Yttrium Aluminum Garnet Transparent Ceramics Research Articles

Fabrication of Yb:YAG Transparent Ceramic by Vacuum Sintering Using Monodispersed Spherical Y2O3 and Al2O3 Powders

In the present work, Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated using monodispersed spherical Y2O3, Al2O3 powders and commercial Yb2O3 nano-powder as raw materials, adding 0.5 wt% tetraethyl orthosilicae (TEOS) through the solid-state method and vacuum sintering technology. The prepared monodispersed Y2O3 and Al2O3 powders adopted by homogeneous co-precipitation showed improved mixing uniformity and lead to the reduced defect of the YAG powders. After sintering in vacuum at 1700 °C for 10 h, the (Y1−xYbx)AG (x = 0, 0.01, 0.10) ceramics with high transparency were obtained. Analysis of the densification rate, micromorphology, and optical properties of the ceramics suggests that the performance of the Yb:YAG ceramics is independent of the doping amount of Yb. Moreover, when the Y2O3, Al2O3, and Yb2O3 mixtures were laid aside for some time in the air after milling and drying, the performances of the as-prepared Yb:YAG ceramics would be affected distinctively. It is likely because the Y2O3 is easily hydrolyzed to Y(OH)3, Y(OH)2+ and Y2(OH)24+, which impinged the sintering activity of the powder mixture.

Effect of heat treatment of green bodies on the sintering and optical properties of large-size and thick transparent YAG ceramics

Large-size and thick yttrium aluminum garnet (YAG) transparent ceramics with complex shapes were successfully fabricated by solid-state reaction and vacuum sintering using commercial high-purity oxide powders as starting materials. Before sintering in vacuum, green bodies were calcined at different temperatures in a muffle furnace. The effects of heat treatment temperature on the particle size of green bodies as well as the compressive strength, densification, phase transformation and optical transparency of YAG ceramics were examined. After calcination at 1000 °C, the compressive strength of the green bodies reached 81.7 MPa. Using this calcinating procedure, large-size and thick YAG transparent ceramic dome (120 mm diameter, 15 mm thickness) was successfully obtained by sintering at 1750 °C for 30 h, without any apparent defects including microcracks. The in-line optical transmittance of the as-prepared ceramic was 83.1% and 76.8% at wavelengths of 1000 nm and 400 nm, respectively. Such transparent ceramic domes can be used as missile domes or transparent protective covers for submersibles, and detection equipment at full ocean depth (maximum depth of 11,000 m).

A novel gelcasting induction method for YAG transparent ceramic

Yttrium aluminum garnet (YAG) transparent ceramics are an attractive material for solid-state laser, optical and high temperature structural applications because of their prominent advantages. However, it is difficult to achieve the high-quality ceramics by acrylamide (AM) aqueous gelcasting due to the inhomogeneous of gel body induced by traditional methods of “catalyst + initiator” or “initiator + heating”. In this work, a series of gel induction contrast experiments were designed to explore a novel gelcasting induction method. With different gelation process, the SEM of surfaces and fracture surfaces and as-fabricated ceramics quality, it was found the effectiveness of the novel gel induction method “vacuum defoaming + initiator”. The method had the ability to improve the uniformity of the gel body by regulating the form of energy injection and having enough time for the stirring of initiator. In addition, the decreasing residual organics would result in contributing to the subsequent debinding and ceramic quality improvement. Therefore, the “vacuum defoaming + initiator” was regarded as an effective induction method to eliminate defect and to achieve the in-line transmittance of as-fabricated YAG ceramic with excellent optical quality at 1064 nm reached 82.8%. This work greatly promotes the application of gelcasting in YAG transparent ceramics.

Manufacturing of Large Size and Highly Transparent Nd:YAG Ceramics by Pressure Slip-Casting and Post-Sintering by HIP: An Experimental and Simulation Study.

This study reports the fabrication of Nd:YAG (i.e., Neodymium-doped Yttrium Aluminum Garnet: Y3-xNdxAl5O12) transparent ceramics of a large size by the pressure slip-casting forming technique. Colloidal suspensions of primary oxides (i.e., Y2O3, Al2O3, Nd2O3, and SiO2 used as sintering aid) were cast under pressure through a porous membrane. Cakes with a good microstructural homogeneity and mean pore diameter of 90 nm were obtained. Modeling of the pressure slip-casting process at the millimetric to centimetric scale based on a computational fluid dynamics simulation showed good agreement with experimental results in terms of the casting kinetics (i.e., cake thickness and fluid flow as a function of time) and cake permeability. As a result, it was possible to better manage pressure casting parameters in order to obtain large size and homogeneous green parts. Finally, transparent Nd:YAG ceramics sintered by vacuum sintering, followed by post-sintering treatment by Hot Isostatic Pressing (HIP), demonstrated laser slope efficiency (51.7%) and optical-to-optical efficiency (44%) with 130 mJ of output laser energy at 1064 nm equivalent to commercial single crystals.

Photoluminescence of Doped YAG Transparent Ceramics Fabricated by Spark Plasma Sintering

AbstractDoped transparent ceramics have potential for a wide range of optical applications derived from their easily controllable fluorescence properties. The aim of this contribution is to present and discuss an effective fabrication method for doped transparent yttrium aluminum garnet ceramics. The powder synthesis route was adapted from common co‐precipitation methods and the powder densification was performed by spark plasma sintering. Parameters of the fabrication process were optimized for different lanthanide dopants in order to obtain highly transparent single‐ and multi‐doped ceramics. The photoluminescence properties of the samples were measured and discussed. The use of lithium fluoride as a sintering additive was confirmed to be favorable and post‐sintering heat treatments were shown to be necessary for certain dopants. Finally, the photoluminescence lifetime of dopants in a multi‐doped sample were measured and the energy transfer efficiency between dopants was determined.

The effects of ball milling time on the rheological, optical, and microstructural properties of YAG transparent ceramics

AbstractStable slurries dispersed mixture of commercial Al2O3 and Y2O3 powders, ammonium poly meta acrylate (Dolapix CE64) as the dispersant, and tetraethyl orthosilicate (TEOS) as the sintering aid were prepared by ball milling process. The effects of the milling time on the fabrication of transparent polycrystalline yttrium aluminum garnet (YAG) ceramics were investigated by slip casting and vacuum sintering. The results showed that the best milling time for the deagglomeration of the powder mixture was 16 hours and the slurry prepared during this time showed a near‐Newtonian behavior due to the better deagglomeration and lower viscosity. X‐ray patterns also showed that all samples were pure YAG phase. The results also revealed that the sample prepared by 16 hours ball milling time suspensions exhibited higher relative green and final density, as well as the maximum transmittance at 1064 nm (≈ 77%). These samples had a more uniform microstructure too.

Influence of Yb and Si on the fabrication of Yb:YAG transparent ceramics using spherical Y2O3 powders

Abstract Yb:YAG (yttrium aluminum garnet) transparent ceramics were fabricated by the solid-state method using monodispersed spherical Y2O3 powders as well as commercial Al2O3 and Yb2O3 powders. Pure YAG phase was obtained at low temperature due to homogeneous mixing of powders. Under the same sintering conditions, the Yb:YAG ceramics with different doping contents of Yb3+ had similar morphologies and densification rates. After being sintered at 1700 °C in vacuum, the ceramic samples had high transparencies. The Yb:YAG ceramics doped with 0.5 wt% SiO2 formed Y–Si–O liquid phase and nonstoichiometric point defects that enhanced sintering. Compared with Nd doping, Yb doping hardly affected the YAG grain growth, sintering densification or optical transmittance, probably because Yb3+ easily entered the YAG lattice and had a high segregation coefficient.

Thermostability and reliability properties studies of transparent Ce:GdYAG ceramic by Gd substitution for white LEDs

Yttrium Aluminum Garnet transparent ceramics with different Gd3+ concentrations ((Ce0.001GdxY0.999-x)3Al5O12) ceramics were prepared by the conventional solid state reaction technology and vacuum sintering method, and the ceramic-based LEDs with blue LED were prepared by flip chips and Chip on board package. The extreme circumstance of −40 °C and 150 °C alternate environments, high humidity and high temperature as well as xenon lamp aging test for a long time are mentioned to explore the reliability of phosphor ceramics, respectively. And the thermostability and the effect of Gd3+ doping on the PL properties of ceramics were also investigated. What is more, the effects on the optical performance of the ceramic-based LEDs have been also investigated. As Gd3+ concentration added from 0 to 0.5, the PL spectra peaks of phosphor shifted from 542 nm to 575 nm under the excitation of 460 nm. It can be summarized that the increase of Gd3+ concentration could not improve the thermostability, but enhance the reliability when the x value is x is not greater than 0.4. What is more, the ceramics-based LED of x = 0.3 instead of x = 0.4 has the best reliability after the xenon lamp aging and thermal cycling test. And the ceramics-based LED of x = 0.45 has the best reliability after the high humidity and high temperature test. Compared with Ce:YAG and Ce:LuAG ceramics, although the increase of Gd3+ concentration decreases the thermostability and luminous efficiency, the Gd substitution can enhance the reliability and solve the lack of red light in the spectrum of Ce:YAG and Ce:LuAG ceramics, which has improved the performance of Ce:GdYAG ceramics for high-power warm white LEDs.

Effect of yttrium variation on phase, transparency, and micro-structure of neodymium doped yttrium aluminum garnet ceramic

Neodymium doped transparent yttrium aluminum garnet ceramics with yttrium excess and deficient (1 to 3 mol%) compositions were fabricated. The samples with stoichiometric and 1.0 mol% yttrium excess are transparent. However, 1.0 mol% yttrium deficiency samples are opaque. The structural and compositional analysis corroborates the presence of alumina as secondary phase for yttrium deficient compositions and yttrium aluminum perovskite for yttrium excess compositions. It is found that yttrium deficiency is highly detrimental to the transparency as compared to yttrium excess. The solubility width of garnet phase is found to be lower than 1.0 mol% for yttrium excess and deficiency.

Quantification of SiO_2 sintering additive in YAG transparent ceramics by laser-induced breakdown spectroscopy (LIBS)

Transparent ceramics are important optical materials with applications in street lighting, high-strength windows, electro- and magneto-optical isolators, high-power laser gain media and radiation detectors. Their fabrication most often relies on powder densification techniques carried out at high temperatures, sometimes promoted by sintering additives. Here, we describe the application of laser-induced breakdown spectroscopy (LIBS) for following the concentration levels of silica used as a sintering agent in the fabrication of yttrium aluminum garnet (YAG) transparent ceramics. The sensitivity limit of our protocol reaches a few tens of ppm of silica in YAG ceramic samples, showing that LIBS can be implemented reliably for the rapid assessment of sintering additives in advanced ceramic processing.

Grain boundary sliding mechanism in plastic deformation of nano-grained YAG transparent ceramics: Generalized self-consistent model and nanoindentation experimental validation

The elastic-plastic deformation behaviors of nano-grained and coarse-grained yttrium aluminum garnet (YAG) transparent ceramics are investigated using nanoindentation. An inverse Hall-Petch relation is observed for the nano-grained YAG ceramic and a forward Hall-Petch relation is observed for the coarse-grained YAG ceramic. In addition, the plastic work ratio as a function of applied load for the nano-grained YAG ceramic shows a different trend than that for the coarse-grained YAG ceramic. These observations suggest that the plastic deformation of the nano-grained YAG ceramic cannot be attributed to the normal dislocation mechanism and is controlled by grain boundary sliding. A generalized self-consistent model for studying the mechanical behavior of the nano-grained YAG ceramic is developed and validated by experimental results. The stress-strain relationship predicted by this model is embedded in finite element simulations which confirmed that the plastic deformation of the nano-grained YAG ceramic indeed can be attributed to grain boundary sliding.

Positron annihilation in transparent ceramics

Transparent ceramics are emerging as excellent candidates for many photonic applications including laser, scintillation and illumination. However achieving perfect transparency is essential in these applications and requires high technology processing and complete understanding for the ceramic microstructure and its effect on the optical properties. Positron annihilation spectroscopy (PAS) is the perfect tool to study porosity and defects. It has been applied to investigate many ceramic structures; and transparent ceramics field may be greatly advanced by applying PAS. In this work positron lifetime (PLT) measurements were carried out in parallel with optical studies on yttrium aluminum garnet transparent ceramics in order to gain an understanding for their structure at the atomic level and its effect on the transparency and light scattering. The study confirmed that PAS can provide useful information on their microstructure and guide the technology of manufacturing and advancing transparent ceramics.

Dy3+/Ce3+ Codoped YAG Transparent Ceramics for Single‐Composition Tunable White‐Light Phosphor

Superior optical, thermal, and mechanical properties of transparent ceramics are very important in the applications of solid lasers, solid‐state lighting, and transparent armors. Herein, a series of (Dy0.03CexY0.97−x)3Al5O12 transparent ceramics were fabricated using vacuum reactive sintering method. Importantly, these Dy3+/Ce3+ codoped yttrium aluminum garnet (YAG) transparent ceramics served as single‐composition tunable white‐light phosphors for UV‐LEDs is developed for the first time. By combining with commercially available UV‐LEDs directly, the optimal chromaticity coordinates and correlated color temperature (CCT) are (x = 0.33, y = 0.35) and 5609 K, respectively. Notably, the codoping of Ce3+ enhances the luminescent intensity of Dy3+ ions while excited at 327 nm. The emission color of YAG transparent ceramics can be tuned from white to yellow through energy transfer between Dy3+ and Ce3+. These new phosphors, possessing of pure CIE chromaticity and environmentally friendly nature, are promising for applications in white UV‐LEDs.

Microstructure characterization via stereological relations — A shortcut for beginners

Stereological relations that can be routinely applied for the quantitative characterization of microstructures of heterogeneous single- and two-phase materials via global microstructural descriptors are reviewed. It is shown that in the case of dense, single-phase polycrystalline materials (e.g., transparent yttrium aluminum garnet ceramics) two quantities have to be determined, the interface density (or, equivalently, the mean chord length of the grains) and the mean curvature integral density (or, equivalently, the Jeffries grain size), while for two-phase materials (e.g., highly porous, cellular alumina ceramics), one additional quantity, the volume fraction (porosity), is required. The Delesse–Rosiwal law is recalled and size measures are discussed. It is shown that the Jeffries grain size is based on the triple junction line length density, while the mean chord length of grains is based on the interface density (grain boundary area density). In contrast to widespread belief, however, these two size measures are not alternative, but independent (and thus complementary), measures of grain size. Concomitant with this fact, a clear distinction between linear and planar grain size numbers is proposed. Finally, based on our concept of phase-specific quantities, it is shown that under certain conditions it is possible to define a Jeffries size also for two-phase materials and that the ratio of the mean chord length and the Jeffries size has to be considered as an invariant number for a certain type of microstructure, i.e., a characteristic value that is independent of the absolute size of the microstructural features (e.g., grains, inclusions or pores).

Optical properties of Ho:YAG and Ho:LuAG polycrystalline transparent ceramics

Lutetium Aluminum Garnet (LuAG) is a garnet isostructure similar to Yttrium Aluminum Garnet (YAG). High quality Ho3+ doped YAG and LuAG transparent polycrystalline ceramics were fabricated successfully by a reactive sintering method under vacuum. The microstructures, absorption spectrum, fluorescence spectrum and the laser performances of Ho:YAG and Ho:LuAG ceramics were systematically investigated. The in-line transmittances of Ho:YAG ceramic in the visible and infrared region are higher than 82% and 84%. The absorption coefficient of 1.0 at.% Ho:LuAG is 0.88 cm−1 at 1906 nm and its absorption cross section is 0.62 × 10−20 cm2.

Reduce synthesis temperature and improve dispersion of YAG nanopowders based on the co-crystallization method

Pure yttrium aluminum garnet (YAG) nanopowders were synthesized at 950°C from the co-crystallization precursor of Y(NO3)3⋅6H2O and Al(NO3)3⋅9H2O (nitrate process). When 17wt.% of Y(NO3)3⋅6H2O was replaced by Y2O3 nanopowders, so as to make up a three-layer core–shell structure of the precursor, the synthesis temperature was reduced to 850°C (Y2O3 process). Based on Y2O3 process, further reducing the synthesis temperature to 700°C was realized by adding polyacrylic acid (PAA, 50% M), which was used to shorten the distance of the metal ions and provide combustion heat (PAA process). TEM characterizations indicated that the powders produced through nitrate and Y2O3 processes agglomerated, while the powders produced through PAA process were dispersed much better. The agglomerate size analysis results demonstrated that the powders produced through PAA process were with smaller agglomerate size and wider agglomerate size distribution than those through nitrate process or Y2O3 process. And they were more likely to be sintered to YAG transparent ceramics.

Gelcasting of transparent YAG ceramics by a new gelling system

Highly transparent yttrium aluminum garnet (YAG) ceramic was prepared through spontaneous gelcasting with an aqueous mixture of commercial Al2O3 and Y2O3 powders and a water soluble co-polymer of isobutylene and maleic anhydride (commercial name Isobam) as the dispersant and gelling agent. The rheological behaviors of the slurries were investigated as a functions of Isobam concentration and solids loading. The optimized formula of the slurry was obtained with a solids loading of 68wt% and Isobam addition 0.3wt% (per powder). The in-line transmittances of the resulted YAG ceramic (thickness: 7.5mm) were about 82.3% and 79.2% at the wavelengths of 1000nm and 400nm respectively. Microstructure observation exhibited a nearly pore-free structure with the average grain size about 15μm. The new gelling system may have potential use for the fabrication of large and complexly shaped high quality YAG transparent ceramics.

Spectroscopic properties of transparent Y_3Al_5O_12: Eu ceramics

Yttrium aluminum garnet transparent ceramics with divalent or trivalent europium ion doping were prepared by vacuum sintering at 1780 °C. Refined XRD data confirmed the pure phase ceramic with little lattice expansion. The transmittance of the 1.1 mm thick blue color divalent europium doped sample was measured to be 61% at wavelength of 900 nm. The photoluminescence spectrum exhibited the typical broad emission related to divalent europium ions. Furthermore, the photoluminescence spectra of the samples with different europium doping concentration and different annealing temperature were compared.

Antisite defect in nonstoichiometric yttrium aluminum garnet: Experimental and first-principles calculation

Abstract Antisite defects in nonstoichiometric yttrium aluminum garnet (YAG) were investigated systematically with experiments and first-principles calculation based on density functional theory. Transparent YAG ceramics with different deviations from stoichiometry have been prepared and the lattice constants increase with the increase of deviation. Calculations show that Y Al,16a is the most preferred antisite defect and the concentration of defects increases as the sintering temperature increased. High sintering temperature results in high concentration of Y Al defects, however Y Al defects may avoid secondary phase in Y 2 O 3 -rich YAG. It is found that formation energy of Al Y antisite defect is very high, therefore the concentration of Al Y antisite defects is very low even at high temperature. For Al 2 O 3 -rich YAG, it is impossible to avoid the formation of secondary phase. The deviation from stoichiometry has great influence on the transmittance and optical quality of transparent ceramics.

Microstructural Evolution During Vacuum Sintering of Yttrium Aluminum Garnet Transparent Ceramics: Toward the Origin of Residual Porosity Affecting the Transparency

Controlling residual amount of defects in transparent ceramics is a major challenge for laser applications. This study was focused on microstructural evolution of Nd:YAG ceramics during their reactive solid‐state sintering which was correlated to their optical transmittance. From microstructural observations, the microstructural maps and grain size‐density and grain size‐pore size sintering trajectories of Nd:YAG ceramics were established as a function of silica content. For densities higher than 99.7%, the occurrence of intragranular porosity was correlated to a critical pore radius of 0.16 μm. Silica appears to favor the formation of intragranular porosity which was attributed to the increasing of the grain growth rate compared with the densification one. An analytical model was established by coupling the analytical laws derived from sintering trajectories and the classical theory of light diffusion, allowing to correlate the microstructural features of transparent Nd:YAG ceramics to their optical properties.