Browsing by Author "Slavov S."
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Item Bi12TiO20 crystallization in a Bi2O3-TiO2-SiO2-Nd2O3 system(2018-04-05) Slavov S.; Jiao Z.Polycrystalline mono-phase bismuth titanate was produced by free cooling from melts heated to 1170 °C. The control over the initial amounts in the starting compositions in the system Bi2O3/TiO2/SiO2/Nd2O3 and over the thermal gradient of the heat process resulted in the formation of specific structures and microstructures of monophase sillenite ceramics. The main phase Bi12TiO20 belongs to the amorphous network groups based on oxides of silicon, bismuth and titanium. In this work, we demonstrated a way to control the crystalline and amorphous phase formation in bulk poly-crystalline materials in the selected system.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 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 Glass-crystall materials containing Bi12Tio20 and Bi4Ti3O12 phases obtained from freelycooled melts of Bi2O3-Tio2-Sio2-Nd2O3 system(2018-01-01) Slavov S.; Jiao Z.Synthesis of selected samples of Bi2O3-TiO2-SiO2-Nd2O3 system was carried out on the ground of freely cooled melts in the range from 1450°C to 1100°C. The initial amount control of the start compositions provided the formation of polyphase glass-ceramics containing Bi12TiO20 and Bi4Ti3O12. The amorphous phases were based on oxides of silicon, bismuth, titanium and aluminum. The present investigation shows a way to control the crystalline and amorphous phase formation in bulk poly-crystalline materials on the ground of a selected system.Item Growing Oriented Layers of Bi4Ti3O12 in Bi2O3/TiO2/SiO2/Nd2O3/Al2O3 Glass-Ceramics by Melt Quenching(2018-12-01) Wisniewski W.; Slavov S.; Rüssel C.; Dimitriev Y.A glass melt with the composition 24Bi2O3/40TiO2/10SiO2/10Nd2O3/16 Al2O3 was prepared and rapidly quenched between two copper blocks (sample A). A part of this glass was subsequently crystallised at 800 °C for 8 h (sample B). For the preparation of another two samples, the melt was slowly cooled on a cooper plate (sample C) or cast into a graphite mould and subsequently thermally treated at 300 °C for 3 h (sample D). As shown by X-ray diffraction (XRD) and scanning electron microscopy (SEM) including energy dispersive X-ray spectroscopy (EDXS) and electron backscatter diffraction (EBSD) measurements, the rapidly cooled samples contained notable amounts of uncrystallised glassy phase next to the Aurivillius phase Bi4Ti3O12. The latter occurred in higher concentrations in all other samples and formed oriented layers. Minor concentrations of Bi2Al4O9 and Al2O3 were also detected in the microstructure.Item Growing Oriented Layers of Bi4Ti3O12 in Bi2O3/TiO2/SiO2/Nd2O3/Al2O3 Glass-Ceramics by Melt Quenching(2018-12-01) Wisniewski W.; Slavov S.; Rüssel C.; Dimitriev Y.A glass melt with the composition 24Bi2O3/40TiO2/10SiO2/10Nd2O3/16 Al2O3 was prepared and rapidly quenched between two copper blocks (sample A). A part of this glass was subsequently crystallised at 800 °C for 8 h (sample B). For the preparation of another two samples, the melt was slowly cooled on a cooper plate (sample C) or cast into a graphite mould and subsequently thermally treated at 300 °C for 3 h (sample D). As shown by X-ray diffraction (XRD) and scanning electron microscopy (SEM) including energy dispersive X-ray spectroscopy (EDXS) and electron backscatter diffraction (EBSD) measurements, the rapidly cooled samples contained notable amounts of uncrystallised glassy phase next to the Aurivillius phase Bi4Ti3O12. The latter occurred in higher concentrations in all other samples and formed oriented layers. Minor concentrations of Bi2Al4O9 and Al2O3 were also detected in the microstructure.Item PICARD-LINDELÖF ITERATIONS AND MULTIPLE SHOOTING METHOD FOR PARAMETER ESTIMATION(2020-01-01) Slavov S.; Tsvetkov T.In this article, we modify the Picard-Lindelöf iteration scheme in order to show an iteration algorithm for parameter estimation of ordinary differential equations. The proposed algorithm inherited the advantages exhibited in the classical algorithms and, moreover, the parameters can be transformed to a form that are convenient and suitable for computation. In the end, a numerical example has also been discused to highlight the results.Item 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.Item Synthesis of doped bismuth titanate ceramics with Nd2O3 and SiO2 and their electrical properties(2013-05-21) Slavov S.; Krapchanska M.; Kashchieva E.; Parvanov S.; Dimitriev Y.Bismuth-titanate ceramics containing SiO2 and Nd2O3 as additives are synthesized by melt quenching method in the system Bi2O3-TiO2-Nd2O3-SiO2 at temperature range 1260 - 1500oC. The phase composition of the obtained materials is determined by X-ray diffraction analysis and energy dispersive spectroscopy. Using scanning electron microscopy different microstructures are observed in the samples depending on the composition. The addition of SiO2 and Nd2O3 allows controlling the crystallization, glass formation ability, melting temperature and Curie temperature. Different values of conductivity, dielectric losses and relative permittivity are obtained depending on the composition. Measurements of the electrical conductivity, capacitance and dielectric losses of selected samples are performed by DC resistible bridge and digital capacity meter (with frequency of 1 kHz) using two-terminal method and a suitable sample holder with graphite electrodes. It is established that all investigated samples are dielectric materials with conductivity between 10-9 and 10-13 (O cm)-1 at room temperature, dielectric permittivity from 1000 to 3000 and dielectric losses tgd between 0,0002 and 0,1.Item The effect of Nd substitution on the glass forming ability and thermal properties in the system Bi2O3-TiO2-SiO2(2015-01-01) Slavov S.; Kashchieva E.; Parvanov S.; Dimitriev Y.The paper reports data on glass formation in wider concentration ranges. The samples amorphous nature is characterized by x-ray diffraction analysis, their thermal characteristics and stability are determined by differential thermal analysis (DTA), while the transmission and absorption characteristics outlined are investigated by ultraviolet-visible spectroscopy (UV-Vis). SiO2 content increase leads to a shift of the absorption border to lower wavelengths. The experimental results show that it is possible to obtain low-melting glasses containing 5 mol % - 20 mol % SiO2 and 30 mol % - 63 mol % Bi2O3 with the synthesis technique applied. Substitution of Bi2O3 with Nd2O3 up to 10 mol% leads to the preparation of a complex multicomponent glasses and glass-crystalline materials of high thermal stability.