Browsing by Author "Yoleva A."
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Item A study on the technological parameters' influence on the properties, polishing and microstructure of corundum ceramic discs for water taps(2015-01-01) Djambazov S.; Yoleva A.; Hristov V.; Ivanov N.Some of corundum ceramics properties like high chemical resistance, hardness and mechanical strength make it an extremely suitable material for water tap gaskets and regulators. Solid fragments that normally get into the water, like sand, rust and limestone, cannot damage corundum ceramics. The latter stability allows these products to remain perfectly smooth even when exposed to varying levels of pressure and extreme water temperature fluctuations. The corundum ceramics properties depend mainly on the purity of the starting powder, the production method and the microstructure. This work investigates how the pressure applied during the semidry pressing, the firing temperature and the source materials composition (percentage of Al2O3) affect the apparent density, the coefficient of abrasion, the polished surface (the degree of polishing) and the microstructure of corundum ceramic water taps discs. Four types of a corundum powder were used as a source material for the process of semidry pressing. Five different pressures (100 MPa 150 MPa, 200 MPa, 250 MPa, 350 MPa) and three firing temperature values (1600°C, 1650°C, 1680°C) were applied in addition to the subsequent procedures of lapping and polishing in the course of production of corundum ceramic discs for water taps. The fired corundum samples underwent characterization aimed to determine the apparent density (using the method of Archimedes), the coefficient of abrasion (determined by the loss of mass in percentage points), as well as the degree of surface polishing. Using SEM and optical microscopy we proved that the technological parameters determine to a large extent the appearance of a microstructure which in turn affects the properties and the degree of polishing of corundum ceramics. Water taps of a homogeneous structure having the highest density, the smallest pores, the lowest coefficient of abrasion and the highest degree of polishing were made from a starting powder containing 98 % of Al2O3 and a glass phase containing CaO, MgO and SiO2. A firing temperature of 1650°C and pressing pressure 350 MPa were applied.Item Amorphous silicate and zeolite effect on the thermal conductivity and porosity of ceramic building bricks(2015-01-01) Djambazov S.; Yoleva A.; Chervenliev P.; Georgiev A.The effect of amorphous silicate and zeolite on the thermal conductivity and porosity of ceramic building bricks is studied. The aim of the present work is to produce porous bricks of reduced thermal conductivity and acceptable compressive strength by using a combination of natural raw materials (amorphous silicate and zeolite) as well as sawdust and coal dust as pore forming agents. Clay from Lucovite deposits is used as the main component of the ceramic bodies (over 60 %). The content of amorphous silicate is increased from 7 mass % to 20 mass %, while that of zeolite - from 7 mass % to 30 mass %. The green samples are prepared by plastic extrusion. After moulding and drying at 100oC, the samples are fired at 800oC and 850oC for one hour. Then their water absorption, apparent density and apparent porosity, mechanical compressive strength and thermal conductivity are analyzed. It is found that the amorphous silicate has a small effect on the properties of the bricks obtained. The bricks containing zeolite have low?r thermal conductivity and higher porosity when compared to those of other compositions. A tendency of thermal conductivity decrease and porosity increase with zeolite quantity increase is found at both firing temperatures. Bricks of improved properties are prepared at the lower firing temperature. The highest porosity (36,00 %) and the lowest thermal conductivity (0,67 W/mK) is obtained in case of bricks containing 30 % zeolite and firing temperature of 800oC. The results of this work show that building bricks of lower thermal conductivity and higher porosity can be produced at a lower firing temperature by introducing zeolite to their composition. They are a potential heat insulating material.Item An effect of a natural zeolite on the characteristics of spring or electrochemically activated water(2018-01-01) Djambazov S.; Yoleva A.The aim of the present experimental investigation is to trace the change of some general characteristics (pH, electro conductivity, mineralization and chemical composition) of spring water from Rila mountain in Bulgaria, as well as of electrochemically activated water after different periods in contact with a natural zeolite from Haskovo region, Bulgaria. The chemical and phase compositions of the zeolite are determined by means of ICP-AES and XRD. The effect of the zeolite-water contact time of 24 h, 48 h, 72 h and 168 h on the ion exchange process in presence of 10 mass % natural zeolite concentration is followed. The experiments are carried out with two types of a zeolite - a non thermally treated zeolite at 25°C and a thermally treated one for 2 h at a maximal temperature of 300°C and 550°C. Electrochemically activated water of different PH values is obtained through electrolysis carried out for time intervals ranging from 5 min to 20 min. The treated waters of different PH values are used for 48 h treatment at 550°C. The chemical composition, the pH changes, the electro conductivity and the mineralization are determined using ICP-AES and Combo pH/Conductivity/TDS meter (Low Range, HANNA instruments HI98129 multiparameter, 14pH, 3999us/cm, 2000 ppm). The optimal conditions of the ion exchange process between the natural zeolite and the spring water are established. The extended ion exchange process and the higher heat treatment temperature of the zeolite result in a higher amount of transmitted K+and Na+c ations improving thus the ion exchange. It is proved that alkaline water has an activating action stimulating the ion exchange of both cations and anions in the water systems.Item 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.Item Effect of wheat straw and sunflower seeds husks as pore forming agents on the properties of porous clay bricks(2017-01-01) Georgiev A.; Yoleva A.; Djambazov S.The interest towards introduction of agricultural biomass wastes to porous clay bricks is increasing lately. These pore-forming agents start to ignite during the firing process providing extra thermal energy inside the product and decreasing the required external energy required. Besides this effect, their combustion increases the porosity of the final product resulting in enhanced thermal insulation properties. This paper presents the effect of adding biomasses (wheat straw and husks of sunflower seeds) in quantity of 3,5 mass % and 8 mass % on the physical and thermal properties of clay porous bricks fired at 900°C. The results show increase of the apparent porosity compared to fired clay and decrease of the thermal conductivity of the products. There is a significant positive correlation between the increasing quantity of organic matter and the porosity. This study shows that 5 mass % of wheat straw and 3 mass % of sunflower seeds husks can be used as pore forming agents in bricks production to improve their thermal conductivity and keep acceptable compressive strength.Item Influense of brewery waste sludge containing diatomite on the physical properties and thermal conductivity of porous clay bricks(2018-01-01) Georgiev A.; Yoleva A.; Djambazov S.Different forms of wastes like sewage sludges, bagasse, sawdust, papermaking sludge, olive oil waste, tobacco residues, urban waste water sludge, brewing industry sludge, biomasses, etc. are used as pore-forming agents in the manufacture of ceramic bricks. These wastes modify the microstructure of clay improving its thermal conductivity. Diatomite is used as a filtration medium the brewing industry. A sludge containing diatomite and organic material retained during the clarification process is used as a by-product. The effect of the dried diatomite containing a brewing industry waste by-product in quantities of 0 mass %, 3 mass %, 5 mass %, 8 mass %, 11 mass % and 14 mass % on the water absorption, apparent density, apparent porosity, compressive strength and thermal conductivity of porous clay samples fired et 900oC is investigated. There is no change of the fired material properties in case of a minimum (3 mass %) content of a brewery waste. The fired clay material properties vary proportionally to further waste amount increase. It is found that the water absorption and the apparent porosity increase, while the apparent density, the thermal conductivity and the compressive strength decrease with increase of the brewery waste content.Item INVESTIGATION OF DECARBONIZATION OF GRANULATED LIMESTONE FROM THE EASTERN RHODOPES DEPOSIT, BULGARIA(2021-01-01) Djambazov S.; Yoleva A.Limestones are widely used in various industries such as cement industry, glass industry, ceramic industry, chemical industry, metallurgy and others. The use of limestone is based on the mineral and chemical composition, structure, texture, physic-mechanical or technological properties. The chemical reactivity of different limestones shows great variation due to their differences in crystal structure and also due to the nature of impurities such as silicon, iron, magnesium, manganese, sodium and potassium. Changes in the structure of the pores play a significant role in the calcination mechanism and the reactivity of calcined lime. The thermal decomposition of limestone has been the subject of intensive research over the years. Limestones are extremely widespread in Bulgaria. They are represented in the form of layers, bundles and sedimentary formations. Thermal treatment of granulated limestone with grain size from 1 to 4 mm from deposit Eastern Rhodopes, Bulgaria at different temperatures in the range 700OC - K)()()``(' was performed, in order to establish the temperature regime of limestone decarbonization. Using the methods of X-ray diffraction and IR spectroscopy, it was proved that the limestone, which contains over 98 % calcite, begins to decarbonize above 800°C and is completely decarbonizes at 1000°C. After its decarbonization, CaO (quicklime) was formed, which was indirectly proven by the presence ofportlandite in the diffratograms and IR spectra.Item LIGHT CERAMIC MATERIALS WITH LOW THERMAL CONDUCTIVITY(2022-01-01) Yoleva A.; Djambazov S.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 ceramic materials by introducing pore-forming additives to the clay for production of lightweight ceramic panels. Compositions and properties of lightweight ceramic panels from marl clay and rice husks as lightweight aggregate were investigated. The lightweight ceramic panels from marl clay and rice husks with sizes 5x5x1 cm were produced by dry pressing and following firing at 1000oC. The influence of the amount of rice husks of 15, 30, 50 and 65 mass % on the properties of light ceramic samples fired at 1000°C has been established. Apparent density, apparent porosity, thermal conductivity and compressive strength were measured. The thermal conductivity is reduced from 1.0 W mK-1 (in ceramic panels without rice husks) to 0.6 W mK-1 (ceramic panels with 30 mass % rice husks) at standard compressive strength of 10 MPa. This quantity of rice husks is recommended for the composition of light porous ceramic materials for heat isolation in intelligent house.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 Organic modification of Bulgarian bentonite by an easy low cost method(2016-01-01) Yoleva A.; Djambazov S.; Michailov G.Bentonite is hydrophilic in nature due to the cations within its interlayer space. This makes natural bentonites ineffective sorbents of organic compounds. The modification of their surfaces by surfactants is an option to prepare bentonitebased sorbents of organic pollutants and bentonite fillers used for polymer nanocomposites production. The present study reports the formation of organobentonites on the ground of Bulgarian bentonite following an easy and cost-effective cation exchange methodology. Octadecylamine is the surfactant used. All reactions are carried out in a single vessel. No drying and grinding of the bentonite's is required prior to the organic modification. The organobentonites obtained are studied by XRD, IR, DTA and TGA. Different organic solvents as ethanol, toluene and xylene are used to investigate their sorption and swelling properties. It is found better organic ions intercalation is obtained that with increase of the amount of the organic modifier. This is evidenced by the increase of the spacing d (001) (Å) from 13,20 (Å) to 20,50 (Å). The best organic cations intercalation is achieved in a bentonite containing 20 mass % of octadecylamine. The organobentonites obtained are hydrophobic in water and hydrophilic in organic media. The organobentonite modified by 20 mass % of octadecylamine shows the best swelling capacity in xylene.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 PREPARATION AND CHARACTERIZATION OF CERAMIC MEMBRANES ON THE BASE OF HALLOYSITE CLAY AND LIMESTONE(2020-01-01) Yoleva A.; Djambazov S.; Sabrieva S.The development of ceramic membranes based on natural raw materials such as kaolins, clays, zeolites, lime-stones, dolomites, feldspars, quartzs and others appears an efficient solution to purification of water at a low cost. The ceramic membranes are prepared in the form of disks of a size of 3,0 cm x 0,4 cm by dry pressing of a homog-enized batch from halloysite clay and limestone of different quantities at pressure of 50 MPa. They are fired for 2 h at 1100°C. The properties of the sintered ceramic membranes such as shrinkage, water absorption, an apparent density, an apparent porosity and a mechanical bending strength are determined by standard methods. Mercury porosimetry is used to determine the pore size distribution and the average pores size. The phase composition is identified by XRD analysis. The microstructure is observed using SEM. The increase of the limestone content results in an increase of the water absorption and the apparent porosity and a decrease of the shrinkage, the apparent den-sity and the compressive strength. The average pore size equals 2,32 µm, while the total porosity amounts to 45,00 % for the ceramic membrane containing 60 wt.% of halloysite clay and 30 wt.% of limestone. The main crystalline phases found in the sintered ceramic samples refer to anorthite (CaO.Al2O2.2SiO2) gehlenite (2CaO.Al2O3.SiO2) mullite (2Al2O3.SiO2) and wollastonite (CaO.SiO2).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 Study of medieval ceramics excavated at the monastery of Karaachteke (Varna, Bulgaria)(2015-01-01) Yoleva A.; Djambazov S.; Djambazov P.Various medieval unglazed and glazed ceramic artifacts discovered during archaeological excavations at the monastery of Karaachteke near Varna (Bulgaria) are for the first time chemically and structurally characterized by ICP-OES, XRD, SEM-EDS and the Archimedes method aiming to understand the technology of medieval ceramics production. The unglazed artifacts water absorption ranges from 10 mass % to 15 mass %, which indicates that the ceramics are well sintered and most probably fired at a high temperature, ca 950°C - 1050°C. The artifacts contain a certain amount of coloring oxides (Fe2O3 + TiO2), which determine their slightly red color. The phase composition indicates the presence of crystalline phases of quartz and plagioclase, the amount of which differs in the different samples. SEM proves the presence in the sintered ceramic body of coarse quartz grains having a size of 0.05 mm to 0.3 mm. This leads to the conclusion that highly sandy clay or ceramic body containing red firing clay and coarse grain quartz sand was used. The study of the glazed artifacts proves that a transparent lead glaze prepared at a firing temperature of ca 950°C - 1050°C was widely used in the Middle Ages. Some of the objects are typical sgraffito pottery, which indicates that the ceramic masters during this period could produce ceramic bodies, engobes and glazes using different raw materials compositions. Other glazed artifacts refer to the type of the famous Preslav ceramics having a white ceramic body and transparent green glaze of the Seger formula PbO.0,16Al2O3·2SiO2 and firing temperature higher than 1000°C.