Phase Relations in the System Fe2O3–Fe3O4–YFeO3 in Air

Measurements were made of temperature and ternary composition for coexisting liquid and crystalline phases on the air isobar in the system Fe₂O₃-Fe₃O₄-YFeO₃ with particular regard to the stability range and compositional limits of yttrium iron garnet. Phase equilibrium relations were determined by conventional quenching techniques combined with measurements of loss in weight at the reaction temperature to locate true ternary compositions. The intersection of the air isobar with the ternary univariant boundary curve for coexisting magnetite, garnet, and liquid phases results in a eutectic-type situation at the composition Y_0.27Fe_1.73 O_2.87 and 1469°± 2°C. A similar intersection of the isobar with the boundary curve for coexisting garnet, orthoferrite, and liquid phases gives rise to a peritectic-type reaction at 1555° 3°C. and Y_0.44Fe_1.56 O_2.89 The yttrium iron garnet crystallizing from liquids within these temperature and composition limits contains up to 0.5 mole % iron oxide in excess of the stoichiometric formula in terms of the starting composition 37.5 mole % Y₂O₃, 62.5 mole % Fe₂O₃. At 1470° C. the garnet phase in equilibrium with oxide liquid contains 2 mole % FeO in solid solution. The small solubility of excess of iron oxide and partial reduction of the garnet phase in air are unavoidable during equilibrium growth from the melt.     

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.     

Phase Equilibria and Ionic Conductivity in the System ZrO2−Yb2O3−Y2O3

The phase equilibria in the zirconia-rich part of the system ZrO₂−Yb₂O₃−Y₂O₃ were determined at 1200°, 1400°, and 1650°C. The stabilizing effects of Yb₂O₃ and Y₂O₃ were found to be quite similar with <10 mol% of either being necessary to fully stabilize the cubic fluorite-structure phase at 1200°C. The two binary ordered phases, Zr₃Yb₄O₁₂ and Zr₃Y₄O₁₂, are completely miscible at 1200°C. These were the only binary or ternary phases detected. The ionic conductivities of ternary specimens in this system were measured using the complex impedance analysis technique. For a given level of total dopant, the substitution of Yb₂O₃ for Y₂O₃ gives only minor increases in specimen conductivity.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

Glycothermal Reaction of Rare-Earth Acetate and Iron Acetylacetonate: Formation of Hexagonal ReFeO3

The reaction of a mixture of iron acetylacetonate and rare-earth (Tm-Lu) acetate in 1,4-butanediol at 300°C yielded a novel phase of ReFeO₃ having a hexagonal crystal system (a0 = 6.06, c₀= 11.74 A) together with small amounts of Fe₃O₄and/or the garnet phase. The particle size of the product distributed in a narrow range and selected area electron diffraction from a particle having apparent polycrystalline outlines suggested that each particle was actually a single crystal grown from one nucleus. On calcination, the hexagonal phase irreversibly transformed into the perovskite phase at around 980°C. The use of ethylene glycol in place of 1,4-butanediol of the present procedure afforded Fe₃O₄, while hydrothermal reaction of the same starting materials yielded a mixture of Fe₂O₃and an amorphous rare-earth phase.     

Properties of Yttrium Oxide Ceramics

The cubic structure of yttrium oxide is stable to 1800°C. in air as indicated by petrographic, X-ray, and differential thermal analyses. A change in lattice parameter of less than ±0.007 a.u. was observed on heating the oxide to 1800°C. The mean specific heat of Y₂O₃ to 1600°C. was 0.13 cal. per gm. per °C. The coefficient of linear expansion to 1400°C. was 9.3 × 10⁻⁶ in. per in. per °C. Compacts of Y₂O₃ required a temperature of 1800°C. for vitrification. In equimolecular binary mixtures heated in the powdered state at 1500°C., Y₂O₃ formed compounds with Al₂O₃ and Fe₂O₃ and solid solutions with ZrO₂ and HfO₂. Y₂O₃ did not react with CaO, MgO, or ThO₂. Crystal types and unit-cell sizes of the reaction products are included.     

Composition Limits and Magnetic Properties of Sintered Y2.66Gd0.34FexAl0.677Mn0.09O12 Garnets

Single-phase garnet solid solutions can be synthesized between the composition limits of x =4.18 and x =4.22 in Y_2.66Gd_0.34Fe _x Al_0.677Mn_0.09O₁₂ at temperatures between 1340° and 1500°C in O₂. Solid solutions occur only on the Y₂O₃-excess side of the stoichiometric garnet composition. Electromagnetic properties and microstructural features of sintered garnets depend critically on small changes in Fe content in the vicinity of the garnet solid-solution region. An intergranular spinel-type second phase exists for compositions when x >4.22 and has a deleterious effect on remanent induction and magnetic loss at 3 GHz. The relative density of powder compacts sintered for 16 h at 1500°C in O₂ increases with increasing Fe content (i.e. as x increases) in the garnet solid solution.     

Phase Relations in the Garnet Region of the System Y2O3–Fe2O3–FeO–Al2O3

Liquidus equilibrium relations for the air isobaric section of the system Y₂O₃–Fe₂O₃–FeO–Al₂O₃ are presented. A Complete solid-solution series is found between yttrium iron garnet and yttrium aluminum garnet as well as extensive solid solutions in the spinel, hematite, orthoferrite, and corundum phases. Minimum melting temperatures are raised progressively with the addition of alumina from 1469°C in the system Y–Fe–O to a quaternary isobaric peritectic at 1547°C and composition Y _0.22 Fe _1.08 Al _0.70 O _2.83* Liquidus temperatures increase rapidly with alumina substitutions beyond this point. The thermal stability of the garnet phase is increased with alumina substitution to the extent that above composition Y _0.75 Fe _0.65 Al _0.60 O ₃ garnet melts directly to oxide liquid without the intrusion of the orthoferrite phase. Garnet solid solutions between Y _0.75 Fe _1.25 O ₃ and Y _0.75 Fe _0.32- Al _0.93 O ₃ can be crystallized from oxide liquids at minimum temperatures ranging from 1469° to 1547°C, respectively. During equilibrium crystallization of the garnet phase, large changes in composition occur through reaction with the liquid. Unless care is taken to minimize temperature fluctuations and unless growth proceeds very slowly, the crystals may show extensive compositional variation from core to exterior.     

Phase Transformation in EB-PVD Yttria Partially Stabilized Zirconia Thermal Barrier Coatings during Annealing

Electron-beam physical-vapor-deposited thermal barrier coatings consisting of ZrO₂ stabilized by 7 wt% Y₂O₃ were investigated in regard to phase transformation after annealing. Free-standing ceramic layers were heat-treated in air, for up to 200 h, in the temperature range 1200°—1400°C and then analyzed by X-ray diffractometry. Based on information obtained from the {111} and {400} peaks, the phase composition and the Y₂O₃ content in the phases were calculated. At the start of transformation, small grains of a low-Y₂O₃ t phase and a c phase formed. After >30 h at 1300°C and at 1400°C, a mixture of a t phase deficient in Y₂O₃, an m phase, and a c phase formed after cooling, with the Y₂O₃ contents in the phases roughly predicted by the phase diagrams. The results of the present study are discussed here in detail and compared with data for plasma-sprayed coatings.     

Preparation of Strontium Ferrite Particles by Spray Pyrolysis

Crystalline, submicrometer strontium ferrite powders, including SrFeO_2.97, SrFe₂O₄, Sr₂FeO₄, Sr₃Fe₂O_6.16, and SrFe₁₂O₁₉, were prepared by spray pyrolysis of an aqueous solution of mixed metal nitrates. The Sr:Fe mole ratio in the precursor solution was retained in the final products. Phase-pure materials were typically obtained only at the highest temperatures investigated (>1100°C) and powders prepared at lower temperatures frequently contained crystalline Fe₂O₃. The as-prepared particles were unagglomerated, polycrystalline, and hollow at lower temperatures, but densified in the gas phase at higher temperatures to give solid particles. The strontium ferrite (SrFe₁₂O₁₉) system was studied in detail as a representative example of the Sr-Fe-O system. At temperatures of 1200°C, dense, phase-pure magnetoplumbite-structure material, SrFe₁₂O₁₉, was obtained, while at lower temperatures, small amounts of Fe₂O₃ were observed. The particles prepared at 800° and 1100°C were 0.1-1.0 μm in diameter, and consisted of crystallites <100 nm, and were nearly solid. The difficulty in forming phase-pure SrFe₁₂O₁₉ was the different thermal decomposition temperatures of Sr(NO₃)₂ (725°C) and Fe(NO₃)₃9H₂O (125°C) as demonstrated by thermogravimetric analysis in the SrFe₁₂O₁₉ system.     

Growth of Single Crystals of Incongruently Melting Yttrium Iron Garnet by Flame Fusion Process

Single crystals of yttrium iron garnet (Y₃Fe₅O₁₂) have been grown using the flame fusion process, even though the compound is reported to melt incongruently. The growth of these single crystals involves a mechanism different from that which has been proposed for the growth of single crystals of incongruently melting mullite. Crystal boules were grown at varying linear growth rates and analyzed with chemical, X-ray, and metallographic techniques. With high linear growth rates, the samples are uniformly polycrystalline and three-phase, containing Fe₂O₃, YFeO₃, and Y₃Fe₅O₁₂. When slow linear growth rates are used, single-crystal Y₃Fe₅O₁₂ can be grown. The mechanism is as follows: At the beginning of growth the first phase to precipitate is YFeO₃, and during this stage in growth the molten cap becomes enriched in Fe₂O₃, compared with the Y₃Fe₅O₁₂ composition. The liquid cap composition thus changes to the limit of the peritectic on the Fe₂O₃-rich side, and Y₃Fe₆O₁₂ then crystallizes from the bottom of the melt as Y₃Fe₅O₁₂ powder is added to the top of the molten cap. The central sections of these boules are single-crystal yttrium iron garnet.     

Subsolidus Reactions in the System Fe2O3- TiO2

A study has been made of the binary system Fe₂O₂-TiO₂ by solid-state reactions under dry and hydrothermal conditions. Under dry conditions only one binary compound, pseudobrookite (Fe₂O₃-TiO₂), was formed and no evidence of solid solution on either side of this compound at temperatures up to 1200°C. was obtained. The system under these conditions is a simple binary with a single binary compound. Under hydrothermal conditions of 300° C. and 1200 Ib. per sq. in. the system is apparently also binary, with a single unstable compound closely resembling, if not identical with, the naturally occurring mineral arizonite (ferric metatitanate, Fe₂(TiO₃)₃).     

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.     

High-Temperature Phase Relations in the System Y2O3-Ta2O5

The phase relations for the system y₂o₃–Ta₂o₅ in the composition range 50 to 100 mol% Y₂O₃ have been studied by solid-state reactions at 1350°, 1500°, or 17000C and by thermal analyses up to the melting temperatures. Weberite-type orthorhombic phases (W2 phase, space group C222₁), fluorite-type cubic phases (F phase, space group Fm3m )and another orthorhombic phase (O phase, space group Cmmm )are found in the system. The W2 phase forms in 75 mol% Y₂O₃ under 17000C and O phase in 70 mol% Y₂O₃ up to 1700°C These phases seem to melt incongruently. The F phase forms in about 80 mol% Y₂O₃ and melts congruently at 2454° 3°C. Two eutectic points seem to exist at about 2220°C 90 mol% Y²O₃, and at about 1990°C, 62 mol% Y₂O₃. A Phase diagram including the above three phases were not identified with each other.     

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.     

Co2+-Substitution Effect in Ce-TZP/La M-type Hexaferrite Composites

A Ce-TZP/platelike La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composite was synthesized in situ while sintering from a mixture of Ce-TZP, La(Fe_0.9Al_0.1)O₃, Fe₂O₃, Al₂O₃, and CoO powders. Platelike La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ crystals were grown in a dense Ce-TZP matrix after sintering at temperatures of 1200°–1350°C. The temperature range for sintering Ce-TZP/La(Fe,Al)₁₂O₁₉ composites was expanded widely by substituting Co²⁺ ions for Fe²⁺ ions in its structure. The highest value of the bending strength of the Ce-TZP/La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composites was 880 MPa, which was higher than that of the Ce-TZP/La(Fe,Al)₁₂O₁₉ composite (780 MPa) and Ce-TZP (513 MPa). The saturation magnetization of the Ce-TZP/La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composite was a constant value of 7.7 emu/g after the composite was sintered at 1200°–1350°C.     

10 kbar Phase Equilibria in the Y-Ba-Cu Oxide System at 950°C

Phase equilibria in the YO_1.5BaO-CuO system have been determined at 950°C at 10 kbar using a piston-cylinder apparatus. The oxide phases stable under these conditions are Y₂O₃, Y₂Cu₂O₅, CuO, Y₂BaCuO₅, YBa₂Cu₃O_6.5, BaCuO₂, Y₂BaO₄, Y₂Ba₃O₆, and YBa₄Cu₂O_7.5. The phase stabilities observed at 950°C at 10 kbar are identical to those observed at 950°C in air or oxygen at 1 atm for compositions with <40% Ba of the cations. In more Ba-rich portions of the phase diagram, carbonates and oxycarbonates are stabilized and a systematic determination of the phase equilibria has not been successful.     

Densification of Nanocrystalline Yttria by Low Temperature Spark Plasma Sintering

We investigated the densification of undoped, nanocrystalline yttria (Y₂O₃) powder by spark plasma sintering (SPS) at sintering temperatures between 650°C and 1050°C at a heating rate of 10°C/min and an applied stress of 83 MPa. In spite of the low sinterability of the undoped Y₂O₃, a remarkable densification of the powder started at about 600°C, and a theoretical density of more than 97% was achieved at a sintering temperature of 850°C with a grain size of about 500 nm. The low temperature SPS is effective for fabricating dense Y₂O₃ polycrystals.     

Compressive Properties of Yttrium Oxide

Compressive properties of polycrystalline yttrium oxide (Y₂O₃) were studied by slow-strain-rate experiments (ε= 5.7 × 10^–6 s⁻¹) between room temperature and 1600°C. It was shown that Y₂O₃ fails in a brittle manner up to 1000°C, and at 1200°C and above plastic deformation becomes dominant. Plastic deformation of Y₂O₃ takes place exclusively by dislocation motion. Maximum stress, yield stress, and elastic modulus decrease with increasing temperature, although the decrease at temperatures above 1000°C is much more pronounced.     

Effect of Additives on the Pressureless Sintering of Aluminum Nitride between 1500° and 1800°C

Combined oxide additives (Y₂O₃, CaO, La₂O₃, CeO₂, SiO₂, TiO₂, and Fe₂O₃) were investigated as AIN sintering aids. AIN can be fully sintered at 1600°C to substantial thermal conductivity (92 W/(m·K)) using a multiple sintering aid of Y₂O₃, CaO, SiO₂, La₂O₃, and CeO₂. This lowtemperature material has small grain size (1 to 3 μm).