Self-Organized Porous Anodic Oxide Formation: Role of Oxide Flow Driven By Surface Stress
Authors: Hebert Kurt , Capraz Omer , Mishra Pratyush , Ide Shinsuke , Shrotriya Pranav 
Published: February 2020

Porous anodic oxide (PAO) films are grown by electrochemical polarization of Al, Ti, and other metals in baths that dissolve the oxide. Procedures to grow films with highly ordered and controllable arrangements of nanoscale pores have led to the extensive use of PAO films in nanofunctional devices. Experiments and calculations show that transport in the oxide occurs by electrical migration coupled with plastic flow driven by mechanical stress in the oxide, and suggest that flow may play a role in formation of self-ordered PAO (1-3). Pores initiate by a morphological instability at the oxide-solution interface when the initially formed barrier oxide reaches a critical thickness (4).

Results of a combined experimental and modeling investigation of the formation and self-ordering of PAO are discussed. Experiments were carried out to identify stress generation mechanisms during galvanostatic formation of Al barrier films in 0.4 M H3PO4. Stress distributions in these films were measured by monitoring stress-induced sample curvature changes during oxide formation followed by open circuit dissolution of the films (5). Elevated compressive stress reaching several GPa was found to build up within a few nanometers of the oxide-solution interface, coinciding with high phosphorus contamination in the same layer revealed by GDOES (6). The compressive surface stress is likely due to electric field-induced phosphate incorporation.

The experimentally characterized surface stress was incorporated in an anodizing model based on transport by coupled viscous flow and electrical migration (2). Morphological stability analysis of the model revealed pattern formation consistent with the characteristic pore spacing to voltage relationship of PAO (7). Pore formation is governed by competition between stabilizing oxide formation at the solution interface and destabilizing flow driven by gradients of anion incorporation-induced surface stress. The model successfully predicts the narrow ranges of anodizing efficiency characterizing PAO formation on many metals (3,4). A compelling analogy exists between the mechanisms of PAO ordering and formation of hexagonally-ordered Marangoni patterns in thin liquid films.

Electrochemical impedance spectroscopy of steady-state porous anodic alumina formed in sulfuric and oxalic acid solutions are presented. It is shown that the characteristic inductive loop of impedance spectra of anodic oxide can be understood quantitatively in terms of electric field-induced homogeneous formation and annihilation of oxygen vacancies. We argue that this reaction can produce oxide displacement in the presence of sufficiently large stress gradients. The possible explanation of viscous oxide flow by such a vacancy creep mechanism is discussed.

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