Amorphous Yttrium Research Articles

Magnetic behavior of yttrium–iron–garnet nanoparticles dispersed in glass composites

By absorbing amorphous yttrium–iron–garnet (YIG) nanoparticles into nanosized pores of a controlled pore glass (CPG) followed by heat treatment, we synthesized YIG nanocrystal-dispersed glass composites. In a sample which was prepared with a 300 nm pore size CPG and heat treated at 800 °C for 2 h, almost all the amorphous YIG nanoparticles transformed to YIG nanocrystals. The mass fraction of YIG was estimated to be 4.7% from the measured saturation magnetization. In a sample which was prepared from a 49 nm pore size CPG with the same heat treatment, the amount of YIG was estimated to be only 0.54%. Magnetization measurements of different samples revealed that heat treatment at higher temperatures or for longer times resulted in reactions of the YIG particles with the SiO2 of the CPG, and the composites became paramagnetic. However, even in those samples, large initial magnetic susceptibilities were measured, implying the presence of a small amount of unreacted YIG nanocrystals.

Molecular dynamics simulation of the structure of yttriaY2O3phases using pairwise interactions

We have studied the structure of yttria (Y2O3) by means of ab initio and molecular dynamics methods. The suggested simple model for the interatomic interaction is shown to produce reasonable results at moderate pressures for a wide range of temperatures. The calculated x-ray structure factor is in good agreement with experimental data obtained by the x-ray levitation technique at the temperature of 2526 K. The quality of the agreement decreases with increasing temperature. We demonstrate that it is not necessary to assume nonstoichiometry of liquid yttria, as was done in a recent publication, to obtain agreement with experiment. The structure of liquid yttria can be considered as a mixture of 4- and 6-coordinated Y atoms. We also show the possibility of a light amorphous yttria phase, which possibly can be obtained by quenching from a vapor instead of conventional amorphous yttria quenched from a liquid.

Physical and electrical characterization of ultrathin yttrium silicate insulators on silicon

This article describes the oxidation of yttrium on silicon to form yttrium silicate films for application as high dielectric constant insulators. The high reactivity of yttrium metal with silicon and oxygen is utilized to form amorphous yttrium silicate films with a minimal interfacial silicon dioxide layer. Yttrium silicate films (∼40 Å) with an equivalent silicon dioxide thickness of ∼11 Å and k∼14 are formed by oxidizing yttrium on silicon. The physical properties of yttrium silicate films on silicon are investigated using x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The oxidation of yttrium silicide results in films nearly identical, although with a higher silicon fraction, to films formed by oxidation of yttrium on silicon. The oxidation of yttrium on silicon results in a competition for yttrium between silicide formation and oxidation. This competition yields yttrium silicate films for thin (<40 Å) initial metal thickness and a Y2O3/silicate bilayer for thick (>80 Å) initial metal thickness. Annealing yttrium films on silicon in vacuum to form yttrium silicide and then oxidizing the silicide is used to eliminate the competition and control the yttrium/silicon reaction. Analysis of the oxidation of yttrium on silicon reveals fast oxidation during silicate formation and a slow rate during oxidation of the silicon substrate to form SiO2. Oxidation of other metals, such as Hf, Zr, and La, on silicon is expected to result in metal silicate films through a similar simultaneous (or controlled sequential) silicide/oxidation reactions.

Crystallization Mechanism of Amorphous Mullite and the Al2O3-SiO2Phase Diagram

ABSTRACTRecent progress in the crystallization of amorphous mullite (3Al2O3•2SiO2) and yttrium aluminate (Y3Al5O12, 5Al2O3•3Y2O3or “YAG”) beads and fibers is reviewed. Studies were made by differential thermal analyses, (DTA, DSC), synchrotron X-ray diffraction and Rietveld profile fitting, HREM, EDS and SEM. Crystallization kinetics, activation energies and microstructures were determined, leading to temperature-time-transformation (T-T-T) curves. The crystallization of mullite from the solid state proceeds by formation of large-grained, pseudo-tetragonal mullite of composition 70 mol% Al2O3, which is highly strained and which contains <10 nm silica-containing inclusions. During further heat treatments, the inclusions react with the first-formed mullite yielding single phase, orthorhombic mullite of 60 mol% Al2O3. Recrystallization also occurs on further heating with a significant reduction of grain size, in a manner similar to recrystallization in brass. These observations (using more modern and sensitive characterization techniques) suggest the presence of a peritectic-like, syntectic or syntectoidal invariant reaction in the Al2O3-SiO2phase diagram. Further work needs to be done to ascertain whether the reaction is an equilibrium or non-equilibrium phenomenon.

Synthesis and Properties of Al-Based Amorphous and Microcrystalline Thin Films

ABSTRACTAmorphous metal alloys are ideally suited for interconnects in micro-electromechanical sys- tems (MEMS) because of their resistance against stress- and electromigration, and their stability in chemically aggressive environments, which should both lead to a substantial improvement of lifetime and reliability of robust sensors. While amorphous refractory metal alloys and amor- phous silicides are excellent interconnect materials for devices operating at elevated tempera- tures, these systems lack the cost-effective and easy interconnect processing of the prevalent polycrystalline aluminum alloy metallizations. Amorphous aluminum alloys are applicable to devices operating at up to 200°C, and their stressmigration resistance and chemical stability is far superior to conventional polycrystalline aluminum alloys. These new metallizations are very promising for processing interconnects, in particular because of their high strength and ductility, though having low density, and their relatively low electrical resistivity compared to other amor- phous metal alloys. Therefore these metallizations are especially suited for surface acoustic wave (SAW) sensors, where the interconnects are exposed to considerable mechanical strains. In this work amorphous Aluminum Yttrium alloy thin film metallizations deposited on appropriate sub- strates at room temperature (R.T.) by ultra-high vacuum (UHV) electron beam evaporation will be presented, and their mechanical and electronic properties together with their temperature sta- bility will be investigated.

Preparation of yttrium–iron-garnet nanocrystals dispersed in nanosize-pore glass

Amorphous yttrium–iron-garnet (YIG) nanoparticles were dispersed in kerosene solvent and these colloids were introduced into the nanosize pores of controlled pore glass (CPG). After heat treatment, a YIG nanocrystal-dispersed glass was obtained and observed by electron microscopy. The compounds prepared with different pore size CPG under different heat treatment conditions were identified by X-ray diffraction. For samples with a 292-nm-pore-size CPG matrix which were calcined for 2 h, only YIG was observed up to a 800°C calcination temperature. However, at a heat treatment of 2 h at 900°C, the YIG particles decomposed and many kinds of iron and yttrium silicate compounds were formed. In a 16 h treatment at 675°C, yttrium silicate was also prepared besides YIG. On the other hand, only YIG nanocrystals formed when a rapid temperature rise up to 890°C was followed by immediate cooling. It was also found that iron and yttrium silicates decomposed and iron oxide and yttrium oxide formed by high-temperature treatment at 1200°C. A very rare iron oxide ε-Fe 2O 3 was also found in some samples.

Peculiar sponge-like structure of yttrium–iron–garnet nanocrystals on quartz substrate surface

Nanoscale amorphous yttrium–iron–garnet (YIG) particles were prepared by the alkoxide method. They were dispersed in a kerosene solvent, coated on a quartz plate substrate, and calcined at a temperature of 1273 K for 2 h. Surface morphology and cross-sectional microstructure of the thin coated films were examined by atomic force microscopy and transmission electron microscopy, respectively. During the calcination, amorphous YIG particles were transformed to YIG nanocrystals of ≈25 nm in mean diameter, and no extended grain growth or fusion of the multigrains was observed. Each particle was individually crystallized, but interconnected to each other, forming a sponge-like structure of 600 nm in thickness. Electron diffraction and energy dispersion x-ray analysis verified that the sponge-like layer consisted of YIG nanocrystalline particles. A rather dense intermediate layer of ≈100 nm in thickness was formed as a result of interfacial reactions between YIG and SiO2 decomposing to α-Fe2O3 and Y2Si2O7. The change in the Si concentration across the interlayer depth was modeled by thermal diffusion. This peculiar sponge-like structure of YIG nanoparticles supports our previous interpretation of the shift of the light absorption spectral peak, i.e., due to this peculiar structure, electrons are localized in each YIG particle, which act as a quantum dot, attributing to the quantum size effect observed in the spectral shift.

Yttrium Acetate-Derived Particle Coatings for Mitigating Oxidation and Corrosion of Inconel 625

“Sol paint” that yields yttrium-based compounds was prepared by mixing four chemical ingredients, yttrium acetate tetrahydrate precursor, diethanolamine, isopropyl alcohol, and hydrochloric acid, and then applied as oxidation/corrosion resistant coatings for Inconel 625 substrates. Annealing the coatings at 500°C developed a coalescent microstructure of coarse particles consisting of amorphous yttrium carbonate as the major component and crystalline yttrium oxide (Y2O3) as the minor one. At 700°C, the yttrium carbonate was transformed into Y2O3 by decarbonation. Increasing the annealing temperature to 900°C led to the formation of the YCrO3 phase yielded by interaction between Y2O3 and the Cr2O3 which had arisen from the oxidation of the underlying Inconel; the YCrO3 phase created a particle coating with a densified microstructure. There were two key factors in mitigating the degree of oxidation of Inconel at 900°C in air: (1) an uptake of oxygen by Y2O3 in the coatings, and (2) a densified coating layer that suppresses the diffusion and permeation of oxygen through it. Furthermore, inhibiting the rate of NaCl-caused corrosion was not only due to the excellent coverage of particle coatings over the entire surfaces of the substrates, but also may be associated with a good adherence of the coatings to the substrates.

Synthesis of yttrium oxide thin films with and without the use of organic self-assembled monolayers

Thin Y-containing films have been deposited at 80 °C from aqueous solutions of YNO3⋅5H2O and urea on bare single crystal Si wafers, and on Si wafers coated with sulfonate-functionalized organic self-assembled monolayers. The as-deposited films are believed to be an amorphous yttrium basic carbonate and can be completely transformed at 600 °C in air to crystalline Y2O3. Capacitance–voltage measurements on these films showed good dielectric properties, with a relative permitivity of 18, more than a factor of four higher than that of SiO2.

Micromorphology, Microstructure and Magnetic Properties of Sputtered Garnet Multilayers

ABSTRACTThe growth technique, the micromorphological and microstructural characterization by means of atomic force microscopy (AFM) and secondary ions mass spectrometry (SIMS) as well as the magnetic properties of a novel class of magnetic multilayers, based on radio frequency (RF) sputtered thin amorphous garnet films, are presented. One, three and five thin film multilayers composed by amorphous pure yttrium iron garnet (a:YIG) and amorphous gadolinium gallium garnet (a:GGG) have been grown on GGG single crystal substrates. The multilayer interfaces have been found to be comparable in both, the three and five-layers structure. Low field susceptibility measurements, showed a paramagnetic behaviour for the single layer YIG film. For the three and five layers samples, irreversibility effects were observed, giving evidence of magnetic clusters at the interface YIG/GGG.

Optical properties of Ca-substituted amorphous yttrium iron garnet films

In this work the optical absorption properties of Ca-substituted yttrium iron garnet films are discussed. In particular the optical absorption of sputtered amorphous garnets film and the contribution of Ca-ions are analyzed. A comparison with the optical response of crystalline garnets is also made.

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 To 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 To= 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.

Kinetics of morphological change during annealing of amorphous yttria

We report a time-at-temperature study of air annealing at 700 °C of amorphous yttria films sputter deposited on fused silica. The results show that crystallization was limited (7%). A striking morphological change involved the formation of 2.5 μm-maximum diameter globules. By comparing changes in globule number with maximum diameter, the time-at-temperature ranges for which nucleation+growth and growth+coarsening processes occurred was determined. The globule formation rate initially followed an Arrhenius curve: dNV/dt = (4 × 105/cc s)exp[ − 0.28 × 10−3/s)t]. The activation energy estimated for globule formation was close to that for Y diffusion in yttria.

High temperature air-annealing behavior of sputter deposited amorphous yttria films on fused silica

This article addresses the response of amorphous yttria films grown on fused silica substrates to high temperature (900 °C) cyclic annealing in air for up to 145.5 h. Surface science and bulk analytical techniques were applied to determine and correlate changes in microstructure, crystallization, optical behavior, and chemistry (atomic concentration and bonding) concurrent with annealing. Limited crystallization occurred after short-term annealing, resulting in regions of nanocrystallinity in an amorphous matrix. X-ray photoelectron spectroscopy data showed an accompanying loss of resolution of Y 3d5/2 − 3d3/2 spin states, but no change in the fundamental optical absorption edge was observed. Long-term annealing resulted in granularization of the matrix itself, with a large increase in optical absorption in the 3–5 eV range. X-ray photoelectron spectroscopy data showed a broadening of the O 1s peak. No evidence of Si diffusion from the interfacial region into the yttria film was seen, even after long-term annealing, demonstrating amorphous yttria’s potential use as a high temperature diffusion barrier for metal-on-Si-based ceramic applications.

Partial oxidation of methane over fine particles of various amorphous oxides including lanthanum

Partial oxidation of methane was carried out over fine particles of amorphous lanthanum oxides including 3B element and amorphous yttrium aluminum oxide. At low temperatures and low methane/oxygen ratios products were CO, CO2, C2 hydrocarbon, hydrogen and water, and no formaldehyde was detected. The discussion on the contribution of catalyst components to the reaction suggested that 3B elements substantially influenced methyl radical formation and consumption while rare earth elements sufficiently affected formaldehyde decomposition.

A semi-wet route to the synthesis of YBa2Cu3O7-y ceramics

Advantages of a semi-wet route to the synthesis of YBa2Cu3O7-y employing coprecipitation of yttrium and barium as precursor carbonates are presented. It is shown that the precursor carbonate is an intimate mixture of fine barium carbonate and amorphous yttrium carbonate particles. YBa2Cu3O7-y powders prepared under identical conditions by the conventional dry route and the present semi-wet route are analysed for compositional inhomogeneities and coherently scattering domain size (D), using principles of X-ray line profile analysis. It is shown that unlike the powders prepared by the conventional method, which contain compositional inhomogeneities and possess domains of about 2200 AA, powders obtained under identical conditions by the semi-wet route are free from compositional inhomogeneities and possess larger domain size (around 4000 AA). Since the actual particle size in both the cases is several mu m, the loss of coherency in each particle is attributed to the presence of twin interfaces. Better compositional homogeneity and lower density of twin interfaces in specimens prepared by the semi-wet route are believed to be responsible for the improved superconducting behaviour of ceramics prepared by the present method.

Thermal transformation of yttrium hydroxides to yttrium oxides

The reaction between aqueous solutions of yttrium salts (chloride and nitrate) at constant concentration and alkaline solutions (sodium and ammonium hydroxides) were investigated under different conditions. As a result, amorphous and hexagonal yttrium hydroxides, Y(OH) 3, were formed and in addition monoclinic yttrium hydroxide, α-Y(OH) 3, was prepared hydrothermally from amorphous yttrium hydroxide. The materials so obtained were examined by thermogravimetry and differential thermal analysis (TG and DTA), and yttrium hydroxides and their thermal decomposition products by X-ray diffraction study and infrared spectrophotometry. The sequence for the thermal decomposition of those materials is proposed on the basis of the results obtained.

Theory of hyperfine- and exchange-field distributions in amorphous speromagnets with application to amorphous yttrium iron garnet

Theory of hyperfine- and exchange-field distributions in amorphous speromagnets with application to amorphous yttrium iron garnet

Crystal field splittings in dilute amorphous rare earth alloys

Inelastic neutron scattering has been used in an attempt to characterize the crystal field of isolated rare earth ions in an amorphous matrix. Amorphous yttrium copper alloys in which 5% of the yttrium was replaced by Ce, Pr, and Er were studied using neutron time-of-flight spectroscopy. Both the Ce and Pr alloys showed quasielastic paramagnetic scattering at all temperatures between 15 and 300 K. The Er alloy however displayed a well defined inelastic peak centered at 2 meV at low temperatures which was identified with a crystal field transition. To our knowledge this is the first observation of a well-defined spectroscopic transition for rare earths in an amorphous environment.

Observation of linewidth asymmetry in the Mössbauer Zeeman spectrum of amorphous yttrium iron garnet

A measurement of the six-line M\ossbauer Zeeman spectrum of $^{57}\mathrm{Fe}$ in amorphous yttrium iron garnet at 4.2 K reveals a linewidth asymmetry involving the three line pairs ${L}_{i}$ and ${L}_{7\ensuremath{-}i}$, $i=1, 2, 3$. The major source of this asymmetry is found to be a positive correlation between isomer shift and hyperfine field with a degree of linearity of 18%. A complete linewidth analysis is given which also leads to separate estimates of the widths of the hyperfine field, isomer shift, and electric-field-gradient distributions in the glass.