Characterization of stress in a thermal barrier coating during CMAS corrosion using Ce3+ photoluminescence spectroscopy

The stress caused by calcium–magnesium–alumino–silicate (CMAS) corrosion is a critical factor in thermal barrier failure of thermal barrier coatings (TBCs). For the service safety of TBCs, it is important to characterize the stress inside TBCs during CMAS corrosion using a nondestructive and accurate method. In this study, photoluminescence spectroscopy technology was applied to characterize the stress in TBCs during CMAS corrosion. First, TBC specimens containing yttrium–aluminum–garnet doped with trace Ce³⁺ ions (YAG:Ce³⁺)/yttrium oxide partially stabilized zirconia double-ceramic-layer were prepared by atmospheric plasma spraying. Then, CMAS corrosion experiments were performed using the TBC specimens, and a mechanical model was derived based on Ce³⁺ photoluminescence spectroscopy to investigate the stress in the TBCs. Finally, the microstructure, extent of CMAS corrosion and stress field in TBC specimens, was characterized. The results reveal that the penetration of CMAS leads to local stress concentration and a nonlinear stress distribution from the outside surface to the inside of the YAG:Ce³⁺ layer. In addition, an increase in corrosion time, temperature, and CMAS concentration can significantly influence the evolution of the stress field in TBCs.     

高显色白光LED用YAG：Ce^3＋，Ln^3＋荧光粉的研究进展

综述了YAG：Ce荧光粉的制备方法,着重介绍了高显色白光LED用YAG：Ce^3＋,Ln^3＋（Ln=Sm,Pr,Gd,Tb）荧光粉的最新研究进展和发光性能。YAG：Ce^3＋,Ln^3＋荧光粉的发射谱带发生红移,或在红光区增加尖锐的红光发射峰,从而提高了白光LED的显色性能。     

蓝光转换白光荧光粉YAG:Ce3+的研究进展

本文综述了YAG：Ce^3 发光粉合成的各种方法。对高温固相反应,溶胶—凝胶法,喷雾热解法和共沉淀法进行了比较,对该荧光粉的合成方法及今后的研究趋势进行了展望。     

Optimization of Ce3+ concentration and Y4MgSi3O13 phase in Mg2+-Si4+ Co-doped Ce: YAG ceramic phosphors

As a promising replacement for nitride red phosphors, Ce: Y₃(Mg_1.8Al_1.4Si_1.8)O₁₂ (Ce: YMASG) ceramic phosphors have attracted significant attention recently for their advantages in inorganic encapsulation and massive red-shifting of Ce³⁺ emission. In this work, Ce: YMASG with different doping concentrations of Ce³⁺ and Al₂O₃, was fabricated by vacuum sintering to investigate its effects on the elimination of the impurity phase and the enhancement of the luminescent properties of white light-emitting diodes (w-LEDs). It was discovered that the emission wavelength redshifts from 592 to 606 nm as the Ce³⁺ concentration increases, while at 450 K, the emission intensity deteriorates from 0.47 to 0.36 of its initial value. The Rietveld analysis revealed the presence of an impurity phase of Y₄MgSi₃O₁₃ with a concentration of 17.021 wt% in Ce: YMASG. With the introduction of Al₂O₃, the impurity phase was eliminated from the matrix completely, the emission peak shifted to a shorter wavelength, and the thermal stability was greatly improved. When the correlated color temperature was controlled at around 3000 K in the packaged w-LEDs, the commission international de l'éclairage (CIE) chromaticity coordinates shifted toward the bottom left corner of the diagram with increasing concentration of Ce³⁺. Conversely, the luminous efficiency (LE) increased from 36 lm/W to 58.6 lm/W as the concentration of Al₂O₃ increased from 0 to 10 wt%, which demonstrated the application prospect of the fabricated phosphor in warm w-LEDs.     

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.     

Characterization of the stress distribution and evolution inside a thermal barrier coating after initial thermal cycles

The residual stress introduced inside a thermal barrier coating (TBC) during manufacturing and service processes is one of the main causes of thermal barrier failure. The formation and evolution of internal stress in the TBC begin at the early stage of service, but studies on the mechanism of the distribution and evolution of the stress in the TBC during the initial thermal cycle are still lacking. To explore the evolution mechanism of the stress in the TBC interior, an experimental study on the regulation mechanism of the initial thermal cycle on the TBC internal stress was carried out in this paper. First, the internal stress of TBC specimens after thermal cycles was characterized based on photoluminescence spectroscopy (PL) and terahertz time-domain spectroscopy (THz-TDS) technologies, in which the homogenization of the near-interface stress field was observed during the initial thermal cycle. Then, the evolution of the microstructure and phase structure of the TBC specimens was characterized. Finally, the phenomenological model of the evolution of the TBC internal structure was established, revealing that the initial thermal cycle regulated the microstructure of the top coating (TC) through phase transformation to realize the homogenization of the near-interface stress field.     

Transient thermal stress due to the penetration of calcium-magnesium-alumino-silicate in EB-PVD thermal barrier coating system

Electron beam-physical vapor deposited (EB-PVD) thermal barrier coating system (TBCs) are vulnerable to the degradations induced by the penetration of calcium- magnesium-alumino-silicate (CMAS). In this work, we conduct a numerical study to investigate the effect of CMAS penetration on the development of transient thermal stress in EB-PVD TBCs with the columnar microstructure. A two-dimensional periodical model is developed, taking into account the columnar microstructure of EB-PVD TBCs and the CMAS penetration. We found that the CMAS penetration would induce a field of high in-plane tensile stress in TC upon the rapid cooling, promoting the initiation of the vertical cracks from top surface toward to the bottom of TC. Meanwhile, the accumulation of out-of-plane tensile stress tends to occur at the side edges of EB-PVD columns near three main regions: closely beneath the top surface of TC, at the interface between CMAS penetrated and non-penetrated zone, and close to the TC/BC interface. Therefore, the horizontal cracks are likely to initiate from the side edges of EB-PVD columns at these three regions, which agrees well with experiments.     

Optical performances of mono‐dispersed spherical YAG:Ce3+ nano‐phosphor achieved by one‐pot synthesis

Mono‐dispersed spherical YAG:Ce³⁺ nano‐phosphors were successfully synthesized by a one‐pot glycol‐thermal process using aluminum isopropoxide, yttrium, and cerium acetate hydrates as the precursor, (1,4)‐butanediol as the solvent, ethidenediamine as the additive agent that can both control the morphology and improve the optical performances of the as‐achieved products. The as‐prepared YAG:Ce³⁺ nano‐phosphors displayed mono‐dispersed spheres of about 150 nm and an improved optical performance with a quantum yield (QY) of 41% and good photostability, indicating that they have a considerable potential to be applied in solid‐state lighting or used as coatings in other optical electronic devices.     

YAG：Ce3＋黄色荧光粉的高温固相法制备

采用高温固相法制备了Ce3＋激活的钇铝石榴石荧光粉Y3Al5O12：Ce3＋（YAG：Ce3＋）。采用X射线衍射法对所制备的样品进行结构分析,采用荧光光谱仪对样品进行发光分析。结果表明,在YAG：Ce3＋的高温固相法制备过程中原材料和助熔剂用量以及稀土离子掺杂量、研磨时间、灼烧温度和时间等,都会对所制备样品的成相和发光性能产生影响。     

Anomalous enhancement of photoluminescence intensity of sintered YAG:Ce particles‐embedded glasses

For high‐power white LED applications, YAG:Ce‐based yellow phosphors were embedded in a low‐T_g Bi₂O₃–B₂O₅–ZnO–Sb₂O₅ glass by sintering route. Effects of sintering temperature (325‐390°C) on the microstructure and photoluminescence properties were investigated. X‐ray diffraction was used to measure the retained fraction of YAG:Ce phase after sintering. Scanning electron microscope and transmission electron microscope, equipped with energy‐dispersive X‐ray spectrometry, were used to examine the microstructure, including the element distribution across the phosphor–glass interface. Photoluminescence properties of the samples before and after sintering were compared. With the increasing sintering temperature, the retained fraction of YAG:Ce decreased from 83.3% to 82%. This effect tends to reduce the luminescence intensity of the samples after sintering. The increasing sintering temperature also enhances the diffusion of cations (esp. Bi) from glass matrix to YAG:Ce. This effect tends to increase the luminescence intensity of the YAG:Ce particles after sintering. When the sintering temperature was lower (325°C), the effect of YAG:Ce loss was dominant, thus the luminescence intensity was reduced after sintering. When the sintering temperature was higher (350‐390°C), the effect of solute dissolution was dominant, resulting in luminescence intensity anomalously higher than that before sintering. Similar result has not been reported in literatures. The maximum luminescence intensity of the sintered samples is 1.57 times as high as that of the samples before sintering.     

Effect of process parameters on YAG:Ce phosphor properties obtained by co-precipitation method

In the present paper we have studied the influence of the process parameters and the possibility of adapting a urea based co-precipitation process by using different types of surfactants in order to improve the optical properties of YAG:Ce phosphors. Practically, the process is based on the concomitant precipitation of the metal cations with the formation of a gelatinous precipitate between the cation salts (Y, Al or Ce nitrate) and a precipitating agent (urea) in basic medium and in the presence of surfactants, followed by maturation, filtration, washing, drying steps for the formation of the precursor, completed with the final thermal treatment at 1000 °C. In order to find the optimal synthesis conditions, we compared the effect of the different parameters on the structural properties (using as characterization techniques FTIR spectrometry, TG-DSC and XRD), morphological (SEM) and optical (PL). The analysis of phosphors samples confirmed utility of the use of the PEG or CTAB surfactants and flux agents, the need of an optimum temperature of 1000 °C for the synthesis of (Y_1-xCe_x)₃A₅O₁₂, with x = 0.05.     

Thermal‐cycle dependent residual stress within the crack‐susceptible zone in thermal barrier coating system

Nondestructive and accurate measurement of residual stress in ceramic coatings is challenging, but it is crucial to the assessment of coatings failure and life. In this study, for the first time, the thermal‐cycle dependent residual stress in an atmosphere plasma sprayed thermal barrier coating system has been nondestructively and accurately measured using photoluminescence piezo‐spectroscopy. Each thermal cycle consists of a 5‐minute heating held at 1150°C and a 3‐minute water quenching. The measurement was performed within a crack‐susceptible zone in the yttria‐stabilized‐zirconia (YSZ) top coat (TC) closely above the thermally grown oxide layer. A YSZ:Eu³⁺ sublayer was embedded in TC as a stress sensor. It was found that the initial residual stress was compressive, with a mean value of 240 MPa, which rapidly increased to 395 MPa after 5 thermal cycles (12.5% life) and then increased gradually to the peak of 473 MPa after 25 thermal cycles (62.5% life). After 30 thermal cycles (75% life), the mean stress dropped abruptly to 310 MPa and became highly heterogeneous, with gradual reduction toward final spallation. The heterogeneous stress distribution indicates that many microcracks nucleated at different locations and the spallation occurred due to the coalescence of the microcracks.     

Preparation and characterization of YAG:Ce thin phosphor films by pulsed laser deposition

We report the use of YAG:Ce phosphor as the raw material to make thin and transparent phosphor films with pulsed laser deposition including the effects of heating temperature, target–substrate distances, annealing times, and annealing atmosphere on the YAG:Ce³⁺ phosphor film crystal types and spectral properties. The results indicated that at a coating temperature of 350°C, the YAG:Ce³⁺ phosphor film had the best crystallinity with an intact film and maximum fluorescence emission. The crystallinity and fluorescence emission intensity of the film gradually decreased as a function of increasing target–substrate distances. As the annealing time increased, the crystallinity and the fluorescence emission intensity of the film first increased and then decreased. The film made with 5 h of annealing had the best crystallinity and the highest fluorescence emission intensity. The crystallinity of the film annealed under air was higher than that made under nitrogen; the fluorescence intensity of the film under air was slightly lower than the film under nitrogen. The emission peak of the prepared film was at 523 nm when excited at 450 nm. This is slightly blue‐shifted versus the emission of commercial phosphor powders. This study offers a theoretical basis for the development of transparent phosphor films.     

The effect of SiC addition on photoluminescence of YAG:Ce phosphor for white LED

An effective way of improving photoluminescence (PL) of YAG:Ce by addition of small amount of SiC and sintering in air was described. The breakdown of SiC during sintering process in air was employed to provide the presence of SiO₂ and CO both of which are known to be beneficial in enhancing the PL of YAG:Ce phosphor. SiC in the form of a fine powder was added to YAG:Ce powder and sintered to densities of >99% of theoretical density. The highest luminescence was measured in sample containing 0.08?wt% SiC. The effect of the formed SiO₂ and CO was discussed and their contribution to the emission intensity was assessed. The enhancement of PL intensity is attributed to the formation of vacancies, both on Y sub-lattice and on oxygen sub-lattice and their ability to release the electrons for subsequent reduction of Ce⁴⁺ to Ce³⁺ which plays a role of luminescence activator.     

Simulation of residual stress in thick thermal barrier coating (TTBC) during thermal shock: A response surface-finite element modeling

The current study demonstrates a well-designed response surface methodology (RSM), based on the generated dataset of finite element method (FEM) to establish an integrated model for simulation of residual stress distribution in a thick thermal barrier coating (TTBC). In this study, typical TTBCs were applied on Hastelloy X Nickel-based superalloy using air plasma spray technique followed by thermal cycling. The recorded stress data of Raman spectroscopy was employed to verify the proposed FEM model. A relatively good agreement was obtained between predicted residual stresses and measured ones. Verified FEM model was used to carry out the parametric studies to evaluate the effects of such various parameters as interface amplitude, wavelength, thermally grown oxide thickness and preheating temperature on the stress distribution in the TTBC during the thermal cycling. The computed data were subsequently used for the development of RSM model. In conclusion, experimentally verified numerical data was used to construct a statistical model based on RSM and successfully used to predict the residual stress distribution field in TTBC during thermal cycling. The obtained results of hybrid FEM- RSM model were in acceptable conformity with Raman spectroscopy measurements.     

Low temperature synthesis of YAG:Ce phosphors by LiF assisted sol-gel combustion method

Ce doped yttrium aluminium garnet (YAG:Ce) was synthesized by LiF assisted sol-gel combustion method. The thermal behavior, composition, and microstructure feature of the precursor were studied by TG/DTA, XRD, PL, and SEM. The using of LiF in the sol-gel combustion process was helpful to decrease the sintering temperature about 200 °C. The X-ray diffraction patterns of the powder sintered at 540 °C were identical with the stoichiometric YAG composition, which was the lowest temperature reported for the synthesis of crystalline and single phase YAG. The PL intensity of the particles was found to be affected by the content of LiF and this phenomenon was thought to be related to the change of phase composition with the increase of flux.     

Solid-state reaction method to 6Li,Ce:YAG ceramics for thermal neutron detection

ABSTRACT

The crystal structures and optical properties of 5% ⁶Li:Ce_0.09Y_2.91Al₅O₁₂ transparent ceramics prepared by solid-state reaction with different vacuum sintering temperature were investigated in this paper. The results reveal that with the increasing of sintering temperature, the transmittance of ⁶Li,Ce:YAG ceramics increases from 36% (1680°C) to 82% (1780°C) at 1000?nm, and the intensity of absorption peaks at 340 and 460?nm increases. The emission peak wavelengths of ⁶Li:Ce_0.09Y_2.91Al₅O₁₂ ceramics have been measured as 534.5?nm, and there is no red shift. The high transmittance and emission peak (at 534.5?nm) suggested that this material could be a candidate for neutron detection applications.     

Mechanical properties,oxygen barrier property,and chemical stability of RE3NbO7 for thermal barrier coating

Rare earth niobate (RE₃NbO₇, RE = Dy, Y, Er, Yb) ceramics have shown extremely low thermal conductivity but remain questionable in high temperature thermal barrier coating (TBC) applications with high thermal, mechanical, and chemical loads. Herein, we comprehensively characterize the properties of rare earth niobates, including mechanical properties, oxygen barrier properties, chemical stability, etc. It is found that the oxygen conductivities of the rare earth niobates are three orders of magnitude lower than 7wt.% yttria-stabilized zirconia (YSZ), indicating a remarkable oxygen barrier property to avoid oxidation of underlying metallic components. The corrosion resistance of rare earth niobate against calcium-magnesium-aluminum silicate (CMAS) is also significantly better than that of YSZ. Together with the extremely low thermal conductivity, the rare earth niobates exhibit a combination of excellent high temperature properties, which may become a promising candidate material of high temperature TBC of next generation gas turbines.     

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.     

Investigation of sintering mechanism for novel composite structural thermal barrier coating

A micro-agglomerated particle embedded–thermal barrier coating (TBC) structure was prepared by an improved plasma spray process to withstand the sintering-induced degradation of TBCs during service. In this study, the sintering resistance and thermophysical and mechanical properties of conventional and novel-structured TBCs were systematically characterized. The results suggested that the thermal conductivity and sintering shrinkage of the novel-structured TBCs were approximately 30% lower than those of conventional air plasma spraying TBCs. The elastic modulus of the novel-structured coating is only 32% of that of the conventional structure after thermal exposure at 1300°C for 100 h. The distinct structure of the coating is the main factor that influences its performance. The relationship between the structural evolution and residual strain of the coating was analyzed using electron backscatter diffraction and transmission electron microscopy. Significant differences were observed in the sintering behavior of the dense matrix and embedded particle regions in the coating. Some columnar grains near the intersplat pores in the dense matrix have similar lattice orientations, and they tend to connect and consequently heal the intersplat pores. The large pores between the agglomerated particles and non-oriented submicron-sized grains that constitute these particles are responsible for the sintering resistance of the coating.