Rhombohedral Phase in Y2O3-Partially-Stabilized ZrO2

Tetragonal-to-rhombohedral stress-induced phase transformation was studied by X-ray diffraction on the ground surfaces of tetragonal zirconia polycrystals and partially stabilized zirconia containing 2.0 to 5.0 mol% Y₂O₃ prepared by hot isostatic pressing. The rhombohedral phase increased with Y₂O₃ content and also with hot isostatic pressing temperature. The stability of the rhombohedral phase was studied with regard to surface finish and thermal annealing. The subsequent heat treatment of the specimens was found to cause the reverse rhombohedral-to-tetragonal transformation.     

Low-Temperature Synthesis of NiFe2O4 by a Hydrothermal Method

Nickel ferrite (NiFe₂O₄) nanoparticles were successfully synthesized via a hydrothermal process and characterized by X-ray diffraction and transmission electron microscope techniques. The effects of reaction temperature, holding time, and RH ratio (isopropyl alcohol/water) were discussed. The NiFe₂O₄ nanoparticles could be obtained at 60°C within 3 h. The crystallization of the spinel ferrites was promoted by the increase in reaction temperature, holding time, and RH ratio.     

Synthesis of Metastable Tetragonal (t') ZrO2-12 mol% YO1.5 by the Organic Polymerized Complex Method

Zirconia-yttria (ZrO₂-12 mol% YO_1.5) powders were prepared via an organic polymerized complex method that was based on the Pechini-type reaction route. A mixed solution of citric acid, ethylene glycol, and zirconium and yttrium ions were polymerized to form a transparent resin, which then became a black powder after charring at ∼450°C. This product was used as a precursor for ZrO₂-12 mol% YO_1.5. The formation of a metastable tetragonal ( t ´) phase with compositional homogeneity occurred when the precursor was heated at the rate of 10°C/min up to a temperature >560°C. X-ray diffraction peaks became sharp with an increase in the heat-treatment temperature. A t ´-form with cell parameters of a = 5.1265(6) Å and c = 5.1559(9) Å were obtained via heat treatment up to 1400°C. This study shows that the organic polymerized complex method is effective for obtaining the compositionally homogeneous metastable t ´-form in the ZrO₂-YO_1.5system for short annealing times at relatively lower temperatures.     

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.     

Fabrication of Grain-Oriented Bi2WO6 Ceramics

Grain-oriented Bi₂WO₆ ceramics were fabricated by normal sintering techniques. Platelike crystallites were initially synthesized by a fused salt process using an NaCl-KCI melt. When calcined at <800°C, the Bi₂WO₆ crystallites are 3∼5 μ m in size and, at >850°C, =100 μm. After dissolving away the salt matrix, the Bi₂WO₆ particles were mixed with an organic binder and tapecast to align the platelike crystallites. Large particles were easily oriented by tapecasting but the sinterability of the tape was poor. Preferred orientation of small particles was increased by tapecasting and grain growth during sintering further improves the degree of orientation. Sintering above the 950°C phase transition, however, results in discontinuous grain growth and low densities. Optimum conditions for obtaining highly oriented ceramics with high density occur at sintering temperatures of 900°C using fine-grained powders which yield orientation factors of =0.88 and densities of 94% theoretical.     

Transformational Superplasticity of Bi2WO6 and Bi2MoO6

Transformational Superplasticity was studied in the compounds Bi₂WO₆ and Bi₂MoO₆. The magnitudes of transitional strain are related to the ( T_t/T_mP ), s of the phase transitions and are proportional to the externally applied stresses. Strain-rate sensitivities were similar, 0.85 and 0.86; however, the Bi₂WO₆ exhibited a strain-axis intercept and the Bi₂MoO₆ a stress-axis intercept. The grain-size effect present in the Bi₂WO₆ supports an accommodated grain-boundary sliding mechanism for the superplastic deformation process.     

Polymerized Complex Route to Synthesis of Pure Y2Ti2O7 at 750°C Using Yttrium-Titanium Mixed-Metal Citric Acid Complex

A polymerized complex technique was used to prepare high-purity pyrochlore Y₂Ti₂O₇ powders at 750°C. Heating of a mixed solution of citric acid (CA), ethylene glycol (EG), and yttrium and titanium ions, with a molar ratio of CA:EG:Y:Ti = 5:20:1:1, at 130°C produced a yellowish transparent polymeric gel without any precipitation; this material was used subsequently as a precursor for Y₂Ti₂O₇. Based on the results of ¹³C-NMR spectroscopy, it was suggested that a mixed-metal (Y,Ti)-CA₃ chelate complex formed in a starting CA/EG solution. The formation of pure pyrochlore Y₂Ti₂O₇ occurred when the precursor was heat-treated in a furnace set at 750°C in static air for 4 h.     

Single-Phase MnFe2O4 Powders Obtained by the Polymerized Complex Method

The present investigation reports on a simple technique for the preparation of single-phase MnFe₂O₄ nanocrystalline powders using the polymerized complex method, starting from manganese carbonate and iron nitrate. A mixed aqueous solution with citric acid, ethylene glycol, Fe, and Mn ions was polymerized. The phases formation, the homogeneity, and the structure of the obtained powders have been investigated by thermogravimetry, X-ray diffraction (XRD), scanning and transmission electron microscopy, and Mössbauer spectroscopy measurements. The magnetic hysteresis loop behavior was measured at 5 K in a vibrating sample magnetometer. XRD results demonstrated that thermally induced crystallization of cubic MnFe₂O₄ from the Mn–Fe polymeric precursor occurred at temperatures as low as 400°C. Pure single-phase MnFe₂O₄ nanocrystallites without any impurity or amorphous phases were obtained when the precursor was treated at 600°C for 1 h.     

Low-Temperature Synthesis Route to MgNb2O6

A solution-derived precursor may be calcined at temperatures >500°C to yield MgNb₂O₆. Calcining temperatures 800°C result in the familiar, cation-ordered columbite structure type. Calcining temperatures <800°C result in a previously unobserved, cation-disordered α-PbO₂ structure type. The low-temperature form of MgNb₂O₆ is a more reactive material than is obtained by conventional solid-state reaction as evidenced by its small particle size and its ability to react with PbO to form lead magnesium niobate at lower temperatures.     

Low-Temperature Aging of t-ZrO2 Polycrystals with 3 mol% Y2O3

A kinetic study of the aging behavior of tetragonal zirconia polycrystals (TZPs) from 100° to 500°C suggests that there exists a cricital grain size of 0.52 μm below which the tetragonal symmetry can be retained. Property degradation of TZPs at low temperatures is connected with transformation-induced microcracking. It is reported here that the temperature range of 100° to 400°C in which degradation occurs could be explained by an infinite incubation time for the lower limit and by the martensite starting temperature for the upper limit.     

Low-Temperature Sol–Gel Synthesis of NaZr2P3O12

The NZP family of new low-expansion materials has attracted wide interest for its potential in advanced technological applications. NaZr₂P₃O₁₂, which is the parent composition of this family, has been synthesized by the solution sol-gel method using special precursor solutions, which led to its formation (although poorly crystalline) at temperatures as low as 120°C. The lowest temperature of formation of a single phase of NaZr₂P₃O₁₂ with a high degree of crystallinity was found to be 600°C.     

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.     

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.     

Formation, Transformation, and Decomposition of Y2TeO6

A metastable modification of Y₂TeO₆ crystallizes at 770°C to 870°C at a heating rate of 10°C·min⁻¹ in air from an amorphous material prepared by the simultaneous hydrolysis of yttrium and tellurium alkoxides. It has a tetragonal unit cell with a=1.0428 nm and c=1.7524 nm. The tetragonal-to-hexagonal phase transformation occurs at 920° to 960°C. Hexagonal Y₂TeO₆ decomposes into Y₆TeO₁₂ and TeO₃↑ at 1220° to 1320°C; only Y₆TeO₁₂ is obtained as the decomposition product. Both modifications of Y₂TeO₆ are built up by octahedral TeO₆ groups.     

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.     

Electronic Structure and Bonding in Crystalline Y10[SiO4]6N2

The electronic structure and bonding of the complex ceramic crystal Y₁₀[SiO₄]₆N₂ is studied by a first-principles method. It is shown that this crystal is an insulator with a direct band gap of 1.3 eV. It has some unique properties related to the one-dimensional chain structure in the c -direction and the planar N-Y bonding in the x - y plane.     

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.