Formation Of γ-Al2O3 Research Articles

Adsorptive desulfurization properties of refractory sulfur compounds onto Al2O3-Ga2O3 nanomaterials

The impact of the synthesis method on the structural, textural, and surface properties of the Al2O3-Ga2O3 sample was investigated, and the adsorbents were evaluated for their adsorptive desulfurization properties. The incorporation of Ga atoms into the Al2O3 framework (via SGM) resulted in an increase in specific surface area and pore volume, but a decrease in average pore size. XRF analysis confirmed Ga loading, while SEM-EDS and STEM-EDS techniques revealed well-distributed elements with irregular spherical morphologies. XRD diffractograms confirmed the formation of γ-Al2O3, with a slight shift of diffraction peaks due to the presence of Ga (via SGM), suggesting that Ga atoms are part of the structure. The obtained solids exhibit values in the Freundlich equation that indicate a degree of nonlinearity in adsorption for Al2O3, while Al2O3-Ga2O3(SGM) and Al2O3-Ga2O3(IM) suggest a possible chemical process and favorable adsorption. The small KF values demonstrate weak adsorption on the adsorbents, which can be explained by sulfur-to-metal interaction, π-complexation, and non-specific interactions such as Van der Waals forces. The adsorbent showed the greatest affinity for BT and the least for DBT. The selectivity order from strongest to weakest adsorption was BT: Al2O3-Ga2O3(SGM) > Al2O3-Ga2O3(IM) > Al2O3, and DBT: Al2O3-Ga2O3(IM) > Al2O3-Ga2O3(SGM) > Al2O3.

Influence of microarc machining on resizing of aluminum parts

Problem. The change of the sizes of the processed samples (aluminum alloys) after oxidation in alkaline-silicate electrolyte at the anode-cathode mode is studied in the work. The two-layer structure of aluminum alloys after MAO processing is shown. The change in the size of the parts is determined by the phase composition of the coating. Goal. The goal is study of the changes in the size of the processed samples (aluminum alloys) after oxidation in alkaline-silicate electrolyte at the anode-cathode mode. The change of the sizes of the processed samples (aluminum alloys) after oxidation in alkaline-silicate electrolyte at the anode-cathode mode is studied in the work. Methodology. X-ray structural analysis (Dron - 3) in radiation Кα-Cu, microhardness measurement (PMT-3) with the load of 100 gr., measurement of coating thickness (vortex thickness gauge BT - 10NTs). Results. It is shown that in the case of predominant α- Al2O3 formation the δ / B ratio is 1.28, in the case of γ-Al2O3 formation, the δ / B ratio is 1.55 and in the case of mullite formation (3Al2O3 • 2SiO2) - 2.23. The calculation showed that the level of change in the size of the sample after MАO significantly depends on the phase composition of the coating. Experimental testing on different alloys and different electrolytes confirmed the different degree of change in size depending on the phase composition of the coating, which is determined by the modes of MAO and the composition of the electrolyte. Thus, the experimental value for alloy D16 subjected to MАO in different modes varies from 1.0 to 2.0; for B96 metal - 1.15-2.76; for AMg6 metal - 1.46-2.55. The results presented above relate to the total thickness of the formed coating. Given the two-layer structure of the coating and the fact that the thickness of the loose layer to be removed is 15 - 50% of the total thickness, the change in the size of the part after the final finishing of the friction surface should be insignificant. It has been experimentally established that from alloys D16, B96, AMg6 at optimal modes of MAO (thickness of wear-resistant coating 100 - 150 μm) the increase in the size of the part to the side is 5 - 10 μm. As for cast alloys (for example, Al9), the structure of the coating which contains a significant amount of mullite, the increase in the size of the part of such alloy after MАO and refining, more (compared to deformed alloys) and is 20 - 30 microns Al2O3. Originality. The calculation and research showed that the level of change in the size of the sample after MDO significantly depends on the phase composition of the coating itself. Considering the two-layer structure of the coating and the fact that the thickness of the loose layer to be removed is 15 - 50% of the total thickness, the change in the dimensions of the part after the final proofing of the surface should be insignificant. Practical value. Changes in the dimensions of the part must be taken into account when processing parts with small tolerances or eliminated by additional finishing by partially removing the main wear-resistant layer.

Generating Self-Shaped 2D Aluminum Oxide Nanopowders.

The thermal-assisted exfoliation phenomena of boehmite particles under moderate heating rates were examined. The exfoliation that generated flakes of 5–6 nm in thickness can be achieved because of the perfect cleavage on the boehmite particles that are stripped when thermal treatments bring about dehydration and γ-Al2O3 formation in sequential phase transformation of boehmite. Examinations of the exfoliation effects were carried out on calcined boehmite single crystal particles, which were about 500 nm in diameter, and obtained at three heating rates 0.5, 1.0, and 2.0 °C/min with the heating schedules. The TEM techniques, BET-N2 measurements, XRD-Scherrer equation, and AFM images were employed in the examination. That the BET values increased as increasing of exfoliated flakes reflected two stages of exfoliation. In the beginning stage, during which the BET values were <40 m2/g, the exfoliation resulted from the stress produced by dehydration. In the second stage, the increased rate of surface area was due to the additional force, which originated from the γ-Al2O3 formation. Exfoliation occurred on the cleavage planes {010}, the side pinacoid of the boehmite particle. The generation of flakes resulted in the thinning of boehmite particles. Some of the flakes preserved the external form of boehmite crystals. From the surface energy evaluations of boehmite and γ-Al2O3, it can be inferred that exfoliation is a natural way of thermal treatment.

Interfacial oxidation of hafnium modified NiAl alloys

The high-temperature oxidation of Hf-modified NiAl alloy at 950 °C results in the formation of γ-Al2O3 and θ-Al2O3. Exclusive formation of HfO2 particles is observed at the adherent γ-Al2O3/NiAl interface. By contrast, the θ-Al2O3/NiAl interface oxidation results in multi-layered oxide growth consisting of alternate thin and thick layers of θ-Al2O3 and NiAl2O4 along with dispersed HfO2 particles in the oxide layers. This difference is elucidated by first-principle calculations showing that the θ-Al2O3 lattice is more favorable than γ-Al2O3 to host Hf and Ni atoms, thereby resulting in the growth of mixed oxides at the θ-Al2O3/NiAl interface.

Tool wear mechanisms of PcBN in machining Inconel 718: Analysis across multiple length scale

Recently, PcBN tooling have been successfully introduced in machining Ni-based superalloys, yet our knowledge of involved wear mechanisms remains limited. In this study, an in-depth investigation of PcBN tool degradation and related wear mechanisms when machining Inconel 718 was performed. Diffusional dissolution of cBN is an active wear mechanism. At high cutting speed oxidation of cBN becomes equally important. Apart from degradation, tool protection phenomena were also discovered. Oxidation of Inconel 718 resulted in formation of γ-Al2O3 and (Al,Cr,Ti)3O4 spinel that were deposited on the tool rake. Also on the rake, formation of (Ti,Nb,Cr)N takes place due to cBN-workpiece interaction. This creates a sandwich tool protection layer forming continuously as tool wear progresses. Such in operando protection enabled counterbalancing tool wear mechanisms and achieved high performance of PcBN in machining.

Strong interfacial charge transfer between hausmannite manganese oxide and alumina for efficient photocatalysis

Well crystalline manganese oxide (Mn3O4) nanoparticles anchored on gamma alumina (γ-Al2O3) have been successfully tailored via a proficient and cost effective chemical process as an efficient material for photo catalysis. XRD indicated the composite formation of γ-Al2O3 and hausmannite structure of Mn3O4. SEM and TEM revealed that hetero structure of Mn3O4/γ-Al2O3 exhibits an amalgam of aggregated nanoparticles and nanorods. XPS demonstrated the chemical states of binary nanocomposite. The band gap tuning has been performed with γ-Al2O3 nanoparticles by assimilating hausmannite Mn3O4 particles into flower like microstructure of Al2O3. The photoluminescence spectra affirmed the enhancement in charge separation in Mn3O4/γ-Al2O3 binary hybrid photocatalyst. The band gap becomes narrow with the increase in concentrations of Mn3O4. The narrowing of band gap is concorded with crystalline domains of primary aggregated particles. To elucidate the mechanism of the photocatalytic activity linear sweep voltammetry was performed. The results showed that Mn3O4/γ-Al2O3 nanocomposite revealed the enhancement in current density as compared to pure γ-Al2O3 which confirmed the electron transfer from Mn3O4 to γ-Al2O3 through the interfacial potential gradient in conduction bands. The optimum concentration of 6.0% Mn3O4/γ-Al2O3 for hybrid structure showed an excellent photocatalytic activity under visible light due to narrow band gap energy. High degree distribution of Mn3O4 nano architects overlying on γ-Al2O3 induces a significant synergic effect between γ-Al2O3 and hausmannite phase of manganese oxide (Mn3O4). This strong interfacial contact between γ-Al2O3 and Mn3O4 endures the quick transfer of photo generated charge carriers across interface.

STRUCTURE AND PROPERTIES OF HETEROGENEOUS CATALYST ON AN OXIDE CARRIER BASED ON ALUMINIUM

In this study, the MAO technology was optimized in order to obtain a high development of coatings and their good adhesion to the substrate for the creation of a highly efficient heterogeneous catalyst on the oxide carrier of the Pt/Al2O3 system. Oxide coatings of different phase composition, surface structure and roughness, with a thickness of 20 –100 μm were obtained by MAO methods on D16 alloy and A97 commercial aluminum. It is shown that the structure and morphology of the surface of the coatings is determined by the conditions of electrolysis, which allows their optimization to improve efficiency. It is determined that the surface coating layer consists of the following phases: γ-Al2O3, α-Al2O3 and 3 Al2O3 • 2SiO2 (mullite). On the D16 alloy there was a predominant formation of γ-Al2O3 and α-Al2O3 phases. Increasing the duration of microarc oxidation increases the content in the coating of the α-Al2O3 phase. It is established that the use of MAO technology allows to change the morphology of the surface and its roughness in a wide range. The formation of two types of surface inhomogeneity in the planar growth region of coatings is revealed: with a relatively small size of such formations (up to 50 μm) and with the formations of the second type, the sizes of which in the planar growth region of MAO coatings reach 300 μm.It is determined that the purification coefficient of nitrogen oxide in Pt/Al2O3 catalysts is temperature dependent. At relatively low (150°C) and high (300°C) temperatures the highest value of α is achieved in systems based on MAO coatings with binary (γ-Al2O3 and α-Al2O3) phase composition. In the temperature range of the most effective cleaning (200–250°C) the greatest influence on their efficiency, apparently, has a surface morphology, namely, the formation of large (up to 300 μm) formations of surface inhomogeneity in the growth plane. In this case, the purification coefficient (for the reduction of nitrogen oxides) α reaches 92 %.The results of the study can be used in the development and creation of a new type of catalysts for the Pt/Al2O3 system.

Effect of Precursors on the Preparation and Texture of Mesoporous γ-Al2O3 Powders

We have studied processes underlying the formation of γ-Al2O3 from different precursors (aluminum hydroxide, aluminum isopropoxide, and aluminum nitrate) and synthesized mesoporous γ-Al2O3 powders at temperatures of 500 and 600°C. Texture parameters of the powders (specific surface area, average crystallite size, pore volume, pore shape, and pore size distribution) have been studied using low-temperature nitrogen adsorption measurements and X-ray diffraction, and the effect of the precursors on the texture parameters of γ-Al2O3 has been examined. We have demonstrated the possibility of obtaining γ-Al2O3 with a unimodal pore size distribution (2.5–4.5 and 4.5–8.0 nm, depending on the precursor) and a pore volume of ~0.550 cm3/g. The experimental results obtained here show that the texture characteristics of the synthesized γ-Al2O3 powders as catalyst supports compare well to those of their foreign analogues.

Effect of Mechanical Activation on Reaction between Boehmite and Lithium Carbonate

We have studied the effect of mechanical activation of a boehmite + lithium carbonate mixture in an AGO-2 planetary activator on phase transformations during heat treatment in air. The results demonstrate that heat treatment of the starting boehmite + lithium carbonate mixture leads to the formation of γ-Al2O3 as a reaction intermediate, which reacts with lithium carbonate to form α-LiAlO2. Heat treatment of the activated mixture leads to the formation of gamma-aluminum oxide and an unidentified intermediate phase, presumably, metastable lithium aluminate, which transforms into α-LiAlO2 at temperatures above 500°C. We have proposed a possible mechanism of the formation of this phase, which involves exchange of protons of boehmite with lithium cations, followed by the release of water and carbon dioxide.

Hydration-dehydration behavior induced densification of porous plasma electrolysis coating

Control of porosity in plasma electrolysis (PE) coating is extremely important for a wide set of applications. Up to this study, utilizing the phase transformation reactions between γ-alumina (γ-Al2O3) and boehmite (AlOOH) to densificate the porous surface of the coating deposited on Al-Mg-Si alloy has been rarely reported. For this aim, the PE coating sealed by boiling water is subjected additionally to an annealing treatment at 633 K for 30 min. The annealing treatment not only decreased the average size of micropores and the degree of porosity in the coating but also promoted the formation of γ-Al2O3 through the dehydration reaction of AlOOH which was formed during the sealing treatment of PE coating by the boiling water. Accordingly, the corrosion resistance of the annealed sample was superior to other cases.

Extreme ion irradiation of oxide nanoceramics: Influence of the irradiation spectrum

Oxide nanoceramics combine the enhanced radiation tolerance of nanocrystalline materials with the chemical inertness of oxides, and are promising materials for highly corrosive and intense radiation environments. In this work, nanocrystalline Al2O3 thin films are irradiated at 600 °C with either 12 MeV Au5++18 MeV W8+ or 4 MeV Ni2+ ions. The radiation damage exposure exceeds 450 displacements per atom. A comprehensive analysis of the irradiated samples is accomplished by X-Ray Diffractometry (XRD), Transmission Electron Microscopy (TEM) and Scanning-TEM (STEM). Results are compared in an effort to establish correlations between the irradiation spectrum and the response of this class of materials to radiation environments. The results show that grain growth is the main microstructural change induced by ion irradiation in the material, regardless of the ion utilized in this work. The phase evolution may be depth-dependent, and depends strongly on the ion utilized and on the irradiation spectrum. 12 MeV Au5++18 MeV W8+ irradiations favor the formation of γ-Al2O3 and α-Al2O3, while 4 MeV Ni2+ irradiations yield mainly δ-Al2O3, accompanied by small α-Al2O3 centers. Molecular dynamics simulations of displacement cascades are used to support discussions on the mass effect brought about by the different ions.

Effect of Carbon in Fabrication Al-SiC Nanocomposites for Tribological Application.

Aluminium-based hybrid composites are a new class of advanced materials with the potential of satisfying the demands in engineering applications. This paper describes the effects of carbon addition on the formation and properties of AMC with SiC nanoparticles reinforcement. The composites were produced via mechanical alloying followed by hot pressing. Three forms of carbon, graphite (GR), multiwalled carbon nanotubes (CNTs), and, for the first time, glassy carbon (GC), were used for the hybrid composites manufacturing and compared with tribological properties of Al-SiC composite without carbon addition. GC and CNTs enhanced formation of Al-SiC composite particles and resulted in a homogeneous distribution of reinforcing particles. On the other hand, GR addition altered mechanochemical alloying and did not lead to a proper distribution of nanoparticulate SiC reinforcement. Hot pressing technique led to the reaction between Al and carbon as well as SiC particles and caused the formation of Al4C3 and γ-Al2O3. The subsistence of carbon particles in the composites altered the predominant wear mechanisms since the wear reduction and the stabilization of the friction coefficient were observed. GC with simultaneous γ-Al2O3 formation in the hybrid Al-SiC(n)-C composites turned out to be the most effective additive in terms of their tribological behaviour.

Elaboration of an Easy Aqueous Sol-Gel Method for the Synthesis of Micro- and Mesoporous &amp;lt;i&amp;gt;γ&amp;lt;/i&amp;gt;-Al&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; Supports

An aqueous sol-gel method for the synthesis of γ-Al2O3 supports has been developed for the use in tar reforming applications. It was determined the influences of two different aluminum precursors (aluminum sec-butoxide (Al[OCH(CH3)CH2CH3]3) and aluminum nitrate (Al(NO3)3)) on the textural and crystallographic properties of Al2O3 supports. Only the formation of γ-Al2O3 is aimed in order to use these alumina materials as catalytic supports, because it presents high specific surface area and pore volume values. Additionally, the synthesis of γ-Al2O3 was realized with the use of a functionalized silicon precursor, [3-(2-aminoethylamino)propyl]trimethoxysilane, called EDAS. By the presence of an ethylenediamine group in this molecule, it is possible to chelate metallic ions and to highly increase their dispersion at a molecular level during the synthesis of metallic catalysts supported on alumina, which is an asset for catalytic applications. So it was developed a synthesis sol-gel procedure for the cogelation between the functionalized silicon alkoxide EDAS and alumina precursor. The alumina supports synthesized with Al(NO3)3 as precursor presented higher porous values than the ones obtained with aluminium sec-butoxide precursor. Since nitrate salts are much easier to handle than alkoxides, these observations allowed validating Al(NO3)3 as aluminum source for the future synthesis procedures for metallic catalysts supported on alumina.

Oxidation of Fe-Al Alloys (5-40 at.% Al) at 700 and 900 °C

Fe-Al alloys with Al contents between 5 and 40 at.% Al were oxidised for 1000 h in synthetic air at 700 and 900 °C to determine their oxidation behaviour. The minimum Al content which is necessary for the formation of protective Al2O3 scales decreases with increasing temperature from about 17 at.% Al at 700 °C to about 12 at.% Al at 900 °C. Established parabolic rate constants for the steady state growth of Al2O3 indicate formation of γ-Al2O3 at 700 °C while at 900 °C α-Al2O3 + Θ-Al2O3 scales form. At lower Al contents scales are predominantly formed by Fe2O3 as revealed by GI-XRD. It is also found that the oxidation behaviour is independent of the crystallographic order of the alloys, i.e. whether they are disordered A2 or ordered B2.

The Appearance of Pitting on Thermally Grown Aluminum Oxide caused by Surface Segregation of Fe Impurities from the Bulk

Abstract The oxidation of 99 wt.-% purity (low purity – main impurities were Fe and Si) and 99.999 wt.-% purity (high purity) Al foil was studied using a thermo-gravimetric method. The Al oxide was characterized with several techniques such as scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS) and x-ray photoelectron spectroscopy (XPS). The Al foils were oxidized in a 50 % O2 – 50 % Ar mixture between 773 and 843 K. Microscopic examination (SEM) of the oxide revealed that “pitting” occurred on the low purity Al. High Si and Fe concentration were found on the surface or near-surface of the oxide. The presence of Fe, as precipitates, on the surface of the oxide accounts for the pitting of the oxide. XPS inspection revealed the formation of γ-Al2O3 with the presence of some Al hydroxide.

Use of a design-of-experiments approach for preparing ceria–zirconia–alumina samples by sol–gel process

In this work we successfully obtained ceria–zirconia–alumina samples by the sol–gel technique. These materials were prepared under acidic or basic conditions, using either nitric acid or ammonium hydroxide as the catalyst. A design-of-experiments approach was used in order to optimize the specific surface area, pore structure, and thermal stability of the prepared samples. It was observed that the addition of ceria and zirconia did not affect the formation of γ-Al2O3. The highest surface areas and smallest pore sizes were observed for specimens obtained under acidic conditions and with low to intermediate concentrations of cerium and water. The increase of the heat treatment temperature from 600°C to 1000°C led to both a decrease of the surface area and an increase of the mean pore size. This behavior is due to the coalescence of pores upon calcination. Samples with a high concentration of ceria showed an expressive thermal instability at high temperatures. On the other hand, the addition of zirconia increased the thermal stability of these materials. In general, samples with improved thermal stabilities were obtained under basic conditions.

NON-CRYSTALLINE COPPER OXIDE HIGHLY DISPERSED ON MESOPOROUS ALUMINA: SYNTHESIS, CHARACTERIZATION AND CATALYTIC ACTIVITY IN GLYCEROL CONVERSION TO ACETOL

Polymeric precursor method and wet impregnation route were applied to synthesize copper and aluminium-based catalysts in order to obtain a material with interesting properties in catalytic reactions. The changes in the structural, morphological and textural properties due to the choice of preparation method were characterized by different techniques, such as XRD, N2 physisorption isotherms and SEM. The XRD results of the solids present the formation of γ-Al2O3 or CuO and β-Al(OH)3, depending on the preparation method. The average crystallite diameters of the alumina were estimated by the Scherrer's formula with a particle size of 5.2 nm. N2 physisorption isotherms analysis shows that the alumina is a mesoporous material with a high specific surface area. An excessive increase in surface area was observed after Cu2+ insertion by wet impregnation from β-Al(OH)3 support, which is explained by the redissolution and recrystallization of bayerite to γ-Al2O3 during the impregnation of Cu2+ and recalcination process, respectively. The SEM images confirmed this phenomenon. Catalytic tests explain that the combination of the two methods improves the activity, selectivity and the stability in the conversion of glycerol to acetol. The results indicate that the way of catalyst preparation affects its structural, textural and morphological properties and consequently the catalytic performance

Alumina/YAG 20 vol% composites prepared by the dawsonite thermal decomposition

The co-precipitation of alumina/YAG precursor from the aqueous solution of aluminum and yttrium nitrates with ammonium carbonate results in ammonium dawsonite (NH4Al(OH)2CO3) with yttrium built into its structure. The decomposition of dawsonite at elevated temperatures leads to the formation of γ-Al2O3. The δ- and θ-Al2O3 modifications appear at higher and higher temperatures. A full transformation to α-Al2O3 and YAG occurs at the temperatures higher than 1230°C. The starting powder for sintering experiments was prepared using the co-precipitated precursor calcination at 600°C. Seeding of such a powder with 5wt% α-Al2O3 proved to be necessary in order to receive the dense composite material at 1500°C. Such a material shows much lower wear susceptibility as compared to the dense pure alumina polycrystal. The unseeded powder led to the high porosity samples and showed symptoms of a discontinuous grain growth.

Formation and distribution of phases during the dehydration of large hydrargillite floccules

The phase composition and phase distribution of large floccules of hydrargillite dehydration products obtained by heat treatment at temperatures from 250 to 500°C have been studied by thermal analysis, X-ray diffraction, and IR spectroscopy. Heat treatment in the range 250–300°C leads to the formation of macrocrystalline and microcrystalline boehmite particles. The macrocrystalline boehmite is formed from large hydrargillite crystals. The microcrystalline boehmite results from the “fragmentation” of the hydrargillite crystals. The IR spectra of the samples point to an increase in the number of hydroxyls with ν(OH) = 3471 cm−1 on the lateral faces of the hydrargillite crystals. The crystallite size of the hydrargillite crystals decreases by a factor of 1.8–2.3. In the floccules, a layer of the macrocrystalline boehmite surrounds a microcrystalline boehmite core. The outer surface is covered predominantly with χ-Al2O3. Heat treatment at t ≥ 350°C leads to γ-Al2O3 formation through sequential dehydration of microcrystalline and then macrocrystalline boehmite particles.

Ab initio and experimental study on the effect of Y additions on the phase formation and thermal stability of Al2O3 thin films deposited by filtered cathodic arc evaporation

The effect of 0.6 to 7.5at.% Y addition on the phase formation and thermal stability of Al2O3 has been investigated using density functional theory and post-annealing of Y containing alumina thin films deposited by filtered cathodic arc evaporation. The calculations indicate the decomposition of the Y containing γ- and α-Al2O3 solid solutions into Y2O3 and the corresponding alumina phase. This prediction is consistent with experiments: The lattice parameters of the γ-Al2O3 thin films with and without Y are comparable and are hence inconsistent with the predicted expansion in equilibrium volume as Y is incorporated into the γ-(Al,Y)2O3 solid solution. The predicted metastable character of γ-(Al,Y)2O3 is also consistent with the formation of γ-Al2O3, Y2O3, and Y3Al5O12 in the as-deposited state as identified by X-ray diffraction. While the phase transition from γ-Al2O3 to α-Al2O3 phase takes place at T≤1100°C, the formation of traces of α-Al2O3 is restrained to T≤1200°C for alumina thin films containing Y and additionally the formation of Y3Al5O12 is observed. The restrained transition of the metastable γ-Al2O3 polymorph to α-Al2O3 may be explained by the segregation of Y at the metastable Al2O3 grain boundaries impeding mass transport and hence retard both formation and grain growth of α-Al2O3.