Fabrication of Fine YAG-Particulate-Dispersed Alumina Fiber

This paper involves novel fabrication processes for polycrystalline α-Al₂O₃-matrix composite fibers that contain nanosized yttrium aluminum garnet (YAG) particles. Dense α-Al₂O₃/YAG nanocomposite fibers with a fine and homogeneous microstructure can be successfully fabricated via a modified sol-gel process and α-Al₂O₃ seed-particle addition. YAG nanoparticles have been homogeneously dispersed within Al₂O₃-matrix grains as well as at grain boundaries. Effects of α-Al₂O₃ seed particles and YAG nanodispersions on crystallization and microstructure development of nanocomposite fibers are discussed.     

Effect of Yttrium and Lanthanum on the Final-Stage Sintering Behavior of Ultrahigh-Purity Alumina

Final-stage sintering has been investigated in ultrahigh-purity Al₂O₃ and Al₂O₃that has been doped individually with 1000 ppm of yttrium and 1000 ppm of lanthanum. In the undoped and doped materials, the dominant densification mechanism is consistent with grain-boundary diffusion. Doping with yttrium and lanthanum decreases the densification rate by a factor of ˜11 and 21, respectively. It is postulated that these large rare-earth cations, which segregate strongly to the grain boundaries in Al₂O₃, block the diffusion of ions along grain boundaries, leading to reduced grain-boundary diffusivity and decreased densification rate. In addition, doping with yttrium and lanthanum decreases grain growth during sintering. In the undoped Al₂O₃, surface-diffusion-controlled pore drag governs grain growth; in the doped materials, no grain-growth mechanism could be unambiguously identified. Overall, yttrium and lanthanum decreases the coarsening rate, relative to the densification rate, and, hence, shifted the grain-size-density trajectory to higher density for a given grain size. It is believed that the effect of the additives is linked strongly to their segregation to the Al₂O₃grain boundaries.     

Yttria Doping and Sintering of Submicrometer-Grained α-Alumina

The sintering behavior of α-alumina powders doped with magnesia (500 or 1500 ppm) and yttria (0, 500, or 1500 ppm) was investigated using constant-heating-rate dilato-metric experiments. The apparent activation energies for the intermediate stage of sintering were 740, 800, and 870 kJ/mol for 0, 500, and 1500 ppm yttria doping levels, respectively; these were independent of magnesia doping. Yttria-doped powder compacts exhibited systematic anomalous second peaks in the densification rate curves at certain grain sizes which were determined only by yttria doping levels. Before the anomalous peak, with lower yttrium contents at grain boundaries, yttrium in an atomic state delays densification and raises the apparent activation energy. Beyond the peak, with higher yttrium contents at grain boundaries, yttria-rich precipitation delays the densification. Within the peak, yttrium segregation near the saturation level enhances densification.     

Synthesis and Characterization of Ce3+:YAG Phosphors by Heterogeneous Precipitation Using Different Alumina Sources

Ce³⁺-doped yttrium aluminum garnet (Ce:YAG) phosphor powders were synthesized by heterogeneous precipitation process using three different aluminum sources: α-phase, θ-phase, and boehmite (AlOOH). Mixtures of yttrium and cerium nitrate solutions containing various aluminum sources were precipitated by ammonia solution in normal and reverse strike methods. The influence of pH was studied in the normal strike method by maintaining the solutions at pH 7, 9, and 11 during precipitation. Dried precipitates were double calcined at 1300°C/16 h and 1300–1500°C/24 h, at a ramping rate of 10°C/min, with an intermittent wet ball milling in water. Structural evolution of the resultant phosphors was studied by powder XRD. In the normal strike method, a highly pure YAG phase was formed by α- alumina (pH 7, 11) and θ-alumina (pH 11) while boehmite source ended up with mixed phases of YAlO₃ (YAP) and Y₄Al₂O₉ (YAM) along with YAG phase at all pH values of precipitation. However, in the reverse strike process, the θ-phase of alumina gave an extremely pure Ce:YAG phase at a relatively lower calcination temperature (1400°C/24 h) compared with the α-phase and also showed more intense emission of yellowish-green light under blue (λ=469 nm) excitation. Scanning electron microscopy revealed 1–2 μm sized particles with least agglomeration in the reverse strike method.     

In Situ-Toughened Silicon Carbide

A new processing strategy based on atmospheric pressure sintering is presented for obtaining dense SiC-based materials with microstructures consisting of (i) uniformly distributed elongate-shaped α-SiC grains and (ii) relatively high amounts (20 vol%) of second-phase yttrium aluminum garnet (YAG). This strategy entails the sintering of β-SiC powder doped with α-SiC, Al₂O₃, and Y₂O₃. The Al₂O₃ and Y₂O₃ aid in the liquid-phase sintering of SiC and form in situ YAG, which has a significant thermal expansion mismatch with SiC. During a subsequent grain-growth heat treatment, it is postulated that the α-SiC "seeds" assist in controlling in situ growth of the elongated α-SiC grains. The fracture pattern in the in situ -toughened SiC is intergranular with evidence of copious crack-wake bridging, akin to toughened Si₃N₄ ceramics. The elongate nature of the α-SiC grains, together with the high thermal-residual stresses in the microstructure, enhance the observed crack-wake bridging. This bridging accounts for a measured twofold increase in the indentation toughness of this new class of in situ -toughened SiC relative to a commercial SiC.     

Structure and Chemistry of Symmetrical Tilt Grain Boundaries in α-Al2O3: II, Bicrystals with Y at the Interface

Symmetrical tilt grain boundaries (STGBs) on the (10¯10), (¯2112), and (01¯18) planes in α-Al₂O₃ have been investigated for their behavior with respect to yttrium doping by performing a combined study of high-resolution transmission electron microscopy and spatially resolved energy dispersive X-ray analysis. Bicrystals have been produced by diffusion bonding under ultrahigh vacuum; yttrium was introduced before the bonding process. The prismatic twin has a bulklike grain boundary (GB) structure and does not accommodate any yttrium at the GB. The yttrium at the Σ17 (¯2112) GB and the Σ37 (01¯18) GB changes the GB structure and the content of other impurities.     

Texture Development by Templated Grain Growth in Liquid-Phase-Sintered α-Alumina

The distribution and orientation of platelet-shaped particles of α-alumina in a fine-grained alumina matrix is shown to template texture development via anisotropic grain growth. The textured microstructure ranges from 4 wt% oriented platelet particles in calcined samples to nearly 100% oriented α-Al₂O₃ grains after sintering at 1400°C. A CaO + SiO₂ liquid phase creates favorable thermodynamic and kinetic conditions for anisotropic grain growth and grain reorientation during sintering. Important criteria for templated grain growth include (1) anisotropic crystal structure and growth, (2) high thermodynamic driving force for template grain growth, and (3) modification of diffusion in the system to continuously provide material to the anisotropically growing template grains.     

Processing and Microstructure Development in Alumina–Silicon Carbide Intragranular Particulate Composites

Al₂O₃–SiC particulate composites were fabricated by hot-pressing mixtures of 5–30 vol% SiC with either α-Al₂O₃, γ-Al₂O₃, or boehmite (γ-AlOOH) to determine whether grain growth or the α-alumina phase transformation could be used to fabricate intragranular particulate composites. Samples starting with α-alumina resulted in primarily intergranular SiC of 0.3 μ and an alumina grain size of 1.5–4.1 μm. Heat treatments resulted in SiC coarsening but no entrapment of SiC by grain boundary breakaway. The α-alumina transformation in the samples starting with γ-alumina resulted in the entrapment of ∼48% of the 5 vol% of SiC added whereas 79% of the SiC was entrapped in the α-alumina grains in samples starting with boehmite. Only SiC particles ≤0.2 SmUm were entrapped in the α-alumina grains during the phase transformation. With increasing SiC content, the relative volume of intragranular SiC decreased, but the amount of intragranular SiC was constant and independent of the amount of SiC added before transformation. The formation of intragranular composites from γ-alumina and boehmite samples was explained with a model that attributes particle entrapment to the vermicular growth of α-alumina into the transition alumina matrix during the α-alumina phase transformation. Seeding the boehmite-based samples did not affect the concentration of entrapped SiC, but did lower the hot-pressing densification temperature by as much as 150°C.     

Effects of Annealing Temperature on Microstructure and Electrical and Optical Properties of Radio-Frequency-Sputtered Tin-Doped Indium Oxide Films

Indium tin oxide (ITO) films (0.3 μm thick), with a doping level of 28 mol% SnO₂, were prepared by a radio frequency magnetron sputtering mehthod. The effects of postannealing on the microstructure and the electrical properties of the ITO films were investigated. The as-sputtered film showed an amorphous structure, whereas the films annealed at 350° and 510°C exhibited crystalline structures with grain sizes of 0.12 and 0.14 μm, respectively. Examination by TEM showed that the postannealing treatment induced SnO₂ precipitates along the grain boundaries. The resistivity increased with increasing postannealing temperatures. The mobility of carriers appears to be responsible for the resistivity increase in these specimens. The mobility change is discussed in connection with the SnO₂ precipitates.     

Ab-initio Calculation of Yttrium Substitutional Impurities in alpha-Al2O3

Yttrium has a very low solubility in bulk α-Al₂ O₃ and a great propensity for segregation to the grain boundaries in polycrystalline Al₂O₃. The segregation of yttrium influ-ences many properties of the material. To understand the nature of the chemical bonding of yttrium within Al₂O₃, we have, as a first step, conducted an ab-initio self-consistent calculation for a yttrium impurity atom replacing an aluminum atom in Al₂O₃. We have used the first-principles orthogonalized linear combination of atomic orbitals (OLCAO) method and a supercell with 120 atoms in the hexagonal lattice. The relaxation of the nearby atoms, be-cause of the presence of yttrium, is studied via total energy calculation within the local density approximation. The nearest-neighbor oxygen atoms of yttrium move outward by 8% and the next-nearest-neighbor aluminum atoms move inward by 5% of the respective separations in the undistorted lattice. A substitutional energy of 4.79 eV is obtained. The yttrium impurity introduces three defect states in the gap near the conduction band edge, thus natu-rally explaining the "donor effect." Two of the defect states are degenerate, and all three are derived mainly from the Y4d orbitals with very different local symmetries. The effec-tive-charge and bond-order calculations show a substantial covalent bonding character between yttrium and the re-laxed oxygen and aluminum atoms in the host.     

Anisotropic Grain Growth in α-Al2O3 with SiO2 and TiO2 Additions

Microstructural changes caused by doping α-Al₂O₃ with small amounts of SiO₂ and TiO₂ added singly or together were investigated. When they were sintered at 1450°C for 120 min, singly doped samples developed equiaxed microstructures, but codoped material developed an anisotropic microstructure that contained platelike grains with an average aspect ratio of 3.4. The development of anisotropy thus resulted from a cooperative effect of silicon and titanium. Amorphous material was present at most grain boundaries in the silicon-doped sample. In the codoped sample, only boundaries that exhibited a basal facet were penetrated by amorphous material. Energy dispersive X-ray spectroscopy analysis showed strong titanium enrichment at the edges of platelets. Additional experiments demonstrated that the volume fraction of highly anisotropic platelike grains interspersed with equiaxed grains could be adjusted by using varying amounts of titanium with a constant amount of silicon content. The fracture toughnesses of such materials increased as the structure became more anisotropic.     

Mullite Transformation Kinetics in P2O5-, TiO2-, and B2O3-Doped Aluminosilicate Gels

Mullite transformation kinetics of sol-gel-derived diphasic mullite gels doped with P₂O₅, TiO₂, and B₂O₃ were studied using quantitative X-ray diffraction and differential thermal analysis (DTA). The mullite transformation temperature initially increased with P₂O₅ doping because of phase separation and formation of α-alumina and cristobalite. In TiO₂-doped samples, the mullite transformation temperature decreased with TiO₂ doping, and the transformation rate increased with decreasing TiO₂ particle size. Kinetic studies showed that titania reduced the activation energy for both nucleation and growth relative to pure diphasic mullite gels by lowering the glass viscosity and/or enhancing the solid-state mass transport through lattice defects. B₂O₃ doping decreased the mullite transformation temperature and lowered the activation energy for both nucleation and growth but especially affected the mullite nucleation process, as indicated by the much smaller grain size.     

Influence of Yttrium Doping on Grain Misorientation in Aluminum Oxide

Oversized dopant ions such as yttrium and lanthanum segregate to grain boundaries and reduce the tensile creep rate of α-Al₂ O₃ by 2-3 orders of magnitude. One explanation for this behavior is that the oversized segregants give rise to a "site-blocking" effect for grain boundary diffusion. It has also been speculated that the dopant ions modify the grain boundary structure in alumina and reduce the creep rate by promoting the formation of special (e.g., coincidence site lattice (CSL)) grain boundaries. In order to test the latter hypothesis, we have used electron backscattered Kikuchi diffraction to characterize the misorientation and special grain boundary distribution for undoped and 1000-ppm-yttrium-doped alumina. The results show that the grain boundary structure in alumina (as characterized by the frequency of selected CSLs and misorientation distribution) was not significantly changed by the addition of yttrium, indicating that creep retardation results mainly from site-blocking.     

Grain Growth Control and Dopant Distribution in ZnO-Doped BaTiO3

ZnO additions to BaTiO₃ have been studied in order to determine the role of this dopant on sintering and microstructure development. As a consequence of a better initial dopant distribution, samples doped with 0.1 wt% zinc stearate show homogeneous fine-grained microstructure, while a doping level of 0.5 wt% solid ZnO is necessary to reach the same effect. When solid ZnO is used as the dopant precursor, ZnO is redistributed among the BaTiO₃ particles during heating. Since no liquid formation has been detected for temperatures below 1400°C in the system BaTiO₃-ZnO, it is proposed that dopant redistribution takes place by vapor-phase transport and grain boundary diffusion. Shrinkage and porosimetry measurements have shown that grain growth is inhibited during the first step of sintering for the doped samples. STEM-EDX analysis revealed that solid solubility of ZnO into the BaTiO₃ lattice is very low, being strongly segregated at the grain boundaries. Grain growth control is attributed to a decrease in grain boundary mobility due to solute drag. Because of its effectiveness in controlling grain growth, ZnO appears to be an attractive additive for BaTiO₃ dielectrics.     

In S/Yu-Toughened Silicon Carbide-Titanium Carbide Composites

A process based on liquid-phase sintering and subsequent annealing for grain growth is presented to obtain in situ -toughened SiC-30 wt% TiC composites. Its microstructures consist of uniformly distributed elongated α-SiC grains, matrixlike TiC grains, and yttrium aluminum garnet (YAG) as a grain boundary phase. The composites were fabricated from β-SiC and TiC powders with the liquid forming additives of A1₂O₃ and Y₂O₃ by hot pressing. During the subsequent heat treatment, the β→α phase transformation of SiC led to the in situ growth of elongated α-SiC grains. The fracture toughness of the SiC-30 wt% TiC composites after 6-h annealing was 6.9 MPa-m^1/2, approximately 60% higher than that of as-hot-pressed composites (4.4 MPa-m^1/2). Bridging and crack deflection by the elongated α-SiC grains appear to account for the increased toughness of this new class of composites.     

Production of Chromium-Doped α-Al2O3 Whiskers Via Vapor Liquid Solid Deposition

The prime objective of this work is to demonstrate that chromium-doped alumina fibers could, for the first time, be obtained via vapor liquid solid (VLS) deposition. Various procedures are described and discussed in the text. The mechanism for effective doping is also discussed, and the resulting fibers are analyzed. A modification of the basic VLS deposition process was investigated with the aim of producing doped α-Al₂O₃ (α-alumina or corundum) whiskers. Chromium-doped (ruby) corundum whiskers were obtained by the introduction of Cr³⁺ in gas form within the argon flow used to attain inert furnace atmospheres. Various procedures are described and discussed in the text, using different chromium compounds, and the mechanism of effective doping is also discussed in each case.     

Pulsed Electric Current Sintering of Silicon Nitride

Pulsed electric current sintering (PECS) has been used to densify α-Si₃N₄ powder doped with oxide additives of Y₂O₃ and Al₂O₃. A full density (>99%) was achieved with virtually no transformation to β-phase, resulting in a microstructure with fine equiaxed grains. With further holding at the sintering temperature, the α-to-β phase transformation took place, concurrent with an exaggerated grain growth of a limited number of elongated β-grains in a fine-grained matrix, leading to a distinct bimodal grain size distribution. The average grain size was found to obey a cubic growth law, indicating that the growth is diffusion-controlled. In contrast, the densification by hot pressing was accompanied by a significant degree of the phase transformation, and the subsequent grain growth gave a broad normal size distribution. The apparent activation energy for the phase transformation was as high as 1000 kJ/mol for PECS, almost twice the value for hot pressing (∼500 kJ/mol), thereby causing the retention of α-phase during the densification by PECS.     

Observation of Coherent Perovskite Particles in Growing Chromia Films

A Co–45 wt% Cr alloy that was implanted with 2×10¹⁶ yttrium ions/cm² was oxidized at 1000°C in pure oxygen for 25 h. The chromia film that formed on the surface of the alloy had a grain size of 200 to 300 nm. Yttrium was segregated to the chromia grain boundaries, and both coherent and incoherent particles of YcrO₃ (<2 vol% of the oxide film) were present in the chromia film.     

Morphological Forms of α-Alumina Particles Synthesized in 1,4-Butanediol Solution

Phase-pure, monodispersed, hexagonal plates of single-crystal α-alumina (∼ 2 μm wide and ∼0.5 μm thick) have been prepared via precipitation by treating an aluminum hydrous oxide precursor in 1,4-butanediol at 300°C under autogenous vapor pressure. Present work shows that KOH is the only reagent that precipitates an aluminum hydrous oxide precursor suitable to synthesize α-alumina in 1,4-butanediol solution. In contrast, the use of NaOH or NH₄OH as the precipitating reagent for the precursor material does not yield the alpha phase. The solution pH at which the precursor materials are precipitated is also a critical factor for the formation of α-Al₂O₃. Phase-pure α-alumina powders were also only synthesized from the aluminum hydrous oxide precursors precipitated in the pH range from 10 to 10.5. The results of X-ray diffraction and scanning electron microscopy indicate that longer reaction times promote the phase transformation from the intermediate boehmite phase to α-alumina. The complete transformation from boehmite to α-alumina requires reaction times of about 12 h.     

Electron Microscopy Study of the Effect of Iron in Reaction-Sintered Silicoa Nitride

The microstructure, crystal structure, and chemical composition of reaction-sintered Si₃N₄ containing iron were studied using conventional and scanning transmission electron microscopy. It was found that the grains of β -Si₃N₄ were large and blocklike with well-developed facets, a series of voids along some grain boundaries, a subgrain of iron silicide near the periphery, and penetration of iron silicide into the three-grain junctions and grain boundaries. At some distance from each β -Si₃N₄ grain was a region of small α-Si₃N₄ grains, with no evidence of iron silicide. Between this region and the β -Si₃N₄ grain was a zone containing both α- and β -Si₃N₄ and iron silicide. These observations suggest that the large β -Si₃N₄ grains grow in liquid iron silicide, that the smaller α-Si₃N₄ grains grow from the vapor, and that the latter are converted to the β form by solution in, and reprecipitation from, liquid iron silicide.