Anodised aluminium oxide

Rose-like and nanopearl-structured [Me4N][Ni(dmit)2]2 films have been grown on anodised aluminium oxide (AAO) template. The difference in morphology of the deposit is attributed to the size of the channels of each AAO template. The nanowires made of nanopearl chains grow inside the channels of the AAO template and accommodate their size to the channel diameter 49 2 and 32 4 nm.f Rose-like structures are obtained on AAO template with smaller channel diameter 15 4 nm. 

Sheet aluminium can be given a colour by a similar process. The aluminium is first made the anode in a bath of chromic acid (p. 377) when, instead of oxygen being evolved, the aluminium becomes coated with a very adherent film of aluminium oxide which is very adsorbent. If a dye is added to the bath the oxide film is coloured, this colour being incorporated in a film which also makes the remaining aluminium resistant to corrosion. This process is called anodising aluminium. 

Mesoporous alumina membranes ( anodic aluminium oxide , or AAO) are prepared by anodic oxidation of aluminium metal [1,2]. The cylindrical pores, perpendicular to the membrane surface, form hexagonal arrays of straight non-intersecting channels with pore densities up to lO Vcm. Their diameters are controllable within the range 5 - 100 nm as a linear function of anodisation voltage. These membranes are used as molecular sieves, and have also found application as templates for metallic nanowires [3,4,5,6], metal elusters and colloids [7,8], and carbon nanotubes [9,10]. 

The anodisation of aluminium is a well-established process [6-10]. For the formation of barrier aluminium oxide films, commonly used electrolytes are citric acid, tartaric acid and ammonium adipate [11]. Barrier films with high capacitance values and breakdown field strength were obtained in tartaric acid ofpH7[8, 12]. 

Concerning the two-layer model, the thickness and properties of each layer depend on the nature of the electrolyte and the anodisation conditions. For the application, a permanent control of thickness and electrical properties is necessary. In the present chapter, electrochemical impedance spectroscopy (EIS) was used to study the film properties. The EIS measurements can provide accurate information on the dielectric properties and the thickness of the barrier layer [13-14]. The porous layer cannot be studied by impedance measurements because of the high conductivity of the electrolyte in the pores [15]. The total thickness of the aluminium oxide films was determined by scanning electron microscopy. The thickness of the single layers was then calculated. The information on the film properties was confirmed by electrical characterisation performed on metal/insulator/metal (MIM) structures. 

The anodisation factor can be considered as an indicator for the quality of the aluminium oxide films. In particular, the barrier aluminium oxide films formed with low anodisation factors exhibit high breakdown field strengths... 

The dependence of the aluminium oxide film properties on the anodisation time was studied. Figure 23.10 shows the current-time transients during anodisation at the formation current density of 2.5 mA/cm. In the constant current... 

The aluminium oxide films formed for different times (marked in Figure 23.10) were characterised by electrochemical impedance spectroscopy. For the aluminium oxide film anodised for 160 s, the open circuit potential (OCP) is not stable. This can be explained by instability of the film structure. The processes of the film formation were not yet completed. The OCP is more stable and positive for films anodised for more than 700 s. This can be explained by the formation of the compact barrier aluminium oxide layer. 

During the period of the current decay, there are two competitive processes, densification to form the barrier layer and dissolution of barrier layer to form the porous layer. Under the high electrical field strength (constant voltage mode), the densification of the aluminium oxide films is favoured for process durations shorter than 3700 s. When the constant voltage was kept for a long anodisation time (beyond 10900 s), the dissolution of the aluminium oxide film becomes more dominant. Thus, the film could be contaminated by inward migration of the electrolyte into the film or by formation of micro-voids. 

The average capacitance and specific resistivity of the barrier aluminium oxide films are determined to be 430. .. 470nF/cm and 1.3. .. 2.4 10 " Qcm, respectively. By using the anodisation factor of 1.2 nm/V for the films formed at low formation voltage, dielectric constants of 5.8. .. 6.4 are calculated from the measured capacitance values. The comparatively low dielectric constant is in agreement with the formation of an amorphous anodic aluminium oxide film as discussed above rather than a crystalline structure for which a higher dielec-... 

Figure 23.12 Current density (/) vs. field strength (E) of the harrier aluminium oxide film/Al(200 nm)/glass formed at a current density of 0.5 mA/cm, a formation voltage of 10 V and an anodisation time of 1800 s.

In a template synthesis, CP is polymerized within the pores or channels of a nanoporous template to obtain the controlled structure and morphology upon removal of the template. Templates can be either a masking of a confined area that CP can grow in or a structured surface that CP is deposited upon. Masking type templates, namely nanoporous anodic aluminium oxide (AAO also commonly known as AI2O3, alumina, or anodise ) [113-118], hydrogels [119,120] and latex particles [121-130], have been used to prepare nanoscale fibrils, particles, and tubules of CPs. 

The layer of aluminium oxide can be made thicker by electrolysis, to give even more protection. This process is called anodising. The aluminium is used as the anode of a cell in which dilute sulphuric acid is electrolysed. Oxygen forms at the anode and reacts with the aluminium, so the layer of oxide grows. 

Anodised aluminium is used for cookers, fridges, cooking utensils, saucepans, window frames and sometimes for wall panels on buildings. The oxide layer can easily be dyed to give bright colours. 

Decorative anodised aluminium or aluminium alloys are, as such, not suitable for adhesive bonding as they have been sealed these types of substrate require stripping prior to use. Stripping is sometimes accomplished by abrasive blasting but this sort of treatment is not recommended. The anodic oxide film is best removed by immersion in the chromic-sulphuric acid solution given above [22,23]. 

The reflectivity of bare and anodised aluminium depends both on the surface aspect and on the wavelength (Figure A.2.1). It increases with purity with bright metal anodised to an oxide thickness of 5 xm, reflectivity increases from 75% on metal with a purity of 99.6 to 85% reflectivity for a 99.99% pure metal (Figure A.2.2). 

The widespread use of aluminium alloys in applications ranging from transport to architecture demands surface treatments for their corrosion protection [1,2], Anodising of aluminium is a widely studied and accepted method that consists of electrochemically forming a protective aluminium oxide layer [2], The growth, morphology, composition and performance of anodic films have been considered in the literature, for both pure aluminium and aluminium alloys [2 7], Furthermore, sealing mechanisms of the porous aluminium oxide layers have also been reported [6-9]. 

The second category was concerned with adhesion to porous or microfibrous surfaces on metals. Aluminium may be anodised to form an oxide surface comprising pores of diameter of tens of nanometers. Electroforming and chemical oxidation can be used to produce microfibrous or needle-like coatings on metals, including copper, steel and titanium. The substrate topography was demonstrated to play an vital part in adhesion to these surfaces [45-48]. 

Since the natural passivity of aluminium is due to the thin film of oxide formed by the action of the atmosphere, it is not unexpected that the thicker films formed by anodic oxidation afford considerable protection against corrosive influences, provided the oxide layer is continuous, and free from macropores. The protective action of the film is considerably enhanced by effective sealing, which plugs the mouths of the micropores formed in the normal course of anodising with hydrated oxide, and still further improvement may be afforded by the incorporation of corrosion inhibitors, such as dichromates, in the sealing solution. Chromic acid films, in spite of their thinness, show good corrosion resistance. 

Anodising the formation of oxide films on metals by anodic oxidation of the metal in an electrolyte solution. The term can be used for thin dielectric films but is more particularly applied to thicker films formed on some metals such as aluminium at higher anodic voltages. 

A great advantage of aluminium is the ability of its oxide film to absorb dyes, this leads to its use in decorative articles such as pen cases, cigarette lighters, etc. The oxide layer is thickened by "anodising , thus producing a porous structure in the oxide film. The dye is absorbed into the pores which are then sealed by hydrolysing the surface oxide. 

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