Mechanical properties of microwave hydrothermally synthesized titanate nanowires

Nonlinear elastic aspects of multi-component iron oxide core-shell nanowires by means of atom probe tomography, analytical microscopy, and nonlinear mechanics

One-dimensional objects as nanowires have been proven to be building blocks in novel applications due to their unique functionalities. In the realm of magnetic materials, iron-oxides form an important class by providing potential solutions in catalysis, magnetic devices, drug delivery, or in the field of sensors. The accurate composition and spatial structure analysis are crucial to describe the mechanical aspects and optimize strategies for the design of multi-component NWs. Atom probe tomography offers a unique analytic characterization tool to map the (re-)distribution of the constituents leading to a deeper insight into NW growth, thermally-assisted kinetics, and related mechanisms. As NW-based devices critically rely on the mechanical properties of NWs, the appropriate mechanical modeling with the resulting material constants is also highly demanded and can open novel ways to potential applications. Here, we report a compositional and structural study of quasi-ceramic one-dimensional objects: α-Fe ⊕ α-FeOOH(goethite) ⊕ Pt and α-Fe ⊕ α-Fe₃O₄(magnetite) ⊕ Pt core-shell NWs. We provide a theoretical model for the elastic behavior with terms accounting for the geometrical and mechanical nonlinearity, prior and subsequent to thermal treatment. The as-deposited system with a homogeneous distribution of the constituents demonstrates strikingly different structural and elastic features than that of after annealing, as observed by applying atom probe tomography, energy-dispersive spectroscopy, analytic electron microscopy, and a micromanipulator nanoprobe system. During annealing at a temperature of 350 °C for 20 h, (i) compositional partitioning between phases (α-Fe, α-Fe₃O₄ and in a minority of α-Fe₂O₃) in diffusional solid-solid phase transformations takes place, (ii) a distinct newly-formed shell formation develops, (iii) the degree of crystallinity increases and (iv) nanosized precipitation of evolving phases is detected leading to a considerable change in the description of the elastic material properties. The as-deposited nanowires already exhibit a significantly large maximum strain (1-8%) and stress (3-13 GPa) in moderately large bending tests, which become even more enhanced after the annealing treatment resulting at a maximum of about 2.5-10.5% and 6-18 GPa, respectively. As a constitutive parameter, the strain-dependent stretch modulus undoubtedly represents changes in the material properties as the deformation progresses.

Beyond linearity: bent crystalline copper nanowires in the small-to-moderate regime

Several models can describe the nonlinear response of 1D objects to bending under a concentrated load. Successive stages consisting of geometrical and, additionally, mechanical non-linearity can be identified in moderately large extensions. We provide an explicit bending moment function with terms accounting for the linearity (Euler-Bernoulli), quasi-linearity, geometrical and finally, mechanical non-linearity as global features of a moderately large elastic deformation. We apply our method, also suitable for other metals, to the experimental data of Cu nanowires (NWs) with an aspect ratio of about 16 under different concentrated loadings. The spatial distribution of strain-hardening/softening along the wire or through the cross-section is also demonstrated. As a constitutive parameter, the strain-dependent stretch modulus represents, undoubtedly, changes in the material properties as the deformation progresses. At the highest load, the Green-Lagrange strain reaches a 12.5% extension with a corresponding ultra-high strength of about 7.45 GPa at the most strained volume still in the elastic regime. The determined stretch modulus indicates a significantly lower elastic response with an approximated Young's modulus (E ≅ 65 GPa) and a third-order elastic constant, C ₁₁₁ ≅ -350 GPa. Surprisingly, these constants suggest a 25-35% of that of the bulk counterparts. Ultimately, the method not only provides a quantitative description of the bent Cu NWs, but also indicates the robustness of the theory of nonlinear elasticity.

Fabrication of hydroxyapatite on pure titanium by micro-arc oxidation coupled with microwave-hydrothermal treatment

Porous hydroxyapatite (HA)-containing composite films were prepared by a novel method consisting of micro-arc oxidation (MAO) combined with microwave-hydrothermal (M-H) treatment. The morphology, composition and phase composition of the bioactive films were investigated with scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction. MTT assay was carried out to investigate the in vitro effects of the different surfaces on bone integration properties. The prepared MAO films consisted mainly of anatase, rutile and tricalcium phosphate along with amorphous calcium (Ca) and phosphorus (P) phases. The M-H-treated composite films were composed primarily of anatase, rutile and HA. As the time and temperature of the M-H treatment increased, the number of HA crystals gradually increased. Using the M-H method, HA was obtained at a lower temperature and in a shorter period of time compared to the conventional hydrothermal method. The results suggest that the M-H method significantly decreases the hydrothermal reaction temperature and also greatly shortens the reaction time. Due to the nanocrystallinity and porosity of the prepared composite films, the method presented here shows promise for the formation of bioactive materials for medical applications.

In Situ Mechanical Property Measurement of Titania Nanowires

One-dimensional (1D) nanostructure such as nanowires (NWs), nanobelts and nanorods have attracted tremendous attention in recent years due to their exceptional micro-structural properties and novel potential applications. In the present investigation, titania (TiO2) nanowires are synthesized by microwave hydrothermal process (MHP) treating TiO2 nano powder with NaOH inside a microwave oven for 5h at 210°C and 350W. The mechanical properties of as synthesized TiO2 nanowires are determined by bending it with a nanomanipulator inside a scanning electron microscope (SEM). Young’s moduli of the nanowires are measured to be approximately 11.870.923GPa.

The fabrication of TiO(2) nanorods from TiO(2) nanoparticles by organic protection assisted template method

We report here the fabrication of TiO(2) nanorods from TiO(2) nanoparticles by using the organic protection assisted template method. After the deposition of perylene nanotubes in the pores of the porous alumina membranes, implantation of TiO(2) nanoparticles resulted in the formation of TiO(2)/perylene composite nanorods. The diameters and lengths of the nanorods correspond well to the diameter of the pores of the membrane and the thickness of the template used. After removing the perylene protection layer by calcination, the TiO(2) nanorods obtained have an anatase structure, the same as that of the original TiO(2) nanoparticles. It is supposed that the organic layers protected the TiO(2) rods from damage during removal of the template by alkaline etching. Such an organic layer protection method presents a new approach to fabricating one-dimensional nanostructures from the corresponding nanoparticles by an AAO template.