Medium effect on electrochemical behaviour of anodized aluminium

The effect of some anions on the growth of the oxide film on aluminium was studied in acid and neutral media, as well as the effect of pH in presence of the same anion. In all the cases studied an inner barrier layer is formed adjacent to the metal and is covered on top with a porous layer. This latter outer layer differentiates into two regions, the one adjacent to the solution being characterized by a more open structure and a higher degree of anion incorporation as compared to the region embedded between it and the inner barrier layer. The rate of dissolution of the barrier layer is not affected by pH or anion type prevailing in the formation medium, since this layer is formed of pure alumina. The dissolution of the outer porous layer, on the other hand, is affected by both pH and anion type.     

Modelling of degradation of nickel-pigmented aluminium oxide photothermal collector coatings

Thermal degradation phenomena occurring in nickel-pigmented aluminium oxide coatings were investigated by combining detailed microstructural analysis with modelling of the optical properties. Scanning electron microscopy and Auger electron spectroscopy sputter profiling showed that the initial film consisted of a nickel-pigmented aluminium oxide layer close to the substrate. This layer supported a porous aluminium oxide layer that had a rough outer surface. While the solar absorptance degraded substantially (from 0.94 to 0.71) after heat treatment at 350°C for tens of hours, the aluminium oxide film morphology and thickness remained virtually unchanged and there was apparently no redistribution of nickel within the coating. Instead, the optical quality of the film degraded through oxidation of the nickel particles. These observations were supported by an optical model of the coating which produced the spectral reflectance properties measured both before and after the thermal ageing.     

Microstructure of porous anodic oxide films on aluminium

Microstructure of porous anodized films of aluminium prepared in sulphuric acid solution are different from those prepared in an oxalic or phosphoric acid solution. Transmission electron microscopy reveals a multilayer or higher order structure in the former films. Infrared spectra and specific surface area were also studied for these films and new functional properties of the films suitable for new materials were found. In contrast to the fibrous colloidal structure in the cells and barrier layer in the conventional films anodized in a sulphuric acid solution at d.c. 15 V, a network structure is formed in the cells and barrier layer in the hard films prepared at higher voltage of d.c. 25 V. The microstructure changes according to the anodizing conditions. A new model for these sulphuric acid films is presented, i.e. the cell walls are constructed from five layers and the fracture of the films occurs at the centre of the cell walls. Centre barrier layer (4 to 6 nm in thickness) composed of aluminium oxide of high crystallinity was found in a barrier layer at the bottom of the pore, and the thickness is independent on the applied voltage of the anodizing. Increase in thickness of the barrier layer due to applied voltage is governed by that of the outer barrier layer.     

How does aluminium foaming agent impact the geopolymer formation mechanism?

Aluminium is widely used as a foaming agent in lightweight geopolymer concretes. The impact of aluminium reaction on the phase evolution of fly-ash based geopolymers is investigated. In-situ Fourier transform infrared spectroscopy (FTIR) is used to track the changes in the nanostructure of geopolymers. By combining the FTIR functional group analysis with Atomic-force microscopy (AFM) results, it is possible to evaluate the phase development during the early hours of reaction. At the initial stages of geopolymerization, aluminium reaction induces the formation of aluminium hydroxide gel which precipitates on fly ash and conceals the reactive surface of ash particles. Therefore, the dissolution rate of fly ash declines and the strength development is delayed. The high release rate of alumina into the solution, as a result of the aluminium metal reaction, leads to a better connectivity of unreacted particles and better microstructural development as shown in SEM images. Also, by consuming part of the sodium hydroxide catalyst at the beginning of the reaction, aluminium powder protects some of the alkali content of geopolymer matrix from carbonation.     

Influence of microstructure on electrograining behaviour of commercial purity aluminiumused for lithographic printing plates

Abstract

High quality lithographic printing plates are made from electrolytically roughened aluminium sheets. A nitric or hydrochloric acid based electrolyte is normally used for the electro graining, and the substrate is most commonly commercial purity (CP) aluminium. Little is known about the relationship between the substrate microstructure and the pitting behaviour during electrograining in nitric acid solutions. The present work used three techniques to study the initiation and growth of surface pits responsible for the surface roughening of CP aluminium. A laboratory microcell simulated commercial graining but with the ability to interrupt the process and to rapidly clean and preserve the partially grained surface. Detailed scanning and transmission electron microscopy has been used to link substrate microstructure to pit initiation and growth morphology. The results demonstrate that the cold worked dislocation structure is an important feature whereas examination of simple binary alloys illustrates the relative roles of solute in solid solution and coarse particles. The present work suggests that the barrier oxide is very important and it has been postulated that chemistry or defect structure modifications to the film control the process of pit site selection.

MST/3137     

Magnesium metallic interlayer as an oxygen-diffusion-barrier between high-κ dielectric thin films and silicon substrate

The deposition of a thin magnesium metallic interlayer on an Si substrate prior to the deposition of an oxide thin film using rf-sputtering was investigated. The deposition of high-κ HfO₂ thin film was more particularly studied and it was demonstrated that the metallic interlayer acts as an oxygen barrier, preventing the formation of a low-κ layer at the high-κ/Si interface during the deposition. A post-deposition annealing treatment performed on the films induced the diffusion of the metal barrier into the HfO₂ film and allowed obtaining a sharp interface. However, the degree of diffusion depends not only on the interlayer thickness, but also on the thickness of the high-κ film. X-ray photoelectron spectroscopy was used to study the degree of oxidation of the Mg interlayer. High resolution transmission electron microscopy and energy filtered transmission electron microscopy were used to characterize the films and the diffusion of the Mg interlayer into the high-κ film after annealing. In this work we will stress on the engineering of the interface via the diffusion of the Mg interlayer during the growth process and on annealing.     

Growth of porous anodic films on FVS0812 aluminium alloy

The formation of porous anodic films on FVS0812 aluminium alloy has been examined by transmission electron microscopy in order to elucidate the processes of film growth. A complex morphology of film material is revealed containing relatively tortuous, branched and terminated porosity and relatively large cavities. The morphology is associated with the differing anodic oxidation behaviour of the aluminium matrix and silicide dispersion regions of the alloy and the differing chemical stabilities of the resultant film regions. The anodic oxidation of the silicide proceeds more slowly than that of the aluminium matrix, with the production of film material of much finer morphology. The reduced rate of oxidation of the silicide is attributed to the effects of alloying element species in the anodic film material and pore solution. The rate of oxidation of the silicide is sufficient for most of the particles to be oxidized completely during anodizing. However, the resultant film material subsequently dissolves in the pore solution leaving relatively large cavities in the film. The differing oxidation rates of the alloy components, coupled with locally differing film properties, leads to a relatively rough alloy/film interface.© 1998 Kluwer Academic Publishers     

Root growth of multi-wall carbon nanotubes by MPCVD

This work examines the relationships among the growth and interlayer reactions of carbon nanotubes (CNTs) to develop an effective process for controlling the nanostructure, orientation and characteristics of CNTs. Vertically oriented CNTs were successfully synthesized by microwave plasma chemical vapor deposition (MPCVD) with CH₄/H₂ as source gases. Additionally, the Ti and SiO₂ barrier layers and the Co catalyst were used in an experiment on the growth of CNTs on the Si wafer. Then, the SiO₂ barrier layer was deposited by low-pressure chemical vapor deposition (LPCVD). The Ti barrier layer and Co catalyst films were deposited on the Si wafer by physical vapor deposition (PVD). The deposited nanostructures were characterized by scanning and transmission electron microscopy, the results of which reveal that the deposited MWCNTs were grown under the influence of a catalyst on Si substrates with or without a barrier layer, by MPCVD. Vertically grown, dense MWCNTs attached to a catalytic film demonstrate that various MWCNTs penetrated the root particles. The diameter of the root particles, of approximately in the order of 100 nm, is larger than those of the tube, 10–15 nm. The well-known model of the growth of CNTs includes base- and tip-root growth. The interaction between the catalytic film and the supporting barrier layer is suggested to determine whether the catalytic particles are driven up or pinned down on the substrate during the growth.     

Study on growth of hollow nanoparticles of alumina

This article addresses the growth of hollow nanocrystalline particles of γ-alumina by the post-oxidation of nano-aluminium particles in air. The nanoparticles of aluminium were synthesized in a DC-transferred arc thermal plasma reactor. The as-synthesized nano-aluminium particles were oxidized, in air, at different temperatures. The as-synthesized parent nano aluminium and their daughter nanoparticles of aluminium oxide were thoroughly characterized with the help of X-ray diffraction analysis, high resolution transmission electron microscopy and thermogravimetric analysis. Two-step oxidation behaviours, unique in nanoparticles, are found to be the main driving force behind the formation of hollow spherical structures. The entire phenomenon is compared with the oxidation behaviour of coarse grain aluminium. The content of γ-alumina, identified by X-ray diffraction, relative to that of unreacted aluminium, has increased almost exponentially with the oxidation temperature in the case of nano aluminium. Similar behaviour is not observed in the case of coarse grain aluminium. The crystalline features of alumina, forming the walls of the hollow sphere, were confirmed by high resolution transmission electron microscopy.     

Direct growth of oxide nanowires on CuOx thin film

Oxide nanowires were directly grown on a CuO(x) thin film deposited by plasma-enhanced atomic layer deposition without additional metal catalysts. Oxide nanowires would exhibit metal-catalyst-free growth on the CuO(x) thin film with oxide materials diffused on the top. Through a focused ion beam and transmission electron microscopy, we could verify that SnO(2) and ZnO nanowires were grown as single-crystalline structures just above the CuO(x) thin film. Bottom-gate structural SnO(2) and ZnO nanowire transistors exhibited mobilities of 135.2 and 237.6 cm(2) V(-1) s(-1), respectively. We anticipate that a variety of large-area and high-density oxide nanowires can be grown at low cost by using the CuO(x) thin film.     

A growth mechanism of porous film formed on Al in 0.6 M oxalic acid electrolyte

Understanding of mechanism of porous film formation is of fundamental importance for anodizing in general because, the onset of pore initiation terminates the barrier film growth process over the macroscopic metal surface. Several mechanisms have been proposed to explain pore formation. They include direct injection of aluminum ions into electrolyte and a field-assisted dissolution mechanism. High-resolution scanning electron microscopy of anodized surfaces and direct TEM of ion beam thinned films and ultrarmicrotomed film sections have been employed to gain further insight into the mechanism of initial porous film growth in 0.6 M oxalic acid. From detailed examination of the behavior of the xenon-tagged layer in the film during pore initiation and development in oxalic acid, the film structure of the barrier layer is found to be unstable during pore initiation and the instability of the film structure is possibly related to the field-assisted structure modification process.     

Thermal Stability of Fe2O3 Nanowires

The thermal stability of α-Fe203 and γ-Fe2O3 nanowires was studied by post annealing the samples at different temperatures. Before and after annealing, the samples were characterized by X-ray diffraction and scanning electron microscopy. The α-Fe2O3 nanowires are stable at the temperatures up to 600℃, and the crystalline structure becomes more perfect after annealing. This behavior supplies a way to improve the quality of the α-Fe2O3 nanowires. The γ-Fe2O3 nanowires become unstable when annealed at 350℃. Raman spectra of both nanowires have been measured, which also indicate that the γ-Fe203 nanowires are transformed into α-Fe2O3 under the strong laser beam.     

Formation of porous anodic alumina in alkaline borate electrolyte

The growth of porous anodic films at 60 V in an alkaline 0.13 M borax electrolyte at 333 K is examined using sputtering-deposited aluminium substrates, with a fine band of incorporated tungsten tracer. The findings reveal amorphous alumina films containing approximately conical major pores incorporating finer secondary pores, with film thicknesses similar to that of the oxidized aluminium. Further, the distribution of the tungsten tracer within the film is mainly consistent with its expected migration behaviour in anodic alumina. The results indicate that pore development under the present growth conditions is dominated by field-assisted dissolution of anodic alumina, with an efficiency of film growth of about 50%. The findings are in contrast with those of porous anodic films formed in phosphoric acid electrolyte, which are significantly thicker than the layer of oxidized metal.     

Structure and composition of oxide film on 5083 alloy at brazing temperature

The structure and composition of the surface oxide film on the 5083 aluminium alloy at a brazing temperature of 500°C were investigated by transmission electron microscopy and X-ray photoelectron spectroscopy. The results showed that the original γ-Al₂O₃ film on the surface of the cold rolled 5083 aluminium alloy was transformed into a complex oxide film consisting of MgO, MgAl₂O₄, and free Al atoms after heating. The thickness of this oxide film, which could be divided into two distinct layers, was approximately 130 nm. The outer layer was mainly composed of an amorphous MgO phase, while the inner layer was MgO based, with a few free Al atoms and a small number of nanocrystalline MgAl₂O₄ particles distributed in it.     

Thermally induced surface changes in aluminium-iron oxide thermites

The aluminium and iron oxide surfaces of Al-Fe₃O₄ thermite in powder and pressed pellet form were studied before and after accelerated ageing at 180° C by X-ray photoelectron spectroscopy. The Al₂O₃ surface film thicknesses on aluminium metal were deduced from the intensity ratio of aluminium K L L Auger signals induced by X-ray radiation on Al₂O₃ and aluminium metal. Based on the mean free path of 1.65 nm for aluminium K L L Auger electrons, the oxide thickness on aluminium flakes before mixing with Fe₃O₄ was estimated to be 0.8 to 0.9 nm. A slight oxidation was observed on the aluminium surface after mixing with Fe₃O₄ at room temperature. Hot pressing of this mixture at 425° C for 7 min increased the oxide film to 3.1 nm. This surface oxide film seemed to protect the aluminium metal, and further ageing at 180° C did not cause significant oxide growth.Mound is operated by Monsanto Research Corporation for the US Department of Energy Under Contract No DE-AC04-76DP00053.     

Unusual W-shaped Galvanic Cell Model of ISCC of α-brass in Neutral Mattsson''''s Solution

Intergranular stress corrosion cracking(ISCC) of α-brass in neutral Mattsson's solutionwas found to be controlled by an unusual"W"-shaped galvanic cell whose cathode is thegrain boundary oxide film (G.B.0. film) andsurface film and the anode is fresh metal atthe cracked tip on both sides of the G.B.0.film. Redox reactions involved in the cellhave been proposed here. According to thismdel, initidtion of ISCC is caused by therupturing of surface film along grain boundaries,thus forming a galvanic cell. Propagation ofISCC resulted from alternate advances of G.B.0.film and dissolution on both sides of G.B.0.film caused by the effect of electrochemicalreaction. This work developed an effective approachto investigate the embrittlement process atthe tip of the crack, by increasing the lengthof the embrittlement region through constantstrain test and distinguishing the morphologyand the nature of the corrosion products byoptical microscopy and scanning electronmicroscopy (SEH).     

Modelling behaviour of oxide film during vibration diffusion bonding of SiCp/A356 composite in air

Abstract

The vibration liquid phase diffusion bonding of SiC_p/A356 composite in air has been investigated. The surface of specimens to be bonded was treated with and without vibration under the bonding condition. It was found by atomic force microscopy analysis that some of the oxide film could be broken down when ridges on the surface of the matrix were ground down. Dissolution of the base metal by the filler metal occurred with removal of the oxide film during vibration liquid phase bonding, and SiC particles in the base metal entered the bond region. A removal process model for vibration bonding has been established with and without filler metal. Results show that shearing and impacting actions are the two main breaking mechanisms during vibration; the oxide film bulk is generally broken down by shear, and dissolution of the base metal by the filler metal promotes particle segregation from the matrix and their entry into the bond region.     

Chemical functionalization of graphene oxide by a hindered amine stabilizer and evaluation of the product as a UV-stabilizer for polypropylene

The present work was devoted to synthesis of a novel UV-stabilizer based on graphene oxide for polypropylene. Hence, we grafted a hindered amine compound on graphene oxide through an amidation reaction. Fourier transform infrared (FTIR) spectroscopy and thermo-gravimetric analysis confirmed the grafting reaction. Also, X-ray diffraction together with scanning electron microscopy showed that the grafting reaction transforms the compact layered structure of graphene oxide into finer aggregation of loosely connected particles. Polypropylene film samples containing either of graphene oxide and the functionalized graphene oxide were prepared by solution mixing/anti-solvent precipitation followed by compression molding. Transmission electron microscopy showed significantly better dispersion of the modified graphene oxide in the polymer matrix compared to the pristine graphene oxide. Afterwards, the film samples were exposed to an artificial sunlight at 25?°C. It was proved by FTIR spectroscopy and tensile test that the modified graphene oxide at a concentration of 0.5?wt. % can efficiently retard the rate of the photo-oxidation of PP.     

Bonding between aluminium and copper in cold spraying: story of asymmetry

Abstract

The bonding mechanism in cold spraying is still a matter of some debate, which requires further investigation. In the present work, aluminium powder was cold sprayed onto a copper substrate and copper powder was cold sprayed onto an aluminium substrate using the same process gas and spray parameters. Separate experiments were performed to produce thick (～400 μm) coatings and isolated particle impacts. Deposits were characterised using scanning electron microscopy and image analysis. The coating–substrate interfacial bonding was assessed via a method in which, following a short heat treatment at 400°C, intermetallics grow at the interface where metal to metal contact has been established. In addition, the bond strength values of deposits were determined using a standard pull-off test. It was found that the copper particles deposited onto an aluminium substrate resulted in significant substrate deformation, whereas aluminium particles caused minimal deformation of the copper substrate. Furthermore, the former displayed a higher degree of metallurgical bonding at the coating/substrate interface in comparison with the latter. These results suggest that the removal of oxide films from the surfaces was greater when copper was the material being sprayed rather than aluminium. The impact behaviour of the two materials and the removal of oxide due to deformation at high strain rate are discussed with the aid of the Johnson–Cook plasticity model.     

泡沫铝合金阳极氧化工艺研究

以草酸为电解液,对泡沫铝合金材料进行阳极氧化处理,制备多孔有序阳极氧化铝阵列模板,采用SEM扫描电镜对其形貌进行分析.研究了电流密度、电解液浓度、电解温度等条件对氧化铝膜结构的影响,并对一步法和两步法制得的多孔氧化铝膜进行了比较,结果表明,氧化膜上的微孔分布均匀,孔径大小基本相同;两步阳极氧化法制备的多孔氧化铝模板的有序性优于一步氧化法.