YTTRIUM ALUMINATE CERAMIC FIBERS VIA PRE-CERAMIC POLYMER AND SOL-GEL ROUTES

Yttrium aluminum garnet (YAG - Y₃Al₅O₁₂) fibers have been prepared by dry spinning solutions of yttrium and aluminum carboxylate polymers (precursor route) and by dry spinning aqueous oxide sols (sol-gel route). Fibers from aqueous diphasic gels are prepared by mixing a colloidal alumina sol containing 50-nm hydrous alumina with a colloidal yttria sol containing 10-nm yttrium oxide, using polyvinylpyrrolidone as a spinning aid. Fibers by the precursor route are made from spinnable THF solutions of yttrium isobutyrate and aluminum isobutyrate or from aqueous solutions of polymeric aluminum formate and yttrium acetate.

The isobutyrate materials decompose between 200-400^°C to an amorphous residue. Crystallization occurs abruptly between 875°C and 900^°C, forming the YAG phase directly. The formate-acetate also decomposes to amorphous residues, which form YAG at 900^°C. In the diphasic gel, YAG forms gradually between 1000 and 1200^°C, with intermediate products YAP (YalO₃ perovskite) and/or YAM (Y₄Al₂O₉ monoclinic). At 1500^°C, single phase YAG is obtained as pore-free fibers with 500 nm grains.     

Preparation of polycrystalline YAG/alumina composite fibers and YAG fiber by sol–gel method

Polycrystalline yttrium–aluminum garnet, Y₃Al₅O₁₂ (YAG) fiber and α-alumina and YAG matrix composite fiber were prepared by the sol–gel method. α-Alumina and YAG matrix composite fiber with fine and homogeneous microstructure could be successfully fabricated by interpenetrating YAG in alumina matrix and adding α-alumina of seed particles to fibers. Effect of α-alumina seed particles and YAG on crystallization and microstructure of composite fiber were discussed. The size of alumina matrix of the composite fibers heated at 1600°C for 4 h was below 2 μm. The tensile of strength alumina fiber heat-treated at 1500°C was 0.2 GPa, while that of the composite fiber was 1.1 GPa.     

An IR and XPS spectroscopy assessment of the physico-chemical surface properties of alumina–YAG nanopowders

Well-dispersed nano-crystalline transition alumina suspensions were mixed with yttrium chloride aqueous solutions, with the aim of producing by spray-drying Al₂O₃–Y₃Al₅O₁₂ (YAG) composite powders of increasing YAG vol.%. Two samples were prepared, with different Y content, corresponding to 5 and 20 YAG vol.%, respectively. Both samples were then treated at either 600 or 1150 °C. The obtained powders were characterized by X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infra Red (FT-IR) spectroscopy and compared to three reference samples: commercial nano-crystalline transition alumina, YAG and Y₂O₃. YAG powders were obtained by co-precipitation route whereas Y₂O₃ powders were yielded by spray-drying of a yttrium chloride aqueous solution. Modification of physico-chemical properties of the surface of alumina nanoparticles were assessed by combining XPS and FT-IR spectroscopies. On the basis of the results obtained, a possible model is proposed for the structure of the obtained composites, in which Y basically reacts with more acidic hydroxyls of alumina, by forming Y-rich surface grains, the extension of which depends on the thermal treatment.     

Continuous production of phosphor YAG:Tb nanoparticles by hydrothermal synthesis in supercritical water

Phosphor YAG:Tb ((Y_2.7Tb_0.3)Al₅O₁₂) nano particles were synthesized by a hydrothermal method at supercritical conditions (400 °C and 30 MPa) using a flow reactor. Hydroxide sol solutions formed by stoichiometric aluminum nitrate, yttrium nitrate, terbium nitrate and potassium hydroxide solutions. The relationship between particle size and experimental variables including pH, concentration of coexistent ions and hydroxide sol were investigated. Particles were characterized by XRD, TEM and photo-luminescence measurements. Particle size of YAG:Tb became finer as pH was increased or potassium nitrate concentration of the starting metal salt solution was increased. By removing the coexisting ions (NO₃⁻, K⁺) from the metal salt solution, single phase YAG:Tb particles with 20 nm particle size were obtained. The emission spectra of YAG:Tb particles of 14 nm shows a blue shift.     

Ultra-Small YPO4-YAG:Ce Composite Nanophosphors with a Photoluminescence Quantum Yield Exceeding 50%

Synthesis of high quality colloidal Cerium(III) doped yttrium aluminum garnet (Y₃Al₅O₁₂:Ce³⁺, “YAG:Ce”) nanoparticles (NPs) meeting simultaneously both ultra-small size and high photoluminescence (PL) performance is challenging, as generally a particle size/PL trade-off has been observed for this type of nanomaterials. The glycothermal route is capable to yield ultra-fine crystalline colloidal YAG:Ce nanoparticles with a particle size as small as 10 nm but with quantum yield (QY) no more than 20%. In this paper, the first ultra-small YPO₄-YAG:Ce nanocomposite phosphor particles having an exceptional QY-to-size performance with an QY up to 53% while maintaining the particle size ≈10 nm is reported. The NPs are produced via a phosphoric acid- and extra yttrium acetate-assisted glycothermal synthesis route. Localization of phosphate and extra yttrium entities with respect to cerium centers in the YAG host has been determined by fine structural analysis techniques such as X-ray diffration (XRD), solid state nuclear magnetic resonance (NMR), and high resolution scanning transmission electron microscopy (HR-STEM), and shows distinct YPO₄ and YAG phases. Finally, a correlation between the additive-induced physico-chemical environment change around cerium centers and the increasing PL performance has been suggested based on electron paramagnetic resonance (EPR), X-ray photoelectron spectrometry (XPS) data, and crystallographic simulation studies.     

Interactions between Y2O3–Al mixture studied by solid-state reaction method

Interactions between Y₂O₃–Al mixture studied by solid-state reaction method were investigated in present paper. Interactions between Y₂O₃–Al mixture was characterized by differential thermal and thermogravimetric analyses and X-ray diffraction, Y₂O₃–Al mixture and yttrium aluminum garnet (YAG) powder as final reaction product were characterized by scanning electron microscopy. The results show Al is isolated with Y₂O₃ by aluminum oxide layer in air, and no opportunity of directional reaction between Y₂O₃–Al systems. With temperature increasing to ∼569 °C, aluminum partly turned into transitional aluminas, Y₂O₃ reacts with transitional aluminas instead of aluminum to form yttrium aluminum monoclinic (YAM) and yttrium aluminum perovkite (YAP) phases after calcination at 600 °C, 800 °C separately, and pure YAG powder is obtained after calcination at 1200 °C. From the point of view of reaction temperature, the reaction between Y₂O₃ and transitional aluminas is easier than that of Y₂O₃ and Al or α-Al₂O₃.     

Influence of yttrium dopant on the synthesis of ultrafine AlN powders by CRN route from a sol–gel low temperature combustion precursor

Y-doped ultrafine AlN powders were synthesized by a carbothermal reduction nitridation (CRN) route from precursors of Al₂O₃, C and Y₂O₃ prepared by a sol–gel low temperature combustion technology. The Y dopant reacted with alumina and thus forming yttrium aluminate of AlYO₃, Al₃Y₅O₁₂ and Al₂Y₄O₉, which formed a liquid at about 1400 °C and promoted the transformation of Al₂O₃ to AlN and the growth of AlN particles. Compared with the conventional solid CRN process, Y dopant reduced the synthesis temperature by 150 °C, and Al₂O₃ transformed to AlN completely at 1450 °C. The content of Y dopant had little effect on the synthesis temperature of AlN whereas it influenced the phase of Y compounds in the products. As the Y/Al molar ratio was in the range of 0.007648–0.022944, the particle sizes of Y-doped AlN powders synthesized at 1450 °C were 150–300 nm.     

Synthesis and sintering of submicron Nd:YAG particles prepared from carbonate precursors

Using high-temperature X-ray diffraction, differential scanning calorimetry, and electron microscopy, we have studied the formation of yttrium aluminates and Nd:YAG (YAG) activated garnet nanoparticles during the thermal decomposition of a poorly crystallized carbonate precursor prepared in the NH₄Al(OH)₂CO₃–(Y,Nd)(ОН)CO₃ nanosystem and the development of the morphological structure of powders during heating to a temperature of 1350°C. The results demonstrate that heat treatment in the temperature range 850–950°C leads to the formation of metastable nonstoichiometric YAlO₃ with a garnet-like structure, which reacts with Al₂O₃ at a temperature of 1000°C to form YAG. The cubic cell parameter a and X-ray density of YAG crystals with the composition Y_2.97Nd_0.03Al₅O₁₂ synthesized at 1200°C are 1.2009 nm and 4.565 g/cm³, respectively, and the average particle size is 108 nm. Using carbonate route, we prepared transparent Nd:YAG ceramics with a relative density of 99.7%, X-ray density of 4.562 g/cm³, and crystallite size in the range 1–7 μm.     

Preparation of continuous alumina gel fibres by aqueous sol–gel process

Continuous alumina gel fibres were prepared by sol–gel method. The spinning sol was prepared by mixing aluminum nitrate, lactic acid and polyvinylpyrrolidone with a mass ratio of 10:3:1· 5. Thermogravimetry–differential scanning calorimetry (TG–DSC), Fourier transform infrared (FTIR) spectra, X-ray diffraction (XRD), and scanning electron microscopy (SEM) were used to characterize the properties of the gel and ceramic fibres. The Al₂O₃ fibres with a uniform diameter can be obtained by sintering gel fibres at 1200 °C.     

YAG and Y2O3 laser ceramics from nonagglomerated nanopowders

Undoped and Nd- or Yb-doped laser-grade yttrium aluminum garnet and (Y,La)₂O₃ ceramics with a transmittance above 80% in the 1-μm lasing region have been prepared by solid-state reactions using nonagglomerated Y₂O₃ and Al₂O₃ nanopowders. The Y₂O₃ nanopowders were prepared via laser evaporation, chemical precipitation from urea solutions, and grinding of commercially available Y₂O₃ in a purposedesigned laboratory-scale attritor at stirrer rotation rates of up to 1500 rpm. The YAG ceramics were prepared using commercially available Al₂O₃. After grinding, all of the powders had a particle size on the order of a hundred nanometers. Green compacts produced from the nanopowders were sintered in a vacuum furnace between 1615 and 1750°C to give highly transparent ceramic samples.     

Sol–gel synthesis of 2D and 3D nanostructured YSZ:Yb<Superscript>3+</Superscript> ceramics

This paper presents results of a detailed study of fundamental aspects of the formation of 2D and 3D nanostructured YSZ:Yb³⁺ ceramics with a cubic structure through a key synthesis step in aqueous solutions of zirconium-containing hydroxy nanoparticles (1–2 nm) modified by Y³⁺ and Yb³⁺ ions, with the use of a sol–gel method and subsequent calcination of the resultant xerogels at temperatures above 350°C. As starting chemicals for the synthesis of ceramic powders, we used zirconyl, yttrium, and ytterbium nitrates and chlorides and aqueous ammonia. Using mixed solutions of these salts and a procedure developed by us, we synthesized sols, gels, and xerogels. To examine the effect of temperature on solid-state transformations, the xerogels were calcined according to a predetermined program in a muffle furnace at temperatures in the range from 350 to 1350°C (rarely, up to 1650°C). We focused primarily on ceramic powders close in composition to 0.86ZrO₂ · 0.10Y₂O₃ · 0.04Yb₂O₃. The ceramics were characterized by high-resolution transmission electron microscopy, electron microdiffraction, electronic diffuse reflectance spectroscopy, energy dispersive X-ray microanalysis, and X-ray fluorescence analysis.     

Microstructure of sol–gel synthesized Al2O3–ZrO2(Y2O3) nano-composites studied by transmission electron microscopy

The Al₂O₃–ZrO₂(Y₂O₃) composite powder was synthesized through a sol–gel process using aluminum sec-butoxide and zirconium butoxide as precursors. The as-received powders in an amorphous phase were crystallized with c-ZrO₂ at around 980 °C. As the calcination temperature increased, the c-ZrO₂ crystalline phase was transformed to t-ZrO₂ at about 1200 °C. However, the Al₂O₃ phase in the Al₂O₃–ZrO₂(Y₂O₃) composite powders still existed in an amorphous phase up to 1050 °C. In the sintered body using the calcined powders at 400 °C, the Al₂O₃ phase was crystallized in an α-phase at 1200 °C during the sintering for 2 h. Using the sol–gel Al₂O₃–ZrO₂(Y₂O₃) powder, a typical nano-composite having a nano-crystalline phase (less than 20 nm) can be successfully obtained by a pressureless-sintering process even at 1200 °C for 2 h.Using the sol–gel Al₂O₃–ZrO₂(Y₂O₃) powder, a typical nano-composite having a nano-crystalline phase (less than 20 nm) can be successfully obtained by a pressureless-sintering process even at 1200 °C for 2 h. The values of relative density and Vickers hardness were comparatively high value with about 96.2% and 1100 Hv, respectively, even though it was made at low temperature. In the composite sintered at 1400 °C, the hardness value was saturated with 1570 Hv and the values of fracture toughness were almost same with about 6 MPa m^1/2.     

Nonhydrolytic sol-gel synthesis and characterization of YAG

Yttrium–aluminum oxides are interesting compounds, which are widely used as hosts for lasers and phosphors due to their stable physical and chemical properties. The manufacture of YAG has been investigated thoroughly. YAG powders are traditionally synthesized through the reaction of aluminum and yttrium powders at high temperatures. The work reported here involved an investigation into the preparation of YAG by a nonhydrolytic sol-gel route and the influence of heating time at low temperatures to obtain YAG from inorganic precursors (yttrium and aluminum chloride). AlCl₃, YCl₃ and ethanol were reflux reacted under an argon atmosphere. Europium III chloride was added as a structural probe. The powder was treated at 800 °C for 1, 2, 4, 8 and 16 h. The YAG structure was analyzed by X-ray diffraction (XRD), nuclear magnetic resonance (NMR), thermal analysis (TA) and photoluminescence (PL). The XRD revealed only peaks corresponding to the YAG phase. PL data showed that the YAG phase was formed in 2 min with samples pretreated at 50 °C. For the samples pretreated at 800 °C, the YAG phase appeared in 30 s. The excitation spectra presented a maximum of 394 nm corresponding to the ⁵L₆ level, while the emission spectra of Eu III ions showed characteristic transition bands arising from the ⁵D₀ → ⁷F _J (J = 1, 2, 3, 4) manifolds at maximum excitation. The magnetic dipole ⁵D₀ → ⁷F₁ transition exhibited greater intensity than the electric dipole ⁵D₀ → ⁷F₂ transition. This methodology proved efficient for obtaining YAG phase.     

Effect of BaF2 powder addition on the synthesis of YAG phosphor by mechanical method

Here, we report on the effect of BaF₂ powder addition on the mechanical synthesis of Ce³⁺-doped Y₃Al₅O₁₂ (Y_2.97Al₅O₁₂:Ce_0.03³⁺, YAG:Ce³⁺) phosphors for white light emitting diodes. The YAG phosphors were synthesized by the mechanical method using an attrition-type mill. When BaF₂ was added at 6 wt% to the raw powder materials and milled, the synthesis of YAG:Ce³⁺ was favorably achieved at the vessel temperature of 255 °C, which was about 1200 °C lower than the YAG phosphor synthesis temperature by solid-state reaction. The synthesized YAG:Ce³⁺ phosphor revealed the maximum internal quantum yield of 57%.     

Flame aerosol deposition and annealing of Y3Al5O12:Tb phosphor coatings

Sub-micrometer-sized powders of Y₃Al₅O₁₂:Tb phosphor (d_SEM = 320 nm) were prepared by flame-assisted spray pyrolysis of aqueous precursors in a premixed propane/air flame and in situ deposited onto quartz substrates. Phosphor screens with densities of up to 0.7 mg cm⁻² could be produced within 20 min. As-deposited coatings were amorphous and required a thermal post-treatment. After annealing in an oven for 2 h (T ≥ 900 °C), the yttrium aluminum garnet phase (YAG:Tb) was obtained. Alternatively, the phosphor coatings were treated by an impinging flame in the same setup used for the deposition. Quasi-amorphous Y₃Al₅O₁₂:Tb coatings demonstrated bright green photoluminescence upon flame annealing at T ≈ 1100 °C for just several minutes and could outperform YAG:Tb when excited in the wavelength ranges 205–220 nm and 230–260 nm. For example, brightness of emission from the quasi-amorphous coatings was up to five times higher than that of the fully crystalline YAG:Tb phosphor at a technically important wavelength of 254 nm.     

Interaction between liquid aluminum and yttria substrate: microstructure characterization and thermodynamic considerations

The systematic microstructure and thermodynamic studies of the reactions between liquid aluminum and dense polycrystalline yttria (Y₂O₃) substrates at 1,273 K are reported in this article. The microstructure observations showed the presence of three reaction product zones extending ~1 mm into the oxide substrate of typical C4 (Co-Continuous-Ceramic-Composites) structure. The first zone starting from the drop side was composed of fine crystalline precipitates of the Al₅Y₃O₁₂ (YAG) phase dispersed in the Al₃Y matrix. The second zone was built of larger AlYO₃ (YAP) crystals. The third zone formed elongated oxide precipitates (YAP) surrounded by the Al₂Y intermetallic channels. The thermodynamic calculation indicated that, depending on the amount of the yttrium dissolved in aluminum, the YAG (up to 5 at.% Y), YAP (5–13 at.% Y), or Al₂Y for higher content of yttrium might form at 1,273 K, while the Al₃Y phase might appear during cooling.     

Ce-doped Al2O3−YAG eutectic and its application for white LEDs

Ce-doped Al₂O₃−YAG (Y₃Al₅O₁₂, yttrium aluminum garnet) eutectic, a resin-free phosphor for white light emitting diodes (WLEDs), was successfully grown by the Czochralski method. X-ray diffraction and scanning electron microscopy show that this material has a typical eutectic structure of interpenetrating sapphire and garnet phases, as well as lamellar spacing in the order of tens of microns. The eutectic has a higher Ce³⁺ segregation coefficient than YAG single crystal. The photoluminescence properties of this eutectic were also investigated. Results show that it is characterized by a wide excitation band, and that the luminous efficiency of a eutectic-packaged LED is higher than that of a phosphor powder-packaged LED. The findings indicate that the Al₂O₃−YAG eutectic is a promising phosphor for WLED applications.     

Elaboration,structural characterization and optical properties of the yttrium alkoxide derived Y2O3 planar optical waveguides

Y₂O₃ optical planar waveguides are fabricated by the sol–gel method and the dip-coating technique. High purity yttrium alkoxide (yttrium 2-methoxyethoxide) is used as starting material for the Y₂O₃ sol and acetylacetone is added to improve the stability of the sol. Highly concentrated sol up to 0.8 M is prepared and six stacked layers are enough to support four propagation modes at wavelength from 488 to 632.8 nm. The cubic phase of Y₂O₃ is crystallized after an annealing treatment at 700 °C. X-ray diffraction is conducted on the films for their structural analysis. Optical properties of the waveguides are determined by multi-wavelength m-lines spectroscopy and attenuation coefficient measurement. The attenuation coefficient of the prepared Y₂O₃ waveguiding thin films is in the range between 1 and 2 dB/cm at 632.8 nm.     

Sol-gel fabrication of compact, crack-free alumina film

Compact, crack-free alumina film was fabricated using an alumina sol with a high Al₂O₃ content. With the addition of ethylacetoacetate (CH₃COCH₂COOC₂H₅, EAcAc), the stable sol could be prepared with a molar ratio of aluminum sec-butoxide (Al(O-sec-Bu)₃, ASB) to water up to 1:25. It was found that EAcAc could notably decrease the surface tension of the liquid in the gel pores. The EAcAc modification layer on the colloidal particle retarded greatly the densification of the Al₂O₃ gel film and provided a long-lasting structural relaxation during heating. Therefore, the formation of cracks was effectively prevented in this alumina material. The alumina gel film contained a high Al₂O₃ content and there was a rather small mass loss during sintering. The critical thickness of Al₂O₃ sol-gel film was eight times higher than that could be achieved via the general sol-gel route and a film thicker than 0.8 μm was prepared by a single-step dipping operation.     

Grain growth and microstructural evolution of yttrium aluminum garnet nanocrystallites during calcination process

An yttrium aluminum garnet (YAG) precursor precipitate was synthesized by urea method using yttria (Y₂O₃) and aluminum nitrate (Al(NO₃)₃·9H₂O) as raw materials. The fresh wet precipitate was dried by supercritical carbon dioxide (CO₂) fluid and the resulting powder was calcined at temperatures from 600 to 1600 °C. Crystallization of YAG was detected at 800 °C, and completed at 900 °C. HRTEM images of the YAG product obtained above 900 °C revealed crystallographically specific oriented attachment along the [1 1 2] direction. Based on the observation of the particle morphology a possible growth mechanism of YAG nanoparticles was presented. The fast increase on the average crystallite size of YAG at temperatures from 900 to 1300 °C is attributed to the crystallographically specific oriented attachment growth process. As the growth process proceeds at higher temperatures, oriented attachment based growth becomes less important because of the increase on particle size, and the self-integration assisted by the Ostwald ripening becomes dominant.