Fracture Characteristics of Czochralski-Grown Y3Al5O12

The fracture surfaces of single-crystal Y₃A1₅O₁₂ (YAG) have been examined in detail using differential interference microscopy and atomic force microscopy. In YAG crystals grown in 1977 by the Czochralski method strikingly regular undulations of microscopic and submicroscopic dimensions (4–20 nm in amplitude, 1–20 μm in wavelength) are observed on every fracture surface. The undulations do not exhibit typical characteristics of Wallner lines; furthermore, they are not observed in identical fracture tests for YAG crystals grown in 1990 by the Czochralski method. Transmission electron microscopy reveals that the crystals grown in 1977 exhibit a periodic fluctuation of the ratio of the concentrations of aluminum to yttrium, suggesting that the fracture surface features are caused by periodic astoichiometry.     

Film Synthesis of Y3Al5O12 and Y3Fe5O12 by the Spray–Inductively Coupled Plasma Technique

Thin films of yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) and yttrium iron garnet (YIG, Y₃Fe₅O₁₂) were synthesized on single-crystal Al₂O₃ substrates by a modification of spray pyrolysis using a high-temperature inductively coupled plasma at atmospheric pressure (spray–ICP technique). Using this technique, films could be grown at faster rates (0.12 μm/min for YAG and 0.10 μm/min for YIG) than using chemical vapor deposition (0.005–0.008 μm/min for YAG) or sputtering (0.003–0.005 μm/min for YIG). The films were dense and revealed a preferred orientation of (211). The growth of YIG was accompanied by coprecipitation of α-Fe₂O₃. The coprecipitation, however, could be largely suppressed by preliminary formation of a Y₂O₃ layer on the substrate.     

Fracture Behavior of Directionally Solidified Y3Al5O12/Al2O3 Eutectic Fiber

The strength and fracture of a directionally solidified Y₃Al₅O₁₂/Al₂O₃ eutectic fiber were investigated. The fiber was grown continuously by an edge-defined film-fed growth technique. The microstructure was characterized using X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy. The tensile strength and Weibull's modulus of the eutectic fibers were determined in the as-fabricated state and after extended thermal exposure at 1460°C in air. Fractographic analysis was used to identify and classify the strength-limiting mechanisms. The fracture toughness and crack growth behavior were characterized by an indentation technique. A fracture mechanics analysis was also used to establish the relationships between surface flaw size, tensile strength, and fracture toughness of the fiber.     

Eutectic Composition in the Pseudobinary of Y4Al2O9 and Y2O3

The eutectic composition between Y₄Al₂O₉ and Y₂O₃ was determined using electron probe microanalysis (EPMA) on directionally solidified specimens with hypo- and hypereutectic compositions. The microstructures of the specimens as a function of composition differ considerably with small deviation from the eutectic composition (70.5 mol% Y₂O₃ and 29.5 mol% Al₂O₃). Based on the current results and other published data, the pseudobinary system between Al₂O₃ and Y₂O₃ is revised.     

Microstructures of Y2O3-Stabilized ZrO2 Electron Beam-Physical Vapor Deposition Coatings on Ni-Base Superalloys

The microstructures of ZrO₂–20 wt% Y₂O₃ thermal barrier coatings formed by electron beam-physical vapor deposition on a Nibase superalloy have been studied by transmission electron microscopy. The coating systems consist of several layers, including a superalloy substrate, a bond coat, an Al₂O₃ scale, and the PVD coating. The overall ceramic thermal barrier coatings were characterized, with special emphasis being given to the α-Al₂O₃ scale which forms between the bond coat and the ZrO₂₋Y₂O₃ coating. The oxide scale exhibited various morphologies in different coating systems; the majority of the porosity formed in this region for all coatings.     

Ta2O5/Nb2O5 and Y2O3 Co-doped Zirconias for Thermal Barrier Coatings

Zirconia doped with 3.2–4.2 mol% (6–8 wt%) yttria (3–4YSZ) is currently the material of choice for thermal barrier coating topcoats. The present study examines the ZrO₂-Y₂O₃-Ta₂O₅/Nb₂O₅ systems for potential alternative chemistries that would overcome the limitations of the 3–4YSZ. A rationale for choosing specific compositions based on the effect of defect chemistry on the thermal conductivity and phase stability in zirconia-based systems is presented. The results show that it is possible to produce stable (for up to 200 h at 1000°–1500°C), single (tetragonal) or dual (tetragonal + cubic) phase chemistries that have thermal conductivity that is as low (1.8–2.8W/m K) as the 3–4YSZ, a wide range of elastic moduli (150–232 GPa), and a similar mean coefficient of thermal expansion at 1000°C. The chemistries can be plasma sprayed without change in composition or deleterious effects to phase stability. Preliminary burner rig testing results on one of the compositions are also presented.     

High-Temperature Phase Transition of Y4Al2O9

Reversible thermal phase transition was observed for Y₄Al₂O₉ at 1377°C with a hysteresis width of 78°C using differential scanning calorimetry. The enthalpy of the transition (Δ H ) was about 300 cal/mol. Powder X-ray diffraction peaks of the high-temperature phase, as well as the peaks of the low-temperature phase, were characterized with a monoclinic unit cell. Thermal hysteresis was also confirmed by the temperature dependence of the lattice parameter. The unit cell volume of the high-temperature phase was 0.5% smaller than that of the low-temperature phase at 1400°C.     

Deformation Behavior of an Al2O3─Y3Al5O12 Eutectic Composite in Comparison with Sapphire and YAG

The relationship between a composite and its constituents with respect to high-temperature deformation behavior was investigated using the Al₂O₃-Y₃Al₅O₁₂ (YAG) eutectic system. The eutectic is essentially a composite with sapphire and YAG as the two phases with a volume fraction of YAG of 0.45. The deformation behavior of the eutectic and those of single-crystal sapphire and YAG were studied under identical conditions, using constant-strain-rate compression tests in air at 1530°C. The composite was also studied at 1650°C and compared with existing data on sapphire and YAG. The stronger YAG phase was found to reinforce the sapphire matrix at higher strain rates, but the composite crept faster than sapphire at lower strain rates. It is suggested using a simple semiempirical analysis that diffusional relaxation at the YAG-sapphire interface boundaries may cause the inferior creep behavior at lower strain rates.     

Hydrothermal Preparation and Morphology Characteristics of Y3Fe5O12

The influences of processing parameters such as mineralizer, temperature, and nonstoichiometry on reaction product and morphology were investigated in the hydrothermal synthesis of yttrium iron garnet, Y₃Fe₅O₁₂ (YIG). A cubic YIG phase was synthesized successfully under hydrothermal conditions of 200°C and 6 h from yttrium and iron hydroxide coprecipitates obtained by adjusting the pH to 10.5 using NH₄OH. The other mineralizer, NaOH, was not effective in forming the garnet phase. Nonstoichiometric compositions utilizing excess Y content tended to increase the yield of the garnet phase, but did not affect the morphology. Different morphology characteristics were observed by changing the synthesis temperature. Isometric YIG particles disappeared at a relatively high temperature of 250°C, resulting in irregular star-shaped particles of YIG due to the preferential orientation during particle growth.     

Solid-Liquid Reactions in the System Si3N4–β-SiAlON–Y3Al5O12

Solid-liquid equilibria in the system Si,Al,Y/N,0 were determined for the compatibility triangle bounded by the β-SiAlON solid-solution line and the compound Y₃Al₅O₁₂. X-ray diffraction was used to determine the crystalline phases present in the equilibrated, rapidly cooled specimens. The liquid phase was quantified with volume fraction measurements performed on scanning electron micrographs. The solid-liquid tie lines at 1650° and 1750°C were established from lattice parameters of the β-SiAlON phase and from the amount of liquid phase in equilibrium with the crystalline solid. The liquid phase was crystallized to verify the location of the starting composition.     

Electrical Conductivity of MgAl2O4 and Y3A15O12

The electrical conductivities of single crystal and polycrystalline MgAl₂O₄ and Y₃Al₅O₁₂ were measured to 1260 K using a three-contact, guard-ring technique. The electrical conduction mechanisms change with temperature, with anomalous oxygen pressure and time-dependent inflections in log σ versus T⁻¹ curves between 900 to 1000 K. The conduction processes of Y₃Al₅O₁₂ and MgAl₂O₄ appear to be similar and possibly related to A1³⁺ ion diffusion.     

Chemical Vapor Deposition of Polycrystalline Al2O3

Polycrystalline Al₂O₃ was chemically vapor-deposited onto sintered Al₂O₃ substrates by reaction of AlCl₃ with (1) H₂O, (2) CO:H₂, and (3) O₂ at 1000° and 1500°C and 0.5 and 5.0 torr. Although the thermodynamics of all these reactions predict the formation of solid Al₂O₃, the deposition rate of the first reaction was considerably greater than that of the second. The third reaction was so slow that no measurable deposit was formed in 6 h at 1500°C. Formation of dense deposits of α-Al₂O₃ was favored by increasing temperature and decreasing pressure. Microstructural examination of the dense deposits showed long columnar grains, the largest of which extended through the deposit from the substrate to the surface.     

Preparation and Optical Properties of Transparent Eu3+:Y3Al5(1−x)Sc5xO12 Ceramics

Using a novel combustion method, Eu-doped Eu:yttrium aluminum garnet (YAG) and Eu:YSAG powders, and transparent Eu:YSAG ceramics were fabricated. The optical properties of these transparent ceramics have been measured, and a reduced peak splitting of Eu³⁺ for ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ was observed when 10 at.% Al³⁺ was substituted by Sc³⁺. The enhanced symmetry of the Eu sites in YAG lattice, which resulted from the expanded YSAG lattice by Sc³⁺ doping, is the main reason for the reduced peak splitting.     

Direct-Current Conductivity of Y2O3 as a Function of Water Vapor Pressure

The dc conductivity of a sintered sample of Y₂O₃ containing an excess of lower-valent metal (Ml) impurities has been measured at 600° to 1100°C in air as a function of the water vapor pressure (0.4 to 3000 Pa). The logarithm of the p -type conductivity vs log P _H2o has a slope of – 1/2 in certain regions of F _H2o and temperature. This reflects a defect situation where interstitial protons from water vapor balance the excess of lower-valent cation impurities: [H_˙] = [M1_']. The activation energy for the p -type conductivity under these conditions is evaluated and interpreted in terms of the enthalpy needed to dissolve interstitial protons in yttria.     

Cation Self-Diffusion in Polycrystalline Y2O3 and Er2O3

A tracer sectioning technique was used to measure cation self-diffusion coefficients in fully dense polycrystalline Y_aO₃ and Er₂O_s under oxidizing conditions. The results are described by the relations for Y₂O₃ (1400° to 1670°C), and for Er₂O₃ (1400° to 1700°C). The greater activation energy for erbium diffusion in erbia may be partly attributable to a mass effect.