Characteristics of Yttrium Iron Garnet Ultrafine Particles Prepared by the Alkoxide Method

Hydrolysis of a mixture of iron ethoxide and yttrium butoxide produced amorphous yttrium iron garnet (YIG) ultrafine particles with a mean diameter of 9 nm. The particles prepared by hot water vapor hydrolysis were less agglomerated than those prepared by plain hot water. DTA revealed that a broad phase transition temperature T _o from amorphous YIG to crystalline YIG ultrafine particles is 691°C with heat discharge of 118 kJ/mol which is attributed to the entropy decrease accompanying the crystallization, whereas magnetic experiments gave a value of T _o= 650°C. The particles calcined at 700°C were single-crystalline ones while those calcined at higher temperatures (at least higher than 800°C) were multicrystalline ones.     

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.     

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.     

Preparation of High-Tc Superconducting Oxides by the Amorphous Citrate Process

The high-T_c superconducting oxide Yba₂Cu₃O_7–y has been prepared by the amorphous citrate process. A citrate-nitrate solution was prepared at room temperature and dehydrated at around 80°C to yield a solid precursor material. The precursor was fired to high temperature to form the desired compound. The decomposition mechanism was studied with thermogravimetric analysis and X-ray diffraction. Intermediate products were formed during the decomposition reactions. Nearly phase-pure Yba₂Cu₃O_7-y powders were obtained by firing the precursor in air to 900°Cfor2 h. Sintered samples were typically over 90% dense and exhibited good superconducting properties.     

Properties of the Solid Electrolyte Gadolinia-Doped Ceria Prepared by Thermal Decomposition of Mixed Cerium-Gadolinium Oxalate

Fine-grained powder of the mixed oxide (CeO₂)_0.9(Gd₂O₃)_0.1, which is an ionic conductor for oxygen ions, was prepared by coprecipitation of the corresponding oxalates followed by calcination. The powder was used to prepare pellets sintered at a relatively low temperature of 1000°C compared with the usual sintering temperature of 1700° to 1800°C. The size of the powder grains was determined from BET surface area (S_BET) measurements. The effect of precipitation conditions and calcination temperature on S_bet was examined. The largest surface area measured was 88 m²/g. Decomposition of the oxalate powder was followed using an optical dilatometer. The decomposition was indicated by a large shrinkage and it was completed below 300°C (for a heating rate of 3.3°C/min). The formation of the oxide was verified by X–ray diffraction analysis. It shows that the product of decomposition is the oxide and that decomposition can be carried to completion at 250°C if the heating lasts for 1 h. The pellets had a density of 83% of theoretical, small grains (0.5 μm), and a conductivity which, at 900°C, is two–thirds of the conductivity of dense samples obtained from the same raw material, but calcined and fired at much higher temperatures.     

Low-Temperature Fabrication of Cordierite Ceramics from Kaolinite and Magnesium Hydroxide Mixtures with Boron Oxide Additions

Thermal reactions of mixtures of ultrafine particles of magnesium hydroxide (Mg(OH)₂) and kaolinite in a composition of MgO:Al₂O₃:2SiO₂ were investigated to obtain dense cordierite ceramics at temperatures <1000°C. While heating the mixture of kaolinite and Mg(OH)₂ with the equivalent of 2 mass% of boron oxide (B₂O₃) (in the form of magnesium borate, 2MgOB₂O₃), an amorphous phase formed at a temperature of ∼850°C after thermal decomposition. Firing the mixture at a temperature of 900°C yielded dense ceramics with an apparent porosity of almost zero. The addition of B₂O₃ promoted the densification at 850°-900°C and accelerated the crystallization of alpha-cordierite. The specimen with 3 mass% of B₂O₃ that was fired at a temperature of 950°C showed a linear thermal expansion coefficient of ∼3 × 10⁻⁶ K⁻¹, a bending strength of >200 MPa, and a relative dielectric constant of 5.5 at 1 MHz. These cordierite ceramics may be used as substrate materials for semiconductor interconnection applications.     

Reaction During Sintering of Barium Titanate with Lithium Fluoride

The reactions occurring during sintering of stoichiometric BaTiO₃ with small additions of LiF were studied at temperatures between 700° and 900°C. BaTiO₃ reacts with LiF to form a cubic solid solution and Li₂TiO₃ During sintering, the cubic solid solution coexists with Li₂Ti0₃ and forms a liquid phase at 740°± 5°C. The occurrence of a liquid phase at this temperature results in an enhancement of the sintering process and leads to the development of a highly dense microstructure.     

Production of Nanosized YAG Powders with Spherical Morphology and Nonaggregation via a Solvothermal Method

Monodispersed nanosized yttrium aluminum garnet (YAG) powder was synthesized via a mixed-solvent thermal method using stoichiometric amounts of inorganic aluminum and yttrium salts. Pure-phase YAG crystalline powder was obtained at low temperature (290°C) and low pressure (10 MPa). The resulting products were characterized by X-ray powder diffraction (XRD), infrared, and transmission electron microscopy (TEM). XRD results showed that single-phase YAG could be formed directly from an amorphous precursor at 280°C and become fully developed at 290°C. TEM images showed that the YAG powder particles in the study were basically spherical in shape and well-dispersed with a mean grain size of about 60 nm.     

Synthesis and Sintering of Cordierite from Ultrafine Particles of Magnesium Hydroxide and Kaolinite

Thermal reactions of magnesium compounds and kaolinite were investigated to obtain dense cordierite ceramics without additives. Magnesium hydroxide was precipitated from aqueous solution in the form of ultrafine hexagonal tabular particles of about 0.1 µm, and heating this mixture with kaolinite in a mole ratio of MgOAl₂O₃2SiO₂ resulted in formation of an amorphous state at about 900°C following thermal decomposition. µ-Cordierite was then crystallized from the amorphous phase at about 950°C, and gradually transformed into alpha-cordierite. Firing the pressed specimens yielded a dense alpha-cordierite ceramic with a relative density of 97.7% at 1350°C.     

Mechanical Properties of Polycrystalline β-Sic

The mechanical properties of chemically vapor-deposited β-Sic were measured in bending between room temperature and 1400°C. Material with grain diameters from less than 1 to 15 μm was tested. No grain-size dependence of the bend strength of dense (<99% of theoretical) Sic was observed at any test temperature. The fracture strength of dense Sic remained approximately constant between room temperature and about 900°C and then increased sharply up to the maximum test temperature of 1215° to 1400°C. This increase in fracture stress coincided with the onset of plastic yielding detectable in the stress-strain curves. The fracture mode of this material was transgranular cleavage at all test temperatures. The fracture stress of Sic of lower density, which was characterized by the presence of grain boundary flaws, decreased slightly at high temperature. The fracture mode of the low-density (3.17 g/cm³) β-Sic underwent a transition from predominantly transgranular at room temperature to predominantly intergranular at high temperature.     

Oxygen Ion Diffusion in Single-Crystal and PoIycrystaIIine Yttrium Iron Garnet

The oxygen ion self-diffusion coefficient was determined for single-crystal and polycrystalline yttrium iron garnet (Y₃Fe₅O₁₂). The rate of exchange between oxygen gas enriched with the stable isotope ¹⁸O and solid yttrium iron garnet was measured. Oxygen ion diffusion rates were found to be the same in single-crystal and 8μ polycrystalline Y₃Fe₅O₁₂ between 1100° and 1400°C. This is in contrast to previous measurements of anion diffusion in several alkali halides and in AI₂O₃ where a strong dependence of diffusion rates on the presence of grain boundaries was found. Enhancing oxidation rates in dense, reduced yttrium iron garnet at low temperature by minimizing the final fired grain size in the sintering process does not appear to be possible on the basis of the results obtained in this investigation. The temperature dependence of the diffusion coefficient of oxygen measured at 100 torr can be represented by D = 0.40 exp (-65.4/RT) cm² per second.     

Highly Reactive β-Dicalcium Silicate

Calcium silicate hydrate was prepared by hydrothermal reaction between calcium oxide and silica (C/S = 2.0) at a temperature of 205°–215°C and a pressure of 17–19 bar. This reaction with decomposition at 900°C produced highly reactive β-dicalcium silicate (specific surface area 4.55 m²/g) contaminated with small amounts of wollastonite as an impurity. Infrared spectral studies have shown that β-dicalcium silicate prepared at 900°C is less symmetric compared with the control prepared at 1450°C using boron trioxide as a stabilizer. The specific surface area of β-dicalcium silicate decreased with temperature. The hydration studies were done by determining the nonevaporable water, calcium hydroxide (CH) contents, and specific surface area of the hydrated samples. X-ray diffraction studies were also done. The results showed that prepared β-dicalcium silicate is highly reactive. Calcium chloride (1.0 wt%) and gypsum retard the hydration. Possible causes of high reactivity have been discussed.     

Preparation of Nanohydroxyapatite by a Sol–Gel Method Using Alginic Acid as a Complexing Agent

Single-phase calcium hydroxyapatite (HAP) was synthesized by a sol–gel route using calcium nitrate solution and ammonium dihydrogen phosphate solution as Ca and P sources and alginic acid as a chelating agent. The dried gel was heat-treated at different temperatures in the range of 110°–900°C. Structural evolution from sol to gel and from the gel to HAP was studied by the powder X-ray diffraction method, Infrared spectroscopy, and thermal analyses. The formation temperature of HAP was confirmed to be ∼300°C. Examination of the product obtained at 300°C under TEM suggested the development of hexagonal-shaped nanoparticles, with the average particle size in the range of 50–100 nm.     

Retrograde Densification in Bi2Sr2CaCu2O8 Superconductors

Bi₂Sr₂CaCu₂O₈ was prepared using the mixed oxide-carbonate method and sintered at temperatures ranging from 850° to 911°C. The samples were characterized for density, mechanical strength, phase composition, microstructure, and superconducting transition temperatures. A unique retrograde densification characteristic is demonstrated in the temperature range 850° to 890°C whereby the material first becomes less dense as the sintering temperature is raised, and only in a narrow temperature range from 900° to 905°C does the material densify then with the formation of a liquid phase. The retrograde densification mechanism is shown to be that of the formation of thin platelike crystallites which grow in a randomly oriented fashion, thus pushing the structure apart. This retrograde densification, coupled with a narrow sintering range overlapping the melting temperature, makes this compound a difficult one to process.     

Crystallization of Fluorite-Type Solid Solution from Alkoxides in the System Y2O3-TiO2

Fluorite-type solid solutions crystallize directly at lower temperatures from amorphous materials prepared by the simultaneous hydrolysis of yttrium and titanium alkoxides. The lattice constant of the solid solution changes continuously as a function of composition. The kinetics of crystallization of the fluorite phase have been studied by X-ray measurements. The initial stage at each temperature proceeds rapidly in a short time. The final stage can be expressed in terms of the contracting cube equation 1—(1 —f)¹¹³= kt, the activation energy being 184 mol⁻¹. A pyrochlore-type solid solution corresponding to the composition Y₂Ti₂O₇ crystallizes at 8l5° to 900°C.     

Sintering and grain growth control of high dense YIG

The densification and grain growth of yttrium iron garnet (YIG) were systematically studied to produce highly densified YIG via conventional solid-state route (CSSR). The percentage of purity and structure of YIG was confirmed by XRD characterization. SEM micrographs revealed that with increasing sintering temperature and time, the grain size and the average pores radius (R_p) increased, while the number of pores per volumes (N_v) decreased. The maximum material density obtained using Archimedes principle was 97.9% of that of theoretical density (ρ_theory). It required approximately 132.55 kJ/mol of energy to produce dense YIG sintered for 6 h at 1420 °C. However, beyond this temperature, a new phase that confirmed the presence of YFe₂O_4-δ phase was found through EDX analysis along the grain boundaries. This occurrence lowered the grain boundary mobility thereby resulting in slight change in density. Therefore, the results suggested that a highly densified YIG (ρ_theory of ≈98%) could be successfully obtained when YIG is sintered at 1420 °C for 6 h.     

Phase Relations Applicable to the Garnet Phase Y3Fe4AlO12

Phase relations of the system Fe₂O₃-Y₂O₃-Al₂O₃ were studied at 1500° and 1525°C in air and in oxygen at 1 atm. Isothermal-isobaric sections indicate that the liquids phase field at 1500°C is larger in oxygen than in air. In either atmosphere, at this temperature, the composition of the garnet phase in equilibrium with a liquid is enriched in aluminum relative to the liquid. In the same manner, yttrium orthoferrite is enriched in aluminum relative to garnet in equilibrium between these two phases. The limit of solid solubility of excess iron-aluminum and/or yttrium in the garnet phase Y₃Fe₄AlO₁₂ was determined by X-ray diffraction techniques to be 0.2 ± 0.05 mole % Y₂.O_3.     

Kinetics of Ceramic-Metal Composite Formation by Reactive Metal Penetration

The rate of composite formation via reactive metal penetration has been determined. The metal penetration depth (i.e., the reaction-layer thickness) was measured from cross sections of partially reacted samples. Samples were fabricated by immersing dense mullite preforms in a bath of molten aluminum at temperatures of 900°–1300°C and reacting the combination for up to 250 min. In general, the reaction-layer thickness increased linearly as the time increased. Penetration rates as high as 6.0 mm/h were measured; however, the aluminum penetration rate varied dramatically with time and temperature. The penetration rate increased when the reaction temperature was increased from 900°C to 1100°C, and the reaction-layer thickness increased linearly as the time increased in this temperature range. At temperatures of 1150°C and above, reaction-layer formation slowed or stopped after a relatively short period of rapid linear growth, because of an increase in silicon concentration near the reaction interface. The duration of the rapid linear growth period decreased from 25 min at 1150°C to <1 min at 1250°C. At temperatures of 1300°C and above, no reaction layer was detected by using optical microscopy. Kinetics data and transmission electron microscopy analysis suggest that the reaction was inhibited at higher reaction temperatures and longer times, because of silicon buildup and saturation at the reaction front. Calculations show that, as the reaction temperature increased, the silicon production increased faster than the silicon transport. The two rates were approximately equal at a temperature of 1100°C.     

Fabrication of Dense, Shaped Barium Cerate by the Oxidation of Solid Metal-Bearing Precursors

Some rare-earth-doped alkaline-earth cerates (e.g., Nd₂O₃-doped BaCeO₃) are proton conductors that can act as solid state electrolytes in hydrogen or steam sensors, hydrogen pumps, steam electrolyzers, and fuel cells. In the present study, dense tapes of neodymia-doped barium cerate were produced by the oxidation of malleable Ba-CeH₃-Nd₂O₃precursor tapes. Precursor tapes were produced by cold-rolling either blended or milled mixtures of Ba, CeH₃, and Nd₂O₃ within a silver can. X-ray diffraction, electron microprobe analyses, and thermogravimetric analyses were used to study the phase transformations resulting from heat treatments conducted at 300° to 900°C in pure, flowing oxygen. Barium peroxide and cerium dioxide, produced during oxidation at ≤300°C, reacted at ≤500°C to form barium cerate. The rate of formation of barium cerate depended on the degree of milling of the solid metal-bearing precursor powder. Dense barium cerate tapes were obtained after heat treatment at 1080°C.     

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.