YAG Glass Ceramics Research Articles

Study of crystallization kinetics, microstructure and optical properties of Ce: YAG glass-ceramics for white LED applications

CeO2 doped Y2O3-Al2O3-SiO2 (YAS) glass composition was synthesized and devitrified to a single phase Ce: YAG glass–ceramics phosphor with controlled heat treatment process. Kinetic predictions for crystallization of the glass were derived from isoconversional analysis of DTA data using AKTS software. Activation energy of crystallization (E) at reaction progress (α = 0.5) estimated 659 kJ mol−1. The kinetic parameters were calculated as n = m = 2, representing two-dimensional nucleation and crystal growth with constant number of surface nuclei in this glass composition. From microstructure analysis of glass–ceramics we have seen the dendritic growth of crystals in the glass matrix originating from the surface creating a glass–ceramics phosphor layer of ~ 50 μm thick. No nucleation or crystal growth observed in the glass beyond 50 μm depth. Surface roughness found to increase the nucleation and accelerate the crystallization in the glass–ceramics layer. Five glass–ceramics samples prepared with increasing isothermal holding time and their photoluminescence spectra obtained by excitation with a blue LED light source. Intensity of broad band yellow emission increased with the increase in holding time which enhances the amount of Ce: YAG crystalline phase in glass–ceramics. A device was assembled for the demonstration of white light generation using an optimized glass–ceramics phosphor and blue LED in tandem.

Efficient near-infrared down conversion in Nd3+-Yb3+ co-doped transparent nanostructured glass ceramics for photovoltaic application

Down conversion involving emission of two near-infrared photons for one ultraviolet photon absorbed was achieved in Nd3+ and Yb3+ co-doped transparent YAG glass ceramics. Upon excitation of Nd3+ ions with an ultraviolet photon at 355nm, Yb3+ ions emitted two near-infrared photons at 980nm through an efficient two-step energy transfer process from Nd3+ to Yb3+. The mechanisms of the two-step energy transfer were verified by spectra and lifetime measurements, and the optimal quantum efficiency achieved was as high as 185.0%, which was close to the theoretical value of 200%. Therefore, the Yb3+ emission centered at 980nm was efficiently enhanced and responded to the strongest absorption of crystalline silicon solar cells. The development of near-infrared down conversion transparent glass ceramics would allow an increase of the energy efficiency of the silicon solar cells.

Erosion mechanism of YAG:Ce3+ phosphor in bismuth borate glasses

The poor thermal stability of current commercial white light emitting diodes (WLED) fabricated using a blue chip, a yellow YAG:Ce3+ phosphor (YAG) and resins, restricts their application in fields where long service time and high power lighting systems are required. Incorporating YAG into glass offers a possible way to resolve the problem by removing the organic resins in LED encapsulation. In the present study, YAG glass-ceramics with bismuth borate glass matrices were prepared using a rapid melt quenching technique. The YAG erosion mechanism was investigated for the first time in various glass matrix compositions at different co-sintering temperatures. The results demonstrate that neither B2O3 nor Bi2O3 individually erodes YAG particles, but the mixtures of B2O3 and Bi2O3 binary glass systems react with YAG and dissolve it almost completely or generate YAl3(BO3)4 (abbreviated as YAB), and they even erode as-produced YAB again under certain conditions. The production of YAB particles increases with the increase of the nBi/nB ratio at a lower temperature (680°C), while it first increases and then decreases at a higher temperature (800°C). The maximum production of YAB can be obtained at the co-sintering temperature of 680°C with an nBi/nB ratio of 55:45 or at 760°C with an nBi/nB ratio of 35:65. The intermediate Bi4B2O9 phase is found during the transformation of YAG into YAB. A simple but efficient approach is proposed to prevent YAG erosion and increase the luminous properties of YAG glass ceramics to a great extent by simply adding Ca2+ into bismuth borate glass matrices. The degree of YAG erosion decreases with increasing Ca2+ concentration until an impurity phase appears when the Ca2+ concentration exceeds 20mol%. The combination of good white light emission and suitable quantum efficiency as well as higher thermal stability makes the as-prepared YAG glass-ceramics equipped with a blue chip a possible alternative to commercial WLED.

High reliable and chromaticity-tunable flip-chip w-LEDs with Ce:YAG glass-ceramics phosphor for long-lifetime automotive headlights applications

Nowadays, major commercial w-LEDs fabricated by the traditionally gold-wire-welding packaging technology have undergone considerable development as indoor/outdoor lighting sources due to its high-energy utilization efficiency, long service life, environmental friendliness, and excellent chromatic stability. While, new generation applications in projections, automotive lighting, street lighting, plaza lighting, and high-end general lighting need further improvements in power handling and light extraction. Herein, transparent Ce:YAG glass-ceramics (GCs) phosphor was prepared by low-temperature co-sintering polycrystalline Ce:YAG phosphor powder and home-made PbO-B2O3-ZnO-SiO2 glass powder. Thereafter, the flip-chip (FC) w-LEDs were fabricated with the GCs phosphor plates and FC blue chips. The GCs-based FC w-LEDs show not only excellent heat- and humidity-resistance characteristics, but also superior optical performances with an LE of 112.8 lm/W, a CRI of 71.2, a CCT of 6103 K as well as a chromaticity coordinate of (0.3202, 0.3298), under a high operation current of 400 mA. The technology route will open a practically commercial feasible approach to achieve excellent performances for advanced high-power FC w-LEDs.

Crystallization and fluorescence properties of Ce:YAG glass-ceramics with low SiO2 content

Ce3+ doped YAG (Ce:YAG) glass-ceramics with 20mol% SiO2 were fabricated by the method of amorphous sintering followed by controlled crystallization (ASCC). An investigation on the crystallization process of as-sintered glasses revealed that surface crystallization occurred first and YAlO3 (YAP) was precipitated as the main crystalline phase; then bulk crystallization continued inside these glasses and YAP phase transited to YAG. The phase transition from YAP to YAG could be promoted by an inert atmosphere instead of reductive one and by the increase of the crystallization temperature and holding time. By combining the Ce:YAG glass-ceramics with a blue-emitting InGaN chip, white LEDs with a relatively high color rendering index of 80.3 and low color temperature of 5098K were produced. Their luminous efficacy decreased with the increasing annealing temperatures, which was probably caused by the degradation of transparency.

Samarium Ions Doped Yttrium Aluminium Garnet Glass Ceramics

Sm3+doped YAG glass ceramics containing single phase of YAG micro-crystals have been obtained by heat-treating the aluminosilicate precursor glasses. The YAG crystals grow with several preferred orientations and a dominant one (444) in the matrix, and present microcrystal sizes in the range of 3~15µm. The pilotaxitic texture existed in internal network of the YAG glass ceramics can increase yield strength and tensile strength of the sample effectively. Typical visible transition emissions of Sm3+have been observed in the YAG glass ceramics under the excitation of short-wavelength visible light, and the spectral intensity ratio between the electric dipole4G5/2→6H9/2and the magnetic dipole4G5/2→6H5/2transitions is lower than the values reported in the optical glasses specifying the enhanced symmetric nature in the Sm3+doped YAG glass ceramics. The obvious Stark splitting exhibited in emission spectrum manifests that rare-earth ions have been incorporated into YAG lattices.

Microcrystal Growth in Heat-Treating Process for Europium Ions Doped Aluminosilicate Glasses

Eu doped YAG glass ceramics containing single phase of YAG micro-cryatals have been obtained by heat-treating the aluminosilicate precursor glasses. The primary crystallite size of YAG phase is calculated to be ~46 nm and the crystal grains grow with preffered orientation (444) in the internal network. The average secondary particle size is estimated to be ~8μm and the larger grain size is due to the particle aggregation. The pilotaxitic texture of YAG glass ceramics presented in SEM micrograph, can increase yield strength and tensile strength of the sample effectively. The emission spectrum of Eu doped YAG glass ceramics consists of a broad blue emission band and several narrow emission bands, indicating the coexistence of Eu3+ and Eu2+ ions in YAG glass ceramics.

Upconversion emissions in YAG glass ceramics doped with Tm3+/Yb3+ ions

Tm3+/Yb3+ doped glass ceramics, containing single phase of YAG micro-crystals with several preferred orientations and a dominant one (444) in the matrix, which present the primary particle size of ∼46nm and the secondary particle size of ∼8μm, have been prepared by heat-treating the Li2O–Y2O3–Al2O3–SiO2 (LYAS) precursor glasses. Judd-Ofelt parameters Ωt (t=2, 4, 6) are derived to be 5.14×10−20, 1.11×10−20, and 1.19×10−20cm2, respectively, indicating high inversion asymmetry and strong covalent environment in the precursor glasses. Efficient near-infrared (NIR), weak red, intense blue and rare UV multi-photon upconversion emissions of Tm3+ are captured in Tm3+/Yb3+ doped LYAS glasses and YAG glass ceramics. Obvious Stark splitting in the glass ceramics manifests that rare earth ions have been incorporated into YAG lattices. The comparison of spectral behaviors of the glass and glass ceramic samples will be helpful for further exploring the novel phosphors applied to illumination and display.

Crystallization of Y2O3–Al2O3 Rich Glasses: Synthesis of YAG Glass-Ceramics

An yttria-alumina glass incorporating 4 wt % of SiO2 was prepared by the use of an aerodynamic levitator equipped with a laser heating device. Upon post synthesis annealing, the glass is observed to exhibit crystallization below the glass transition temperature. In the current study, this unusual behavior is examined using powder X-ray diffraction and infrared spectroscopy to follow crystallinity and optical absorbance as a function of annealing treatment. Double-resonance 27Al/29Si NMR spectroscopy and synchrotron diffraction experiments demonstrate that Si cations are incorporated in the formed Y3Al5O12 (YAG) crystals on the 4-fold coordination Al site during crystallization. This has important consequences on the resulting microstructure of the YAG glass-ceramics.

PREPARATION AND SPECTROSCOPIC PROPERTIES OF Yb:YAG GLASS-CERAMICS

Yb 3+-doped 32 CaO -12 Y 2 O 3-24 Al 2 O 3-31 SiO 2-1 Yb 2 O 3 glasses were prepared by melting method, and then annealed at 950°C, 1050°C and 1100°C, respectively. We analyzed the crystallographic structures of the glasses annealed at different temperatures by XRD, and observed the glass annealed at 1050°C by TEM. The spectroscopic properties of glass before and after annealing were also investigated. Results show that the glasses annealed at 1050°C are YAG glass-ceramics with no impurity crystalline phase. Variances in the spectroscopic properties before and after heat treatment indicate that Yb 3+ ions were preferentially partitioned into the YAG crystal phase, and the transparent Yb : YAG glass-ceramics were obtained.

Fluorescence of Er, Yb: YAG glass-ceramics

The Er3+, Yb3+-doped CaO-Y2O3-Al2O3-SiO2 glasses were prepared by melting, and then annealed. The emission spectra and upconversion spectra of glass-ceramics have been investigated. The upconversion luminescence mechanisms of Er3+ in YAG glass-ceramics were analyzed. The results show that the Er,Yb:YAG glass-ceramics are obtained after annealing of the glasses. A broad emission spectrum of width 200nm in the wavelength range of 1450—1650nm was obtained in Er,Yb:YAG glass-ceramics at room temperature. The full width at half-maximum (FWHM) of the 4I13/2→4I15/2 transition is ～180nm. This may be as a result of the luminescence of the Er3+ ions in YAG crystal phase and the remnant Er3+ ions in glass phase acting together. The upconversion luminescence intensity increases because the phonon energy of YAG crystallite is lower than that of aluminosilicate glass. The infrared to green or to red upconversion luminescence intensities for the glass-ceramics are evaluated to be nearly 7 or 3 times as high as that of aluminosilicate glasses, respectively. The relative intensities of green and red upconversion emission changed because the microscopic environment around Er3+ and Yb3+ ions changed upon heat treatment.

Phase-selective cathodoluminescence spectroscopy of Er:YAG glass-ceramics

Phase-selective excitation by a field-emission electron gun was performed for an Er 3+-doped Y 3Al 5O 12 (YAG) glass-ceramics, and the emission spectra due to the 4S 3/2→ 4I 15/2 transition of Er 3+ were measured. Powder X-ray diffraction analysis revealed that a single phase of YAG was precipitated in the glass-ceramics. Field-emission scanning electron microscope (FE-SEM) images indicated that two phases existed in the glass-ceramics. There was a large difference in the intensity of cathodoluminescence (CL) between these two phases. All of the spectral peaks of the phase with higher CL intensity were attributed to the 4S 3/2→ 4I 15/2 transition of Er 3+ in YAG crystal. The phase with lower CL intensity is considered to be composed of the glass phase including a small amount of YAG crystals. The CL from Er 3+ in the glass phase could not be detected, implying that almost all of Er 3+ ions are incorporated into YAG phase.