2020 г.

Автори V. Lyutov and V. Tsakova

Резюме The redox behavior of polyaniline (PANI) doped with polysulfonic acids is studied in slightly acidic and neutral solutions (from pH 4 to pH 7). The dopants used are polystyrene sulfonic acid (PSSA), poly(2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA) and poly-(4,4’-(2,2′-disulfonic acid)-diphenylene-tere-phthalamide) (t-PASA). Thin PANI layers are used in order to identify clearly the redox peak structure. Based on deconvolution of the reductive currents the pH dependence of the constituent peaks is studied and pathways of the reductive transitions are suggested. With increasing pH a loss of electroactivity is observed for all three dopants but to a different extent: the largest for PAMPSA- and the smallest PSSA-doped PANIs. In the same time a stabilization of the conductive emeraldine salt state is observed at pH 7 for all PANIs with largest extent of stabilization in the PANI/PAMPSA case. The electrocatalytic properties of the polysulfonic acids-doped PANIs are comparatively studied for ascorbic acid (AA) and dopamine (DA) oxidation at pH 7. It is found that all three types of PANI are good electrocatalysts for both reactions with advantage of the PAMPSA-doped layers showing higher absolute oxidative currents for both reactions. The results concerning DA oxidation demonstrate the possibility to involve polysulfonic acid-doped PANIs in electrocatalytic reactions in neutral solutions occurring at high positive potentials beyond the peak potential of the PANI oxidative transition

Списание Journal of Solid State Electrochemistry, in print

Автори: M. V. Abrashev,  V. G. Ivanov,  B. S. Stefanov,  N. D. Todorov, J. Rosell, and  V. Skumryev

Резюме: Synthetic powder, ore samples, and mineral single crystals of goethite (αα-FeOOH) were investigated with polarized Raman spectroscopy at temperatures from 293 K to 473 K. The symmetry of the vibrational modes, observed in different scattering configurations, was determined unequivocally. The assignment of the Raman-active modes to definite atomic vibrations is supported by two types of lattice-dynamical calculations: empirical shell model and ab initio density functional theory. The temperature dependencies of the line shape parameters of some Raman-active vibrations at temperatures near to the antiferromagnetic–paramagnetic phase transition infers for a significant spin–lattice coupling in this compound. The most informative in this aspect is the B3gB3g phonon at 387cm−1387cm−1, which overlays a broad scattering background and displays a pronounced asymmetric Fano-line shape. The asymmetry increases in the paramagnetic state above the Néel temperature (TN=393KTN=393K), indicating a strong interaction of this mode with the underlying excitation continuum. The origin of the excitation background is discussed in light of our experimental results and the existing data for the magnetic structure and transport properties of αα-FeOOH. By using the molecular-orbital dimer approach, we calculate the spin–phonon coupling constants for the B3gB3g mode as J′=−0.2eV/ÅJ′=−0.2eV/Å and J″=+0.18eV/Å2J″=+0.18eV/Å2, respectively. Thus, we rationalize that, most probably, the scattering background stems from magnetic excitations, and the asymmetric shape of the B3gB3g band is a result of a linear spin–phonon coupling of this mode with the Fe–O1–Fe spin dimers. Another mechanism, a phonon interaction with thermally activated charge carriers above TNTN, is also considered.

Списание: Journal of Applied Physics 127, 205108 (2020);

Автори: St. Todorova, V. Rangelova, L. Mihaylov, T. Spassov

Резюме: MmNi5 hydrogen storage alloy with an average particle size of 120 μm was subjected to hydriding treatment at 50oC, under 40 atm. pure hydrogen atmosphere. As a result, a large density of particle cracks was formed, which led to an increase in the particle surface area. The enlarged particles surface, in turn, led to substantially improved hydrogen sorption kinetics compared to the as-received (untreated) alloy. It was also found that hydrogen-induced decrepitation of the MmNi5 powder results in enhancement of the hydrogen diffusivity mainly due to shortening of its diffusion distances.

Списание: International Journal of ELECTROCHEMICAL SCIENCE

Int. J. Electrochem. Sci., 15 (2020) 4900 – 4907, doi: 10.20964/2020.06.29

2019 г.

Автори N.M. Ferreira, A.R. Sarabando, S. Atanasova-Vladimirova, R. Kukeva, R. Stoyanova, B.S. Ranguelov, F.M. Costa

Резюме Mg-Al-Si-O glassy systems have a great importance in a wide range of industrial applications, specifically as an electrolyte for molten oxide electrolysis processes in steelmaking. Understanding how the iron oxidation state of the raw material (Fe2+/Fe3+) and its corresponding amount influence this glassy system’s properties will be the aim of the current work. Iron oxides (as Fe2O3 or Fe3O4) were used to dope Mg-Al-Si-O system obtaining amorphous materials through an unconventional method: Laser Floating Zone (LFZ). Above 8% mol of Fe formation of magnetic phases or iron clusters, were observed in the glass matrix. Samples with Fe2O3 showed a higher crystal concentration, when compared with Fe3O4. The electron paramagnetic resonance measurements show a strong dependence on the iron source (Fe3O4 or Fe2O3). In addition, the magnetization decreases linearly with iron content, independently of iron oxidation state, except for samples with a higher concentration of Fe2O3(15% mol), due to sample crystallization. Moreover, with Fe3O4 as raw material there is an improvement (~250 times) of the electrical conductivity when compared with Fe2O3. The results show that the presence of Fe2+ on the glass influences the electrical conductivity, which could have impact in the efficiency of molten oxide electrolysis process.

Списание Ceramics International, 45 (17) Part А, 21379-21384, doi.org/10.1016/j.ceramint.2019.07.125

Автори Miroslav V. Abrashev, Petko Chernev, Paul Kubella, Mohammad Reza Mohammadi, ChiaraPasquini, Holger Dau and Ivelina Zaharieva,

Резюме Catalysis of the oxygen evolution reaction (OER) by earth-abundant materials in the near-neutral pH regime is of great interest as it is the key reaction for non-fossil fuel production. To address the pertinent stability problems and insufficiently understood structure–activity relations, we investigate the influence of moderate annealing (100–300 °C for 20 min) for two types of electrodeposited Mn oxide films with contrasting properties. Upon annealing, the originally inactive and structurally well-ordered Oxide 1 of birnessite type became as OER active as the non-heated Oxide 2, which has a highly disordered atomic structure. Oxide 2 also improved its activity upon heating, but more important is the stability improvement: the operation time increased by about two orders of magnitude (in 0.1 M KPi at pH 7). Aiming at atomistic understanding, electrochemical methods including quantitative analysis of impedance spectra, X-ray spectroscopy (XANES and EXAFS), and adapted optical spectroscopies (infrared, UV-vis and Raman) identified structure–reactivity relations. Oxide structures featuring both di-μ-oxo bridged Mn ions and (close to) linear mono-μ-oxo Mn3+–O–Mn4+ connectivity seem to be a prerequisite for OER activity. The latter motif likely stabilizes Mn3+ ions at higher potentials and promotes electron/hole hopping, a feature related to electrical conductivity and reflected in the strongly accelerated rates of Mn oxidation and O2 formation. Poor charge mobility, which may result from a low level of Mn3+ ions at high potentials, likely promotes inactivation after prolonged operation. Oxide structures related to the perovskite-like ζ-Mn2O3 were formed after the heating of Oxide 2 and could favour stabilization of Mn ions in oxidation states lower than +4. This rare phase was previously found only at high pressure (20 GPa) and temperature (1200 °C) and this is the first report where it was stable under ambient conditions.

Списание J. Mater. Chem. A, 2019, 7, 17022

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Автори L. Nedelchev, D. Ivanov, B. Blagoeva, D. Nazarova

Резюме Azopolymers are highly efficient materials, able to register the polarization state of light. They have broad range of applications from diffractive optical elements with unique polarization properties to biophysics. We present a study of birefringence kinetics, induced with five different pump lasers with wavelengths 355, 442, 491, 514 and 532 nm in thin films of widely used amorphous azopolymer PAZO – poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzene sulfon amido]-1,2-ethanediyl, sodium salt]. During the real-time measurement, the value of the photoinduced birefringence (Δn) is determined at 635 nm, using a DPSS laser. When saturation is reached, the spectrum of the birefringence is measured. Our results indicate that the investigated azopolymer can be used for applications in a wide wavelength range.

Списание, Journal of Photochemistry & Photobiology A: Chemistry, 376, 1–6, doi.org/10.1016/j.jphotochem.2019.02.009

Автори G. Mateev, L. Nedelchev, A. Georgiev and D. Nazarova

Резюме We present a study of the photoinduced birefringence in nanocomposite films of the azopolymer PAZO (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfon amido]-1,2-ethanediyl, sodium salt]) doped with TiO2 nanoparticles (NP) with different concentrations before and after thermal annealing. The NP represent nanopowder with primary particle size 21 nm. The concentration of the NP was varied from 0% (non-doped azopolymer film) to 5 wt%. The thermal process, applied to the nanocomposite films, includes 1 h heating at 200°C. Previous studies of PAZO show that the polymer is stable up to 270°C. We study the dependence of the maximal birefringence induced with He-Cd laser (λ= 442 nm) on the concentration of the TiO2 NP in the azopolymer thin films as well as thermal effect on the absorbance spectra of the thin films. As indicated by our results, the birefringence is higher for the thermally annealed samples. An increase of the photoinduced birefringence is also observed for the nanocomposite layers with 1% NP for the non-annealed films, and with 2% NP for the annealed films.

Списание Open Materials Science, 4, 1–5 doi.org/10.1515/oms-2019-0001

Автори K. Lazarova, D. Christova , R. Georgiev, B. Georgieva and T. Babeva

Резюме Thin films with nanometer thicknesses in the range 100–400 nm are prepared from double hydrophilic copolymers of complex branched structures containing poly(N,N-dimethyl acrylamide) and poly(ethylene oxide) blocks and are used as humidity sensitive media. Instead of using glass or opaque wafer for substrates, polymer thin films are deposited on Bragg stacks and thin (30 nm) sputtered Au–Pd films thus bringing color for the colorless polymer/glass system and enabling transmittance measurements for humidity sensing. All samples are characterized by transmittance measurements at different humidity levels in the range from 5% to 90% relative humidity. Additionally, the humidity induced color change is studied by calculating the color coordinates at different relative humidity using measured spectra of transmittance or reflectance. A special attention is paid to the selection of wavelength(s) of measurements and discriminating between different humidity levels when sensing is performed by measuring transmittance at fixed wavelengths. The influence of initial film thickness, sensor architecture, and measuring configuration on sensitivity is studied. The potential and advantages of using top covered Bragg stacks and polymer/metal thin film structures as humidity sensors with simple optical read-outs are demonstrated and discussed.

Списание Nanomaterials, 9, 875, doi:10.3390/nano9060875

Автори B. Blagoeva, G. Mateev, D. Nazarova, L. Nedelchev

Резюме In this work we study the dependence of the photoinduced birefringence on the initial temperature of the layer, as well as the possibility of recording and termal erasure in thin azopolymer films. To determine the temperature dependence, the sample is placed on a hot stage, which can maintain or vary the temperature of the sample with a high precision. Birefringence is induced with a linearly polarized light from a laser at 442 nm, and its value is determined by measuring the Stokes parameters of a probe laser beam after the sample. The birefringence is erasured by placing the sample on a hot stage.

Списание Journal of Physics and Technology 3(1), 3-7

Автори D. Ivanov, V. Strijkova, L. Nedelchev, D. Nazarova and E. Borisova

Резюме This study presents optical and chemical methods to visualize healthy and malignant sections of histological samples, by using polarized light imaging and staining with different contrast agents. This approach complements the diagnostic conclusions made by the physicians and improves qualitatively healthy versus tumor tissue differentiation, which may be in practical use for pathologistsin their diagnostic conclusions.

Списание Journal of Physics and Technology, 3(1) 14-17

Автори G. Mateev, D. Nazarova and L. Nedelchev

Резюме In this article, we investigate the photoinduced birefringence in commercially available azo-polymer PAZO doped with TiO2 spherical nanoparticles (21 nm size). The films are prepared by spin-coating water solution of the polymer mixed with the nanoparticles with different concentrations from 0 to 10 wt %. To induce the birefringence we use He-Cd laser with wavelength 442 nm. For measurement of the effect, probe laser with wavelength 635 nm is used. The experimental setup includes also polarimeter, which measures the Stokes parameters in real time. We observe increase of the photo-induced birefringence in azopolymer films doped with TiO2 nanoparticles. The maximal value of n is achieved for the sample with 1 wt. % concentration.

Списание Journal of Physics and Technology, 3(1) 18-21

Автори S. Kozhukharov, V. Zhelev and S. Tchaoushev

Книга Advances in Materials Science Research Vol. 37, Ed. By: M. Wythers, NOVA Sci. Publ. pp.101-166; ISBN: 978-1-53615-038-4

Автори Stephan V. Kozhukharov and Christian A. Girginov

Книга Phenomena and Theories in Corrosion Science. Methods of Prevention, ed. By A. Gergely NOVA Sci. Publ. pp. 3 – 150; ISBN 978-153-615253-1