Fast Fabrication of Self-ordered Anodic Porous Alumina on Oriented Aluminum Grains

Abstract

As mentioned in Chap. 1, under the conventional mild anodization (MA) condition, self-ordered anodic porous alumina is obtained only under certain narrow processing windows with specific values of the interpore distance (D int ) [1][2][3][4]. Also, due to the slow oxide growth rate, prolonged anodization time of typically more than 2 days is required under MA [1,[5][6][7]. Recently, hard anodization (HA), which is conducted under low temperatures similar to MA but high voltages or current densities, is regarded as a promising approach to replace the MA for fast and selfordered anodic porous alumina fabrication [5][6][7][8][9][10]. However, because of the high anodization voltages of HA [6,9], the very rapid heat generation caused by the high electric field across the barrier layer is difficult to be dissipated quickly [11,12], and as a result, many macroscopic burns or cracks, observable even by the naked eye, may form on the alumina surface [5,8]. Furthermore, direct anodization under HA may result in disordered pore arrangements [6]. Therefore, fast and mechanically stable fabrication of highly self-ordered anodic porous alumina is still a challenge.In this chapter, [13] we systematically investigate the key influencing factors for self-ordered anodic porous alumina formation, in order to find alternative processing windows for self-ordering with fast and mechanically stable alumina growth features. These factors are electrolyte (H 2 C 2 O 4 ) concentration, temperature, voltage, time, and substrate grain orientation, especially the last factor was recently found independently by Ng and Ngan [14][15][16] and Beck et al. [17,18], and further confirmed by Napolskii et al. [19]. However, systematic investigation of the combined effects of the above factors on the self-ordered anodic porous alumina formation has not been reported before the present work [13]. The ordering qualities of porous patterns obtained under a series of anodization conditions were quantitatively evaluated by the angular orientation distribution (AOD) method developed in Chap. 6, and the calculation program is shown in Appendix II. This chapter will show that beside the MA and HA conditions, an alternative condition, referred to as