Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations

In titanium alloys, the ω(hexagonal)-phase transformation has been categorized as either a diffusion-mediated isothermal transformation or an athermal transformation that occurs spontaneously via a diffusionless mechanism. Here we report a diffusionless isothermal ω transformation that can occur even above the ω transformation temperature. In body-centered cubic β-titanium alloyed with β-stabilizing elements, there are locally unstable regions having fewer β-stabilizing elements owing to quenched-in compositional fluctuations that are inevitably present in thermal equilibrium. In these locally unstable regions, diffusionless isothermal ω transformation occurs even when the entire β region is stable on average so that athermal ω transformation cannot occur. This anomalous, localized transformation originates from the fluctuation-driven localized softening of 2/3[111]β longitudinal phonon, which cannot be suppressed by the stabilization of β phase on average. In the diffusionless isothermal and athermal ω transformations, the transformation rate is dominated by two activation processes: a dynamical collapse of {111}β pairs, caused by the phonon softening, and a nucleation process. In the diffusionless isothermal transformation, the ω-phase nucleation, resulting from the localized phonon softening, requires relatively high activation energy owing to the coherent β/ω interface. Thus, the transformation occurs at slower rates than the athermal transformation, which occurs by the widely spread phonon softening. Consequently, the nucleation probability reflecting the β/ω interface energy is the rate-determining process in the diffusionless ω transformations.Tane M., Nishiyama H., Umeda A., et al. Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations. Physical Review Materials 3, 043604 (2019); https://doi.org/10.1103/PhysRevMaterials.3.043604

Effect of Nanosheet Surface Structure of Titanium Alloys on Cell Differentiation

Titanium alloys are the most frequently used dental implants partly because of the protective oxide coating that spontaneously forms on their surface. We fabricated titania nanosheet (TNS) structures on titanium surfaces by NaOH treatment to improve bone differentiation on titanium alloy implants. The cellular response to TNSs on Ti6Al4V alloy was investigated, and the ability of the modified surfaces to affect osteogenic differentiation of rat bone marrow cells and increase the success rate of titanium implants was evaluated. The nanoscale network structures formed by alkali etching markedly enhanced the functions of cell adhesion and osteogenesis-related gene expression of rat bone marrow cells. Other cell behaviors, such as proliferation, alkaline phosphatase activity, osteocalcin deposition, and mineralization, were also markedly increased in TNS-modified Ti6Al4V. Our results suggest that titanium implants modified with nanostructures promote osteogenic differentiation, which may improve the biointegration of these implants into the alveolar bone

Crystal Growth of Thiol-Stabilized Gold Nanoparticles by Heat-Induced Coalescence

A monolayer of dodecanethiol-stabilized gold nanoparticles changed into two-dimensional and three-dimensional self-organized structures by annealing at 323 K. Subsequent crystal growth of gold nanoparticles occurred. Thiol molecules, although chemisorbed, form relatively unstable bonds with the gold surface; a few thiols desorbed from the surface and oxidized to disulfides at 323 K, because the interaction energy between thiol macromolecules is larger than that between a thiol and a nanoparticle. The gold nanoparticles approached each other and grew into large single or twinned crystals because of the van der Waals attraction and the heat generated by the exothermic formation of disulfides

A novel method for synthesis of titania nanotube powders using rapid breakdown anodization

The present paper describes a new method utilizing rapid anodization to quickly synthesize high-quality, high aspect ratio, robust titanium dioxide nanotube powders. TiO2 nanotube powders, with a typical nanotube outer diameter of approximately 40 nm, wall thickness of approximately 8−15 nm, and length of about 10−35 μm, were synthesized by potentiostatic rapid breakdown anodization of titanium foils in aqueous electrolytes of 0.3 M NaCl or 0.1 M HClO4 under an applied potential of 20 V. High reactivity and ultrahigh reaction rate are cornerstones responsible for periodic release of TiO2 nanotubes into solution and formation of a white precipitate of TiO2 nanotubes. The reaction yield is approximately 4−6 g in less than 3 h, and the approximate cost of the material is $3.50/g, based on the laboratory-scale production. Various characterization techniques, including FESEM, HRTEM, EDX, XRD, XPS, FT-IR, UV−visible diffuse-reflectance, and N2 adsorption, have been used to probe morphology, microstructure, crystallographic, composition, bond configuration, optical properties, and surface area of the nanotubes. XPS and EDX investigations show that nanotubes formed in NaCl/phosphate electrolyte solutions contain a significant amount of phosphorus species, which strongly affects crystallization and phase transformation of TiO2. Namely, phosphate-incorporating nanotubes stabilized the anatase phase, and initiation of the rutile phase was observed at annealing temperatures ≥700 °C. The resulting nanotube powders have a significant level of OH groups with a band gap ranging from 3.04 to 3.23 eV. Our results indicate that rapid breakdown anodization is highly efficient in the production of good-quality TiO2 nanotube powders, which makes it an alternative to well-documented conventional methods

Dye-sensitized solar cell based on anodic TiO2 nanotubes produced from anodization in fluoride-free electrolyte

An increasing energy demand and environmental pollution create a pressing need for clean and sustainable energy solutions. T1O2 semiconductor material is expected to play an important role in helping solve the energy crisis through effective utilization of solar energy based on photovoltaic devices. Dye-sensitized solar cells (DSSCs) are potentially lower cost alternative to inorganic siliconbased photovoltaic. In the present work, we report about the fabrication of dye-sensitized solar cell (DSSCs) from anodic T1O2 nanotubes powder, produced by potentiostatic anodization of Ti foil in 0.1 M HCIO4 electrolyte, as photoanode. The counter electrode was made by electrodeposition of Pt from an aqueous solution of 5 mM F^PtCU onto fluorine-doped tin oxide glass substrate (FTO-glass). The above frontside illuminated DSSCs were compared with back-side-illuminated DSSCs fabricated from anodic T1O2 NTs that were grown on the top of Ti foil as photoanode. The highest cell efficiency was 3.54 % under 100 mW/cm2 light intensity (1 sun AM 1.5 G light, Jsc = 14.3 mA/cm2, Foe = 0.544 V, fill factor = 0.455). To the best of our knowledge, this is the first report on the fabrication of dyesensitized solar cell from anodic T1O2 NTs powder. The T1O2/FTO photoanodes were characterized by FE-SEM, XRD and Uv-visible spectroscopy. The catalytic properties of Pt/FTO counter electrodes have been examined by cyclic voltammetry

Preparation and characterization of high aspect ratio TiO2 nanotube powders using rapid anodisation method in chloride-based electrolytes

This new method describes, for the first time, the application of rapid anodization in chloride-based electrolytes to quickly synthesize high quality, high-aspect ratio and robust titanium dioxide nanotube powders. This titania nanotubes powder produced from potentiostatic anodization of titanium foil in an electrolyte containing perchlorate or chloride ions. This would result in a more efficient usage of the titanium foil and in the production of large quantities (of the order of grams) titanium oxide tubes in less than 1 h. Further optimization of this route may provide a fast alternative method for the production of titanium oxide nanotube powders, now routinely synthesized via a hydrothermal method derived from the one pioneered by Kasuage et al. Various characterization techniques (viz., TEM, FESEM, XRD< DRUV-Visible, XPS) are used to study the morphology, phase, band gap and chemical composition

Biomimetic hydroxyapatite formation on silica-loaded anodic TiO2 nanotubes

Abstract not available

Fundamental photo-electrochemical studies of TiO2 and doped TiO2 nanotubes photo-electrode materials

Titanium dioxide nanotubes and titanium dioxide nanotubes doped with various chemical species such as phosphorus, carbon, boron or silica were prepared by the electrochemical rapid anodization approach. The presence of boron as a dopant affects the band gap energy of TiO2 NTs in the form of 0.18eV red shift. This finding is considered as an advantage to overcome the wide band gap of TiO2 (pure anatase 3.25eV), therefore B-doped TiO2 NTs can absorb a large part of the solar energy spectrum. The photocurrent response of TiO2 NTs with and without doping was investigated by the electrochemical linear sweep voltammetry (LSV). The results will indicate that the doped TiO2 NTs prepared by rapid anodization method will provide larger photocurrents, thus improve their performance as a photoanode in dye-sensitizer solar cells