Browsing by Author "Avramova I."

Now showing 1 - 4 of 4
  • No Thumbnail Available
    Item
    Addition of transition elements to the phosphate sealing of cerium oxide protective primer deposited on AA2024-T3 alloy
    (2016-01-01) Girginov C.; Avramova I.; Kozhukharov S.
    A concise investigation was undertaken to determine the impact of transitional elements on the phosphate sealing of cerium oxide primer layers deposited on AA2024-T3 aircraft alloy. The cerium oxide primer layers were deposited electrochemically on preliminary treated alloy specimens under potentiostatic conditions. The subsequent phosphate sealing layer deposition was performed in equimolar solutions of NaMoO4, NaVO3 or ZrO(NO3)2 in (NH4)2HPO4 medium for 30 min at 90°C. The already coated and sealed specimens were submitted to comparative measurements via electrochemical impedance spectroscopy and linear voltammetry after 168 h of exposure to 3.5% NaCl model corrosive medium. The obtained bi-layer coatings were characterized by systematic analyses using scanning electron microscopy, energy dispersion X-ray spectroscopy and X-ray photoelectron spectroscopy. The latter analysis evinced that cerium present in a cerium phosphate coating was predominantly in Ce(III) state, while doping by Mo, V and Zr caused oxidation to Ce(IV) species. The results obtained reveal successful inclusion of these elements inside the phosphate sealing layer.
  • No Thumbnail Available
    Item
    Adsorption of Ag+ ions on hydrolyzed lignocellulosic materials based on willow, paulownia, wheat straw and maize stalks
    (2016-05-01) Vassileva P.; Radoykova T.; Detcheva A.; Avramova I.; Aleksieva K.; Nenkova S.; Valchev I.; Mehandjiev D.
    In the present work, the adsorption of Ag+ ions on hydrolyzed plant biomass (willow, paulownia, wheat straw and maize stalks) was investigated. Chemical analyses were performed to establish the composition of the obtained materials. Adsorption mechanism, adsorption sites and specific surface areas of these materials were examined by BET analysis, IR spectroscopy, XPS and EPR. The effects of contact time, acidity of initial solutions and Ag+ ion concentrations were followed. Pseudo-first-order, pseudo-second-order and intra-particle diffusion models were used to analyze kinetic data. In all cases, the adsorption was significantly affected by the pH value. Different types of adsorption isotherms of Ag+ ions (either Langmuir or Freundlich) were registered depending on the adsorbing material. The adsorption mechanism is complex, and the process passes through different stages as clustering of Ag+ ions and formation of elemental Ag. The maximal adsorption capacities varied from 2.05 to 6.07 mg g−1. The obtained results revealed that the examined waste lignocellulosic materials are promising adsorbents for Ag+ ions.
  • No Thumbnail Available
    Item
    PHASE COMPOSITION AND MICROSTRUCTURE CHARACTERIZATION OF STRONTIUM-MODIFIED BARIUM TITANATE GLASS-CERAMICS
    (2023-01-01) Harizanova R.; Pernikov M.; Mihailova I.; Tatchev D.; Avdeev G.; Avramova I.; Rüssel C.
    Dielectric glass-ceramic materials find various applications as parts of sensors, electronic components and even in biomedicine. The present work reports on the synthesis of glass-ceramic materials in the complex oxide system (23.1-z)Na2O/17.1BaO/6SrO/23TiO2/17.4SiO2/7.6B2O3/5.8Fe2O3/zAl2O3, z = 0 and 3 mol %. The melts show spontaneous crystallization during quenching. The phase composition is studied using X-ray diffraction and reveals crystallization of Ba1-xSrxTiO3 solid solutions with varying Ba/Sr ratio. The valence states and the local environments of the Ba, Ti and Fe ions of the prepared materials are investigated by utilizing X-ray photoelectron spectroscopy. It is shown that all the elements in the composition are present in their oxidized states and the formation of barium titanate based compounds is possible. The microstructure of the obtained samples is imaged by scanning electron microscopy and shows bright polygon-like crystalline structures embedded in a dark amorphous matrix. The further evaluation of the microstructure as average crystal size and volume fraction of the precipitated crystals is done by using microcomputed tomography. Thus, a relatively high crystalline volume fraction is observed for the samples without alumina. The samples with alumina possess comparable as size and volume fraction crystals.
  • No Thumbnail Available
    Item
    Two dimensional polymerization of graphene oxide: Bottom-up approach
    (2015-08-01) Atanasov V.; Russev S.; Lyutov L.; Zagraniarsky Y.; Dimitrova I.; Avdeev G.; Avramova I.; Vulcheva E.; Kirilov K.; Tzonev A.; Abrashev M.; Tsutsumanova G.
    We demonstrate a bottom-up synthesis of structures similar to graphene oxide via a two dimensional polymerization. Experimental evidence and discussion are conveyed as well as a general framework for this two dimensional polymerization. The proposed morphologies and lattice structures of these graphene oxides are derived from aldol condensation of alternating three nucleophilic and three electrophilic centers of benzenetriol.