Theoretical Pore Growth Models for Nanoporous Alumina

Abstract

Nanoporous alumina has been extensively used in a wide range of applications, including template materials for various types of nanomaterials, high surface-area structures for energy conversation and storage, bio/chemo sensors, electronic/photonic devices, and so on. However, the formation mechanism of the nanopores and the subsequent pore growth process towards self-ordered pore arrangements have been under investigation for several decades without clear conclusions. The present models may be divided into two main groups in terms of the driving force for pore initialization, as well as the subsequent pore growth process. One group considers that the driving force is the high electric field across the oxide barrier layer at the bottom of the pore channels, which assists metal oxidation at the metal/oxide interface, and oxide dissolution at the oxide/electrolyte interface. The other group of models assumes that the driving force is mechanical stress originating from the volume expansion of the metal oxidation process. This chapter reviews the development of these models for nanoporous alumina formation, and discusses their advantages and shortcomings. A recent model proposed by us is also described, and potential directions for further development are discussed.

LIPSS‐Sticks: Laser Induced Double Self Organization Enhances the Broadband Light Absorption of TiO2 Nanotube Arrays

“…A more complete model is being developed to consider the effects of the electrolyte and surface stresses evolved in the presence of a laser templated surface. Despite that, we believe that the qualitative nature of the explanation still holds, because the electric field strength and electric currents remain the most important contributors to the initial stage of nanotube formation and pore size 43,44 .…”

“…The valley acts as the shortest path from the bottom surface at 60 V to the "ground" at 0 V, hence most of the current density passes through this region. This enhances the electric field-assisted dissolution process, deepens pores, and enlarges the pore size 42,43 . Therefore, at a later stage of anodization, the pore that is initiated in the valley has a larger diameter and the pores near the crests have a smaller diameter (Figure 7b).…”

Plasma Electrolytic Oxidation Coatings on Aluminum Alloys: Microstructures, Properties, and Applications

“…In pore formation and oxide growth via anodic oxidation the mechanism has been proposed to be governed by electric-field interface evolution, mechanical stressguided repulsion and viscous flows of the oxide species [34]. The electric-field assisted mechanism involves oxide decomposition which results in direct ejection of molten aluminum (Al 3+ ) into the solution followed by oxide formation via transport of oxygen ion (O 2-) at oxide/electrolyte interface and Al 3+ transport at metal/ oxide interface in the opposite direction [35,36]. Direct ejection of Al 3+ from metal/oxide interface into electrolyte has been indicated by coating-ratio measurement and tracer experiments [36].…”

“…The electric-field assisted mechanism involves oxide decomposition which results in direct ejection of molten aluminum (Al 3+ ) into the solution followed by oxide formation via transport of oxygen ion (O 2-) at oxide/electrolyte interface and Al 3+ transport at metal/ oxide interface in the opposite direction [35,36]. Direct ejection of Al 3+ from metal/oxide interface into electrolyte has been indicated by coating-ratio measurement and tracer experiments [36]. The reaction at the anode and cathode are listed below [35,36].…”

“…Direct ejection of Al 3+ from metal/oxide interface into electrolyte has been indicated by coating-ratio measurement and tracer experiments [36]. The reaction at the anode and cathode are listed below [35,36].…”