The research objective of this award is to develop new methods for the rational design of highly ordered porous anodic oxide (PAO) films. These films are formed electrochemically by applying voltages to metals such as aluminum and titanium, in electrochemical cells. The films contain self-ordered hexagonal arrays of nanoscale pores, covering macroscopic areas. While much attention has focused on the use of PAO as templates for functional devices, these structures have been developed empirically, in the absence of robust understanding of processes controlling film growth. The goal of this research is to develop a fully predictive model of pore formation and self-ordering. The work will be guided by the concept that oxide material is transported by the combined influence of the electric field and mechanical stress. Experimental stress measurements will determine the operative balances of viscous, electrostatic and oxidation-induced stress governing interface motion in PAO films. Using this knowledge, descriptions of transport processes and driving forces will be formulated as a predictive simulation PAO growth, revealing the relations between electrochemical polarization, bath chemistry, and the dynamics of the self-ordering.
If successful, the chemical-mechanical model of PAO formation will provide a fundamental basis for model-based design of self-organized porous anodic films. Thus, the model may permit rational manipulation of process conditions for high-rate fabrication of defect-free films, thus enhancing the commercial potential of PAO-based functional devices for solar energy conversion, catalysis, and biomedical applications. Student training will benefit from the unique aspects of this project: close integration of chemistry and mechanics; rigorous combination of a variety of experimental and modeling approaches; and on-site collaboration with a materials characterization group in England. Student training at Iowa State will take advantage of the strong presence of graduate and undergraduate programs promoting underrepresented groups, both at the Department and University levels.
Self-ordered porous anodic oxide films are formed by electrochemical oxidation of aluminum, titanium and other metals in solutions in which the oxide is soluble. Films produced under particular process conditions contain highly regular arrangements of ten to several hundred nanometer diameter cylindrical pores. The geometry of the porous structures may be tuned precisely by changing the electrochemical voltage and the compostion of the bath. The anodizing process is inexpensive and can in principle be scaled to large areas and high fabrication rates. Taking advantage of the diverse range of oxides that can be produced, porous structures have been fabricated with favorable properties for many electronic, optical and biological applications, such as TiO2 based dye-sensitized solar cells and battery electrodes. Despite intense research activity over the past fifteen years, the commercial potential of porous anodic oxides has not been fully realized. The difficulty in transitioning from research to production can be attributed to incomplete quantiitative understanding of the mechanism by which these structures form. Thus, the strategy of empirical design of process conditions, while effective in the research laboratory, has not proved adequate for scaleup to commercially viable production rates. The focus of our effort in this grant was to develop a quantitative model of porous oxide formation to provide the basis for effective scaleup. For this purpose, it was necessary to resolve key fundamental questions about the process of pore formation involving the relative importance of electrical, chemical and mechanical variables: (1) What are the roles of electrical voltage vs mechanical stress in pore growth? (2) Why does the geometry of the porous structure depend on solution chemistry as well as on electrical variables? (3) What mathematical rules govern the dependence of the self-organized porous layer geometry on electrical voltage? For this purpose, we carried out the first comprehensive study of stress generation during anodic oxide growth on aluminum. Sources of stress generation in both the oxide and metal during film growth were identified and related to the emergence of the porous layer geometry and its composition. We found that stress builds up to high levels at the oxide surface as a result of electric field-induced incorporation of anions from the solution (such as, in our specific case phosphate ions from the phosphoric acid anodizing bath). We showed, that pores initiate due to a process involving bulk motion or flow of oxide when a critical stress level is reached. Thus, pore formation is initated directly by a mechanical mechanism, but depends on the chemical process of anion incorporation from the solution. Additionally, we discovered the origin of the dependence of pore geometry on electical voltage lies in the competition of different factors that influence the shape evolutiion of the oxide layer: migration of ions within the film and deposition of oxide from solution. The pore separation in the porous oxide is influenced by the different depenendences of these processes on the electric field in the oxide. By identifying the key physical and chemical processes involved in pore formation, this work lays the basis for development of quantitative models of porous oxide growth. These models will provide the framework for rational desgin of porous oxides at commercially acceptable production rates.
