γ-Al2O3 Matrix Research Articles

Phase transitions in alumina films during post-sparking anodising of Al alloys

Formation of the high-temperature α-Al2O3 phase during Plasma Electrolytic Oxidation of aluminium at ambient bulk temperatures has been previously attributed to local microdischarge events providing multiple melting-solidification cycles in micro-volumes of the surface oxide layer. In this work, it is demonstrated that the α phase can be formed even if the microdischarge is fully suppressed under specific processing conditions. Oxide layers produced in the post-sparking anodising mode were studied by FIB, TEM, EBSD, EDS and GDOES techniques to reveal microstructural and chemical evolutions that accompany the γ to α alumina transition. Our results provide strong evidence that the α phase can form spontaneously in regions of oxide with the appropriate temperature, grain size and impurity distributions in the γ-Al2O3 matrix that allow sufficient mobility of α/γ grain boundaries. Ionic migration within the oxide and hydrothermal dissolution/precipitation in the associated microporous network that facilitate species mobility at the grain boundaries allow the critical temperature for activation of γ→α transition to be reduced. Overall, it is suggested that oxide layer growth can be considered in terms of a relatively simple Plug Flow Reactor model. This can help predict the phase transition kinetics depending on key processing parameters such as current density and frequency of pulse polarisation, thus enabling optimum control of coating microstructure for specific application requirements.

Role of zeolite properties in bio-oil deoxygenation and hydrocarbons production by catalytic cracking

A comparison between the behavior of Y and ZSM-5 zeolites having the same Si/Al ratio was established in the catalytic cracking of bio-oil for producing fuels and platform chemicals. The catalysts were prepared by agglomerating each zeolite with a mesoporous γ-Al2O3 matrix. Properties of the zeolites and the catalysts were characterized by N2 ads/desorption, SEM-EDX, pyridine-FTIR, tBA-TPD, and NH3-TPD techniques. The experiments were conducted in a continuous two-step catalytic cracking system (TS-CC) at 450 °C. Both catalysts are highly selective to C5-C12 hydrocarbons, with a composition strongly affected by the porous structure and acidity of the zeolite. The HZSM-5 catalyst (high acid strength and Brønsted/Lewis ratio) promotes cracking and deoxygenation reactions that lead to C2−C4 olefins which condensate into aromatics, yielding 9% of 1-ring aromatics (3% BTX) and 4% of naphthalenes for a feed conversion of 85%. These reactions are less favored over the HY catalyst, which promotes olefins oligomerization and hydrogen-transfer reactions, yielding 14% of highly aliphatic gasoline (> 30% linear paraffins) and 5% of C13−C20 diesel fraction (> 90% long chain paraffins). The remaining upgraded bio-oil obtained with both catalysts is composed of light oxygenates, with ketones, acids and esters as main components and low concentration of phenolic compounds.

Shock impedance adjustment using anion and cation components: From γ-Al2O3 to Al(8+)/3□(1/3-/3)O4-N and Mg1-Al2+O4-N transparent ceramic windows

Abstract In shock wave precision physical measurements, the shock impedance of sapphire (Al2O3) transparent windows and Fe samples cannot be completely matched, which seriously affects the measurement accuracy. Adjusting the components of the anion and cation in the γ-Al2O3 structure is a possible solution. N3- and Mg2+ were introduced into the γ-Al2O3 matrix to form Al(8+x)/3□(1/3-x/3)O4-xNx (□ denotes the vacancy, AlON) or Mg1-yAl2+yO4-yNy (MgAlON). Using the first-principles method coupled with the shock wave theory, the shock response behaviors and optical properties of AlON and MgAlON were studied in this work. Due to a more obvious effect was induced by Mg2+ than N3-, MgAlON had a wider shock impedance adjustment range compared with AlON. The simulation results of the shock pressure-particle velocity curves (σxx-U) demonstrated that the shock impedance of MgAlON can be completely matched with that of the Fe samples in a wide pressure range (0–20 GPa) by tuning its composition. In addition, AlON and MgAlON show good transparency between 0 and 20 GPa. This discovery provides theoretical guidance for selecting transparent ceramic windows with optimized structures and compositions that match the shock impedance of the Fe samples.

Facile synthesis of spray pyrolysis-derived CuCl/γ-Al2O3 microspheres and their properties for CO adsorption and CO/CO2 separation

In this study, we developed a facile and scalable preparation method of [email protected]γ-Al2O3 composite microspheres for CO storage and separation by combining sol-gel and spray pyrolysis methods, followed by metal salt impregnation. γ-Al2O3 microspheres were first prepared by spray pyrolysis of a mixture containing various ratios of boehmite sol and citric acid (CA). Pore-size distribution analyses indicated that the mesopores that are generated inside of the γ-Al2O3 microspheres and their pore volumes could be fine-tuned by adjusting the CA/boehmite ratio. Importantly, the newly formed pore structures have been identified to accelerate the thermal dispersion of CuCl species into the γ-Al2O3 matrix, resulting in improved CO sorption. Gas-adsorption experiments showed that CO-adsorption capacity and CO/CO2 selectivity were strongly affected by the CA/boehmite ratio, CuCl-dispersion temperature, and CuCl-loading amount. The optimized adsorbent exhibited a maximum CO-adsorption capacity of 1.69 mmol g−1 and CO/CO2 selectivity of 84 (at 25 °C and 100 kPa), exceeding the benchmarks for CO-selective adsorbents. Moreover, the spherical [email protected]γ-Al2O3 adsorbent showed good regenerative ability after a number of adsorption–desorption cycles. This study provided facile and scalable synthesis method for low-cost CO-selective material with high CO performance.

Nickel Poisoning of a Cracking Catalyst Unravelled by Single-Particle X-ray Fluorescence-Diffraction-Absorption Tomography.

Ni contamination from crude oil in the fluid catalytic cracking (FCC) process is one of the primary sources of catalyst deactivation, thereby promoting dehydrogenation-hydrogenation and speeding up coke growth. Herein, single-particle X-ray fluorescence, diffraction and absorption (μXRF-μXRD-μXAS) tomography is used in combination with confocal fluorescence microscopy (CFM) after thiophene staining to spatially resolve Ni interaction with catalyst components and study zeolite degradation, including the processes of dealumination and Brønsted acid sites distribution changes. The comparison between a Ni-lean particle, exposed to hydrotreated feedstock, and a Ni-rich one, exposed to non-hydrotreated feedstock, reveals a preferential interaction of Ni, found in co-localization with Fe, with the γ-Al2 O3 matrix, leading to the formation of spinel-type hotspots. Although both particles show similar surface zeolite degradation, the Ni-rich particle displays higher dealumination and a clear Brønsted acidity drop.

γ-Al2O3 sheet-stabilized isolate Co2+ for catalytic propane dehydrogenation

Co-based catalysts are promising alternatives to replace Pt- and Cr-based catalysts for propane dehydrogenation (PDH) in view of the catalytic activity for C–H bond activation, low cost, and low toxicity. However, the reduction of unstable CoOx species and coke deposition cause serious catalyst deactivation. In this work, a sheet-shaped Co–Al2O3 catalyst comprising of isolate Co2+ sites shows a high intrinsic activity (specific reaction rate > 16 mmol g−1 h−1) and catalytic stability in highly selective PDH reaction (>97% C3H6 selectivity). Extensive characterization demonstrates that the tetrahedral Co2+ isolated site is stabilized by the γ-Al2O3 matrix, facilitating propylene desorption to inhibit formation of coke and other byproducts and to free the active Co sites.

Photoluminescence imaging of europium (III)-doped γ-Al2 O3 nanofiber structures.

Aluminium oxide (Al2 O3 ) has widely been used for catalysts, insulators, and composite materials for diverse applications. Herein, we demonstrated if γ-Al2 O3 was useful as a luminescence support material for europium (Eu) (III) activator ion. The hydrothermal method and post-thermal treatment at 800°C were employed to synthesize Eu(III)-doped γ-Al2 O3 nanofibre structures. Luminescence characteristics of Eu(III) ions in Al2 O3 matrix were fully understood by taking 2D and 3D-photoluminescence imaging profiles. Various sharp emissions between 580 to 720nm were assigned to the 5 D0 →7 FJ (J=0, 1, 2, 3, 4) transitions of Eu(III) activators. On the basis of X-ray diffraction crystallography, Auger elemental mapping and the asymmetry ratio, Eu(III) ions were found to be well doped into the γ-Al2 O3 matrix at a low (1mol%) doping level. A broad emission at 460nm was substantially increased upon higher (2mol%) Eu(III) doping due to defect creation. The first 3D photoluminescence imaging profiles highlight detailed understanding of emission characteristics of Eu(III) ions in Al oxide-based phosphor materials and their potential applications.

Preparation of Pt/γ-Al2O3-C heterogeneous catalyst for highly selective hydrogenation of cinnamaldehyde towards cinnamyl alcohol

A Pt/γ-Al2O3-C heterogeneous catalyst with improved catalytic performance was successfully prepared. Bayberry tannin(BT), an abundant natural plant polyphenol, was utilized to modifiy the surface of the γ-Al2O3 matrix and then stabilize Pt nanoparticles(NPs). The catalyst was systematically and specifically characterized by a collection of analytic tools including XRD, XPS, FTIR, TEM, and TG. It was found that the Pt NPs were envenly dispered on the γ-Al2O3-C matrix. Meanwhile, when the catalyst was applied for selective hydrogenation of cinnamalde-hyde(CMA) in the presence of KOH promoter in solvent, excellent catalytic performance was obtained. The selectivity to cinnamyl alcohol(CMO) reached 97.88% with 88.45% CMA conversion within 30 min in the presence of 0.037 mol/L KOH, without significant loss of catalytic activity even under cyclic measurements for 5 times. The excellent catalytic performances should be ascribed predominantly to the uniform and stable dispersion of Pt NPs on the γ-Al2O3-C matrix and the excellent heat stability of BT.

Optical properties versus temperature of Cr-doped γ- and α-Al2O3: Irreversible thermal sensors application

A thorough investigation by X-ray diffraction, UV–vis and luminescence spectroscopy is carried out to demonstrate how the chromium content of alumina matrices impacts the temperature of the γ→α irreversible phase transition. The Cr3+ contents influence slightly the phase transition temperature but control the brightness of the powders. Nice colorimetric contrasts from green to pink are observed between the two allotropic forms. Furthermore, drastic changes of the spectral distribution and of the intensity of luminescence are observed, thus allowing to use this pigment as a both thermochromic and luminescent thermal sensor. Additional measurements at low temperature revealed that the Cr3+ emission of the γ-Al2O3 matrix is constituted by a large band. A configurational diagram schematic approach suggested for the first time that this emission is due to spin-allowed 4T2→4A2 transition on this largely investigated γ-Al2O3 compound.

In situ Formed α-Al2O3 Nanocrystals Repaired the Preexisting Microcracks in Plasma-Sprayed Al2O3 Coating via Stress-Induced Phase Transformation

In the present study, the phase composition and generation mechanism of the nanocrystals located in the microcracks of plasma-sprayed Al2O3 coating were reevaluated. The Al2O3 coatings were investigated using transmission electron microscopy and x-ray diffraction. We supply the detailed explanations to support the new viewpoint that in situ formation of α-Al2O3 nanocrystals in the preexisting microcracks of the as-sprayed Al2O3 coating may be due to the stress-induced phase transformation. Owing to the partially coherent relationship, the phase interfaces between the α-Al2O3 nanocrystals with the preferred orientation and the γ-Al2O3 matrix may possess better bonding strength. The α-Al2O3 nanocrystals could repair the microcracks in the coating, which further strengthens grain boundaries. Grain boundary strengthening is beneficial to the coating fracture toughness enhancement.

On the high temperature stability of γ-Al2O3/Ti0.33Al0.67N coated WC–Co cutting inserts

Abstract The high temperature stability of γ-Al2O3 films deposited using filtered cathodic arc and plasma assisted chemical vapor deposition on Ti0.33Al0.67N coated WC–Co cutting inserts is investigated. X-ray diffractometry reveals that filtered cathodic arc deposited films transform partially into the thermodynamically stable α-Al2O3 phase at a temperature of 1000°C. The γ to α-Al2O3 transformation for plasma assisted chemical vapor deposition grown films is observed at 900°C. These results are in qualitative agreement with differential scanning calorimetry measurements. Transmission electron microscopy on filtered cathodic arc and plasma assisted chemical vapor deposition films annealed at 900°C reveals the existence of hexagonal AlN in the Ti0.33Al0.67N interlayer, as well as Al depletion at the Al2O3/Ti0.33Al0.67N interface. After annealing the plasma assisted chemical vapor deposition sample at 900°C, α-Al2O3 grains with a size of ∼100 nm are observed inside the γ-Al2O3 matrix, while for filtered cathodic arc samples only the γ-phase is identified. Transmission electron microscopy analysis on both filtered cathodic arc and plasma assisted chemical vapor deposition samples annealed at 1000°C shows that the original Al2O3/Ti0.33Al0.67N/WC–Co layer architecture is no longer intact. The formation of TiO2 is detected along the growth direction of the Al2O3 films. The present study suggests that not only the morphology and the impurities incorporated into γ-Al2O3 but also stability of the Ti0.33Al0.67N interlayer determine the high temperature stability of γ-Al2O3/Ti0.33Al0.67N coated hardmetal.

Influence of autogenous seeding on densification and microstructure in processing of γ-alumina nanopowders

The effect of α-Al2O3 seeds and MgO on the densification behavior and microstructure of the nanocrystalline γ-Al2O3 powder was investigated. The required α-Al2O3 seeds were produced by ‘autogenous seeding’ where the seeds were obtained by calcination of the initial nanosized γ-Al2O3 powders above its transformation temperature and incorporated into γ-Al2O3 matrix. The seeds were characterized by means of laser particle size analyzer, X-ray diffraction, and Brunauer–Emmett–Teller surface analyzer. The simultaneous influence of α-Al2O3 seeds and MgO on γ-Al2O3 behavior was evaluated by differential thermal analysis, densitometry, and electron microscopy. The results showed that simultaneous presence of MgO and seeds in γ-Al2O3 matrix has a great impact on densification and microstructural evolution. The main role of MgO and the seeds are densification enhancement and grain refinement, respectively. The grain morphology is more equiaxed and desirably finer by using smaller seeds prepared using shorter calcination times.

Fine-Scale Nanostructure in γ-Al2O3

We have applied a local structural technique, atomic pair distribution function (PDF) analysis of powder diffraction data to γ-Al2O3, to obtain a quantitative structure. Refinements of the PDF support previous findings that nonspinel model representations are more suitable to describe the average structure of γ-Al2O3, as opposed to the spinel model. Surprisingly, also we find a previously unknown fine-scale nanostructure with a domain size ∼ 1 nm. Modeling suggests that within these nanodomains the oxygen sublattice is modified from the average structure and retains aspects of the boehmite precursor that reflects a stacking fault in the γ-Al2O3 matrix. This results in a novel and unexpected view of the γ-Al2O3 structure because earlier controversies about it centered on the arrangement of Al ions among different cation sites, whereas the oxygen sublattice arrangement was rarely questioned. This average oxygen sublattice structure is recovered in our models on longer length scales by introducing aperiodic ...

Preparation and Characterization of FeCo-Al2O3and Al2O3Aerogels

Nanocrystalline γ-Al2O3 and FeCo-Al2O3 nanocomposites containing FeCo alloy nanoparticles well dispersed in the nanocrystalline γ-Al2O3 matrix were successfully obtained by the sol-gel method using aluminum tri-sec-butoxide and iron and cobalt nitrates as precursors. The gels were submitted to high temperature supercritical drying which allowed to obtain aerogels with high surface areas and pore volumes.

Evolution of the Structure and Magnetic Properties of FeCo Nanoparticles in an Alumina Aerogel Matrix

Nanocrystalline γ-Al2O3 and FeCo−Al2O3 nanocomposite aerogels with high surface areas and pore volumes were prepared by high-temperature supercritical drying of alcogels obtained by a fast sol−gel procedure. The formation of γ-Al2O3 occurs via a sequence of stages: in the parent aerogel an alkyl derivative of boehmite is observed whose calcination gives rise to a disordered phase and finally to γ-Al2O3 which is stable up to 1000 °C. In the presence of iron and cobalt, calcination of the aerogel gives rise to a spinel phase similar to γ-Al2O3 where metal ions partially fill the vacancies. Nanocomposites constituted of FeCo alloy nanoparticles dispersed into γ-Al2O3 matrix are obtained via reduction in hydrogen flow of the aerogels containing iron and cobalt. The amount and average size of the nanoparticles depends both on the temperature and time of the reduction treatment and affects the resulting magnetic properties. All the reduced aerogels show superparamagnetic behavior, but the collective magnetic p...

Thermal Processes of Volatile RuO2 in Nanocrystalline Al2O3 Matrixes Involving γ→α Phase Transformation

Thermal processes of the volatile compound RuO2 in Al2O3 matrixes involving phase transformation have been investigated. In this model system, the secondary phase, rutile RuO2, can be formed and preserved in Al2O3 matrixes throughout a wide temperature range (400−1000 °C) due to the nanocrystalline Al2O3 that turns on or off matrix reactions with the surrounding RuO2. At 300−400 °C, nanocrystallites (2 nm) of γ-Al2O3 are formed in the presence of ruthenium. Single-phase RuO2 is formed from its precursor compound at 400 °C, while the intermediate species of Ru0 and RuO2·xH2O are detected at lower temperatures. The imbedded ruthenium shows a catalytic effect on the early formation of nanocrystalline γ-Al2O3 matrix, and the resultant high-surface-area matrix in turn allows the ruthenium to be fully oxidized to RuO2 in air. The γ-Al2O3 matrix is converted to α-phase at ca. 1000 °C. This transformation is promoted by the nanosized RuO2. A lowering of 60 °C in the transformation temperature has been observed wi...

Nucleation and Abnormal Grain Growth of Alpha-A12O3 in Gamma-Alumina Matrix

AbstractThe microstructures of reactive sputter-deposited alumina films have been studied by transmission electron microscopy. The as-deposited films contained γ-A12O3 phase in an amorphous alumina matrix. Annealing of the films at 1200° C for 2 h resulted in nucleation and concurrent anomalous grain growth of α-A12O3 in a polycrystaUine γ-Al2O3 matrix which exhibited a layered microstructure and was strongly textured along [001]. The grain sizes of α-A12O3 varied from 3 to 20 μm, while the average grain size of γ-A12O3 was only about 50 nm. It appears that the nucleation kinetics of a-A12O3 was slow. As a result, the abnormal grain growth of α-A12O3 proceeded by consuming surrounding γ-Al2O3 grains. An atomic model is presented to explain the origin of layered structure in γ-A12O3. The nucleation mechanism of a-A12O3 in γ-alumina matrix is suggested. Orientation relationships between γ- and α-A12O3 are reported. The anomalous grain growth of α- A12O3 is discussed in terms of γ/α interface boundary migration.