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.     

Hydrothermal Synthesis of Submicrometer α-Alumina from Seeded Tetraethylammonium Hydroxide-Peptized Aluminum Hydroxide

Submicrometer α-alumina powder was successfully synthesized from seeded aluminum hydroxide peptized with tetraethylammonium hydroxide (TENOH) and hydrothermally treated at 200°C, using α-alumina particles as seeds. The powders were characterized by XRD, SEM, DTA-TG, and BET analyses. Results showed that seeding could greatly enhance the transition to α-alumina at 200°C without formation of other transient alumina phases. α-Alumina with some amount of boehmite formed in the seeded samples, whereas boehmite was the exclusive phase formed in the nonseeded sample. The morphology of α-alumina embedded in the boehmite matrix for the seeded samples suggests a direct transition from aluminum hydroxide to α-alumina without the formation of transient alumina phases. The formation of α-alumina in the seeded samples at temperatures as low as 200°C could be attributed to a favored nucleation in the TENOH-peptized aluminum hydroxide and to the subsequent hydrothermal treatment that supplies the necessary activation energy for crystal growth. Transition of boehmite to α-alumina in the hydrothermally treated samples with low-seed contents was significantly promoted by heat-treating the samples at 500°C.     

Preparation of Nanometer-Sized α-Alumina Powders by Calcining an Emulsion of Boehmite and Oleic Acid

This study proposes a method to form ultrafine α-Al₂O₃ powders. Oleic acid is mixed with Al(OH)₃ gel. The gel is the precursor of the Al₂O₃. After it is mixed and aged, the mixture is calcined in a depleted oxygen atmosphere between 25° and 1100°C. Oleic acid evaporates and decomposes into carbon during the thermal process. Residual carbon prevents the growth of agglomerates during the formation of α-Al₂O₃. The phase transformation in this process is as follows: emulsion →γ-Al₂O₃→δ-Al₂O₃→θ-Al₂O₃→α-Al₂O₃. This process has no clear θ phase. Aging the mixed sample lowers the formation temperature of α-Al₂O₃ from 1100° to 1000°C. The average crystallite diameter is 60 nm, measured using Scherrer's equation, which is consistent with TEM observations.     

Synthesis of Magnesium Aluminate Spinel Platelets from α-Alumina Platelet and Magnesium Sulfate Precursors

Spinel platelets were formed from a powder mixture of 3–5 μm wide and 0.2–0.5 μm thick α-Al₂O₃ and 1–8 μm (average 3 μm) MgSO₄ heated 2 h at 1200°C. The hexagonal platelet shape of the original α-Al₂O₃ platelet was maintained in the spinel, although their size was slightly increased and their surface roughened. When a mixture of α-Al₂O₃ platelets and MgO powder was heated 3 h at 1400°C, the spinel formed lost the platelet morphology of the alumina.     

Nanocrystalline α-Al2O3 Using Sucrose

Nanocrystalline α-Al₂O₃ ceramic powders have been prepared from an aqueous solution of aluminum nitrate and sucrose. Soluble Al ion-sucrose solution forms the precursor material once it is completely dehydrated. Heat treatment of the dehydrated precursors at low temperature (600°C) results in the formation of porous single-phase α-Al₂O₃. The precursor and heat-treated powders have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and BET surface area analysis. The phase-pure nanocrystalline α-Al₂O₃ particles had an average specific surface area of >190 m²/g, with an average pore size between 18 and 25 nm.     

Kinetics and Microstructural Evolution of Heterogeneous Transformation of θ-Alumina to α-Alumina

Ultrafine (<0.1 μm) high-purity θ-Al₂O₃ powder containing 3–17.5 mol%α-Al₂O₃ seeds was used to investigate the kinetics and microstructural evolution of the θ-Al₂O₃ to α-Al₂O₃ transformation. The transformation and densification of the powder that occurred in sequence from 960° to 1100°C were characterized by quantitative X-ray diffractometry, dilatometry, mercury intrusion porosimetry, and transmission and scanning electron microscopy. The relative bulk density and the fraction of α phase increased with annealing temperature and holding time, but the crystal size of the α phase remained ∼50 nm in all cases at the transformation stage (≤1020°C). The activation energy and the time exponent of the θ to α transformation were 650 ± 50 kJ/mol and 1.5, respectively. The results implied the transformation occurred at the interface via structure rearrangement caused by the diffusion of oxygen ions in the Al₂O₃ lattice. A completely transformed α matrix of uniform porosity was the result of appropriate annealing processes (1020°C for 10 h) that considerably enhanced densification and reduced grain growth in the sintering stage. The Al₂O₃ sample sintered at 1490°C for 1 h had a density of 99.4% of the theoretical density and average grain size of 1.67 μm.     

Nanocrystalline γ-Alumina Synthesized by Sonohydrolysis of Alkoxide Precursor in the Presence of Organic Acids: Structure and Morphological Properties

γ-Al₂O₃ nanoparticles with an average size of 5 nm were synthesized by the hydrolysis of aluminum triisopropoxide under the influence of power ultrasound (100 W/cm²) and in the presence of formic or oxalic acids as peptizers, followed by calcination. The structural and morphological properties of the as-prepared precursor hydroxides and calcined nanocrystalline powders were characterized by XRD, SEM, TEM, TGA, IR, and BET. The ultrasound-driven cavitation process has been shown to affect the agglomeration of the precursor nanoparticles by condensation of interparticle hydroxyls. The oxalate anions were strongly adsorbed on the surface of the precursor nanoparticles and thus retarded the ultrasound-driven condensation of interparticle hydroxyls. Formic acid showed a lesser degree of adsorption on the surface of the precursor particles. The ultrasound-driven agglomeration of the primary particles as well as the role of organic modifiers on the microstructural properties of the precursor and the target alumina phases have been discussed.     

Preparation and Characterization of Macroporous α-Alumina

α-Al₂O₃ powders with three-dimensionally ordered or randomly positioned macropores were synthesized by templating with poly(methyl methacrylate) colloidal crystals. Aluminum nitrate was precipitated with ammonium hydroxide within the interstices of the template; calcination removed the polymer and converted the inorganic precursors into a macroporous skeleton of α-Al₂O₃. Subsequent calcination at higher temperatures and hot stage transmission electron microscopy experiments were performed to study sintering effects on the product morphology. These materials combine the thermal and chemical stability of corundum with a very open structure of uniform macropores that can permit facile transport of guest molecules in potential catalysis, filtration, and sensing applications.     

Preparation of α-Alumina Platelets by Laser Scanning

A laser scanning with gas jet process was developed to prepare alumina platelets from an alumina powder. When the carbon-dioxide laser scanned the alumina powdery coatings prepared using an electrospraying technique, the alumina particles were heated to a melting state. The coaxial gas ejection force pushed the melting particles to obtain tabular shape grains that recrystallized into alumina platelets in the subsequent rapid-cool solidification. The phase and morphologies of powder bed were characterized by XRD and SEM. Results show that only α-alumina platelets were formed in the scanning process and the average edge length and thickness is 10 μm and 1–2 μm, respectively. Laser processing parameters such as laser energy density, scanning speed, and gas pressure were expected to play a vital role in the melting-crystallization-solidification process for obtaining platelike grains from powder beds. The preliminary experiment showed that the laser-scanning technique could be an effective means of tailoring the morphologies of particles to meet application requirements.     

Preparation Method of Submicrometer-Sized α-Alumina by Surface Modification of γ-Alumina with Alumina Sol

A method is introduced to prepare almost-spherical submicrometer-sized α-alumina via surface modification of γ-alumina with an alumina sol. Milled γ-alumina, in the presence of 3 wt% of α-alumina with a median particle size ( d ₅₀) of 0.32 μm (AKP-30), produced irregularly shaped α-alumina with d ₅₀∼0.3 μm after heat treatment at 1100°C for 1 h. γ-alumina that had been surface-modified by milling in the presence of 3 wt% of the alumina sol resulted in almost-monosized, spherical α-alumina ∼0.3 μm in size after heat treatment at 1100°C for 1 h. Furthermore, almost-spherical α-alumina 0.1—0.2 μm in size was obtained by milling γ-alumina with 3 wt% of AKP-30 alumina in the presence of 3 wt% of the alumina sol, followed by heat treatment at 1100°C for 1 h. The alumina sol that has been introduced in this work seems to act as a dispersant, in addition to helping to form a spherical shape.     

Synthesis of Zinc Aluminate Spinel Film through the Solid-Phase Reaction between Zinc Oxide Film and α-Alumina Substrate

We synthesized spinel ZnAl₂O₄ film on α-Al₂O₃ substrate using a solid-phase reaction between the pulsed-laser-deposited ZnO film and α-Al₂O₃ substrate. Auger electron spectroscopy showed that the atomic distribution in the spinel ZnAl₂O₄ was inhomogeneous, which indicated that the reaction was diffusion controlled. Based on X-ray fluorescence measurements, the apparent growth activation energy of ZnAl₂O₄ was determined as 504 kJ/mol. X-ray diffractometry spectra showed that, as the growth temperature increased, the ZnAl₂O₄ film became disoriented from the single (111) orientation. The ZnAl₂O₄ (333) diffraction peak shifted toward a small angle, and its full-width at half-maximum decreased from 1.30° to 0.37°. At the growth temperature of 1100°C, the morphology of the ZnAl₂O₄ was initially transformed from islands to stick structures, then to bulgy-line structures with increased growth time. X-ray diffractometry spectra showed that these transformations were correlated with changes of ZnAl₂O₄ orientation.     

Tribological Characteristics of α-Alumina at Elevated Temperatures

The tribological characteristics of a high-purity α-alumina sliding on a similar material under unlubricated conditions are divided into four distinct regimes. At low temperatures, T < 200°C, tribochemical reactions between the alumina surface and water vapor in the environment control the tribological performance. The coefficient of friction in this temperature range is approximately 0.40 and the wear coefficient is less than 10⁻⁶, independent of contact load. At intermediate temperatures, 200°C < T < 800°C, the wear behavior depends on the contact load. At low loads, wear occurs by plastic flow and plowing; the coefficient of friction is approximately 0.60 and the wear coefficient is less than 10⁻⁶. At loads larger than a threshold value, severe wear occurs by intergranular fracture. The coefficient of friction increases to 0.85 and the wear coefficient increases to a value greater than 10⁻⁴. At temperatures above 800°C, formation of a silicon-rich layer on the wear track by diffusion and viscous flow of the grain-boundary phase reduces the coefficient of friction to 0.40, and the wear coefficient is reduced to a value less than 10⁻⁶. The results of the wear tests and observations of the fundamental mechanisms controlling the tribological behavior of this material are consolidated in a simple wear transition diagram.     

Sintering of Mixtures of Seeded Boehmite and Ultrafine α-Alumina

α-Alumina was fabricated by dry pressing mixtures of seeded boehmite and fine α-alumina (i.e., 0.2 and 0.3 μm diameter) to reduce the large shrinkage of boehmite-derived α-alumina. The maximum green density was obtained with mixtures containing ∼70%α-alumina for both alumina powders. The ∼15% linear shrinkage and microstructures of these samples were comparable to 100% alumina powder samples. Samples with 0.2 μm alumina sintered to densities >95% at 1300°C whereas 1400°C was needed for samples with 0.3 μm alumina. These results indicate that boehmite can be used as a substitute for relatively expensive ultrafine α-alumina powders.     

Method for Production of α-Alumina Whiskers via Vapor-Liquid-Solid Deposition

α-alumina (α-Al₂O₃, corundum) fibers exhibit high thermal and chemical stability, as well as good mechanical properties, even at high temperatures. Such characteristics make them good candidates for use in composites. Nevertheless, very few methods of producing α-Al₂O₃ fibers are available. In the present work, we describe a method that uses aluminum pieces deposited on SiO₂ powder, in an argon atmosphere, at temperatures in the range 1300°–1600°C. The α-Al₂O₃ fibers are obtained via vapor-liquid-solid deposition. The novel addition of nickel and cobalt (or their oxides) allows the use of temperatures >1500°C, resulting in improved fiber production. We demonstrate that the metals do not contaminate the fibers produced in this way. Finally, we also estimate the tensile strength of the Al₂O₃ fibers produced through this method.     

Formation of Microcrystalline α-Alumina by Glycothermal Treatment of Gibbsite

Microcrystalline α-aluminas (hexagonal plates, ∼0.1 μm wide and ∼0.025 μm thick) were prepared by treating fineparticle gibbsite in glycol at 300°C under the spontaneous vapor pressure of glycol (glycothermal treatment). The α-alumina was formed by the collapse of the glycol derivative of boehmite.     

Solid-Phase Epitaxy of Boehmite-Derived α-Alumina on Hematite Seed Crystals

The seeded transformation of boehmite-derived alumina was studied by transmission electron microscopy. Crystallographic analysis confirmed that the growth of α-Al₂O₃ on α-Fe₂O₃ seed crystals occurs by solid-phase epitaxy, with the orientation relationship [0001]_Al2O₃||[0001]_Fe2O₃ and [11 2 0]_Al2O₃||[11 2 0_Fe2O₃.     

Topotactic Growth of β-Alumina on α-Alumina

Thin foils of polycrystalline α-alumina were reacted with a potassium-rich vapor at ≤900°C. Potassium β-alumina formed along α-alumina grain boundaries and protruded from holes in the foils. Conventional transmission electron microscopy was used to analyze the α-alumina/β-alumina phase boundary for possible orientation relations.     

Size Control of α-Alumina Particles Synthesized in 1,4-Butanediol Solution by α-Alumina and α-Hematite Seeding

The effects of seed particles and shear rate on the size and shape of α-Al₂O₃ particles synthesized in glycothermal conditions are described. It is proposed that seed particles provide a low-energy, epitaxial surface in solution to lower the overall surface energy contribution to the nucleation barrier, thus increasing nucleation frequency and subsequently reducing the particle size of hexagonal α-Al₂O₃ platelets or polyhedra, depending on synthesis conditions, in 1,4-butanediol solution. Seeds have a significant effect on the size of hexagonal α-Al₂O₃ platelets in samples with high seed concentration. The particle size of α-Al₂O₃ platelets decreases from 3 to 4 µm to 100 to 200 nm by increasing the number concentration of seeds. In the case of α-Fe₂O₃ seeding, the effect of seeding on the size of α-Al₂O₃ particles closely resembles the effects obtained with α-Al₂O₃ seeding. Regardless of seed concentration, high stirring rate promotes the formation of hexagonal platelets with high aspect ratio, whereas medium and low stirring rates promote the formation of elongated platelets and polyhedra with 14 faces, respectively.     

Grain Growth Phenomena in the Interface Region of α-Alumina Bilayer Composites

Microstructural development in the interface region of α-Al₂O₃ bilayer composites has been systematically investigated in terms of the sintering additive CaO–SiO₂, residual impurity level in the starting powders (particularly MgO), and sintering conditions. The interfacial microstructure is strongly related to relative CaO–SiO₂ doping levels in the two constituting layers and to residual impurities in the starting powders. The presence of high levels of impurities in the starting powder can substantially modify the features of CaO–SiO₂-Al₂O₃ liquid at the interface region, thereby strongly influencing α-Al₂O₃ grain growth across the interface. Three grain growth modes in the interface region thus have been identified for different combinations of impurity level and CaO–SiO₂ dopant in the α-Al₂O₃ bilayer. This provides an important mechanism for controlling two-dimensional structures in coatings, films, and layered ceramic materials for various engineering applications.