March 23, 2012

Anodizing Aluminum History

The history of electrochemical oxidation of aluminum dates back to the beginning of the last century. Anodic treatment of aluminum was intensively investigated to obtain protective and decorative films on its surface. More recently, applications of porous alumina with a huge surface area and a relatively narrow pore size distribution have been exploited. For example, several attempts to fabricate inorganic membranes have been reported. Nowadays, porous alumina is one of the most prominent template materials for synthesis of nano wires or nano tubes with mono-disperse controllable diameter and high aspect ratios. Moreover, it can be employed as a 2-D photonic crystal.

Numerous patents had been published before the 1950’s, concerning the anodizing of aluminum for coloring. Since the early years, anodic processes at DC or AC current based on either chromium, sulfuric acid or oxalic acid as electrolytes has been paid attention to. 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 the physical and mechanical properties as well as of the colors of the oxide. Bengough’s and Stuart’s patent in 1923 is recognized as the first patent for protecting Al and its alloys from corrosion by means of an anodic treatment. In 1936, Caboni invented the famous coloring method consisting of two sequential processes: anodizing in sulfuric acid, followed by the application of an alternating current in a metal salt solution.

The development of electron microscopy led to a deeper understanding of the 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. Also, they demonstrated the relationship of an applied potential and the geometric features of the hexagonal porous structures such as the interpore distance. This model was the basis for initial studies that aimed at better a understanding of the physical and chemical properties of porous alumina.

A review paper dealing with anodic oxide film on aluminum was already published in 1968. Structural features concerning anion incorporation and water content in the oxide and theoretical models of formation mechanisms of both the barrier-type oxide and the porous-type oxide were described in detail in this paper.

Between 1970 and 1990, studies by the Manchester group (led by Thompson and Wood) resulted in a deep insight in the growth mechanisms of alumina oxide film. This was possible by the uses of new techniques such as Transmission Electron Microscopy (TEM), marker methods and microtome sectioning. A corresponding publication by O’Sullivan and Wood is one of the most cited articles on anodizing of aluminum to obtain porous alumina structures.

Efforts on theoretical modeling of porous oxide growth were carried out by several groups. Fundamentally, an instability mechanism in terms of a field focusing phenomenon was attributed to create pores in the barrier oxide. In these papers, it is claimed that the theoretic modeling of the pore formation mechanism in alumina is analogous to that for other 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 synthesized by Masuda and Fukuda in 1995. This discovery was a break through in the preparation of 2D-polydomain porous alumina structures with a very narrow size distribution and extremely high aspect ratios. Two years later, they combined the aluminum anodizing method with nanoimprint technologies, which allowed for the first time the preparation of a monodomain porous alumina structure.

Numerous other groups, not mentioned here specifically, have also contributed to an improvement of porous alumina structures. The purpose of this dissertation is to understand self-assembly of porous alumina under specific conditions. In addition, nanoimprint methods are developed to obtain monodomain porous alumina structures. These nanoimprint methods are further advanced to fabricate porous alumina arrays with various configurations.

Furthermore, the combination of nanoimprint and anodizing will be applied to obtain porous oxides grown on titanium, which is also a valve metal. Finally, in the last two chapters, applications of the alumina templates will be discussed, for example, templates for monodisperse silver nanowires and 2D-photonic crystals.