Anodic Oxide Nanostructures Research Articles

Passivation and dissolution mechanisms in ordered anodic tantalum oxide nanostructures

Tantalum oxide (Ta2O5) nanostructures exhibit outstanding electrical and optical properties, as well as, high chemical resistance and stability. These materials have great potential for biomedical, catalysis, semiconductors and energy applications due to their large surface area and high specific charge, when arranged in nanoporous or nanotubular morphologies. In order to obtain these structures, an anodization process, which is inexpensive, reproducible and easy to scale up, is used. Yet, depending on the anodization conditions, the formation of a nanoporous or nanotubular layer is difficult to stabilize during the anodization process. In this regard, anodized tantalum oxide nanostructures were produced to understand the effect of the anodization conditions, including electrolyte concentration, potential and time. The nanopores or nanotubes morphologies, their chemical composition and structure were investigated by FIB-SEM, double-corrected TEM-STEM and EDS. We found that it is necessary to have high acid concentrations (mixture of H2SO4 with HF) to be able to form nanoporous or nanotubular structures. Despite the capacity of HF to dissolve and create anodic oxide nanostructures, the amount of H2SO4 concentration in the mixture is very important, leading to a dimple morphology. Furthermore, the increase of the anodization potential/electrical field clearly leads to an increase in the dimples diameter.

Surface characteristics and adhesive strength to epoxy of three different types of titanium alloys anodized in NaTESi electrolyte

Three different types of titanium alloys (near-alpha, near-beta and alpha–beta) were anodized in NaTESi electrolyte to compare the surface characteristics and adhesive strength between the anodic oxide nanostructures and the epoxy resin. FESEM, XRD and EDS were used to evaluate the morphology and crystalline structure of the anodic oxide films. The wettability and roughness of the anodic oxide films were measured using a contact angle meter and a digital video microscope. The correlation between the adhesive strength, surface morphology, phase composition, roughness and wettability of three different types of anodic oxide films is discussed in detail. The results showed that a uniform, dense oxide film was formed on the TA15 samples and a micro-protrusion with pore oxide film was formed on the TB8 samples, whereas an easily dissolved oxide film with large-scale pits was formed on the TC4 samples. The maximum shear strengths of TA15 (15.5MPa), TB8 (19.2MPa) and TC4 (17.6MPa) were obtained after they were anodized at 20V, 15V and 10V, respectively. The wettability of the anodic oxide films increased with increasing voltage and a notable increase of the roughness is obtained after anodization. The TB8, which is of the near-beta titanium alloy type can provide a better surface topography for adhesion.

Biomedical Polymer Surface Modification of Beta-Type Titanium Alloy for Implants through Anodic Oxide Nanostructures

Anodic oxide nanostructures (nanopores and nanotubes) were formed on a biomedical β-type titanium alloy, Ti–29Nb–13Ta–4.6Zr alloy (TNTZ), in order to improve adhesive strength by the anchor effect of a segmented polyurethane (SPU) with soft tissue compatibility. The nanotube structure was formed beneath the nanoporous structure. The adhesive strength between the SPU coating and the nanoporous structure formed on TNTZ by anodization is more than 1.5 times that of an SPU coating on as-polished TNTZ with a mirror finish. After removal of the nanoporous structure by etching with HF solution, the adhesive strength of the SPU coating on the exposed nanotube structure is decreased.

Adhesive strength of medical polymer on anodic oxide nanostructures fabricated on biomedical β-type titanium alloy

Anodic oxide nanostructures (nanopores and nanotubes) were fabricated on a biomedical β-type titanium alloy, Ti–29Nb–13Ta–4.6Zr alloy (TNTZ), by anodization in order to improve the adhesive strength of a medical polymer, segmented polyurethane (SPU), to TNTZ. TNTZ was anodized in 1.0M H3PO4 solution with 0.5 mass% NaF using a direct-current power supply at a voltage of 20V. A nanoporous structure is formed on TNTZ in the first stage of anodization, and the formation of a nanotube structure occurs subsequently beneath the nanoporous structure. The nanostructures formed on TNTZ by anodization for less than 3600s exhibit higher adhesive strengths than those formed at longer anodization times. The adhesive strength of the SPU coating on the nanoporous structure formed on top of TNTZ by anodization for 1200s improves by 144% compared to that of the SPU coating on as-polished TNTZ with a mirror surface. The adhesive strength of the SPU coating on the nanotube structure formed on TNTZ by anodization for 3600s increases by 50%. These improvements in the adhesive strength of SPU are the result of an anchor effect introduced by the nanostructures formed by anodization. Fracture occurs at the interface of the nanoporous structure and the SPU coating layer. In contrast, in the case that SPU coating has been performed on the nanotube structure, fracture occurs inside the nanotubes.

Nucleation and growth of the nanostructured anodic oxides on tantalum and niobium under the porous alumina film

Anodic oxidation of Ta–Al (aluminium deposited on tantalum) and Nb–Al (aluminium deposited on niobium) has been performed in organic and inorganic acid electrolytes for porous alumina formation. Arrays of tantalum nanoscale oxide ‘hillocks’ and niobium oxide ‘goblets’ derived from the anodised Ta–Al and Nb–Al bilayer samples have been investigated by scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy depth profiling. Anodising proceeds in the sequence of growth of porous anodic alumina and, when the aluminium layer is consumed up to the underlying metal, growth of anodic tantalum (niobium) oxide under the bottoms of the alumina pores. The oxidation of the underlying metal results from metal ions migrating outward and oxygen ions transported through, and released from, the alumina barrier layer, which dissolves at the tantala/alumina interface, i.e. without tantalum being in direct contact with the electrolyte. The shape and mutual arrangement of the anodic oxide nanostructures depend on the nature of the underlying metal, anodising solution, and are consistent with the difference between the resistivities of the tantalum (niobium) oxide formed and the barrier layer of the overlying alumina cells, which is influenced by incorporated electrolyte-derived species.