Chemical composition and structural changes of porous templates obtained by anodising aluminium in phosphoric acid electrolyte

Abstract

Ordered anodic aluminium oxide (AAO) films were first prepared by anodising in a phosphoric acid electrolyte and then studied extensively and characterised by field emission gun-scanning electron microscopy (FEG-SEM), X-ray diffraction, Raman and infrared spectroscopy at a macroscopic scale. These analyses showed that the as-prepared AAO film is in fact amorphous, partially hydrated and that its initial global chemical composition can be described, in agreement with previous works, as: Al 2 O 3 , 0.186AlPO 4 · 0.005H 2 O. Additional analyses (thermogravimetric analysis, differential thermal analysis and FEG-SEM) showed geometrical changes of the film structure at different scales, explained by various steps of dehydration and allotropic transformations of the resulting crystallised alumina. However, because their structure remains unchanged up to 900• C, the phosphoric templates appear to be particularly suitable for applications or processes at medium or high temperatures, such as the preparation of carbon nanotubes or oxide rods. Copyright c 2010 John Wiley & Sons, Ltd.Keywords: anodic aluminium film; anodising; porous template; chemical characterisation; thermal stability IntroductionThe porous anodising of aluminium is a well-established process, widely used in various industrial applications. About 20 years ago, there was an academic renewal of interest in this process due to the possibility of experimentally observing the porosity inside the anodic film using microscopic techniques [field emission gunscanning electron microscopy (FEG-SEM), transmission electron microscope (TEM) or atomic force microscopy (AFM)]. In the following research the ability to control and optimise the mesoor nanoporosity was at stake; the pioneering research works of Masuda [1] and Gösele [2] showed that, under particular anodising conditions, it is possible to obtain porous anodic aluminium oxide (AAO) templates.Despite many previous studies, [3 -5] some aspects of anodising still remain to be clarified. Thus the chemical composition of the anodic films is open to discussion because it depends on many of the operational parameters of anodising. For example, previous works have shown that the chemical composition of the anodic films depends especially on the anodising electrolyte, as well as on the electrical conditions applied during the anodising.Many authors consider that the anodic films obtained on aluminium substrates are made of alumina (Al 2 O 3 ) and are consequently pure insulators. However, previous studies [6,7] revealed that the anodic films in fact act as n-type semiconductors. This electrical behaviour was explained by the formation of different types of aluminium oxy-hydroxides such as hydroxyl Al(OH) 3 , oxy-hydroxyl AlOOH or hydrated oxides Al 2 O 3 · xH 2 O during the anodising process. [5,6,8 -15] The crystallised phases of alumina (Al 2 O 3 ), i.e. the metastable (δ, η, θ , γ , κ, χ , ξ , etc.) or stable (α) allotropic forms, are in fact obtained only after additional thermal treatments. Thus, Mardilovich ...

The Sealing Step in Aluminum Anodizing: A Focus on Sustainable Strategies for Enhancing Both Energy Efficiency and Corrosion Resistance

“…Figure 1 illustrates the structure of the anodized layer on aluminum prior to sealing and changes that occur during sealing. The chemical and phase composition of the anodized layer on aluminum has been a subject of much interest [64][65][66][67][68][69][70][71][72]. From analysis of these numerous reports on the subject, there appears to be a consensus that the porous anodized layer on aluminum prior to sealing is predominantly comprised of amorphous alumina [64,69,73,74], some boehmite (AlOOH) [66,75,76], and aluminum oxide compounds whose actual compositions are determined by the bath electrolyte anion chemistry.…”

“…Hashimoto et al [82] recently investigated the local structure around aluminum atoms in anodic alumina using nuclear magnetic resonance spectroscopy, critiqued earlier studies on the subject matter, and reported that aluminum cations coordinated with oxygen anions in three types of coordination; AlO4 (tetra-coordination), AlO5 (penta-coordination), and AlO6 (octahedralcoordination), and that penta-coordination was predominant while the proportion of octahedrally coordinated (AlO6) units decreased with removal of physisorbed water. They [82] also evaluated the range of ratios of each mode of coordination to be 30%-40% for AlO4, 50%-60% for AlO5, and 4%- The chemical and phase composition of the anodized layer on aluminum has been a subject of much interest [64][65][66][67][68][69][70][71][72]. From analysis of these numerous reports on the subject, there appears to be a consensus that the porous anodized layer on aluminum prior to sealing is predominantly comprised of amorphous alumina [64,69,73,74], some boehmite (AlOOH) [66,75,76], and aluminum oxide compounds whose actual compositions are determined by the bath electrolyte anion chemistry.…”

“…Lee et al [94] had reported about 88% higher anionic impurities (mostly SO 4 2− ) in hard anodized anodic aluminum oxide samples (formation voltage = 35 to 55 V), compared to mild anodized anodic aluminum oxide samples (formation voltage = 25 V), and explained the observation using the high-field conduction theory. For porous oxide layers on aluminum anodized in phosphoric acid, the presence of AlPO 4 [77,95] or Al 2 PO 4 (OH) 3 [68], Al 2 O 3· 17P 0.072 [69], and (Al 2 O 2.88 ) 100 (PO 4 ) 8 (OH) 0.1 H 2 O [96] have been reported. With respect to the presentation of electrolyte species incorporated into porous anodic oxides layers in aluminum anodized in baths containing carboxylic acids like oxalic acid there is apparently no consensus.…”

Accurate evaluations of both porosity and tortuosity of anodic films grown on rolled AA 1050 and on rolled or machined AA 2024 T3

“…The porosity of the anodic films grown on aluminium substrates depends on various operating conditions, related to the anodization electrolyte and to the applied electrical parameters, as well as the substrate itself. Pure substrates thus allow anodic aluminium oxide (AAO) templates with highly ordered porosities to be produced; these latter have been extensively studied . In contrast, complex and tortuous porosities are usually obtained for multiphase aluminium alloys, eg, aeronautical AA 2XXX or 7XXX …”

Fabrication of Cellulose Nanofiber/AlOOH Aerogel for Flame Retardant and Thermal Insulation

“…The inorganic AlOOH particle is one of the most widely used inorganic flame retardant additives due to its demonstrated properties such as being tasteless, non-toxic, heat resistance, and non-volatile. It decomposes at 400 °C [3] according to the following reaction [4]: 2AlOOH (s) → Al 2 O 3 (s) + H 2 O (g) …”