prepared samples could be used as γ dosimeters within this range.     

The effects of Mg2+/Si4+ co-substitution for Al3+ on sintering and photoluminescence of (Gd,Lu)3Al5O12:Ce garnet ceramics

A series of phase-pure [(Gd_0.6Lu_0.4)_0.99Ce_0.01]₃[Al_1-z(Mg/Si)_z]₅O₁₂ (z = 0-0.10) garnet phosphor powders were prepared via gel-combustion, which were then sintered into ceramics (up to 1550 °C) under atmospheric pressure. Dilatometry revealed that equimole of Mg²⁺/Si⁴⁺ substitution for Al³⁺ accelerates densification and lowers the activation energy for grain boundary diffusion in the intermediate stage of sintering (∼1150–1370 °C), which was assayed to be ∼353 kJ/mol for z = 0 and ∼289 kJ/mol for z = 0.10. The acceleration effects of Mg²⁺/Si⁴⁺ on sintering and grain growth were further demonstrated by the results of ramp and holding sintering. Firing at 1550 °C for 4 h also produced ∼99 % dense ceramics for the Mg²⁺/Si⁴⁺ codoped garnet powders. Through considering crystal splitting of the Ce³⁺ 5d energy level, photon-phonon coupling, and crystal structure/microstructure, the influences of Mg²⁺/Si⁴⁺ content and material form on Ce³⁺ luminescence, including intensity, external/internal quantum efficiencies, emission wavelength, CIE color coordinates and decay time, were clarified in detail.     

Formation of β-Silicon Nitride Crystals from (Si,Al,Mg,Y)(O,N) Liquid: I, Phase, Composition, and Shape Evolutions

Precipitation, growth, and coarsening of Si₃N₄ crystals in (Si,Al,Mg,Y)(O,N) liquids at 1680°C has been studied. The initial nucleation of β-Si₃N₄ occurs mostly on α-Si₃N₄ because of the very high supersaturation of the liquid. After a brief period of growth, the crystals then undergo accelerated coarsening, decreasing the crystal concentration by almost 100 times with little change in the total crystal volume. Meanwhile, the crystals gradually transform from β-Si₃N₄, by substituting Si-N with Al-O, to β'-SiAlON of various compositions. The evolution of aspect ratio strongly depends on the Si/(Al,Mg,Y) ratio, which is rationalized by cation segregation to the interface driven by the acidity-basicity differential between the liquid and the crystal.     

Decay Behavior in Ce3+‐doped La3Si6N11 and Lu3Al5O12 Phosphors

Ce³⁺‐activated light emitting diode (LED) phosphors have been extensively examined for photoluminescence, and have been the focus of many detailed structural studies. However, reports of the decay curves of Ce³⁺‐activated LED phosphors are rare. Although we have reported the decay behaviors of several Eu²⁺‐activated LED phosphors such as Sr₂SiO₄, Sr₂Si₅N₈, and CaAlSiN₃, we have never conducted an in‐depth study into the decay behavior for Ce³⁺‐activated LED phosphors. For this study, we investigated the decay curves of well‐known Ce³⁺‐activated LED phosphors such as La₃Si₆N₁₁ and Lu₃Al₅O₁₂. Similar to Eu²⁺‐activated LED phosphors, the decay behavior of Ce³⁺‐activated LED phosphors was sensitive to the Ce³⁺ concentration and to the detection wavelength. There was active nonradiative energy transfer between the Ce³⁺ activators located at different sites.     

Highly Stable Red‐Emitting Sr2Si5N8:Eu2+ Phosphor with a Hydrophobic Surface

One of the biggest problems in white light‐emitting diodes (WLEDs) is the moisture‐induced degradation of phosphors. This paper proposes a simple and feasible surface modification method to solve it, whereby a hydrophobic surface layer is developed on the surface of the phosphors. The particular case of orange‐red‐emitting Sr₂Si₅N₈:Eu²⁺ (SSN) phosphor was investigated. The mechanism to develop the hydrophobic layer involves hydrolysis and polymerization of tetraethylorthosilicate (TEOS) and polydimethylsiloxane (PDMS). The experimental results showed that the surface layer of SSN phosphor was successfully modified to a hydrophobic nanolayer (8 nm) of amorphous silicon dioxide that contains CH₃ groups in the surface. This hydrophobic surface layer gives the modified phosphor superior stability in high‐pressure water steam conditions at 150°C.     

Fabrication of transparent Ce3+-doped (Gd,Lu)3Al5O12 ceramics by two-step spark plasma sintering

Transparent Ce³⁺:(Gd,Lu)₃Al₅O₁₂ with microstructure control was fabricated by two-step spark plasma sintering. In the two-step profile, the heating rate was changed from 50 to 5°C/min at the first step temperatures. During the initial stage of shrinkage, the holding time of the first step sintering could induce densification by suppressing the microstructure coarsening. As compared to the single-step profile, the two-step profile showed a smaller grain size, which decreased with a decrease in the first step temperature. The porosity of the two-step profile was lower than that of the single-step profile, and the lowest porosity was obtained at the first step temperature of 1000°C, which was the starting point of shrinkage. The TS-1000 specimen showed the highest transmittance among all specimens because of the microstructure control offered by the two-step profile. Thus, by employing the two-step profile, the transmittance could be increased from 50.1% (SS-1250) to 56.5% (TS-1000).     

Near infrared mechanoluminescence from Sr3Sn2O7: Nd3+ for in situ biomechanical sensor and dynamic pressure mapping

Mechanoluminescence (ML) is a phenomenon upon external mechanical stimuli and it has found diverse applications such as stress sensing, structure health diagnosis, 3-D signature, energy harvesting etc because of its unique properties of in situ and real-time response to the stimuli. However, ML of most of the state-of-art phosphors primarily appears within the spectral range from ultraviolet to visible, which does not lie in the biological transparent windows, and, therefore, limits its applications in biological field. Here, we report a strong near infrared (NIR) ML from orthorhombic Cmcm perovskite Sr₃Sn₂O₇: Nd³⁺, which is peaked at ~ 900 nm and located exactly within the first optical transparent window of tissues. The ML apparates after gradual discharge of the traps deeper than 0.73 eV triggered by the external mechanical stimuli and subsequently excitation of Nd³⁺ to the state of ⁴F_3/2. The rechargable ML presents well repeatable linearity to loaded force at least up to 5000 N, and it can penetrate tissues easily that are thick up to 30 mm such as pigskin and the ceramic disk of hydroxy apatite which is the main constituent of bone. These results demonstrate the potential application for in situ biomechanical sensor. Meanwhile, by recording and processing these ML signals during signing on a pellet sample, for the first time, we provide a novel signature system based on NIR ML. This could raise the security level of existed signature anti-countering to a higher level.     

Tunable luminescence and energy transfer properties of MgY4Si3O13: Ce3+, Tb3+, Eu3+ phosphors

The tricolor-emitting MgY₄Si₃O₁₃: Ce³⁺, Tb³⁺, Eu³⁺ phosphors for ultraviolet-LED have been prepared via a high-temperature solid-state method. X-ray diffraction, photoluminescence emission, excitation spectra and fluorescence lifetime were utilized to characterize the structure and the properties of synthesized samples. Two different lattice sites for Ce³⁺ are occupied from the host structure and the normalized PL and PLE spectra. The emissions of single-doped Ce³⁺/Tb³⁺/Eu³⁺ are located in blue, green and red region, respectively. The energy transfer from Ce³⁺ to Tb³⁺ and from Tb³⁺ to Eu³⁺ has been validated by spectra and decay curves and the energy transfer mode from Tb³⁺ to Eu³⁺ was calculated to be electric dipole-dipole interactions. By adjusting the content of Tb³⁺ and Eu³⁺ in MgY₄Si₃O₁₃: Ce³⁺, Tb³⁺, Eu³⁺, the CIE coordinates can be changed from blue to green and eventually generate white light under UV excitation. All the results indicate that the MgY₄Si₃O₁₃: Ce³⁺, Tb³⁺, Eu³⁺ phosphors are potential candidates in the application of UV-WLEDs.     

Temperature-Dependent Viscosity of SiREAl-Based Glasses as a Function of N:O and RE:Al Ratios (RE = La, Gd, Y, and Lu)

Tailoring glass compositions can raise the viscosity of SiREAl oxynitride glasses. In the present study, viscosity data are obtained by determining the compressive creep response of bulk glasses in air. The findings reveal that increasing both the nitrogen-to-oxygen and the yttrium-to-aluminum ratios of the glasses shifts the glass viscosity to higher temperatures. In addition, the substitution of progressively smaller rare earths in the glass composition results in a further increase in the glass viscosity. These effects have important implications in the creep resistance of silicon nitride ceramics where the amorphous intergranular films are a major factor in creep resistance.     

White‐emitting phosphor Ba2B2O5:Ce3+, Tb3+, Sm3+: Luminescence,energy transfer,and thermal stability

A series of Ba₂B₂O₅: RE (RE=Ce³⁺/Tb³⁺/Sm³⁺) phosphors were synthesized using high‐temperature solid‐state reaction. The X‐ray diffraction (XRD), luminescent properties, and decay lifetimes are utilized to characterize the properties of the phosphors. The obtained phosphors can emit blue, green, and orange‐red light when single‐doped Ce³⁺, Tb³⁺, and Sm³⁺. The energy can transfer from Ce³⁺ to Tb³⁺ and Tb³⁺ to Sm³⁺ in Ba₂B₂O₅, but not from Ce³⁺ to Sm³⁺ in Ce³⁺ and Sm³⁺ codoped in Ba₂B₂O₅. However, the energy can transfer from Ce³⁺ to Sm³⁺ through the bridge role of Tb³⁺. We obtain white emission based on energy transfer of Ce³⁺→Tb³⁺→Sm³⁺ ions. These results reveal that Ce³⁺/Tb³⁺/Sm³⁺ can interact with each other in Ba₂B₂O₅, and Ba₂B₂O₅ may be a potential candidate host for white‐light‐emitting phosphors.