Characterization of metal-supported Al2O3 thin films by scanning electrochemical microscopy

In this paper, physiochemical properties of amorphous alumina thin films, grown by the metal organic chemical vapour deposition process on the surface of platinum (Pt/Al₂O₃) and stainless steel (SS/Al₂O₃), were investigated in aqueous media. The study was performed by the use of scanning electrochemical microscopy (SECM), which allowed obtaining information on uniformity, topography and chemical stability/reactivity of the alumina coatings with high spatial resolution. In particular, the effects due to local acid, base and fluoride ions attack on alumina layers of thickness of about 250 nm (in the Pt/Al₂O₃ sample) and 1000 nm (in the SS/Al₂O₃ sample) were investigated. In the acid and base attack, high concentrations of H₂SO₄ and KOH were electrogenerated locally by the use of a 25 μm diameter platinum microelectrode. The latter was also used as SECM tip to monitor the chemical effect on the alumina layers. It was found that, regardless of the thickness of the film, alumina provided good resistance against local attack of concentrated H₂SO₄; instead, the film dissolved when subjected to KOH attack. The dissolution rate depended on several experimental parameters, such as SECM-tip to substrate distance, electrolysis time and alumina film thickness. The alumina layer proved also relatively poor resistance to etching in 0.1 M NaF solutions.     

Mesoporous structure of Al2O3 prepared from poly(N-vinylpyrrolidone)-modified sols of hydrous metal oxides

Mesoporous alumina with a narrow effective pore diameter distribution has been prepared using poly(N-vinylpyrrolidone)-modified sols of hydrous Al₂O₃ and Al₂O₃-ZrO₂. We compare the microstructures of nanoporous aluminas prepared from electrochemically produced unmodified and modified sols of hydrous oxides and describe the formation of highly ordered mesoporous structures from a mixture of modified sols of hydrous metal oxides differing in chemical nature. Microstructural studies of uni-and biporous permeable nanosystems during heat treatment demonstrate that the pore diameter distribution in the mesoporous oxides prepared from the modified sols remains unchanged at calcination temperatures of 700°C and lower. The microstructure and phase composition of the oxides depend on the initial properties of the sols. Original Russian Text ? T.M. Zima, N.I. Baklanova, N.Z. Lyakhov, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 2, pp. 189–197.     

Sol-gel fabrication of compact, crack-free alumina film

Compact, crack-free alumina film was fabricated using an alumina sol with a high Al₂O₃ content. With the addition of ethylacetoacetate (CH₃COCH₂COOC₂H₅, EAcAc), the stable sol could be prepared with a molar ratio of aluminum sec-butoxide (Al(O-sec-Bu)₃, ASB) to water up to 1:25. It was found that EAcAc could notably decrease the surface tension of the liquid in the gel pores. The EAcAc modification layer on the colloidal particle retarded greatly the densification of the Al₂O₃ gel film and provided a long-lasting structural relaxation during heating. Therefore, the formation of cracks was effectively prevented in this alumina material. The alumina gel film contained a high Al₂O₃ content and there was a rather small mass loss during sintering. The critical thickness of Al₂O₃ sol-gel film was eight times higher than that could be achieved via the general sol-gel route and a film thicker than 0.8 μm was prepared by a single-step dipping operation.     

Preparation and Analysis of Porous Anodic Alumina Template on Silicon Substrate

Porous anodic alumina (PAA) template is widely used to prepare ordered nanostructure materials. But conventional PAA templates have been restricted for application in micro-electro-mechanical systems (MEMS) technology due to limitations such as shape and brittleness. In this article, a novel process of fabricating alumina porous template based on silicon wafer is described. Porous alumina films were formed by two-step anodization of aluminum layers sputter deposited on silicon wafer. The pore diameters range from 80 to 100 nm. The Pilling–Bedworth ratio of Al/Al₂O₃ was measured and calculated. Thickness of PAA template can be precisely controlled. This research provides an effective tool to nanofabrication in MEMS technology.     

The role of Ag particles deposited on TiO2 or Al2O3 self-organized nanoporous layers in their behavior as SERS-active and biomedical substrates

In this paper, we present recent results of investigations of hybrid materials consisting of nanoporous oxides layers loaded with Ag nanoparticles: Ag/TiO₂-n/Ti and Ag/Al₂O₃-n/Al (where “n-“stands for nanotubes), which could be used as active SERS substrates or as bioactive materials in medicine (implants). Simple electrochemical, chemical and physical methods appear suitable for fabricating such hybrid materials having different functional properties.     

Porous alumina ceramics prepared by slurry infiltration of expanded polystyrene beads

To produce highly porous MgO-doped alumina (Al₂O₃) ceramics, expanded polystyrene (EPS) beads were packed as a pore former and well-dispersed alumina slurry was used to infiltrate the pore space in the EPS bead compacts. The alumina particle-EPS bead green compacts were then heated to 1550°C in air to burn out the pore former and subsequently densify the MgO-doped alumina struts. The porous Al₂O₃ ceramics were featured with uniformly distributed open pore structures with porosities ranging from 72 to 78% and a pore interconnectivity of about 96%. The macropore size and the pore window size could be controlled by adjusting the size of the EPS beads and the contacting area between the EPS beads. The compressive strengths of the porous Al₂O₃ ceramics were in the range of 5.5–7.5 MPa, similar to those of cancellous bones (2–12 MPa). The porous alumina ceramics were further made bioactive after the dip coating of a sol-gel derived 58 S bioglass powder, followed by sintering at 1200°C.     

New roots to formation of nanostructures on glass surface through anodic oxidation of sputtered aluminum

New processes for the preparation of nanostructure on glass surfaces have been developed through anodic oxidation of sputtered aluminum. Aluminum thin film sputtered on a tin doped indium oxide (ITO) thin film on a glass surface was converted into alumina by anodic oxidation. The anodic alumina gave nanometer size pore array standing vertically on the glass surface. Kinds of acids used in the anodic oxidation changed the pore size drastically. The employment of phosphoric acid solution gave several tens nanometer size pores. Oxalic acid cases produced a few tens nanometer size pores and sulfuric acid solution provided a few nanometer size pores. The number of pores in a unit area could be changed with varying the applied voltage in the anodization and the pore sizes could be increased by phosphoric acid etching. The specimen consisting of a glass substrate with the alumina nanostructures on the surface could transmit UV and visible light. An etched specimen was dipped in a TiO₂ sol solution, resulting in the impregnation of TiO₂ sol into the pores of alumina layer. The TiO₂ sol was heated at ∼400 °C for 2 h, converting into anatase phase TiO₂. The specimens possessing TiO₂ film on the pore wall were transparent to the light in UV–Visible region. The electro deposition technique was applied to the introduction of Ni metal into pores, giving Ni nanorod array on the glass surface. The removal of the barrier layer alumina at the bottom of the pores was necessary to attain smooth electro deposition of Ni. The photo catalytic function of the specimens possessing TiO₂ nanotube array was investigated in the decomposition of acetaldehyde gas under the irradiation of UV light, showing that the rate of the decomposition was quite large.     

New roots to formation of nanostructures on glass surface through anodic oxidation of sputtered aluminum

New processes for the preparation of nanostructure on glass surfaces have been developed through anodic oxidation of sputtered aluminum. Aluminum thin film sputtered on a tin doped indium oxide (ITO) thin film on a glass surface was converted into alumina by anodic oxidation. The anodic alumina gave nanometer size pore array standing vertically on the glass surface. Kinds of acids used in the anodic oxidation changed the pore size drastically. The employment of phosphoric acid solution gave several tens nanometer size pores. Oxalic acid cases produced a few tens nanometer size pores and sulfuric acid solution provided a few nanometer size pores. The number of pores in a unit area could be changed with varying the applied voltage in the anodization and the pore sizes could be increased by phosphoric acid etching. The specimen consisting of a glass substrate with the alumina nanostructures on the surface could transmit UV and visible light. An etched specimen was dipped in a TiO₂ sol solution, resulting in the impregnation of TiO₂ sol into the pores of alumina layer. The TiO₂ sol was heated at ~400 °C for 2 h, converting into anatase phase TiO₂. The specimens possessing TiO₂ film on the pore wall were transparent to the light in UV–Visible region. The electro deposition technique was applied to the introduction of Ni metal into pores, giving Ni nanorod array on theglass surface. The removal of the barrier layer alumina at the bottom of the pores was necessary to attain smooth electro deposition of Ni. The photo catalytic function of the specimens possessing TiO₂ nanotube array was investigatedin the decomposition of acetaldehyde gas under the irradiation of UV light, showing that the rate of the decomposition was quite large.     

Metal distribution in alumina/aluminium composites synthesized by directed metal oxidation

The directed oxidation of molten aluminium alloys by vapour phase oxidants can be used to produce Al₂O₃/Al ceramic matrix composites. The toughness of these composites is determined by the amount and the nature of metal distribution in the composite. This paper addresses the problem of understanding the metal distribution in Al₂O₃/Al composites and its dependence on growth temperature. Electrical conductivities and microstructures of Al₂O₃/Al composites synthesized by directed oxidation of Al-5056 alloy are investigated. The high conductivity of the Al₂O₃/Al composite compared to sintered Al₂O₃-4 wt% MgO is shown as a proof of the presence of some continuous metal channels in the composite. The activation energy forthe diffusion of the dominant charge carrier in the oxide matrix is found to be 1.36 eV from the analysis of the conductivity data. Both the amount of metal in the composite and the extent of interconnection of the metal channels decrease with increasing growth temperature. The observed changes in microstructure with temperature can be explained by considering temperature variations of grain boundary energies in alumina and the alumina/aluminium interfacial energy. The metal content of the Al₂O₃/Al composites, prepared by directed oxidation of Al-5056 alloys, can be tailored by the choice of the growth temperature.     

Apparent zeta potential of nano-porous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> film deposited on different substrates in streaming potential method

Al₂O₃ films were coated on SUS304L stainless steel and fused silica substrates using chemical solution deposition. Continuous pores with a diameter of approximately 2 nm were observed through the measurement of the pore diameter distribution in the Al₂O₃ films using N₂ gas adsorption. The zeta potential of the Al₂O₃ film was measured using the streaming potential method, and the effect of the substrate material on the zeta potential was investigated. Initially, the measured zeta potential of the Al₂O₃ films was +?40 to +?50 mV, which was the same for both the SUS304L and fused silica substrates. However, the zeta potential of the Al₂O₃ film on the fused silica substrate decreased significantly with repeated measurements. Elemental analysis of the Al₂O₃ film in the depth direction using dynamic secondary ion mass spectroscopy showed that both K and Cl contents increased after zeta potential measurements were taken. Moreover, the zeta potential of a specimen impregnated with KCl electrolyte solution under vacuum exhibited no dependence on the number of measurements taken. It was thereby considered that the decrease in the zeta potential with repeated measurements was caused by the gradual penetration of the electrolyte solution into the pores, which eventually reached the fused silica substrate. This is a characteristic phenomenon observed when the zeta potential of a film that contains continuous pores is measured using the streaming potential method.     

Aluminium powder particles produced by SAMD technique: typical characteristics and correlations between processing conditions and powder size

Abstract

The present paper consists of two parts. In the first part the principles of a new method of metal powder production, termed 'solid assisted melt disintegration (SAMD)' are discussed and the typical characteristics of the produced powder are outlined. In the second part the effects of some processing parameters on the size distribution and mean diameter of the powder are reported. The SAMD method involves mixing solid particles (i.e. alumina) with the liquid aluminium alloy aided by mechanical agitation. The shear force induced by the impeller is transferred to the metal via the non-wetting solid medium and results in melt disintegration. The resulting mixture of aluminium droplets and alumina particles are subsequently cooled in air and screened through 300 μm sieve to separate alumina from solidified aluminium powder particles. The SAMD technique has demonstrated the capability to produce a wide particle size distribution. The small sized particles (i.e. <53 μm) exhibited irregular shapes, but larger ones were mostly spherical. These powder particles were dense (pore free) without attached satellite particles and exhibited a relatively coarse microstructure. The processing parameters investigated include the size of Al₂O₃ particles, Al₂O₃/Al weight ratio, stirring speed and stirring time. It was concluded that there exists an optimum value for each of the aforementioned parameters corresponding to a minimum in the mean particle size.     

The fabrication of tunable nanoporous oxide surfaces by block copolymer lithography and atomic layer deposition

Patterned nanoscale materials with controllable characteristic feature sizes and periodicity are of considerable interest in a wide range of fields, with various possible applications ranging from biomedical to nanoelectronic devices. Block-copolymer (BC)-based lithography is a powerful tool for the fabrication of uniform, densely spaced nanometer-scale features over large areas. Following this bottom-up approach, nanoporous polymeric films can be deposited on any type of substrate. The nanoporous periodic template can be transferred to the underlying substrate by dry anisotropic etching. Nevertheless the physical sizes of the polymeric mask represent an important limitation in the implementation of suitable lithographic protocols based on BC technology, since the diameter and the center-to-center distance of the pores cannot be varied independently in this class of materials. This problem could be overcome by combining block copolymer technology with atomic layer deposition (ALD): by means of BC-based lithography a nanoporous SiO2 template, with well-reproducible characteristic dimensions, can be fabricated and subsequently used as a backbone for the growth of perfectly conformal thin oxide films by ALD. In this work polystyrene-b-poly(methylmethacrylate) (PS-b-PMMA) BC and reactive ion etching are used to fabricate hexagonally packed 23 nm wide nanopores in a 50 nm thick SiO2 matrix. By ALD deposition of Al2O3 thin films onto the nanoporous SiO2 templates, nanostructured Al2O3 surfaces are obtained. By properly adjusting the thickness of the Al2O3 film the dimension of the pores in the oxide films is progressively reduced, with nanometer precision, from the original size down to complete filling of the pores, thus providing a simple and fast strategy for the fabrication of nanoporous Al2O3 surfaces with well-controllable feature size.     

Effect of Nb?+?Ti coating on the wetting behavior, interfacial microstructure, and mechanical properties of Al/Al2O3 joints

The subject of the work was to study the effect of Nb + Ti thin film deposited by PVD method on alumina substrates on the wetting behavior, bond strength properties, and structure of interface in the Al/Al₂O₃ joints. Applying the sessile drop method, the wetting behavior of molten Al (99,999%) on coated alumina substrates was studied in the temperature range between 953 and 1373 K under a vacuum of 0.2 mPa for 30 min of contact. The sessile drop samples were used to examine the interface structure, shear strength, and interfacial fracture toughness under the concentrated load. The introduction of the thin Nb + Ti film layer of 900 nm thickness: (1) greatly improves the wettability of alumina by molten Al at above 1223 K and the shear strength of Al/Al₂O₃ joints produced at 1223 K, (2) has positive effect on structure transformation in the interface and leads to fabrication of reliable metal–ceramic joints. Microstructural investigations of the interface indicated that the precipitates of Nb and Ti-rich intermetallic phases were formed at the Al/Al₂O₃ interface, which influenced strengthening of these joints. Hence a conclusion can be drawn that the interface structure influences the durability increase in Al/Al₂O₃ joints.     

Origins of hindrance in densification of Ag/Al2O3 composites

The densification and microstructure development were studied for Ag-matrix composites containing dispersed Al₂O₃ particles to examine the effect of inclusions on the densification of composites. The incorporation of Al₂O₃ particles into Ag matrix hindered densification. Microstructure observation revealed that pores larger than those in the matrix formed around Al₂O₃ particles during sintering. The pore morphology was dependent on the number density of Al₂O₃ particles. When the number density was low, pores remained around an Al₂O₃ particle and coalesced to a large circumferential void at high temperatures. When the number density was intermediate, clusters made of a few Al₂O₃ particles formed and pores within and around clusters remained up to high temperatures. When the number density was high, the distance between clusters became small and the clusters were connected, forming continuous pores (pore channels). The three-dimensional connectivity of Ag was decreased, and the shrinkage between Ag particles resulted in thickening of pore channels. The presence of these large pores was the origin of the hindrance in densification.     

Bond strength and microstructure investigation of Al2O3/Al/Al2O3 joints with surface modification of alumina by titanium

Joints of Al₂O₃/Al/Al₂O₃ are formed by liquid-state bonding of alumina substrates covered with thin titanium film of 800 nm thickness using an Al interlayer of 30 or 300 μm at 973 K under a vacuum of 0.2 mPa for 5 min and an applied pressure of 0.01 MPa. The bond strength of the joints is examined by a four-point bend testing at room temperature coupled with optical, scanning and transmission electron microscopy. Results show that: (i) bonding occurs due to the formation of a reactive interface on the metal side of the joint with the presence of Al₃Ti precipitates (ii) a decrease in Al layer thickness leads to stronger Al₂O₃/Al/Al₂O₃ bonds accompanied by a change of both the distribution of reaction products (Al₃Ti) in the region of the interface and the failure surface characteristics.     

The effects of β-alumina on the production of Al2O3/Al composites by the directed melt oxidation process

Directed melt oxidation (DMOX) of pure aluminium has been used to produce Al/Al₂O₃ composites by growth into a particulate alumina filler in the absence of any dopants apart from a -Al₂O₃ impurity in the filler. The microstructural development and mechanisms of growth of these composites have been investigated. It is shown that the Al₂O₃ filler used in this work has both chemical and physical effects on the reaction process. The -Al₂O₃ impurity introduces sodium into the system; this increases the wettability of alumina (both filler and oxidation reaction product) by molten aluminium, and initiates DMOX reactions. In addition, the filler particle size has an effect on the directed oxidation reaction. If the particle size is too fine, no oxidation growth takes place. Filler particles limit the ingress of oxygen through the reaction front so that AIN instead of Al₂O₃ may be formed in regions behind the main reaction front. Although such AIN production is seen when magnesium is used as a dopant to initiate DMOX reactions in the Al/Al₂O₃ system, it is more marked with sodium, because the latter has a greater effect on the wettability of alumina by aluminium.     

The influence of porous silica substrate on the properties of alumina films studied by X-ray reflection spectroscopy

The structure of alumina (Al₂O₃) films of various thicknesses grown by the atomic layer deposition method on porous silica (por-SiO₂) substrates has been studied using soft X-ray reflection spectroscopy. It is established that the synthesized films are amorphous and that the ratio of Al atoms with tetrahedral and octahedral coordinations in a film depends on its thickness. It can be suggested that thicker Al₂O₃ films contain a greater proportion of Al atoms with tetrahedral coordination.     

Conduction in MIM structures with an organic monomolecular layer at high electric fields

The influence of one organic monolayer (30 Å thick) on the electronic conduction through Al/Al₂O₃/Al structures at very high d.c. electric fields (E > 10⁶ V cm^?1) between 20 and 120°C was studied.One interposed monolayer reduces the injected current from aluminium into alumina by a factor of 100–1000. This reduction in the conduction current cannot be explained by one single potential barrier associated with the organic layer. It is necessary to take into account the presence of a natural oxide layer on the counterelectrode and the trapping of electrons in the organic layer or at the interface.     

Atomistic Modeling of Corrosion Resistance: A First Principles Study of O2 Reduction on the Al(111) Surface Covered with a Thin Hydroxylated Alumina Film

In the early stage of corrosion of Al or Al alloys (i.e., during the initiation of localized corrosion), an oxide film is generally present on the surface. This work investigates the possibility for a cathodic reaction to occur on these oxide films. We discuss realistic models of supported oxide films on Al(111) in order to disentangle the factors determining the reactivity towards O₂. Three components of the complex film formed on Al(111) can be identified: an ultrathin under‐stoichiometric Al_xO_y interface layer, an intermediate Al₂O₃ phase with γ‐alumina structure, and an hydroxylated AlOOH surface termination with boehmite structure. The electron transfer to O₂ molecules depends on the workfunction, Φe, of the metal/oxide interface and on the thickness of the inner Al₂O₃ phase. The electron transfer takes place both from the metal‐oxide interface and the oxide surface to the adsorbed O₂ molecule. Very important is the role of the hydroxyl groups at the surface: they eliminate the Al surface states and stabilize the surface; they allow the reduced O₂ ⁻ species to capture protons and transform into hydrogen peroxide in a non‐activated process. H₂O₂ is further reduced to two water molecules, in a series of two‐electron mechanisms. These reactions take place only when the internal alumina phase is ultrathin (here 0.2 nm). As soon as an Al₂O₃ inner layer develops (film thickness of about 1 nm), the film becomes unreactive and passivates the Al(111) surface. The results help to shed light on the complex reactions responsible for metal corrosion.     

Influence of surface modification of alumina on bond strength in Al2O3/Al/Al2O3 joints

The subject of the work was to study the effect of Ti thin film on alumina ceramic on mechanical strength and fracture character of Al₂O₃/Al/Al₂O₃ joints. The joints were formed by liquid state bonding of alumina substrates covered with titanium thin film of 800 nm thickness using Al interlayer of 30μm thickness at temperature of 973 K in a vacuum of 0.2 mPa for 5 min. The bend strength was measured by four–point bending test at room temperature. Scanning and transmission electron microscopy were applied for detailed characterization of interface structure and failure character of fractured joint surfaces. Result analysis has shown that application of the Ti thin film on alumina leads to decrease of bond strength properties of Al₂O₃/Al/Al₂O₃ joints along with the change either of structure and chemistry of interface or of failure character.