Effect of surfactants on preparation of nanoscale α-Al2O3 powders by oil-in-water microemulsion

The nanoscale α-Al₂O₃ powders have been synthesized by the O/W microemulsion system using cyclohexane as the oil phase, ultrapure water as the water phase, OP-10 and alcohol as the surfactant and co-surfactant. It has been found that the nature of surfactants played an important role to regulate the size and morphologies of the α-Al₂O₃ nanoparticles. Three different nonionic surfactants (OP-10, Triton X-100 and Tween-80) have been used for the preparation of microemulsions. The microemulsions were characterized by Zeta Potential Analyzer. The results showed that the OP-10 system possesses wide and stable microemulsion phase regions. The synthesized α- Al₂O₃ powders have been comprehensively characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD). The powders calcined at 900–1150 °C showed the presence of alumina phase with crystal structure, The SEM images showed that the powders was an average diameter of 30–100 nm.     

The effect of a Cr2O3-addition on the phase transformation and catalytic properties of γ-Al2O3 in treatment of lean-burn exhausts

The solid-state thermal behaviour of -Al₂O₃ doped with 10 mol% Cr (oxide) was studied with respect to phase-transition behaviour and the co-ordination of the dopant Cr cations. A series of transformations: -Al₂O₃ -Al₂O₃ -Al₂O₃ -Al₂O₃ was observed for Cr₂O₃-doped alumina samples between 500 °C and 1100 °C. Rapid grain growth occurred at temperatures close to 1100 °C. The electron spin resonance (ESR) spectra for the sample heat-treated at 500 °C corresponded to the resonance of -Cr³⁺ in an amorphous Cr oxide impregnated onto the - and -alumina support. The change of ESR spectrum indicated the existence of Cr³⁺, suggesting the formation of a solid solution with the same structures as -Al₂O₃ and/or -Al₂O₃ at 800–1000 °C. The evaluation of catalytic activities for model exhaust was performed under lean-burn (an excess of oxygen) condition of air/fuel ratio A/F = 18 and space velocity SV = 100 000 h^–1 . The modified Al₂O₃ catalyst heat-treated at 1000 °C in air showed removal conversion of 100% for hydrocarbon (C₃H₆), 92% for CO and 5% for NO at 550 °C. Present results suggest that Cr-modification to Al₂O₃ leads to catalytic improvement with good thermal durability.     

Adsorption of Mo on γ-Al2O3 Samples of Various Structures

Adsorption of Mo on -Al₂O₃ samples of various structures was studied with the aim of using these sorbents in production of chromatographic 99mTc generators. The sorption capacities of the oxides for Mo were determined. The optimal concentrations of HCl and HNO₃ for converting the sorbent to the H⁺ form were found. In all the cases, treatment with HCl ensures higher sorption capacity than treatment with HNO₃. The sorption capacity of aluminum oxides prepared by ac electrochemical synthesis was studied in relation to pH of sodium polymolybdate solution and sorption time. The sorption capacity of these samples for Mo is several times higher than that of the chromatographic oxide traditionally used in the generator technology.     

Effect of mechanical activation on the synthesis of α-Fe2O3-Cr2O3 solid solutions

Continuous -Fe₂O₃-Cr₂O₃ solid solution series have been synthesized by two methods: (i) direct heating of coprecipitated hydroxides, and (ii) mechanical pre-treatment followed by heating. It is shown that mechanical treatment leads to a decrease in the preparation temperature of the solid solutions to 623 K. The formation of a continuous solid solution series by direct heating begins only at 773 K. The formation of the solid solutions was established by X-ray diffraction analysis, infrared and Mössbauer spectroscopy. The decrease in synthesis temperature of the -Fe₂O₃-Cr₂O₃ solid solutions is attributed to activation of the samples during their mechanical treatment. The samples obtained have large specific surface areas (up to 130 m² g^–1).     

Sintering of pseudo-boehmite and γ-Al2O3

Sintering of pseudo-boehmite, acicular-Al₂O₃ produced by dehydration of pseudo-boehmite, and-Al₂O₃ ex alum was investigated. The sintering process was studied by X-ray diffraction, transmission electron microscopy with selected area electron diffraction and BET surface area measurements. The solid state reaction to-Al₂O₃ causes a steep drop of the surface area to less than 10 m²g^–1. The acicular pseudo-boehmite and-Al₂O₃ supports exhibit an intermediate state where the acicular particles assume a rod-like shape and the surface area falls from about 300 to 100 m²g^–1. It was established that reaction to -Al₂O₃ and, hence, sintering proceeds via a nucleation and growth mechanism. The rate-limiting step is nucleation of -Al₂O₃. Consequently, the contacts between the elementary alumina particles dominate the sinter process. The contact between the acicular elementary particles of pseudoboehmite and-Al₂O₃ studied leads to the reaction to -Al₂O₃ to be almost complete after keeping samples for 145 h at 1050 °C. Decomposition of alum produces very small particles showing negligible mutual contacts. Consequently an elevated thermal stability is exhibited. Treatment of the alumina ex alum with water and drying results in a xerogel in which contact between elementary particles is much more intimate. Accordingly, treatment at 1050 °C causes a sharp drop in surface area.     

Microwave sintering of nanocrystalline γ-Al2O3

The densification and phase transformation behavior of gas condensation synthesized nanocrystalline γ-A1₂O₃ sintered with microwave radiation has been studied. The polymorphic nucleation and growth phase transformations which occurred as the material was heated through the temperature range of 800–1300°C present significant obstacles in the achievement of specimens which possess high bulk densities. These phase transformations are accompanied by a change in particle morphology, crystallite size, and surface area. Alumina derived from a chemically synthesized boehmite precursor has been shown to exhibit the same nucleation and growth phase transformation behavior when conventionally heated. It is concluded that nanocrystalline γ or δ alumina will not be a viable starting material for the production of dense bodies with grain sizes of less than 100 nm.     

Structural behavior of laser-irradiated γ-Fe2O3 nanocrystals dispersed in porous silica matrix : γ-Fe2O3 to α-Fe2O3 phase transition and formation of ε-Fe2O3

The effects of laser irradiation on γ-Fe₂O₃ 4 ± 1 nm diameter maghemite nanocrystals synthesized by co-precipitation and dispersed into an amorphous silica matrix by sol-gel methods have been investigated as function of iron oxide mass fraction. The structural properties of γ-Fe₂O₃ phase were carefully examined by X-ray diffraction and transmission electron microscopy. It has been shown that γ-Fe₂O₃ nanocrystals are isolated from each other and uniformly dispersed in silica matrix. The phase stability of maghemite nanocrystals was examined in situ under laser irradiation by Raman spectroscopy and compared with that resulting from heat treatment by X-ray diffraction. It was concluded that ε-Fe₂O₃ is an intermediate phase between γ-Fe₂O₃ and α-Fe₂O₃ and a series of distinct Raman vibrational bands were identified with the ε-Fe₂O₃ phase. The structural transformation of γ-Fe₂O₃ into α-Fe₂O₃ occurs either directly or via ε-Fe₂O₃, depending on the rate of nanocrystal agglomeration, the concentration of iron oxide in the nanocomposite and the properties of silica matrix. A phase diagram is established as a function of laser power density and concentration.     

Effect of the starting Al2O3 and of the method of preparation on the characteristics of Li-stabilized β″-Al2O3 ceramics

The influence of the morphology and the size of the particles of various types of starting Al₂ O₃ material on the synthesis and characteristics of Li-stabilized'-Al₂O₃ ceramics have been investigated. The use of highly dispersed oxides makes it possible to attain higher densities in the fired ceramic bodies due to their higher reactivity. In the case of oxides obtained from ammonium alum, the degree of dispersion and the reactivity may be increased by raising the amount of-Al₂O₃ up to a certain limit. Alumina prepared from Al₂ (OH)₅ NO₃ by slurry solution spray-drying also gives satisfactory results despite its lower degree of dispersion. This is connected with the morphology of the particles. In the case of synthesized materials containing an insufficient amount ofAl₂O₃-NaAlO₂ eutectic, high densities may also be achieved by applying a two-step firing schedule at temperatures above the melting point of the eutectic.     

Study on the effects of Cr2O3 on the reduction behavior of γ-Fe2O3

XRD and TG reduction analysis show that -Fe₂O₃ Fe-Cr catalysts, which contains 0.0 to 14.0 wt %. Cr₂O₃ and prepared by coprecipitating method, consist of crystalline -Fe₂O₃ and non-crystalline Fe₂O₃. Between 150–450 °C, three reduction stages are observed in the catalyst. The first stage is non-crystalline Fe₂O₃ reduced to non-crystalline Fe₃O₄, the second is crystalline -Fe₂O₃ to crystalline Fe₃O₄ and the third is non-crystalline Fe₂O₃ reduced to non-crystalline FeO. About 5 wt %. Cr₂O₃ can enter the lattices of -Fe₂O₃ to form solid solution. With the increasing of Cr₂O₃ content, the relative abundance of non-crystalline Fe₂O₃ and the amount of soluble Cr₂O₃ in non-crystalline increases, while the crystalline size of -Fe₂O₃ decreases.     

Effect of zinc on the magnetic properties of acicular cobalt-modified γ-Fe2O3 particles

Acicular -FeOOH particles with a particle length of about 0.35 m and an axial ratio of about 7 were synthesized by the coprecipitation method using the reaction of FeCl₂-NaOH. The (Co, Zn)-modified -F₂O₃ particles were produced by absorbing Co²⁺ and Zn²⁺ ions on the surfaces of -FeOOH particles followed by dehydration, reduction and oxidation. The saturation magnetization and thermal stability of the coercivity of (Co, Zn)--Fe₂O₃ particles were all higher than those of Co--F₂O₃ particles. For the same (Co+Zn) content, the saturation magnetization of (Co, Zn)--Fe₂O₃ particles increased with increasing zinc content but the coercivity decreased.     

Heteroepitaxial growth of the δ-Ta2O5 films on α-Al2O3 (0001)

Journal of Materials Science: Materials in Electronics - The heteroepitaxial δ-Ta2O5 films were deposited on α-Al2O3 (0001) by MOCVD. As the growth temperature increases from...     

Auto ignition processing of nanocrystalline α-Al2O3

This paper reports the results of an investigation aiming at synthesizing nanocrystalline α-alumina using the “Auto Ignition” processing technique. The process utilizes in-situ exothermic reactions occurring between a suitable oxidizer (e.g., aluminum nitrate) and a fuel (urea). Due to generation of high temperatures for a short time, the resulting crystalline phase(s) is(are) able to nucleate but cannot grow. This work systematically examines and characterizes the resulting powders obtained from various ratios of the oxidizer and fuel. Both X-ray diffraction and transmission electron microscopy results confirm that under certain conditions it is possible to obtain pure α-alumina in nanocrystalline size. It is expected that sinterability of such powders will be better because the question of γ to α phase transformation does not arise.     

Electron emission measurements and the defect structure of α-Al2O3

In this article, electron emission is used to study the defect structure of alumina. The need of a direct measurement of the position of the Fermi level (or the electron concentration in the conduction band) is shown by discussing the actual electrical data on alumina. The emission has been measured over a large temperature range (1400 to 2400 K) and the emission of a technical polycrystalline alumina is reported up to the melting temperature under a controlled oxygen partial pressure. Additional results are reported for titanium- and iron-doped polycrystalline aluminas. The results are discussed from two points of view. First the quantitative data concerning the work function are taken into account and the contribution of the surface layer is discussed. Secondly, the dependency of the electron emission on the oxygen partial pressure is explained by the defect chemistry of the oxide. The absence of variation of the electron concentration in a certain range of is due to a self compensation between donor and acceptor impurities.     

Application of lanthanum to pseudo-boehmite and γ-Al2O3

Elements such as lanthanum, zirconium, thorium and cerium are known to raise the thermostability of -Al₂O₃. Among these elements lanthanum is the most effective. Three procedures have been used to apply lanthanum homogeneously to the surface of pseudo-boehmite and -Al₂O₃, namely, incipient wetness impregnation, deposition-precipitation from a homogeneous solution, and specific adsorption of a La(EDTA)-complex. The latter procedure results in supports covered homogeneously with lanthanum. The minimum amount of lanthanum required to render the catalyst supports thermostable, depends on the procedure to apply the lanthanum. The specific adsorption procedure offers the possibility to obtain thermostable supports with loadings as low as 0.5 wt % La.High-resolution electron microscopy was used to assess the mechanism of the stabilization by application of foreign metal ions in amounts less than a monolayer. It appeared that addition of foreign metal ions brings about crystallization of the amorphous surface layer of -Al₂O₃.     

Wetting and adhesion of Ni-Al alloys on α-Al2O3 single crystals

The effect of Al additions on the wetting and adhesion of Ni on an -Al₂O₃ single crystal was studied. Contact angles were measured by the sessile drop technique under vacuum or in He atmosphere. The morphological and chemical features of metal-vapour and metal-oxide interfaces were determined by scanning electron microscope (SEM), microprobe analysis and profilometry. The work of adhesion of Ni-Al alloys on Al₂O₃ substrates was significantly higher than for pure Ni and Al components. This result was explained by co-operative adsorption of aluminium and oxygen atoms at the Ni-Al₂O₃ interface. The influence of oxidation of the alloy on wetting and bonding is also discussed.     

Effect of Al2O3 and Fe3Al on the phase formation and mechanical properties of β-sialon

The paper evaluated the mechanical properties of β-sialon composites prepared by hot-pressing sintering at 1600 °C in N₂ atmosphere using α-Si₃N₄, Al₂O₃, Y₂O₃ and Fe₃Al as raw materials. The influence of Al₂O₃ and Fe₃Al content on flexure strength, fracture toughness, hardness, and relative density was investigated. And phase formation and morphology of the composites were characterized by X-ray diffraction and electron microscopy. The experimental results indicate that the raw material Fe₃Al reacts with α-Si₃N₄ to form silicides at elevated temperature, and supplies more liquid phase to assist densification. Besides, the variation of flexure strength, fracture toughness and hardness is mainly consistent, and also in good agreement with the relative density measurements. The values all increase firstly, and then decrease when the Al₂O₃ content increases. Scanning electron microscopy illustrates that the metal particles act to inhibit the crack propagation.     

Fine Structure of α-Al2O3-Based Solid Solutions

The fine structure of solid solutions between isomorphic p-block and 3dtransition-metal oxides was investigated experimentally and theoretically in the technologically important systems -Al₂O₃–M₂O₃(M = Ti, V, Cr, Fe). It was shown that the thermodynamic theory of isomorphic miscibility and conventional approaches to estimating solubility limits fail to explain and, at best, only predict the phase relations in the systems examined. The proposed magnetochemical method offers the possibility of studying in detail the M–M interactions in first-row oxides M₂O₃and (M _x Al_{1 – x} )₂O₃solid solutions, probing their homogeneity, and revealing their salient structural features. The marked difference in the solubilities of isomorphic M₂O₃oxides in -Al₂O₃is interpreted in terms of the magnetic structure of the oxides.     

Chemiomechanical effects on crack propagation: Polycrystalline α-Al2O3

The role of environment on microcracks in Al₂O₃ is important as the material has applications in the biomedical field, such as artificial bones. The indentation method of producing radial cracks and measuring hardness is good in that it is reproducible. The radial crack lengths in -Al₂O₃ are found to be sensitive towards the environment, and the load applied on the indentor. Long-time Vickers' microhardness and radial crack-length measurements in the presence of bio-simulating, aqueous and non-aqueous environments on the surface of -Al₂O₃ are analysed. In each environment considered, the crack propagation showed a linear variation with the applied load, and the microhardness decreased similarly with time. Evidence was sought to show that the presence of water in bio-simulating liquid helps crack propagation under load. The results are explained in terms of possible interactions between the molecules of Al₂O₃ and the chemical environment and surface energies in the presence of that environment.     

Energetics of formation of lamellar porous microstructures in γ-Al2O3

The formation of a lamellar porous microstructure in -Al₂O₃ during the dehydration of boehmite (-AlOOH) has been studied as a function of the temperature and water vapour pressure conditions under which the dehydration was performed. The -Al₂O₃ samples were characterized by transmission electron microscopy and nitrogen adsorption. A marked dependence of the scale of the microstructure, and of the adsorption characteristics, on water vapour pressure was observed. This pressure dependence of scale and surface area has been discussed in terms of a model, similar to those derived for cellular growth of lamellar microstructures during eutectic and eutectoid reactions, in which the dehydration mechanism is diffusion controlled. The inter-lamellar spacing is determined by the energy available for the formation of a new surface, and an estimate of surface energy for -Al₂O₃ has been made. The partial pressure, , dependence observed has important implications for the possibility of tailoring pore structures.