A Simple Way to Synthesize Yttrium Aluminum Garnet by Dissolving Yttria Powder in Alumina Sol

A precursor for Y₃Al₅O₁₂ was synthesized as a YAG sol by simply dissolving Y₂O₃ powder in an alumina sol. Phase-pure Y₃Al₅O₁₂ powder was obtained by precipitating the YAG sol with an aqueous dilute ammonia solution followed by calcination at 1100°C. TG/DTA analysis showed an exotherm at 938°C attributed to formation of YAG phase and weight loss of 44% at 1000°C. XRD and FT-IR analysis showed that phase-pure YAG can be formed through noncrystalline and metastable hexagonal YAlO₃ without forming either yttrium or aluminum formate intermediate.     

Low-Temperature Synthesis of Nanocrystalline Yttrium Aluminum Garnet Powder Using Triethanolamine

Nanocrystalline yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) was synthesized by pyrolysis of complex compounds of aluminum and yttrium with triethanolamine [(HOCH₂CH₂)₃N, (TEA)]. Loose and porous precursor was obtained on complete dehydration of the metal ion–triethanolamine complexes. Pure YAG powder was obtained by calcination of the precursor at 950°C. The precursor was characterized by simultaneous thermogravimetry, differential scanning calorimetry, and mass spectra analyses (TG–DSC–MS). The heat-treated powders were characterized by X-ray diffractometry (XRD), specific surface area measurements, and transmission electron microscopy (TEM). The average crystallite size as determined from X-ray line broadening and transmission electron microscopy studies was ∼40 nm. The effects of the calcination temperature and the ratio of triethanolamine to mixed metal ions were also studied.     

Synthesis and Characterization of Ce3+:YAG Phosphors by Heterogeneous Precipitation Using Different Alumina Sources

Ce³⁺-doped yttrium aluminum garnet (Ce:YAG) phosphor powders were synthesized by heterogeneous precipitation process using three different aluminum sources: α-phase, θ-phase, and boehmite (AlOOH). Mixtures of yttrium and cerium nitrate solutions containing various aluminum sources were precipitated by ammonia solution in normal and reverse strike methods. The influence of pH was studied in the normal strike method by maintaining the solutions at pH 7, 9, and 11 during precipitation. Dried precipitates were double calcined at 1300°C/16 h and 1300–1500°C/24 h, at a ramping rate of 10°C/min, with an intermittent wet ball milling in water. Structural evolution of the resultant phosphors was studied by powder XRD. In the normal strike method, a highly pure YAG phase was formed by α- alumina (pH 7, 11) and θ-alumina (pH 11) while boehmite source ended up with mixed phases of YAlO₃ (YAP) and Y₄Al₂O₉ (YAM) along with YAG phase at all pH values of precipitation. However, in the reverse strike process, the θ-phase of alumina gave an extremely pure Ce:YAG phase at a relatively lower calcination temperature (1400°C/24 h) compared with the α-phase and also showed more intense emission of yellowish-green light under blue (λ=469 nm) excitation. Scanning electron microscopy revealed 1–2 μm sized particles with least agglomeration in the reverse strike method.     

Synthesis, Characterization, and Optical Properties of Pristine and Doped Yttrium Aluminum Garnet Nanopowders

Pristine, Si-doped, and Si/Nd-codoped yttrium aluminum garnet (YAG) nanoparticles were synthesized by pyrolysis of complex compounds of aluminum and yttrium with triethanolamine. It was found that the coexistence of Si⁴⁺ and Nd³⁺ increased the solubility of both ions and promoted the formation of YAG phase. Single-phase, nanocrystalline Si/Nd:YAG powders were obtained at calcination temperatures as low as 920°C. The optical behavior of the Si/Nd:YAG nanopowders was similar to that of single-crystal Nd:YAG.     

Synthesis of Bulk, Dense, Nanocrystalline Yttrium Aluminum Garnet from Amorphous Powders

Amorphous powders of Al₂O₃—37.5 mol% Y₂O₃ (yttrium aluminum garnet (YAG)) were prepared by coprecipitation, decomposed at 800°C, and hot-pressed uniaxally at low temperature (600°C) and a moderate pressure (750 MPa). Optimum conditions yielded microstructures with only 2% porosity and partial crystallization of YAG. Further processing using high quasi-hydrostatic pressure (1 GPa) at 1000°C enabled the production of fully crystallized YAG with >96% relative density and a nanocrystalline grain size of ∼70 nm.     

Bulk, Dense, Nanocrystalline Yttrium Aluminum Garnet by Consolidation of Amorphous Powders at Low Temperatures and High Pressures

Amorphous Al₂O₃–37.5% Y₂O₃ powders, prepared using spray pyrolysis followed by partial or complete thermal decomposition, were hot-pressed at 315°–640°C and 500 or 750 MPa uniaxial pressure. Hot pressing of fully decomposed amorphous powder at 450°–640°C at pressures up to 750 MPa led to densification (up to 96%) as well as nanocrystallization of yttrium aluminum garnet (YAG). When the pressure was applied during heating, instead of after reaching the final temperature, higher relative densities resulted. Fully crystalline starting powder did not densify. The low true density of the amorphous phase (3.1 g·cm⁻³) was believed to be responsible for the densification through enhanced ionic mobilities.     

Low-Temperature Fabrication of Transparent Yttrium Aluminum Garnet (YAG) Ceramics without Additives

A carbonate precursor of yttrium aluminum garnet (YAG) with an approximate composition of NH₄AlY_0.6(CO₃)_1.9(OH)₂·0.9H₂O was synthesized via a coprecipitation method from a mixed solution of ammonium aluminum sulfate and yttrium nitrate, using ammonium hydrogen carbonate as the precipitant. The precursor precipitate was characterized using chemical analysis, differential thermal analysis/thermogravimetry, X-ray diffractometry, and scanning electron microscopy. The sinterability of the YAG powders was evaluated by sintering at a constant rate of heating in air and vacuum sintering. The results showed that the precursor completely transforms to YAG at ∼1000°C via the formation of a yttrium aluminate perovskite (YAP) phase. YAG powders obtained by calcining the precursor at temperatures of ≤1200°C were highly sinterable and could be densified to transparency under vacuum at 1700°C in 1 h without additives.     

Reactive Laser Ablation Synthesis of Nanosize Aluminum Nitride

An aluminum (Al) target was laser ablated in a nitrogen (N₂) atmosphere, producing aluminum nitride (AlN) powder. These powders were calcined at 900°C for 2 h. Powders were produced at various nitrogen pressures, and the calcined powders were tested for unreacted aluminum content, using differential thermal analysis (DTA). The AlN powder, produced at a laser fluence of 12 J/cm² and a nitrogen pressure of 10.0 kPa (75 torr), showed no evidence of unreacted aluminum by DTA and was phase-pure AlN by X-ray diffraction (XRD). The surface area of this powder is 82 m²/g, corresponding to a particle size of ∼11 nm, which is in good agreement with TEM observations.     

Structure formation in yttrium aluminum garnet (YAG) fibers

Polycrystalline yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) fibers were prepared from aqueous solutions of aluminum chlorohydrate and yttrium chloride. Fiber processing was accomplished via dry spinning. Poly(vinylpyrrolidone) (PVP) was used as spinning aid. Polycrystalline YAG fibers were obtained by pyrolysis of the green fibers followed by sintering at defined temperatures in air. Ceramic fibers were 9–16 μm in diameter. Differential scanning calorimetry/thermogravimetric analysis coupled with mass spectrometry (DSC/TGA-MS) showed an exothermic peak at 920 °C assigned to the crystallization of YAG and an overall ceramic yield of 38% at 1400 °C. X-ray diffraction (XRD) analysis showed that phase-pure YAG can be obtained at 1600 °C after intermediate formation of Y₂O₃ and monoclinic yttrium aluminum oxide (YAM, Y₄Al₂O₉) phases.     

Combined effect of MgO sintering additive and stoichiometry deviation on YAG crystal lattice defects

This study aims to investigate the combined effect of the magnesium oxide (MgO) sintering additive and the deviations of the yttrium aluminum garnet (YAG) stoichiometry towards the excess of Al³⁺ and Y³⁺ on crystal lattice defects. YAG ceramic powders with an excess of aluminum (up to 4.79 mol.%) or yttrium (up to 2.87 mol.%) and various concentrations of the MgO sintering additive (from 0 to 0.2 wt%) were obtained using a two-stage co-precipitation method. The samples calcined at 1600 °C were studied by X-ray powder diffraction (XRD), scanning electron microscopy, diffuse reflectance, Raman and IR-spectroscopy.The cumulative analysis of theoretical calculations of the composition of the samples was performed based on a change in the lattice parameters. Experimental data obtained suggested that, depending on the YAG stoichiometry, magnesium can substitute both the dodecahedral positions of yttrium (in the case of aluminum excess), as well as the octahedral positions of aluminum (in the case of yttrium excess). The resulting substitution defects give pronounced absorption bands in the diffuse reflectance spectra. An increase in the concentration of introduced magnesium leads to a shift in the IR-transmission maximum of ≈620 cm⁻¹ towards wave numbers increasing with an excess of aluminum and towards wave numbers decreasing with an excess of yttrium. In addition, when magnesium was introduced, a decrease in the intensity of incidental peaks in the Raman spectra was observed, which may be due to the relaxation of stresses due to a decrease in the number of intrinsic crystal lattice defects.     

The effects of chelating agent type on the morphology and phase evolutions of alumina nanostructures

In this work, pure aluminum oxide nanoparticle was obtained by the chemical method at low temperature. In this approach, alpha-alumina nanoparticle was prepared by aluminum nitrate nonahydrate and triethanolamine (TEA) as the Al³⁺ and both gel agent, respectively. FESEM images show that TEA has better efficiency in controlling particle size and agglomeration as compared with citric acid as a chelating agent. Then, yttrium aluminum garnet (YAG) powder was prepared by yttrium nitrate, urea, and tetraethyl orthosilicate (TEOS) as a Y³⁺, precipitation agent, and flux agent, respectively. Furthermore, results show that by using TEOS, the YAG particles were obtained, while in the absence of this agent, the impurely YAG phase was achieved at the simillar annealing temperature.     

A Novel Way to Synthesize Yttrium Aluminum Garnet from Metal–Inorganic Precursors

Yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) was synthesized by sol–gel processing from the stoichiometric amounts of aluminum pellets, Y(NO₃)₃·6H₂O, and Al(NO₃)₃·9H₂O or AlCl₃·6H₂O, with suitable kinds of acid (citric acid, acetic acid, etc.) as catalysts. Polycrystalline YAG powder was obtained by drying the YAG precursor followed by calcination at temperatures above 900°C. Thermogravimetry/differential thermal analysis and Fourier transform infrared specotrscopic analyses in air showed an exothermic peak at ∼900°C, attributed to the formation of a polycrystalline YAG phase and weight loss of 60% at 1000°C, caused by the decomposition of hydroxyl and NO₃⁻, etc. X-ray diffraction analysis showed that YAG can be formed at 900°C, and no other intermediate was observed. In particular, the YAG sol can be used for dry-spinning fibers with the aid of some organic polymer.     

Effect of Moisture on the High-Temperature Stability of Unidirectionally Solidified Al2O3/YAG Eutectic Composites

The corrosion resistance of a unidirectionally solidified alumina/yttrium aluminum garnet (Al₂O₃/YAG) eutectic mixture was investigated at high temperature. Samples were exposed to high temperature (1200°–1800°C) in different atmospheres, which included argon, argon/water vapor, air, and air/water vapor. The most important microstructural changes occurred at the interface between the YAG and the Al₂O₃. Those changes consisted of localized thermal grooving, especially when the corrosive atmosphere contained water vapor. The samples exhibited significant weight loss at high temperature (1800°C) after 20 h of exposure. The calculated volume gain that was induced by the increased surface relief was low and limited, except when the corrosive atmosphere contained air, which indicated that the presence of air (particularly oxygen) induced a more-active corrosion process. On the other hand, no change in the flexural strength was observed, even after 100 h at 1800°C in a humid atmosphere, because of the cross-linked structure of the composite, which limited propagation of the groove.     

Polycrystalline Yttrium Aluminum Garnet Fibers from Colloidal Sols

Polycrystalline yttrium aluminum garnet (YAG) fibers were prepared from commercially available colloidal sols of Y₂O₃ and AlOOH and water-soluble polymers. The fibers were dry spun and all processing was performed in air. Transformation to YAG was complete by ≅1300°C, and the fibers were mostly dense by 1600°C with a final fired diameter of 120 μm. A bend test was used to characterize mechanical strength, and an average of 522 ± 186 MPa with a Weibull modulus of 3.5 was determined. The bend stress relaxation (BSR) test was used to characterize creep properties. The creep resistance was better than that of all commercially available oxide fibers with the exception of Saphikon single-crystal alumina ( c -axis oriented). The creep strain of the YAG fibers compared well with that calculated for YAG monoliths with roughly the same grain size.     

Synthesis of Yttrium Aluminum Garnet from a Mixed-Metal Citrate Precursor

Yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) was synthesized using a polymeric precursor derived from a mixed-metal citric acid/ethylene glycol/ethanol solution. YAG was found to crystallize directly from an amorphous precursor beginning at temperatures as low as 600°C within 1 h in air. The polymer resin concentration was found to have an effect on the temperature of crystallization initiation. However, all precursors produced a well-crystallized YAG powder within 1 h at 800°C in air. Formation of phase-pure YAG in an argon atmosphere did not occur until heating for 1 h at 1000°C. An optimum cation-to-resin ratio to maximize reactivity provides a polymeric network to ensure a homogeneous dispersion of cations, yet minimizes cation diffusion distances within the char by limiting excess free carbon after polymer pyrolysis.     

Agglomeration Control of Nd:YAG Nanoparticles Via Freeze Drying for Transparent Nd:YAG Ceramics

Neodymium-doped yttrium aluminum garnet (Nd:YAG) nanopowders were synthesized by the carbonate coprecipitation method. The effects of freeze drying and conventional oven drying of the precursor on the agglomeration of the Nd:YAG nanopowders were compared. The optical properties of the Nd:YAG nanopowders and the corresponding sintered Nd:YAG transparent ceramics were also investigated. The Nd:YAG nanopowders synthesized from freeze-dried precursor showed better dispersion and narrower particle size distribution compared with the powders synthesized from conventional oven drying. As a result, the Nd:YAG nanopowders synthesized from freeze-dried precursor have good sinterability, and Nd:YAG transparent ceramics were fabricated by vacuum sintering at 1750°C for 5 h.     

Precipitation and Calcination Processes for Yttrium Aluminum Garnet Precursors Synthesized by the Urea Method

Yttrium aluminum garnet (YAG) precursors for transparent ceramics were synthesized by the urea method under various [urea]/[metal ions] ([U]/[M]) conditions. Monophase YAG was obtained from solutions with a high [U]/[M] ratio after calcination at a temperature of 1200°C. The condition of the precipitates seemed to indicate that the yttrium compounds had precipitated onto the aluminum compounds. The surface morphology and size of the particles were controlled by the [U]/[M] ratio. The different reaction sequences of YAG crystallization for low- and high-ratio samples were dependent on morphology, size, and the quantity of chemical species that was precipitated as carbonate and/or sulfate compounds.     

Effect of Yttrium Aluminum Garnet Additions on Alumina-Fiber-Reinforced Porous-Alumina-Matrix Composites

The effects of incorporating yttrium aluminum garnet (YAG) into a porous alumina matrix reinforced with Nextel 610 alumina fibers were investigated. Composites with various amounts of YAG added to the matrix were prepared to determine its effect on retained tensile strengths after heating to 1100° and 1200°C. Strengths of YAG-containing composites were slightly lower than those of an all-alumina-matrix composite after heating for 5 h to 1100°C. However, after heating for 5 or 100 h at 1200°C, all the YAG-containing composites displayed greater strengths and greater strains to failure than the all-alumina composite. At the higher temperature, the presence of YAG is believed to inhibit the densification of the matrix, which helps to maintain higher levels of porosity and weaker interparticle bonding that allows for crack-energy dissipation within the matrix. A reduction in grain growth of the fibers by the presence of segregated Y was also observed, which may also contribute to higher fiber strength, thereby increasing the retained strengths of the YAG-containing composites.     

Decomposition-Crystallization of Polymer-Derived Si-C-N Ceramics

Monolithic polymer-derived Si-C-N ceramics were processed by blending an oligomeric Si-C-N precursor (liquid polysilazane) with 70 vol% of crosslinked or pyrolyzed Si-C-N powder particles, which were obtained from the same liquid precursor preheated at 300° or 1000°C, respectively. Powder compacts subsequently were annealed at 300°C to crosslink the liquid precursor acting as a binder between the powder particles, thus yielding monolithic green bodies. Heat treatment at 1540°C was performed to initiate crystallization in the various samples. Microstructure development and, in particular, crystallization behavior were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and preliminary nuclear magnetic resonance (NMR) spectroscopy. The material containing 300°C polymer powder (with oligomeric binder, also crosslinked at 300°C) revealed a homogeneous amorphous microstructure after exposure to temperatures of 1540°C. In contrast, the specimen containing powder particles preheated at 1000°C exhibited a high volume fraction of SiC crystallites within regions that were previously filled by the binder; however, the Si-C-N powder particles themselves remained amorphous. SEM observations as well as XRD studies showed the formation of idiomorphic SiC and Si₃N₄ crystallites on specimen surfaces as well as along internal crack walls. This finding suggested that vapor-phase reactions at the surface were involved in the formation of crystalline phases at temperatures >1250°C. Moreover, NMR spectroscopy data indicated a phase separation process, implying structural rearrangement prior to crystallization.     

Low-Temperature Chemical Synthesis of Nanocrystalline MgAl2O4 Spinel Powder

Nanocrystalline MgAl₂O₄ spinel powder was synthesized by pyrolysis of complex compounds of aluminum and magnesium with triethanolamine (TEA). The soluble metal ion–TEA complexes formed the precursor material on complete dehydration of the complexes of aluminum–TEA and magnesium–TEA. Single-phase MgAl₂O₄ spinel powder resulted after heat treatment of the precursor material at 675°C. The precursor and the heat-treated powders were characterized by X-ray diffractometry (XRD), differential thermal and thermogravimetric analysis, and transmission electron microscopy (TEM). The average crystallite size as measured from the X-ray line broadening was around 14 nm and the average particle size from TEM studies was around 20 nm.