Showing papers on "Aluminium oxide published in 2011"

The influence of carbon nanotube (CNT) morphology and diameter on the processing and properties of CNT-reinforced aluminium composites

Abstract: Ball milling was used to disperse MWCNTs of two different morphologies (stiff and straight vs. bent and entangled) and diameters (very large diameter and 3.5 times smaller diameter) in aluminium powders, which were subsequently hot consolidated by hot extrusion. Characterization of the produced composites revealed that the CNT morphology plays an important role in affecting dispersion. It was found that the smaller diameter bent and entangled CNTs were more difficult to disperse with increase in CNT content compared to the larger diameter stiff and straight ones; which in turn affected the tensile properties and hardness of the composites. Furthermore, cold welding of the milled powders as well as carbide formation in the final composite was found to depend on the CNT diameter. The smaller diameter CNTs – having a larger effective interfacial contact area with the aluminium matrix compared to the larger ones for a given CNT wt.% – were found to reduce particle welding during milling and to be more affected by carbide formation. Nano-sized particles of aluminium oxide as well as nano-rods of aluminium carbide, in addition to CNT damage were observed upon TEM analysis of the smaller diameter CNTs.

Ultra-Thin Aluminium Oxide Films Deposited by Plasma-Enhanced Atomic Layer Deposition for Corrosion Protection

Abstract: We have employed plasma-enhanced and thermal atomic layer deposition (ALD) within the temperature range of 50-150°C for the deposition of ultra-thin (10-50 nm) Al 2 O 3 films on 100Cr6 steel and aluminium Al2024-T3 alloys. [Al(CH 2 ) 3 ] was used as the precursor with either an O 2 plasma or water as co-reactants. Neutral salt spray tests showed that the thicker films offered the best corrosion-resistance. Using cyclic voltametry, the 50 nm films were found to be the least porous (<0.5%). For 10 nm thick films, plasma-enhanced ALD afforded a lower porosity and higher film density than thermal ALD. ToF-SIMS measurements on 100Cr6 showed that the main 'bulk' of the films contained very few impurities, but OH and C were observed at the interfaces. TEM confirmed that the films were conformal on all substrates and the adhesion was excellent for the films deposited by plasma-enhanced ALD but not for thermal ALD, as delamination was observed. On the basis of these and other results, the prospects of the application of ALD films for corrosion protection, and the use of plasma-enhanced ALD to promote their nucleation, is discussed.

Modelling of the thermal conductivity in polymer nanocomposites and the impact of the interface between filler and matrix

Abstract: In this paper the thermal conductivity of epoxy-based composite materials is analysed. Two and three-phase Lewis–Nielsen models are proposed for fitting the experimental values of the thermal conductivity of epoxy-based polymer composites. Various inorganic nano- and microparticles were used, namely aluminium oxide, aluminium nitride, magnesium oxide and silicon dioxide with average particle size between 20 nm and 20?m. It is shown that the filler–matrix interface plays a dominant role in the thermal conduction process of the nanocomposites. The two-phase model was proposed as an initial step for describing systems containing 2 constituents, i.e. an epoxy matrix and an inorganic filler. The three-phase model was introduced to specifically address the properties of the interfacial zone between the host polymer and the surface modified nanoparticles.

Phase separation of a binary liquid in anodic aluminium oxide templates: a structural study by small angle neutron scattering.

Abstract: The radial nanostructure of the binary liquid triethylamine/water confined in 60 nm diameter independent cylindrical pores of anodic aluminium oxide membranes is studied by small angle neutron scattering. It is shown that composition inhomogeneities are present in the confined mixtures well below the bulk critical point. An analysis of the neutron scattering form factor reveals the existence of an adsorbed water layer of a few nanometers at the liquid/alumina interface, coexisting with a TEA-rich phase in the core.

Effect of solvent composition on oxide morphology during flame spray pyrolysis of metal nitrates

Abstract: The effect of solvent composition on particle formation during flame spray pyrolysis of inexpensive metal-nitrates has been investigated for alumina, iron oxide, cobalt oxide, zinc oxide and magnesium oxide. The as-prepared materials were characterized by electron microscopy, nitrogen adsorption, X-ray diffraction (XRD) and disc centrifugation (XDC). The influence of solvent parameters such as boiling point, combustion enthalpy and chemical reactivity on formation of either homogeneous nanoparticles by evaporation/nucleation/coagulation (gas-to-particle conversion) or large particles through precipitation and conversion within the sprayed droplets (droplet-to-particle conversion) is discussed. For Al2O3, Fe2O3, Co3O4 and partly also MgO, the presence of a carboxylic acid in the FSP solution resulted in homogeneous nanoparticles. This is attributed to formation of volatile metal carboxylates in solution as evidenced by attenuated total reflectance spectroscopy (ATR). For ZnO and MgO rather homogeneous nanoparticles were formed regardless of solvent composition. For ZnO this is attributed to its relatively low dissociation temperature compared to other oxides. While for MgO this is traced to the high decomposition temperature of Mg(NO3)2 together with Mg(OH)2 ↔ MgO transformations. Cobalt oxide (Co3O4) nanoparticles made by FSP were not aggregated but rather loosely agglomerated as determined by the excellent agreement between XRD- and XDC-derived crystallite and particle sizes, respectively, pointing out the potential of FSP to make non-aggregated particles.

Origin of self-organisation in porous anodic alumina films derived from analogy with Rayleigh–Bénard convection cells

Abstract: Porous anodic aluminium oxide (PAOX) has different practical applications (e.g. filters with uniform pore sizes, adsorbers, porous templates for functional nanomaterials), but the formation mechanism is still poorly understood. Equal-sized hexagonally ordered pores are formed during anodic oxidation of aluminium in water solutions of some acids at certain concentrations and temperatures, and comparatively high electrode potentials. Today, a limited range of pore diameters and the degree of hexagonal ordering are reached with empirically found conditions. Here, a theoretical model explaining the appearance of honeycomb structure in porous anodic alumina is presented. The proposed mechanism is based on a dissipative self-organization process, but not on the earlier accepted field-assisted dissolution of pre-formed dense alumina. Our analysis rests on the concept that electrolyte currents near aluminium anode are organized in the same way as well-known Rayleigh–Benard convection currents. A simple yet effective way to predict pore formation in unexplored electrolytes is suggested. The validity of theoretical considerations is experimentally confirmed by the growth of hexagonally arranged porous alumina in a new electrolyte–aqueous formic acid solution.

Controlled growth of CNT in mesoporous AAO through optimized conditions for membrane preparation and CVD operation.

Abstract: Anodic aluminium oxide (RAAO) membranes with a mesoporous structure were prepared under strictly controlling experimental process conditions, and physically and chemically characterized by a wide range of experimental techniques. Commercial anodic aluminium oxide (CAAO) membranes were also investigated for comparison. We demonstrated that RAAO membranes have lower content of both water and phosphorus and showed better porosity shape than CAAO. The RAAO membranes were used for template growth of carbon nanotubes (CNT) inside its pores by ethylene chemical vapour deposition (CVD) in the absence of a catalyst. A composite material, containing one nanotube for each channel, having the same length as the membrane thickness and an external diameter close to the diameter of the membrane holes, was obtained. Yield, selectivity and quality of CNTs in terms of diameter, length and arrangement (i.e. number of tubes for each channel) were optimized by investigating the effect of changing the experimental conditions for the CVD process. We showed that upon thermal treatment RAAO membranes were made up of crystallized allotropic alumina phases, which govern the subsequent CNT growth, because of their catalytic activity, likely due to their Lewis acidity. The strict control of experimental conditions for membrane preparation and CNT growth allowed us to enhance the carbon structural order, which is a critical requisite for CNT application as a substitute for copper in novel nano-interconnects.

Utilisation of continuous atomic layer deposition process for barrier enhancement of extrusion-coated paper

Abstract: The feasibility of a novel continuous atomic layer deposition process in improving the barrier properties of extrusion-coated papers was investigated. The polymer coatings on paper were low-density polyethylene, polypropylene, polyethylene terephtalate and polylactide. The new method was tested by depositing 100 nm aluminium oxide layers on the polymer side of the structures. According to test results, the aluminium oxide layer produced significant barrier improvements. The water vapour and oxygen transmission rates measured were approximately 3–5 times lower than those of the untreated samples. Even better improvements were found for the water vapour transmission rates of polyethylene terephtalate and polylactide coated papers being over 10 times lower than for the untreated structures. It is proposed that the better water vapour barrier improvements were found because of these polymers' disposition to water sorption in the non-processed samples. The continuous atomic layer deposition process caused also considerable changes in the topography and morphology of some polymers, which reduced the barrier properties and applicability of the structures. Further research is needed to enable the use of lower process temperatures in the continuous atomic layer deposition process, which would improve the feasibility of the new method.

Opposite effect of Al on the performances of MoO3/SiO2-Al2O3 catalysts in the metathesis and in the partial oxidation of propene

Abstract: The selective oxidation and the metathesis of light alkenes are two important catalytic reactions for the petrochemical industry. This paper highlights the contrasting effect of alumina on the catalytic behaviour of silica and silica-alumina supported molybdenum oxide catalysts in these two reactions. Model MoO3/SiO2-Al2O3 catalysts with ca. 6% MoO3 weight loading are prepared via wet impregnation of ammonium heptamolybdate on a set of amorphous mesoporous silica-alumina supports with silica weight content comprised between 100% and 75%. The samples are characterized by ICP-AES, N2-physisorption, XRD, NH3-chemisorption and XPS and are evaluated in the selective oxidation of propene to acrolein and in its self-metathesis to form butene and ethene. The addition of aluminium oxide into silicon oxide increases the acidity of the support and of the catalyst. It also affects the nature of the deposited species and the dispersion of Mo. Overall, the effect of Al is negative in the case of the partial oxidation of propene because it favours over-oxidation towards carbon oxides. On the contrary, the presence of Al is crucial for the metathesis reaction. It appears that the acidity, created by the presence of alumina in silica, is beneficial for the metathesis reaction at low temperature. An optimum of activity is found for the catalyst supported on the silica-alumina containing 15 wt% of Al2O3.

Phase separation of a binary liquid in anodic aluminium oxide templates : A structural study by small angle neutron scattering (Regular Article)

Abstract: The radial nanostructure of the binary liquid triethylamine/water confined in 60 nm diameter independent cylindrical pores of anodic aluminium oxide membranes is studied by small angle neutron scattering. It is shown that composition inhomogeneities are present in the confined mixtures well below the bulk critical point. An analysis of the neutron scattering form factor reveals the existence of an adsorbed water layer of a few nanometers at the liquid/alumina interface, coexisting with a TEA-rich phase in the core.

Morphology, thermal, electrical and electrochemical stability of nano aluminium-oxide-filled polyvinyl alcohol composite gel electrolyte

Abstract: In the present work, an attempt has been made to develop nano aluminium oxide (Al2O3)-filled polyvinyl alcohol (PVA) composite gel electrolytes. Surface morphological studies, thermal behaviour, electrochemical stability and electrical characterization of these composite gel electrolytes have been performed. An increase in the concentration of Al2O3 in composite gel electrolytes increases the amorphous characteristics of pure PVA. Bulk conductivity of composite gel electrolytes increases by an order of magnitude on addition of a nano filler. Maximum conductivity of 5·81 × 10−2 S/cm is observed for 6 wt% Al2O3-filled polymer gel composite electrolytes. Temperature dependence of electrical conductivity shows a combination of Arrhenius and Vogel-Tamman-Fulcher (VTF) nature. Maximum current stability during oxidation and reduction cycle is noticed for 6 wt% Al2O3-filled PVA composite electrolyte, viz. ±1·65 V.

Pore diameter control in anodic titanium and aluminium oxides

Abstract: A nonlinear relation between the pore diameter and anodising voltage is established for nanoporous anodic titanium oxide (ATO) and anodic aluminium oxide (AAO). The pore diameters of both ATO and AAO have been found to increase with the anodising voltage and drop down when the voltage exceeds a critical value. The origin for the existence of this maximum value of pore diameter in AAO and ATO is discussed.

Optical and structural properties of aluminium oxide thin films prepared by a non-aqueous sol–gel technique

Abstract: Clear aluminium oxide sols without precipitation were synthesized via a non-aqueous sol–gel technique using three different alcohols (ethanol, isopropanol and n-butyl alcohol) as solvent, aluminium sec-butoxide as a precursor and acetyl acetone as a chelating agent. Although all sols could be successfully used to prepare thin films, the most stable one was prepared with n-butyl alcohol. Highly transparent, homogenous and amorphous aluminium oxide thin films were obtained on Si substrates after a heat treatment at 500 °C. X-Ray photoelectron spectroscopy (XPS) and Fourier transform infrared absorption (FT-IR) spectroscopy revealed all films were hydroxide free. The optical and structural properties of the films were particularly investigated. Any significant difference except from thickness on the film properties was not observed by changing the alcohol. Refractive index was used as an indication of the porosity of the films and ranged from 1.54 to 1.60.

Reflective interferometric gas sensing using nanoporous anodic aluminium oxide (AAO)

Abstract: This Letter reports the use of nanoporous anodic aluminium oxide (AAO) for reflective interferometric gas sensing. Gas detection is based on changes of the interference signal from AAO porous layer as a result of the specific adsorption of gas molecules on a metal coated surface. Two different metal coatings – gold (Au) and platinum (Pt) – were used and their application for the detection of hydrogen sulphide (H2S) and hydrogen (H2) is shown. The practical application of a microchip Au-AAO sensor for measurement of H2S concentration and monitoring of malodour is also presented. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In situ study of the adsorption of humic acid on the surface of aluminium oxide by QCM-D reveals novel features

Abstract: Novel features of the adsorption of humic acid on the surface of aluminium oxide were revealed by means of an in situ study by quartz crystal microbalance with dissipation (QCM-D). The adsorption of humic acid on the surface of aluminium oxide shows a special case at pH 3 where the adsorption takes place in two steps. Each step has characteristic thickness, kinetics and viscoelasticity. While the first step shows low thickness, fast kinetics and almost no viscoelastic character, indicating a rigidly adsorbed humic acid monolayer, the second step is of much higher thickness, slower kinetics and with significant viscoelastic character, indicating a large diffuse layer with significant bulk contributions. In contrast, adsorption at pH 5 and 6.8 was found to be limited to a monolayer which is more rigid at pH 5 than at pH 6.8. Ellipsometry and X-ray photoelectron spectroscopy (XPS) were used to prove and quantify the adsorption in the dry state. The monolayer thickness measured by ellipsometry is close to the Sauerbrey thickness obtained by QCM-D. Adsorption takes place at a dilute concentration of 1 mg L−1, followed by saturation at a concentration of 10–20 mg L−1. XPS spectra of the carbon 1s peak indicated almost no desorption of the adsorbed humic acid layer after immersion in water for 48 hours, implying irreversible adsorption based on a strong binding between humic acid and aluminium oxide.

Non-vacuum solar spectrum selective absorption coating and preparation method thereof

Abstract: The invention relates to a non-vacuum solar spectrum selective absorption coating and a preparation method thereof. The preparation method comprises the following steps: (1) selecting copper or stainless steel with low infrared emissivity as a base material; (2) selecting oxide resistant to high-temperature oxidation, nitride and complex or doped oxide as a film material, wherein a metal or an alloy serves as a bonding force increased layer, metal nitride or pure metal serves as a high infrared reflecting layer, an absorption layer is composed of two conducting particle ceramic layers with different metal nitride conducting particle volume fractions, and aluminium nitride and aluminium oxide serve as an antireflection layer; (3) controlling the components and contents of different film materials by controlling gas flow and sputtering power; (4) cleaning the base material before the base material is placed into a vacuum chamber, and carrying out argon ion bombarding on the surface of the base material before sputtering is carried out; and (5) obtaining a multilayer coating, wherein the thickness of the coating is less than 500nm, and the coating has high absorption rate alpha (0.9-0.97) in the solar spectrum range (0.3-2.5microns) and has extremely low emissivity epsilon (0.02-0.18) in the infrared region (2.5-50microns).

Sintering behaviour of porous ceramic kaolin–corundum composites: Phase evolution and densification

Abstract: Kaolinite–corundum (derived from bauxite) associations were assessed as candidate matrices in the field of porous ceramics composites. Particles of corundum were expected to behave as non reactive second phase, deflecting the matrix cracks and increasing the toughness. Porosity and densification were monitored by developing coarse grains (67 wt.% of grains 2 O 3 > 56 wt.%), the secondary expansion ( 3 ). Such materials are promising low-cost solutions for the production of porous ceramics composites.

Effect of process parameters on growth rate and diameter of nano-porous alumina templates

Abstract: Anodic aluminium oxide (AAO) template with hexagonal shaped nano-pores with high aspect ratio was fabricated by two-step anodization processes from high purity aluminium foil. It was observed that pore dimensions were affected by anodizing voltage, electrolyte temperature and the duration of anodization time. The vertical growth rate of the pores (10–250 nm/min) was found to vary exponentially with anodizing voltage; however, it exhibits linear increment with the electrolyte temperature. The measured pore diameter (50–130 nm) shows a linear variation with anodizing voltage. The bottom barrier oxide layer was etched out by pore widening treatment to obtain through holes.

Catalyst for hydrogen production

Abstract: The invention provides a catalyst for the production of hydrogen by steam reforming. The catalyst is a porous catalyst which is based on at least aluminium oxide and preferably magnesium oxide, and further comprises boron and nickel. The porous catalyst comprises pores having an average pore size in the range of 0.1-50 nm. The activity of the catalyst may be further enhanced by addition of a noble metal such as Rh, Ru, Pd, Ir or Pt. The catalyst can be broadly used in hydrogen production processes, and is especially suitable for reforming using a membrane which is selective for a predetermined reaction product. Such process can be operated at relatively low temperatures of about 450-700 oC.

Synthesis and characterization of Fe doped mesoporous Al2O3 by sol-gel method and its use in trichloroethylene combustion

Abstract: Two mesoporous alumina samples were synthesized using the sol–gel method, and these samples were tested as catalysts in trichloroethylene combustion reaction. One alumina sample was doped with Fe to study the influence of a small amount of this agent on the characteristics and properties of alumina as a catalyst. Both catalysts (pure alumina and alumina doped with Fe) were thoroughly characterized by different techniques, such as DTA/TGA, FT-IR, XRD, SEM and TEM, and the porous characterization was conducted using a N2 physisorption technique. The doping agent presented a particular influence on the morphology and textural porosity in the alumina catalyst and therefore, it exhibited different catalytic behavior than the pure alumina catalyst. For both catalysts, the crystalline phase of γ-alumina was reported using XRD technique, and the crystallite size ranged from 7.8 to 12.8 nm. Using TEM images, the alumina catalyst doped with Fe revealed to contain a mixture of three types of iron oxide (maghemite, magnetite and hematite), mainly as roughly spherical nanoparticles. For both alumina catalysts, trichloroethylene catalytic combustion was conducted on a packed bed reactor in air at a temperature range of 50 to 600 °C. The alumina catalyst doped with Fe showed a higher catalytic activity than pure alumina, mainly due to the presence of micropores and grain morphology of flat faces.

Silver coated aluminium microrods as highly colloidal stable SERS platforms

Abstract: We report on the fabrication of a novel material with the ability to remain in solution even under the very demanding conditions required for structural and dynamic characterization of biomacromolecule assays. This stability is provided by the increase in surface area of a low density material (aluminium) natively coated with a very hydrophilic surface composed of aluminium oxide (Al2O3) and metallic silver nanoparticles. Additionally, due to the dense collection of active hot spots on their surface, this material offers higher levels of SERS intensity as compared with the same free and aggregated silver nanoparticles.