On the contribution of electrostriction to charge-induced stresses in anodic oxide films

A review: research progress on the formation mechanism of porous anodic oxides

Owing to the great development potential of porous anodic oxides (PAO) in many fields, research on their formation mechanisms, fabrication processes and applications has a history of more than ten years. Although compared with research on the fabrication processes and applications of PAO, research on their formation mechanisms started late, several mainstream theories have been formed in the academic community, including the field-assisted dissolution (FAD) theory, the field-assisted ejection (FAE) theory, the self-organization theory, the ionic and electronic current theory and the oxygen bubble mould effect. This review will focus on summarizing the core views of the mainstream mechanisms mentioned above and comparing the explanations for some of their classical experimental phenomena.

Old is Gold: Electrolyte Aging Influences the Topography, Chemistry, and Bioactivity of Anodized TiO2 Nanopores

Titanium dioxide (TiO₂) nanostructures including nanopores and nanotubes have been fabricated on titanium (Ti)-based orthopedic/dental implants via electrochemical anodization (EA) to enable local drug release and enhanced bioactivity. EA using organic electrolytes such as ethylene glycol often requires aging (repeated anodization of nontarget Ti) to fabricate stable well-ordered nanotopographies. However, limited information is available with respect to its influence on topography, chemistry, mechanical stability, and bioactivity of the fabricated structures. In the current study, titania nanopores (TNPs) using a similar voltage/time were fabricated using different ages of electrolyte (fresh/0 h to 30 h aged). Current density vs time plots of EA, changes in the electrolyte (pH, conductivity, and Ti/F ion concentration), and topographical, chemical, and mechanical characteristics of the fabricated TNPs were compared. EA using 10-20 h electrolytes resulted in stable TNPs with uniform size and improved alignment (parallel to the underlying substrate microroughness). Additionally, to evaluate bioactivity, primary human gingival fibroblasts (hGFs) were cultured onto various TNPs in vitro. The findings confirmed that the proliferation and morphology of hGFs were enhanced on 10-20 h aged electrolyte anodized TNPs. This pioneering study systematically investigates the optimization of anodization electrolyte toward fabricating nanoporous implants with desirable characteristics.

Nonlinear evolution of a thin anodic film

The formation of pores in anodic aluminium oxide films is treated with a model equation. The model treats the oxide layer as a thin viscous liquid in two dimensions. Surface tension on the top boundary, electrostriction due to the external electric field and mass flow through the bottom boundary due to oxide formation are all included. Viscous flow is treated with the creeping flow assumption. The model equation is solved numerically using a Fourier spectral method in space and Adams–Bashforth/Adams–Moulton methods in time. Initial conditions include sinusoidal shapes as well as random shapes. The results show that pores form at the trough of the initial sinusoidal shape. Random shapes get smoothed before forming pore structures with spacing different than predicted by linear theory.

Aligned metal oxide nanotube arrays: key-aspects of anodic TiO2 nanotube formation and properties

Over the past ten years, self-aligned TiO₂ nanotubes have attracted tremendous scientific and technological interest due to their anticipated impact on energy conversion, environment remediation and biocompatibility. In the present manuscript, we review fundamental principles that govern the self-organized initiation of anodic TiO₂ nanotubes. We start with the fundamental question: why is self-organization taking place? We illustrate the inherent key mechanistic aspects that lead to tube growth in various different morphologies, such as ripple-walled tubes, smooth tubes, stacks and bamboo-type tubes, and importantly the formation of double-walled TiO₂ nanotubes versus single-walled tubes, and the drastic difference in their physical and chemical properties. We show how both double- and single-walled tube layers can be detached from the metallic substrate and exploited for the preparation of robust self-standing membranes. Finally, we show how by selecting specific growth approaches to TiO₂ nanotubes desired functional features can be significantly improved, e.g., enhanced electron mobility, intrinsic doping, or crystallization into pure anatase at high temperatures can be achieved. Finally, we briefly outline the impact of property, modifications and morphology on functional uses of self-organized nanotubes for most important applications.

Self-ordering dual-layered honeycomb nanotubular titania: a study in formation mechanisms

A model for the development of a novel self-ordering honeycomb morphology of TiO₂ is presented using a unique anodization process.

On the origin of damped electrochemical oscillations at silicon anodes (revisited)

Electrochemical oscillations accompanying the formation of anodic silica have been shown in the past to be correlated with rather abrupt changes in the mechanical stress state of the silica film, commonly associated with some kind of fracture or porosification of the oxide. To advance the understanding on the origin of such oscillations in fluoride-free electrolytes, we have revisited a seminal experiment reported by Lehmann almost two decades ago. We thereby demonstrate that the oscillations are not stress-induced, and do not originate from a morphological transformation of the oxide in the course of anodisation. Alternatively, the mechanical features accompanying the oscillations can be explained by a partial relaxation of the field-induced electrostrictive stress. Furthermore, our observations suggest that the oscillation mechanism more likely results from a periodic depolarisation of the anodic silica.

TiO2 nanotubes: synthesis and applications

TiO(2) is one of the most studied compounds in materials science. Owing to some outstanding properties it is used for instance in photocatalysis, dye-sensitized solar cells, and biomedical devices. In 1999, first reports showed the feasibility to grow highly ordered arrays of TiO(2) nanotubes by a simple but optimized electrochemical anodization of a titanium metal sheet. This finding stimulated intense research activities that focused on growth, modification, properties, and applications of these one-dimensional nanostructures. This review attempts to cover all these aspects, including underlying principles and key functional features of TiO(2), in a comprehensive way and also indicates potential future directions of the field.

On the use of a multiple beam optical sensor for in situ curvature monitoring in liquids

Several methods have been developed since the early 1900 to extract thin film stresses from the curvature of the substrate to which it is attached. One robust method particularly suitable for in situ curvature monitoring is the multiple beam optical sensor, which consists in measuring the change in relative spacings between parallel laser beams reflecting off the curved substrate. Although the technique is already well established for curvature monitoring in low pressure, gaseous environments, its use in liquid media has not yet received similar attention. Moreover, in the majority of the published work so far, spot spacings have been assumed to depend linearly on curvature. In this paper, it is first shown that this assumption may induce significant errors, particularly at large curvature. A more accurate set of equations is proposed. Next, the relationship between spot spacings and curvature is established when the substrate of interest is in a liquid, and a constitutive formula is proposed in that case as well. Finally, some practical aspects of the multiple beam technique for performing curvature measurements in a liquid are discussed. Various factors disturbing the measurement resolution are identified, with a specific interest for thin film anodizing, and a cell design is proposed to minimize their effect.

The role of viscous flow of oxide in the growth of self-ordered porous anodic alumina films

Porous anodic alumina (PAA) films are widely used as templates for functional nanostructures, because of the high regularity and controllability of the pore morphology. However, growth mechanisms have not yet been developed that can explain quantitative relationships between processing conditions and oxide layer geometry. Here, we present a model for steady-state growth of these amorphous films, incorporating the novel feature that metal and oxygen ions are transported by coupled electrical migration and viscous flow. The oxide flow in the model arises near the film-solution interface at the pore bottoms, in response to the constraint of volume conservation. The hypothesis of viscous flow was successfully validated through detailed comparisons to observations of the motion of tungsten tracers in the film. Predictions of localized tensile stress near nanoscale ridges at the metal-film interface were supported by observations of voids at these sites. We suggest that the ordering of PAA may be explained by a mechanism in which metal-film interface motion is regulated by the combination of ionic migration in the oxide and stress-driven interface diffusion of metal atoms.