Browsing by Author "Mihailova I."
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Item Batio3 Structure as a Function of the Preparation Method(2020-01-01) Nicheva D.; Harizanova R.; Ilcheva V.; Mihailova I.; Petkova T.; Petkov P.Barium titanate is synthesized by sol-gel and hydrothermal methods. The sol-gel technique allows the preparation of amorphous powders which are subsequently subjected to appropriate time-temperature programs and barium titanate is precipitated. The allotropic modification of the barium titanate from the obtained sol-gel powder depends on the temperature applied. It is cubic for heat treatment at 900 °C for 4 h, while in case of crystallization at 1100 °C for 3 h the tetragonal modification occurs as witnessed by X-ray diffraction analyses. Barium titanate is also successfully prepared by the hydrothermal method which results in the crystallization of the cubic allotrope as observed by X-ray diffraction. The microstructure of the formed barium titanate powders is imaged by scanning electron microscopy and reveals the formation of large fraction of crystals which are polydispersed for the samples prepared via sol-gel method and tend to agglomerate in case of hydrothermal synthesis. The infrared spectroscopy investigation of the powders obtained by both synthesis methods shows the presence of the absorption peak characteristic for the barium titanate phase in the range 540–580 cm−1.Item Calcareous clays: Phase composition and adsorption capacity for heavy metal ions(2019-01-01) Mihailova I.; Vassileva P.; Mehandjiev D.The purpose of this study was to investigate the prospective use of calcareous clay in its natural state and after thermal treatment for removal of heavy metals from aquatic environment. The study included 6 natural products from clay deposits in Bulgaria, five of which were calcareous clays and the sixth one was an iron-rich clay. The structural characterization of the samples was carried out using inductively coupled plasma optic emission spectrometry, X-ray powder diffraction analysis, Fourier-transform infrared spectroscopy, thermogravimetric analysis and differential scanning calorimetry. The clay minerals contained in the samples were identified as illite, chlorite, kaolinite and smectite. Quartz, calcite and feldspar were identified in the samples too. The thermal treatment has led to changes in the phase composition of clays as a result of partial dehydration and decomposition of minerals. There was an increase in the adsorption capacity of the calcareous clays relative to the metal ions of Cu(II) and Zn(II) after thermal treatment, regardless of the reduction of their specific surface area, which is determined by the increase of the adsorption centers upon thermal treatment.Item Carbonate - apatite forming ability of polyphase glass - ceramics in the CaO - MgO - SiO2 system(2015-01-01) Mihailova I.; Radev L.; Aleksandrova V.; Colova I.; Salvado I.M.M.; Fernandes M.H.V.Several types of glass-ceramics in the CaO - MgO - SiO2 system are shown to be bioactive, biocompatible and exhibiting various advantages for bone regeneration. A polyphase calcium-magnesium-silicate glass-ceramics is prepared by the sol-gel method including a two-step thermal treatment procedure aiming to investigate the effect of its phase composition on the in-vitro bioactivity observed. The dried gels of a chemical composition 3CaO.MgO.2SiO2 corresponding to merwinite are thermally treated at 700°C and 1100°C. The structural behavior of the synthesized samples is examined by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). Larnite as the main crystalline phase, accompanied by merwinite, akermanite and periclase are detected after heat treatment at 1100°C. The in-vitro bioactivity of the glass-ceramics is detected by investigating the apatite formation ability in Simulated Body Fluid (SBF) for different times of soaking. The changes on the surface of the immersed samples and the formation of Mg- and Si-substituted carbonated apatites are verified by FTIR, SEM and Energy Dispersive Spectroscopy (EDS) techniques. The dissolution behavior of the glass-ceramics in SBF is also carried out by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). The results indicate fast degradation in SBF and high reactivity of the polyphase glass-ceramics. The latter might be used as a bioactive implant material.Item Effect of the thermal treatment on the Pb (II) adsorption ability of blast furnace slag(2013-05-21) Mihailova I.; Dimitrova S.; Mehandjiev D.The aim of this study is to evaluate the possibilities for improving the adsorption capacity of blast furnace slag in processes for removal of heavy metal ions from aqueous media. Three samples of slag have been prepared by heating of granulated blast furnace slag. The initial slag and thermally treated slag samples have been characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and BET analysis. It has been found that the main crystalline phase of the analyzed slag samples is melilite. Merwinite and pseudowollastonite have been also identified. The Pb(II) adsorption isotherms have been obtained under bath conditions. All samples demonstrate a good Pb(II) removal ability. The equilibrium adsorption data are described by the Langmuir model. The highest Pb(II) adsorption capacity has been achieved by the blast furnace sample treated at 860 °C. The obtained results suggest that it is possible to improve the adsorption properties of blast furnace slag and slag based materials, by control of the phase composition.Item Glass–crystalline materials with high iron oxide concentration: Phase composition, redox ratio and magnetic properties Vitrocerámicas con alta concentración de óxido de hierro: Composición de fases, relación redox y propiedades magnéticas(2024-01-01) Harizanova R.; Mihailova I.; Cherkezova-Zheleva Z.; Paneva D.; Georgieva M.; Tzankov D.; Avdeev G.; Rüssel C.The synthesis of glass–crystalline materials in the system Na2O/CaO/SiO2/Fe2O3 with high concentrations of Fe2O3 (20, 25 and 30 mol%) by applying the melt-quenching technique is reported. The melts spontaneously crystallize during pouring and the formation of magnetite (Fe3O4), hematite (α-Fe2O3) and ɛ-Fe2O3, as identified by X-ray diffraction (XRD) is observed. The microstructure and the elemental composition of the prepared materials are further investigated by scanning electron microscopy (SEM) and two different morphological types of Fe-containing crystals – needle-like and dendrite-shaped are detected. Mössbauer spectroscopy showed the presence of Fe3+ and Fe2+, as well as the existence of iron ions both in tetrahedral and octahedral coordination and the precipitation of hematite, ɛ-Fe2O3 and magnetite. The magnetic measurements on the prepared samples reveal ferrimagnetic properties with well defined hysteresis curves, although due to relatively small volume fraction of the iron-rich crystalline phases, the net magnetic moment is quite low compared to the bulk values for magnetite.Item IR-SPECTROSCOPY, THERMOPHYSICAL AND MECHANICAL PROPERTIES OF GLASSES IN THE SYSTEM Na2O/BaO/TiO2/B2O3/SiO2/Al2O3(2023-01-01) Harizanova R.; Gaydarov V.; Gugov I.; Zamfirova G.; Mihailova I.; Pernikov M.; Rüssel C.Glasses with the compositions (23.1-x)Na2O/23.1BaO/23TiO2/9.8B2O3/21SiO2/xAl2O3 x = 3, 7, 11 and 15 mol% were melted using a traditional melt-quenching technique. The thermophysical properties of the obtained glasses, i.e., glass transition temperature, Tg, and crystallization peak maximum temperature, Tc, were determined by differential scanning calorimetry and showed that the increasing alumina concentration leads to an increase of both the Tg and Tc values, while the difference Tg-Tc, used as a glass stability criterium, changes in the opposite manner, being highest for the glass with 3 mol % Al2O3 and smallest for the composition with 15 mol % Al2O3. The structure of the prepared series of glasses was studied by infrared spectroscopy and showed that the main structural units present are isolated and interconnected SiO4 tetrahedra. Both BO4 and BO3 structural units, which number initially increases with the increasing alumina concentration and then starts to decrease, are also present. The conducted depth-sensing indentation on the obtained glasses allowed to determine mechanical properties such as universal (HMV) and indentation (Hit) hardness, elastic modulus and elastic part of indentation work. The change of the Na2O/Al2O3 ratio resulted in increasing HMV and Hit up to 11 mol% Al2O3 and then a slight decrease of the respective parameters was observed.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.Item Phase composition, structure and catalytic activity in oxidation reactions of fayalite waste from the flotation of copper slag(2017-01-01) Mihailova I.; Radev L.; Mehandjiev D.The study examines the phase composition and structure of fayalite waste (FW), as well as the changes that occur during heat treatment applied in the temperature range of 500 - 1000 °C. Differential thermal analysis, X-Ray diffraction analysis, Fourier transform infrared spectroscopy, Optical microscopy, Scanning electron microscopy and Energy dispersive spectroscopy have been used. The main crystalline phase in waste was fayalite, accompanied by magnetite, amorphous silicate phase and a small amount of copper-iron sulfides. Phase-structural changes were found in all heat treated samples. Their extent was temperature dependent. Fayalite transformed to hematite, magnetite and silica. Magnetite transformed to hematite through oxidation. Glass phase underwent oxidation too. In oxidation at 1000°C FW baseline phases were fully replaced by hematite, magnetite, cristobalite and amorphous silicate phase. At this heat treatment temperature the sample exhibited the highest catalytic activity towards the oxidation of CO.Item PHYSICOCHEMICAL AND STRUCTURAL CHARACTERIZATION OF SILICATE GLASSES CONTAINING IRON OXIDES(2023-01-01) Mihailova I.; Harizanova R.; Tasheva T.; Dimova N.; Rüssel C.The interest in glasses containing iron oxides is determined by the possibility for application in optics, sensors, nanoelectronics, biotechnology and medicine. Subject of the present investigation is the preparation of glasses in the system 16Na2O/10CaO/(74-x)SiO2/xFe2O3, x = 0, 5, 10 and 15 mol % by applying the traditional melt-quenching technique. X-ray diffraction analysis proves that the materials with up to 15 mol % Fe2O3 are amorphous while for higher concentrations, the crystallization of magnetite and hematite occurs during melt quenching. For the physicochemical characterization of the glasses, the density, molar volume and oxygen packing density have been determined. The Becke line method has been utilized for the refractive indices measurement. The structural evolution in dependence of the Fe2O3 concentration has been studied by Fourier Transformed Infrared Spectroscopy. The data obtained from the different methods correlates well and reveals a depolymerization tendency of the silicate network with the increasing Fe2O3 concentration, i.e. an increase in the number of SiO4 tetrahedra with more non-bridging oxygens as a result of the occurrence of Fe-containing polyhedra in the glass structure.Item POSSIBILITIES FOR CONSTRUCTION OF PHOTOVOLTAIC PARKS ON INDUSTRIAL LANDS(2022-01-01) Stanev R.; Mihailova I.; Lazarov P.; Shtaplev I.It is carried out an overview of current European strategic documents regulating the guidelines for transition to a green, resource-efficient and competitive economy. The main place is given to industry and power engineering. It is pointed out that the elimination of the existing problems in these branches requires the most serious concentration of material and intellectual resources, as well as an entirely new approach to their solution. It is emphasized that in addition to a major overhaul of large power stations, this large-scale task can be supported by the diversification of electricity generation into smaller, flexible and based on modern environmentally friendly technologies generating facilities. On the ground of a critical overview of the literature on renewable energy sources (RES), as well as on our own previous studies, solar installations have been identified as especially suitable for such integration into factory circumstances. Finding suitable areas for their location is often associated not only with financial and technical, but also with significant infrastructure, logistics, urban, social and other difficulties. Given the above and in view of the desire for synergy between industry and energetics in solving their general and specific problems, as an aim of this study is chosen to be studied the possibilities for building of photovoltaic parks on industrial lands. Based on the analysis of three of the most widespread types of solar systems, conclusions and recommendations have been made for the choice of a concrete construction, as well as for the way of integrating such power generating equipment to the energy system of the respective plant. Three criteria are derived, according to which the technical-economical indicators of different variants for design and building of photovoltaic parks can be assessed under the restrictions imposed by the circumstances of the production area in the considered enterprise.Item SOLID-STATE SYNTHESIS OF HYDROXYAPATITE FROM BLACK SEA RAPANA VENOSA SHELLS(2023-01-01) Yoleva A.; Mihailova I.; Djambazov S.Hydroxyapatite (HA) is a major non-organic component of bones and is widely used as orthopaedic and dental material. It is widely used in biological materials due to the apatite-like structure of enamel, dentin, and bones. Black Sea Rapana venosa shells may be used as natural raw material for solid-state synthesis of hydroxyapatite. Huge amounts of rapana shells fall on the shores of the Black Sea and lead to environmental pollution. The wide availability and low cost of rapana shells, as well as the fact that they contain mainly calcium carbonate, make them a low-cost raw material for the solid-state synthesis of hydroxyapatite. In this paper a solid-state synthesis of hydroxyapatite from the Black Sea Rapana venosa shells and monocalcium phosphate monohydrate Ca(H2PO4)2.H2O is discussed. The raw materials previously were calcined at high temperature of 1000°C to increase their reactivity and formation of a good crystal structure. Then they were homogenized at a 1 to 1 weight ratio in ball mill. Samples with dimensions of 3 x 0.5 cm were pressed at a pressure of 50 MPa on a hydraulic press and then fired at 1180°C with 2 hours hold at maximum temperature. Synthesized hydrоxyapatite was confirmed by XRD and FTIR spectroscopy analyses.Item Valorization of Peanut and Walnut Shells through Utilisation as Biosorbents for the Removal of Textile Dyes from Water(2023-08-01) Lazarova S.; Tonev R.; Dimitrova S.; Dimova G.; Mihailova I.The present research focuses on the application of peanut and walnut shells, in their natural state, for the removal of methylene blue (MB) from water solutions in batch and dynamic (flow-through) conditions. Under batch conditions, at a 100 mg/L MB concentration of aqueous solution, the optimum dose of the studied biosorbents was determined to be 2.5 g/L, reaching about 95–97% efficiency of MB removal for both materials. Langmuir and Freundlich isotherms were used to model and evaluate the experimental data under different initial concentrations of MB (25 to 100 mg/L). The determined maximum adsorption capacities are 41.50 mg/g for walnut shells and 46.80 mg/g for peanut shells. Under flow-through conditions, breakthrough curves are determined for three sizes of fractions (<0.25 mm, 0.315–0.5 mm, and 0.8–1.6 mm). For peanut shells, the smaller the particle size, the higher the adsorption capacity and the column breakthrough time. For walnut shells, however, the particle size seems to have a more complex influence on the adsorption process parameters, and this phenomenon deserves future investigation. The adsorption capacity for one and the same fraction size of 0.315–0.5 mm and initial MB concentration of 50 mg/L is higher under dynamic flow conditions, i.e., 51 mg/g compared to 20 mg/g for walnut shells and 46 mg/g compared to 17.5 mg/g for peanut shells.Item Waste Copper Slag/Aluminium Dross-Based Geopolymer(2021-01-01) Mihailova I.; Uzunov I.; Mehandjiev D.One of the important issues of the modern world is the ecological problems, which raise the issues of rational and efficient use of natural resources and the reduction of industrial waste. Materials synthesized from industrial waste are an example of producing new useful materials for various applications as well as efficient utilization of waste. The present study aims to synthesize amorphous aluminosilicates from metallurgical waste - waste copper slag, aluminum dross as well as from agricultural waste (rice husks). A synthesis method was applied by which the necessary silica and alumina were extracted from the waste products by chemical treatment in an alkaline environment. Two types of aluminosilicate materials were obtained depending on the experimental procedure. In the first case during the synthesis, there was a stage of polymerization and gel formation and in the second - of co-precipitation. Accordingly, the materials designated as geopolymer and amorphous aluminosilicate were characterized experimentally. They were found to be X-ray amorphous by XRD. FTIR spectroscopy proved that their structure was characterized by a three-dimensional aluminosilicate network with a high degree of connectivity of silicon and aluminum tetrahedra. The microstructure of the materials was established by SEM-EDS and their homogeneity was confirmed. However, due to the difference in synthesis, some differences in the chemical composition and structure of the geopolymer and the amorphous aluminosilicate were experimentally established. Their composition, the structural and textural features of the obtained geopolymer and amorphous aluminosilicate show that such materials can find application as catalyst carriers.Item α/β-TCP silicate glass-ceramic obtained by sol–gel: Structure and in vitro bioactivity Vitrocerámica α/β-TCP obtenida por método sol-gel: estructura y bioactividad in vitro(2024-01-01) Mihailova I.; Dimitrova P.; Radev L.A glass-ceramic in the CaO–P2O5–SiO2 system, which contains two polymorphic modifications of tricalcium phosphate – whitlockite, β-Ca3(PO4)2 and α-Ca3(PO4)2, has been synthesized by the sol–gel method and thermal treatment up to 1200 °C. The phase composition and microstructure of the glass-ceramic were investigated with X-ray diffraction analysis (XRD), Fourier-transformed infrared spectroscopy (FTIR), scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS). An in vitro bioactivity test of the glass-ceramic in a simulated body fluid (SBF) was conducted for up to 21 days. α-Ca3(PO4) dissolved almost completely in SBF after 7 days. The experimental results of XRD, FTIR, SEM and EDS clearly validated the ability of the glass-ceramic samples to form a layer of hydroxyapatite on their surface in an SBF environment. We also studied the cytotoxic effect of the glass-ceramic on murine bone marrow (BM) cells and pre-osteoclasts in vitro. The glass-ceramic reduced the viability of BM cells in a dose-dependent manner being less toxic at concentrations below 0.1 mg/ml. It modestly affected the viability of pre-osteoclasts cultured in osteoclast differentiation media. The obtained sample increased the percentage of pre-osteoclasts expressing the receptors involved in osteoclastogenesis (RANK) and apoptosis (TRAIL). In conclusion, the glass-ceramic showed the potential to affect the survival/differentiation of pre-osteoclasts at early stage of osteoclastogenesis. It might be suitable for tissue engineering including implants coating or scaffold as it can interfere with early stage of osteoclastogenesis which is required for proper bone remodelling and repair upon a long-term application of biomaterials.