The influence of the activity of water on the phase composition of aluminum hydroxides obtained by reaction of amalgamated aluminum with water has been studied. The reaction was carried out in solutions of water and dioxane, and in sodium chloride solutions of different concentrations; amorphous aluminum hydroxide was precipitated initially. The aging of this primary product to bayerite, pseudoboehmite, or an amorphous gel was controlled by the water content of the system. The pseudoboehmite has shown a significant reactivity when used as a source of alumina in the hydrothermal syntheses of kaolinite at relatively low temperature and pressure.

Alumina is produced from bauxite, which contains a mixture of various oxides, such as aluminum (Al), iron (Fe), silicon (Si), and titanium (Ti). Bauxite can also be considered a source of several other valuable metals, such as scandium (Sc), vanadium (V), and gallium (Ga). The composition and mineralogy of alumina determine their economic value, but their characteristics vary by locality. The physicochemical characteristics of bauxites can also be influenced largely by weathering processes, even within the same locality. For this reason, the present study was undertaken with the objective of comparing the characterization data of three bauxite samples collected, which will be referred to as D, E, and F, from the Cruz Alta do Pará plateau in northern Brazil. The samples were solubilized by multi-acid digestion and fusion with lithium metaborate to quantify their metal compositions by inductively coupled plasma optical emission spectrometry (ICP-OES). The mineralogical characterization was conducted by X-ray diffraction (XRD), and the phase changes of minerals in bauxite were detected by thermogravimetric analysis (TGA/DTG). The total organic carbon (TOC) technique was used to quantify the C in the samples, and the moisture content was also measured. Alumina was 30 wt.% on average for all samples, good for producing high-purity alumina by hydrometallurgical processes. The results, however, showed high (~20 at.%) silica concentrations in two samples and ~3 wt.% Fe in one sample, which can pose a challenge in the Bayer process. The X-ray diffraction (XRD) analysis showed that gibbsite (Gbs), kaolinite (Kln), anatase (Ant), and hematite (Hem) were the major mineral phases in these samples. The study showed that the samples from the same mine vary in their metal content, especially with regard to Si, and they, thus, need to be processed selectively to maximize their economic value.

The nature of Cu2+ adsorption by boehmite, gibbsite, and noncrystalline alumina was studied over a range of equilibrium pH (4.5–7.5) and Cu2+ concentration (10−3–10−8 M) by electron spin resonance (ESR). Available chemisorption sites at pH 4.5 were the most numerous for noncrystalline alumina (~1 mmole/100 g), less for boehmite, and least for gibbsite as indicated by the relative strength of the rigid-limit ESR signal attributed to Cu2+ adsorbed at discrete sites. The chemisorption process involved immobilization of Cu2+ by displacement of one or more H2O ligands by hydroxyl or surface oxygen ions, with the formation of at least one Cu-O-Al bond. As the pH was raised from 4.5 to 6.0, essentially all of the solution Cu2+ appeared to be adsorbed by the solids. However, the noncrystalline alumina and boehmite chemisorbed much of the total adsorbed Cu2+ (10 mmole/100 g), whereas precipitation or nucleation of Cu(OH)2 in the gibbsite system was indicated. Precipitated Cu2+ was more readily redissolved by exposure to NH3 vapor than chemisorbed Cu2+.

The sorption of 8-hydroxyquinoline onto some clays and oxides was studied as a function of concentration, pH, and time. The sorption reaction reached equilibrium in about 5 hr, was irreversible, and reached a maximum at pH 5. The decrease in sorption on both sides of the maximum was attributed to electrostatic interactions of charged molecules with similarly charged surfaces and/or neighbors as well as solvent and proton competition. X-ray powder diffraction of dried clays showed that a one-layer complex formed in which the molecules lay flat between the clay interlayers. The sorption onto the clays included physical and exchange sorption and was accompanied by exchangeable cation hydrolysis. At high surface coverage the silicate structure deteriorated in the direction of the Z-axis due to chemical aggressiveness of the reagent.

Cu-doped alumina-pillared montmorillonite samples have been prepared and characterized with X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), electron spin resonance (ESR) and inductive coupled plasma atomic emission spectroscopy (ICP AES) techniques. The results show that the catalysts are porous materials with copper species located in the interlayer, present either as isolated Cu2+ ions anchored at alumina pillars or as patches of amorphous CuO. Catalytic tests with hydroxylation of phenol show that the clay samples possess significant activity for dihydroxybenzene (DHB) formation, comparable with the reference TS-1 catalyst. Experiments with changing the substrate dosing indicate that adsorption and activation of phenol molecules is a necessary condition for the reaction to occur.

In the production of alumina (Al2O3) from clays by acid leaching, thermal activation by calcination is applied widely as a pre-treatment to improve the dissolution of aluminum. Previous studies have focused only on the thermal behavior of high-purity kaolinite and pyrophyllite, individually. However, thermal activation of complex clay ores containing several silicate minerals and their effect on aluminum extraction have not been studied. The purpose of the present study was to characterize the thermal behavior of a clay ore containing mainly pyrophyllite (Al2Si4O10(OH)2), kaolinite (Al2Si2O5(OH)4), muscovite (KAl2(AlSi3O10)(OH)2), quartz (SiO2), and kyanite (Al2SiO5) from the Pütürge clay deposits (Malatya, Turkey) for possible use in alumina (Al2O3) production by acid leaching. The ore and its calcination products obtained at various temperatures were characterized with respect to their mineral paragenesis, crystal structure, surface morphology, and thermal, calorimetric, and aluminum dissolution properties in order to understand the changes that occurred. Aluminum recovery in the leach solution increased in direct proportion to the dehydroxylation degree (Dtg) of the ore as the calcination temperature increased to 900°C. A maximum aluminum extraction of 90.57% was achieved by leaching of the product from calcination at 900°C. Aluminum extraction decreased sharply above that temperature, even though Dtg increased a little. By increasing the calcination temperature; the structures of pyrophyllite, kaolinite, and muscovite were destroyed by dehydroxylation, resulting in the exfoliation of the mineral layers, thus, a mixture of dehydroxylated phases formed. Depending mainly on the temperature range at which each of the dehydroxylated phases is durable, aluminum could be leached to some extent. The sharp decrease in the extraction of aluminum, iron, and potassium at higher temperatures was attributed to compaction of previously exfoliated layers of the minerals through re-crystallization to form mullite-like structures which seemed insensitive to acid attack during the leaching.

High-performance mullite-based composite ceramics were prepared successfully using natural kaolin and alumina as raw materials and ZrO2 as an additive. The influence of sintering temperature and ZrO2 content on the sintering behaviour and mechanical properties of zirconia-toughened mullite ceramics was studied systematically. With increasing sintering temperature from 1450°C to 1560°C, the primary phases of as-sintered composite ceramics were mullite and corundum with a small amount of ZrO2, and the bulk density of the composite ceramics increased from 2.29 to 2.72 g cm–3. Furthermore, the ZrO2 phase transition promoted transgranular fracture, and ZrO2 grains were pinned at the grain boundaries, thereby enhancing the mechanical strength of the composite ceramics. Moreover, the AZS12 sample, with 12 wt.% ZrO2 and sintered at 1560°C, had the greatest flexural strength and fracture toughness of 91.6 MPa and 2.47 MPa m–1/2, respectively. Adding ZrO2 to the composite ceramics increased their flexural strength by ~37.6%.

The scope of this work includes detailed microstructural investigations using high-resolution scanning transmission electron microscopy of the oxide scales and deposits formed on nozzle ring vanes made of IN713LC alloy during the operation of an auxiliary power unit under real conditions. The paper presents the differences of oxidation processes between the suction and the pressure sides of the vanes. It has been shown that in the pressure side of the vane, where a greater flow of exhaust gases from the combustion chamber is present, the oxidation process is accompanied by a deposition of among others (i.a.) Fe, Mg, Ca, Na, and P, while on the suction side of the vane, the thermal stresses and the mechanical loading most likely lead to a cracking of the oxide scale. A segregation of Cr, Ni, Ta, and Ti to the boundaries of α-Al2O3 grains is observed, which may indicate their diffusion from a metal to a gas atmosphere.

Asymptotic Giant Branch (AGB) stars contribute a major part to the global dust budget in galaxies. Owing to their refractory nature alumina (stoichiometric formula AlO) is a promising candidate to be the first condensate emerging in the atmospheres of oxygen-rich AGB stars. Strong evidence for that is supplied by the presence of alumina in pristine meteorites and a broad spectral feature observed around ∼ 13 μm. The emergence of a specific condensate depends on the thermal stability of the solid, the gas density and its composition. The evaluation of the condensates is based on macroscopic bulk properties. The growth and size distribution of dust grains is commonly described by Classical Nucleation Theory (CNT). We question the applicability of CNT in an expanding circumstellar envelope as CNT presumes thermodynamic equilibrium and requires, in practise, seed nuclei on which material can condense. However, nano-sized molecular clusters differ significantly from bulk analogues. Quantum effects of the clusters lead to non-crystalline structures, whose characteristics (energy, geometry) differ substantially, compared to the bulk material. Hence, a kinetic quantum-chemical treatment involving various transition states describes dust nucleation most accurately. However, such a treatment is prohibitive for systems with more than 10 atoms. We discuss the viability of chemical-kinetic routes towards the formation of the monomer (Al2O3) and the dimer (Al4O6) of alumina.

We employed correlative atom probe tomography (APT) and transmission electron microscopy (TEM) to analyze the alumina scale thermally grown on the oxide dispersion-strengthened alloy MA956. Segregation of Ti and Y and associated variation in metal/oxygen stoichiometry at the grain boundaries and triple junctions of alumina were quantified and discussed with respect to the oxidation behavior of the alloy, in particular, to the formation of cation vacancies. Correlative TEM analysis was helpful to avoid building pragmatically well-looking but substantially incorrect APT reconstructions, which can result in erroneous quantification of segregating species, and highlights the need to consider ionic volumes and detection efficiency in the reconstruction routine. We also demonstrate a cost-efficient, robust, and easy-handling setup for correlative analysis based solely on commercially available components, which can be used with all conventional TEM tools without the need to modify the specimen holder assembly.

A promising candidate to initiate dust formation in oxygen-rich AGB stars is alumina (Al2O3) showing an emission feature around ∼13μm attributed to Al−O stretching and bending modes (Posch+99,Sloan+03). The counterpart to alumina in carbon-rich AGB atmospheres is the highly refractory silicon carbide (SiC) showing a characteristic feature around 11.3μm (Treffers74). Alumina and SiC grains are thought to represent the first condensates to emerge in AGB stellar atmospheres. We follow a bottom-up approach, starting with the smallest stoichiometric clusters (i.e. Al4O6, Si2C2), successively building up larger-sized clusters. We present new results of quantum-mechanical structure calculations of (Al2O3)n, n = 1−10 and (SiC)n clusters with n = 1−16, including potential energies, rotational constants, and structure-specific vibrational spectra. We demonstrate the energetic viability of homogeneous nucleation scenarios where monomers (Al2O3 and SiC) or dimers (Al4O6 and Si2C2) are successively added. We find significant differences between our quantum theory based results and nanoparticle properties derived from (classical) nucleation theory.

Nano-structured yttria stabilized zirconia (YSZ)/Al2O3 nano-composite coatings were prepared by electrophoretic deposition (EPD) in acetyl-acetone/ethanol solvents under the constant voltage of 40 V. High sintering temperature may damage the metal parts and also lead to high production costs. To overcome the disadvantages of high sintering temperatures, reaction bonding of Al was taken as the approach. It was found that a powder mixture of Al and YSZ can lower the sintering temperature. YSZ/Al green composites were deposited on the MCrAlY layer applied on Inconel alloy cathode. Iodine was added to the solutions as the stabilizing agent. According to differential thermal analysis (DTA) results, embedded Al particles oxidation started at 660 °C. Sintering process for YSZ/Al2O3 nano-composite coating occurred at 1150 °C for 4 h. A lower pinholes coating with the highest density due to the constraint of the substrate was obtained.

A novel type of alumina (Al2O3)-doped molybdenum (Mo) alloy sheet was prepared by a hydrothermal method and a subsequent powder metallurgy process. Then the characterization of α-Al2O3 was investigated using high-resolution transmission electron microscopy as the research focus. The tensile strength of the Al2O3-doped Mo sheet is 43–85% higher than that of the pure Mo sheet, a very obvious reinforcement effect. The sub-micron and nanometer-scale Al2O3 particles can increase the recrystallization temperature by hindering grain boundary migration and improve the tensile strength by effectively blocking the motion of the dislocations. The Al2O3 particles have a good bond with the Mo matrix and there exists an amorphous transition layer at the interface between Al2O3 particles and the Mo matrix in the as-rolled sheet. The sub-structure of α-Al2O3 is characterized by a number of nanograins in the $\left[ {2\bar{2}1} \right]$ direction. Lastly, a new computer-based method for indexing diffraction patterns of the hexagonal system is introduced, with 16 types of diffraction patterns of α-Al2O3 indexed.

Direct coagulation casting of alumina via controlled release of high valence counter ions using ammonium polyphosphate (APP) chelate complex as the coagulating agent was proposed. APP chelate complex suspension was prepared from APP and calcium chloride. Calcium used as high valence counter ions was chelated by APP. The average particle size of the chelate complex is 0.13 μm with a narrow particle size distribution which is close to the size of alumina particles. Glycerol diacetate was used to tailor the pH value of the suspension by hydrolysis which produces acetic acid. The lowering of the pH value helps to decompose the chelate complex, and enhance to release the calcium chelated. It is indicated that the viscosity of the suspensions with the addition of APP chelate complex suspension and glycerol diacetate increases to approximately 20 Pa s after heating at 40–70 °C for 1.5–5 h, which is high enough to coagulate the suspension. 55 vol% alumina suspension with a addition of 3 vol% APP chelate complex suspension and 5 vol% glycerol diacetate treated at 60 °C could coagulate completely within 2 h with a compressive strength of 2.1 MPa. Dense alumina with a relative density of above 97% and a flexural strength of 388 ± 23 MPa can be prepared by this method from 55 vol% alumina suspensions without a burnout process.

The mechanisms of densification and creep were examined during spark plasma sintering (SPS) of alumina doped with a low and high level of zirconia or yttria, over a temperature range of 1173–1573 K and stresses between 25 and 100 MPa. Large additions of yttria led clearly to in situ reactions during SPS and the formation of a yttrium-aluminum garnet phase. Dopants generally lead to a reduction in the densification rate, with substantial reductions noted in samples with ∼5.5 vol% second phase. In contrast to a stress exponent of n ∼ 1 for pure alumina, the doped aluminas displayed n ∼ 2 corresponding to an interface-controlled diffusion process. The higher activation energies in the composites are consistent with previous data on creep and changes in the interfacial energies. The results reveal a compensation effect, such that an increase in the activation energy is accompanied by a corresponding increase in the pre-exponential term for diffusion.

Microstructural features of nanostructured copper-matrix composites produced via high-energy milling were studied. Copper–graphite–alumina batches were planetary ball milled up to 16 h; copper–graphite batches were also prepared under the same conditions to evaluate the effect of contamination from the milling media. The microstructure of the produced materials was characterized by field emission gun scanning electron microscopy/energy-dispersive spectroscopy and related to Raman, X-ray diffraction, and particle size analysis results. Results showed that alumina was present in all milled powders. However, size reduction was effective at shorter times in the copper–graphite–alumina system. In both cases the produced powders were nanostructured, containing graphite and alumina nanoparticles homogeneously distributed in the copper matrix, especially for longer milling times and in the presence of added alumina. Copper crystallite size was significantly affected above 4 h milling; nanographite size decreased and incipient amorphization occurred. A minimum size of 15 nm was obtained for the copper crystallite copper–alumina–graphite composite powders, corresponding to 16 h of milling. Contamination from the media became more significant above 8 h. Results suggest that efficient dispersion and bonding of graphite and alumina nanoparticles in the copper matrix is achieved, envisioning high conductivity, high strength, and thermal stability.

A tomographic heating holder for transmission electron microscopy that can be used to study supported catalysts at temperatures of up to ~1,500°C is described. The specimen is placed in direct thermal contact with a tungsten filament that is oriented perpendicular to the axis of the holder without using a support film, allowing tomographic image acquisition at high specimen tilt angles with minimum optical shadowing. We use the holder to illustrate the evolution of the active phases of Pt nanoparticles on carbon black and PtPd nanoparticles on γ-alumina with temperature. Particle size distributions and changes in active surface area are quantified from tilt series of images acquired after subjecting the specimens to increasing temperatures. The porosity of the alumina support and the sintering mechanisms of the catalysts are shown to depend on distance from the heating filament.

In the last few decades, X-ray diffraction (XRD) systems have been paramount and irreplaceable in controlling bauxite exploration, as well as Bayer and reduction processes. XRD quantitative phase analysis in the aluminum industry witnessed a steady deployment of the Rietveld method, which at present progressively replaces existing methodologies in research and plant laboratories. Rietveld analysis not only helped to surpass traditional XRD calibration methods, it also opened the door for new applications previously not possible. The use of the Rietveld method to characterize selected materials unique to the aluminum industry, such as bauxite, red mud, and alumina is demonstrated and discussed. This paper also presents how synchrotron-based diffractograms obtained for bauxite and red mud samples allowed a much better understanding of mineralogical representation, and made it possible to leverage their Rietveld quantification. Despite clear advantages, the Rietveld method also has limitations that are revealed. For alumina phase quantification, a dedicated Rietveld analytical program was built with structure data for eight alumina mineralogical phases: alpha, beta (β-Al2O3 = Na2O•11Al2O3), delta, gamma (2), kappa, sigma, and theta. The paper gives unique examples of phase quantification in aluminas of various origins and phase composition.

This paper deals with the implementation of a theoretically described method to determine residual stresses in real space directly by means of small gauge volumes. For this purpose, beam limiting masks were designed, manufactured, and investigated in first experiments. Image series taken with a position sensitive CCD camera demonstrate the ability to detect interferences from gauge volumes beneath the sample surface by defined slit geometries. The experiments show that due to the highly absorbing masks the amount of detectable photons is poor, and thus long exposure times are necessary to receive suitable data. For increasing measurement depths (altering masks) a decrease in the intensity can be detected which leads to the assumption that the diffracted photons originate from deeper regions in the material. A model was developed to simulate the diffraction conditions with different mask layouts and material properties. Modeling yields consistent results with experimental data, and thus provides a basis for further improvements of the experimental setup and the realization and assessment of residual stress measurements.