A theory of avalanche breakdown during anodic oxidation

Simulation and Optimization of the Auxiliary Cathode for Inter-Electrode Discharge Electric Field in Microarc Oxidation

Currently, research on the edge effect issue in the micro-arc oxidation process primarily focuses on investigating process conditions and enhancing additives. However, some scholars have utilized finite element analysis software to simulate the edge effect during the simulation process, overlooking the investigation of the morphology of the auxiliary cathode. This study analyzes the growth characteristics of the oxide film on aluminum alloy 2A12 during micro-arc oxidation. Additionally, the inter-electrode discharge electric field is simulated using the finite element analysis method. The auxiliary cathode is optimized to mitigate the influence of the edge effect on the film layer. The findings indicate that employing a cylindrical shape as the auxiliary cathode instead of a rectangular groove leads to an increased thickness of the micro-arc oxidation film. However, it also results in an augmented length of the film layer affected by the edge effect at both ends of the workpiece. Decreasing the distance between the auxiliary cathode and the workpiece surface leads to a higher thickness of the obtained micro-arc oxidation film. Decreasing the length of the auxiliary cathode results in a reduction in both the thickness of the film layer on the workpiece surface and the area affected by the edge effect. Increasing the eccentric cone ratio of the auxiliary cathode enhances the uniformity of the micro-arc oxidation film layer. In this study, we present a novel auxiliary cathode model that incorporates a smaller cylindrical shell at the center and eccentric cone shells on each side. This model has the potential to enhance the optimization rate of the micro-arc oxidation film layer on cylindrical workpieces by 17.77%.

Application of Micro-Arc Discharges during Anodization of Tantalum for Synthesis of Photocatalytic Active Ta2O5 Coatings

Ta₂O₅ coatings were created using micro-arc discharges (MDs) during anodization on a tantalum substrate in a sodium phosphate electrolyte (10 g/L Na₃PO₄·10H₂O). During the process, the size of MDs increases while the number of MDs decreases. The elements and their ionization states present in MDs were identified using optical emission spectroscopy. The hydrogen Balmer line H_β shape analysis revealed the presence of two types of MDs, with estimated electron number densities of around 1.1 × 10²¹ m^-3 and 7.3 × 10²¹ m^-3. The effect of MDs duration on surface morphology, phase and chemical composition, optical absorption, and photoluminescent, properties of Ta₂O₅ coatings, as well as their applications in photocatalytic degradation of methyl orange, were investigated. The created coatings were crystalline and were primarily composed of Ta₂O₅ orthorhombic phase. Since Ta₂O₅ coatings feature strong absorption in the ultraviolet light region below 320 nm, their photocatalytic activity is very high and increases with the time of the MDs process. This was associated with an increase of oxygen vacancy defects in coatings formed during the MDs, which was confirmed by photoluminescent measurements. The photocatalytic activity after 8 h of irradiation was around 69%, 74%, 80%, and 88% for Ta₂O₅ coatings created after 3 min, 5 min, 10 min, and 15 min, respectively.

A phosphoric anion layer inhibits electronic current generation and nanotube growth during anodization of titanium

Nowadays the formation mechanism of anodic TiO₂ nanotubes has attracted extensive attention. Field-assisted dissolution (TiO₂ + 6F^- + 4H⁺ → [TiF₆]^2- + 2H₂O) has been considered as the causal link to the formation and growth of nanotubes. But it is hard for this theory to explain three stages of the current-time curve. Here, the anodization of titanium was studied by adding different concentrations of H₃PO₄ (0%, 4 wt%, 6 wt%, 8 wt%, and 10 wt%) in ethylene glycol containing the same concentration of NH₄F (0.5 wt%). The results prove that under the action of the same concentration of NH₄F, the growth rate of nanotubes decreases obviously with the increase of H₃PO₄ concentration, and the second stage of the current-time curve is also prolonged simultaneously. These experimental facts cannot be interpreted by field-assisted dissolution theory and the viscous flow model. Here, an anion layer formed by H₃PO₄ and the electronic current theory are ably used to explain these facts reasonably for the first time.

Fluoride-free synthesis of anodic TiO2 nanotube layers: a promising environmentally friendly method for efficient photocatalysts

TiO₂ nanotube (TNT) layers are generally prepared in fluoride-based electrolytes via electrochemical anodization that relies on the field-assisted dissolution of Ti metal forming nanoporous/nanotubular structures. However, the usage of fluoride ions is considered hazardous to the environment. Therefore, we present an environmentally friendly synthesis and application of TNT layers prepared in fluoride-free nitrate-based electrolytes. A well-defined nanotubular structure with thickness up to 1.5 μm and an inner tube diameter of ∼55 nm was obtained within 5 min using aqueous X(NO₃)_Y electrolytes (X = Na⁺, K⁺, Sr²⁺, Ag⁺). For the first time, we show the photocatalytic performance (using a model organic pollutant), HO˙ radical production, and thorough characterization of TNT layers prepared in such electrolytes. The highest degradation efficiency (k = 0.0113 min^-1) and HO˙ radical production rate were obtained using TNT layers prepared in AgNO₃ (Ag-NT). The intrinsic properties of Ag-NT such as the valence band maximum of ∼2.9 eV, surface roughness of ∼6 nm, and suitable morphological features and crystal structure were obtained. These results have the potential to pave the way for a more environmentally friendly synthesis of anodic TNT layers in the future using the next generation of fluoride-free nitrate-based electrolytes.

Understanding the influence of electrolyte aging in electrochemical anodization of titanium

Titania nanotubes or nanopores self-ordered on electrochemically anodized (EA) titanium have been widely applied towards photocatalysis, solar cells, purification and biomedical implants. As a result, significant research has been focused towards optimizing anodization to fabricate controlled, stable and reproducible nanostructures. Among these, the use of organic-based electrolyte, like ethylene glycol (with NH₄F and water), to anodize Ti has been widely applied and researched. Interestingly, among the various influencing EA factors, electrolyte aging (repeated EA using non-target Ti, prior to EA of target Ti substrate) has been underexplored, with only few studies aiming to optimize electrolyte aging and its influence on the nanostructures fabricated. Moreover, many research laboratories utilize electrolyte aging in Ti anodization, but this practice is seldom reported. In this extensive and pioneering review, we discuss and detail electrolyte aging in Ti anodization to fabricate controlled nanostructures, and its influence on nanostructure characteristics including morphology, chemistry, stability and application-specific performance. This review will inform future research aimed at optimizing electrolyte aging and Ti anodization to fabricate controlled nanostructures catering to specific application needs.

Addition of Organic Acids during PEO of Titanium in Alkaline Solution

This research study describes recent advances in understanding the effects of the addition of organic acids, such as acetic, lactic, citric and phytic acids, on the process of plasma electrolytic oxidation (PEO) on Ti using an alkaline bath. As the plasma developed over the workpiece is central to determine the particular morphological and structural features of the growing oxide, the focus is then on the inter-relationships between the electrolyte and the resultant plasma regime established. In situ optical emission spectroscopy (OES) allowed us to verify a marked plasma suppression when adding low-molecular-weight anions such as acetates, resulting in short-lived and well-distributed discharges. Conversely, when more bulky anions, such as lactates, citrates and phytates, were considered, a less efficient shielding of the electrode caused the build-up of long-lasting and destructive sparks responsible for the formation of thicker coatings, even >30 µm, at the expense of a higher roughness and loss of compactness. Corrosion resistance was tested electrochemically, according to electrochemical impedance spectroscopy (EIS), and weight losses evidenced the coatings produced in the solution containing acetates to be more suitable for service in H2SO4.

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.

Unraveling the six stages of the current-time curve and the bilayer nanotubes obtained by one-step anodization of Zr

The application and growth mechanism of anodic TiO2 nanotubes have been a hot topic in the last ten years, but the formation mechanism of anodic ZrO2 nanotubes has rarely been studied. In one-step constant voltage anodization of Al and Ti, the typical current-time curve has three stages. Moreover, the current-time curves of the three stages can last for 10 min or even 10 hours, resulting in a single layer of nanotubes with the same diameter due to the constant voltage in one-step anodization. However, in this paper, it was found for the first time that the three stages of the current-time curve appeared twice in succession during one-step constant voltage anodization of Zr for only 900 seconds, and bilayer nanotubes with increased diameter were obtained. This six-stage current-time curve cannot be explained by classical field-assisted dissolution and field-assisted flow or stress-driven mechanisms. Here, the formation mechanism and growth kinetics of bilayer ZrO2 nanotubes have been clarified rationally by the theories of ionic current, electronic current and oxygen bubble mold. The interesting results presented in this paper are of great significance for revealing the anodizing process of various metals and the formation mechanism of porous structures.

Debunking the essential effect of temperature and voltage on the current curve and the nanotube morphology

The formation mechanism of anodic TiO2 nanotubes remains to be unclear till now. Many researchers study the influence of temperatures above 0 °C instead of below 0 °C. Few papers before have explained the relationship between the current-time curve and the morphology of the nanotubes. In this study, the innovative 'oxygen bubble model' and the ionic current and electronic current theories were introduced to explain the growth of nanotubes below 0 °C. The length of anodic TiO2 nanotubes at 15 °C, 0 °C, -10 °C were 1.28 μm, 0.93 μm and 0.21 μm, respectively, but the diameter of anodic TiO2 nanotubes was almost the same, at about 164 nm. When the temperature was low, the magnitude of electronic current and the ionic current was small, the mold effect was weak and nanotubes could not be formed. At the same time, this study shows that the dissolution reaction of the field-assisted solution theory has no electron gain or loss, and it has nothing to do with the current, which negates the field-assisted dissolution theory. A novel two-step anodization was used to verify the conclusion. It was found that nanotubes could be obtained when the anodizing current was decreasing or increasing. Also, ginseng-shaped nanotubes are formed at a particular voltage sequence. Based on the 'oxygen bubble model' and the ionic current and electronic current theories, the formation process of nanotubes of two-step anodization is explained clearly.

Anodic TiO2 Nanotubes on 3D-Printed Titanium Meshes for Photocatalytic Applications

In this work, large 3D Ti meshes fabricated by direct ink writing were wirelessly anodized for the first time to prepare highly photocatalytically active TiO₂ nanotube (TNT) layers. The use of bipolar electrochemistry enabled the fabrication of TNT layers within the 3D Ti meshes without the establishment of an electrical contact between Ti meshes and the potentiostat, confirming its unique ability and advantage for the synthesis of anodic structures on metallic substrates with a complex geometry. TNT layers with nanotube diameters of up to 110 nm and thicknesses of up to 3.3 μm were formed. The TNT-layer-modified 3D Ti meshes showed a superior performance for the photocatalytic degradation of methylene blue in comparison to TiO₂-nanoparticle-decorated and nonanodized Ti meshes (with a thermal oxide layer), resulting in multiple increases in the dye degradation rate. The results presented here open new horizons for the employment of anodized 3D Ti meshes in various flow-through (photo)catalytic reactors.

Peculiarities of Aluminum Anodization in AHAs-Based Electrolytes: Case Study of the Anodization in Glycolic Acid Solution

The anodization of aluminum (Al) in three alpha-hydroxy acids (AHAs): glycolic (GC), malic (MC), and citric (CC), was analyzed. Highly ordered pores in GC were obtained for the first time. However, the hexagonal cells were characterized by a non-uniform size distribution. Although common features of current density behavior are visible, the anodization in AHAs demonstrates some peculiarities. The electric conductivity (σ) of 0.5 M GC, MC, and CC electrolytes was in the following order: σ(CC) > σ(MC) > σ(GC), in accordance with the acid strength pK_a(CC) < pK_a(MC) < pK_a(GC). However, the anodization voltage, under which a self-organized pore formation in anodic alumina (AAO) was observed (U_max), decreased with increasing pK_a: U_max(CC) > U_max(MC) ≥ U_max(GC). This unusual behavior is most probably linked with the facility of acid ions to complex Al and the active participation of the Al complexes in the AAO formation. Depending on the AHA, its tendency and different modes to coordinate Al ions, the contribution of stable Al complexes to the AAO growth is different. It can be concluded that the structure of Al complexes, their molecular mass, and the ability to lose electrons play more important roles in the AAO formation than pK_a values of AHAs.

Evidence of oxygen bubbles forming nanotube embryos in porous anodic oxides

Anodic TiO2 nanotubes have been studied widely for two decades because of their regular tubular structures and extensive applications. However, the formation mechanism of anodic TiO2 nanotubes remains unclear, because it is difficult to find convincing evidence for popular field-assisted dissolution or field-assisted injection theories and the oxygen bubble model. Here, in a bid to find direct evidence that oxygen bubbles form nanotube embryos, a new method is applied to handle this challenge. Before nanotube formation, a dense cover layer was formed to make nanotubes grow more slowly. Many completely enclosed nanotube embryos formed by oxygen bubbles were found beneath the dense cover layer for the first time. The formation of these enclosed and hollow gourd-shaped embryos is convincing enough to prove that the nanotubes are formed by the oxygen bubble mold, similar to inflating a football, rather than by field-assisted dissolution. Based on the 'oxygen bubble model' and ionic current and electronic current theories, the formation and growth process of nanotube embryos is explained clearly for the first time. These interesting findings indicate that the 'oxygen bubble model' and ionic current and electronic current theories also apply to anodization of other metals.

Fabrication and application of nanoporous anodic aluminum oxide: a review

Due to the unique optical and electrochemical properties, large surface area, tunable properties, and high thermal stability, nanoporous anodic aluminum oxide (AAO) has become one of the most popular materials with a large potential to develop emerging applications in numerous areas, including biosensors, desalination, high-risk pollutants detection, capacitors, solar cell devices, photonic crystals, template-assisted fabrication of nanostructures, and so on. This review covers the mechanism of AAO formation, manufacturing technology, the relationship between the properties of AAO and fabrication conditions, and applications of AAO. Properties of AAO, like pore diameter, interpore distance, wall thickness, and anodized aluminum layer thickness, can be fully controlled by fabrication conditions, including electrolyte, applied voltage, anodizing and widening time. Generally speaking, the pore diameter of AAO will affect its specific application to a large extent. Moreover, manufacturing technology like one/two/multi step anodization, nanoimprint lithography anodization, and pulse/cyclic anodization also have a major impact on overall array arrangement. The review aims to provide a perspective overview of the relationship between applications and their corresponding AAO pore sizes, systematically. And the review also focuses on the strategies by which the structures and functions of AAO can be utilized.

Bioactive multi-elemental PEO-coatings on titanium for dental implant applications

Bioactive PEO (Plasma Electrolytic Oxidation) coatings were generated on Grade I commercially pure titanium for dentistry applications using a Ca/P-based electrolyte with added Si, Mg, Zn or F species. Surface characteristics, chemical composition and ion liberation of the coatings were characterized using SEM/EDS, X-ray diffraction, optical profilometry, contact angle and ICP-OES. Corrosion resistance (OCP and DC polarization) was evaluated in SBF. Osteoblastogenesis and osteoclastogenesis processes on PEO-coated Ti and non-coated Ti controls were assessed after 7 days and 5 days of cell culture, respectively. Monolayer formation and metabolic activity were evaluated for the MC3T3 preosteoblastic cell line. All PEO coatings favoured differentiation processes over proliferation and presented three times greater quantity of secreted collagen than non-coated Ti control. All coating enabled osteoclast differentiation, with differences in number and size of the osteoclasts between the materials.

Electron-Donor and -Acceptor Agents Responsible for Surface Modification Optimizing Electrochemical Performance

The electrochemical roles of electron-donor and -acceptor agents in surface reforming of magnesium alloy were investigated via plasma electrolysis. The surface modification was performed in an aluminate-based electrolyte, having urea and hydrazine with inherent molecular structures, which might act as electron acceptor and donor during plasma-assisted electrochemical reaction. The presence of hydrazine working as donor would promote the formation of magnesium aluminates in the oxide layer, resulting in superior compactness of the oxide layer to that when urea was used as the working as acceptor since the precipitation of MgCO₃ was favored in the electrolyte with urea. The thickness of the oxide layer formed by a combination of urea and hydrazine was higher than urea, while the porosity was higher than hydrazine. The electrochemical performance was enhanced in the order of hydrazine, urea and hydrazine combined, and urea, which was discussed on the basis of impedance interpretation.

Simulation of anodizing current–time curves and the morphology evolution of TiO2 nanotubes obtained in phosphoric electrolytes

The simulation and separation of anodizing current density–time curves obtained in mixed electrolytes.

Surface characterization and corrosion behavior of calcium phosphate-base composite layer on titanium and its alloys via plasma electrolytic oxidation: A review paper

In recent years, calcium phosphate-base composites, such as hydroxyapatite (HA) and carbonate apatite (CA) have been considered desirable and biocompatible coating layers in clinical and biomedical applications such as implants because of the high resistance of the composites. This review focuses on the effects of voltage, time and electrolytes on a calcium phosphate-base composite layer in case of pure titanium and other biomedical grade titanium alloys via the plasma electrolytic oxidation (PEO) method. Remarkably, these parameters changed the structure, morphology, pH, thickness and crystallinity of the obtained coating for various engineering and biomedical applications. Hence, the structured layer caused improvement of the biocompatibility, corrosion resistance and assignment of extra benefits for Osseo integration. The fabricated layer with a thickness range of 10 to 20 μm was evaluated for physical, chemical, mechanical and tribological characteristics via XRD, FESEM, EDS, EIS and corrosion analysis respectively, to determine the effects of the applied parameters and various electrolytes on morphology and phase transition. Moreover, it was observed that during PEO, the concentration of calcium, phosphor and titanium shifts upward, which leads to an enhanced bioactivity by altering the thickness. The results confirm that the crystallinity, thickness and contents of composite layer can be changed by applying thermal treatments. The corrosion behavior was investigated via the potentiodynamic polarization test in a body-simulated environment. Here, the optimum corrosion resistance was obtained for the coating process condition at 500 V for 15 min in Ringer solution. This review has been summarized, aiming at the further development of PEO by producing more adequate titanium-base implants along with desired mechanical and biomedical features.

Theoretical derivation of anodizing current and comparison between fitted curves and measured curves under different conditions

Anodic TiO2 nanotubes have been studied extensively for many years. However, the growth kinetics still remains unclear. The systematic study of the current transient under constant anodizing voltage has not been mentioned in the original literature. Here, a derivation and its corresponding theoretical formula are proposed to overcome this challenge. In this paper, the theoretical expressions for the time dependent ionic current and electronic current are derived to explore the anodizing process of Ti. The anodizing current-time curves under different anodizing voltages and different temperatures are experimentally investigated in the anodization of Ti. Furthermore, the quantitative relationship between the thickness of the barrier layer and anodizing time, and the relationships between the ionic/electronic current and temperatures are proposed in this paper. All of the current-transient plots can be fitted consistently by the proposed theoretical expressions. Additionally, it is the first time that the coefficient A of the exponential relationship (ionic current j(ion) = A exp(BE)) has been determined under various temperatures and voltages. And the results indicate that as temperature and voltage increase, ionic current and electronic current both increase. The temperature has a larger effect on electronic current than ionic current. These results can promote the research of kinetics from a qualitative to quantitative level.