Browsing by Author "Vladimirova L."

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    Antimycotic activity of sumac extract in composites based on epoxidized natural rubber for application in footwear soles production
    (2020-01-01) Dishovsky N.; Marinkova D.; Raykova R.; Vladimirova L.
    Biofilms are attached to different kinds of surfaces or associated with interfaces. The microbial communities are often composed of multiple species, which interact between each other. Attachment is complex process, regulated by diverse characteristics of the growing media, substrates and cell surface. Biofilms have different effects, which can affect changes in surface properties of polymer carriers. Accumulated biomass may provoke negative effects by direct attack, which leads to destruction of polymer matrixes. An option to solve the problem is introduction of bioactive ingredient into the composite. The aim of our study was to find and propose bioactive ingredients, derived from renewable sources, particularly such exhibiting of antimycotic influence, which can be included to elastomeric composite materials. These materials can be used for obtaining of winter footwear soles with adhesion, increasing to various types of icy surfaces. The influence of introduced antimycotic additive, extracted from Cotinus coggygria plant, was investigated. Antimycotic activity of obtained plant extracts against two eukaryotic strains was investigated by diffusion method in agar broth. The formation of biofilms from Candida lypolitica and Candida albicans strains onto natural rubber vulcanizates, kinetics and growth of biofilms strains were also observed.
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    Assessment of Physical-Chemical Characteristics of Surface Water from Key Sites of the Mesta River: State and Environmental Implications
    (2018-06-12) Georgieva S.; Gartsiyanova K.; Ivanova V.; Vladimirova L.
    The anthropogenic source pollution of the Mesta River was assessed during the period 2011 and 2016 in terms of pH, conductivity, chemical oxygen demand (COD), anions and heavy metals in key sites of the Mesta river pointed as: S1 (the Mesta river before the Iztok river); S2 (the Iztok river before the Mesta river); S3 - the river Mesta after the Iztok river); S4 - the river Mesta at Momina Klisura, near Bukovo) and S5 - the river Mesta before the Greece border, after the Matnitsa river. The application of multivariate clustur analysis (CA) for the interpretation of a large and complex data matrix obtained during a monitoring program of surface water in Mesta river is presented in this study. The dataset consists of analytical results from a 6-yrs survey conducted in selected points of the river system. The physical-chemical characterization in the water samples were made in accordance with the Directive 2000/60/EU-Water Framework Directive (WFD) and its equivalent criteria transposed into the Water Law (WL) in Bulgaria. In water, concentrations (mg L-1) during 2016 of NO2 - (0.006 to 0.052), NO3 -(0.01 to 1.33) and total contents in μg L-1 of Cu (<0.002), Pb (<0.003), Co(<0.002), Ni(<0.003), S(<0.050) and Zn(<0.02), pH (5.60 to 8.00), and electrical conductivity (0.12 to 48.60 mS.cm-1) were agreed with environmental standards except cadmium Cd (>0.15 μg L-1) and PO4 3-(0.15 mg L-1). During analyzed period the cadmium concentration was much higher than recommended limit only. In this sense, it was possible to demonstrate relatively good quality of river water even with numbers of industrial and touristic activities in the analyzed area but also to consider a future concept on cadmium sources and their eliminated.
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    ELECTRODEPOSITION OF Sn-Co POWDERS BY STATIONARY AND PULSE MODE
    (2021-01-01) Ignatova K.; Lilova D.; Vladimirova L.; Alakushev M.
    Sn-Co powders with homogeneous distribution of components in powder particles were obtained by constant and pulse potential modes from a fluoride-chloride electrolyte. Morphology, elemental and phase compositions by SEM, EDX, SEM-Mapping andXDR analysis were investigated. Under stationary conditions, the potential increment leads to an increase in cobalt content from 3.6 wt.% to 56.4 wt.%, and to a structural alteration of the powders from network-shaped to a dendritic form with an average particle size between 15 and 30 µm, depending on the total metal concentration. The powders, obtained by pulse mode at a lower pulse frequency (100 Hz), contain particles with an irregular dendritic structure, characterized by the greatest thickness of the main axes of the dendrites. Increasing the pulse frequency to 1000 Hz results in a finer dispersed structure (r < 10 µm) and the cobalt content achieves 70.5 wt.% under these conditions. The obtained Sn-Co powders are mixed structures of amorphous phase and inclusions of microcrystalline phases of tetragonal β-Sn, hexagonal α-Co, CoO and SnO2 and only in stationary mode an additional phase of SnF2 exists.
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    Spectrophotometric method for rapid estimation of the oxygen stoichiometry in YBCO superconducting samples
    (2013-03-01) Georgieva S.; Nedeltcheva T.; Vladimirova L.; Stoyanova-Ivanova A.
    Detailed investigations on the possibility for determination of the oxygen stoichiometry in YBa2Cu3Oy superconducting samples by measuring the absorbance of the colored Co(III)-EDTA and Cu(II)-EDTA complexes in one solution have been performed. The absorptivity of the complexes at pH 2.5 and 4.9 as well as Beer's law have been studied. The content of the solution for the sample dissolving has been established. The stoichiometry of the oxidation-reduction reaction between Cu(III) from the sample and Co(II) in the solution has been proved. The possibility to dissolve the sample in air has been tested. As a result a simple and rapid spectrophotometric method for the determination of the δ non-stoichiometric oxygen coefficient in YBa 2Cu3O6.5+δ samples has been developed without precise measurement of the sample mass. The standard deviation of the δ- and y-coefficient results is 0.017. The reproducibility of the y-values in relative units (s r ) is 0.2%. The accuracy of the results has been confirmed by the method, which is based on the absorbance measurement of the I 3 - -starch compound. © 2012 Versita Warsaw and Springer-Verlag Wien.
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    Studies on natural rubber based composites filled with products from renewable resources with improved adhesion to icy surfaces
    (2018-01-01) Dishovsky N.; Mitkova F.; Angelov Y.; Kandeva M.; Vladimirova L.
    Slips and falls on icy surfaces can cause serious injuries of people. The primary risk factor for slipping incidents is undoubtedly the decreased friction coefficient between the shoe sole and the ice or snow surface. Nowadays environmental protection has been gaining significance and becoming highly important for the various innovation strategies. In rubber industry the concept of environmental protection is more often associated with the maximum use of elastomers and ingredients from renewable sources in the manufacture of rubber products. The aim of this work is to investigate the possibilities of using elastomers and ingredients from renewable sources - natural rubber, silica obtained by rice husks incineration and microcrystalline cellulose - as fillers and rapeseed oil as a process additive in compositions, intended for the manufacture of soles for winter footwear having an increased coefficient of friction to various types of icy surfaces. Dynamic mechanical thermal analysis has been used to predict the adhesion of the materials to ``dry ice``, ``wet ice`` and melting ice. It has been found that, in all the cases examined, the composites containing natural rubber and a combination of microcrystalline cellulose (20 phr) and silica (40 phr) as fillers are the most suitable for the purpose. The silica used can be both synthetic and from renewable sources. The tribological tests carried out have confirmed these composites under the conditions of ``dry ice`` and ``wet ice`` fall into the class of the highest resistance against slip but on melting ice they do not have the necessary degree of resistance. It is believed that the use of a polar elastomer and hydrophobilizing the surface of microcrystalline cellulose particles will solve the problem.
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    Study Of The Potential Of Silver/Sulfide Ion Selective Electrode In Solutions Of Silver And Thiosulfate Ions When The Thiosulfate Is In Excess
    (2016-01-01) Vladimirova L.; Nedeltcheva T.
    A dependence of the potential of silver ion-selective electrode with silver sulfide membrane versus the analytical concentrations of silver ions and thiosulphate is derived theoretically. Equations, valid at a ratio between the molar concentrations of thiosulfate and silver ions at least 20 times and a concentration of thiosulfate from 0.001 to 0.2 mol L-1 are found theoretically and proven experimentally.
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    Synthesis and superconducting properties of Nd0.33 Eu0.08Gd0.58Ba2Cu3 Oz materials
    (2008-03-01) Stoyanova-Ivanova A.; Terzieva S.; Shivachev B.; Mikli V.; Vladimirova L.
    We have studied the effect of the ratio of different rare-earth element on the superconducting properties and phase formation of (Nd0.33Eu0.08Gd0.58)Ba2 Cu3Oz ceramic. Bulk NEG samples were prepared using the solid-state reaction process. The superconducting transition for Nd0.33Eu0.08Gd0.58Ba2 Cu3Oz is Tc ≈ 90 K and the value of the critical current density (Jc) is 13.9 A/cm2 at 77 K under zero magnetic fields. This value is twice as high when compared with the (Jc) value of YBCO systems (J3 = 7.31 A/cm2). The obtained bulk sample was used for the production of superconducting Ag-sheathed tapes by OPIT method including hot rolling. The critical current density of the obtained tape (337 A/cm2) is one order higher than the one of the bulk sample. © Versita Warsaw and Springer-Verlag Berlin Heidelberg 2008.