Alexandre Mikowski

Tribo-mechanical characterization of rough, porous and bioactive Ti anodic layers.

Rough and porous titanium oxide layers, which are important features for improving the osseointegration of Ti implants with bone tissues, are obtained through the technique of anodic oxidation. The thicknesses of such coatings are typically in the order of micrometers, and their mechanical characterization can be assessed by instrumented indentation, provided that the composite nature of the surface is considered. Titania anodic layers were produced on Ti under galvanostatic mode using Ca-P-based electrolytes (a mixture of (CH3COO)2Ca⋅H2O and NaH2PO(4)⋅2H2O), employing current densities (J) of 150 mA/cm2 and 300 mA/cm2. The structure and morphology were characterized by X-ray diffraction (XRD), scanning electron microscopy with electron dispersive X-ray spectroscopy (SEM/EDS), and profilometry, and the chemical features were characterized by X-ray photoelectron spectroscopy (XPS). TiO2 layers presented the crystalline phases rutile and anatase, and incorporation of Ca and P presented as a calcium phosphate compound. The porosity, roughness, and thickness increased with J. Analytical methods were employed to obtain the modified layers elastic modulus and hardness from instrumented indentation data, deducting the substrate and roughness effects. The elastic moduli were about 40 GPa for both values of J, which are similar to the values for human bones (10-40 GPa). The hardness decreased with indentation load, varying from 5 GPa at the near surface to 1 GPa at the layer-substrate interface. Such hardness behavior is a consequence of the surface brittleness under normal loading. Additional scratch tests using an acute tip indicated that the layer integrity under shear forces was 220 mN (J=150 mA/cm2) and 280 mN (J=300 mA/cm2). TiO2 layers produced with both current densities presented good results for in vitro bioactivity tests using simulated body fluid (SBF) solution, which can be attributed to a combined effect of the microstructure, layer porosity, and hydroxyl radicals in plenty at the near surface.

Effect of adsorbed/intercalated anionic dyes into the mechanical properties of PVA: layered zinc hydroxide nitrate nanocomposites.

Zinc hydroxide nitrate (ZHN) was adsorbed with anions of blue dyes (Chicago sky blue, CSB; Evans blue, EB; and Niagara blue, NB) and intercalated with anions of orange dyes (Orange G, OG; Orange II, OII; methyl orange, MO). Transparent, homogeneous and colored nanocomposite films were obtained by casting after dispersing the pigments (dye-intercalated/adsorbed into LHSs) into commercial poly(vinyl alcohol) (PVA). The films were characterized by XRD, UV-Vis spectroscopy, and mechanical testing. The mechanical properties of the PVA compounded with the dye-intercalated/adsorbed ZHN were evaluated, and reasonable increases in Youngs modulus and ultimate tensile strength were observed, depending on the amount and choice of layered filler. These results demonstrate the possibility of using a new class of layered hydroxide salts intercalated and adsorbed with anionic dyes to prepare multifunctional polymer nanocomposite materials.

A method to measure fracture toughness using indentation in REBa2Cu3O7-δ superconductor single crystals

A method is proposed to estimate indentation fracture toughness based on the critical load for crack nucleation calculated by the derivative of the loading curve. This method is applied to REBa2Cu3O7-δ superconductor single crystals based on xenotime, where is difficult to observe well defined radial crack lengths due to its lamellar structure. The indentation fracture toughness agrees well with that obtained from the traditional method based on radial crack propagation. The proposed procedure is an alternative technique to estimate indentation fracture toughness without the necessity of measuring radial cracks lengths while keeping the advantages of the traditional method.

A Mini-Review on the Progress of Spherical Bacterial Cellulose Production

Bacterial cellulose (BC) is a polymer produced by some bacteria and it is highly pure when compared to plant cellulose. Its structure and properties are unique, which makes it a material of a large commercial interest due to its broad potential of applications. The biosynthesis of BC may be carried out in two ways either static or agitated culture. Static culture usually generates BC membranes whereas agitated tends to produce spherical shapes. The production in agitated culture often enables an increase of yield for cellulose production. However, it provides a material with lower mechanical properties compared to static culture. The processing parameters and the phenomena governing the formation of BC in agitated cultivation have not fully established. A greater understanding of the phenomena and parameters inherent in the production of BC in agitated culture is necessary to achieve a technological progress. Some limitations concerning molecular weight control and BC structure made in agitated culture can improve with a better understanding of the culture conditions and the biosynthesis evolution. The ex-situ or in-situ insertion of additives may be performed to increase some specific properties of BC over agitated culture. Thus, the principal objective of this work is to discuss and provide a broad literature review on the techniques of BC production by agitated culture.

Magnetic and mechanical properties of rolled-up Au/Co/Au nanomembranes with multiple windings

Rolled-up Au/Co/Au microtubes with up to three windings were fabricated by the combination of strain engineering, conventional photolithography, and electron beam deposition. First, magnetization properties of the initial 2D film arrays and the corresponding tube arrays were studied and strong influences of magnetostrictive and shape anisotropy are observed. Second, the mechanical deformation was examined by an instrumented indentation technique at the nanoscale and analyzed by contact mechanics theory. The loading curve fitting in the elastic regime by the Hertz model provides a first approximation of the nanomembrane radial elastic modulus of about 135 GPa.

Preparation and Characterization of Bacterial Cellulose/TiO2 Hydrogel Nanocomposite

Bacterial cellulose (BC) is a biopolymer with interesting properties, such as biocompatibility, high tensile strength, high absorption capacity, water retention and high crystallinity. Nanoparticles of titanium dioxide (TiO2) are extremely important in electrical applications, photocatalysis, sensors and biomedical areas. Multifunctional materials, based on bacterial cellulose, with differentiated properties can be designed from the BC/TiO2 nanocomposite by ex situ method of sol-gel immersion. It was manufactured as a nanocomposite consisting of BC/TiO2 hydrogel. Characterizations were carried out by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and fourier transform infrared spectroscopy (FTIR). The morphological analysis of nanocomposite revealed the existence of molecular interaction and adhesion between TiO2 nanoparticles and cellulosic nanofibers matrix, where the presence of Ti peaks in EDS spectra was discovered, proving the successful incorporation of nanoparticles. The FTIR showed modification on the functional groups, suggesting interaction between the components. The manufacturing of a BC/TiO2 nanocomposite by method of sol-gel immersion has a great potential for future applications.

Depth-Sensing Indentation on REBa2Cu3O7−δ Single Crystals Obtained from Xenotime Mineral

A natural mixture of heavy rare-earth oxides extracted from xenotime mineral have been used to prepare large single crystals of the high-temperature REBa2Cu3O7−δ superconductor, grown using the CuO–BaO self-flux method. Its mechanical properties along the ab-plane were characterized using instrumented indentation. Hardness and elastic moduli were measured by the Oliver and Pharr method, which yielded 7.4u2009±u20090.2xa0GPa and the range 135–175xa0GPa at small depths, respectively. Increased loads promote the nucleation of lateral cracks, which reduce hardness and measured elastic modulus, as indicated by instrumented indentation at higher loads. The indentation fracture toughness, estimated by measuring the radial crack length from cube corner indentations at various loads, was found to be 0.8u2009±u20090.2xa0MPa m1/2. The observed slip systems of REBa2Cu3O7−δ single crystals were [100](001) and [010](001), the same as for YBa2Cu3O7−δ single crystals. The initial stages of deformation and fracture in the indentation process were investigated. The hardness and elastic modulus are not strongly modified by the crystallographic orientation in the ab-plane. This was interpreted in terms of resolved shear stresses in the active slip systems. Evidence of cracking along the {100} and {110} planes on the ab-plane was observed. In conclusion, the mechanical properties of REBa2Cu3O7−δ single crystals prepared from xenotime are equivalent to those of YBa2Cu3O7−δ single crystals from conventional rare-earth oxides.