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.     

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.     

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).     

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.     

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.     

Fabrication and mechanical properties of α-Al2O3/β-Al2O3/Al/Si composites by liquid displacement reaction

Al₂O₃/metal composites were fabricated by heating three kinds of commercial mullite refractories in contact with Al, and their mechanical properties were investigated. Aluminum reacted with the mullite and SiO₂-glass constituting the mullite refractories and changed them into -Al₂O₃ and Si. Simultaneously, -Al₂O₃ was formed by the reactions among -Al₂O₃ and sodium and potassium oxides in the glassy phase. Also, Al penetrated into the -Al₂O₃/-Al₂O₃/Si composite by partly dissolving Si. Finally, the mullite refractories were changed into -Al₂O₃/-Al₂O₃/Al/Si composites. The phase contents, microstructures and mechanical properties of the resulting composites varied with the composition of the refractories. The content of -Al₂O₃ in the composite was lowest at the lowest Na₂O and K₂O contents in the refractories. Silicon in the composite had its highest content at the highest SiO₂ content. The composite fabricated from SiO₂/Al₂O₃ (in mol) (SAR)=1.85 consisted of 2–5 m Al₂O₃ grains embedded in metal, but that from SAR=1.05 showed a complicated microstructure with small and large grains. The bending strength of the composites fabricated from the refractories of SAR=1.85, 1.24, and 1.05 were 327, 405 and 421 MPa, respectively. Also, the corresponding fracture toughness values were 5.2, 6.1, and 5.5 MPam , respectively.     

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...     

Changing the surface mechanical properties of silicon and α-Al2O3 by ion implantation

Microhardness indentation testing has been used as a means of introducing controlled localized deformation and fracture in both ion-implanted and unimplanted {1 1 1} silicon and {1 0 ˉ1 2} sapphire single crystal surfaces. The microstructural alterations due to implantation with N ₂ ⁺ and Al⁺ into silicon and Y⁺ into sapphire have been characterized using channelled Rutherford backscattering, transmission electron microscopy and electron channelling in the scanning electron microscope. It was found that sapphire only became amorphous at doses ⪞3×10¹⁶ Y⁺cm⁻² which corresponds to a total energy deposition of ∼3×10²³ keV cm⁻³ (∼44 kJ mm⁻³). The low-load microhardness (<50 gf) was found to be sensitive to the thickness of the amorphous layer produced by implantation into both silicon and sapphire. Compared with the parent crystal, this layer was found both to be softer and to behave in a relatively plastic manner with considerable plastic pile-up occurring around indentations in the higher dose specimens. The indentation fracture behaviour was found to be dominated by the presence of implantation-induced compressive stresses. The resulting effects were: (a) a decrease in the size of the radial crack traces (henceK _IC is apparently increased when evaluated using indentation fracture mechanics), (b) a decrease in the frequency of occurrence of lateral break-out in silicon and subsurface lateral cracking in sapphire, (c) initiation of lateral cracks further below the surface in both silicon and sapphire. Thus in general, it is concluded that hardness and surface plasticity are associated with softer amorphous layers whilst indentation fracture modifications are principally stress related.     

High-purity disperse α-Al2O3 nanoparticles synthesized by high-energy ball milling

The preparation of disperse fine equiaxed α-Al₂O₃ nanoparticles with narrow size distribution, high purity, and high yield is essential for producing Al₂O₃ nanocrystalline ceramic of fine grains which may exhibit a good toughness. In this work, micron-sized α-Al₂O₃ particles were directly ball-milled and subsequently washed with hydrochloric acid at room temperature. Fracture of large α-Al₂O₃ particles and cold welding of fine α-Al₂O₃ nanoparticles occur simultaneously during ball milling. It leads to the reduction of particle size with increasing milling duration below 80?h and reaches to a dynamic equilibrium with a minimal average particle size of 6.4?nm for milling durations over 80?h. Using the optimized high-energy ball milling parameters, we prepared high-purity disperse equiaxed α-Al₂O₃ nanoparticles with an average particle size of 8?nm and a purity of 99.96% (mass percent) in a high yield. After fractionated coagulation separations, disperse fine equiaxed α-Al₂O₃ nanoparticles with narrow size distribution were obtained. Finally, Al₂O₃ nanocrystalline ceramic with a relative density of 99.8% and an average grain size of 34?nm was sintered from the disperse fine equiaxed α-Al₂O₃ nanoparticles with an average particle size of 4.8?nm and a size distribution of 2–10?nm by pressureless two-step sintering.     

Formation of mixed TiC/Al2O3 layers and αa- and κ-Al2O3 on cemented carbides by chemical vapour deposition

Al₂O₃ and Ti(C, O) were codeposited as a mixed chemical vapour deposition (CVD) layer from AlCl₃-TiCl₄-CH₄-CO₂-H₂ gas mixtures on cemented carbides and pure alumina substrates. A thermodynamical approach of this CVD system is presented. The coatings were described by SEM and X-ray diffraction analysis. They consist of large facetted-Al₂O₃ crystals containing some titanium and surrounded by a fine grained Ti(C, O) matrix. Carbon diffusing from the cemented carbide substrate can considerably influence the morphology and the composition of the mixed coating.Methane in a AlCl₃-CO₂-H₂ environment stabilizes the-Al₂O₃ phase which can be deposited as a compact layer without whisker formation on a WC-Co substrate even without a TiC underlayer.     

Self-diffusion in α-Al2O3 and growth rate of alumina scales formed by oxidation: effect of Y2O3 doping

In many cases, alumina scales are assumed to grow predominantly by oxygen diffusion, but some authors have found that the growth can be controlled by aluminium diffusion. These mechanisms can be modified by active elements. The problem with alumina is that there is a lack of data about self-diffusion coefficients, and, due to the stoichiometry of alumina, diffusion data correspond to an extrinsic diffusion mechanism so that it is not possible to compare oxygen and aluminium diffusion coefficients. In order to obtain information about the alumina scale growth mechanism, oxygen (¹⁸O) and aluminium (²⁶Al) self-diffusion coefficients in Al₂O₃ were determined in the same materials and in the same experimental conditions, thus allowing a direct comparison. For both isotopes, bulk and sub-boundary diffusion coefficients were determined in single crystals of undoped alumina. Grain-boundary diffusion coefficients have been computed only for oxygen diffusion in polycrystals. Oxygen diffusion has been also studied for yttria-doped -alumina in the lattice, sub-boundaries and grain boundaries. Oxygen and aluminium bulk diffusion coefficients are of the same order of magnitude. In the sub-boundaries, aluminium diffusion is slightly faster than oxygen diffusion. Yttria doping induces a slight increase of the oxygen bulk diffusion, but decreases the grain-boundary diffusion coefficients on account of segregation phenomena. These results are compared with the oxidation constants of alumina former alloys (alloys which develop an alumina scale by oxidation). It appears that neither lattice self-diffusion nor grain boundary self-diffusion can explain the growth rate of alumina scales. Such a situation is compared to the case of Cr₂O₃.     

Sol-gel preparation and thermal stability of Pd/γ-Al2O3 catalysts

A series of Pd/-Al₂O₃ catalysts with a designed loading of 1 wt % were prepared by the sol-gel method using boehmite (AlOOH) sols and various palladium compounds. These materials were developed for later use as inorganic membranes. The samples were calcined at 400 °C in flowing O₂ to decompose the Pd complex and change the phase of the support to -Al₂O₃. H₂ pulse chemisorption was used to determine the average Pd particle sizes, while particle size distributions were obtained from TEM micrographs. XRD was used to determine the crystallinity of the -Al₂O₃ support after extended treatments at 650 °C in O₂ and H₂ atmospheres. The physical properties of the support were studied using nitrogen adsorption-desorption at 77 K. Initial BET surface areas were about 350 m²/g. Pore size distributions were very narrow and centered at 3.6 nm, but broadened and became bimodal during the 650 °C treatments. An ion-exchanged sample prepared by traditional methods was used as a basis for comparison to the sol-gel samples.     

Improvement of structural and mechanical properties of alumina coatings by incorporation of TiO2 and α-Al2O3 nanoadditives

Abstract

Alumina coatings embedded with different nanoadditives were fabricated on aluminium alloy by microarc oxidation (MAO). Incorporation of nanograins into the prepared coatings was accomplished by dispersing nanoadditives into different electrolytes during the MAO process. Our results show that nanograins are successfully embedded in the ceramic coatings, and the embedded coatings are compact and have lower porosity. The mechanical properties of the nanograin embedded coatings such as hardness, adhesion and wear resistance are consequently improved, and the samples prepared in aluminate electrolyte with α-Al₂O₃ nanoadditive have better mechanical properties than those prepared in other electrolytes. Our results also show that the mechanical properties of MAO coatings are closely related to the surface structure. The introduction mechanism of nanograins into the ceramic coatings resulted from the reactions occurring in the microarc discharge channels such as diffusion and electrophoresis, which is believed to improve the structure of the prepared coatings.     

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.     

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.     

Effects of chelation reactions between metal alkoxide and acetylacetone on the preparation of MgAl2O4 powders by sol–gel process

Magnesium aluminate spinel (MgAl₂O₄) were synthesized via the sol–gel method, using magnesium aluminum double n-butoxide as precursors. The n-butoxide, prepared by the direct reaction of stoichiometric magnesium, aluminum and adequate n-butanol, was modified by acetylacetone as the chelating agent to control the speed of hydrolysis. Gel samples were identified as the mixture of Mg₂Al(OH)₇ and MgAl₂(OH)₈ phases, which converted to pure MgAl₂O₄ spinel phase at 800 °C. Then dense spinel ceramics with 96% of relative density were prepared by sintering in air at 1600 °C. Besides, the morphology of the powders and sintered ceramics has been investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM).     

Effect of glycerol on the preparation of phosphogypsum-based CaSO4·0.5H2O whiskers

Phosphogypsum-based CaSO₄·0.5H₂O whiskers were successfully prepared in the presence of glycerol by hydrothermal method. The action mechanism of glycerol on the morphology of CaSO₄·0.5H₂O whiskers was investigated and discussed in detail. The as-prepared products were characterized by X-ray powder diffraction spectrometer, scanning electron microscopy, Fourier transform infrared spectrometer, and thermal analyzer. The main phase of the as-prepared products was CaSO₄·0.5H₂O, and the morphology was uniform fiber. Moreover, the aspect ratio and productivity of the as-prepared CaSO₄·0.5H₂O whiskers increased with the concentration of glycerol. Even whiskers with mean length of 38–83 μm, mean width of 1.33–0.73 μm, aspect ratio of 29–118, and productivity of 89–96 % were obtained under the condition of glycerol/water volume ratio of 10–100 %. As far as mechanism was concerned, the hydroxyl groups of glycerol adsorbed and occupied the vacancies of Ca sites on (001) plane of CaSO₄·0.5H₂O whiskers, and however, on (100) plane, hydroxyl groups of glycerol adsorbed mainly on Ca sites and formed hydrogen bond layered structure on both sides of that, which should be the reason that CaSO₄·0.5H₂O whiskers grew along the c-axis observed in this study.