High-temperature creep of yttrium-aluminium garnet single crystals

High-temperature creep in single crystals of Y₃Al₅O₁₂ (YAG) was studied by constant strainrate compression tests. The creep resistance of YAG is very high: a stress of ~ 300 MPa is needed to deform at a strain rate of 10^–6 (s^–1) at a temperature as high as 1900 K (~0.84 T _m, (melting temperature)). YAG deforms using the 111 {1¯10} slip systems following a power law with stress exponent n ~ 3 and activation energy E* ~ 720 kJ mol^–1. However, a small dependence of n on temperature and of E* on stress was observed. This stress-dependence of activation energy combined with the observed dislocation microstructures suggests that the high creep resistance of YAG is due to the difficulty of dislocation glide as opposed to the difficulty of climb. Present dislocation creep data are compared with diffusion creep data and a deformation mechanism map is constructed. Large transition stresses (2–3 GPa for 10 m grain size) are predicted, implying that deformation of most fine-grained YAG will occur by diffusion creep.     

Microhardness and brittle fracture of garnet single crystals

Hardness and fracture toughness were measured using the Vickers microhardness test in the low load range from 25 to 100 g near to the fracture threshold for near-perfect single crystals of garnets. The influence of crystal growth parameters, calcium impurity content and crystallographic orientation of Gd₃Ga₅O₁₂ (GGG) and Ca₃Ga₂Ge₃O₁₂ (CaGeGG) samples was investigated. Fracture starts with radial cracking from indent corners followed by lateral fracture of two distinct modes. The mean hardness of [111] oriented GGG isH=13 GN m^–2, for [111] oriented CaGeGG it is 12 GN m^–2, the average fracture toughness beingK _c=1.2 and 0.8 MN m^–3/2, respectively for the two crystals. Impurity doping slightly increases the strength of the material. Among the investigated crystals (111) faces are the least strong, the (100) face has maximumH andK _c values for CaGeGG. The constraint factor,, and yield stress,Y, were deduced from the measured hardness data, giving=2.2 andY about 7 GN m^–2.     

Slip systems in gadolinium gallium garnet single crystals

Gd₃Ga₅O₁₂ single crystals with compression axes of different orientations have been deformed under creep conditions in air at 1450° C and 1550° C (0.86 T _M and 0.92 T _M, respectively, where T _M is the melting temperature). After a few per cent of permanent strain the deformation substructure has been studied by optical and transmission electron microscopy. Etching and bi-refringence patterns indicate that slip on {110}, {112} and {123} can be activated depending on the orientation of the compression load. Dislocations with a/2 111 Burgers vectors have been observed to glide in {110 planes. They exhibit a segmented aspect suggesting their dissociation out of the glide plane.     

The dislocation-free growth of gadolinium gallium garnet single crystals

It is established that a substantial number of the microscopic strain centres observed in Czochralski-grown gadolinium gallium garnet single crystals are due to dislocations whose propagation is markedly dependent upon the shape of the solid/liquid interface. It is shown that the dislocation density can be minimized by using facets to block dislocation propagation and that crystals substantially free from dislocations can be produced.     

Growth striae in single crystals of gadolinium gallium garnet

Growth striae on (111) planes were examined in core and facet-free low dislocation density boules of gadolinium gallium garnet (GGG). Lang x-ray transmission topography of (111), (21?1?) and (011?) slices revealed a nearly ideal flat growth interface except for a 1–2 mm periphery. Striae were separated by 10–60 μm, of non-uniform width, and discontinous across the full boule diameter. Electron microprobe scans along [111] across the striae demonstrated fluctuations in Gd and Ga concentrations. Our observations are consistent with a proposed growth mechanism of interface attachment by layer motion.     

Luminescence of gadolinium garnet single crystals excited by synchrotron radiation

Single crystals of gadolinium-containing garnets with the compositions Gd₃Ga₅O₁₂, Gd₃(Sc,Ga)₅O₁₂:Cr, and (Gd,Ca)₃(Mg,Zr,Ga)₅O₁₂ were grown using the Czochralski technique, and their luminescence was studied under the conditions of excitation with “white” synchrotron radiation. The observed differences in the luminescence spectra are discussed.     

Interface shape changes during the Czochralski growth of gadolinium gallium garnet single crystals

It is shown that interface shape changes, often induced during the growth of gadolinium gallium garnet single crystals in order to control defects, can occur naturally under specific conditions. Changes in both crystal weight and melt temperature, associated with changes in interface shape, are consistent with a melting-back process arising from modification to the flow patterns in the melt.     

An X-ray diffraction topographic study of single crystals of melt-grown yttrium aluminium garnet

Melt-grown crystals of yttrium aluminium garnet usually exhibit an optically imperfect central core region associated with the formation of facets on the solid/liquid interface during growth. The nature of the optical inhomogeneity of the core region, and the general defect structure of the crystals, was investigated by the technique of X-ray diffraction topography. The core region, which showed an elastic strain effect, contained straight growth striations reflecting the development of facets. Comparison was made between undoped and neodymium-doped crystals, and between crystals grown on <111> and <110> growth axes. Impurity striations were thought to be caused by temperature oscillations in the melt ahead of the growing interface. Dislocations, which were observed in the outer regions of the crystals, were sometimes nucleated at inclusions.     

Electromagnetic showers in oriented garnet crystals

The orientation dependences of the energy release in showers formed in transparent garnet crystals with thicknesses of 23 and 50 mm are measured, and the influence of the orientation of the crystals on the absorption of a shower in an amorphous radiator is determined. Pis’ma Zh. Tekh. Fiz. 24, 60–64 (June 12, 1998)     

Luminescence of bulk and thin-film single crystals of gadolinium gallium garnet excited by UV radiation

A difference between the luminescence spectra of bulk and thin-film single crystals of Gd₃Ga₅O₁₂ gadolinium gallium garnet excited by UV radiation from a deuterium lamp has been studied. The films were grown by liquid phase epitaxy from supercooled melts based on the PbO-B₂O₃ and Bi₂O₃-B₂O₃ solid solution systems.     

On the relationship between emission color and Ce concentration in garnet phosphors

It is known that the emission color of Ce³⁺ doped garnets is strongly redshifted at higher Ce³⁺ concentrations. In this report, we study the cause of this redshift in Lu₃Al₅O₁₂:Ce³⁺ (LuAG:Ce) phosphors. These changes in emission color with Ce³⁺ concentration are mainly attributed to a combination of inhomogenous broadening for Ce³⁺ in LuAG and energy transfer from high energy Ce³⁺ ions to low energy Ce³⁺ ions. Evidence for inhomogenous broadening and energy transfer is given through time resolved measurements. Potential reasons for inhomogenous broadening of Ce³⁺ in these phosphors are also discussed.     

Macroscopic strain in facetted regions of garnet crystals

Morphological studies of Czochralski-grown gadolinium gallium garnet single crystals have revealed facetting characteristics identical to those observed in yttrium aluminium garnet. Accurate lattice parameter measurements made at various points within single crystals of these materials have been used to determine the strains associated with such facets. A possible explanation for the origin of the strain is proposed and the effect of facet imperfections upon the use of garnets in optical and magnetic thin film devices is discussed.     

Effect of inter-ionic interaction on photoluminescence property of yttrium aluminate garnet phosphors

The photoluminescence of Terbium-activated yttrium aluminate with the general formula Y_3-xTb_XAl₅O₁₂was investigated as a function of Tb³⁺concentration. The main attention is focused on the ⁵D₃/⁵D₄fluorescence and its energy transfer behavior. The emission and excitation spectra were measured in terms of Tb³⁺concentration and analyzed in view of the application for plasma display panel (PDP). Also, the diffuse reflectance spectra were measured and analyzed in the range from VUV to UV. As a result, the yttrium aluminate was found to have a broad excitation band extending from VUV to UV range and to show the typical concentration quenching behaviors both for ⁵D₃and ⁵D₄fluorescence. The energy transfer was investigated by analyzing the decay curve of ⁵D₃and ⁵D₄emission based on the multipolar interaction or the exchange interaction. The decay curves of ⁵D₃emission, for which well known cross relaxation has been accepted as a main factor, were analyzed by Inokuti and Hirayama's formula based on the direct quenching scheme. The decay curves of ⁵D₄emission were analyzed by Yokota and Tanimoto's formula based on the diffusion limited quenching scheme.     

Synthesis and optical properties of green to orange tunable garnet phosphors for pcLEDs

Y_3−xLu_xAl₃MgSiO₁₂:Ce³⁺ phosphors were prepared by aqueous sol-gel technique. Samples with 0.25, 0.5, 0.75 and 1 mol-% of Ce³⁺ were fabricated and characterized by powder X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, and temperature dependent luminescence to reveal the thermal quenching. Moreover, luminous efficacies (LE), CIE 1931 color points and quantum efficiencies (QE) were calculated and discussed. XRD patterns confirmed the presence of single phase garnet for all samples independent of Ce³⁺ concentration and Y/Lu ratio. Phosphors showed broad band emission in the range 500-750 nm. The emission maximum shifts from 555 to 585 nm depending on the Ce³⁺ concentration and Y/Lu ratio. Quantum efficiency of the phosphors decreased with rising Ce³⁺ concentration and increased with increasing Lu content.     

On the relationship between emission color and Ce3+ concentration in garnet phosphors

It is known that the emission color of Ce³⁺ doped garnets is strongly redshifted at higher Ce³⁺ concentrations. In this report, we study the cause of this redshift in Lu₃Al₅O₁₂:Ce³⁺ (LuAG:Ce) phosphors. These changes in emission color with Ce³⁺ concentration are mainly attributed to a combination of inhomogenous broadening for Ce³⁺ in LuAG and energy transfer from high energy Ce³⁺ ions to low energy Ce³⁺ ions. Evidence for inhomogenous broadening and energy transfer is given through time resolved measurements. Potential reasons for inhomogenous broadening of Ce³⁺ in these phosphors are also discussed.     

The refractive indices of some garnet crystals at 1.15 μm

Refractive index measurements for Y₃Fe₅O₁₂, (LaY)₃Ga_.33Fe_4.67O₁₂ and Ga₃Gd₅O₁₂ were obtained at 1.152 μm by observing optical waveguiding in epitaxial thin films.