Anodizing of aluminium

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1 Anodizing of aluminium Posted on Jan 30, Posted by P&A International Category General Talk The history of electrochemical oxidation of aluminium dates back to beginning of last century. Anodic treatment of aluminum were intensively investigated to obtain protective and decorative films on its surface [1]. More recently, applications of porous alumina with a huge surface area and a relatively narrow pore size distribution have been exploited [2]. For example, several attempts to fabricate inorganic membranes have been reported [3, 4, 5]. Nowadays, porous alumina is one of most prominent template materials for synsis of nanowires or nanotubes with monodisperse controllable diameter and high aspect ratios [6, 7, 8, 9, 10, 11,12, 13, 14]. Moreover, it can be employed as a 2-D photonic crystal. Numerous patents had been published before 1950 s, concerning anodization of aluminum for coloring [15]. Since early years, anodic processes at DC or AC current based on eir chromic, sulfuric or oxalic acid as electrolytes have been paid attention to [15]. Consequently, it was observed that additives such as metal salts like copper, nickel, silver, arsenic, antimony, bismuth, tellurium, selenium or tin lead to a change of physical and mechani- cal properties as well as of colors of oxide. Bengough s and Stuart s patent in 1923 is recognized as first patent for protecting Al and its alloys from corrosion by means of an anodic treatment [16]. In 1936, Caboni invented famous coloring method consisting of two sequential processes: anodization in sulfuric acid, followed by application of an alternating current in a metal salt solution [17]. The development of electron microscopy led to a deeper understanding of porous alumina structures. In 1953, Keller and his coworkers described a porous alumina model as a hexagonally close-packed duplex structure consisting of porous and barrier layers [18]. Also, y demonstrated relationship of an applied potential and geometric features of hexagonal porous structures such as interpore distance. This model was basis for initial studies that aimed at better a understanding of physical and chemical properties of porous alumina. A review paper dealing with anodic oxide films on aluminum was already published in 1968 [19]. Structural features concerning anion incorporation and water content in oxide and oretical models of formation mechanisms of both barrier-type oxide and porous-type oxide were described in detail in this paper. Between 1970 and 1990, studies by Manchester group (led by Thompson and Wood) resulted in a deep insight in growth mechanisms of alumina oxide films. This was possible by uses of new techniques such as Transmission Electron Microscopy (TEM), marker methods and microtome sectioning. [20, 21, 22, 23]. A corresponding publication by O Sullivan and Wood is one of most cited articles on anodization of aluminum to obtain porous alumina structures [24]. Efforts on oretical modelling of porous oxide growth were carried out by several groups [25, 26, 27, 28, 29, 30, 31, 32, 33]. Fundamentally, an instability 1 / 6

2 mechanism in terms of a field focusing phenomenon was attributed to create pores in barrier oxide. In se papers, it is claimed that oretic modelling of pore formation mechanism in alumina is analogous to that for or porous materials which can be obtained via an anodic treatment, for example, mciroporous silicon. Based on a two-step replicating process, a self-ordered porous alumina membrane with 100 nm interpore distance was synsized by Masuda and Fukuda in 1995 [34]. This discovery was a breakthrough in preparation of 2D-polydomain porous alumina structures with a very narrow size distribution and extremely high aspect ratios. Two years later, y combined aluminum anodization method with nanoimprint technologies, which allowed for first time preparation of a monodomain porous alumina structure [35]. Numerous or groups, not mentioned here specifically, have also contributed to an improve- ment of porous alumina structures. The purpose of this dissertation is to understand self-assembly of porous alumina under spe- cific conditions (Chapter 1 and Chapter 2). In addition, nanoimprint methods are developed to obtain monodomain porous alumina structures (Chapter 3). These nanoimprint methods are furr advanced to fabricate porous alumina arrays with various configurations (Chapter 4). Furrmore, combination of nanoimprint and anodization will be applied to obtain porous oxides grown on titanium, which is also a valve metal1 (Chapter 5). Finally, in last two chapters, applications of alumina templates will be discussed, for example, templates for monodisperse silver nanowires (Chapter 6) and 2D-photonic crystals (Chapter 7). 1.2 Electrochemistry of anodic alumina Thermodynamics The spontaneous reaction leading to formation of aluminum oxide in air can be ascribed to large negative Gibb s free energy changes [36]. 2Al(s) +2 O2(g) αal2o3(s) ; ΔG = 1582kJ/mol (1.1) 2Al(s) + 3H2 O(l) αal2 O3(s) + 3H2(g) ; ΔG = 871kJ/mol (1.2) If aluminum is electrochemically anodized, an oxide grows at anode electrode [37], 2Al(s) + 3H2 O(l) = Al2O3 (s) + 6H + + 6e, (1.3) 2 / 6

3 and hydrogen evolves at cathode 6H + + 6e = 3H2(g). (1.4) Assuming re are no complex anions, Nernst equation reads E = E0 ( zf )ln( [ox] ) (1.5) where R is universal gas constant, T is absolute temperature in Kelvin, z is charge number of electrode reaction, and F is Faraday constant (96,500 C mol 1). The electrode potential E at anode can be written as E = pH (1.6) Table 1.1: Alumina oxide forms [39] Name Crystalline form Density (g/cm3) / Crystal system Remark Corundum α-al2o / hexagonal found in nature 3 / 6

4 Boehmite α-al2 O3 H2O 3.44 / ortho-rhombic Gibbsite α-al2o3 (H2O) / monoclinic Diaspore β-al2o3 (H2O) 3.4 / ortho-rhombic no occurrence in nature Bayerite β-al2o3 (H2O) / monoclinic Gamma alumina γ-al2o3 anhydrous alumina with ill-defined structure This explains that reaction at anode electrode (Al) rmodynamically depends on ph value, which is determined by electrolyte and temperature Kinetics The current density passing across oxide film can be written as [19, 37] j = ja + jc + je (1.7) where ja, jc and je are anion-contributing, cation-contributing and electron-contributing cur- rent density, respectively. Since electronic conductivity in aluminum oxide is very low, ionic current density ( ji = ja + jc) is predominant mode to transport charges. The relationship between ionic current, j i, and electric field, E, can be expressed in terms of Gunrschultze-Betz equation ji = j0 exp(βe) (1.8) where both j0 and β are temperature- and metal-dependent parameters. For aluminum oxide, electric field E, j0 and β are in range of 106 to 107 V/cm, to ma/cm2 and to cm/v, respectively [38]. Based on Gunrschultze- Betz equation, rate-limiting steps of film formation are determined by ionic transport eir at metal/oxide interface, within bulk oxide or at oxide/electrolyte interface [19]. Nowadays, it is generally accepted that oxides simultaneously grow at both interfaces, e.g., at metal/oxide interface by Al3+ transport and at oxide/electrolyte interface by oxygen ion transport [21, 24]. For example, transport number2 of Al3+ anions, tal3+, and cation transport number, to2, were reported as 0.45 and 0.55, respectively, for 5mA/cm2 [22]. 1.3 Composition of oxide Due to a number of polymorphs, hydrates, and incorporated ions, anodic Al2O3 can exist in various forms, e.g., Al2O3 (H2 O)n where n = 0 to 3 [37, 40]. Six forms are mostly discussed 4 / 6

5 (see Table 1.1). Gibbsite and Boehmite are converted to several transition alumina minerals such as γ series (e.g., γ, ρ, ξ) and δ series, (e.g., δ, κ, θ) by heating. Corundum, which rmodynamically is most stable form among alumina oxides, is generated above 1100 C, regardless of transition course. Boehmite heated at between C yields γ-al2o3 consisting of irregular structures [40]. Generally speaking, anodic Al2 O3 was mostly reported as a form of X -ray amorphous solid [19, 24, 37]. For barrier layer, presence of nanocrystallites of γ0 -Al2O3 with sizes of 2-10 nm was demonstrated by several authors. γ0 -Al2O3 is considered as an intermediate form between amorphous and γ-crystalline Al2O3. Thompson and Wood suggested that aluminum oxides may consist of nanoocystallites, hydrated alumina, anions, and water moleculars[20, 24]. 1.4 Barrier-type and porous-type alumina Depending on several factors, in particular electrolyte, two types of anodic films can be produced. Barrier type films can be formed in completely insoluble electrolytes ( 5 + 3/4 C2H5OH (Fig. 1.6 (b)). Electropolishing is a prerequisite for formation of selfordered porous alumina with large domain size. Note that caution is needed when perchloric acid/ethanol is used due to its explosiveness at moderate temperatures. After pretreatment, anodization is performed eir at 19 V in 2 M H2SO4, at 25 V in 0.3 M H2SO4, at 40 V in 0.3 M (COOH)2, at 160 V in 1 M H3PO4, or at 195 V in 0.1 M H3PO4 for more than 1 day (Fig. 1.6 (c)). Since pores are randomly created on surface, initial pore arrangement is very irregular (Fig. 1.7 (a)). However, due to repulsive forces between neighboring pores during long-anodization, self-organization occurs. As a result, hexagonally close-packed arrays are obtained at interface between porous alumina layer and aluminum substrate ((Fig. 1.7 (b))). Then, porous alumina film is selectively dissolved in a solution containing chromic acid (Fig. 1.6 (d)) [53]. Patterns that are replicas of hexagonal pore array are preserved on fresh aluminium surface. This allows preparation of pores with a high regularity by a subsequent second anodization under same conditions as first anodization (Fig. 1.6 (e)). If needed, resulting pores can be isotropically widened by chemical etching with M phosphoric acid (Fig. 1.6 (f)). 5 / 6

6 Figure by Jessensky hexagonally-ordered oxygen electrolyte/oxide formed a first 1.7: 4. In Pores membrane (c), oxidation dissolution respectively. oxide when containing case grow anodization The without et Scanning layer. Al3+ al. of perpendicular mechanical after takes barrier and interface contributing ions selective pore Li (in Electron place thinning (O2 et oxide drift 0.1 contribute al. arrays stress M through to proposed removal growth or phosphoric to Microscopy of OH [48, entire surface model to without oxide 52]. ) of from a Al, metal/oxide mechanical They acid (SEM) when formation. layer which pore layer. at considered electrolyte. 195 images formation, correspond mainly Some field-enhanced stress interface V). On (a) of due of model a all m porous to mainly to following or Al3+ surface, are dissolution hand, hydration explain ions by alumina ejected factors; and reaching porous migration T (b) sample reaction and into formation alu- B bottom elec- in produced of mina of Fig. of of anodized 6. ide/metal According pansion rmation (ξmax Potential alumina. potential =ku of nterpore The This (1.9) applied formed volume of coefficient As with interface. alumina 2), In Anodization to self-ordered by seen addition, potential, ir a alumina expansion factor proportionality pores studies, expands ξ The Fig. (see of is are U is pores parameters expansion equilibrated 21.4,, thickness assumed Eq. leads lower is generated. to one or about 2.5), degree constant to than interpore of compressive varies to of formation twice influencing be If of most k metallic Al2O3 vertical of barrier with distance, stress approximately volume important original of growth. stress aluminium. disordered direction Therefore, self-ordering applied Dint expansion too volume. can during factors small, potential pushes 2.5 pores. This linearly oxide/metal approximately (ξ to of atomic k(nm/v adjust means aluminum, If and proportional pore density formation self-assembly determines ) stress that interface. walls 2.8 estimated of is [18, volume upwards. maximal aluminium eir 24]. of applied as ox- ex- of Dint half inis Type The properly ranges phosphoric conductivity acid oxide size dissolution diameters using Temperature During formed 40 restricted. type of and layer sulfuric a (ca. oxide to distance high and are concentration pores. anodization, obtain takes acid and 5 Usually, structure dependence formed 40 ph place pore concentration The (Dint [24, self-ordered V), high value by lower tip. oxalic 41]. from (note = very potential using anodization 2DB of This being often. that acid, pore where ph leads electrolyte. sulfuric ranges dissolved In phosphoric value, used growth. should addition, of electrolyte DB aluminium a acid (ca. smaller be For medium In 80 has lower acid, acidic kept or example, for barrier-layer a ph-value 200 size and lower very a carried electrolytes. words, given V). of potential small high if than This aluminum of potential out pores. conductivity), thickness). ranges room restriction choice threshold For electrolyte sulfuric diameter temperature Therefore, has instance, is (ca. anodized to is acid 30 breakdown be due are determines type field-enhanced selected anodization large 120 to obtained to low of prevent sulfuric V) electrolyte potential pore of and by fo 195 avoid field In temperature electrolytes fact, V case V distribution a cracks oxalic local phosphoric of affected are controlling. heating acid and used, is bursts by acid. performed bottom, If of electrolyte temperature. a A bottom high second leading oxide potential). 5 of reason may film 18 C to The local pores freeze. are The lower to and too generated electrical keep during local In low 0 addition, (just heat temperature, 2 C breakdown if course below causes porous speed zero of case alumina of anodization inhomogeneous degree) lower of oxide anodization is formed is growth possible and (see (specially, growth diluted without Fig. of electric atporous is 1.8). to rate. i Figure Enlarged Influence Aluminum alumina. defects knowledge next Or Additionally, parameters main Fig.1.6 point electropolishing hydrogen into out that chapter sizes. of aluminum important 1.8: (a)). since It of with re to (a) annealing, Usually, 680 (see impurity image impurities reasonable be a SEM substrate high are C). parameters considered chapter is image purity shown electro-polishing, have investigations annealing 3), to of ( annealed expect to we a porous different fabricate Fig. will process that 1.8 discuss %) oxide below about (b). volume anodization self-ordered agitating, usually after se performed expansion effect melting electrical recommended effects. and porous of aluminum point at impurities breakdown coefficient 500 first alumina to anodizing obtain to having C on for obtain than structures caused 3h impurities large time (note self-ordering. self-ordered aluminum. by are grain that with local important leads size large To heating. P&A During The that pore International surface first average channels bubbles anodizing domain anodization, roughness is of performed and [49]. domain size time local increases affects smood sizes heat electrolyte for 1 decreased with size 4 by min. should surface, electropolishing of first domains again be anodizing and vigorously if Al of to self-ordered allow anodized (see time stirred a Fig.1.6 [54]. homogenous over However, porous to (b)) effectively a critical [49]. alumina. diffusion Usually, was time. remove also It of turned porous In melting anions found my to (see do- Tags: aluminium, Anodizing, Anodizing of aluminium 6 / 6