Γ-alumina Film Research Articles

How Does “Wormhole” Mesoporous γ-Alumina Matrix Direct the Morphology of Pt Nanocrystals?

This study demonstrates experimental evidence of formation of different discrete shaped Pt nanocrystals inside a mesoporous γ-alumina film at 500 °C. Shape and size of the finally formed nanocrystals are found to be dependent on the concentration of surfactant used to form the porous structure, as well as the precursor salt solution. Electron microscopic characterization at different temperature intervals during the growth of nanocrystals reveals the nucleation of small nanoparticles and their clustering and growth toward the formation of pyramidal, rhombus, cuboid, and bean-shaped nanocrystals in the mesoporous alumina matrix. The study finds that “wormhole” type mesoporous γ-alumina is the key medium for the generation of these nanostructures which are not formed while using hexagonally ordered mesoporous alumina. From the experimental evidence, we attempted to understand the formation mechanism of Pt nanocrystals in the mesoporous γ-alumina film matrix. This study is expected to open up new direction i...

Coloration of the aluminum alloy surface with dye emulsions while growing a plasma electrolytic oxide layer

Abstract Coloring a dense oxide layer is accomplished during plasma electrolytic oxidation (PEO) with an electrolyte mixed with dye emulsions. We demonstrate this coating technique using the aluminum disks of 6061 alloy and five synthetic dyes of red, yellow, orange, blue, and black color. The prepared dye emulsions are kinetically stable, containing negatively charged oil droplets of ~ 50 nm in size. To avoid color speckles, a barrier oxide layer must be first grown under the normal PEO conditions with an electrolyte without dye, followed by the growth of colored γ-alumina film with a stepwise increasing voltage of positive polarization in the electrolyte containing dye emulsion. Although the presence of surfactant in emulsion causes a large amount of bubbles that obscures the light of electric sparks, the spark sound can be heard clearly on site. These electric discharges can produce an uneven surface with many volcano craters of micrometer size which is the icon of PEO. Fortunately this typical unevenness can be smoothed out by synthetic dyes with proper voltage adjustment, resulting in a glossy surface.

A Mesoporous γ‐Alumina Film with Vertical Mesoporosity: The Unusual Conversion from a Im${\bar 3}$m Mesostructure to Vertically Oriented γ‐Alumina Nanowires

A Mesoporous γ‐Alumina Film with Vertical Mesoporosity: The Unusual Conversion from a <i>Im</i>${\bar 3}$<i>m</i> Mesostructure to Vertically Oriented γ‐Alumina Nanowires

Synthesis of highly ordered mesoporous alumina thin films and their framework crystallization to γ-alumina phase

Here we report the preparation of highly ordered mesoporous alumina films existing both as P6(3)/mmc and Fm-3m mesostructures by using triblock copolymer Pluronic P123 as the structure-directing agent. 2D grazing-incidence small-angle X-ray scattering (GI-SAXS) completely proves the existence of two different mesopore structures (i.e., [001]-oriented P6(3)/mmc and [111]-oriented Fm-3m symmetries). After calcination at 1000 °C, the amorphous alumina framework is successfully converted to γ-alumina crystals. During the crystallization process, large uniaxial shrinkage occurs along the direction perpendicular to the substrate with the retention of horizontal mesoscale periodicity, thereby resulting in formation of partially vertical mesoporosity in the film. Through detailed electron microscopic study, we discuss the formation mechanism for the vertical mesoporosity upon calcination. The obtained mesoporous γ-alumina film shows high thermal stability up to 1000 °C, which is highly useful in wide research areas such as catalyst supports and separators.

Ab-initio study on the γ-Al2O3 surfaces and interfaces.

ABSTRACTThe controlled growth of alumina films by atomic layer deposition (ALD) is of great interest to the electronics industry, as high-k dielectrics are being sought for the next-generation of MOSFETS. We present a theoretical study of the alumina polymorph γ-Al2O3 based on density functional theory (DFT). The predicted bulk structure for γ-Al2O3 has the Al vacancy sites widely separated, which is in agreement with other theoretical predictions. We estimated the energy of several γ-Al2O3 surfaces namely: (111), (110) and (110). The atomic and electronic structure of the most stable (111) surface is discussed and compared with α-Al2O3 (0001). The adsorption and dissociation of the H2O onto the (111) surface is considered and H-diffusion is simulated. Finally, a model for the γ-Al2O3 (111)/aluminium-hydroxide interface is proposed and considered as an intermediate stage in the γ-alumina film growth mechanism.

Gas permeation characteristics of silica/alumina composite membrane prepared by chemical vapor deposition

Amorphous silica membranes were deposited by thermal decomposition of tetraethoxysilane at 600–650 ‡C on a porous α-alumina tube with pore size of 110–180 nm or γ-alumina coated α-alumina tube with pore size of 6–8 nm.The forced cross-flow through the porous wall of the support was very effective in plugging macropores. The membranes formed on γ-alumina coated oc-alumina tube showed H2 permeances much higher than the SiO2 membranes formed on the α-alumina tube. This indicated that the γ-alumina film was effective in improving the H2 permeance and H2/N2 selectivity. The permeation tests with CO2, N2, CH4, C3H8 and i-C4H10 showed that a very small number of mesopores remained unplugged by the CVD. Permeation of hydrogen was explained by activated diffusion, and that of the other gases by Knudsen diffusion through the unplugged pores. Thus, the total permeance was composed of permeances due to the activated and Knudsen diffusion mechanisms. The contribution of Knudsen diffusion pores decreased to 0.02 when the γ-alumina film was modified at 650 ‡C until Pfe=50 Pa.

Preparation of a γ-Alumina Film Doped with Fine γ-Iron(III) Oxide Particles

A poly(vinyl alcohol)−alumina hydrate composite film prepared by using a sol−gel process was transparent in the visible region. The composite film was insoluble in water after being heated at about 100 °C but exhibited hydrophilicity. These characteristics are suitable for the deposition of Fe2O3·nH2O in the film by a counter diffusion method. By heating at 600 °C in air, this deposited film was transformed to a γ-Fe2O3-doped γ-Al2O3 film. No absorption peaks were observed in the visible region at a wavelength of >550 nm. The crystallite size could be controlled in the range 3−8 nm by changing the counter diffusion time. Due to the orientation of the alumina hydrate matrix, the γ-Fe2O3-doped film was more easily magnetized in the direction parallel to the film plane than to the perpendicular.

Pore size control and gas permeation kinetics of silica membranes by pyrolysis of phenyl-substituted ethoxysilanes with cross-flow through a porous support wall

A silica membrane was produced by chemical vapor deposition using tetraethoxysilane (TEOS), phenyltriethoxysilane (PTES) or diphenyldiethoxysilane (DPDES) as the Si source. Amorphous silica was deposited in the mesopores of a γ-alumina film coated on a porous α-alumina tube, by evacuating the reactant through the porous wall. Hydrogen permeance at a permeation temperature of 600°C was of the order of 10 −7 mol m −2 s −1 Pa −1, and was not greatly dependent on the Si sources. The silica membrane produced using TEOS contained micropores permeable to both helium and hydrogen, but CO 2 and larger molecules were only slightly permeated through those mesopores which were left unplugged. The silica membrane produced from DPDES showed a single-component CO 2 permeance equivalent to that of single-component He, and CO 2 N 2 selectivity was approximately 9 at a permeation temperature of 30°C. When a mixture of CO 2 and N 2 was fed, however, CO 2 permeance decreased to the level of N 2 permeance. The H 2 N 2 selectivity, determined from single-component permeances to H 2 and N 2, was approximately 100, and these permeances remained unchanged when an equimolar mixture of H 2 and N 2 was fed. Thus, the DPDES-derived membrane possessed two types of micropores, abundant pores through which helium and hydrogen permeated and a small number of pores in which molecules of CO 2 and N 2 were permeable but not able to pass one another. Neither meso or macropores remained in the DPDES membrane.

Formation of hydrogen permselective silica membrane for elevated temperature hydrogen recovery from a mixture containing steam

A silica membrane was formed by chemical vapor deposition using tetraethylorthosilicate in macropores of an α-alumina tube or a γ-alumina film coated on the α-alumina tube. The reactant was evacuated through the porous wall, and silica was deposited in the macropores at 600–700 °C. When the silica membrane was formed in a γ-alumina film coated on the α-alumina tube, hydrogen permeance at a permeation temperature of 600 °C was 3 × 10 −7 mol m −2 s −1 Pa −1, which was one order of magnitude higher than that of a membrane formed directly on the α-alumina tube. H 2/N 2 selectivity determined from the permeance of each component was 100–1000. To separate hydrogen selectively from abundant steam, however, a higher permselectivity was required. The membrane formed in the γ-alumina film at 650 °C showed a hydrogen permeance of 3 × 10 −8 mol m −2 s −1 Pa −1 and an H 2/H 2O selectivity of 7.6 at 400 °C.

Preparation of supported composite membrane by pyrolysis of polycarbosilane for gas separation at high temperature

The outer surface of an α-alumina tube was coated with a γ-alumina film formed by a sol-gel process. Then the tube was coated with polycarbosilane (PC), which was cured at 473 K and pyrolyzed at 623–823 K. The composite membrane prepared was ca. 1–1.4 μm in thickness and had no pinholes larger than several nm. For the composite membrane pyrolyzed at 623 K, the permeation of H 2 and N 2 at 283–673 K was controlled by the activated diffusion mechanism, and for the membrane pyrolyzed at 823 K by the Knudsen diffusion mechanism. The permeation of CO 2 was facilitated by the surface diffusion mechanism for all membranes prepared. The optimum pyrolysis temperature was 723 K from the viewpoints of the permeance of H 2 and the separation factor of H 2 to N 2, which were respectively 5.5 × 10 −7 mol m −2 s −1 Pa −1 and 7.2 at 673 K.

Experimental studies on pore size change of porous ceramic membranes after modification

Experimental results on pore size change of a microfiltration (MF) α-alumina membrane and an ultrafiltration (UF) γ-alumina membrane after modification by chemical vapor deposition (CVD) of solid oxides in the membrane pores are presented and explained using the results of a theoretical analysis. With an approx. 10-fold reduction in permeability, the average pore size of the MF membrane is found to increase after CVD modification, due to its relatively broader initial pore size distribution with a small amount of large pores and due to the particular CVD conditions (heterogeneous deposition mechanism) which give a pore narrowing rate independent of pore size. The effective pore size of the UF membrane appears to remain unchanged after modification with an approx. 50-fold reduction in permeability, as a result of the slit-shaped pores of the γ-alumina film and the particular modification conditions. The experimental and theoretical results suggest that, in order to reduce effectively the average pore size of a membrane by a modification process, the membrane should have a rather uniform pore size distribution, or the modification process should be conducted under conditions which give a pore narrowing rate proportional to the pore size.

Gas transport and separation with ceramic membranes. Part II. Synthesis and separation properties of microporous membranes

Non-supported microporous silica (amorphous) and titania thin films were made by the polymeric gel route. The titania system consisted of particles smaller than 5 nm. Reproducible modification of supported γ-alumina films with silica demands a strict control of every modification step. Silica films of 30–60 nm thickness on top of and presumably partly inside the γ-alumina film were realised. The permeabilities of helium and hydrogen through this film are activated, while the propylene permeability was below the detection limit. Separation factors of a H 2C 3H 6 mixture are larger than 200 at 200 °C with a flux of the preferentially hydrogen of 1.6 × 10 −6 mol/m 2-sec-Pa. The pores must be of molecular dimensions to realise this (< 1 nm diameter). Preliminary research shows that changes in the synthesis parameters result in higher activation energies and improved separation properties. The relation between synthesis, resulting microstructure and gas separation properties, however, is not yet fully understood.

Gas transport and separation with ceramic membranes. Part I. Multilayer diffusion and capillary condensation

Multilayer diffusion and capillary condensation of propylene on supported γ-alumina films greatly improved the permeability and selectivity. Multilayer diffusion, occurring at relative pressures of 0.4 to 0.8 strongly increased the permeability of 6 times the Knudsen permeability, yielding permeabilities of 3.2 × 10 −5 mol/m 2-sec-Pa. The occurrence of a maximum in the permeability coincides with blocking of the pore by adsorbate (capillary condensation). This point could be predicted, employing adsorption data and the slit shape form of the pore. Separation factors of 27 were obtained with a N 2N 3H 6 mixture and a supported γ-alumina film, with C 3H 6 the preferentially permeating component. This very effective separation is due to pore blocking by adsorbate. The separation factor increased to 85 after modification of the system with magnesia by the reservoir method. However, the permeability of propylene decreased by a factor of 20 to 1.6 × 10 −6 mol/m 2-sec-Pa.

Synthesis of ceramic membranes

Non-supported γ-alumina films are prepared from a boehmite colloidal suspension. After calcination at 600°C, the microstructure is characterized by a mean pore diameter of 3 nm, a porosity of 50% and a tortuosity of 5.5. The structure is formed by card packed, plate-shaped particles, giving rise to slit-shaped pores. Supported γ-alumina films, made by a slipcasting process using the same boehmite precursor, have a similar structure as the non-supported films. The slipcasting process is very sensitive to support characteristics and frequently yields defect films. Polyvinylalcohol (PVA) (molecular weight 72000 g mol-1) is added to the boehmite precursor (0.25 gg-1 film) to improve this process. This addition results in a less critical and better controllable drying and calcining procedure. The addition of PVA is necessary to slipcast defect free γ-alumina films on supports with pore diameters of 0.4 μm or smaller and on multi-layer supports. It has no significant effect on the resultant microstructure of the γ-alumina film, provided all PVA is removed by appropriate thermal treatment. The slipcasting rate is slower, resulting in thinner films of 3-5 μm at identical slipcasting times.

Synthesis of ceramic membranes

Non-supported γ-alumina films are prepared from a boehmite colloidal suspension. After calcination at 600°C, the microstructure is characterized by a mean pore diameter of 3 nm, a porosity of 50% and a tortuosity of 5.5. The structure is formed by card packed, plate-shaped particles, giving rise to slit-shaped pores. Supported γ-alumina films, made by a slipcasting process using the same boehmite precursor, have a similar structure as the non-supported films. The slipcasting process is very sensitive to support characteristics and frequently yields defect films. Polyvinylalcohol (PVA) (molecular weight 72000 g mol−1) is added to the boehmite precursor (0.25 gg−1 film) to improve this process. This addition results in a less critical and better controllable drying and calcining procedure. The addition of PVA is necessary to slipcast defect free γ-alumina films on supports with pore diameters of 0.4 μm or smaller and on multi-layer supports. It has no significant effect on the resultant microstructure of the γ-alumina film, provided all PVA is removed by appropriate thermal treatment. The slipcasting rate is slower, resulting in thinner films of 3–5 μm at identical slipcasting times.