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.     

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.     

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

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

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

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

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

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

A far-red-emitting (Gd,Y)3(Ga,Al)5O12:Mn2+ ceramic phosphor with enhanced thermal stability for plant cultivation

As for plants, far-red (FR) light with wavelength from 700 nm to 740 nm is critical for processes of photosynthesis and photomorphogenesis. Light-controlled development depends on light to control cell differentiation, structural and functional changes, and finally converge into the formation of tissues and organs. Phosphor converted FR emission under LED excitation is a cost-effective and high-efficient way to provide artificial FR light source. With the aim to develop an efficient FR phosphor that can promote the plant growth, a series of gadolinium yttrium gallium garnet (GYGAG) transparent ceramic phosphors co-doped with Mn²⁺ and Si⁴⁺ have been fabricated via chemical co-precipitation method, followed sintered in O₂ and hot isostatic pressing in this work. Under UV excitation, the phosphor exhibited two bright and broadband red emission spectra due to Mn²⁺: ⁴T₁ → ⁶A₁ spin-forbidden transition, and one of which located in the right FR region. And then, Ce³⁺ ions were co-doped as the activator to enhance the absorption at blue light region and the emission of Mn²⁺. It turns out that the emission band of GYGAG transparent ceramic phosphors matches well with the absorption band of phytochrome P_FR, which means they are promising to be applied in plant cultivation light-emitting diodes (LEDs) for modulating plant growth. Besides, the thermal stability of this material was investigated systematically, and an energy transferring model involves defects was also proposed to explain the phenomenon of abnormal temperature quenching.     

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

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

溶胶凝胶法制备的Y3Al5O12:Tb3+,Ce3+的光学性能及能量传递研究

用溶胶凝胶法制备了一系列不同掺杂浓度的Y3Al5O1 2(YAG):Tb3+,Ce3+荧光粉,对其物相、光学性能和能量传递进行了研究.多晶粉末X-射线衍射结果表明,所有样品均为YAG晶相,没有其它杂相.当样品在Tb3+的特征激发峰273 nm激发时,除了Tb3+的特征发射外,还观察到位于467 nm的YAG基质的电荷迁...     

The influence of Gd doping on thermophysical properties,elasticity modulus and phase stability of garnet-type (Y1-xGdx)3Al5O12 ceramics

In this work, Gd³⁺ was selected to partially substitute the Y³⁺ in yttrium aluminum garnet (YAG) in order to improve the thermophysical properties of YAG. A series of (Y_1-xGd_x)₃Al₅O₁₂ (x = 0, 0.1, 0.2, 0.3, 0.4) ceramics were synthesized through chemical co-precipitation route. The microstructure, thermophysical properties and elasticity modulus of (Y_1-xGd_x)₃Al₅O₁₂ were investigated. The (Y_1-xGd_x)₃Al₅O₁₂ ceramics was comprised of single garnet-type Y₃Al₅O₁₂ phase. The thermal conductivities of (Y_1-xGd_x)₃Al₅O₁₂ bulk samples decreased with increasing doping concentration to 0.2, but increased with furthering increasing the concentration to 0.4. The thermal conductivity of (Y_0.8Gd_0.2)₃Al₅O₁₂ was 1.51 W m⁻¹ K⁻¹ at 1200 °C. The average thermal expansion coefficient of (Y_0.8Gd_0.2)₃Al₅O₁₂ was slightly larger than that of Y₃Al₅O₁₂. (Y_0.8Gd_0.2)₃Al₅O₁₂ bulk sample exhibited the lowest elasticity modulus among the investigated (Y_1-xGd_x)₃Al₅O₁₂. In addition, (Y_0.8Gd_0.2)₃Al₅O₁₂ ceramic remained good phase stability from room temperature to 1600 °C.     

Influence of Yb3+ doping on phase stability and thermophysical properties of (Y1-xYbx)3Al5O12 under high temperature

In this work, Yb³⁺ was selected to replace the Y³⁺ in yttrium aluminum garnet (YAG) in order to reduce its thermal conductivity under high temperature. A series of (Y_1-xYb_x)₃Al₅O₁₂ (x=0, 0.1, 0.2, 0.3, 0.4) ceramics were prepared by solid-state reaction at 1600 °C for 10 h. The microstructure, thermophysical properties and phase stability under high temperature were investigated. The results showed that all the Yb doped (Y_1-xYb_x)₃Al₅O₁₂ ceramics were comprised of a single garnet-type Y₃Al₅O₁₂ phase. The thermal conductivities of (Y_1-xYb_x)₃Al₅O₁₂ ceramics firstly decreased and subsequently increased with Yb ions concentration rising from room temperature to 1200 °C. (Y_0.7Yb_0.3)₃Al₅O₁₂ had the lowest thermal conductivity among investigated specimens, which was about 1.62 W m⁻¹ K⁻¹ at 1000 °C, around 30% lower than that of pure YAG (2.3 W m⁻¹ K⁻¹, 1000 °C). Yb had almost no effect on the coefficients of thermal expansion (CTEs) of (Y_1-xYb_x)₃Al₅O₁₂ ceramics and the CTE was approximate 10.7×10⁻⁶ K⁻¹ at 1200 °C. In addition, (Y_0.7Yb_0.3)₃Al₅O₁₂ ceramic remained good phase stability when heating from room temperature to 1450 °C.     

Inorganic remote glass film based on yellowish-green (Y,Ba)3(Al,Si)5O12:Ce garnet phosphor for warm white LEDs

Compared with other fluorescent crystal phases, garnet has better structural stability in a glass matrix and renders precisely controllable emissions due to the abundant lattice control positions. In this work, we regulate the coordination field of Ce³⁺ ion based on the co-substitution method and achieve the spectra regulation in the yellow–green range. We used Ba²⁺–Si⁴⁺ cations to replace Y³⁺–Al³⁺ cations in Y₃Al₅O₁₂ (YAG) matrix to obtain blue-shift of the emission peak from 552 to 539 nm. The centroid shift and crystal field splitting decrease with the decreasing covalency of the bond between the Ce³⁺ ion and the surrounding anions owing to the higher electronegativity of Si⁴⁺ ions than Al³⁺ ions. The corresponding fluorescent films were prepared by a low-temperature co-sintering process based on the as-made Ba²⁺–Si⁴⁺ co-substituted phosphor. X-ray powder diffractometer and scanning electron microscopy images showed that the fluorescence crystals were less eroded and evenly dispersed in the glass matrix. Spectral analysis showed that the garnet phase was protected by using lead-free borosilicate glass with a low melting point, and the quantum efficiency of phosphor-in-glass (PiG) retains 98% of the corresponding phosphor. By adjusting the ratio of garnet phosphor to commercial red nitride phosphors, a warm white fluorescence with a color rendering index of 80.3 and color temperature of 3899 K was obtained. The prepared warm white film has potential application value in the whole spectra field.     

Effects of Yb and Sc co-doping on the thermal properties and CMAS corrosion of garnet-type (Y3-xYbx)(Al5-xScx)O12 ceramics

Thermal barrier coatings with excellent thermal performance and corrosion resistance are essential for improving the performance of aero-engines. In this paper, (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ (x = 0, 0.1, 0.2, 0.3) thermal barrier coating materials were synthesized by a combination of sol-gel method and ball milling refinement method. The thermal properties of the (Y_3-xYb_x)(Al_5-xSc_x)O₁₂ ceramics were significantly improved by increasing Yb and Sc doping content. Among designed ceramics, (Y_2.8Yb_0.2)(Al_4.8Sc_0.2)O₁₂ (YS-YAG) showed the lowest thermal conductivity (1.58 Wm^?1K^?1, at 800 °C) and the highest thermal expansion coefficient (10.7 × 10^?6 K^?1, at 1000 °C). In addition, calcium-magnesium- aluminum -silicate (CMAS) corrosion resistance of YS-YAG was further investigated. It was observed that YS-YAG ceramic effectively prevented CMAS corrosion due to its chemical inertness to CMAS as well as its unique and complex structure. Due to the excellent thermal properties and CMAS corrosion resistance, YS-YAG is considered to be prospective material for thermal barrier coatings.     

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).     

Directionally solidified Al2O3/(Y0.2Er0.2Yb0.2Ho0.2Lu0.2)3Al5O12 eutectic high-entropy oxide ceramics with well-oriented structure,high hardness,and low thermal conductivity

Directionally solidified Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic high-entropy oxide ceramics (HEOCs) were successfully prepared with an optical floating zone furnace. The Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic HEOCs were pure phases with uniform distribution of rare-earth elements. The preferred growth orientation relationships were <10−10 > {0001}Al₂O₃ // <110 > {211}(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂. The indentation fracture toughness and Vickers hardness were 6.8 ± 0.9 MPa·m^1/2 and 16.1 ± 0.3 GPa, which were higher than that of Al₂O₃/Y₃Al₅O₁₂ eutectic ceramics. The room temperature bending strength was 333 ± 42 MPa. Crack bridging, deflection and bifurcation were the main toughening mechanism. Hardness and reduced modulus mapping results illustrated that the hardness of (Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ was close to that of Al₂O₃. Thermal expansion coefficient of Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic HEOCs was very similar to that of Al₂O₃/Y₃Al₅O₁₂ but thermal conductivity was as low as 4.9 Wm⁻¹ K⁻¹ due to strong lattice distortion. These results suggest that high-entropy Al₂O₃/(Y_0.2Er_0.2Yb_0.2Ho_0.2Lu_0.2)₃Al₅O₁₂ eutectic ceramics are promising candidates for structural components application in gas turbine engines.     

Phase stability and thermal conductivity of RE2O3 (RE=La,Nd, Gd,Yb) and Yb2O3 co-doped Y2O3 stabilized ZrO2 ceramics

Y₂O₃ stabilized ZrO₂ (YSZ) has been considered as the material of choice for thermal barrier coatings (TBCs), but it becomes unstable at high temperatures and its thermal conductivity needs to be further reduced. In this study, 1 mol% RE₂O₃ (RE=La, Nd, Gd, Yb) and 1 mol% Yb₂O₃ co-doped YSZ (1RE1Yb–YSZ) were fabricated to obtain improved phase stability and reduced thermal conductivity. For 1RE1Yb–YSZ ceramics, the phase stability of metastable tetragonal (t′) phase increased with decreasing RE³⁺ size, mainly attributable to the reduced driving force for t′ phase partitioning. The thermal conductivity of 1RE1Yb–YSZ was lower than that of YSZ, with the value decreasing with the increase of the RE³⁺ size mainly due to the increased elastic field in the lattice, but 1La1Yb–YSZ exhibited undesirably high thermal conductivity. By considering the comprehensive properties, 1Gd1Yb–YSZ ceramic could be a good potential material for TBC applications.     

Thermal properties of (Zr0.2Ce0.2Hf0.2Y0.2RE0.2)O1.8 (RE= La,Nd and Sm) high entropy ceramics for thermal barrier materials

The high-entropy ceramic materials (Zr_0.25Ce_0.25Hf_0.25Y_0.25)O_1.875 (H-0) and (Zr_0.2Ce_0.2Hf_0.2Y_0.2RE_0.2)O_1.8 (H-RE) (RE = La, Nd and Sm) with fluorite structure and homogeneous element distribution were prepared. With fluorite structure, fine grain size and high density, the H-0 and H-RE ceramics displayed low thermal conductivity, suitable thermal expansion coefficient, high hardness and fracture toughness. The effect of La, Nd and Sm on the mechanical, heat conductivity and heat expansion properties of high entropy ceramics were discussed. The single-phase high-entropy ceramic materials in this work are very suitable for application as thermal barrier materials.     

Insight into luminescent properties,energy transfer and thermal stability of NaBa4(AlB4O9)2Cl3:Ce3+, Tb3+ phosphors

A series of Ce³⁺ and Tb³⁺ singly- and co-doped NaBa₄(AlB₄O₉)₂Cl₃ (NBAC) phosphors have been synthesized via high-temperature solid state route. The crystal structure, morphology, photoluminescent properties, thermal properties and energy transfer process between Ce³⁺ and Tb³⁺ were systematically investigated. The structure refinements indicated that the phosphors based on NBAC crystallized in P42nm polar space group in monoclinic phase. The emission color could be tuned from blue (0.1595, 0.0955) to green (0.2689, 0.4334) via changing the ratio of Ce³⁺/Tb³⁺. The energy transfer mechanism of Ce³⁺/Tb³⁺ was verified to be dipole–quadrupole interaction via the examination of decay times of Ce³⁺ based on Dexter's theory. The good thermal stability showed the intensities of Ce³⁺ at 150°C were about 66.9% and 64.88% in NBAC:0.09Ce³⁺ and NBAC:0.09Ce³⁺, 0.07Tb³⁺ of that at room temperature, and the emission intensities of Tb³⁺ remained 102.41% in NBAC:0.11Tb³⁺ and 95.22% in NBAC:0.09Ce³⁺, 0.07Tb³⁺ due to the nephelauxetic shielding effect and the highly asymmetric rigid framework structure of NBAC. The maximum external quantum efficiency (EQE) of Ce³⁺ in NBAC:0.09Ce³⁺, yTb³⁺ phosphors could reach 43.38% at y = 0.13. Overall, all the results obtained suggested that NBAC:Ce³⁺, Tb³⁺ could be a promising option for n-UV pumped phosphors.