Transparent Yttrium Aluminum Garnet Research Articles

Spark plasma sintering of transparent YAG:Ce ceramics with LiF flux

Transparent yttrium aluminum garnet (YAG) ceramics doped with cerium ions (Ce3+) were successfully synthesized using the spark plasma sintering (SPS) process at a temperature of 1600 °C, isothermal exposure 10 minutes, pressure 100 MPa and a heating rate 10 °C/min from a mechanical mixture of powders of yttrium, aluminum, and cerium oxides. Transmittance in the visible range for 1 mm thick samples is above 40 %; in the near infrared is above the 60 %. Vickers hardness was 15±1.05 GPa. The optimal SPS conditions have been discussed and suggested in order to obtain a good combination of density and transmittance.

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).

3D printed transparent ceramic YAG laser rods: Matching the core-clad refractive index

Abstract Yttrium Aluminum Garnet (YAG) solid state laser gain media rods with an active Neodymium-doped core and an optically-clear cladding region were additively manufactured via direct-ink-writing (DIW), followed by sintering and hot isostatic pressing to form fully dense optical ceramics. Lutetium and Gadolinium were chosen as optically-inert co-doping ions in the clad to match the increase in refractive index caused by the Neodymium in the core. Either 11.6% Lutetium or 3.8% Gadolinium can be used to match the index change from 2% neodymium; however, differences in diffusion distances across the core-clad interface lead to large fluctuations in index in that region. These index fluctuations can be minimized either by matching dopants with similar diffusion distances, or by implementing a gradual gradient in the doping profile, possible through DIW, rather than a sharp compositional interface attainable via more standard fabrication methods. This improvement in index homogeneity resulted in a 40% improvement in lasing performance compared with that of a core-clad rod fabricated with a sharp interface between the doped and undoped regions.

Demonstration and aging test of a radiation resistant strontium-90 betavoltaic mechanism

In this work, a betavoltaic combined with photovoltaic mechanism using strontium-90 as a radioactive source was built up. A transparent yttrium aluminum garnet ceramic doped with a cerium ion is used as an intermediate conversion layer to avoid direct irradiation to a semiconductor conversion device and translates most of the high energy beta particles into photons. The traditional crystalline silicon conversion device with an N+PP+ junction structure functions as a betavoltaic and photovoltaic conversion device simultaneously. Radiation resistance is demonstrated by a 0.9 MeV electron beam irradiation aging experiment. In the optimized betavoltaic setup with a 0.5 mm transparent yttrium aluminum garnet ceramic, the crystalline silicon conversion device can bear 5 × 1015 e/cm2 by degrading output power less than 15%. Loaded with 1.3 mCi strontium-90, this betavoltaic setup produces a short circuit current of 282 nA, an open circuit voltage of 0.168 V, a fill factor of 0.58, and a total conversion efficiency of 0.32%.

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.

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.

On the densification mechanism of nano grained Yttrium aluminum garnet transparent ceramic during high pressure sintering process

The sintering densification mechanism of nano grained Yttrium aluminum garnet transparent ceramic during high pressure sintering process can be divided into three period. The first sintering stage can be described as plastic deformation after yield point. The intermediate stage is conducted by grain boundary viscous sliding. The grain boundary diffusion is the significant contribution for the final sintering densification period.

Stereology of dense polycrystalline materials—from interface density and mean curvature integral density to Rayleigh distributions of grain sizes

Using transparent yttrium–aluminum garnet (YAG) as an example, the microstructure of dense polycrystalline ceramics is investigated by microscopic image analysis. The mean chord length and Jeffries grain size determined via stereological relations are 9.9–11.3μm and 19.0–21.3μm after 2h and 16h of firing at 1735°C, respectively. Distributions of grain size sections are determined via image analysis and transformed via Saltykov, Cruz-Orive and Woodhead transformations. None of the weighted means or deciles of these distributions corresponds to the mean chord length or Jeffries size exactly, but the distributions of 2D grain size sections are almost identical to the 3D grain size distributions obtained after transformation, which is possible only for Rayleigh distributions. It is shown that the grain size distributions determined here are indeed close to Rayleigh distributions that evidently arise as a result of certain grain growth and ripening processes (Ostwald ripening).

Tool Rake Face Temperature Distribution When Machining Ti6Al4V and Inconel 718

This paper is concerned with experimentally obtained full field temperature distributions at the interface between chip and tool – the tool rake face – for the machining of Ti6Al4V and Inconel 718. As part of the work presented herein, temperature distributions at the chip-tool interface were obtained using a novel technique, near infrared thermography, which provides high accuracy for the target temperature range above 1250K. During the study, transparent yttrium aluminum garnet (YAG) tools were used to provide an optical path to the chip-tool interface.

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.

Influence of non-stoichiometry on solid-state reactive sintering of YAG transparent ceramics

Transparent yttrium aluminum garnet (Y3(1+x)Al5O12) ceramics with different deviations from stoichiometry were fabricated by solid-state reactive sintering of commercial powders. The influence of non-stoichiometry on phase composition, microstructure, grain growth kinetics and optical transparency of the ceramics were studied. It was found that for Al2O3-excess samples, the transmittance and the average grain size decreased sharply due to the Al2O3 secondary phase. For the stoichiometric sample sintered at 1760°C for 10h, the highest transmittances of 83.1% at 1064nm and 80.2% at 400nm were obtained. For Y2O3-excess samples, when the excessive content was smaller than 0.7mol%, the transmittance decreased slightly and the average grain size increased. However, when the deviation was larger than 0.7mol% the transmittance and the average grain size decreased due to the secondary phases.

Environmentally benign processing of YAG transparent wafers

Transparent yttrium aluminum garnet (YAG) wafers were successfully produced via aqueous tape casting and vacuum sintering techniques using a new environmentally friendly binder, a copolymer of isobutylene and maleic anhydride with the commercial name ISOBAM (noted as ISOBAM). Aqueous YAG slurries were mixed by ball-milling, which was followed by de-gassing and tape casting of wafers. The final YAG green tapes were homogenous and flexible, and could be bent freely without cracking. After the drying and sintering processes, transparent YAG wafers were achieved. The microstructures of both the green tape and vacuum-sintered YAG ceramic were observed by scanning electronic microscopy (SEM). Phase compositions were examined by X-ray diffraction (XRD). Optical transmittance was measured in UV–VIS regions with the result that the transmittance is 82.6% at a wavelength of 800nm.

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).

Effect of nonstoichiometry on the terahertz absorption of Y3Al5O12 optical ceramics

Abstract

Thermo-optic properties of ceramic YAG at high temperatures

The thermo-optic coefficient dn/dT at 632.8 nm and thermal expansion coefficient α of transparent ceramic yttrium aluminum garnet (YAG) were measured between room temperature and 600 K. The data showed that dn/dT increases with temperature and α is in good agreement with that of single-crystal YAG. To the best of our knowledge, these are the first experimental data of the thermo-optic properties of highly transparent ceramic YAG above room temperature. We also present, using previously reported values measured below room temperature, fitting parameters for dn/dT that are valid over a wide temperature range (70–600 K) with an average error of 2.0%.

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.