Transformation, Microstructure Development, and Densification in α-Fe2O3-Seeded Boehmite-Derived Alumina

Addition of α-Fe₂O₃ seed particles to alkoxide-derived boehmite sols resulted in a 10-fold increase in isothermal rate constants for the transformation of γ- to α-Al₂O₃. Changes in porosity and surface area with sintering temperature showed no effect of seeding on coarsening of the transition alumina gels, but the 200-fold decrease in surface area associated with transformation to α-Al₂O₃ occurred ∼ 100°C lower in seeded gels compared with unseeded materials. As a result of high nucleation frequency and reduced microstructure coarsening, fully transformed seeded alumina retained specific surface areas >22 m²/g and exhibited narrow pore size distributions, permitting development of fully dense, submicrometer α-Al₂O₃ at ∼ 1200°C.     

Growth of Single-Crystal and Polycrystalline Thin Films of MgAl2O4 and MgFe2O4

Single-crystal and polycrystalline films of Mg-Al₂O₄ and MgFe₂O₄ were formed by two methods on cleavage surfaces of MgO single crystals. In one procedure, aluminum was deposited on MgO by vacuum evaporation. Subsequent heating in air at about 510°C formed a polycrystalline γ-Al₂O₈ film. Above 540°C, the γ-Al₂O, and MgO reacted to form a single-crystal MgAl₂O₄ film with {001} MgAl₂O₄‖{001} MgO. Above 590°C, an additional layer of MgAl₂O₄, which is polycrystalline, formed between the γ-Al₂O₃ and the single-crystal spinel. Polycrystalline Mg-Al₂O₄ formed only when diffusion of Mg²⁺ ions proceeded into the polycrystalline γ-Al₂O₃ region. Corresponding results were obtained for Mg-Fe₂O₄. MgAl₂O₄ films were also formed on cleaved MgO single-crystal substrates by direct evaporation, using an Al₂O₃ crucible as a source. Very slow deposition rates were used with source temperatures of ∼1350°C and substrate temperatures of ∼800°C. Departures from single-crystal character in the films may arise through temperature gradients in the substrate.     

Dense Al2O3/ZrO2 Particulate Composites by Free Sintering of Coated Powders

Composite powders, prepared by coating coarse ZrO₂ particles with fine Al₂O₃ powder using a chemical precipitation technique, were compacted and sintered freely at a constant heating rate of 4°C/min to ∼1600°C. Composites containing up to ∼30 vol% inclusions were sintered to nearly full density under the same conditions used for the unreinforced matrix. Furthermore, the sintering kinetics were not influenced significantly by the inclusion volume fraction. The sinterability of the composites formed from the coated powders was significantly better than that for similar composites formed from mechanically mixed powders. The present data provide a further demonstration that the use of coated powders may have widespread applicability for the fabrication, by free sintering, of dense ceramic particulate composites.     

Spark-Plasma Sintering of Silicon Carbide Whiskers (SiCw) Reinforced Nanocrystalline Alumina

The combined effect of rapid sintering by spark-plasma-sintering (SPS) technique and mechanical milling of γ-Al₂O₃ nanopowder via high-energy ball milling (HEBM) on the microstructural development and mechanical properties of nanocrystalline alumina matrix composites toughened by 20 vol% silicon carbide whiskers was investigated. SiC_w/γ-Al₂O₃ nanopowders processed by HEBM can be successfully consolidated to full density by SPS at a temperature as low as 1125°C and still retain a near-nanocrystalline matrix grain size (∼118 nm). However, to densify the same nanopowder mixture to full density without the benefit of HEBM procedure, the required temperature for sintering was higher than 1200°C, where one encountered excessive grain growth. X-ray diffraction (XRD) and scanning electron microscopy (SEM) results indicated that HEBM did not lead to the transformation of γ-Al₂O₃ to α-Al₂O₃ of the starting powder but rather induced possible residual stress that enhances the densification at lower temperatures. The SiC_w/HEBMγ-Al₂O₃ nanocomposite with grain size of 118 nm has attractive mechanical properties, i.e., Vickers hardness of 26.1 GPa and fracture toughness of 6.2 MPa·m^1/2.     

Densification of Cr2O3-ZrO2 Ceramics by Sintering

Cr₂O₃ and ZrO₂ were mixed in various ratios and pressed to form compacts, which were then sintered in carbon powder. Compacts with >30 wt% Cr₂O₃ were sintered to densities >98% of true density at 1500°C. This method of sintering in carbon powder can be used to prepare very dense Cr₂O₃-ZrO₂ ceramics at a relatively low temperature, (∼1500°C) without additives.     

Effects of Heating Rate on the Sintering Behavior and the Dielectric Properties of Ba0.7Sr0.3TiO3 Ceramics Prepared by Boron-Containing Liquid-Phase Sintering

The effects of heating rate on the sintering behavior and the dielectric properties of Ba_0.7Sr_0.3TiO₃ ceramics prepared by boron-containing liquid-phase sintering were investigated. When 0.5 wt% B₂O₃ was added to Ba_0.7Sr_0.3TiO₃, sintering was achieved at ∼1150°C, and the overdoped B₂O₃ did not form an adequate amount of liquid phase or volatilize; it remained in the samples and formed a secondary phase. A transition broadening was observed as the heating rate increased. As the heating rate increased, the Curie temperature increased and the maximum dielectric constant ( k _max) at the Curie temperature decreased. This result is attributable to a decrease in the diffuseness parameter (δ) and the tetragonality ( c / a ).     

Preparation of Porous Cr3C2 Grains with Cr2O3

Porous Cr₃C₂ grains (∼300 to 500 μm) with ∼10 wt% of Cr₂O₃ were prepared by heating a mixture of MgCr₂O₄ grains and graphite powder at 1450° to 1650°C for 2 h in an Al₂O₃ crucible covered by an Al₂O₃ lid with a hole in the center. The porous Cr₃C₂ grains exhibited a three-dimensional network skeleton structure. The mean open pore diameter and the specific surface area of the porous grains formed at 1600°C for 2 h were ∼3.5 (μm and ∼6.7 m²/g, respectively. The present work investigated the morphology and the formation conditions of the porous Cr₃C₂ grains, and this paper will discuss the formation mechanism of those grains in terms of chemical thermodynamics.     

Low-Temperature High-Pressure Consolidation of Amorphous Al2O3–15 mol% Y2O3

This study examined pressure consolidation of amorphous Al₂O₃–15 mol% Y₂O₃ powders prepared by co-precipitation and spray pyrolysis. The two amorphous powders had similar true densities and crystallization sequences. Uniaxial hot pressing was carried out at 450°–600°C with a moderate pressure of 750 MPa. The co-precipitated powder could be hot pressed to a maximum relative density of 98% and remained amorphous. Pressure adversely affected the densification of the spray-pyrolyzed powder by favoring an early crystallization of γ-Al₂O₃ phase at 580°C. Plastic deformation of the amorphous phase is believed to be responsible for the large densification of the amorphous powders.     

Fabrication of Transparent, Sintered Sc2O3 Ceramics

We report here the fabrication of transparent Sc₂O₃ ceramics via vacuum sintering. The starting Sc₂O₃ powders are pyrolyzed from a basic sulfate precursor (Sc(OH)_2.6(SO₄)_0.2·H₂O) precipitated from scandium sulfate solution with hexamethylenetetramine as the precipitant. Thermal decomposition behavior of the precursor is studied via differential thermal analysis/thermogravimetry, Fourier transform infrared spectroscopy, X-ray diffractometry, and elemental analysis. Sinterability of the Sc₂O₃ powders is studied via dilatometry. Microstructure evolution of the ceramic during sintering is investigated via field emission scanning electron microscopy. The best calcination temperature for the precursor is 1100°C, at which the resultant Sc₂O₃ powder is ultrafine (∼85 nm), well dispersed, and almost free from residual sulfur contamination. With this reactive powder, transparent Sc₂O₃ ceramics having an average grain size of ∼9 μm and showing a visible wavelength transmittance of ∼60–62% (∼76% of that of Sc₂O₃ single crystal) have been fabricated via vacuum sintering at a relatively low temperature of 1700°C for 4 h.     

Formation and Transformation of TiO2 (Anatase) Solid Solution in the System TiO2—Al2O3

In the system TiO₂—Al₂O₃, TiO₂ (anatase, tetragonal) solid solutions crystallize at low temperatures (with up to ∼ 22 mol% Al₂O₃) from amorphous materials prepared by the simultaneous hydrolysis of titanium and aluminum alkoxides. The lattice parameter a is relatively constant regardless of composition, whereas parameter c decreases linearly with increasing Al₂O₃. At higher temperatures, anatase solid solutions transform into TiO₂ (rutile) with the formation of α-Al₂O₃. Powder characterization is studied. Pure anatase crystallizes at 220° to 360°C, and the anatase-to-rutile phase transformation occurs at 770° to 850°C.     

Preparation of Submicrometer A12O3 Powder by Gas-Phase Oxidation of Tris (acetylacetonato) aluminum (111)

Fine A1₂O₃ powder was prepared by the gas-phase oxidation of aluminum acetyl-acetonate. The reaction products were amorphous material at 600° and 800°C, γ-Al₂O₃ at 1000° and 1200°C, and δ-Al₂O₃ at 1400°C. The powders consisted of spherical particles from 10 to 80 nm in diameter; particle size increased with increasing reaction temperature and concentration of chelate in the gas.     

Fabrication of Nano-Scaled α-Al2O3 Crystallites Through Heterogeneous Precipitation of Boehmite in a Well-Dispersed θ-Al2O3-Suspension

The possibility of eliminating finger or vermicular growth of α-Al₂O₃ particles obtained by calcination of boehmite was examined. Heterogeneous precipitation of boehmite in a well-dispersed θ-Al₂O₃ suspension was first prepared, in which the mass ratio of boehmite to θ-crystallite was evaluated to form agglomerates of similar sizes that will form α-Al₂O₃ crystallites of <100 nm in diameter. θ- to α-phase transformation of alumina experiences a nucleation and growth mechanism, with the critical size of nucleation being ∼25 nm for θ-Al₂O₃ and the size for accomplishment of transformation followed by finger growth being ∼100 nm. Hence, fabricating agglomerates that would form α-Al₂O₃ crystallites with sizes <100 nm accompanied with appropriate thermal treatments can be a method for obtaining α-Al₂O₃ crystallites free of finger growth. It is found that proper preparation of the agglomerate with appropriate size may initiate a simultaneous and lower temperature θ- to α-Al₂O₃ phase transformation for such powder systems, substantially limiting the mass transfer among the newly formed α-Al₂O₃ particles. Moreover, α-Al₂O₃ crystallites free of finger growth can be obtained.     

Preparation, Sintering, and Properties of Translucent Er2O3-Doped Tetragonal Zirconia

Tetragonal zirconia doped with 3 mol% Er₂O₃ was prepared by the gel-precipitation wet-chemical method. The compaction response of ultrafine (∼8.5 nm), calcined, deagglomerated powders was studied. Initial sintering was studied using both isothermal and nonisothermal techniques, and an activation energy of 270 ± 40 kJ/mol was obtained; therefore, grain-boundary diffusion was probably the predominant mechanism in the sintering stage. The microstructural development in the high-temperature-aged, sintered samples and its effect on density and mechanical properties were also studied. A theoretically dense body of tetragonal zirconia solid solution of 3 mol% Er₂O₃–ZrO₂, obtained from sintering below 1400°C, was translucent.     

Solid-State Sintering of YBa2Cu3Ox in Different Oxygen Partial Pressures

The effect of oxygen partial pressure, ranging from 0.001 to 1.0 atm, and temperature, in the range of 930°–955°C, on the solid-state sintering kinetics of the superconducting ceramic YBa₂Cu₃O_7-δ has been studied. Isothermal compaction rates between 930° and 955°C reached a maximum at some critical PO₂ (PO₂^CRIT), with decreasing rates both above and below this oxygen partial pressure. This behavior was not observed for YBa₂Cu₃O_7-δ sintered at 960°C, when a liquid phase is present. The activation energy for sintering above PO₂^CRIT has been estimated to be ∼190 kJ/mol, whereas below PO₂^CRIT it was found to be ∼130 kJ/mol. The oxygen ion diffusion was considered to be the rate-determining step above PO₂^CRIT, while it is hypothesized that lattice strain caused by the formation of oxygen ion vacancies below PO₂^CRIT affected the rate of sintering.     

Molten Salt Synthesis of Magnesium Aluminate (MgAl2O4) Spinel Powder

MgAl₂O₄ (MA) spinel powder was synthesized by heating an equimolar composition of MgO and Al₂O₃ in LiCl, KCl, or NaCl. The synthesis temperature can be decreased from >1300°C (required by the conventional solid–solid reaction process) to ∼1100°C in LiCl, or to ∼1150°C in KCl or NaCl. The molten salt synthesized MA powder was pseudomorphic and retained, to a large extent, the size and morphology of the original Al₂O₃ raw material, indicating that a "template formation mechanism" plays an important role in the synthesis process.     

Factors Affecting the Electrical Conductivity of Donor-Doped Bi4Ti3O12 Piezoelectric Ceramics

High-temperature piezoelectric ceramics based on W⁶⁺-doped Bi₄Ti₃O₁₂ (W-BIT) were prepared by both the conventional mixing oxides and the chemical coprecipitation methods. Sintering was carried out between 800° and 1150°C in air. A rapid densification, >99% of the theoretical density (rho_th) at 900°C/2 h, took place in the chemically prepared W⁶⁺-doped Bi₄Ti₃O₁₂ ceramics, whereas conventionally prepared BIT-based materials achieved a lower maximum density, ∼94% of rho_th, at higher temperature (1050°C). The microstructure study revealed a platelike morphology in both materials. Platelike grains were larger in the conventionally prepared W-BIT-based materials. The sintering behavior could be related both to the agglomeration state of the calcined powders and to the enlargement of the platelets at high temperature. The W⁶⁺-doped BIT materials showed an electrical conductivity value 2-3 orders of magnitude lower than undoped samples. The electrical conductivity increased exponentially with the aspect ratio of the platelike grains. The addition of excess TiO₂ produced a further decrease of the electrical conductivity.     

Reaction Sintering of ZnO-AI2O3

The reaction sintering of equimolar mixtures of ZnO and A1₂O₃ powders was investigated as a function of primary processing parameters such as the temperature, heating rate, green density, and particle size. The powder mixtures were prepared by two different methods. In one method, the ZnO and A1₂O₃ powders were ball-milled. In the other method, the ZnO powder was chemically precipitated onto the A1₂O₃ particles dispersed in a solution of zinc chloride. The sintering characteristics of the compacted powders prepared by each method were compared with those for a prereacted, single-phase powder of zinc aluminate, ZnAl₂O₄. The chemical reaction between ZnO and A1₂O₃ occurred prior to densification of the powder compact and was accompanied by fairly large expansion. The mixing procedure had a significant effect on the densification rate during reaction sintering. The densification rate of the compact formed from the ball-milled powder was strongly inhibited compared to that for the single-phase ZnAl₂O₄ powder. However, the densification rate of the compact formed from the chemically precipitated mixture was almost identical to that for the ZnAl₂O₄ powder. The difference in sintering between the ball-milled mixture and the chemically precipitated mixture is interpreted in terms of differences in the microstructural uniformity of the initial powder compacts resulting from the different preparation procedures.     

Growth of Tabular α-Al2O3 Grains on Porous Alumina Substrate

Gradient, porous alumina ceramics were prepared with the characteristics of microsized tabular α-Al₂O₃ grains grown on a surface with a fine interlocking feature. The samples were formed by spin-coating diphasic aluminosilicate sol on porous alumina substrates. The sol consisted of nano-sized pseudo-boehmite (AlOOH) and hydrolyzed tetraethyl orthosilicate [Si(OC₂H₅)₄]. After drying and sintering at 1150°–1450°C, the crystallographic and chemical properties of the porous structures were investigated by analytical electron microscopy. The results show that the formation of tabular α-Al₂O₃ grains is controlled by the dissolution of fine Al₂O₃ in the diphasic material at the interface. The nucleation and growth of tabular α-Al₂O₃ grains proceeds heterogeneously at the Al₂O₃/glass interface by ripening nano-sized Al₂O₃ particles.     

Synthesis of AlN Nanopowder from γ-Al2O3 by Reduction–Nitridation in a Mixture of NH3–C3H8

Aluminum nitride (AlN) powders were synthesized by gas reduction–nitridation of γ-Al₂O₃ using NH₃ and C₃H₈ as the reactant gases. AlN was identified in the products synthesized at 1100°–1400°C for 120 min in the NH₃–C₃H₈ gas flow confirming that AlN can be formed by the gas reduction–nitridation of γ-Al₂O₃. The products synthesized at 1100°C for 120 min contained unreacted γ-Al₂O₃. The ²⁷A1 MAS NMR spectra show that Al–N bonding in the product increases with increasing reaction temperature, the tetrahedral AlO₄ resonance decreasing prior to the disappearance of the octahedral AlO₆ resonance. This suggests that the tetrahedral AlO₄ sites of the γ-Al₂O₃ are preferentially nitrided than the AlO₆ sites. AlN nanoparticles were directly formed from γ-Al₂O₃ at low temperature because of this preferred nitridation of AlO₄ sites in the reactant. AlN nanoparticles are formed by gas reduction–nitridation of γ-Al₂O₃ not only because the reaction temperature is sufficiently low to restrict grain growth, but also because γ-Al₂O₃ contains both AlO₄ and AlO₆ sites, by contrast with α-Al₂O₃ which contains only AlO₆.     

Influence of Cr and Fe on Formation of α-Al2O3 from γ-Al2O3

The effect of Cr and Fe in solid solution in γ-Al₂O₃ on its rate of conversion to α-Al₂O₃ at 1100°C was studied by X-ray diffraction. The δ form of Al₂O₃ was the principal intermediate phase produced from both pure γ-Al₂O₃ and that containing Fe³⁺ in solid solution, although addition of Fe greatly reduced crystallinity. Reflectance spectra and magnetic susceptibilities showed that Cr exists as Cr⁶⁺ in γ-Al₂O₃ and as Cr³⁺ in α-Al₂O₃, with θ-Al₂O₃ as the intermediate phase. The intermediates formed rapidly, and the rates of their conversion to α-Al₂O₃ were increased by 2 and 5 wt% additions of Fe and decreased by 2 and 4 wt% additions of Cr. An approximately linear relation observed between α-Al₂O₃ formation and decrease in specific surface area was only slightly affected by the added ions. This relation can be explained by a mechanism in which the sintering of δ- or θ-Al₂O₃, within the aggregates of their crystallites, is closely coupled with conversion of cubic to hexagonal close packing of O^2- ions by synchro-shear.