Al2O3 Clusters Research Articles

Microstructure and High‐temperature Oxidation Behavior of Ti3AlC2/W Composites

Using spark plasma sintering, Ti3AlC2/W composites were prepared at 1300°C. They contained “core‐shell” microstructures in which a TixW1−x “shell” surrounded a W “core”, in a Ti3AlC2 matrix. The composite hardness increased with W addition, and the hardening effect is likely achieved by the TixW1−x interfacial layer providing strong bonding between Ti3AlC2 and W, and by the presence of hard W. Microstructural development during high‐temperature oxidation of Ti3AlC2/W composites involves α‐Al2O3 and rutile (TiO2) formation ≥1000°C and Al2TiO5 formation at ~1400°C while tungsten oxides appear to have volatilized above 800°C. Likely due to exaggerated, secondary grain growth of TiO2‐doped alumina and the effect of W addition, fine (<1 μm) Al2O3 grains formed dense, anisomorphic laths on Ti3AlC2/5 wt%W surfaces ≥1200°C and coarsened to large (>5 μm), dense, TiO2‐doped Al2O3 clusters on Ti3AlC2/10 wt%W surfaces ≥1400°C. W potentially affects the oxidation behavior of Ti3AlC2/W composites beneficially by causing formation of TixW1−x thus altering the defect structure of Ti3AlC2, resulting in Al having a higher activity and by changing the scale morphology by forming dense Al2O3 laths in a thinner oxide coating, and detrimentally through release of volatile tungsten oxides generating cavities in the oxide scale. For Ti3AlC2/5 wt%W oxidation, the former beneficial effects appear to dominate over the latter detrimental effect.

Phase stability and initial low-temperature oxidation mechanism of Ti2AlC thin films

Abstract Ti2AlC thin films deposited onto Al2O3 by magnetron sputtering were used as model for studying the early stages ( ( 1 0 1 ¯ 3 ) and ( 1 0 1 ¯ 6 ) as shown by X-ray diffraction and s electron microscopy. During oxidation, Al2O3 clusters and areas of C-containing titania (TiOxCy) are formed on the surface. A mechanism is proposed in which the locations of the Al2O3 clusters are related to the migration of Al atoms diffusing out of Ti2AlC. The Al2O3 is initially formed in valleys or on plateaus where Al atoms have been trapped while TiOxCy forms by in-diffusion of oxygen into the Al-deficient Ti2AlC. At 500 °C, the migration of Al atoms is faster than the oxidation kinetics; explaining this microstructure-dependent oxidation mechanism.

Growth of thin Al2O3 films on biaxially oriented polymer films by atomic layer deposition

The effects of thin film nucleation and initial growth on roughness, chemistry and thermomechanical properties of polymer film surfaces were studied. Al2O3 was deposited onto commercial biaxially oriented polypropylene and polylactic acid films at 80°C by using atomic layer deposition technique. Both substrates, especially the more hydrophobic polypropylene, showed initial growth through Al2O3 clusters. There was a faster deposition of Al2O3 on polylactic acid film than on polypropylene at the early stages of the Al2O3 deposition. There were also indications of chemical interactions between polylactic acid and trimethyl aluminum used as a precursor for Al2O3. Changes in the thermo-mechanical properties of the polymer surfaces with Al2O3 also evidenced the differences between the substrate polymer films. The near surface interphase formed in polylactic acid probably contributed to the strong increase and scattering in the softening temperature during the early thin film growth.

Preparation and characterization of aluminum matrix composites based on Al–WO3 system

The (WAl12+Al2O3)/Al composites were fabricated by powder metallurgy based on the in situ reaction between WO3 and Al powders in this paper. The microstructure and properties of the composite were investigated and the reaction mechanism was determined. The produced reinforcements including WAl12p and W precipitated Al2O3 clusters are distributed mainly by a network in the matrix, where the majority of reinforcements lied within the wall of network with a thickness of 5–10μm, and while a small part of fine reinforcements in the interior of network. The results show that the WAl12 and Al2O3 clusters were formed by the reaction between WO3 and Al during the sintering process, and WAl12 has the orientation relationship with Al: [110]Al‖[001]WAl12, (1−11)Al‖(310)WAl12, meanwhile, nanosized W precipitates was formed and embedded in Al2O3 particles. The mechanical properties of (WAl12+Al2O3)/Al composites were improved as compared with Al matrix due to the introduction of reinforcement particles, which is due to Orowan strengthening mechanism of reinforcements. The WAl12 and Al2O3 containing nanosized W precipitates reinforcement phases possess an outstanding hybrid effect on the mechanical properties of the composite, including strength and plasticity.

A first principle study of SO3 decomposition on silver nano-clusters: Implications toward hydrogen production

In this work, based on first principles density functional theory, we have investigated the interaction of SO3 molecule on three different substrates; (i) clean Al2O3 surface (0001) (ii) an isolated Ag6 cluster and (iii) Ag6 clusters deposited on the Al2O3 surface. All calculations were carried out using the plane wave based pseudopotential method under the framework of density functional theory. For the clean Al2O3 surface, the SO3 molecule was adsorbed in parallel orientation on the surface resulting in an elongation of the S–O bond from 1.44 to 1.52 Å with interaction energy of 1.67 eV. In contrast, the interaction of SO3 with Ag6 was found to be weak with 0.4 eV interaction energy and 1.47 Å as the largest S–O bond length. Remarkably, when SO3 molecule interacted with Ag6 cluster deposited on the Al2O3 support, the binding was found to be higher than both Al2O3 and Ag6 clusters in their isolated state. In particular, upon adsorption of SO3 on Ag6/@Al2O3, the S–O bond length was found to increases from 1.44 to 1.64 Å and the interaction energy was estimated to be 2.00 eV. As the bond elongation bears the signature of bond weakening, a comparison of the above three results clearly suggests that the dissociation barrier of S–O bond on the Ag6@Al2O3 support will be significantly lower than that on the isolated Ag6 or Al2O3 surface. The nature of chemical interaction of SO3 on these three systems has been discussed based on the electronic density of states analysis.

Experimental Investigation of Magnesium-Base Nanocomposite Produced by Friction Stir Processing: Effects of Particle Types and Number of Friction Stir Processing Passes

In this research, nanosized SiC and Al2O3 particles were added to as-cast AZ91 magnesium alloy, and surface nanocomposite layers with ultrafine-grained structure were produced via friction stir processing (FSP). Effects of reinforcing particle types and FSP pass number on the powder distribution pattern, microstructure, microhardness, and on tensile and wear properties of the developed surfaces were investigated. Results show that the created nanocomposite layer by SiC particles exhibits a microstructure with smaller grains and higher hardness, strength, and elongation compared to the layer by Al2O3 particles. SiC particles do not stick together and are distributed separately in the AZ91 matrix; however, distribution of SiC particles is not uniform in all parts of the stirred zone (SZ), which causes heterogeneity in microstructure, hardness, and wear mechanism of the layer. Al2O3 particles are agglomerated in the different points of matrix and create alumina clusters. However, distribution of Al2O3 clusters in all parts of the SZ is uniform and results in a uniform microstructure. In the specimen produced by one-pass FSP and SiC particles, the wear mechanism changes in different zones of SZ due to the nonuniform distribution of particles. However, in the specimen produced by Al2O3 particles, the wear mechanism in all parts of the SZ is the same and, in addition to the abrasive wear, delamination also occurs. Increasing FSP pass number results in improved distribution of particles, finer grains, and higher hardness, strength, elongation, and wear resistance.

Effects of Process Parameters and Substrate Structures on Growth of Single-Walled Carbon Nanotubes by Catalytic Decomposition of Ethanol

Single-walled carbon nanotubes (SWNTs) were prepared by ethanol chemical vapor deposition (CVD), using Co and Mo as metal catalysts deposited on Si/SiO2 substrates with and without an Al underlayer. The effects of Mo addition, catalyst particle size and the Al underlayer were discussed on the basis of the results of scanning electron microscopy (SEM), atomic force microscopy (AFM) and Raman measurement. In substrates with Si/SiO2/Co/Mo structures, a Co layer with a thickness of 1 nm produced smaller catalyst particles and SWNTs with a thinner diameter and at a higher yield than a Co layer with a thickness of 2 nm. The addition of Mo to Co films in substrates was shown to have the effect of suppressing Co aggregation and keeping catalyst particles small, even with high-temperature CVD growth. It was also found that the introduction of an Al underlayer promoted SWNT growth because of the formation of Al2O3 clusters after heat treatment.

Preparation and Characterization of Pt/Al-ZSM-5 Catalysts and their Reactivities for the Oxidation of CO with N2O at Low Temperatures

Al-ZSM-5 was prepared by treating H-ZSM-5 with an aqueous solution of Al(NO3)3 and used as a support for Pt catalysts. The Pt-loaded Al-ZSM-5 acts as an efficient catalyst for CO oxidation with N2O at 273 K. TEM investigations revealed that Pt clusters with an average particle size of around 1–1.5 nm were homogeneously dispersed within Al-ZSM-5. Moreover, FT-IR and XPS analyses indicated that the small Al2O3 clusters formed within Al-ZSM-5 plays a significant role in the formation of highly dispersed Pt clusters within the pore structure of the ZSM-5 zeolite, leading to the high catalytic activity of Pt/Al-ZSM-5 as compared to Pt/ZSM-5.

Silicon-nitride-based nanoceramic materials

Based on the principle of maximum bond energy, a structural model is proposed for Si3N4 + 60 wt % MoSi2 + 5 wt % Y2O3 + 1.5 wt % Al2O3 ceramics. In this model, Si3N4 clusters form a star-like structure with columnar pores 0.45 and 1.5 nm in size. The smaller pores are filled with MoSi2 molecules, and the larger pores are filled with Y2O3 and Al2O3 clusters. The solubility of MoSi2 molecules in silicon nitride is 70 wt %, and those of Y2O3 and Al2O3 are 5.45 wt %. The energies of intercluster interaction in two mutually perpendicular directions are 3.2 and 5.5 eV, which is comparable to bond energies in structural metallic materials. The lattice parameters of molybdenum disilicide are determined by x-ray diffraction. The formation of MoO3 and SiO2 molecules during thermal cycling has an insignificant effect on the structure of the material.

Role of surface Al2O3 clusters on the kinetics of Al(111) oxidation

The effect of aluminum-oxide clusters on the Al(111) surface on subsequent oxygen adsorption at 300 K was investigated for both low and high degrees of oxidation using x-ray photoelectron spectroscopy and high-resolution electron energy-loss spectroscopy. Oxide clusters on the Al(111) surface were produced by annealing chemisorbed O layers of various coverage up to 773 K. Only the chemisorbed oxygen phase was found to form at 243–300 K after additional O2 exposure on surfaces containing both low- and high-oxide-cluster populations. The chemisorbed O adatoms produced were not influenced by the oxidic phase, exhibiting spectroscopic features similar to those observed upon chemisorption on the clean surface without oxide clusters. Oxygen adsorption on the surface with both low- and high-oxide-cluster populations proceeded with the same rate as found on the clean Al(111) surface. The observed effect is postulated to be due to an increase of the number of the vacant Al adsorption sites liberated upon transformation of chemisorbed O adatoms into oxide clusters.

Structures, Energies, and Electrostatics for Methane Complexed with Alumina Clusters

Ab initio calculations were used to investigate properties of complexes formed from the association of CH4 with Al2O3, Al4O6, and Al8O12 alumina clusters. Methane attaches to a surface Al atom of the cluster to form a complex with an AlC separation that varies between 2.2 and 2.5 Å. The rotational motion for methane in these complexes is highly fluxional. Extrapolated G2MP2 well depths for the CH4- - -Al2O3, CH4- - -Al4O6, and CH4- - -Al8O12 complexes are 21, 14, and 17 kcal/mol, respectively. These different well depths are determined by the accessibility of the Al atom to which CH4 binds and the size of the alumina cluster. The electrostatics of the three alumina clusters are very similar, with a charge on the surface Al atom of +2.2 to 2.3. The potential energy surface for a CH4- - -Al2nO3n cluster is represented semiquantitatively by an analytic function consisting of two-body potentials. The results of this study suggest that the adsorption energy for alkane molecules binding to alumina materials depends very strongly on the structure of the binding site.

Photoacoustic infrared spectroscopy of nanoclusters Al2O3 clusters and cluster-assembled solids.

Stoichiometric nanometer-sized ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ clusters were generated by direct rf magnetron sputtering from an \ensuremath{\alpha}-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ target in an inert atmosphere. Nanostructured compacts were derived by compacting the clusters in situ under ultrahigh vacuum. Photoacoustic infrared spectroscopy was applied in the vibrational study of the ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ nanoclusters and nanostructured compacts. The two samples were associated with considerable spectral broadening of the phonon band, compared to conventional \ensuremath{\alpha}- and \ensuremath{\gamma}-${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ polycrystals. These were attributed to the finite-crystal effects of the nanometer-sized grains and to the disorder in the grain boundaries which made up a significant volume fraction of the samples, particularly in the nanostructured compacts.

Surface properties and cracking activity of delaminated clay catalysts

Delaminated clays are solid supports and catalysts characterized by a house-of-card like structure containing, in addition to micropores of the type found in zeolites and pillared clays, mesopores and macropores characteristic of mixed oxides, aluminas and silicas. Electron microscopy results have indicated that only short-range (3 to about 15 units) face-to-face stacking of the silicate layer occurs. These aggregates are believed to contain pillared clay-like microporosity and are held together by cross-linking particles forming a house-of-cards structure containing regions of macroporosity in which large molecules like perfluorotributylamine can be sorbed. Particles less than ≈ 100 nm in size and having lath-shaped morphology have been found to facilitate the preparation of delaminated clay catalysts. Delaminated clays prepared by reacting a synthetic hectorite (Laponite B) with polyoxocations of aluminium are, after a mild heat pretreatment, somewhat less active than pillared clay catalysts, but exhibit greater selectivity to gasoline, light cycle oil (LCGO) and coke make. The improved coke selectivity is probably due to the weaker acidity and the macroporosity of the catalyst which facilitate the desorption of aromatics which otherwise would be retained (occluded) as coke. The lack of Fe 2O 3 impurities is believed to be responsible for the gasoline advantage exhibited by the delaminated clay catalyst. Delaminated clays containing Al 2O 3-clusters are stable to 600 °C in air; however, in the presence of steam their stability is significantly reduced. Therefore, like pillared clays, delaminated clays do not have the hydrothermal stability of zeolite-containing fluid cracking catalysts. However, their thermal stability and macroporous structure could be particularly useful in the preparation of hydrotreating catalysts for the upgrading of heavy oil to useful products.

Catalytic activity of aluminum fluoride

The development of catalysts for the resource utilization of potent greenhouse gases, hydrofluorocarbons (HFCs) by catalytic pyrolysis is of great significance to alleviate the greenhouse effect. Fabricating small catalyst particles with high specific surface area leading to more Lewis acid sites is the effective strategies to achieve highly functional catalysts. However, catalysts tend to sinter at elevated temperatures. Herein, we synthesized carbon matrix confined Al2O3 clusters via the calcination of MIL-53-Al under N2 atmosphere. Then the Al2O3 clusters were in situ fluorinated and turned into AlF3 clusters accordingly. The derived carbon confined AlF3 clusters exhibited outstanding sintering resistance even following thermal treatment at 600 °C. Moreover, the carbon confined AlF3 clusters presented superior catalytic performance for 1,1-difluoroethane (HFC-152a) conversion with near 40% higher than that of AlF3 prepared by traditional precipitation. The excellent catalytic activity and stability were mainly ascribed to the abundant under-coordinated Al species with strong interaction with carbon matrix and high specific surface area. Thus, in this paper, we developed new practicable method to synthesize highly efficient Lewis acid catalysts with strong sintering resistance and abundant acid sites.