Display omitted
•Raman and NEXAFS and TEM are used for the study of the C. granulata exoskeleton.•Calcite and amorphous calcium carbonate (ACC) are detected in the crab cuticle.•They are unevenly ...distributed across the exoskeleton.•The Ca L-edge crystal field splitting is a measure of ACC in the total CaCO3 content.•ACC has a structural role in the exoskeleton.
The exoskeleton of crustaceans consists of chitin biopolymers where the embedded inorganic biominerals, mainly CaCO3, affect strongly its mechanical properties. Raman and Near Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopies and Transmission Electron Microscopy (TEM) are applied to investigate the CaCO3 structure in various parts of the Calappa granulata crab exoskeleton. The shape of the main Raman peak of CaCO3 reveals the presence of two phases which are identified as calcite and amorphous calcium carbonate (ACC). The relative concentration of the two phases in various parts of the exoskeleton is determined from the area ratio under the corresponding peaks. The results of the Ca L3,2-edge NEXAFS analysis are in line with the Raman findings, since the energy separation of peaks that appear in the lower frequency region of the main L2 and L3 peaks due to crystal field splitting, is directly related to the percentage of the ACC phase in the total CaCO3 mineral content. The C K-edge spectra are used for the determination of the extent of calcification of the exoskeleton. Furthermore, dark and bright field TEM images reveal the presence of nanocrystallites with an average size of 20 nm. The structure of the nanocrystallites, as derived from the Selected Area Electron Diffraction patterns, is calcite. The results suggest that ACC plays a structural role in the exoskeleton of Calappa granulata.
We report here our recent electron transport results in spatially separated two-dimensional electron and hole gases with nominally degenerate energy subbands, realized in an InAs(10 nm)/GaSb(5 nm) ...coupled quantum well. We observe a narrow and intense maximum (∼500 k ) in the four-terminal resistivity in the charge neutrality region, separating the electron-like and hole-like regimes, with a strong activated temperature dependence above T = 7 K and perfect stability against quantizing magnetic fields. We discuss several mechanisms for that unexpectedly large resistance in this zero-gap semi-metal system including the formation of an excitonic insulator state.
Chemical structure of polyaniline electrodeposited on iron from oxalic acid solution has been studied by means of NEXAFS, Raman, and XPS spectroscopies. The effect of the duration and synthesis ...conditions (polarization mode, potentials, stirring of the solution) has been analyzed. The as-formed PANI films have revealed a relatively low degree of protonation. It has been shown that stirring of electrolyte has the greatest effect on the chemical structure of the polymer. The deposition from the stirred solution provides smooth and chemically homogeneous films, whereas the deposition from quiescent solutions favors the precipitation of polyaniline particles enriched in pernigraniline fragments. The obtained XPS results verify the adsorption of the polymer through the N2
p
-Fe3
d
donor-acceptor interaction between iron atoms and amine groups of polyaniline chains in the film nuclei. The nitrogen K edge NEXAFS spectra, which are very sensitive to the protonation of chains and electronic delocalization, vary significantly, depending on the conditions of PANI electrodeposition.
A facile and environmentally benign method for single-phase barium titanate synthesis in a water vapor medium was studied to reveal the mechanism of phase transformation of the initial simple oxide ...mixture and estimate the capability of the product to be used as a raw material for low-frequency dielectric ceramics. The composition and structure of the reactants’ mixture, treated in vapor at 130–150 °C as well as at 230 °C for various time periods, were investigated by means of XRD, SEM, TEM, EDX, and FTIR methods. The kinetics of the occurring phase transformation can be described using the Johnson–Mehl–Avrami–Erofeev equation. The reaction between the initial oxides was considered as a topochemical process with an apparent activation energy of 75–80 kJ mol−1. A crucial role in this process belonged to the water vapor medium, which facilitated the generation of the reaction zone and the spreading inward of the solid particles. The synthesized tetragonal barium titanate powder (mean particle size of 135 nm) was sintered using a conventional technique at 1250 °C to obtain ceramics with grains of about 2 μm. Capacitance measurements identified a permittivity and dielectric loss factor of the ceramics that reached 3879 and 6.7 × 10−3, respectively, at 1 kHz and room temperature.
Chemical interaction and changes in local electronic structure of Cr, Fe, Co, Ni and Cu transition metals (TMs) upon formation of an Al8Co17Cr17Cu8Fe17Ni33 compositionally complex alloy (CCA) have ...been studied by X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. It was found that upon CCA formation, occupancy of the Cr, Co and Ni 3d states changes and the maximum of the occupied and empty Ni 3d states density shifts away from Fermi level (Ef) by 0.5 and 0.6 eV, respectively, whereas the Cr 3d empty states maximum shifts towards Ef by 0.3 eV, compared to the corresponding pure metals. The absence of significant charge transfer between the elements was established, pointing to the balancing of the 3d states occupancy change by involvement of delocalized 4s and 4p states into the charge redistribution. Despite the expected formation of strong Al–TMs covalent bonds, the Al role in the transformation of the TMs 3d electronic states is negligible. The work demonstrates a decisive role of Cr in the Ni local electronic structure transformation and suggests formation of directional Ni–Cr bonds with covalent character. These findings can be helpful for tuning deformation properties and phase stability of the CCA.
Display omitted
•A compositionally complex alloy was studied by soft X-ray spectroscopy methods.•Directional Ni–Cr bonding was found in Al8Co17Cr17Cu8Fe17Ni33.•Ni–Cr bonding is responsible for the ‘pseudo-gap’ formation at Fermi level.•Tuning of Ni–Cr bond covalence/metallicity is promising for properties tailoring.•Al-TM bonding was found to be delocalized in Al8Co17Cr17Cu8Fe17Ni33.
This work considers corrosion processes as one of the main causes of destruction of materials and structures. Anti-corrosion protection is of great importance in ensuring the maximum service life of ...structural elements. Today, various methods of anti-corrosion protection are used, including coating with materials containing corrosion inhibitors. The article provides information about corrosion inhibitors. A classification is given by the inhibition mechanism: passivation (anodic protection) and cathodic protection; classification by the inhibitor nature, and the main groups of inhibitors are described. The conditions for the adsorption of inhibitory components onto a metal surface are indicated.
PDF
