Browsing by Author "Dimitrov D."

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    EFFECT OF EPOXY MATRIX VISCOSITY ON THE DISPERSION OF GPL GRAPHENE LAYER SEPARATION FOR COATING APPLICATIONS
    (2025-11-02) Dimitrov D.; Staneva A.
    Coatings application is among the most accessible methods of surface modification. Many materials exhibit the desired bulk properties (e.g. mechanical strength), but their surface needs to be protected from the environment to have an additionally functionalized surface - heat and electrically conducting or isolating, optical or aesthetic considerations. When adding nanosized materials to a matrix the main concern is the retention of separation of nanoparticles and the prevention of aggregation. In the following study, dispersion of Graphene nanoplatelets (GPL) in bisphenol-a based epoxy is investigated. GPL is initially sonicated in methanol and introduced into a series of liquid epoxies with varying dry weight concentration and thus, varying viscosity. The working hypothesis that increased viscosity aids to stabilize the suspension and delamination of graphene layers, has been confirmed. The samples have been characterized by Transmission Electron Microscopy (TEM) and Raman spectroscopy. TEM confirms that the best delamination is retained in the most concentrated matrix. Increased Raman signal intensity testifies to the increase of stabile suspended fraction of GPL before coating, which leads to a higher concentration of active additives in the coatings. Furthermore, adhesion and delamination testing by the crosshatch method confirms that adding GPL does not negatively affect the mechanical properties of the coatings, as compared to coatings without the addition of GPL.
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    Effect of expanded vermiculite and expanded perlite as pore forming additives on the physical properties and thermal conductivity of porous clay bricks
    (2018-01-01) Georgiev A.; Yoleva A.; Djambazov S.; Dimitrov D.; Ivanova V.
    The way to achieve better energy efficiency in buildings is to improve their thermal insulation properties. This can be done by increasing the porosity of the clay bricks by introducing pore-forming additives to the clay or production of perforated bricks. In general, thermal conductivity of the bulk clay bricks is about 1.0 W/mK depending on their raw materials, processing, firing temperatures and fired densities. These values can be reduced by addition of various inorganic and organic pore-forming agents into the brick raw material mixtures before firing. This paper presents the effect of adding of expanded vermiculite and expanded perlite in quantity of 0, 3, 5 and 8 mass % on the physical properties and thermal conductivity of clay porous bricks fired at 900oC for 2 hours. Expanded vermiculite and expanded perlite were selected as pore former as they have very low density and good thermal insulation properties. The results show an increasing of water absorption and apparent porosity compared t? fired pure clay bricks and lower thermal conductivity of the fired at 900oC products with expanded vermiculite and expanded perlite. At small amounts of pore former, the properties of the fired clay materials slightly changed and more pronounced is the influence of vermiculite. Expanded vermiculite and expanded perlite in amount of 8 mass % can be used as a pore forming agents in bricks production to improve thermal conductivity of clay bricks and keep acceptable the compressive strength. Keywords: expanded vermiculite, expanded perlite, porous clay bricks, porosity, thermal conductivity.
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    Electric properties of new composite materials based on RGO, nanosized ZnO and Cu nanoparticles
    (2021-02-16) Martinov B.; Slavov S.; Staneva A.; Dimitrov D.; Mateeva J.
    Graphene nanostructures combine properties that make them an attractive material for various technical applications. The objective of this study is to obtain a series composites based on Reduced graphene oxide (RGO), nano-sized ZnO and Cu nanoparticles. RGO is synthesized by a modified Hammer's method and chemical reduction after sonication. ZnO nanoparticles are obtained by the Sol-Gel method. The composites' phases and structures are characterized by XRD, SEM, TEM, and EDX analysis. It is shown that ZnO is monophasic with a particle size of 48 nm. A series of composite materials based on RGO, ZnO and Cu nanoparticles is obtained by ultrasonic treatment. Electrical and dielectric properties are measured by Impedance analyser and RLC bridge. For comparison, the conductivity for 100% RGO is 309.513 S/m and for 100 % Cu nanosized is 0.0056 S/m. Therefore, it should be initially concluded that increasing the RGO in the mixture will increase the conductivity, and increasing nanosized Cu will decrease it. The presence of cluster regions in the composites with different conductivity is found: 1) nano grains of conductive copper which are partially or completely surrounded by 2) low-conductivity shells of copper oxide and a minimal number of RGO sheets. The present work shows promising results for future research for the creation of new composite materials with controlled shape and size of copper oxide - RGO clusters. Dielectric measurements for RGO/ZnO composites are made in range from 5 mass% to 100 mass% content of RGO, the composites show variable conductivity. For lower concentration of RGO, selected samples are dielectrics.
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    Structural Characterization, Cytotoxicity and Microbiological Activity of One-Step-Synthesized RGO/AuNPs Nanocomposites
    (2025-10-01) Martinov B.; Dimitrov D.; Foteva T.; Kostadinova A.; Staneva A.
    This study presents a green, single-step method for synthesizing nanocomposites based on reduced graphene oxide (RGO) and gold nanoparticles (AuNPs), using sodium citrate as a mild reducing and stabilizing agent. AuNPs were generated from chloroauric acid (HAuCl4) directly on the surface of graphene oxide (GO), which was simultaneously reduced to RGO. Structural characterization via Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM) and Selected Area Electron Diffraction (SAED) confirms spherical AuNPs (10–60 nm) distributed on RGO sheets, with indications of nanoparticle aggregation. Dynamic Light Scattering (DLS) and zeta potential analysis support these findings, suggesting colloidal instability with higher RGO content. Biological evaluation demonstrates dose-dependent cytotoxicity in HaCaT keratinocytes, with IC50 values (half maximal inhibitory concentration) decreasing as RGO content is increased. At moderate dilutions (1–25 µL/100 µL), the composites show acceptable cell viability (>70%). Antibacterial assays reveal strong synergistic effects against Escherichia coli, Staphylococcus aureus, and Bacillus subtilis, with sample RGO/Au 0.500/0.175 g/L showing complete E. coli inhibition at low Au content (0.175 g/L). The composite retained activity even in protein-rich media, suggesting potential for antimicrobial applications. These findings highlight the potential of RGO/AuNPs composites as multifunctional materials for biomedical uses, particularly in antimicrobial coatings and targeted therapeutic strategies.
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    Synthesis and characterization of nanosized ZnTiO3doped with reduced graphene oxide (RGO)
    (2021-02-16) Martinov B.L.; Staneva A.D.; Vlakhov T.E.; Slavov S.; Dimitrov D.; Marinov Y.G.; Hadjichristov G.B.
    Composite materials based on nanosized zinc titanate (ZnTiO3) doped with reduced graphene oxide (RGO) were obtained by means of ultrasonically assisted precipitation. In these composites, the concentration of RGO nanopartiles was varied from 1 wt.% to 20 wt.%. The ZnTiO3 ceramic was produced by sol-gel method. RGO was prepared by a modified Hammers method and subsequent chemical reduction with sodium borohydride. Structural and phase characterization of the fabricated composites was performed by XRD, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). It was identified that for all of the studied samples of the RGO/ZnTiO3 series, the two phases are uniformly distributed over the observed areas, which proves the formation of homogeneous nanocomposite materials. The electrical properties of the series of RGO/ZnTiO3 samples were characterized by complex electrochemical impedance spectroscopy (EIS) in the frequency range from 0.1 Hz to 1 MHz. The results obtained by EIS for the electrical conductance of RGO-doped ZnTiO3 as a function of the concentration of RGO nanoparticles were correlated with the data from structural studies.
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    Voting Abroad in Bulgaria: Facts, Problems and Solutions
    (2022-09-06) Konstantinov M.; Peychev D.; Stanev A.; Stoichev S.; Dimitrov D.
    A detailed analysis of the impact of voting abroad on the result of parliamentary, European and presidential elections is presented. The effect of the possible introduction of a special electoral region for votes cast abroad is analyzed.