YAG Transparent Research Articles

Conversion into single-phase YAG ceramics at low temperature by stoichiometry adjustment

In the case of low-temperature fabrication aiming at transparent polycrystalline YAG ceramics with fine grain size, it is easy to retain trace amounts of the intermediate YAP phase, thereby reducing the optical transmittance. To address this issue, adding a small amount of Al2O3 to the starting YAG powder was found to promote the transformation of the residual YAP phase to the YAG phase at low temperatures. The study investigated the effect of sintering temperature and excess Al2O3 amount on preparing transparent YAG ceramics by hot pressing. The transmittance of the Al2O3-aided single-phase YAG ceramic (2 mm thick) prepared using the two-step sintering reached 43.4 % at 400 nm and 71.8 % at 1064 nm, with a grain size of about 2 μm, while the transmittance of the multiphase YAG ceramic without Al2O3 addition was less than 1 % at both wavelengths.

Analysis of Er3+ diffusion within transparent YAG ceramics manufactured through various routes by SPS

Different bonding approaches were considered to develop layered composite ceramics of erbium doped yttrium aluminium garnet (Er:YAG/YAG) by Spark Plasma Sintering. More precisely three different strategies were envisaged, starting either from a stack of powders, pre-sintered or sintered ceramics. Highlighted by Electron Probe Micro Analysis (EPMA) analyses, these approaches led to different designs of interface i.e. doping profiles, linked to the different thermo-mechanical treatments applied, which have an influence on the grain boundary diffusion of erbium ions. The lengths of the diffusion were found to be of 12 μm, 63 μm and 140 μm respectively for fully dense, powder and pre-sintered assembly approaches.

Nd3+→ Cr4+ energy transfer in co-doped Nd,Cr:Y3Al5O12 garnet transparent ceramics

Herein, we quantitatively analyze the Nd3+ → Cr4+ energy transfer efficiency in the co-doped transparent ceramic YAG host and determine that it can be maintained at less than a 20 % loss of Nd3+ excited states when the Cr4+ absorption coefficient at 1064 nm is less than 1.0 cm−1. Furthermore, we conclude that the energy transfer mechanism can be fully accounted for on the basis of classic Förster-Dexter theory, as the transfer efficiency deduced from the emission kinetics is consistent with that independently determined from the spectroscopic properties of Cr4+ absorption and Nd3+ emission. Systematic studies of trends for Cr3+ → Cr4+ conversion are reported as a function of the oxygen annealing temperature, initial Cr3+ concentration, and incorporation of divalent (Mg2+ and Ca2+) charge compensators. Lastly, no effect of energy migration among the Nd3+ ions was observed.

Generation of 2 μm few-cycle laser pulse with dual laser filaments nonlinear processes

For the first time, we propose a new scheme for generating few-cycle 2 μm seed laser pulse in an optical parametric chirped pulse amplifier (OPCPA) front-end. The 2 μm ultrafast seed laser pulse is generated via a difference frequency generation (DFG) process between the visible and near-IR supercontinuum from two different filamentation processes inside the transparent YAG crystals. The generated DFG pulse spectrum has a broad spectrum range of 1200 nm from 1500 nm to 2700 nm. By further scaling up the pulse energy of the seed laser pulse via a non-collinear optical parametric amplification (OPA) stage, the single pulse energy is boosted to 15.6 μJ. After dispersion compensation, the pulse duration is compressed to 14.3 fs with its spectrum covering from 1600 nm to 2600 nm.

Al2O3-YAG:Ce/YAG composite ceramic phosphor in a transmissive configuration for high-brightness laser-driven lighting.

High-power, high-brightness laser lighting promotes new requirements for light-conversion materials, such as high thermal conductivity, high saturation threshold and compact encapsulation. In this paper, we designed and fabricated a novel composite structure ceramic including a 1.0 × 1.0 mm2 Al2O3-YAG:Ce ceramic and a φ=16.0 mm transparent YAG ceramic for the transmissive configuration in laser lighting. When pumped by blue laser from 0∼60 W mm2, all the samples exhibited no luminous saturation phenomenon, and the 10.0 wt.%Al2O3-YAG:Ce/YAG composite ceramic with the thickness of 0.3 mm maintained white light with a luminous efficacy over 200 lm/W. Moreover, a maximal luminous flux over 1000 lm, a correlated color temperature (CCT) of 5471 K, and an operating temperature as low as 92.3 °C were obtained under the excitation power density as high as 60 W/mm2. The configuration that Al2O3-YAG:Ce encapsulated by YAG reflects an excellent optical and thermal properties by using transparent and highly thermally conductive YAG materials. These results indicate that Al2O3-YAG:Ce/ YAG composite ceramic phosphor is a promising candidate in transmissive configuration for automotive lighting and laser searchlight.

A useful approach to understand the origin of defects in transparent YAG ceramics

A useful approach to understand the origin of defects in transparent YAG ceramics

Effect of complex Si4++Mg2+ additive on sintering and properties of undoped YAG ceramics

The paper is devoted to studying of Si4++Mg2+ complex additive for obtaining transparent YAG ceramics for laser applications. Ceramics were fabricated by reactive vacuum sintering of commercial Y2O3, Al2O3 powders taken in a stoichiometric mixture with TEOS and MgO as sintering aids. Microstructure and optical properties of YAG:Si4+,Mg2+ ceramics were investigated as a function of the Si4+/Mg2+ ratio. It was found that the influence of complex additive does not correspond to the direct superposition of known Si4+- and Mg2+-induced sintering mechanisms and involves interaction between Si4+ and Mg2+ ions during sintering. It was shown that CSi/CMg> 1 provides more effective pore elimination and uniform microstructure when CSi/CMg< 1 gives more intense inhibition of grain grown which may be important for scaling the size of ceramics.

Direct ink writing of cellulose-plasticized aqueous ceramic slurry for YAG transparent ceramics

Aqueous transparent ceramic YAG slurry with high solids loading up to 52 vol.% is proposed in this work for 3D direct ink writing. Celluloses are readily adjusted in the slurry design to enhance the plasticity, through which both printability of ceramic slurries and the optical quality of transparent ceramics are considerably improved. We demonstrate that the in-line transmittance at 1064 nm for a 1.2-mm-thick YAG ceramic can approach about 81.0%. This research provides a facile 3D printing routine for manufacturing transparent ceramic from water-based slurry.Graphical abstract

Magnesium fluoride (MgF2) – A novel sintering additive for the preparation of transparent YAG ceramics via SPS

The effect of MgF2 as a sintering additive for the preparation of YAG ceramics via spark plasma sintering (SPS) is investigated with promising results, as nearly complete densification (0.58% porosity) is achieved at relatively low temperature and moderate pressure. Higher temperature and dwell time resulted in a translucent/transparent body. On the other side, significant grain growth was observed with MgF2 addition.

Transparent YAG ceramic/sapphire composite fabricated by pressureless direct thermal diffusion bonding

Transparent YAG ceramic/c-plane sapphire composites were successfully fabricated by pressureless direct thermal diffusion bonding at 1650 ℃ for 10 h in a vacuum. The bonding interfaces could be well defined between YAG ceramic and sapphire, and effective diffusion lengths from YAG to sapphire were calculated to be about 0.5 μm for Nd, Yb and Y ions by Fick’s second diffusion law. Around the bonding interface, neither crystallization of YAG ceramic nor ceramization of sapphire were observed, and besides, growth of YAG grains as well as variation of sapphire growth striation under heating probably induced the instability or even crack of the composite. In addition, precisely parallel polished contact surfaces of ceramic and sapphire are required for enhancing bonding strength. This work presented a clear vision for the bonding interfaces of YAG ceramic/sapphire composite, and will pave the way for robust and efficient composite laser media.

Transparent YAG:Ce ceramic with designed low light scattering for high-power blue LED and LD applications

Yellow-emitting YAG:Ce transparent ceramic is recognized as an ideal color converter in high-power blue LEDs and LDs, but the absence of scattering centers in its microstructure leads to a low light extraction efficiency and poor light uniformity. Here, taking advantage of the scattering effect and the transparency of YAG:Ce ceramics, Ce-free YAG phase was used as a second component to form a composite with YAG:Ce phosphor. The sintered YAG:Ce-YAG ceramic possessed a high transparency of ∼63 % and a thermal conductivity of 8.9 Wm−1 K−1. Due to its beneficial thermal properties and high external quantum efficiency of 70.2 %, the YAG:Ce-YAG ceramic could be excited under a high blue-laser flux density of up to 9.60 W/mm2 and showed a luminous emittance of 1220 lm/mm2. Due to light scattering arising from the slightly different refractive indices of the two phases, the designed YAG:Ce-YAG ceramic showed better lighting effects than a single-phase transparent YAG:Ce ceramic.

3D printing of transparent YAG ceramics using copolymer-assisted slurry

Transparent YAG ceramics were successfully formed by 3D printing using a 2.33 wt% ISOBAM per mixed powder assisted aqueous slurry with 72 wt% solids-loading. The 3D printing process enabled the fabrication of complex-shaped transparent YAG ceramics and did not require the use of cold isostatic pressing (CIP), hot isostatic pressing (HIP), or additional tooling. The fabricated ceramics achieved an in-line transmittance of 70% in the visible wavelength range and relative density of 99.7%, which was similar to that achieved by CIP ceramic processing methods. 3D printing ceramics with an ISOBAM assisted slurry could simplify the fabrication process of transparent ceramics with complex shapes.

The van de Hulst approximation for light scattering and its use for transmittance predictions in transparent ceramics

The in-line transmittances of transparent spinel and YAG ceramics with a low volume fraction of monosized spherical pores calculated via the exact Mie solution and the so-called van de Hulst approximation are compared. It is shown that the transmittances calculated via the van de Hulst approximation are in surprisingly good qualitative agreement with the exact Mie solutions. In particular, all relevant trends and features of transmittance-versus-wavelength-and-pore-size plots are correctly predicted, so that the van de Hulst approximation may be considered as a perfect tool for material design and process optimization purposes. It avoids the “unphysical” results that can occur in the Rayleigh and Fraunhofer approximations for pores sizes beyond their validity and is much simpler than the Rayleigh-Gans-Debye approximation. Tables listed in the Appendix to this paper allow the reader reconstruct the exact Mie solution results from the easily calculated van de Hulst approximation, if required.

Effect of citric acid on the microstructure and optical properties of transparent YAG ceramics by reactive-SPS

Commercial α-Al2O3 and Y2O3 powders with 0.5 wt% TEOS were used to fabricate transparent polycrystalline YAG ceramics by reactive spark plasma sintering. The powder mixtures were dispersed with dispersants of PAA and citric acid in aqueous slurry. YAG specimens sintered at 1400 °C for 3 min were used to investigate the effect of processing parameters on the microstructure and the optical properties. It is found that a pretreatment of the powders mixture with 0.6 wt% citric acid can significantly improve the microstructure homogeneity of the as-sintered YAG ceramics. After post-annealing in wet air, the specimen sintered from the treated powders mixture was transparent, while the specimen from the untreated powders mixture was hazy. The effects of citric acid for this were investigated in this paper.

Investigation of YAP/YAG powder sintering behavior using advanced thermal techniques

The fabrication process of transparent YAG (Y3Al5O12) ceramics with an emphasis on examination of the sintering process and microstructural analysis is reported herein. Nanometric powder with an equivalent spherical diameter dBET = 60 nm consists mainly of hexagonal YAP phase (YAlO3). According to sintering analysis carried out using a dilatometer and high-temperature XRD analysis, the phase composition and phase transformation taking place at elevated temperatures accompany the sintering process and the fastest sintering rate was observed at temperatures of YAG formation. Based on the dilatometric and XRD analyses vs temperature, three temperatures were selected for pressureless sintering. Presintered samples were subjected to sintering under isostatic pressure using a hot isostatic pressing, after which transparent YAG was obtained. The best results were obtained for samples presintered at a relatively low temperature of 1600 °C, and for thus obtained sample, the in-line transmittance was 64% at λ = 808 nm.

Transparent YAG material prepared from nano-powder with core-shell morphology

Abstract A powder with core-shell morphology was used to obtain a dense yttrium-aluminium garnet material. Precipitation of a Y 2 O 3 precursor in an aluminium oxide slurry, using ammonium carbonate, allowed nanometric particles of the yttrium oxide precursor to be deposited on submicron Al 2 O 3 grains. It was found that washing of the precipitation product changes the chemical composition of the precursor which has a significant impact on the evolution of a phase composition of powders during the heat treatment process and finally on their morphology. Pressureless sintering at 1650 °C for 2 h of the unwashed powder led to a single-phased material without open porosity reaching approx. 96% of the theoretical density while during sintering under the same conditions the compact of the washed powder did not completely close pores and its density was not higher than 92%. The use of isostatic hot-pressing process allowed a complete elimination of porosity in the materials obtained from the unwashed powder to occur.

Advances in the monitoring of the SiO2 evaporation loss in transparent YAG ceramics by LIBS

Abstract The effect of silica on the densification of transparent YAG ceramics has long been investigated. The present paper makes a step forward on the quantification of the Si amount in YAG. The study shows how to measure with a high level of accuracy the silicon concentration in YAG in different steps of the fabrication process. The Laser-Induced Breakdown Spectroscopy (LIBS) has been used to quantify the amount of silicon in a set of samples with 0–0.275 wt% silica. Nanometric silica soot or its precursor, TEOS, has been used as silica source. The analysis shows that more than 80 wt % of SiO2 evaporates during reactive-sintering under vacuum, most probably because SiO gas forms during sintering under low oxygen partial pressure. In-depth concentration maps were obtained by laser drilling with LIBS pulses. The results show the possibility to modify the amount of residual Si in YAG ceramics by adjusting the sintering and annealing process.

Influence of the ceramic powder morphology and forming conditions on the optical transmittance of YAG:Yb ceramics

The influence of the dispersity and morphology of ceramic powders on the characteristics of green bodies and the optical transmittance of YAG:Yb (20 at%) ceramics was considered. The effect of the specific surface area on the relative density of compacts was studied. An increase in the specific surface area from 1.45 to 12.38 m2/g led to a decrease in the relative density of green body compacts from 52% to 38% under the fixed uniaxial pressure of 50 MPa. An increase in the uniaxial pressing value up to 150 MPa provided a maximum increase of the optical transmission of ceramics. However, an increase in the specific surface area and uniaxial pressure led to the appearance of macrodefects in ceramic samples. Cold isostatic pressing at 200 MPa after uniaxial pressing at 50 MPa resulted in an increase of optical transparency and the elimination of the macrodefect formation in ceramics. Dispersity and morphology of ceramic powders have a significant effect on the optical transparency of ceramics. Forming conditions had insignificant influence on optical characteristics. Highly transparent YAG:Yb ceramics with 80% transmittance were developed.

Residual porosity and optical properties of spark plasma sintered transparent polycrystalline cerium-doped YAG

Transparent cerium-doped yttrium aluminum garnet (Ce:YAG) phosphors are promising candidates for high-power white light emitting diode applications. In the present study, Ce:YAG powder was synthesized by a co-precipitation method and highly transparent ceramics were fabricated by spark plasma sintering. The effects of temperature and pressure, as well as post-sintering treatments (annealing or hot isostatic pressing), on residual porosity were studied by electron and confocal laser microscopy. Correlation between residual porosity characteristics (pore size and volume fraction) and optical properties (in-line transmittance and photoluminescence intensity) of the luminescent transparent ceramics was established.

Influence of TEOS and MgO addition on slurry rheological, optical, and microstructure properties of YAG transparent ceramic

Stable dispersed slurries were prepared from a mixture of commercial Al2O3 and Y2O3 powders by the ball milling process. Ammonium poly meta acrylate (Dolapix CE64) as the dispersant and different amounts of tetraethyl orthosilicate (TEOS)+0.1 wt% MgO as the sintering aids were added to the slurries. The effects of TEOS and MgO addition on the fabrication of transparent polycrystalline YAG ceramics were investigated by slip casting and vacuum sintering. The results showed that almost all slurries containing TEOS and TEOS + MgO had a near-Newtonian behavior, but the slurry containing only MgO had shear thickening with a thixotropic behavior. Also, by increasing the amounts of TEOS, due to the creation of steric repulsion, the viscosity was decreased. In comparison, TEOS + MgO slurries had lower viscosity than the TEOS ones. This was due to both the release of more ethanol and MgO action as a dispersant agent among the particles of yttria and alumina. All samples were pure YAG phase. The maximum transmittance at 1064 nm (≈83%) was obtained for the sample of 0.5 wt% TEOS+0.1 wt% MgO. A nearly pore-free microstructure with high relative final density (99.99%), a narrow grain size distribution (2.5–7.5 μm), with the average grain size of 4.5 μm, was achieved in this sample.