Browsing by Author "Mateeva J."
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Item DIELECTRIC PROPERTIES OF NEW COMPOSITES BASED ON GRAPHENE OXIDE AND NANO-SIZED ZnO(2021-01-01) Staneva A.; Martinov B.; Slavov S.; Karashanova D.; Mateeva J.; Melo B.M.G.; Costa L.C.The objectives of this study were to obtain a complete series of graphene oxide and nano sized zinc oxide and to make a structural and phase characterization of the resulting composites. XRD, SEM, EDX, TEM and impedance analyses were used to investigate these materials as energy storage devices. The complex dielectric permittivity was measured for all samples as a function of frequency. All dielectric characteristics were studied (frequencies from from 100 Hz to 1 MHz and temperatures from 150 to 400 K). The composite GO/ZnO - 80/20 is closer to the desired properties.Item EFFECT OF GRAPHENE OXIDE AND REDUSED GRAPHENE OXIDE ON ELECTRICAL PROPERTIES OF YBCO AND YBCO/Ag COMPOSITES(2020-01-01) Staneva A.; Mateeva J.; Martinov B.; Blagoev B.; Nurgaliev T.Series of bulk composites based on YBCO superconductors, silver and graphene oxide/reduced graphene oxide are synthesized. The aim of this study is to determine the effect of GO and RGO on the superconducting properties of YBCO composites. Series of YBCO and YBCO/Ag phases are synthesized by the ceramic method. GO and RGO are obtained by the modified Hammer’s method. Series of YBCO, YBCO/Ag composites with GO or RGO are studied. The temperatures of the superconducting transition of the obtained composites are determined and the effect of GO/ RGO is investigatedItem 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.Item NANOCOMPOSITE BIFILLERS BASED ON GRAPHENE OXIDE AND ORGANOBENTONITE(2022-01-01) Mateeva J.; Yoleva A.; Djambazov S.; Martinov B.Nanocomposites and in particular polymer nanocomposites play a significant role in adsorption treatment of wastewaters. The use of nanobifillers in polymer nanocomposites is relatively less explored area in materials science. Graphene oxide (GO) and its nanocomposites have drawn increasing interests in the application of water treatment, especially for the removal of metal-containing pollutants. GO has large surface area, due to its sheet-like structure. Recently, GO and clay minerals like bentonite (montmorillonite) find an application as adsorbents because of their good adsorption capacity and low cost. GO nano-sheets were synthesized using a modified Hummers’ method from graphite powder. A natural bentonite from Bulgaria deposit was activated and organically modified by using octadecylamine (ODA) to obtain organobentonite (OB). Nanocomposites based on organobentonite and graphene oxide (OB: GO = 10: 1 and OB: GO = 5: 1) were prepared through a facile ultrasonic-assisted method and exhibited excellent synergetic adsorption behavior for methylene blue (MB). The synthesized nanocomposite bifillers were characterized by XRD and FTIR and their adsorption capacities to methylene blue (MB) were determined.Item PHASE STUDY OF RAPANA VENOSA SHELLS FROM THE BLACK SEA(2023-01-01) Mateeva J.; Yoleva A.; Djambazov S.Sea shells contribute to large amounts of waste. Reuse and recycling of such waste can be an excellent alternative to disposal. Recycling sea shells waste offers many benefits and has potential applications in various fields. Some applications are up-to-date, well-established, widely used, large-scale and sustainable. On the other hand, there are many more strategies that have not yet been fully explored or commercialized. Growing global research shows significant potential for the use of sea shells such as regulators of soil acidity, catalysts for biodiesel production, adsorbents to remove toxic metals from wastewater, building materials, calcium supplements, biomaterials, antibacterial protection, bone implants, etc. The present study represents a phase investigation of Black Sea Rapana venosa shells with a view to their possible application.Item Synthesis and characterization of ybco and ybco/ag superconducting ceramic composites containing reduced graphene oxide(2019-01-01) Mateeva J.; Staneva A.; Martinov B.; Blagoev B.; Nurgaliev T.The high-temperature ceramic superconductors and nanomaterials are among the most promising groups of materials that are intensively studied. Graphene is a new material with unique electrical, optical and biological properties. The high mobility of graphene charges implies the possibility of using it as a suitable additive improving the superconductors' physical characteristics. The idea of this study is to synthesize series of bulk composites based on YBCO superconductors, silver and RGO with superconducting properties. The YBCO phase is obtained by a ceramic technology. It includes Ag aiming better sintering. Graphene is synthesized by a modified Hammer's method. XRD is applied to verify the obtaining of the starting phases. The synthesized composites are characterized by XRD, SEM and EDX analysis. All composites prepared are superconductive and have a Meissner effect.Item SYNTHESIS OF NANOCOMPOSITES WITH PARTICIPATION OF BSCCO SUPERCONDUCTING PHASES, RGO AND AgNPs(2023-01-01) Mateeva J.; Staneva A.Reduced graphene oxide (RGO) and silver nanoparticles (AgNPs) have been used to prepare (Bi-2223) composites. The structural and morphological properties of the synthesized initial components and the composites are characterized using X-ray diffraction and Transmission electron microscopy. Two high-temperature superconducting orthorhombic phases of Bi-2223 and 2212 are synthesized at a temperature of 830°C for 16 hours in air. TEM images of the composite confirm the presence of silver nanoparticles (Ag-cubic) with an approximate size of about 10 nm and the specific structure of reduced graphene oxide sheets. Significant exfoliation of the graphene oxide and successful reduction leading to the observation of smooth and singular RGO layers.Item SYNTHESIS OF NdMnO3, Nd0.6Pb0.4MnO3 AND (La0,7Nd0,3)0.6Pb0.4MnO3 NANOPOWDERS BY MECHANO-CHEMICAL ACTIVATION(2023-01-01) Staneva A.; Mateeva J.Nanosized powders of rare-earth manganites NdMnO3, Nd0.6Pb0.4MnO3 and (La0,7Nd0,3)0.6Pb0.4MnO3obtained by mechano-chemical synthesis under different conditions (speed, milling time, temperature) in planetary ball mill were investigated. The starting oxides for the mechanochemical reaction La2O3, Nd2O3, MnO2, Pb3O4, were mixed in an appropriate ratio corresponding to the stoichiometric compounds. The phase formation was studied by X-ray diffraction, SEM, TEM, DTA and IR-spectroscopy. Phase (La0,7Nd0,3)0.6Pb0.4MnO3 was successfully synthesized at 1000 rpm for 3 hours without the application of additional thermal treatment. The phases NdMnO3 and Nd0.6Pb0.4MnO3 were obtained after 5 hours mechano-chemical treatment at 1000 rpm. The particle size of the powders are in the range of 18 - 20 nm. Magnetic properties of the powders were studied. The phase (La0,7Nd0,3)0.6Pb0.4MnO3 is ferromagnetic at room temperature and it is appropriate for the preparation of the bulk sintering polyfunctional ceramics composites