Alumina Walls Research Articles

Insights into formation and properties of amorphous mesostructured alumina

Here, using liquid crystals as template, an EISA (evaporation-induced self-assembly) method has been used to synthesize amorphous mesostructured alumina. The effect of the synthesis parameters such as the presence of a chelating agent, the amounts of both porogen agent and alumina precursor on the materials properties have been investigated in details. Under the optimum conditions, after Soxhlet extraction, the obtained Al2O3 presents an HMS (Hexagonal Molecular Sieves) structure and a high specific surface area (≈400 m2/g) with a mesopore size distribution centered around 8.3 nm.Investigation of the thermal stability shows that the crystallization into γ-Al2O3 of the amorphous alumina walls delimiting mesopores begins after heating at 800 °C. We have also demonstrated that half of the carbon present in the as-synthesized material can be removed by washing with water or ethanol at room temperature instead of Soxhlet extraction.On the basis of SAXS, XRD and N2 adsorption-desorption analyses, we have shown that when treated into boiling water or exposed to water vapor the alteration of the mesostructure occurs quickly. Its collapse has been attributed to the crystallization of the amorphous alumina walls into aluminum hydroxides, mainly boehmite, upon the hydration of the alumina.Dual mesoporous alumina have been also obtained from micelles through a cooperative templating mechanism at low P123 amount and in the presence of citric acid. In that case both mesopores networks adopt a wormhole-like mesostructure.

Dual-Scale Porosity Alumina Structures Using Ceramic/Camphene Suspensions Containing Polymer Microspheres

This study demonstrates the utility of thermo-regulated phase separable alumina/camphene suspensions containing poly(methyl methacrylate) (PMMA) microspheres as porogens for the production of multi-scale porosity structures. The homogeneous suspension prepared at 60 °C could undergo phase separation during freezing at room temperature. This process resulted in the 3D networks of camphene crystals and alumina walls containing PMMA microspheres. As a consequence, relatively large dendritic pores with several tens of microns size could be created as the replica of frozen camphene crystals. In addition, after the removal of PMMA microspheres via heat-treatment, micron-sized small spherical pores could be generated in alumina walls. As the PMMA content with respect to the alumina content increased from 0 vol% to 40 vol%, while the camphene content in the suspensions was kept constant (70 vol%), the overall porosity increased from 45.7 ± 0.5 vol% to 71.4 ± 0.5 vol%. This increase in porosity is attributed to an increase in the fraction of spherical pores in the alumina walls. Thus, compressive strength decreased from 153 ± 18.3 MPa to 33 ± 7.2 MPa. In addition, multi-scale porosity alumina objects with a honeycomb structure comprising periodic hexagonal macrochannels surrounded by dual-scale porosity walls were constructed using a 3D plotting technique.

Fabrication of unique porous alumina films with extremely high porosity and an ultra-flat barrier layer by anodizing aluminum in sodium metaborate

Unique porous anodic aluminum oxide (PAAO) films with a completely flat barrier layer and an extremely porous loofah-like structure were fabricated by anodizing aluminum in an alkaline sodium metaborate solution. High-purity aluminum plates were anodized in a 0.3 M sodium metaborate solution at various applied voltages in the range of 0.1–200 V. A typical PAAO film with a spherical cap barrier layer was formed at voltages lower than 50 V, whereas a PAAO film with a flat barrier layer was formed at voltages higher than 100 V; this film formation was not based on the Keller-Hunter-Robinson model with the spherical barrier layer. The growth interface of the flat barrier layer exhibited an ultra-flat morphology with a minimum roughness value of 0.4 nm, which is much smaller than that of an electropolished aluminum surface. Such barrier layer morphology is expected to be formed by field-assisted dissolution without oxide flow. Although the current density slightly decreased with the applied voltage, a relatively higher current density of 6.8 Am−2 was still measured at the lowest voltage of 0.1 V. The alumina walls and the bottom barrier layer gradually thinned as the applied voltage decreased, and a loofah-like PAAO film with an ultra-high porosity of 0.93 was successfully fabricated at 0.1 V. The anodic oxide consisted of amorphous, anion-free aluminum oxide.

Discrete‐Continuum Duality of Architected Materials: Failure, Flaws, and Fracture

Abstract3D nano‐ and micro‐architected materials are resilient under compression; their susceptibility to flaws and fracture remain unexplored. This work reports the fabrication and tensile‐to‐failure response of hollow alumina nanolattices arranged into 5 µm octet‐truss unit cells. Some specimens contained through‐thickness center notches oriented at different angles to the loading direction, with a length‐over‐sample‐width ratio of 0.45. In situ tensile experiments reveal that for all orientations, failure initiates at the notch root, followed by instantaneous crack propagation along lattice planes orthogonal to extension. A tensile strength of 27.4 ± 0.7 MPa is highest for unnotched samples and decreases as notch orientation varies from 0° to 90° to its minimum, 7.2 ± 0.4 MPa; their specific tensile strength is ≈4 × higher than that for all other low‐density materials. Finite element simulations reproduce observed strengths and failure mechanisms: initial cracks always initiate at the nodal junctions with highest stress concentrations by tearing of alumina walls at the nodes. Subsequent crack propagation shifts maximum stress concentration to the nodes along lattice plane orthogonal to the loading direction. A modified analytical fracture model based on the effective notch length predicts tensile strengths consistent with experiments. These findings imply that continuum fracture mechanics can predict failure in nano‐architected materials, which helps develop advanced materials through informed architectural design.

Exploration for the Self-ordering of Porous Alumina Fabricated via Anodizing in Etidronic Acid

Ordered porous alumina (OPA) with large-scale circular and hexagonal pores was fabricated via etidronic acid anodizing. High-purity aluminum plates were anodized in 0.2–4.2M etidronic acid solution at 145–310V and 288–323K. Self-ordering of porous alumina was observed at 165V and 313K in 4.2M, at 205V and 303K in 1.0M, and at 260V and 298K in 0.2M, and the cell diameter was measured to be 400–640nm. The ordering potential difference decreased with the electrolyte concentration increasing. OPA without an intercrossing nanostructure could be fabricated on a nanostructured aluminum surface via two-step anodizing. Subsequent pore-widening in etidronic acid solution caused the circular dissolution of anodic oxide and the expansion of pore diameters to 470nm. The shape of the pores was subsequently changed to a hexagon from a circle via long-term pore-widening, and a honeycomb structure with narrow alumina walls and hexagonal pores measuring 590nm in its long-axis was formed in the porous alumina. Transition of the nanostructure configuration during pore-widening corresponded to differences in the incorporated phosphorus distribution originating from the etidronic acid anions.

Ceramic/polymer interpenetrating networks exhibiting increased ionic conductivity with temperature control of ion conduction for thermal runaway protection

A novel polymer electrolyte nanostructure consisting of poly(ethylene oxide), PEO, complexed with lithium triflate placed in a nanoporous ceramic membrane was fabricated. The PEO electrolyte was placed in 200 nm pores of AAO filtration membranes and took the form of “sleeves” of polymer lining the 200 nm pores. The confinement of the electrolyte in this nanostructure somewhat increased the ion conduction of the polymer electrolyte compared to a non-confined PEO electrolyte. This increase was attributed to reduction in PEO crystallinity and polymer chain ordering resulting from interaction with the alumina walls of the nanoporous membranes. As expected, when the non-confined PEO sample was heated to the melting temperature, a large increase in ion conduction, resulting from the molten phase, was seen. Interestingly, the polymer confined in nanopores did not exhibit a large increase in conductivity. This was once again attributed to interaction with the alumina walls of the nanoporous membranes, hindering ion motion as compared to molten pure PEO polymer. The value to thermal runaway prevention of lower conductivities after going through the melt will be discussed. TGA studies indicate that the nanocomposite electrolyte is more thermally stable at high temperatures than pure PEO electrolyte. At 450 °C, 80% of the pure PEO was lost to thermal degradation while the nanocomposite electrolyte lost only 30% of its weight by this degradation process. This increased thermal stability was attributed to polymer confining wall interactions and a nanoscale passive thermal management system created by the heat absorbing AAO matrix and the transfer of heat to the gas in the open tubes of the polymer electrolyte.

Three‐dimensional Ceramic/Camphene‐based Coextrusion for Unidirectionally Macrochanneled Alumina Ceramics with Controlled Porous Walls

We report the utility of three‐dimensional ceramic/camphene‐based coextrusion, newly developed in this study, for the production of unidirectionally macrochanneled alumina ceramics with three‐dimensionally interconnected porous alumina walls. In this technique, a continuous ceramic/camphene filament with a diameter of 1 mm, comprised of a pure camphene core and a frozen alumina/camphene shell, was produced by the coextrusion process and then deposited in a layer‐by‐layer sequence using a computer‐controlled 3‐axis moving table. Unidirectionally aligned macrochannels (~400 μm in diameter) and three‐dimensionally interconnected pores (several tens of micrometers in size) in the alumina walls were created by removing the camphene core and the camphene dendrites formed in the alumina/camphene region, respectively. The sample showed much higher compressive strength in the macrochannel direction than in the perpendicular direction. In addition, the compressive strength of the sample could increase with an increase in initial alumina content owing to a decrease in the total porosity.

Incorporation of copper chelate ions into anodic alumina walls

Anodic aluminum oxide (AAO) is typically applied as a template for nanofabrication. It was found that during anodization the anions of electrolytes are being incorporated into the nanopore walls giving rise to an emission in the UV/VIS range. In this paper, incorporation of copper-EDTA chelate ions was studied. The anodization of all studied samples was carried out in 0.02M CuSO4+0.02M EDTA in 2wt% sulfuric acid, which has provided suitable conditions for self-organized two-step process of AAO growth. Nanoporous alumina was formed and studied with Scanning Electron Microscopy. Chemical composition analysis of the anodic alumina revealed that the greater the oxide growth rate, the more copper is being incorporated. The photoluminescence studies confirmed the presence of copper in the AAO pore walls.

Fabrication of anodic aluminum oxide with incorporated chromate ions

The anodization of aluminum in 0.3M chromic acid is studied. The influence of operating conditions (like anodizing voltage and electrolyte's temperature) on the nanoporous anodic aluminum oxide geometry (including pore diameter, interpore distance, the oxide layer thickness and pores density) is thoroughly investigated. The results revealed typical correlations of the anodic alumina nanopore geometry with operating conditions, such as linear increase of pore diameter and interpore distance with anodizing voltage. The anodic aluminum oxide is characterized by a low pores arrangement, as determined by Fast Fourier transforms analyses of the FE-SEM images, which translates into a high concentration of oxygen vacancies. Moreover, an optimal experimental condition where chromate ions are being successfully incorporated into the anodic alumina walls, have been determined: the higher oxide growth rate the more chromate ions are being trapped. The trapped chromate ions and a high concentration of oxygen vacancies make the anodic aluminum oxide a promising luminescent material.

Novel Ceramic/Camphene‐Based Co‐Extrusion for Highly Aligned Porous Alumina Ceramic Tubes

We herein propose a novel way of producing highly aligned porous alumina tubes by co‐extruding an initial feed rod, which comprised of a pure camphene core and a frozen alumina/camphene shell. This new and simple technique produced alumina tubes (~4.4 ± 0.1 mm in outer diameter) having uniform alumina walls (~1.1 ± 0.12 mm in thickness) with highly aligned pores, which were created by removing the extensively elongated camphene dendrites in the extruded alumina/camphene shell. Furthermore, the heat treatment of the extruded bodies at 33°C, which is close to the freezing point of the alumina/camphene slurry, led to a considerable increase in the size of the aligned pores formed in the alumina walls, from ~<5 to ~54 μm with increasing heat‐treatment time from 0 to 6 h. The compressive strength also increased significantly from 21 ± 1.6 to 61 ± 11.6 MPa, which was attributed mainly to the achievement of well densified alumina walls.

Advances in Steady Inductive Helicity Injection for Plasma Startup and Toroidal Current Drive

The Helicity Injected Torus with Steady Inductive Helicity Injection (HIT-SI) has achieved a breakthrough in the development of a new method of toroidal plasma startup and current drive. HIT-SI accomplishes helicity injection current drive on a spheromak of major radius 0.3 m with two injectors driven sinusoidally at 14.5 kHz. Results include the first sustainment of toroidal plasma current of over 50 kA at up to 3 times the injected currents added in quadrature. Separatrix toroidal currents—currents not linking the helicity injectors—are sustained at up to 40 kA. Toroidal currents persist for up to 0.65 ms after the injectors are turned off. Results are achieved after helium operations condition the alumina plasma-facing surface. The conditioned alumina walls then act to pump the deuterium, thereby reducing the spheromak density. High performance discharges (Itor/Iinj ≥ 2) are characterized by a decrease in the n = 1 mode activity measured by surface probes near the midplane of the confinement volume. Suppression of internal magnetic fields (measured by an internal probe) below the predicted Taylor equilibrium indicates that the magnetic fields in HIT-SI can no longer be described by a zero pressure, constant λ equilibrium.

Porous alumina ceramics with highly aligned pores by heat-treating extruded alumina/camphene body at temperature near its solidification point

Abstract This study reports a novel way of increasing the pore size of highly aligned porous alumina ceramics by heat-treating an extruded alumina/camphene body at a temperature near its solidification point. The pore size obtained increased remarkably from 51 ± 8 to 125 ± 27 μm with increasing heat-treatment time from 1 to 24 h, due to the continuative overgrowth of the camphene dendrites during heat-treatment, while a highly aligned porous structure was preserved. In addition, interestingly, this heat-treatment enabled alumina walls to be densified quite well, whereas porous walls were observed in the sample produced without heat-treatment, which led to a considerable increase in compressive strength. The sample produced with a heat-treatment time of 12 h showed a high compressive strength of 11.6 ± 1.2 MPa at a porosity of approximately 84 vol%, which was much higher than that (0.28 ± 0.1 MPa) of the sample produced without heat-treatment.

Macroporous Alumina Ceramics with Aligned Microporous Walls by Unidirectionally Freezing Foamed Aqueous Ceramic Suspensions

This paper reports a novel way of producing macroporous ceramics with aligned microporous walls by unidirectionally freezing foamed aqueous ceramic suspensions. The stability of the air bubbles introduced by magnetic stirring was improved remarkably using polyvinyl alcohol (PVA) as both a binder and emulsifying agent. Furthermore, the unidirectional freeze casting method could not only trap the air bubbles in the solidified sample successively but also induce the preferential growth of ice dendrites. This endowed the samples with uniform macropores surrounded by aligned microporous alumina walls after freeze drying and sintering. The sample produced with a PVA content of 8 wt% showed a high compressive strength of 4.6 ± 1.6 MPa at a high porosity of 91 vol%.

Synthesis and acidic catalytic properties of ordered mesoporous alumina–tungstophosphoric acid composites

A series of well-ordered mesoporous alumina–tungstophosphoric (HPW) acid composite frameworks has been prepared by a sol–gel copolymerization route in the presence of non-ionic surfactants. The resulting materials feature a high loading of HPW acids (up to ∼53 wt%) in composite framework and possess hexagonal p6mm pore structure with uniform large pores. The mesoscopic order of these structures was evidenced by SAXS analysis, TEM images and N2 physisorption measurements. The composite materials exhibited BET surface areas in the range of 54–71 m2 g−1, total pore volumes in the range of 0.11–0.14 cm3 g−1 and quite narrow pore size distributions with peak maxima in the 7.1–8.3 nm range. The Keggin clusters were incorporated in mesoporous alumina walls by strong chemical bonds according to the FT-IR and UV/vis spectroscopy analysis. This chemical linkage of HPW to the alumina matrix is responsible for the outstanding stability of these materials against water-leaching. The mesoporous surfaces exhibited exceptional acidity that arises from the unique alumina–HPW composite structure. As the loading of HPW increases, the surface acidic character of the composites enhanced, and this is reflected in the higher catalytic activity towards isopropanol conversion.

Thermal degradation of iron chelate complexes adsorbed on mesoporous silica and alumina

Nanometric Fe2O3 particles could be inserted inside the internal pore volume of SBA-15 mesoporous silica and mesoporous alumina supports, when Fe(III) chelates (EDTA, gluconate or citrate) were used as impregnating precursors. The oxidative degradation of the chelating anions was followed by combined TG-DTA. Strong chelate-SiOH interactions (case of bulky EDTA), favored by the mesopore curvature, yield sub-nanometric extremely well dispersed Fe2O3 particles preferentially located at the micropore mouths (confinement effect). Fe2O3 even more strongly interacts with alumina walls, generating either (Fe,Al)2O3 mixed phases or Fe-aluminate micro domains. These iron-based mesoporous alumina composites proved very active catalysts in total oxidation of phenol at ambient conditions, with extremely low iron leaching (0.2%).

Characterization and field emission properties of lanthanum monosulfide nanoprotrusion arrays obtained by pulsed laser deposition on self-assembled nanoporous alumina templates

Three distinct types of nanostructures—nanodomes, nanodots, and nanowires—have been simultaneously self-assembled by pulsed laser deposition of lanthanum monosulfide on anodic alumina films containing hexagonal arrays of pores about 50nm wide and 500nm deep. The nanostructures have been characterized by x-ray diffraction, atomic force microscopy (AFM), and field emission scanning electron microscopy (FE-SEM). Nanodomes preferentially grow on the boundary separating regions (grains) of the alumina template that have near perfect pore ordering, and their density is ∼109∕cm2. The diameter of a nanodome at the base is about 100nm and their aspect ratio (height/diameter at the base) is between 1 and 3. Additionally, nanodots nucleate on top of the alumina walls that separate adjacent pores. They have a diameter of ∼50nm, a density equal to the pore density (1010∕cm2), and an aspect ratio less than 1. Finally, cross sectional FE-SEM images of the templates indicate that LaS nanowires grow inside the pores with a density of 1010∕cm2. They have a diameter of 50nm and a maximum length equal to the length of the pores (∼500nm). The field emission properties of the LaS nanodomes and nanodots have been analyzed via the scanning anode field emission microscopy technique (SAFEM). For a fixed SAFEM probe to cathode distance, the applied voltage necessary to extract the same FE current is found to be ∼3.5 times less for a LaS thin film deposited on alumina templates compared to the value recorded for LaS thin films deposited on Silicon substrates. Assuming a LaS work function of ∼1eV (as recorded for LaS thin films grown on silicon substrates), a field enhancement factor of ∼5.8 is extracted for the nanoscale emitters from Fowler-Nordheim plots of the FE data. The field enhancement effect accrues from the concentration of electric field lines at the tip of the nanodome and nanodot emitters. The value of 5.8 can be correlated to an aspect ratio of 2.7 for the dominant emitter, which is in good agreement with the aspect ratio of the tallest nanodomes observed in AFM measurements.

Effect of Polystyrene Addition on Freeze Casting of Ceramic/Camphene Slurry for Ultra‐High Porosity Ceramics with Aligned Pore Channels

We investigated the effect of polystyrene (PS) addition on the freezing behavior of a very dilute alumina/camphene slurry with an initial solid loading of 5 vol% for the fabrication of ultra‐high porosity ceramics with aligned pore channels. To accomplish this, slurries with various PS contents (10, 20, and 30 vol% in relation to the alumina powders) were prepared by ball milling at 60°C and then cast into molds at a constant temperature of 20°C. After removing the frozen camphene, the samples were heat treated to burn out the organic phases and sinter the alumina walls. The addition of the PS binder remarkably enhanced the green strength of the sample, making it possible to handle it without difficulty. All of the sintered samples showed ultra‐high porosities of >88% without the collapse of the porous structure, wherein the pore channels were completely interconnected. Three distinctive zones having different pore structures were observed, viz. the outer shell with elongated pores, the inner zone with long aligned pore channels, and the center with equiaxed pores. The content of the PS polymer significantly affected the pore morphologies in the three distinctive zones. In addition, it was found that the addition of the PS polymer was highly beneficial to the alignment of the pore channels.

Freezing Dilute Ceramic/Camphene Slurry for Ultra‐High Porosity Ceramics with Completely Interconnected Pore Networks

Highly porous alumina ceramics with completely interconnected pore channels were fabricated by freezing dilute alumina/camphene slurries with solid loadings ranging from 5 to 20 vol%. This method fundamentally made full use of the three‐dimensional camphene dendritic network for producing interconnected pore channels and the concentrated alumina powder network for achieving dense alumina walls. Firstly, alumina/camphene slurries were prepared at 60°C using ball milling and then cast into molds at 20°C. After subliming the frozen camphene, the samples were sintered at 1400°C for 5 h. This method enabled us to freeze very dilute ceramic slurries with a low solid loading of ≤20 vol% without the collapse of the sample after sintering. As the initial solid loading decreased from 20 to 5 vol%, the porosity linearly increased from 66% to 90% with an increase in the pore size, while completely interconnected pore networks were obtained in all cases. In addition, the free surfaces of the alumina walls showed full densification after sintering even at a low temperature of 1400°C, while some pores were present in the inner regions of the alumina walls.

Vertically Aligned Nanopillar Arrays with Hard Skins Using Anodic Aluminum Oxide for Nano Imprint Lithography

Master molds with nano-pillar structures were fabricated by using anodic aluminum oxide (AAO) for nano-imprint lithography (NIL). The proposed method consists of (1) AAO fabrication, (2) transfer of the thin AAO layer onto a substrate, (3) aluminum sputtering, (4) aluminum melting, and (5) removal of the thin AAO layer. Since the sputtered aluminum penetrated into the porous alumina walls of the AAO template during the melting step, the pillars had skin layers of aluminum/alumina composite which added additional mechanical strength and chemical resistance. The diameter of a pillar in the nano-pillar master mold was larger than that of the hole diameter of the original AAO template because of the hard skin of aluminum/alumina composite layer which covers the pillar surface. The pillar structure could be used as a mold for NIL, resulting in a regular pore array on a polymer film.

Anodized Aluminum Oxide Films as Nanostructured Template for the Grow of Carbon Nanotubes

AbstractA summary of our recent work related to the growth of carbon nanotubes (CNTs) inside the pores of anodized alumina oxide (AAO) films is presented. The surface topography of the AAO film was studied by atomic force microscopy, while the internal pore structure of the AOO templates was concluded from the shape of the CNTs obtained. The effect of the different experimental approaches and of the anodization voltage value on the features of the pores and CNTs is also discussed, as well as the catalytic effect in the acetylene pyrolysis of the Co particles and internal alumina walls. The CNTs wall structure observed by HRTEM and its implication on some physico-chemical properties of the CNTs are also presented.