Browsing by Author "Georgiev A."
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Item 8-(Pyridin-2-yl)quinolin-7-ol as a platform for conjugated proton cranes: A dft structural design(2020-10-01) Georgiev A.; Antonov L.Theoretical design of conjugated proton cranes, based on 7-hydroxyquinoline as a tautomeric sub-unit, has been attempted by using ground and excited state density functional theory (DFT) calculations in various environments. The proton crane action request existence of a single enol tautomer in ground state, which under excitation goes to the excited keto tautomer through a series of consecutive excited-state intramolecular proton transfer (ESIPT) steps with the participation of the crane sub-unit. A series of substituted pyridines was used as crane sub-units and the corresponding donor-acceptor interactions were evaluated. The results suggest that the introduction of strong electron donor substituents in the pyridine ring creates optimal conditions for 8-(pyridin-2-yl)quinolin-7-ols to act as proton cranes.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 Complex activity and sensor potential toward metal ions in environmental water samples of n-phthalimide azo-azomethine dyes(2021-10-01) Georgieva S.; Bezfamilnyi A.; Georgiev A.; Varbanov M.Herein, the spectral and electrochemical characterizations of three different substituted N-phthalimide azo-azomethine (NAA) dyes (L) containing an o-hydroxy group and their NAA-M(II) chelates [M(II): Cu, Ni, Co, Pb] were reported by using UV–Vis and fluorescence spectroscopy and potentiometric and voltamperometric techniques. The pK value of the dyes as well as the stoichiometry and stability of the NAA-metal chelates were studied, and the stoichiometry was found to be mostly 1:2 (ML2 ) with high complex stability constant values. The sensor activity of N-phthalimide azo-azomethine derivatives toward pH and metal ions has been also investigated and tested for indicator application in acid–base analysis and detection of Cu(II) ions in real samples of surface river water using voltamperometric detection. The results showed that one of the ligands possesses the highest electrochemical response upon binding to copper ions and could be successfully used in the analysis of copper in water at a concentration range of the analyte from 3.7 × 10−7 to 5.0 × 10−6 mol L−1, with analytical characteristics of the method being Sr = 1.5%, LOD = 3.58 µg L−1 and LOQ =11.9 µg L−1.Item Directed modification of the properties of vacuum deposited polymer layers by energy-stimulated processes(2008-05-01) Dimov D.; Spassova E.; Georgiev A.; Karamancheva I.; Assa J.; Danev G.Vacuum deposited thin layers were investigated for obtaining nanocomposite films based on a polyimide matrix and copper phthalocyanine particles as ``guests``. The paper presents a study related to energy-stimulated processes using microwave treatment as an alternative to the classical thermal processing. FTIR spectroscopy investigations demonstrated the possibility for a considerable shortening of the entire treatment process. The best results were obtained by combining the thermal and microwave treatments (15 min microwave + 15 min thermal treatment). © 2008 IOP Publishing Ltd.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 Indirect solvent assisted tautomerism in 4-substituted phthalimide 2-hydroxy-Schiff bases(2020-08-15) Yordanov D.; Deneva V.; Georgiev A.; Crochet A.; Fromm K.M.; Antonov L.The paper presents the synthesis and characterization of two 4-substituted phthalimide 2-hydroxy-Schiff bases containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties. The structural differences of 2-hydroxyaryl substituents, resulting in different enol/keto tautomeric behaviour, depending on the solvent environment were studied by absorption UV–Vis spectroscopy. Compound 5 is characterized by a solvent-dependent tautomeric equilibrium (KT in toluene = 0.12, acetonitrile = 0.22 and MeOH = 0.63) while no tautomerism is observed in 4. Ground state theoretical DFT calculations by using continuum solvation in MeOH indicate an energy barrier between enol/keto tautomer 5.6 kcal mol−1 of 4 and 0.63 kcal mol−1 of 5, which confirms the experimentally observed impossibility of the tautomeric equilibrium in the former. The experimentally observed specific solvent effect in methanol is modeled via explicit solvation. The excited state intramolecular proton transfer (ESIPT) was investigated by steady state fluorescence spectroscopy. Both compounds show a high rate of photoconversion to keto tautomers hence keto emissions with large Stokes shifts in five alcohols (MeOH, EtOH, 1-propanol, 1-butanol, and 1-pentanol) and various aprotic solvents (toluene, dichlormethane, acetone, AcCN). According to the excited state TDDFT calculations using implicit solvation in MeOH, it was found that enol tautomers of 4 and 5 are higher in energy compared to the keto ones, which explains the origin of the experimentally observed keto form emission.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 Ion - Beam assisted process in the physical deposition of organic thin layers(2010-01-01) Dimov D.; Georgiev A.; Spassova E.; Assa J.; Danev G.A novel method was developed for physical deposition of thin polyimide layers by applying an argon plasma assisted process. The influence was investigated of the plasma on the combined molecular flux of the two thermally evaporated precursors - oxydianiline and pyromellitic dianhydride. The effects observed on the properties of the deposited films are explained with the increased energy of the precursor molecules resulting from the ion-molecular collisions. As could be expected, molecules with higher energy possess higher mobility and thus determine the modification of the films structure and their electrical properties. © 2010 IOP Publishing Ltd.Item On monitoring of mechanical characteristics of hot rolled S355J2 steel(2016-01-01) Angelova D.; Yordanova R.; Georgiev A.; Yankova S.Hot rolling normalization technology for producing sheets from low-carbon Steel S355J2, used in the Bulgarian Metallurgical Plant ``STOMANA Industry SA`` is under investigation. A newly introduced automatic application optimization procedure to this technology is an important step that leads to avoiding traditional heat treatment, improving steel mechanical characteristics, increasing production efficiency, all resulting in high quality final products. On the basis of the final mechanical rolled-sheet characteristics - yield strengths, Re, ultimate tensile strengths, Rm, absorbed energies in impact tests, K, and elongations after fracture - some energy-stress and energy-stress-elongation constructions-spaces have been plotted. These spaces can be used for general evaluation of the applied rolling technology and for prediction of steel-sheet mechanical behaviour.Item On monitoring of mechanical characteristics of hot rolled S355J2 steel(2016-06-01) Angelova D.; Yordanova R.; Georgiev A.; Yankova S.Hot rolling normalization technology for producing sheets from a low-carbon steel, Steel S355J2, used in the Bulgarian Metallurgical Plant “STOMANA Industry SA” is under investigation. A newly introduced automatic application optimization procedure in this technology is an important step that leads to avoiding of traditional heat treatment, improving of steel mechanical characteristics, increasing of production efficiency, all resulting in high quality final products. On the basis of the final mechanical rolled-sheet characteristics - yield strengths, Re, ultimate tensile strengths, Rm, absorbed energies in impact tests, K, and elongations after fracture - some energy-stress and energy-stress-elongation constructions-spaces have been plotted. These spaces can be used for general evaluation of the applied rolling technology and for prediction of steel-sheet mechanical behaviour.Item Physical deposition of thin polyimide layers by applying an argon plasma assisted process(2010-01-01) Dimov D.; Georgiev A.; Spassova E.; Assa J.; Dineff P.; Danev G.A novel method for physical deposition of thin polyimide layers by applying an argon plasma assisted process has been developed. The influence of the plasma on the combined molecular flux of the two thermally evaporated precursors - 4,4'- oxydianiline and pyromellitic dianhydride was investigated. The process parameters were changed in the limlts 0,4 - 2 A for the anode current and 80 - 170 V for the anode voltage. Their influence was studied using FTIR spectroscopy and electron microscopy techniques. It was proposed that the plasma flux crossing the molecular flows of the polyimide precursors enhances the imidization process by partly activating the precursor molecules in the gas phase. © 2010 IOP Publishing Ltd.Item Tautomeric influence on the photoinduced birefringence of 4-substituted phthalimide 2-hydroxy Schiff bases in PMMA matrix(2021-05-01) Blagoeva B.; Stoilova A.; Dimov D.; Yordanov D.; Nazarova D.; Georgiev A.; Antonov L.The photoinduced birefringence of two 4-substituted phthalimide 2-hydroxy Schiff bases, containing salicylic (4) and 2-hydroxy-1-naphthyl (5) moieties has been investigated in PMMA matrix. Their optical behaviour as nanocomposite films was revealed by combined use of DFT quantum chemical calculations (in ground and excited state) and experimental optical spectroscopy (UV–Vis and fluorescence). The results have shown that solid-state reversible switching takes place by enol/keto tautomerization and Z/E isomerization. Birefringence study was performed in the PMMA nanocomposite films using pump lasers at λrec = 355 nm and λrec = 442 nm. Fast response time and high stability of anisotropy up to 58% for (4) and 95% for (5) after turning off the excitation laser, was observed, which makes these materials appropriate candidates for cutting-edge optical information technology materials. The possibility for multiple cycles of recording, reading and optical erasure of the photoinduced birefringence at λrec = 442 nm in 5 has been demonstrated. The obtained results have shown that the maximum value of the measured birefringence is close to the anisotropic characteristics of the most frequently used azo materials.Item Thin dye layers - Vacuum deposition and structure(2012-01-01) Dimov D.; Assa J.; Georgiev A.; Kazakov R.; Karashanova D.; Danev G.; Spassova E.Vacuum-deposited layers of zinc phthalocyanine and perylene-3,4,9,10- tetracarboxylic dianhydride were obtained with regard to the development of converters of solar radiation into electricity, namely, organic solar cells. It was shown that there were no differences in the composition of the initial dyes and the deposited ones, which is a proof of their thermal stability and a prerequisite for the reproducibility of the composition of the single and mixed layers under study. The results obtained are interpreted in view of the possible applicability of the layers studied for the fabrication of organic solar cells in which the processes of creation, the lifetime and the diffusion paths of the current carriers depend significantly on the film composition and the defects formed at the time of deposition of the active dye layers. © Published under licence by IOP Publishing Ltd.