Yttrium Aluminum Garnet Single Crystals Research Articles

Determination of Thermal-Diffusivity Dependence on Temperature of YAG Single Crystals with Different Concentrations of Yb3+ and V3+ Doping Ions

Thermal diffusivities of pure and doped yttrium aluminum garnet single crystals were measured as a function of temperature. Samples doped with rare earth ions (3 at% and 25 at% of Yb 3+, and 0.8 at% and 2.1 at% of V 3+) were investigated in the temperature range from 34 °C to 300 °C. Determination of the thermal diffusivity was based on an analysis of propagation of a thermal wave in the sample. The frequency of the thermal wave was 100 mHz. A temperature disturbance connected with the thermal wave propagating in the sample was detected using the mirage effect. The results showed that the thermal diffusivity of all investigated samples decreases with an increase of sample temperature. A drop in the thermal diffusivity is more pronounced for pure and low-doped crystals.

Preparation and optical properties of sol–gel derived thick YAG:Ce3+ phosphor film

A sol–gel process for the preparation of thick YAG:Ce3+ films has been developed. YAG:Ce3+ phosphor films with a typical yttrium aluminum garnet (YAG) photoluminescence (PL) profile were prepared successfully. Single phase YAG films with a maximum thickness of 2μm were prepared in the present work. The refractive index of the YAG films prepared (∼1.85 @ 633nm and ∼1.80 @ 1540nm) was very close to that of YAG single crystal. It was confirmed that the PL efficiency of the prepared films was strongly related to the degree of YAG phase formation in the samples and the rate of YAG phase evolution strongly depended on the film thickness. To achieve high YAG:Ce3+ PL efficiency, a minimum 1.2μm of thickness is necessary. It was also found that the optical quality of the YAG films decreased following the deposition and thermal annealing processes, probably as a result of the growth of YAG crystals inside. As an example of a compromise between high PL efficiency and good optical quality, prism coupling was achieved on a 30-layer planar waveguide sample annealed at 1000°C for 30min. It was proved that this sample could support multiple waveguide modes at visible wavelengths and its propagation loss at 1540nm was estimated at 4.2dB/cm. The temperature quenching effect in the films of the present work was also measured, being the same as that of commercial YAG:Ce3+ phosphors with relatively high Ce concentration. The present technology is intended for YAG:Ce3+ phosphor films that may be applied on white LED and it may also be of benefit to any areas where YAG films may be of interest.

On the optical properties of undoped and rare-earth-doped yttrium aluminium garnet single crystals

Optical absorption and photoluminescence (PL) measurements were performed on single crystals of undoped Y3Al5O12 (YAG) and a number of rare-earth-doped YAG to study the effect of dopant type and concentration, growth atmosphere, post-growth annealing and UV irradiation on the optical properties of YAG crystals. The presence of hydrogen in the growth atmosphere was found to be essential for enhancing the incorporation of Ce ions in the Ce3+ state in Ce-doped YAG (Ce : YAG). Annealing in air was shown to have no effect on the PL emission of Ce : YAG crystals. An absorption peak around 256 nm was observed in the undoped YAG and Ce : YAG crystals after air anneal at 1200 °C. Optical absorption and annealing experiments support the association of the 256 nm peak with Fe impurities and oxygen ions. UV irradiation modifies the valency of impurities and generates electronic defects leading to an increase in the optical density of YAG crystals. Optimizing the growth and annealing conditions is critical in order to develop Ce : YAG single crystals as efficient scintillators.

Strong visible and near infrared luminescence in undoped YAG single crystals

Strong luminescence peaks were observed at 700 and 800 nm in undoped yttrium aluminum garnet (YAG) single crystals. They were attributed to low level of iron impurities as confirmed by Glow Discharge Mass Spectrometry analysis. The 800 nm was only excited by high energy band at 270 nm; the reason behind that was discussed. Photoluminescence measurements revealed a large number of luminescence peaks in all YAG crystals regardless of the growth conditions due to native defects and low-level impurities. These luminescence centers have significant effects on the optical properties of rare-earth doped YAG crystals and their performance in laser and scintillation applications. Excitons released in the lattice may be easily captured by iron ions instead of Ce3+ ions and the scintillation output is substantially decreased. Nevertheless, Undoped YAG crystals may have the potential to be developed into efficient scintillators.

Determination of Thermal-Diffusivity Dependence on Temperature of Transparent Samples by Thermal Wave Method

The use of a typical measuring cryostat with a standard temperature controller was proposed for investigation of the temperature dependence of the thermal diffusivity of transparent samples. The basic idea is to use the cryostat heater to control the mean sample temperature and to generate the thermal wave in it, simultaneously. Because of the relatively high thermal inertia of the system, the measurements are carried out at frequencies not exceeding 50 mHz. The periodic temperature disturbance in the sample was detected optically by the use of the mirage effect. The proposed method was used for determination of the thermal diffusivity of yttrium aluminum garnet single crystals in a temperature range from 20 °C to 200 °C.

Microstructuration induced differences in the thermo-optical and luminescence properties of Nd:YAG fine grain ceramics and crystals

The temperature and compositional dependences of thermo-optical properties of neodymium doped yttrium aluminum garnet (YAG) crystals and fine grain ceramics have been systematically investigated by means of time-resolved thermal lens spectrometry. We have found that Nd:YAG ceramics show a reduced thermal diffusivity compared to Nd:YAG single crystals in the complete temperature range investigated (80-300 K). The analysis of the time-resolved luminescent properties of Nd(3+) has revealed that the reduction in the phonon mean free path taking place in Nd:YAG ceramics cannot be associated with an increment in the density of lattice defects, indicating that phonon scattering at grain boundaries is the origin of the observed reduction in the thermal diffusivity of Nd:YAG ceramics. Finally, our results showed the ability of the time-resolved thermal lens to determine and optimize the thermo-optical properties of Nd:YAG ceramic based lasers.

Microwave synthesis of YAG:Eu by sol–gel methodology

Yttrium–aluminum oxides are interesting compounds and they have been extensively used as host for lasers and phosphors, due to their stable physical and chemical properties. The fabrication of yttrium–aluminum garnet (YAG) has been investigated thoroughly. Single-crystal YAG is expensive and to produce it a new way has been investigated. This process consists of modifying the methodology of reagents mixture and the process of heating them. The microwave irradiation is used to heat-treat the oxide mixture. The traditional synthesis of YAG powders occurs through the reaction of aluminum and yttrium powders at high temperatures. With this work we investigated the preparation of YAG by non-hydrolytic sol–gel route as an alternative methodology to obtain yttrium–aluminum matrix from inorganic precursors (yttrium and aluminum chloride). The preparation of the gel was carried out in an oven-dried glassware. The AlCl 3, YCl 3 and ethanol were reacted in reflux under argon atmosphere. Europium III chloride was added as a structural probe. The powder was dried and heat-treated in modified microwaves. The samples were pre-treated at 50 and 800 oC during 1 h and then heated in microwaves for 30 s, 2 and 4 min. The formation process and structure of the powders were studied by means of X-ray diffraction (XRD), photoluminescence (PL) and transmission electronic microscopy (TEM). XRD presents only picks corresponding to the YAG phase and confirmed by TEM. PL date showed that the YAG phase was formed in 2 min with the samples pre-treated at 50 °C. For the samples pre-treated at 800 °C, the YAG phase appears in 30 s. The excitation spectra present a maximum of 394 nm corresponding to the 5L 6 level and emission spectra of Eu III ion present bands characteristic transitions arising from the 5D 0→ 7F J ( J=1, 2, 3, 4) monifolds excited at their maximum. The magnetic dipole 5D 0→ 7F 1 transition presents more intensity than the electric dipole 5D 0→ 7F 2 transition. This methodology showed efficiency in obtaining YAG phase.

Comparison of the Mechanical Properties of Single Crystal and Polycrystalline Yttrium Aluminum Garnet

Polycrystalline neodymium‐doped yttrium aluminum garnet (YAG), Nd: Y3Al5O12, has been produced in a transparent form and is being considered as a substitute for single crystal YAG in laser applications. To determine whether such a substitution could impact mechanical reliability, it is critical to compare the baseline mechanical properties of the single crystal and polycrystalline materials. In this study, elastic constants, hardness, fracture toughness and strength properties were measured using standard experimental procedures. In addition, fractography was used to gain insight into the fracture process and the nature of the flaws controlling the strength behavior. Overall, it was determined that the mechanical behavior of the polycrystalline YAG was very similar to that of the single crystal material. The elastic constants of polycrystalline YAG were almost the same as the single crystal and the polycrystalline material showed slight advantages in hardness and fracture toughness. Surface machining flaws were found to control the strength behavior with the single crystal material being the more sensitive to contact damage and, hence exhibiting a lower strength.

Erbium-doped yttrium aluminum garnet as a magnetic refrigerant for low temperature x-ray detectors

Garnets doped with rare-earth elements can be used in adiabatic demagnetization refrigerators. We have measured the specific heat and magnetization of a single crystal yttrium aluminum garnet (YAG) doped with 30% Er3+ ion at temperatures between 93 mK and 8 K under magnetic fields up to 8.0 T along the 〈111〉 crystal axis. From the specific heat and magnetization, we derived consistent temperature and magnetic-field dependence of the magnetic entropy. Under zero magnetic field, the magnetic entropy begins to decrease below 2 K and becomes half of R ln 2 at ∼160 mK. This decrease is considered to be due to an antiferromagnetic short-range ordering among Er3+ ions. This behavior of the specific heat in the measured temperature range can be explained by a model in which both the crystalline-electric-field ground state and the first excited state are included. The operating temperature of the Er3+-doped YAG as a magnetic coolant is estimated to extend down to ∼100 mK, which is lower than those with nonsubstituted garnets such as gallium–gadolinium–garnet used in the range ∼4.2–15 K. With a doping level of 30%, we estimate that ∼6 kg of Er3+-doped YAG exhibits the same cooling performance at 60 mK as the 916 g of ferric–ammonium–alum salt used for the x-ray spectrometer (microcalorimeter detectors) on the Astro-E satellite.

Study of sensitization of Cr4+ ions by Yb3+ ions in yttrium-aluminum garnet crystals

The possibility of using ytterbium ions as sensitizers of luminescence of tetravalent chromium in yttrium-aluminum garnet single crystals is studied. It is found that average values of the microparameters of electronic energy transfer between ytterbium and chromium ions are CDA=7.05×10−36 cm6/s and CDD=4.55×10−43 cm6/s.

High luminance YAG single crystal phosphor screen

Epitaxial phosphors of Ce,Tb:YAG containing Lu 3+ and Ga 3+ on YAG (yttrium aluminum garnet) single crystal faceplates have properties which are suitable for projection CRTs. The external efficiency of the single-crystal YAG screen is relatively low. Truncated cone geometry can increase the external efficiency by a factor of 5.2, if such a shape can be formed in the phosphor layer. An anisotropy etching process of YAG has been developed for the fabrication of a YAG luminescent screen with truncated square mesas. A large fraction of the cathodoluminescent can be directed through the untextured front surface of the crystal. An improvement in phosphor efficiency by a factor of 2.3 has been achieved.

Effect of divalent cations on the synthesis of citrate-gel-derived lanthanum hexaluminate powders and films

Low-temperature synthesis of hexaluminate phases for fiber–matrix interphases in ceramic–matrix composites is necessary to minimize processing temperatures to prevent fiber-strength degradation. Citrate-gel-derived lanthanum hexaluminate was synthesized using divalent transition-metal cations to stabilize the magnetoplumbite structure. Pure, undoped LaAl11O18was obtained in 1 h only at temperatures >1500 °C after the formation and subsequent consumption of the intermediate perovskite, LaAlO3, which first appeared at ∼1150 °C. Powders of LaMAl11O19, where M = Mg, Mn, Fe, Co, Ni, Cu, and Zn, were prepared at much lower temperatures. Highly crystalline, phase-pure powders of LaMnAl11O19and LaCuAl11O19were obtained at 1000 °C in 1 h directly from the amorphous powder without the formation of the intermediate perovskite. All other compositions could be obtained at 1100 °C in 1 h except for the nominal LaNiAl11O19, which formed primarily LaAlO3, NiAl2O4, and Al2O3. Powders containing dopants all had similar grain sizes and morphologies at 1200 and 1500 °C; the grain size of powders with dopants was significantly greater than that of pure LaAl11O18powder. The introduction of a second charge-compensating quadrivalent dopant for excess divalent cation did not greatly influence synthesis or grain growth below the eutectic temperature but did seem to enhance the [0001] texture of films on single-crystal yttrium-aluminum garnet substrates compared with singly doped films.

Convective heat transfer in the melt in Czochralski growth of garnet-structure single crystals

The results of investigating the hydrodynamic processes in the melt during garnet-structure Czochralski crystal-growing (yttrium-aluminum garnet single crystals 25–40 mm in diameter) using an induction heater are presented. A mathematical model of the hydrodynamics and heat transfer in the melt based on the two-dimensional Navier-Stokes equations in the Boussinesq approximation is proposed. The effect of the convective heat transfer on the shape of the crystallization front, which is an important characteristic of the quality of single crystals, is determined in terms of the rotation velocity of the crystal and the induction heating parameters.

The fracture behavior of single-crystal Y3Al5O12

Abstract The fracture behavior of single-crystal yttrium aluminum garnet in ambient air has been studied as a function of temperature for two different crystallographic orientations. A Chevron notch specimen geometry was used so that stable crack propagation could be achieved. It was found that the room temperature fracture toughness, K Ic = 1.8 MPa m 1 2 , increased to K Ic = 4.0 MPa m 1 2 at 1600°C. At 1750°C, samples did not fracture even when the load was brought to an equivalent stress intensity factor of 11.3 MPA m 1 2 . The fracture toughness was found not to depend significantly on crystallographic orientation. The increase in toughness with temperature was attributed to the activation of slip systems, which were evaluated by trace analysis. It was found that the degree of plasticity is extremely sensitive to the strain rate. Finally, periodic striations were observed on the fracture surfaces which are believed to be related to periodic chemical or structural fluctuations which arise during the Czochralski growth process.

Computer simulation of light emission by high-energy electrons in YAG single crystals

Elastic and inelastic multiple scattering in a thin yttrium aluminum garnet (YAG) single crystal attached on a glass plate has been calculated by a Monte Carlo method. The simulation has been made for several accelerating voltages ranging from 100 kV to 1 MV. Nearly all of the energy of 100–200 keV electrons is absorbed in the YAG crystal but high-energy electrons penetrate the crystal and are then scattered in the base glass. The absorbed energy in YAG crystal produces the light pulse which is converted into an electric pulse by a photomultiplier and amplifiers. The distribution of the pulse height and the mean pulse height are calculated from the total light emission by individual electrons. Some of the backscattered electrons stimulate the YAG crystal but the scintillation light of this contribution is not large. The spatial distribution of the electron beam spread in the crystal is also determined. The comparison of the calculational results with experiments shows that the simulation gives a good representation of scintillator properties.

Preparation of single-crystal Y3Al5O12 thin film by metalorganic chemical vapor deposition

Single-crystal yttrium aluminum garnet (Y3Al5O12 or YAG) thin films have been grown on (111) Gd3Ga5O12 substrate by metalorganic chemical vapor deposition. X-ray diffraction, selective-area electron diffraction, and transmission electron microscopy were used to confirm the single-crystal nature of [111]-oriented single-crystal Y3Al5O12 thin films. We determined that the epitaxial relationship between the film and the substrate is (111)[11̄0]Y3Al5O12//(111)[11̄0]Gd3Ga5O12.

Creep Mechanism of Polycrystalline Yttrium Aluminum Garnet

The high‐temperature deformation behavior of a fine‐grained polycrystalline yttrium aluminum garnet (YAG) was studied in the temperature range of 1400° to 1610°C using constant strain rate compression tests under strain rates ranging from 10−5/s to 10−3/s. The stress exponent of the creep rate, the activation energy in comparison with that for single‐crystal YAG, and the grain size dependence suggest that Nabarro–Herring creep rate limited by the bulk diffusion of one of the cations (Y or Al) is the operative mechanism.

Yag Single Crystals for Scintillators of Stem

Although powdered phosphor screens are usually used for scintillators of STEM, it has been found that the phosphor screen of appropriate thickness should be used depending on the accelerating voltage, in order to keep high detective quantum efficiency. 1 It has been also found that the variation in sensitivity, due to granularity of phosphor screens, makes the measurement of fine electron probe difficult and that the sensitivity reduces with electron irradiation specially at high voltages.In order to find out a preferable scintillator for STEM, single crystals of YAG (yttrium aluminum garnet), which are used for detecting secondary and backscattered electrons in SEM were investigated and compared with powdered phosphor screens, at the accelerating voltages of 100kV and 1 MV. A conventional electron detection system, consisting of scintillator, light guide and PMT (Hamamatsu Photonics R268) was used for measurements. Scintillators used are YAG single crystals of 1.0 to 3.2mm thicknesses (with surfaces matted for good interface to the light guide) and of 0.8mm thickness (with polished surface), and powdered P-46 phosphor screens of 0.07mm and 1.0mm thicknesses for 100kV and 1MV, respectively. Surfaces on electron-incidence side of all scintillators are coated with reflecting layers.

Defect-property correlations in garnet crystals. V. Energy transfer in luminescent yttrium aluminum–yttrium iron garnet solid solutions

The absorption and luminescence spectra of yttrium aluminum garnet single crystals doped with iron have been studied. At low levels of iron doping, the intensity of the defect photoluminesence at 320 nm decreases with increasing iron concentration, as a result of energy transfer to Fe+3 ions which luminesce at 800 nm. Further increases in iron concentrations eliminate luminescence at all wavelengths. Differences between the absorption bands of iron in octahedral and tetrahedral sites, and inter-ion charge transfer processes are discussed.

Anisotropic broadening of the linewidth in the EPR spectra of Cr3+ ions in various doped yttrium aluminum garnet single crystals.

The linewidth in the EPR spectra of ${\mathrm{Cr}}^{3+}$ ions in yttrium aluminum garnet single crystals is investigated at room temperature, and an anisotropic line broadening is observed. The characteristics of broadening resulting from the mosaic effect, internal strains, and fluctuations in the crystalline field parameter CFP are analyzed. A reasonable fit to experimental data is obtained using the hybrid model wherein the mosaic effect, internal strains, and fluctuations in CFP were considered, and the results indicate that the internal strains and fluctuations in CFP are the dominant contributions to linewidth, and the mosaic effect is negligible. The effect of the internal strains caused by impurities in the crystal on linewidth is discussed.