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.     

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.     

Low-Temperature Sintering of Seeded Sol—Gel-Derived, ZrO2-Toughened Al2O3 Composites

Seeding a mixture of boehmite (AIOOH) and colloidal ZrO₂ with α-alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ-to α-alumina transformation, homogeneous mixing, and the uniform structure of the sol-gel system. Complete retention of pure ZrO₂ in the tetragonal phase was obtained to 14 vol% ZTA because of the low-temperature sintering. The critical grain size for tetragonal ZrO₂ was determined to be ∼0.4 μm for the 14 vol% ZrO₂—Al₂O₃ composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication.     

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.     

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.     

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.     

Synthesis of Aluminum Oxide Platelets

Aqueous solutions of boehmite and hydrofluoric acid (HF) were used to prepare homogeneous mixtures of alumina and aluminum fluoride. Calcination at temperatures as low as 1000°C resulted in the formation of well-defined hexagonal-shaped α-alumina platelets. Containment of the aluminum fluoride by covering the calcination crucible promoted crystal growth presumably by a reaction of continuous evaporation–condensation of aluminum fluoride. Hexagonal-shaped platelet α-alumina was observed with average diameters ranging from 7 to 33 μm. Large platelets with a narrow size distribution and average diameter of over 25 μm were prepared by controlling the initial concentration of HF and the calcination time, temperature, and atmosphere.     

Preparation of Monodisperse, Spherical Alumina Powders from Alkoxides

Monodisperse, spherical alumina powders were prepared by the controlled hydrolysis of aluminum alkoxide in a dilute solution consisting of octanoi and acetonitrile. The reagent concentration influenced the particle size, size distribution, morphology, and state of agglomeration. The addition of acetonitrile and octanol affected both particle shape and the state of agglomeration and prevented gelation during hydrolysis. Sub-micrometer-sized alumina particles grew to 10 μm with increased concentration of hydroxypropylcellulose dispersant. As-prepared amorphous powders crystal-lized to γ-alumina at 1000°C and converted to α-alumina at 1150°C without intermediate phases. The particle morphology was retained after crystallization to γ-alumina.     

Effect of Pore Size Distribution of Carbon-Covered Alumina on the Preparation of Submicrometer α-Alumina Powders

Calcining carbon-covered alumina (CCA) samples at 800°C in an oxygen flow is an efficient method to prepare α-alumina powders. It is found that the pore size distribution of CCA samples, which depends on the carbon content and the pore size distribution of the precursor alumina used, is one of the key factors for the total conversion of γ-alumina to α-alumina and the complete combustion of carbon in the pores of alumina. No matter how high the carbon content, total conversion does not occur for CCA samples prepared from alumina possessing the most probable pore size of about 5.2 nm. Using γ-alumina with the most probable pore size of 6.1 nm as the precursor of CCA samples, total transformation occurs when the carbon content of CCA ranges from 11.9 to 17.3 wt%, but the color of as-prepared α-alumina is not pure white but light gray. Polyethylene glycol (PEG 20 000), added to the sucrose/γ-alumina system, can expand the pores of CCA samples after carbonization, and calcining of thus-prepared CCA results in a complete transformation of γ-alumina to pure white α-alumina with a particle size of about 1 μm when the carbon content of CCA is between 6 and 19 wt%.     

Synthesis of Thermally Stable χ-Alumina by Thermal Decomposition of Aluminum Isopropoxide in Toluene

Thermal decomposition of aluminum isopropoxide in toluene at 315°C resulted in χ-alumina that had high thermal stability, whereas the reaction at lower temperatures resulted in formation of an amorphous product. The χ-alumina thus obtained directly transformed to α-alumina at ∼1150°C, bypassing the other transition alumina phases, whereas the amorphous product transformed to γ-alumina and then to θ-alumina before final transformation to α-alumina. When the χ-alumina, solvothermally synthesized at 315°C, was recovered by the removal of the solvent at the reaction temperature, thermal stability of the product was improved further. This procedure is convenient because it avoids bothersome work-up processes that yield large-surface-area and large-pore-volume alumina.     

Preparation and Characterization of High-Temperature Thermally Stable Alumina Composite Membrane

A crack- and pinhole-free composite membrane consisting of an α-alumina support and a modified γ-alumina top layer which is thermally stable up to 1100°C was prepared by the sol–gel method. The supported thermally stable top layer was made by dipcoating the support with a boehmite sol doped with lanthanum nitrate. The temperature effects on the microstructure of the (supported and unsupported) La-doped top layers were compared with those of a common γ-alumina membrane (without doping with lanthanum), using the gas permeability and nitrogen adsorption porosimetry data. After sintering at 1100°C for 30 h, the average pore diameter of the La-doped alumina top layer was 17 nm, compared to 109 nm for the common alumina top layer. Addition of poly(vinyl alcohol) to the colloid boehmite precursor solution prevented formation of defects in the γ-alumina top layer. After sintering at temperatures higher than 900°C, the common alumina top layer with addition of poly(vinyl alcohol) exhibits a bimodal pore distribution. The La-doped alumina top layer (also with addition of poly(vinyl alcohol)) retains a monopore distribution after sintering at 1200°C.     

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.     

Effect of Calcination on the Sintering of Gel-Derived, Zirconia-Toughened Alumina

The densification behavior of ZrO₂ (+ 3 mol% Y₂O₃)/85 wt% Al₂O₃ powder compacts, prepared by the hydrolysis of metal chlorides, can be characterized by a transition- and an α-alumina densification stage. The sintering behavior is strongly determined by the densification of the transition alumina aggregates. Intra-aggregate porosity, resulting from calcination at 800°C, partly persists during sintering and alumina phase transformation and negatively influences further macroscopic densification. Calcination at 1200°C, however, densifies the transition alumina aggregates prior to sintering and enables densification to almost full density (96%) within 2 h at 1450°C, thus obtaining a microstructure with an alumina and a zirconia grain size of 1 μm and 0.3–0.4 μm, respectively.     

Enhanced Densification of Boehrmte Sol-Gels by α-Alumina Seeding

Boehmite sol-gels were seeded by adding <2.0 wt%α-alumina powder to a 20% boehmite hydrosol at pH =3. The seeded gel sintered to 98% of theoretical density after 100 min at 1200°C, whereas the unseeded gel had to be sintered at 1600°C to reach 94% of theoretical density. This difference results primarily from nucleation by the α-alumina seeds and enhanced transformation of the boehmite to α-alumina, such that a uniform, well-ordered, fine-grained α-alumina micro-structure forms prior to densification.     

Mechanical-Activation-Triggered Gibbsite-to-Boehmite Transition and Activation-Derived Alumina Powders

Mechanical activation of monoclinic gibbsite (Al(OH)₃) in nitrogen led to the formation of nanocrystalline orthorhombic boehmite (AlOOH) at room temperature. The boehmite phase formed after merely 3 h of mechanical activation and developed steadily as the mechanical-activation time increased. Forty hours of mechanical activation resulted in essentially single-phase boehmite, together with α-alumina (α-Al₂O₃) nanocrystallites 2–3 nm in size. The sequence of phase transitions in the activation-derived boehmite was as follows: boehmite to γ-Al₂O₃ and then to α-Al₂O₃ when flash-calcined at a heating rate of 10°C/min in air. γ-Al₂O₃ formed at 520°C, and flash calcination to 1100°C led to the formation of an α-Al₂O₃ phase, which exhibited a refined particle size in the range of 100–200 nm. In contrast, the gibbsite-to-boehmite transition in the unactivated gibbsite occurred over the temperature range of 220°–330°C. A flash-calcination temperature of 1400°C was required to complete the conversion to α-Al₂O₃ phase, with both δ-Al₂O₃ and θ-Al₂O₃ as the transitional phases. The resulting alumina powder consisted of irregularly shaped particles 0.4–0.8 μm in size, together with an extensive degree of particle agglomeration.     

Submicrometer Transparent Alumina by Sinter Forging Seeded γ-Al2O3 Powders

Seeding boehmite with α-Al₂O₂, followed by calcination at 600°C, results in an agglomerated alumina powder (<53 μm) that can be sinter forged to full density at 1250°C. Compressive strains as high as ɛ_x=−0.9, and radial flow (ɛ_x= 1.0) during sinter forging remove large, interagglomerate pores. The fully dense alumina has a grain size of 0.4 pm and is visually transparent. It is proposed that deformation of dense agglomerates is the primary mecha- nism responsible for large pore elimination and compact densification. The sinter forging of sol-gel-derived alumina powders offers a new technology to prepare highly transparent, optical ceramics at lower temperatures than conventional routes.     

Transitions in Vapor-Deposited Alumina from 300° to 1200°C

The transition of amorphous alumina to α-alumina was studied by X-ray diffraction, electron diffraction, DTA, TGA, and microscopic observation. The amorphous alumina was prepared by condensing vapor from evaporating molten alumina in vacuo onto the glass envelope of the vacuum chamber. The amorphous alumina was transformed to a poorly crystalline material by heating for 16 hr between 570° and 670°C. Between 670° and 1200°C, the poorly crystalline alumina was converted to α-alumina via two parallel series of transition aluminas. The principal series was γ-alumina to δ-alumina to α-alumina. A minor amount of θ-alumina developed from the initial crystallization and persisted throughout the duration of the principal series as a parallel path. Some conversion of δ- to θ-alumina was detected above 900°C. DTA produced an unexplained exothermic peak at 320°C and a second exothermic peak at 860°C which corresponded to formation of metastable aluminas.     

Efficient Preparation of Submicrometer α-Alumina Powders by Calcining Carbon-Covered Alumina

A simple method is described to prepare submicrometer α-alumina by burning carbon supported on the surface of γ-alumina in oxygen flow at a temperature of 800°C. The burning of carbon generates a large amount of heat and leads to a rapid increase in the local temperature inside the pores of alumina. When the temperature is high enough for the phase transformation, α-alumina is obtained in a very short time. It was found that, for carbon contents between 6 and 10 wt%, all the γ-alumina could transform into α-alumina after burning of carbon in oxygen for a short time, and the transformed particle sizes of α-alumina were mostly no more than 1 μm.     

Time-Temperature-Transformation Curves for Kaolinite-α-Alumina

The C-shaped time-temperature-transformation curves (T-T-T curves) of cristobalite formation and the L-shaped T-T-T curves of α-alumina reaction were established for a high-purity kaolinite-α-alumina mixture during heating. The results revealed that cristobalite formation in kaolinite was retarded by the presence of α-alumina between 1250° and 1350°C and was totally prohibited above 1380°C due to the reaction of kaolinite with α-alumina to form secondary mullite. The reaction of α-alumina with kaolinite was initiated at about 1250°C. It became quite extensive above 1380°C and was extremely fast at 1600°C and above, indicating the strong effect of the eutectic liquid formation at ∼1587°C in silica-alumina. The effectiveness of the established T-T-T curves was demonstrated and discussed.     

Microwave Sintering of Alumina at 2.45 GHz

The sintering kinetics and microstructural evolution of alumina tubes (∼17 mm length, ∼9 mm inner diameter, and ∼11 mm outer diameter) were studied by conventional and microwave heating at 2.45 GHz. Temperature during microwave heating was measured with an infrared pyrometer and was calibrated to ±10°C. With no hold at sintering temperature, microwave-sintered samples reached 95% density at 1350°C versus 1600°C for conventionally heated samples. The activation energy for microwave sintering was 85 ± 10 kJ/mol, whereas the activation energy for conventionally sintered samples was 520 ± 14 kJ/mol. Despite the difference in temperature, grains grew from ∼1.0 μm at 86% density to ∼2.6 μm at 98% density for both conventionally sintered and microwave-sintered samples. The grain size/density trajectory was independent of the heating source. It is concluded that the enhanced densification with microwave heating is not a consequence of fast-firing and therefore is not a result in the change in the relative rates of surface and grain boundary diffusion in the presence of microwave energy.