Yttrium Silicate Research Articles

Οxygen tracer diffusion in single crystalline yttrium silicate

Low resistance of C/C-SiC composites against oxidation at high temperatures created the need to develop materials that could be used as oxygen protective layers. Yttrium silicate (Y2SiO5) has been proposed as an excellent candidate for this purpose. In order to investigate whether Y2SiO5 is a sufficient barrier against oxygen, the transport parameters of oxygen in this material should be determined. In the present study, 18O2 tracer diffusion experiments have been conducted in the temperature range between 1000°C and 1300°C. Secondary ion mass spectrometry (SIMS) was used to determine tracer diffusivities D⁎ and oxygen incorporation rates k⁎ in undoped and in 0.3at.% praseodymium doped yttrium silicate. Activation enthalpies EA(D⁎) and EA(k⁎) were determined. An indication of anisotropic oxygen diffusion in yttrium silicate has been found.

Theoretical Predictions on Elastic Stiffness and Intrinsic Thermal Conductivities of Yttrium Silicates

Yttrium silicates are promising candidates for environmental and thermal barrier coatings owing to their excellent high‐temperature performances. Previous works have experimentally attested to their low thermal conductivity, nevertheless, the experimental data were significantly affected by measurement inaccuracy and the existence of defects such as point defects, dislocations, grain boundaries, and pores in measured samples. In this study, the temperature dependences of intrinsic lattice thermal conductivities of γ‐Y2Si2O7 and Y2SiO5 are predicted based on the first‐principles calculations of crystal structure, elastic moduli, Debye temperature, and Grüneisen constant. Both silicates display very low thermal conductivities over the range of 300–2000 K; and in addition, Y2SiO5 exhibits relatively lower thermal conductivity than γ‐Y2Si2O7. We also show certain discrepancies between experimental and theoretical thermal conductivities, and it strengthens the important role of theoretical prediction of intrinsic lattice thermal conductivities of promising materials.

Production and quality control of radioactive yttrium microspheres for medical applications

In this paper, a method for production of yttrium silicate microspheres is reported. Yttrium silicate microspheres with approximate sizes of 20–50µm were obtained when an aqueous solution of Y(NO3)3 was added to tetraethyl orthosilicate (TEOS) and was pumped into silicone oil under constant stirring. The shapes of the particles produced by the proposed method were regular and nearly spherical. The spherical shapes, composition and element distribution were investigated by scanning electron microscopy (SEM), carbon/sulfur analysis and SEM/EDS mapping analysis. Paper chromatography was used to identify radiochemical impurities in the radioactive microspheres. The radionuclide purity was determined using a gamma spectrometry system and an ultra-low-level liquid scintillation spectrometer.The results indicated that the proposed silicone oil spheroidization method is suitable for the production of yttrium silicate microspheres.

Synthesis of yttrium silicates by annealing r.f. sputtered glassy coatings

In this work, the composition evolution of yttrium silicate coatings was investigated during r.f. sputtering from a γ-Y2Si2O7 compound target, focusing on the effect of Ar pressure, target-substrate distance and r.f. power. By controlling the number of collisions between Ar gas and target species, glassy coatings with the stoichiometry of Y2SiO5 and Y2Si2O7 were obtained. Finally, the formation of X2-Y2SiO5 on SiC substrates was observed after annealing the glassy coatings in air at 1250 °C. After annealing at 1550 °C γ-Y2Si2O7 crystallized with high quality and grain boundaries appeared to be crack-free.

Rapid microwave preparation of cerium-substituted sodium yttrium silicate phosphors for solid state white lighting

The preparation of two cerium-substituted sodium yttrium silicate phosphors, NaY9Si6O26:Ce3+ and Na3YSi2O7:Ce3+, was achieved using rapid heating via a modified microwave-assisted preparation technique. Activated carbon is used as a microwave susceptor allowing reaction temperatures greater than 1000 °C to be reached in minutes while producing the reducing atmosphere necessary to achieve Ce3+. Because this method immediately heats the sample and does not employ a H2/N2 gas mixture to maintain Ce3+ in its reduced state, volatilization of starting materials is limited, allowing the products to be prepared from a stoichiometric mixture of starting materials. The samples prepared were characterized using synchrotron X-ray powder diffraction and fluorescence spectroscopy. Photoluminescence measurements indicate NaY9Si6O26:Ce3+ contains a broad excitation band in the UV ranging between 280 nm and 330 nm that produces a blue emission between 350 nm and 500 nm with a 10% quantum yield. Na3YSi2O7:Ce3+ has a strong absorption band at 350 nm and also emits in the blue (375 nm–500 nm) with a quantum yield of 28%.

Evidence of yttrium silicate inclusions in YSZ‐porcelain veneers

This report introduces the discovery of crystalline defects that can form in the porcelain veneering layer when in contact with yttria-stabilized zirconia (YSZ). The focus was on dental prostheses and understanding the defects that form in the YSZ/porcelain system; however the data reported herein may have broader implications toward the use and stability of YSZ-based ceramics in general. Specimens were cut from fully sintered YSZ plates and veneering porcelain was applied (<1 mm thick) to one surface and fired under manufacturer's recommended protocol. Scanning electron microscopy (SEM) with integrated electron dispersive X-ray (EDAX) was used for microstructural and elemental analysis. EDAX, for chemical analysis and transmission electron diffraction (TED) for structural analysis were both performed in the transmission electron microscope (TEM). Additionally, in order to spatially resolve Y-rich precipitates, micro-CT scans were conducted at varying depths within the porcelain veneer. Local EDAX (SEM) was performed in the regions of visible inclusions and showed significant increases in yttrium concentration. TEM specimens also showed apparent inclusions in the porcelain and selected area electron diffraction was performed on these regions and found the inclusions to be crystalline and identified as either yttrium-silicate (Y2 SiO5 ) or yttrium-disilicate (Y2 Si2 O7 ). Micro-CT data showed that yttrium-silicate precipitates were distributed throughout the thickness of the porcelain veneer. Future studies are needed to determine whether many of the premature failures associated with this materials system may be the result of crystalline flaws that form as a result of high temperature yttrium diffusion near the surfaces of YSZ.

EBSD and EDX Analyses of a Multiphase Glass-Ceramic Obtained by Crystallizing an Yttrium Aluminosilicate Glass

A glass with the mol % composition 23.82 Y2O3·39.82 Al2O3·28.50 SiO2·2.91AlF3·4.95 ZrO2 doped with 2 mol % CeF3 is crystallized at 1250 °C for 20 h. The crystallized samples are studied using X-ray diffraction (XRD), the SEM-based methods EBSD and EDX, as well as fluorescence microscopy. Six crystalline phases are detected in the residual glass including alumina, YAG, Y-stabilized zirconia (YSZ), and three different yttrium silicates of the composition Y2Si2O7. Chemistry-assisted indexing (ChI) is successfully applied to separate YAG and YSZ in EBSD-scans. YAG displays polygon as well as dendritic growth. Some crystals show both mechanisms at opposite ends, indicating that the growth mechanism is influenced by the chemistry of the surrounding glass matrix.

Crystal growth and spectroscopic properties of praseodymium and cerium co-doped Y2SiO5

Praseodymium and cerium co-doped yttrium silicate (Pr,Ce:YSO) crystal with high optical quality was grown by the Czochralski method. The segregation coefficient of Pr3+ and Ce3+ is measured to be 0.547 and 0.136, respectively. The polarized absorption bands centered at 268nm, 299nm and 356nm were assigned to 4f (2F5/2)→5d inter-configuration transitions of Ce3+, while bands centered at 220nm and 250nm were assigned to 4fn–4fn−15d transitions of Pr3+. The VUV–UV excitation spectra for monitoring Pr3+ emissions centered at 610nm (1D2→3H4) and 489nm (3P0→3H4), as well as Ce3+ emission centered at 450nm were recorded at T=8K in the range of 100nm–320nm. Polarized excitation spectra for Ce3+ emission centered at 424nm and Pr3+ emission centered at 611nm were recorded at T=300K. Polarized emission spectra were obtained upon excitation at 365nm and 456nm at T=300K. Lifetimes were estimated to be 170μs for Pr3+ (1D2), 3μs for Pr3+ (3P0) and 39ns for Ce3+ (5d1).

Luminescence and Energy Transfer in Europium and Bismuth Codoped Trisodium Yttrium Silicates

Bi3+ and Eu3+ doped Na3 YSi2 O7 phosphors are prepared with solid state reactions. The luminescence properties of Bi3+ and Eu3+ in Na3YSi2O7 and the energy transfer (ET) from Bi3+ to Eu3+ are investigated in detail. The ET mechanism is also discussed. Bi3+ in Na3 YSi2 O7 emits near-ultraviolet (UV) light in a broad band peaking at 394 nm which can be attributed to the transition from excited states 3P0,1 to ground state 1S0 under the excitation of UV light with a wavelength of 309 nm. Eu3+ in Na3 YSi2 O7 emits orange and red light. The co-doping of Bi3+ into Eu3+ doped Na3YSi2O7 introduces an intense absorption band from 270 to 350 nm due to ET from Bi3+ to Eu3+. The sensitization effect of Bi3+ on Eu3+ makes it possible that Bi3+ and Eu3+ co-doped Na3 YSi2 O7 could be used in AlGaN-based UV light emitting diodes as a red phosphor.

Investigation of Native Point Defects and Nonstoichiometry Mechanisms of Two Yttrium Silicates by First‐Principles Calculations

First‐principles method is used to study the native point defects in Y2SiO5 and Y2Si2O7 silicates. The calculated defect formulation energies show similar native point defect behaviors in Y2SiO5 and Y2Si2O7: the oxygen Frenkel defect is predominant; and it is followed by the cation antisite and Schottky defects. The possible chemical potential range of each constituent is further considered in the calculation of defect formation energy. Oxygen interstitial (Oi) and oxygen vacancy (VO) are the predominant native point defects under O‐rich and O‐poor condition, respectively. In addition, the mechanisms of accommodating composition deviations from stoichiometric Y2SiO5 and Y2Si2O7 are investigated. For Y2SiO5, Y2Si2O7 impurity may appear, together with the defects of SiY antisite, Oi interstitial, and/or VY vacancy when SiO2 is excess; while YSi antisite appears together with Yi interstitial and/or VO vacancy in Y2SiO5 when Y2O3 is excess. For Y2Si2O7, the main process is the formation of SiY antisite accompanied by Oi interstitial and/or VY vacancy when SiO2 is excess; but Y2SiO5 impurity forms, together with YSi antisite, VO vacancy, and/or Yi interstitial in Y2Si2O7 when Y2O3 is excess. We expect that the results are useful to control of processing conditions and further to optimization of performance of the two silicates.

An Efficient, Thermally Stable Cerium-Based Silicate Phosphor for Solid State White Lighting

A novel cerium-substituted, barium yttrium silicate has been identified as an efficient blue-green phosphor for application in solid state lighting. Ba9Y2Si6O24:Ce(3+) was prepared and structurally characterized using synchrotron X-ray powder diffraction. The photoluminescent characterization identified a major peak at 394 nm in the excitation spectrum, making this material viable for near-UV LED excitation. An efficient emission, with a quantum yield of ≈60%, covers a broad portion (430-675 nm) of the visible spectrum, leading to the blue-green color. Concentration quenching occurs when the Ce(3+) content exceeds ≈3 mol %, whereas high temperature photoluminescent measurements show a 25% drop from the room temperature efficiency at 500 K. The emission of this compound can be red-shifted via the solid solution Ba9(Y(1-y)Sc(y))(1.94)Ce(0.06)Si6O24 (y = 0.1, 0.2), allowing for tunable color properties when device integration is considered.

Quantum efficiency of silica-coated rare-earth doped yttrium silicate

The photoluminescent properties of rare earth-activated white-emitting Y2SiO5:Ce,Tb nanocrystalline phosphor prepared by two different methods, pressure-assisted combustion synthesis and sol–gel, were studied. The synthesized phosphor samples were post-annealed at 1373K and 1623K in order to obtain the X1-Y2SiO5 and X2-Y2SiO5 phases, respectively, which were confirmed by X-ray diffraction measurements. Photoluminescence analysis showed the contribution of two blue-emission bands within the 380–450nm region originating from 5d–4f transitions in Ce3+ ions and a well-defined green emission of Tb3+ ions located at 545nm corresponding to 5D4→7F5 electronic transitions. Thereafter, Y2SiO5:Ce,Tb powders were coated with colloidal silica in order to investigate the effect of silica coatings on their luminescent properties. Absolute fluorescence quantum efficiency measurements were carefully performed, which revealed an increase of 12% of efficiency in coated compared with bare-Y2SiO5:Ce,Tb phosphor.

Luminescence Properties of Europium Ions-Doped Yttrium Silicate (Y2SiO5:Eu3+) Nanocrystalline Phosphors: Effect of Eu3+ Ion Concentration and Thermal Annealing

The trivalent europium ions-doped yttrium silicate (Y2SiO5:Eu3+) nanocrystalline phosphors were synthesized via a sol-gel method, followed by post thermal annealing. The effects of thermal annealing temperature and doping concentration on the structural and luminescent properties of Y2SiO5:Eu3+ nanocrystalline phosphors were systematically investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and photoluminescence measurements. The nanocrystalline phosphors with a high crystallinity were obtained at an annealing temperature of 1300 degrees C. The luminescent spectra were affected strongly by the Eu3+ ion concentration and annealing temperature. The Eu3+ ion concentration was optimized at 5 mol%, exhibiting excellent red emission (-612 nm) corresponding to the 5D0 --> 7F2 transition of Eu3+ ions at the excitation wavelengths of 262 and 396 nm. For the optimized Y2SiO5:Eu3+ nanocrystalline phosphors, the lifetimes were also estimated from the decay curves under the ultraviolet excitations.

Discovery of novel PDE10 inhibitors by a robust homogeneous screening assay

To develop a homogeneous assay for high-throughput screening (HTS) of inhibitors of phosphodiesterase 10 (PDE10). Purified human PDE10 enzyme derived from E coli, [(3)H]-cAMP and yttrium silicate microbeads were used to develop an HTS assay based on the scintillation proximity assay (SPA) technology. This method was applied to a large-scale screening campaign against a diverse compound library and subsequent confirmation studies. Preliminary structure-activity relationship (SAR) studies were initiated through limited structural modifications of the hits. The IC50 value of the control compound (papaverine) assessed with the SPA approach was comparable and consistent with that reported in the literature. Signal to background (S/B) ratio and Z' factor of the assay system were evaluated to be 5.24 and 0.71, respectively. In an HTS campaign of 71 360 synthetic and natural compounds, 67 hits displayed reproducible PDE10 inhibition, of which, 8 were chosen as the scaffold for structural modifications and subsequent SAR analysis. The homogeneous PDE10 SPA assay is an efficient and robust tool to screen potential PDE10 inhibitors. Preliminary SAR studies suggest that potent PDE10 inhibitors could be identified and developed through this strategy.

Photoluminescence and energy transfer in cerium and terbium co-doped trisodium yttrium silicates

Abstract Ce 3+ and Tb 3+ doped Na 3 YSi 2 O 7 were synthesized with solid state reactions. The luminescence properties of Ce 3+ and Tb 3+ in Na 3 YSi 2 O 7 and energy transfer (ET) from Ce 3+ to Tb 3+ were investigated in detail. Measurements of the luminescence lifetimes of Ce 3+ and calculation of ET efficiency prove that ET from Ce 3+ to Tb 3+ is efficient. Optimal composition was experimentally ascertained to be 10% and 20% mol for Ce 3+ and Tb 3+ , respectively. The integrated green emission intensity of Na 3 YSi 2 O 7 : 10%Ce 3+ , 20%Tb 3+ is 93.2% of commercial green phosphor LaPO 4 : 10%Ce 3+ , 20%Tb 3+ . It proves that Na 3 YSi 2 O 7 : Ce 3+ , Tb 3+ is a promising green phosphor for AlGaN-based ultraviolet light-emitting diodes.

The Prospect of Y2SiO5-Based Materials as Protective Layer in Environmental Barrier Coatings

Bulk yttrium monosilicate (Y2SiO5) possesses interesting properties, such as low thermal expansion coefficient and stability in water vapor atmospheres, which make it a promising protective layer for SiC-based composites, intended for the hottest parts in the future gas turbines. Because protective layers are commonly applied by thermal spraying techniques, it is important to analyze the changes in structure and properties that these methods may produce in yttrium silicate coatings. In this work, two SiO2-Y2O3 compositions were flame sprayed in the form of coatings and beads. In parallel, the beads were spark plasma sintered at relatively low temperature to obtain partially amorphous bulk specimens that are used as model bulk material. The thermal aging—air and water vapor atmosphere—caused extensive nucleation of Y2SiO5 and Y2Si2O7 in both the bulk and coating. The rich water vapor condition caused the selective volatilization of SiO2 from Y2Si2O7 at the specimen surface leaving a very characteristic micro-ridged Y2SiO5 zones—either in coatings or sintered bodies. An important increase in the thermal conductivity of the aged materials was measured. The results of this work may be used as a reference body for the production of Y2SiO5 coatings using thermal spraying techniques.

Yttrium Bearing Silicon Carbide Matrices for Robust Ceramic Composites

Application of SiC‐based ceramic matrix composites (CMCs) in combustion environments demands the use of an environmental barrier coating (EBC) to prevent volatilization of the protective SiO2 scale in flowing water vapor. The EBC only provides protection while present on the surface; cracking and spallation of the coating leaves the underlying SiC vulnerable to the oxidation–volatilization processes. A robust matrix material chemically tailored to regrow a yttrium silicate scale in the event of EBC loss has been developed by incorporating yttrium bearing species including YB2, Y2O3, and Y5Si3 into the SiC. During oxidation a borosilicate glass helps seal cracks while Y2O3 and SiO2 react to form Y2Si2O7 for environmental protection. Candidate compositions were oxidized for 10 min to 100 h at 1400°C and for 24 h at 1500°C to understand the scale growth. The prospects for effectively applying this approach in CMCs are discussed.

Microstructure and properties of highly porous Y2SiO5 ceramics produced by a new water-based freeze casting

This study introduced a new structural/functional porous yttrium silicate (Y2SiO5) ceramic, which was prepared using a water-based freeze-casting technique. Moreover, the effects of sintering temperature on the microstructure, compressive strength, thermal conductivity, and dielectric properties of porous Y2SiO5 ceramics were investigated. The microstructure consisting of 3-D interconnected pores were created using the freezing process, and the formation mechanism was discussed. The microstructure observation indicated an increase in the sintering temperature from 1250°C to 1450°C and a decrease in porosity from 71% to 62%. In addition, the pore channel size decreased from 50μm to 25μm, but the average grain size increased from 1.0μm to 3.0μm. Consequently, the compressive strength, thermal conductivity, and dielectric constant of the porous Y2SiO5 ceramics increased from 3.34MPa to 16.51MPa, from 0.07W/mK to 0.22W/mK, and from 1.89 to 2.22, respectively. Porous Y2SiO5 ceramics with such properties can be applied to ultra-high temperature broadband radome structures and thermal insulators.

Low temperature dielectric dispersion and electrical conductivity studies on Fe2O3 mixed lithium yttrium silicate glasses

Lithium yttrium silicate glasses mixed with different concentrations of Fe2O3 of the composition (40−x) Li2O–10Y2O3–50SiO2: x Fe2O3, with x=0.3, 0.5, 0.8, 1.0, 1.2 and 1.5 (all in mol%) were synthesized. Electrical and dielectric properties including dielectric constant, ε′(ω), loss, tan δ, ac conductivity, σac, impedance spectra as well as electric moduli, M(ω), over a wide continuous frequency range of 40Hz to 106Hz and in the low temperature range 100 to 360K were measured as a function of the concentration of Fe2O3. The dc conductivity is also evaluated in the temperature range 100 … 360K. The temperature and frequency dispersions of dielectric constant as well as dielectric loss have been analyzed using space charge polarization model. The ac and dc conductivities have exhibited increasing trend with increasing Fe2O3 content beyond 0.5mol%, whereas the activation energy for the conductivity demonstrated decreasing tendency in this dopant concentration range. Both quantum mechanical tunneling (QMT) and correlated barrier hopping models (CBH) were used for clarification of ac conductivity origin and the corresponding analysis has indicated that CBH model is more appropriate for this glass system. For the better understanding of relaxation dynamics of the electrical properties we have drawn the scaling plots for ac conductivity and also electric moduli. The plots indicated that the relaxation dynamics is independent on temperature but depends on concentration of Fe2O3. The dc conductivity is analyzed using small polaron hoping model. The increase of conductivity with the concentration of Fe2O3 beyond 0.5mol% is explained in terms of variations in the redox ratio of iron ions in the glass network. The results were further analyzed quantitatively with the support of experimental data from IR, optical absorption and ESR spectral studies. The overall analysis has indicated that Li2O–Y2O3–SiO2 glasses containing more than 0.5mol% of Fe2O3 are more suitable for achieving good electrical conductivity in these glasses.

Thermally Stimulated Luminescence in Ce-Doped Yttrium Oxyorthosilicate

We analyze thermally stimulated luminescence (TSL) above room temperature in Ce-doped Y2SiO5 oxyorthosilicate single crystal. We perform detailed TSL glow peak analysis based on the initial rise technique to evaluate trap depths and other characteristics associated with TSL peaks. The tunneling process previously proposed to be at work in recombination mechanism of Ce-doped lutetium oxyothosilicate was not confirmed in presently studied isostructural yttrium silicate. The charge carrier transfer between traps and Ce recombination centers is rather accomplished via conduction band.