Synthesis of Y<Subscript>3</Subscript>Al<Subscript>5</Subscript>O<Subscript>12</Subscript>:Ce<Superscript>3+</Superscript> phosphor in the Y<Subscript>2</Subscript>O<Subscript>3</Subscript>–Al metal–CeO<Subscript>2</Subscript> ternary system

Garnet phosphor Y₃Al₅O₁₂:Ce³⁺ is prepared in the Y₂O₃–Al metal–CeO₂ ternary system by the solid-state reaction method in the air. For the first time, metal Al is used as a source of aluminum for the reaction instead of traditional oxide Al₂O₃. It is shown that the chemical reaction can be realized at lower temperatures and without use of special reducing atmosphere. The structural and spectroscopic properties of the prepared powder phosphor are very close to those earlier reported for the Y₃Al₅O₁₂:Ce³⁺ single crystal.     

High-temperature oxidation of yttrium silicides

Current silicon melt-infiltrated (SMI) ceramic matrix composites (CMCs) are limited by the melting temperature of silicon (1414 °C) and the volatility of the thermally grown SiO₂ scale in high-temperature water vapor environments. Replacement of the melt-infiltrated (MI) silicon with a rare-earth silicide offers the potential to address both limitations of SMI CMCs. This study focuses on the ability of yttrium silicides to form yttrium silicates (phases with greater stability in high-temperature water vapor than SiO₂) in high-temperature oxidizing environments. Yttrium silicides with compositions of 41, 67 and 95 at.% Si–Y were fabricated using arc melting and oxidized in air at 1000 and 1200 °C for up to 24 h. Oxidation resulted in the rapid formation of a non-protective Y₂O₃ scale and rejected Si. Additional minor oxide phases of Y–Si–O, Y₂SiO₅, Y₂Si₂O₇ and SiO₂ were observed to form on and beneath the specimen surface. Characterization of the microstructural evolution with time and temperature helped elucidate the diffusion mechanisms that control oxide growth rates. Replacement of MI silicon with a MI yttrium silicide would significantly compromise the high-temperature performance of a CMC due to Y₂O₃ CTE mismatch with SiC, high oxygen permeability and the large volume change associated with its rapid subsurface formation. Results are utilized to examine the viability of other rare-earth silicides as MI materials for CMCs.     

Metal composition of Y2O3:Eu powder evaluated using particle analyzer

Y₂O₃:Eu red phosphor powder was synthesized with powders of metal–ethylenediaminetetraacetic acid (EDTA) complexes as a starting material. The compositional analysis of each metal–EDTA complexes particle and Y₂O₃:Eu particle was performed using a particle analyzer. In this study, first, the particles of a mixture of Y– and Eu–EDTA complexes were obtained by a spray drying method from solution consisting of Y– and Eu–EDTA·NH₄. Then, the metal–EDTA complex powder was fired in obtaining the Y₂O₃:Eu red phosphor. The metal composition of each particle was scattered for the powder of the metal–EDTA mixture, while it became narrow for the Y₂O₃:Eu powder. The intensity of cathodoluminescence obtained from the Y₂O₃:Eu powder increased with increasing the fired temperature. In addition, the maximum intensity was obtained from the sample with x=0.12 for Y_2−xO₃:Eu_x.     

Phase transformations in yttrium –aluminium oxides in friction stir welded and recrystallised PM2000 alloys

Abstract

PM2000 is an Fe –Cr – Al oxide dispersion strengthened (ODS) alloy containing 0.5wt% of fine, uniformly dispersed, yttrium oxide particles in a ferritic matrix. The alloys are attractive candidates for high temperature applications since nano-dispersoids improve the creep resistance of the alloys at high temperatures. Friction stir welding (FSW) has been used successfully for the joining of PM2000 sheet without oxide particle agglomeration and significant change in the microstructure. However, it has been reported that the initial Y₂O₃ particles may sometimes oxidise the aluminium from the surrounding matrix to form mixed Y–Al oxides. Hence, in this study, we have been using extraction replication plus high-spatial resolution scanning transmission electron microscopy (STEM) to investigate phase transformations and oxidation of Y–Al during FSW processing and recrystallisation treatments (1380°C, 1 hour in laboratory air).

High-resolution SuperSTEM images indicate that the Y₂O₃ can transform to Y₃Al₅O₁₂ garnet (YAG) and YAlO₃ perovskite (YAP) particles even in the consolidated PM2000. These dispersoids appear to be stable during the FSW process, but most of the Y₂O₃ or YAG particles transform into YAP particles after the high temperature recrystallisation treatment at 1380°C. In some cases partially transformed particles were observed and these may enable the details of the oxidation/transformation mechanisms to be elucidated.     

Metal composition of Y2O3:Eu powder evaluated using particle analyzer

Y₂O₃:Eu red phosphor powder was synthesized with powders of metal–ethylenediaminetetraacetic acid (EDTA) complexes as a starting material. The compositional analysis of each metal–EDTA complexes particle and Y₂O₃:Eu particle was performed using a particle analyzer. In this study, first, the particles of a mixture of Y– and Eu–EDTA complexes were obtained by a spray drying method from solution consisting of Y– and Eu–EDTA$NH₄. Then, the metal–EDTA complex powder was fired in obtaining the Y₂O₃:Eu red phosphor. The metal composition of each particle was scattered for the powder of the metal–EDTA mixture, while it became narrow for the Y₂O₃:Eu powder. The intensity of cathodoluminescence obtained from the Y₂O₃:Eu powder increased with increasing the fired temperature. In addition, the maximum intensity was obtained from the sample with xZ0.12 for Y_2KxO₃:Eu_x.     

Interactions between Y2O3–Al mixture studied by solid-state reaction method

Interactions between Y₂O₃–Al mixture studied by solid-state reaction method were investigated in present paper. Interactions between Y₂O₃–Al mixture was characterized by differential thermal and thermogravimetric analyses and X-ray diffraction, Y₂O₃–Al mixture and yttrium aluminum garnet (YAG) powder as final reaction product were characterized by scanning electron microscopy. The results show Al is isolated with Y₂O₃ by aluminum oxide layer in air, and no opportunity of directional reaction between Y₂O₃–Al systems. With temperature increasing to ∼569 °C, aluminum partly turned into transitional aluminas, Y₂O₃ reacts with transitional aluminas instead of aluminum to form yttrium aluminum monoclinic (YAM) and yttrium aluminum perovkite (YAP) phases after calcination at 600 °C, 800 °C separately, and pure YAG powder is obtained after calcination at 1200 °C. From the point of view of reaction temperature, the reaction between Y₂O₃ and transitional aluminas is easier than that of Y₂O₃ and Al or α-Al₂O₃.     

Improvement in fatigue strength while keeping low Young's modulus of a β-type titanium alloy through yttrium oxide dispersion

Improvement in fatigue strength in spite of maintaining low Young's modulus was achieved in Ti-29Nb-13Ta-4.6Zr (TNTZ) by hard-particles dispersion. A certain amount of Y₂O₃ additions was added into TNTZ. TNTZ with 0.05-1.00mass%Y consists of a β-phase with a small amount of Y₂O₃. Young's moduli of TNTZ with 0.05-1.00mass%Y are maintained low, and are almost similar to that of TNTZ without Y₂O₃. The tensile strength of TNTZ with 0.05-1.00mass%Y is slightly improved and the elongation does not deteriorate by Y₂O₃ additions. However, the 0.2% proof stress decreases with the increase in Y concentration. Although tensile properties are not changed drastically, the fatigue strength is significantly improved by Y₂O₃ additions. The dispersion of Y₂O₃ particle increases the resistance to fatigue initiation. However, Y₂O₃ with too large diameter at the surface of the specimen works harmfully as the fatigue initiation site. The Y₂O₃ diameter and volume fraction increase with the increase in Y concentration. As a result, the fatigue limit of the alloys with 0.05-1.00mass%Y firstly increases and then decreases with the increase in Y concentration. TNTZ with 0.1mass% Y exhibits the best combination of higher fatigue strength and low Young's modulus.     

Effect of boron concentration in Y2O3:(Eu,B) phosphor on luminescence property

Y₂O₃:(Eu,B) red phosphor was obtained by a thermal decomposition technique of a mixture of yttrium–ethylenediaminetetraacetic acid (Y–EDTA), Eu–EDTA and boric acid. The doping ratio of boron, [B]/([Y]C[Eu]), in the starting material was varied from 0 to 1. The properties of morphology, crystallization behavior, metal composition and photoluminescence of resulting Y₂O₃:(Eu,B) powder were examined. The Y₂O₃:(Eu,B) powder had spherical shape with a diameter of 1–5 mm with some hollows on the surface. No secondary cohesive particles were seen. X-ray diffractometry revealed that although the crystallinity of powder was improved with addition of small amounts of boron, it decreased with excess amounts of boron. Several peaks indexed as the impurity phases of Y₃BO₆ and YBO₃ were observed on the samples obtained under condition of excess amounts of boron. The photoluminescence intensity observed at 611 nm was dependent upon crystallinity of the sample.     

Luminescence and crystallinity of flame-made Y2O3:Eu3+ nanoparticles

Cubic and/or monoclinic Y₂O₃:Eu³⁺ nanoparticles (10–50 nm) were made continuously without post-processing by single-step, flame spray pyrolysis (FSP). These particles were characterized by X-ray diffraction, nitrogen adsorption and transmission electron microscopy. Photoluminescence (PL) emission and time-resolved PL intensity decay were measured from these powders. The influence of particle size on PL was examined by annealing (at 700–1300°C for 10 h) as-prepared, initially monoclinic Y₂O₃:Eu³⁺ nanoparticles resulting in larger 0.025–1 μm, cubic Y₂O₃:Eu³⁺. The influence of europium (Eu³⁺) content (1–10 wt%) on sintering dynamics as well as optical properties of the resulting powders was investigated. Longer high-temperature particle residence time during FSP resulted in cubic nanoparticles with lower maximum PL intensity than measured by commercial micron-sized bulk Y₂O₃:Eu³⁺ phosphor powder. After annealing as-prepared 5 wt% Eu-doped Y₂O₃ particles at 900, 1100 and 1300°C for 10 h, the PL intensity increased as particle size increased and finally (at 1300°C) showed similar PL intensity as that of commercially available, bulk Y₂O₃:Eu³⁺ (5 μm particle size). Eu doping stabilized the monoclinic Y₂O₃ and shifted the monoclinic to cubic transition towards higher temperatures.     

Eu stabilized α-sialon ceramics derived from SHS-synthesized powders

The characteristics of Eu-stabilized α-sialon ceramics derived from self-propagating high-temperature synthesis (SHS) Eu α-sialon powders without and with the addition of Y₂O₃ are investigated. The results showed that the amount of α-sialon phase formed in sintered Eu α-sialon composition was much less than that in SHS-ed powder when the composition was hot-pressed at 1800 °C for 1 h, while the transformation of α-sialon to β-sialon phase did occur at the same time, which could be attributed to the metastability of SHS-ed powder because of the high heating and cooling rate during the SHS process and the reduction of Eu³⁺ to Eu²⁺ under the reduction conditions during hot pressing. By addition of Y₂O₃ into SHS-ed Eu α-sialon powder, thus to form (Y,Eu) α-sialon phase in the sintered sample, the stability of α-sialon phase was improved, as the ratio of α-sialon to α-sialon was increased from 70 wt.% in SHS-ed powder to 83 wt.% in the sintered product by 50 mol% of Y₂O₃ added into SHS-ed powder.     

Synthesis,physical, ultrasonic waves,mechanical, FTIR,and dielectric characteristics of B2O3/Li2O/ZnO glasses doped with Y3+ ions

Six specimens of glasses with formula (70???x)B₂O₃/15Li₂O/15ZnO/xY₂O₃: x?=?0.0, 0.5, 1.0, 1.5, 2.0, and 2.5 mol%) have been synthesized via a conventional melt quenching technique. The produced specimens were named as BLZY0.0–BLZY2.5 according to x values. The physical, ultrasonic longitudinal (V_L) and shear (V_S) velocities, FTIR, and dielectric (50 Hz to 5 MHz) characteristics of the prepared glasses have been examined. With increasing content of Y₂O₃ from 0.0 to 2.5 mol%, the density (ρ) of the system increases linearly from 2512?±?11 to 2695?±?14 kg/m³, while the molar volume (V_M) decreases linearly from 2.6?±?0.011 to 2.57?±?0.013?×?10^?5 m³/mol. The oxygen packing density (OPD) as a number of the oxygen per unit composition in the glass sample is describing the packing tightness of the oxide network and thoroughly the compactness of the glass matrix. Values of the average boron–boron separation (d_B–B) decrease from 4.162 to 4.035?×?10^?10 (m) with 0 to 2.5 mol% Y₂O₃. Increasing formation of Y³⁺ ionic bonds with [BO_4/2]^1? may have an effect of lowering bond strength of B–O and thus shifting the absorption IR peak position. By increasing Y₂O₃ content in the investigated samples, the (V_L) and (V_S) increase linearly for the full-studied compositional range. The increasing number of strengthened bonds due to change coordination of B ions from 3 to 4 due to the increasing field strength of inserted accumulated Y³⁺ ions has the incentive impact to higher mechanical properties. The dielectric constant was decreased for Y₂O₃ content up to 1.5 mol% referring to cross-linkage formation with other elements, while the reduction in porosity at high content of Y₂O₃ is the main responsible for gradual enhancement in dielectric constant.

     

Effect of test atmosphere composition on high-temperature oxidation behaviour of CoNiCrAlY coatings produced from conventional and ODS powders

Abstract

The oxidation behaviour of free-standing CoNiCrAlY coatings produced by low-pressure plasma spraying using conventional powder and oxide dispersion strengthened (ODS) powder containing 2 wt. % Al-oxide dispersion was investigated. Thermogravimetric experiments at 1100 °C in Ar-20%O₂ and Ar-4%H₂-2%H₂O showed lower oxidation rates of the ODS than the conventional coating. In the latter material the scale growth was enhanced by extensive Y-incorporation of Y/Al-mixed oxide precipitates in the scale and apparently by Y-segregation to oxide grain boundaries. In the ODS coating the alumina dispersion bonded Y in the form of Y-aluminate thereby effectively suppressing scale ‘overdoping’. SEM/EBSD studies of all alumina scales revealed a columnar grain structure with the lateral grain size increasing approximately linearly with depth from the oxide/gas interface. For both coatings the alumina scale growth was slower in Ar–H₂–H₂O than in Ar–O₂. The result is believed to be related to a lower oxygen potential gradient and to slower grain boundary diffusion in the scale forming in H₂/H₂O containing gas.     

The role of transient oxides during deposition and thermal cycling of thermal barrier coatings

Abstract

High temperature coating systems, consisting of a René N5 superalloy, a Ni–23Co–23Cr–19Al–0.2Y (at.%) bond coating (BC) and a partially yttria stabilised zirconia (PYSZ) thermal barrier coating (TBC), were thermally cycled to failure for three different pre-oxidation treatments performed for 1 h at 1373 K and a partial oxygen pressure (pO₂) of 20 kPa, 100 Pa and 0.1 Pa, respectively. These pre-treatments resulted in the formation of different thermally grown oxide (TGO) layers prior to TBC deposition with respect to the presence of the transient oxides NiAl₂O₄, θ-Al₂O₃, and Y₃Al₅O₁₂ at the TGO surface. The TGO microstructures after TBC deposition and thermal cycling were investigated with a variety of analytical techniques and compared with those after pre-oxidation. For all pre-oxidation treatments, a double-layered TGO developed on the BC during thermal cycling. The TGO adjacent to the TBC consisted of small Zr-rich oxide crystallites embedded in an Al₂O₃ matrix when the TGO surface after pre-oxidation comprised of Y₃Al₅O₁₂ plus α-Al₂O₃. When the TGO surface constituted of θ-Al₂O₃, the Zr-rich oxide crystallites were embedded in a NiAl₂O₄ spinel layer after thermal cycling. Zr was absent in the oxide layer when the TGO surface prior to TBC deposition was composed of NiAl₂O₄ spinel. The TGO contiguous to the BC consisted in all cases of α-Al₂O₃ with Y₃Al₅O₁₂ crystallites. The roughness of the α-Al₂O₃/BC interface increased for a higher density of Y-rich oxide protrusions (i.e. pegs) along this interface.     

Heterovalent cation substitutions in the layered compound YBaCuFeO<Subscript>5 + δ</Subscript>

(Y,M)BaCuFeO_{5 + δ} (M = Ce, Ca, Na), Y(Ba,K)CuFeO_{5 + δ}, YBa(Cu,Co)FeO_{5 + δ}, YBaCu(Fe,M)O_{5 + δ} (M = Zn, Nb), (Y,Ca)BaCu(Fe,Zn)O_{5 + δ}, and (Y,Ca)(Ba,La)Cu(Fe,Zn)O_{5 + δ} solid solutions have been prepared by ceramic processing techniques and have been characterized by x-ray diffraction, IR absorption spectroscopy, and thermal expansion and electrical conductivity (σ) measurements in air at temperatures from 300 to 1100 K. It is shown that, in the range 650–700 K, the linear thermal expansion coefficient of the (Y,M)BaCuFeO_{5 + δ} phases rises from (11–12) × 10^?6 to (14–15) × 10^?6 K^?1, while that of the YBa(Cu,Co)FeO_{5 + δ} solid solution decreases from 18 × 10^?6 to 14 × 10^?6 K^?1. The conductivity data (an increase in σ upon Ca²⁺ → Y³⁺ and Zn²⁺ → Fe³⁺ substitutions and a reduction in σ upon Ce⁴⁺ → Y³⁺ and Nb⁵⁺ → Fe³⁺ substitutions) demonstrate that the transport properties of YBaCuFeO_{5 + δ} can be tuned by electron-hole doping.     

Oxidation of nickel-based alloys in dry and water vapour containing air

Abstract

Ni-based alloys are widely used in power generation and their oxidation behaviour in the uncoated state is of interest, especially the impact of water vapour in the air on the formation of a protective underlying alumina scale. High-temperature X-ray diffraction was applied to investigate in situ the oxide scale formation in the initial state on the alumina formers CM247DS and CMSX4 in comparison to the chromia formers IN792 and SCA425+. Post-oxidation analysis was performed by field emission scanning electron microscopy. The samples were oxidised for 100 h at 950°C in dry air and in air with 20% relative moisture in the high temperature device on the X-ray diffractometer. In dry air, CM247 and CMSX-4 form α-Al₂O₃ from the beginning simultaneously with spinels and nickel oxide. The alumina scale underlies the spinels which spall partially on cooling. When adding water vapour, the same oxides were formed simultaneously resulting in a comparable oxide scale. IN792 forms in dry air mainly NiAl₂O₄ and transitionally CrO₂ under laid by an alumina scale. With water vapour, Cr₂O₃ forms and the underlying alumina scale shows a non coherent dendritic structure. SCA425+ forms in dry air Cr₂O₃ and Cr containing mixed oxides and with water vapour a more coherent alumina scale than IN792.     

The Effect of Using Different Y<Subscript>2</Subscript>O<Subscript>3</Subscript> Layers on the Activation Energy and Irreversibility Line of MPMG YBCO Bulk at 1050 °C Growth Temperature

In this study, three kinds of YBCO samples which are named Y1, Y2 and Y3 were fabricated by a melt–powder–melt–growth (MPMG) method. The Y1 sample was placed into a platinum (Pt) crucible without Y₂O₃, the Y2 sample was located on a Al₂O₃ crucible with a freely poured Y₂O₃ powder and the Y3 sample was located on a Al₂O₃ crucible with a 1-mm-thick buffer layer of Y₂O₃. YBCO samples were investigated by magnetoresistivity (ρ–T) measurements in dc magnetic fields (parallel to the c-axis) up to 5 T. The effect of the Y₂O₃ layer on the activation energy and irreversible flux of the samples was studied. The activation energies (U) were determined using the Arrhenius activation energy law from ρ–T. The power law relationship for U with H^?α was investigated. α was calculated to find out which defects were dominant in the samples. Irreversibility fields (H_irr) and upper critical fields (H_c2) were obtained using 10 and 90% criteria of the normal-state resistivity value from ρ–T curves. Irreversibility lines (ILs) were estimated from the equation H_irr ～ (1 ? T_irr(H)/T_irr(0))ⁿ. The fitting results to giant flux creep and vortex glass models were discussed.     

Effects of some rare earth oxides and/or SiO2 additions to a commercial ZrO2 paint for coating SiGe thermoelectric materials

The effects of some rare earth oxides (Y₂O₃, CeO₂, Sm₂O₃ or Er₂O₃) and/or SiO₂ additions to a commercial oxide paint (mainly ZrO₂) were studied as coatings for SiGe thermoelectric materials. No spalling occurred in the samples prepared from coatings containing small amounts of Er₂O₃, SiO₂ or both CeO₂ and SiO₂ while heating for 1322 h at 1080° C in vacuum, and the sublimation of silicon and/or germanium was reduced significantly. The improvements imparted by these oxide additions to the commercial oxide paint is thought to be due to decreased internal stress of the surface oxides. The addition of Y₂O₃ and Sm₂O₃ had a detrimental effect.     

Capability of mechanical alloying and SPS technique to develop nanostructured high Cr,Al alloyed ODS steels

Abstract

Oxide dispersion strengthened (ODS) Fe alloys were produced by mechanical alloying (MA) with the aim of developing a nanostructured powder. The milled powders were consolidated by spark plasma sintering (SPS). Two prealloyed high chromium stainless steels (Fe–14Cr–5Al–3W) and (Fe–20Cr–5Al+3W) with additions of Y₂O₃ and Ti powders are densified to evaluate the influence of the powder composition on mechanical properties. The microstructure was characterised by scanning electron microscope (SEM) and electron backscattering diffraction (EBSD) was used to analyse grain orientation, grain boundary geometries and distribution grain size. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) equipped with energy dispersive X-ray spectrometer (EDX) were used to observe the nanostructure of ODS alloys and especially to observe and analyse the nanoprecipitates. Vickers microhardness and tensile tests (in situ and ex situ) have been performed on the ODS alloys developed in this work.     

Reactively hot-pressed yttrium iron garnets

Reactive hot-pressing of co-precipitated oxalates of Yttrium and Iron, under 6000–8000 psi, to a maximum temperature of 1100°C, produced high density specimens. Only the stooichiometric mixture (Fe/Y=1.66) produced some garnet phase. Off-stoichiometric specimens were mainly composed of orthoferrites. On annealing in air or oxygen at 1400°C, the orthoferrite phase was converted to the garnet phase (with excess Fe₂O₃ or Y₂O₃ in the lattice) with an accompanying increase in magnetic moment.     

Abradability and hardness in rare-earth-oxide stabilized hafnia

The dry abradability and diamond pyramid hardness of HfO₂ fully-stabilized by variable additions of Er₂O₃ were determined on sintered polycrystalline specimens of near-theoretical densities. Measurements on fused silica and on a single crystal of Y₂O₃-stabilized ZrO₂ were included for comparison. In general, the hardness was decreased and the abradability was increased by increasing additions of stabilizing oxide. These effects were attributed mainly to the increase in defect concentration of the anion sublattice with increasing amounts of modifier oxide. Solution of HfO₂ in Er₂O₃ (rare-earth-oxide Type C structure) increased hardness and decreased abradability: effects which also seemed to be related to the defect concentration of the anion sublattice.