Synthesis of Yttrium Aluminum Garnet by the Glycothermal Method

The reaction of a stoichiometric mixture of aluminum isopropoxide and yttrium acetate in 1,4-butanediol at 300°C yielded crystalline yttrium aluminum garnet having an approximate particle size of 30 nm. No other phases were detected. The use of ethylene glycol in place of 1,4-butanediol afforded an amorphous product.     

Structural and Optical Properties of Highly Nd-Doped Yttrium Aluminum Garnet Ceramics from Alkoxide and Glycolate Precursors

Yttrium aluminum garnet (YAG) nanopowders doped with high neodymium (Nd) content (3 at.%) were synthesized by the sol–gel processing of (i) alkoxide precursors and (ii) metal chelates formed by complexing the cations with polyethylene glycol. A stoichiometric YAG composition was obtained following both procedures; however, the agglomeration of particles was significantly higher in glycolate synthesis, which shielded residual organics from oxidation (elemental analyses). Distribution of Nd³⁺ ions in the YAG matrix, as shown by the absorption of pump energy and photoluminescence spectra of Nd:YAG ceramics, was more homogeneous in alkoxide-derived powders. The segregation of Nd centers in the glycolate-derived sample was supported by the precipitation of a crystalline Nd₂O₃ phase (X-ray diffraction) during sintering. High-resolution absorption spectra (⁴I_9/2(1)→⁴F_9/2(1)) of the powders showed that a higher absorption cross-section of glycolate-derived powders is due to Nd³⁺–Nd³⁺ ion pairing, which leads to the quenching of photoluminescence. Owing to the better dispersion of optically active centers, the photoluminescence signal was found to be substantially enhanced in alkoxide-derived Nd:YAG ceramics.     

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 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.     

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.     

Yttrium Doped Single-Crystalline Orthorhombic Molybdenum Oxide Micro-Belts: Synthesis,Structural, Optical and Photocatalytic Properties

Molybdenum trioxide (MoO₃) micro-belts were successfully synthesized via the sol–gel coprecipitation method. The synthesized material was, then, doped with different concentrations of yttrium element, Y. The influence of the doping concertation on crystallographic, microstructural, optical, and photocatalytic properties has been studied. Good thermal stability has been revealed by thermogravimetric analysis. The X-ray diffraction analysis identified the orthorhombic phase. The peaks were shifted toward the lower 2θ angle position after doping, confirming the Y³⁺ substitution in the MoO₃ crystal structure. Scanning electron microscope measurements revealed a belt-like shape with a decreasing size when increasing the Y³⁺ ions concentration. SEM clearly showed a non-homogeneous distribution of second-phase particles, depending on the concentration of yttrium doping, which could be attributed to the Y₂Mo₄O₁₅ secondary phase. The band-gap energy was shifted to the lower-energies of an amount of 0.61 eV when going from 0 to 10% of yttrium. The photocatalytic performances were monitored through photodegradation of methylene blue under different radiations. It was observed that Y-dopant significantly improved the photocatalytic activity of MoO₃, the longer the wavelength the better the removal efficiency. This enhancement could be associated with the doping-induced modification of the band-gap or to the microstructure evolution.

     

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.     

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.     

Segregation in Yttrium Aluminum Garnet: II, Theoretical Calculations

Calculations based on ionic space-charge models of doped yttrium aluminum garnet (YAG) have been compared to experimental measurements of surface segregation in crystals of various compositions. The comparison allows limits for vacancy-formation energies to be set. The range for anion:cation formation-energy ratios has been established to be 0.20-0.23, based on the reasonable assumptions that the formation energy of the yttrium ion is 75% of that of the aluminum ion and the Schottky defect formation energy is 4.2 eV. The model explains the experimental observation of calcium at the surface regardless of net acceptor excess or net donor excess. The relationship between vacancy-formation energy and dopant excess has been used to construct segregation maps for YAG, which are useful in materials design strategies.     

Segregation in Yttrium Aluminum Garnet: I, Experimental Determination

Single crystals of yttrium aluminum garnet (YAG) that were doped with various cations were annealed in air at different temperatures for varying amounts of time. Dopants were chosen to probe the effect of size, charge, and site occupancy on surface segregation. Of the dopants that were chosen for the study (calcium, silicon, neodymium, chromium, and strontium), calcium was the only one that consistently segregated upon annealing in air. Calcium enrichment to the (111) surface was measured using Auger electron spectroscopy, and the segregation enthalpy was determined to be δ H _seg≈−32 ± 10 kJ/mol. Enrichment occurred according to variations in valence, as opposed to variations in size; therefore, it is suggested that surface segregation is electrostatically driven. The results indicate that aliovalent substituents could be used for interface property tailoring, whereas isovalent dopants would not be useful.     

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.     

铝掺杂纳米氧化锆的合成与改性研究

氧化锆是一种具有特殊作用的氧化物并受到越来越多的关注。作者利用阳离子表面活性剂(CTAB)的辅助途径,通过前期的制备工艺成功实现了金属铝对纳米氧化锆的掺杂改性。借助XRD,TEM,EDS,Uv-vis以及N2吸附-脱附等方法对样品进行了表征分析;研究表明:金属铝离子能成功地进入氧化锆当中,掺杂改性后的氧化锆相对未掺杂的氧化锆有明显的蓝移,热稳定性显著提高,并且有一定的微孔结构。     

Microwave Synthesis of Yttrium Iron Garnet Powder

A 28 GHz microwave heating method was used to react an Fe₂O₃+ Y₂O₃ powder mixture to form yttrium iron garnet (YIG, Y₃Fe₅O₁₂) powder. The minimum temperature to form YIG was lower than the conventional (external) heating method. YIG began to form after only 70 s of irradiation, which means that the solid-state reaction proceeded very rapidly. The amounts of byproducts were controlled by the starting composition and by the Y₂O₃ particle size. The resultant YIG particle size also was controlled by the Y₂O₃ particle size.     

Mechanical Properties of Melt-Grown Alumina–Yttrium Aluminum Garnet Eutectics up to 1900 K

Alumina/yttrium aluminum garnet (YAG) eutectic rods of 1 mm in diameter were grown by the laser-heated floating zone method at different rates to obtain microstructures with the same morphology but of very different domain size. The mechanical properties of the rods (hardness, toughness, strength) were measured at ambient temperature in the longitudinal and transverse directions and, in addition, the longitudinal flexure strength was determined up to 1900 K. The fracture resistance and the hardness of the eutectics at ambient temperature were isotropic and independent of the domain size. On the contrary, the longitudinal strength was significantly higher than the transverse one and increased linearly with the growth rate, reaching almost 2 GPa in the rods grown at 750 mm/h, which presented a submicrometer homogeneous microstructure. The critical defect size was equivalent to that of Al₂O₃ and YAG domains in the microstructure, and the strength was proportional to the inverse of the square root of the domain size. In addition, the strength retention of the eutectics was remarkable, and the rods with the finest microstructure withstood 1.53 GPa at 1900 K. The moderate reduction in strength at very high temperature was induced by the homogeneous coarsening of the microstructure.     

Porous Yttrium Aluminum Garnet Fiber Coatings for Oxide Composites

A porous oxide fiber coating was investigated for Nextel^™ 610 fibers in an alumina matrix. Polymeric-solution-derived yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) with a fugitive carbon phase was used to develop the porous fiber coating. Ultimate tensile strengths of tows and minicomposites following heat treatments in argon and/or air were used to evaluate the effect of the porous fiber coating. The porous YAG fiber coatings did not reduce the strength of the tows when heated in argon, and they degraded tow strength by only ∼20% after heating in air at 1200°C for 100 h. Minicomposites containing porous YAG-coated fibers were nearly twice as strong as those containing uncoated fibers. However, after heating at 1200°C for 100 h, the porous YAG coatings densified to >90%, at which point they were ineffective at protecting the fibers, resulting in identical strengths for minicomposites with and without a fiber coating.     

Synthesis and Characterization of Rutile TiO2 Nanopowders Doped with Iron Ions

Titanium dioxide nanopowders doped with different amounts of Fe ions were prepared by coprecipitation method. Obtained materials were characterized by structural (XRD), morphological (TEM and SEM), optical (UV/vis reflection and photoluminescence, and Raman), and analytical techniques (XPS and ICP-OES). XRD analysis revealed rutile crystalline phase for doped and undoped titanium dioxide obtained in the same manner. Diameter of the particles was 5–7 nm. The presence of iron ions was confirmed by XPS and ICP-OES. Doping process moved absorption threshold of TiO₂ into visible spectrum range. Photocatalytic activity was also checked. Doped nanopowders showed normal and up-converted photoluminescence.