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.     

High-Pressure Experiments on Yttrium Iron Garnet

High-pressure-high-temperature experiments carried out on YIG and on 3Y₂O₃+5Fe₂O₃ mixtures gave expected results and do not corroborate the recent report by Shimada, Kume, and Koizumi indicating the existence of a "dense ferrimagnetic perovskite allotropic form of yttrium iron garnet." The results of experiments by Marezio, Remeika, and Jayaraman on the high-pressure decomposition of synthetic garnets are discussed. The garnet {Y₃}[Sc_0.6Fe_1.4](Fe₃)O₁₂ is more stable than YIG under the same conditions of pressure and temperature; this alone disproves the Marezio et al. hypothesis relating stability vs pressure to ionic site preference. The stability of YIG is more sensitive to oxygen fugacity at high temperatures and atmospheric pressure than are those of YAG and YGaG. Thus relative compressibility and equilibrium oxygen vapor pressure are the important parameters.     

Microstructure Development in Yttrium Iron Garnet

Sintering and grain-growth rates were determined for yttrium iron garnet as a function of the yttrium/iron ratio. Rates decreased with an increase in this ratio; this behavior is interpreted as a result of the oxygen content variation through the garnet field. For constant sintering time, density and grain size, as well as microstructure-dependent properties, varied through the garnet field. Remanent magnetization and coercive force in particular depended on composition and sintering temperature. The rf field for nonlinear spin-wave excitation, h _crit, measured for dense samples with grain sizes from ∼4 to 30 μm, varied by a factor of four.     

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.     

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.     

Reaction Kinetics of Polycrystalline Yttrium Iron Garnet

The formation of yttrium iron garnet, Y₃Fe₂-(FeO₄)₃, starting with (1) Fe₂O₃ and Y₂O₃ and (2) Fe₃O₄ and Y₂O₃, was studied as a function of temperature and time by means of magnetic moment and X-ray measurements. The reaction began at 600°C. and was completed at 1200°C. The perovskite phase appeared only between 600° and 800°C. Above 1200°C. only the garnet phase was present. The microwave line width and g -factor at 9303 mc. per second were also measured and related to the preparation variables.     

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.     

Precipitation and Calcination Processes for Yttrium Aluminum Garnet Precursors Synthesized by the Urea Method

Yttrium aluminum garnet (YAG) precursors for transparent ceramics were synthesized by the urea method under various [urea]/[metal ions] ([U]/[M]) conditions. Monophase YAG was obtained from solutions with a high [U]/[M] ratio after calcination at a temperature of 1200°C. The condition of the precipitates seemed to indicate that the yttrium compounds had precipitated onto the aluminum compounds. The surface morphology and size of the particles were controlled by the [U]/[M] ratio. The different reaction sequences of YAG crystallization for low- and high-ratio samples were dependent on morphology, size, and the quantity of chemical species that was precipitated as carbonate and/or sulfate compounds.     

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.     

Chemical Vapor Deposition of Epitaxial Films of Yttrium Iron Garnet and Gallium-Substituted Yttrium Iron Garnet and a Thermodynamic Analysis

Epitaxial single-crystal films of yttrium iron garnet (YIG) and Ga: YIG (Y₃Fe_{5- x} Ga _x O₁₂) were grown by chemical vapor deposition. The garnet is deposited by the reaction of gaseous metal chlorides with O₂ at 1150°C and 5 torr. The chlorides are generated in-line by reacting Cl₂ gas with a Y-Fe(-Ga) alloy. A computer-aided thermodynamic calculation was developed to determine which of several possible solids in a multicomponent CVD system will be deposited for a given set of growth conditions. The calculation determines an intermediate equilibrium among all gaseous species and then identifies the solid phase most favored to be deposited from the supersaturated gas phase. The results of the calculations agreed well with experiment and have been useful in finding the conditions for optimum growth of garnet. This type of calculation can be applied to other multicomponent CVD systems.     

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.     

Yttrium Iron Garnet/Barium Titanate Multiferroic Composites

Dense multiferroic 0‐3 type composites encompassing BaTiO₃ and Y₃Fe₅O₁₂ were fabricated by the solid‐state reaction method. X‐ray diffraction data combined with scanning electron microscopy imaging show virtual immiscibility between the two phases, with the Y₃Fe₅O₁₂ ferrimagnetic phase well dispersed in the tetragonal BaTiO₃ ferroelectric matrix. Raman spectroscopy analyses corroborate the polar nature of the BaTiO₃ matrix in composites with a Y₃Fe₅O₁₂ content as great as 40 wt%. Ferrimagnetism is detected in all composites and no additional magnetic phases are distinguished. Although these dense ceramics can be electrically poled, they exhibit a very weak magnetoelectric response, which slightly increases with Y₃Fe₅O₁₂ content.     

Characteristic Imperfections in Flux-Grown Crystals of Yttrium Iron Garnet

Crystals of yttrium iron garnet grown from lead oxide and lead oxide-lead fluoride flux systems are inhomogeneous and contain characteristic imperfections that may be significant in the evaluation of physical properties. A petrographic study of single-crystal sections has revealed dendritic nuclei, oriented layers of submicroscopic foreign material located in the outer regions of the crystals, and oriented occlusions in intermediate positions. Changes in the appearance of thin sections resulting from the use of an oxygen atmosphere during synthesis combined with other observations indicate that divalent iron contributes to inhomogeneities and imperfections present in yttrium iron garnet crystals. Growth mechanisms involving an initial dendritic stage with (100) and (111) fast-growth directions followed by layered growth parallel to major crystal planes, normally (110) and (211), can be deduced from the appearance and location of crystal imperfections.     

Spherical Yttrium Aluminum Garnet Embedded in a Glass Matrix

Containerless processing was used to investigate the glass-forming behavior of Al₂O₃–Y₂O₃ glass. The amorphous bulk samples were obtained at compositions with 25–37.5 mol% yttria when the melt was cooled at a cooling rate of ∼250 K/s. Although small spherical particles (∼10 μm) with the same composition of the matrix were detected in the amorphous samples with 32.5–37.5 mol% yttria, the microfocus X-ray diffraction result indicated that the small spherical particles were crystalline Y₃Al₅O₁₂ garnet (YAG), rather than being amorphous. This observation suggested that small YAG particles could not grow larger after their nucleation, because of the high viscosity at high undercooling and the high cooling rate, which would graze the nose of the continuous cooling temperature diagram of YAG.     

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.