Melting behaviour and metastability of yttrium aluminium garnet (YAG) and YAlO3 determined by optical differential thermal analysis

The melting point of yttrium aluminium garnet (YAG), reinvestigated by optical differential thermal analysis (ODTA), was found to be 1940±7° C. Above this temperature YAG liquids are opaque, suggesting the presence of two immiscible liquids. In the composition range 10.0 to 47.5 mol% Y₂O₃, crystallization of the equilibrium phases can only occur in the presence of YAG nuclei; otherwise solidification of YAlO₃ and Al₂O₃ will take place. A metastable phase diagram has been defined with a metastable eutectic at 23 mol% Y₂O₃-77 mol% Al₂O₃ and 1702±7° C. YAlO₃ (perovskite) was found to melt incongruently with a peritectic temperature of 1916±7° C and a liquidus temperature of 1934±7° C. YAlO₃ formed during metastable solidification transforms to YAG in the presence of Al₂O₃ at 1418±7° C. It is suggested that the metastability arises from the difficulty of the aluminium to attain four-fold co-ordination in the YAG structure.     

Selective growth of yttrium iron garnet and yttrium ferrite by combinatorial pulsed-laser ablation of common precursors

In this paper, we demonstrate the capability of growing two alternative complex oxides with different stoichiometries and crystal structures, by choosing the appropriate substrate and adjusting the target ablation ratio, and tuning their composition and properties by combinatorial pulsed-laser ablation of their precursors. In particular, we successfully grew epitaxial crystalline yttrium iron garnet (Y₃Fe₅O₁₂) on yttrium aluminium garnet (Y₃Al₅O₁₂) substrates and polycrystalline yttrium ferrite (YFeO₃) on sapphire (α-Al₂O₃) substrates by co-ablation of yttrium oxide and iron oxide targets.     

Formation of alkoxy-derived yttrium aluminium oxides

Monoclinic Y₄Al₂O₉ and hexagonal YAlO₃ crystallize at low temperatures from amorphous materials prepared by the hydrolysis of yttrium and aluminium double alkoxides. Hexagonal YAlO₃ transforms to the cubic phase with a garnet structure as an intermediate product at elevated temperatures. The formation process of YAlO₃ is described. Solid solutions of hexagonal YAlO₃ crystallize between 50 and 62.5 mol % Al₂O₃. Yttrium aluminium garnet Y₃Al₅O₁₂(YAG) is formed by transformation of the solid solution.     

Structure and composition of oxides in FeCrAl ODS alloy with Zr addition

FeCrAl ODS (oxide dispersion strengthened) alloy with Zr addition was investigated with the main focus on the structure and composition of oxide particles. The results reveal that almost all the small particles (diameter <2?nm) were found to be consistent with trigonal δ-phase Y₄Zr₃O₁₂, with cubic Y₃Al₅O₁₂ (YAG) oxides occasionally detected. The large particles (>2?nm) were mainly identified as monoclinic Al₂O₃ oxides. A core/shell structure is found in slightly large oxide particles. Oxide cores are Al and O while shell regions are enriched in Y. This leads us a promising way to control the structure and size of the oxides and develop the high-performance FeCrAl ODS alloys.     

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.     

YTTRIUM ALUMINATE CERAMIC FIBERS VIA PRE-CERAMIC POLYMER AND SOL-GEL ROUTES

ABSTRACT

Yttrium aluminum garnet (YAG - Y₃Al₅O₁₂) fibers have been prepared by dry spinning solutions of yttrium and aluminum carboxylate polymers (precursor route) and by dry spinning aqueous oxide sols (sol-gel route). Fibers from aqueous diphasic gels are prepared by mixing a colloidal alumina sol containing 50-nm hydrous alumina with a colloidal yttria sol containing 10-nm yttrium oxide, using polyvinylpyrrolidone as a spinning aid. Fibers by the precursor route are made from spinnable THF solutions of yttrium isobutyrate and aluminum isobutyrate or from aqueous solutions of polymeric aluminum formate and yttrium acetate.

The isobutyrate materials decompose between 200-400^°C to an amorphous residue. Crystallization occurs abruptly between 875°C and 900^°C, forming the YAG phase directly. The formate-acetate also decomposes to amorphous residues, which form YAG at 900^°C. In the diphasic gel, YAG forms gradually between 1000 and 1200^°C, with intermediate products YAP (YalO₃ perovskite) and/or YAM (Y₄Al₂O₉ monoclinic). At 1500^°C, single phase YAG is obtained as pore-free fibers with 500 nm grains.     

Debond coating requirements for brittle matrix composites

The cause of improved fracture toughness in Y₂O₃-coated niobium-toughened TiAl relative to either uncoated niobium or Al₂O₃-coated niobium was examined. Reactively sputtered Y₂O₃ coatings, 1–2 m thick, were deposited on to rock salt (NaCl), polished single-crystal (0001) Al₂O₃, and polished polycrystalline niobium. Sputtered niobium coatings, 1–2 m thick, were also deposited on to polished single-crystal Y₂O₃ substrates for comparison. The oxide coating was characterized and consisted of stoichiometric bcc Y₂O₃ witha ₀=1.0602 nm. Indentation tests were performed to correlate the fracture toughness and debond characteristics of as-deposited Y₂O₃ coatings on Al₂O₃ and polycrystalline niobium, and niobium coatings on single-crystal Y₂O₃, to that found in TiAl/Nb and Al₂O₃/Al₂O₃ laminates. The calculated fracture toughness of sputtered Y₂O₃ on sapphire was similar to reported values for bulk Y₂O₃. However, a wide variation in interfacial fracture toughness was obtained by indentation methods, and is attributed to the microstructure of as-deposited coatings and to weak bonding between as-deposited yttria and the sapphire substrate. These results are related to factors that affect debonding and fracture toughness of brittle matrix composites. Reactive and non-reactive metal/ceramic systems were reviewed in an effort to understand why Y₂O₃ coatings perform well. It is postulated that yttrium oxide coatings applied to niobium have an atomically sharp interface that has a lower fracture energy compared to Nb/Al₂O₃, resulting in improved interfacial debonding and composite fracture toughness.     

Sol-gel synthesis and characterization of sub-microsized lanthanide (Ho, Tm, Yb, Lu) aluminium garnets

Sub-microsized and nanosized holmium aluminium garnet (Ho₃Al₅O₁₂, HoAG), thulium aluminium garnet (Tm₃Al₅O₁₂, TmAG), ytterbium aluminium garnet (Yb₃Al₅O₁₂, YbAG) and lutetium aluminium garnet (Lu₃Al₅O₁₂, LuAG) powders were prepared by a simple aqueous sol-gel method using aluminium nitrate nonahydrate, lutetium oxide, thulium oxide, holmium oxide and ytterbium oxide as starting materials. Ethane-1,2-diol was used as complexing agent. The powder X-ray diffraction (XRD) patterns of the specimens sintered at 1000 °C revealed the formation of monophasic HoAG, TmAG, YbAG, and LuAG. The phase composition of the samples was also characterized by infrared (IR) spectroscopy. Microstructural features of the polycrystalline garnets were studied by scanning electron microscopy (SEM).     

Interaction between liquid aluminum and yttria substrate: microstructure characterization and thermodynamic considerations

The systematic microstructure and thermodynamic studies of the reactions between liquid aluminum and dense polycrystalline yttria (Y₂O₃) substrates at 1,273 K are reported in this article. The microstructure observations showed the presence of three reaction product zones extending ~1 mm into the oxide substrate of typical C4 (Co-Continuous-Ceramic-Composites) structure. The first zone starting from the drop side was composed of fine crystalline precipitates of the Al₅Y₃O₁₂ (YAG) phase dispersed in the Al₃Y matrix. The second zone was built of larger AlYO₃ (YAP) crystals. The third zone formed elongated oxide precipitates (YAP) surrounded by the Al₂Y intermetallic channels. The thermodynamic calculation indicated that, depending on the amount of the yttrium dissolved in aluminum, the YAG (up to 5 at.% Y), YAP (5–13 at.% Y), or Al₂Y for higher content of yttrium might form at 1,273 K, while the Al₃Y phase might appear during cooling.     

Crystallization of the glassy phase in an Si3N4 material by post-sintering heat treatments

It is shown that post-sintering heat treatments in air in the temperature range 1100 to 1400° C result in substantial crystallization of the glassy phase in an Si₃N₄ material which was produced by the nitridation pressureless sintering (NPS) method using Y₂O₃ and Al₂O₃ as sintering aids. X-ray diffraction combined with analytical electron microscopy showed that the secondary crystalline phases which form are strongly dependent upon time and temperature of heat treatment as well @S depth below the oxide scale. This effect is primarily due to the outward diffusion of cations (yttrium, aluminium and impurities) as well as the inward diffusion of oxygen. Small glassy pockets and thin amorphous intergranular films remain in the microstructure after heat treatment.     

Influence of yttrium dopant on the synthesis of ultrafine AlN powders by CRN route from a sol–gel low temperature combustion precursor

Y-doped ultrafine AlN powders were synthesized by a carbothermal reduction nitridation (CRN) route from precursors of Al₂O₃, C and Y₂O₃ prepared by a sol–gel low temperature combustion technology. The Y dopant reacted with alumina and thus forming yttrium aluminate of AlYO₃, Al₃Y₅O₁₂ and Al₂Y₄O₉, which formed a liquid at about 1400 °C and promoted the transformation of Al₂O₃ to AlN and the growth of AlN particles. Compared with the conventional solid CRN process, Y dopant reduced the synthesis temperature by 150 °C, and Al₂O₃ transformed to AlN completely at 1450 °C. The content of Y dopant had little effect on the synthesis temperature of AlN whereas it influenced the phase of Y compounds in the products. As the Y/Al molar ratio was in the range of 0.007648–0.022944, the particle sizes of Y-doped AlN powders synthesized at 1450 °C were 150–300 nm.     

Neodymium substitution effects in sol–gel derived Y3−xNdxAl5O12

Yttrium aluminium garnet (YAG) powders substituted by neodymium Y_3−xNd_xAl₅O₁₂ (x = 0.1, 0.25, 0.35, 0.5, 0.6, 0.7, 0.8, 1.5, 2.0, 2.5, and 3.0) were prepared by a simple aqueous sol–gel method using aluminium nitrate nonahydrate, yttrium oxide, neodymium oxide as the starting materials and ethane-1,2-diol as complexing agent. The powders annealed at 1000 °C in air were characterized by X-ray diffraction (XRD) analysis, infrared (IR) spectroscopy and scanning electron microscopy (SEM). It was demonstrated, however, that the total substitution of yttrium by neodymium does not proceed in the YAG. Pure cubic garnet phase was formed only at low concentration of neodymium (x = 0.1, 0.25, 0.35, 0.5, 0.6, 0.7, 0.8 and 1.5). With further substitution, when the amount of neodymium was x = 2.0, 2.5 and 3.0 the main part of garnet phase transformed in to the perovskite neodymium aluminate (NdAlO₃) phase.     

Microstructure of sol–gel synthesized Al2O3–ZrO2(Y2O3) nano-composites studied by transmission electron microscopy

The Al₂O₃–ZrO₂(Y₂O₃) composite powder was synthesized through a sol–gel process using aluminum sec-butoxide and zirconium butoxide as precursors. The as-received powders in an amorphous phase were crystallized with c-ZrO₂ at around 980 °C. As the calcination temperature increased, the c-ZrO₂ crystalline phase was transformed to t-ZrO₂ at about 1200 °C. However, the Al₂O₃ phase in the Al₂O₃–ZrO₂(Y₂O₃) composite powders still existed in an amorphous phase up to 1050 °C. In the sintered body using the calcined powders at 400 °C, the Al₂O₃ phase was crystallized in an α-phase at 1200 °C during the sintering for 2 h. Using the sol–gel Al₂O₃–ZrO₂(Y₂O₃) powder, a typical nano-composite having a nano-crystalline phase (less than 20 nm) can be successfully obtained by a pressureless-sintering process even at 1200 °C for 2 h.Using the sol–gel Al₂O₃–ZrO₂(Y₂O₃) powder, a typical nano-composite having a nano-crystalline phase (less than 20 nm) can be successfully obtained by a pressureless-sintering process even at 1200 °C for 2 h. The values of relative density and Vickers hardness were comparatively high value with about 96.2% and 1100 Hv, respectively, even though it was made at low temperature. In the composite sintered at 1400 °C, the hardness value was saturated with 1570 Hv and the values of fracture toughness were almost same with about 6 MPa m^1/2.     

Synthesis and crystallization of yttrium-aluminium garnet and related compounds

Amorphous oxide combustion products with compositions corresponding to Y₄Al₂O₉, YAlO₃, and Y₃Al₅O₁₂ were synthesized by the glycine-nitrate process and heat-treated to induce crystallization. The crystalline structure of the resulting powders was determined by powder X-ray diffraction techniques. The phase stabilities of the crystalline phases were investigated as functions of the glycine-to-nitrate ratio, the yttrium-to-aluminium ratio, and the heat-treatment conditions. Heat treatment for short durations resulted in incompletely crystalline powders that consisted of a mixture of Y₄Al₂O₉, YAlO₃, and Y₃Al₅O₁₂ phases, regardless of the chemical composition of the amorphous combustion product. However, heat treatment for longer durations or higher temperature generated both pure-phase, monoclinic Y₄Al₂O₉ and Y₃Al₅O₁₂ with the garnet structure. Prolonged heat treatment at high temperature failed to generate pure-phase orthorhombic YAlO₃. Subsequent analysis revealed a sluggish, complex crystallization process involving the formation and decomposition of several phases.     

Ionic liquid mediated green synthesis of Ag-Au/Y2O3 nanoparticles using leaves extracts of Justicia adhatoda: Structural characterization and its biological applications

In the present work, a facile synthesis was applied for the silver-gold decorated yttrium oxide nanoparticles with the use of Justicia adhatoda (leaves extract) and [BMIM] PF₆ (Ionic liquid) as a capping/stabilizing agent. The XRD analysis showed that Ag-Au/Y₂O₃ nanoparticles have a face-center cubic structure and crystallite size of 30 nm. The Y-O stretching bands were observed in the FT-IR spectrum at 464 to 495 cm^?1. The band gap of the silver-gold decorated Y₂O₃ nanoparticles was estimated as 5.75 eV from the UV-DRS spectrum. In the SEM and TEM images, the morphology of silver-gold/Y₂O₃ nanoparticles shows a nanoflake-like structure. The presence of silver, gold, yttrium and oxygen of elements has been confirmed by the EDX spectrum. The antibacterial activity of the nanoparticles was evaluated for the Staphylococcus aureus (Gram-positive) and Escherichia coli (Gram-negative) bacteria. The anticancer activity was also studied by the human cervical cancer cell line. The silver-gold decorated yttrium oxide nanoparticles revealed an exceptional microbicidal and antitumor activity when compared with yttrium oxide, silver decorated yttrium oxide and gold decorated yttrium oxide.     

Thermally induced surface changes in aluminium-iron oxide thermites

The aluminium and iron oxide surfaces of Al-Fe₃O₄ thermite in powder and pressed pellet form were studied before and after accelerated ageing at 180° C by X-ray photoelectron spectroscopy. The Al₂O₃ surface film thicknesses on aluminium metal were deduced from the intensity ratio of aluminium K L L Auger signals induced by X-ray radiation on Al₂O₃ and aluminium metal. Based on the mean free path of 1.65 nm for aluminium K L L Auger electrons, the oxide thickness on aluminium flakes before mixing with Fe₃O₄ was estimated to be 0.8 to 0.9 nm. A slight oxidation was observed on the aluminium surface after mixing with Fe₃O₄ at room temperature. Hot pressing of this mixture at 425° C for 7 min increased the oxide film to 3.1 nm. This surface oxide film seemed to protect the aluminium metal, and further ageing at 180° C did not cause significant oxide growth.Mound is operated by Monsanto Research Corporation for the US Department of Energy Under Contract No DE-AC04-76DP00053.     

Preparation of ZrO2-Al2O3 powder by thermal decomposition of gels produced from an aluminium chelate compound and zirconium butoxide

Organic precursors containing Al and Zr atoms were synthesized from an aluminium chelate compound and zirconium n-butoxide. A ZrO₂-Al₂O₃ composite powder was prepared by the thermal decomposition of these precursors. An amorphous phase exists to higher temperatures for this ZrO₂-Al₂O₃ powder than for a comparable powder prepared from aluminium sec-butoxide and zirconium n-butoxide. In addition the tetragonal ZrO₂ phase was stabler in this ZrO₂-Al₂O₃ powder than in a comparison powder. The ZrO₂ grains were 50–500 nm in diameter and were homogeneously dispersed in the Al₂O₃ matrix after heating at 1400 °C.     

The influence of the molar ratio of Al2O3 to Y2O3 on sintering behavior and the mechanical properties of a SiC–Al2O3–Y2O3 ceramic composite

The influence of the molar ratio of Al₂O₃ to Y₂O₃ (i.e. M_Al2O3/M_Y2O3) on sintering densification, microstructure and the mechanical properties of a SiC–Al₂O₃–Y₂O₃ ceramic composite were studied. It was shown that the optimal value of M_Al2O3/M_Y2O3 was 3/2, not 5/3, which is customarily considered the optimal molar ratio for the formation of YAG (Y₃Al₅O₁₂) phase. When M_Al2O3/M_Y2O3 is 5/3, materials existed in two phases of YAG and very little YAM phases. The sintering mechanism of the solid phase occurred at 1850 °C. When M_Al2O3/M_Y2O3 was 3/2, materials existed in the two phases YAG (Y₃Al₅O₁₂) and YAM (Y₄Al₂O₉). The formation of the low melting point eutectic liquid phase (YAG + YAM) increased sintering densification. Flexure strength, hardness and relative density were all higher.     

Thermisches Spritzen neuartiger SiC und TiC Pulver mit oxidischer Binderphase

Processing of newly developed SiC and TiC Powders with Oxide‐Ceramic Matrix by means of Thermal Spraying Intermediate results of an actual project are presented. The investigations concern newly developed SiC and TiC powders, their processing by means of thermal spraying and the characterization of produced coatings. Development and optimization of powders, thermal spray process and spray parameter optimization are carried out on permanent feedback. The powder production line is spray‐drying, sintering and conditioning. The binder matrix phase is aluminum oxide / yttrium oxide. The produced powders SiC – Al₂O₃/Y₂O₃ and TiC – Al₂O₃/Y₂O₃ show different specific chemical compositions and morphologies each. Carbide contents of >65 % are aimed at. Applied thermal spray processes are atmospherical plasma spraying and high velocity oxygen fuel spraying. The results demonstrated characterize feedstock powders as well as produced coatings. The investigations are done by means of light and scanning electron microscopy.     

Alumina scale growth at zirconia-MCrAlY interface: a microstructural study

High-temperature oxide scale growth at the ceramic-metal interface is a major contributor to the thermomechanical resistance of thermal barrier coatings for hot stages of gas turbines. In order to better understand this phenomenon, microstructural observations of the alumina scales formed at 1100 and 1200 °C under air, between low-pressure plasma-sprayed NiCrAlY and air plasma-sprayed ZrO₂-8.5 wt % Y₂O₃, have been performed by classical and analytical transmission electron microscopy on transverse thin foil specimens. The evolution of the oxide grain morphology from the metal-oxide to the oxide-oxide interface suggests that the scale growth principally takes place at the metal-oxide interface. Segregation of yttrium at oxide grain boundaries has been detected as well as significant quantities of zirconium inside the alumina grains. The oxide growth seems to be dominated by a classical grain-boundary oxygen diffusion mechanism. The presence of zirconium inside the alumina grains also suggests that Al₂O₃ partially forms by chemical reduction of ZrO₂ by AI. The comparison between the microstructures observed and that of alumina scales grown under similar conditions on bare MCrAlY alloys gives some insight into how the ceramic top-coat modifies NiCrAlY high-temperature oxidation mechanisms.