YAG Matrix Research Articles

Comparison of YAG:Nd3+-Yb3+ nanothermometers synthesized by Pechini and solvothermal methods

Luminescence intensity ratio-based nanothermometry is a widely studied thermal sensing technique in the literature. Regarding biological purposes, it is essential to have thermal probes that are efficient in this type of environment. Thermal bioprobes demand highly crystallized nanocrystals, commonly smaller than 100 nm, with luminescence emissions in the near-infrared range that are not significantly absorbed by biological tissues. Several nanomaterials that have been studied for nanothermometry do not meet the requirements for this type of applications. Accordingly, researches are needed to develop suitable and reliable nanothermometers for thermal sensing. Therefore, our goal was to investigate the impact of Nd3+-Yb3+ co-doping on the thermometric performance of YAG matrix, a promising crystal because it presents a host structure favoring the insertion of lanthanide ions, which provide its luminescent features. In order to achieve this purpose, we first synthesized YAG:Nd3+-Yb3+ nanocrystals through a generic route - called modified Pechini method - to screen their thermal properties. Our results show that YAG:Nd3+-Yb3+ nanocrystals have the potential to work in vivo environments. The nanothermometers investigated here are excited in the first biological window (BW-I) at 805 nm with luminescence emissions within the BW-II, at 1030.5 and 1064 nm. By co-doping the YAG matrix with different Nd3+-Yb3+ concentrations, we studied the energy transfer process between the dopant ions and their impact on thermometry efficiency. By the efficient coupling of the Nd3+-Yb3+ pair, we improved the Sr value by a factor of 3 of YAG compounds up to 0.6 %.K−1. We then synthesized YAG:Nd3+-Yb3+ nanocrystals using a second type of synthesis, by solvothermal means, in order to obtain individual nanocrystals, well dispersed in aqueous solutions, and to adapt their morphology and size for biological purposes. Therefore, we compared the structural and luminescence properties and thermometry efficiencies of YAG:Nd3+-Yb3+ nanocrystals obtained through two distinct processes and showed that the nanothermometry properties are not affected by the synthesis method.

Comparative VUV Synchrotron Excitation Study of YAG: Eu and YAG: Cr Ceramics

Using synchrotron radiation, a comparative VUV excitation study of YAG ceramics doped with Eu3+ and Cr3+ ions under VUV excitation (10.5–3.7 eV) at 9 K was conducted in this work. Both ceramics exhibit distinct excitation peaks in the VUV region, indicating high-energy transitions related to the internal electronic levels of the dopants and interband transitions within the YAG matrix. For YAG:Eu, the main excitation peaks at 6–7 eV correspond to transitions within the 4f-shell of Eu3+ and Eu3+-O2− charge transfer states, showing weak dependence on the crystal field and high energy conversion efficiency. In contrast, YAG:Cr shows broad excitation bands due to transitions between levels influenced by strong crystal field interactions, resulting in lower luminescence efficiency. The study highlights the importance of crystal structure and dopant interactions in determining the spectral characteristics of YAG-based ceramics, offering potential for their application in advanced optoelectronic devices.

Chemical vapor deposition of ordered structures in YAG–alumina eutectic system

AbstractThe eutectic structure of metals and ceramics is the result of a self‐organization phenomenon in which multiple solid phases solidify with an ordered structure from a liquid phase. Hence, a melt‐solidification process was the only way to generate ordered structures in a ceramic eutectic system. Here, we prepared ordered structures of Y3Al5O12 (YAG)–Al2O3 composites via chemical vapor deposition in the YAG–Al2O3 eutectic system. Spatially periodic YAG rod and lamellar structures were generated in an alumina matrix homoepitaxially grown on the sapphire substrates on the Al2O3‐rich side of the eutectic composition, whereas α‐Al2O3 rod and lamellar structures were generated in a YAG matrix on the Y2O3‐rich side. The results reveal that the pattern formation of ordered structures in a ceramic eutectic system can occur not only during in the melt‐solidification process but also during the vapor deposition process. The Ce3+‐doped YAG–Al2O3 composite film converts some of the blue light into yellow light, allowing some of it to pass through, and emitting white light. The Eu3+‐doped YAG–Al2O3 composite film can be utilized as a scintillation screen for a high‐resolution X‐ray imaging test to see though a semiconductor storage device.

YAG:Yb crystal with non-linear doping ions distribution as promising active element for high average power laser systems

The influence of optical centers concentration profiles on thermal fields under high-power end-face diode pumping is analyzed using numerical model. It is shown that non-uniform distribution profiles of optical centers concentration are very promising for high average pump power laser amplifiers. The possibility of creating concentration profiles of optical centers in gradient-activated crystals with YAG matrix with a specified distribution along the growth axis of the crystal is demonstrated. A technology of growing gradient-activated single crystal involving active mirror as active element is proposed and discussed.

Rapid synthesis of Ce3+:YAG via CO2 laser irradiation combustion method: Influence of Ce doping and thickness of phosphor ceramic on the performance of a white LED device

A new, rapid method was reported to produce Y3Al5O12 phosphors doped with different concentrations of Ce3+ (YAG:Ce3+) within 1.5 ​min via continuous-wave CO2 laser irradiation, in which a CO2 laser is utilised as the combustion source. X-ray powder diffraction confirmed the formation of pure cubic phase in the as-synthesised powder. Thus, laser irradiation plays an important role in optimising reaction kinetics to obtain high-quality crystal with high luminescence intensity. Furthermore, the incorporation of Ce3+ ions into the YAG matrix can be enhanced through sintering powders at 1050 ​°C for 5 ​h. Optical properties were examined comparatively using photoluminescence emission. Additionally, the performance of white light-emitting diode (LED) can be regulated easily by altering the Ce3+ doping concentration and YAG:Ce3+ layer thickness on blue InGaN LED chip. Excellent white light, which has a CIE coordinate of (0.31, 0.36) that approached the standard WLED values, a colour rendering index of about 69.4, a correlated colour temperature of about 6299 ​K and a luminous radiation efficiency exceeding 169 lm/W, was obtained for YAG doped with 2 ​mol% Ce concentration with a coating layer thickness of 1 ​mm.

Enhancement of light extraction in Y3Al5O12:Tb3+ thin films through nanopatterning

Light extraction is a key parameter to improve the performances of optical devices. Nanopatterned Y3Al5O12:Tb3+ luminescent coatings usable in such devices have been elaborated and have shown enhanced emission efficiency compared to their unpatterned counterparts. These nanostructured films were obtained by using the colloidal lithography combined with the Langmuir-Blodgett technique. It is the first time to our knowledge that this patterning technique is directly applied on YAG matrix. Resulting nanostructuring is a hexagonal network capable of modifying the light travelling path within the substrate. Conventional and angular-resolved photoluminescence were investigated on both unpatterned and patterned samples. Due to the nanostructuring, the extraction efficiency is improved by 26% and 131% depending on the crystallinity of the sample. Noticeably, nanostructuring is found to have an influence on the angular distribution of photoluminescence whose intensity has been evaluated to its maximum normal to the film surface.

Transport properties of p-type Ca3-xLnxCo4O9-Ag (Ln = Lu, Yb; 0.1≤x≤0.2) oxides

Thermoelectric transport properties of p-type Ca3-xLnxCo4O9/yAg oxides (Ln = Lu & Yb; 0.1 ≤ x ≤ 0.2; 0.05 ≤ y ≤ 0.1) synthesized by sol-gel methodology were investigated in this paper. The structural analyses (SEM, XRD and TEM) confirmed the presence of two phases, viz, Ca3-xLnxCo4O9 and Ag-metallic phases. The contribution of rare earth doping in one hand and presence of Ag as secondary phase on the other hand were studied. The resistivity measurements indicated the reduction of electrical resistance at the grain boundary leading to an overall decrease in electrical resistivity with increasing Ag-concentration. The enhancement of Seebeck coefficient is attributed to the substitution of Ln3+ at Ca2+ sites that in turn reduces hole concentration through formation Co3+ for charge concentration counter balance in Ca3-xLnxCo4O9/yAg matrix. The tuning of electrical transport properties through Ca3-xLnxCo4O9 and Ag-metallic bi-phasic formation resulted high power factor of 582 μW m−1 K−2 for Ca2.8Ln0.2Co4O9/0.05Ag and 548 μW m−1 K−2 for Ca2.8Yb0.2Co4O9/0.05Ag at 950 K highlighting its potential application on small scale energy harvesting to power sensor and wireless sensor network where requirement of power is in the milliwatt range.

Investigation of sintering temperature and Ce3+ concentration in YAG:Ce phosphor powder prepared by microwave combustion for white-light-emitting diode luminance applications

Abstract Cerium-doped transparent Y3Al5O12 (YAG:Ce3+) is the most widely used phosphor for converting blue to white light in light-emitting diodes (LEDs). Here, YAG:Ce3+ powders were synthesised through microwave-assisted combustion by using organic fuel urea. In YAG system applications, the powder structure and properties play key roles in the performance of terms of white LEDs (WLEDs). Accordingly, this study aimed to determine the effects of thermal treatment and Ce3+ concentration on the structure, morphology, chemical composition and luminescence properties of YAG powders. A direct conversion from the monoclinic and hexagonal phases to the cubic one was achieved at >850 °C. This conversion was accompanied by increased particle size in the range of 50–100 nm and decreased lattice strain. However, powder sintering at high temperature was required to incorporate Ce3+ into the YAG matrix and eliminate residual impurity phases. The highest electroluminescence (EL) emission was achieved for the sintered powders at 1050 °C with low dopant concentration (2.0 mol%), offering daylight WLED with a correlated colour temperature of 6834 K and corresponding colour-rendering index of 68.2. Finally, a slightly decreased intensity and red-shift in the EL peak position were observed. These findings were attributed to the presence of a separated CeO2 cubic phase (Ce4+) and concentration quenching for YAG ceramics with >2.0 mol% cerium content.

Microstructure and mechanical properties of carbon fiber needled felt reinforced sol-derived YAG composite

The study concerning the second phase reinforced YAG matrix composites is few although YAG is well known as a desirable thermo structural ceramic. In this paper, low-cost 3D carbon fiber needled felt was used as reinforcement to improve the fracture toughness of YAG, and the carbon fiber needled felt reinforced YAG (C/YAG) composite was fabricated through vacuum impregnation-drying-heat treatment, using the Y2O3-Al2O3 sol with a high solid content as raw material. The microstructure, mechanical properties, thermal stability and oxidation resistance of C/YAG composite were carefully investigated. Due to the characteristics of reinforcement and the microstructure, the as-fabricated C/YAG composite showed non-catastrophic failure behavior and much higher fracture toughness compared with monolithic YAG ceramic. The flexural strength of C/YAG composite was well retained after annealed at 1700 °C and 1800 °C under inert atmosphere. Although the chemical inertness between carbon fiber and YAG was confirmed up to 1800 °C, the physical bonding of interface was enhanced during annealing, which led to the decrease of fracture work, especially at 1800 °C. The oxidation resistance of C/YAG composite was related to the cracks and pores since YAG is immune to oxidation. The changes of microstructure and flexural strength after oxidation at 1200 °C, 1400 °C and 1600 °C were characterized and analyzed.

New life of recycled rare earth-oxides powders for lighting applications.

In this work we analysed the optical and structural properties of Ce:YAG regenerated phosphors. The concentrate resulted as the final product of an industrial recycling process of waste electrical and electronic equipment (WEEE), and in particular fluorescent powders coming from spent lamps treatment plant. The waste pristine materials were re-utilized without any further purification and or separation process as starting materials to obtain a YAG matrix (Y2Al5O12) doped with Cerium ions. We tested out the recovered concentrate against commercial Ce:YAG phosphors comparing their structural and optical properties by means of XRD measurements and steady time and time resolved luminescence. The analysis reveals that the new phosphors obtained by scrap powder have the same crystal structure as the commercial reference sample and comparable optical properties. In particular, the Ce-related emission efficiency has a quantum yield of about 0.75 when excited at 450 nm, in good agreement with our reference sample and with the one of commercial powder presently exploited in white LED. This achievement strongly suggests the possibility of a new life for the exhausted phosphors and a possible step forward to a complete circular process for lighting equipment.

Sol–gel-derived Yb:YAG polycrystalline ceramics for laser applications

Yb-doped YAG single crystal is commonly used as the active material in high power lasers. Sol–gel processing is a possible alternative to prepare a similar active polycrystalline ceramic at lower cost. This work describes the preparation of YAG matrix ceramics by sol–gel with a continuous replacement of Y3+ by Yb3+ in the YAG structure. X-ray diffraction showed that the cubic YAG phase was obtained for all Yb doping levels. Structural characterization by infrared and Raman spectroscopies followed the replacement of Y by Yb. Finally, Yb3+ photoluminescence spectra were recorded, together with lifetime measurements, which yielded values in the range of 0.5–0.9 ms. Using Er co-doping as an internal standard, the absolute intensities of the Yb3+ photoluminescence peaks were compared and the optimum Yb concentration for maximum photoluminescence efficiency was determined at ~50 mol% Yb. Based on the photoluminescence efficiency and the lifetime values, a figure of merit for laser operation was maximum for the 40 mol% Yb concentration.

Effect of cerium doping on optical and scintillation properties of transparent YAG ceramic

Abstract Cerium doped transparent YAG ceramics 0.15–2 at% cerium) were prepared using nano-powder technique and vacuum sintering. The effective solubility limit of cerium in YAG was found to lie between 1.0 and 1.5 at%. The PL intensity increases with the cerium content and attains a maximum for 1.0 at% doping. Lifetime of PL is not very sensitive to cerium content, however, a slight decrease in the life-time from 66 ns to 55 ns was observed with increase in cerium content from 0.15 to 2.0 at%. This decrease in PL intensity and life-time is attributed to concentration quenching for the YAG ceramics with cerium content higher than 1.0 at%. A red-shift in the PL peak position was observed and attributed to the local symmetry distortion in YAG matrix.

Composite phase ceramic phosphor of Al₂O₃-Ce:YAG for high efficiency light emitting.

We present our achievement which is a ceramic plate phosphorable to produce white light when directly combined with commercially available blue light emitting diodes. The ceramic phase structure is that the Al₂O₃ particle is uniformly distributed in the Ce:YAG matrix. The Al₂O₃-Ce:YAG ceramic phosphor has a better luminous efficacy than the transparent Ce:YAG ceramic phosphor under the same test condition. The Al₂O₃ particle plays an important role in promoting the luminous efficacy. The Al₂O₃ particle changes the propagation of the light in ceramic, and it reduces the total internal reflection. That is why the composite phase ceramic phosphor improves extraction efficiency of light.

Analysis of the upconversion processes of Nd3+ ions in transparent YAG ceramics

Optical properties of Nd3+ ions in YAG transparent ceramics have been analysed. The samples were prepared by a high temperature (1650 and 1680°C) sintering followed by hot isostatic pressing (HIP) at 1650 or 1700°C in order to obtain a well-homogeneous diffusion of the ions and functional transparency. The emission and luminescence decay curves from the 4F3/2 metastable laser level of these samples are very similar to the ones obtained on crystal samples grown with the Czochralski method. However, excitation upconversion spectra obtained detecting at 590nm for the transparent ceramic samples present important differences compared to the bulk YAG crystal, which can be explained by a change in the upconversion mechanism involved and the different arrangements of the Nd3+ ions in the YAG matrix. The analysis of the upconversion excitation spectra is shown to be a good method to detect the differences between the positions occupied by the Nd3+ ions in different matrices.

Energy transfer processes in YAG:Er3+-Ce3+ crystal

In the work results of research of nonradiative energy transfer processes taking place in the impurity subsystem of the YAG: Er3+-Ce3+ crystal under optical excitations of Er3+ and Ce3+ ions are presented at room temperatures. By means of time-resolved fluorescence spectroscopy, some kinetic characteristics of fluorescence responses of the crystal under optical excitations at 405 and 808 nm wavelengths are investigated. Comparison of the measured characteristics of fluorescence signals at 860, 980 and 1555 nm wavelengths with the theoretical values numerically calculated from appropriate sets of rate equations allowed to estimate the positive contributions of energy transfer processes into population dynamics of energy levels responsible for generation near 1.5 μn of Er3+ active ions in the YAG matrix.

Microstructural Evolution of Aluminum Nitride - Yttrium Aluminum Garnet Composite Coatings by Plasma Spraying from Different Feedstock Powders

A high thermal conductive AlN composite coating is attractive in thermal management applications. In this study, AlN-YAG composite coatings were manufactured by atmospheric plasma spraying from two different powders: spray-dried and plasma-treated. The mixture of both AlN and YAG was first mechanically alloyed and then spray-dried to obtain an agglomerated powder. The spray-dried powder was primarily spherical in shape and composed of an agglomerate of primary particles. The decomposition of AlN was pronounced at elevated temperatures due to the porous nature of the spray-dried powder, and was completely eliminated in nitrogen environment. A highly spherical, dense AlN-YAG composite powder was synthesized by plasma alloying and spheroidization (PAS) in an inert gas environment. The AlN-YAG coatings consisted of irregular-shaped, crystalline AlN particles embedded in amorphous YAG phase, indicating solid deposition of AlN and liquid deposition of YAG. The PAS-processed powder produced a lower-porosity and higher-hardness AlN-YAG coating due to a greater degree of melting in the plasma jet, compared to that of the spray-dried powder. The amorphization of the YAG matrix was evidence of melting degree of feedstock powder in flight because a fully molten YAG droplet formed an amorphous phase during splat quenching.

Nanopowders and Crystals in (Y1‐xNdx)3Al5O12 System: Preparation and Properties.

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Nanopowders and crystals in (Y 1− xNd x) 3Al 5O 12 system: Preparation and properties

Total substitution of yttrium by neodymium assuming garnet phase of the YAG:Nd was investigated. Components were synthesized by sol–gel method and the limit solubility of Nd in the single phase of YAG was equal to 27.5 at.%. At higher concentration of Nd two phases were found, garnet and perovskite. Samples of Nd–Al–O stoichiometric garnet exhibit only the perovskite phase. Crystals were grown by micro-pulling method and at concentration of Nd larger than 1 at.% Nd, the crystals show two phases: garnet and solid solution of perovskite. Luminescence decay curves of nanopowders depend on thermal treatment. The increase in Nd 3+ concentration in YAG matrix over 10 at.% is not recommended for optical applications. It was found that the lifetime value for the powder YAG sample containing 1% of Nd 3+ is larger than the reported lifetimes for sample of the YAG single crystal.

Combinatorial synthesis and sputter parameter optimization of chromium-doped yttrium aluminum garnet photoluminescent thin films

Chromium-doped Y3Al5O12 or YAG thin films emitting at 688 nm due to the 4A2–4E2 transition were synthesized in a combinatorial fashion using reactive sputtering approach. Multilayer YAG and Cr thin films were sputter deposited with a gradient in the Cr thickness and subsequently annealed to diffuse the Cr dopant into the YAG matrix. The combinatorial thin film sputtering technique was used to rapidly determine that ∼0.69 at.% is the optimum chromium concentration for maximum photoluminescence (PL) intensity in YAG. This result was consistent with previous work in YAG:Cr powders. Design of experiments was conducted to determine the effects of various sputter parameters, namely, substrate bias, substrate temperature and oxygen flow rate on PL intensity and crystallinity of co-sputter deposited Cr-doped YAG thin films. The optimum sputtering condition consisted of high substrate bias and low oxygen flow rate and was independent of substrate temperature. The PL temperature dependent behavior of YAG:Cr film was also investigated. Thermal quenching was observed at ∼110 K where the total integrated PL emission intensity was found to rapidly decrease. A non-radiative activation energy of 25.2 meV was determined and is attributed to electron–phonon coupling.

Structural and optical properties of YAG:Ce 3+ phosphors by sol–gel combustion method

High-quality Ce 3+-doped Y 3Al 5O 12 (YAG:Ce 3+) phosphors were synthesized by a facile sol–gel combustion method. In this sol–gel combustion process, citric acid acts as a fuel for combustion, traps the constituent cations and reduces the diffusion length of the precursors. The XRD and FT-IR results show that YAG phase can form through sintering at 900 °C for 2 h. This temperature is much lower than that required to synthesize YAG phase via the solid-state reaction method. There were no intermediate phases such as YAlO 3 (YAP) and Y 4Al 2O 9 (YAM) observed in the sintering process. The average grain size of the phosphors sintered at 900–1100 °C is about 40 nm. With the increasing of sintering temperature, the emission intensity increases due to the improved crystalline and homogeneous distribution of Ce 3+ ions. A blue shift has been observed in the Ce 3+ emission spectrum of YAG:Ce 3+ phosphors with increasing sintering temperatures from 900 to 1200 °C. It can be explained that the decrease of lattice constant affects the crystal field around Ce 3+ ions. The emission intensity of 0.06Ce-doped YAG phosphors is much higher than that of the 0.04Ce and 0.02Ce ones. The red-shift at higher Ce 3+ concentrations may be Ce–Ce interactions or variations in the unit cell parameters between YAG:Ce 3+ and YAG. It can be concluded that the sol–gel combustion synthesis method provides a good distribution of Ce 3+ activators at the molecular level in YAG matrix.