Browsing by Author "Avdeev G."
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Item Corrosion of Stainless Steel in Simulated Nuclear Reactor Primary Coolant—Experiments and Modeling(2024-03-01) Bojinov M.; Betova I.; Karastoyanov V.; Avdeev G.In the present paper, the effect of the evolution of primary water chemistry during power operation on the corrosion rate and conduction mechanism of oxide films on stainless steel is studied by in situ impedance spectroscopy at 300 °C/9 MPa during 1-week exposure periods in an autoclave connected to a recirculation loop. At the end of the exposure period, the samples were anodically polarized in a wide range of potentials to evaluate the stability of the passive oxide. Separate samples of the same steel were simultaneously exposed to the coolant and subsequently analyzed by glow discharge optical emission spectroscopy (GDOES) in order to estimate the thickness and the in-depth composition of the formed oxides. Impedance data were quantitatively interpreted using the mixed-conduction model for oxide films (MCM) to estimate the rates of metal oxidation at the alloy/oxide interface, oxide dissolution and restructuring at the film/coolant interface, and ion transport in the protective corrosion layer.Item Crystallization behaviour of the systems Na2O/BaO/TiO2/SiO2/B2O3/Al2O3 and Na2O/BaO/TiO2/SiO2/B2O3/Fe2O3/Al2O3(2015-01-01) Harizanova R.; Mazhdrakova A.; Vladislavova L.; Avdeev G.; Bocker C.; Gugov I.; Rüssel C.This work reports data on the synthesis of glasses in three selected compositions of each of the systems Na2O/BaO/ TiO2/SiO2/B2O3/Al2O3 and Na2O/BaO/TiO2/SiO2/B2O3/Fe2O3/Al2O3 and the preparation of glass-ceramics by applying appropriate time-temperature regimes. Crystallization of a large volume fraction of cubic BaTiO3 is observed in both systems. The phase compositions of the obtained annealed specimens are investigated by x-ray diffraction, while the microstructure of the obtained glass-ceramics is studied by optical and scanning electron microscopy. The barium titanate crystals in the samples containing iron have a size varying from ca 100 nm to some hundreds of nm. They form interconnected structures which tend to grow further for all temperature-time schedules applied. The microstructure of the annealed samples containing no iron, as proved by optical and electron scanning microscopy, is characterized by barium titanate crystals of a size of some micrometers which are uniformly dispersed in the amorphous matrix. They start to grow independently and tend to merge upon longer annealing times.Item Effect of Electrolyte PH And Pulse Potential Frequency on The Properties of Ni-co Powders(2021-01-01) Ignatova K.; Avdeev G.In this work, we have synthesized finely dispersed Ni-Co powders through two current modes - constant and pulse potential modes in a chloride electrolyte with different pH values and at different pulse frequencies. The powder formation has been found to be difficult when increasing the pH of the solution from 1.4 to 6.8 and by raising the pulse frequency above 500 Hz. With increasing the pulse frequency “anomalous” cobalt deposition and decrease in particle size is observed. The Ni-Co powders, containing more than 45 - 50 wt. % Co, represent three-phase systems based on cubic, f.c.c. β-Ni and β-Co phases and hexagonal, h.c.p. α-Co-phase. Nickel and cobalt are evenly distributed in the structure of the powder particles. The results obtained prove that Ni-Co powders are to a very large extent alloy systems, not a mechanical mixture of individual elements.Item Effect of H3Po3 and Nah2Po2 in the electrolyte on the composition and microstructure of Ni-Co-P alloys(2019-01-01) Ignatova K.; Alakushev M.; Kozhukharov S.; Marcheva Y.; Vladimirova S.; Avdeev G.The effect of two phosphorus-containing additives (phosphorous acid, H3PO3 and sodium hypophosphite, NaH-2PO2) in modified Watt's electrolyte (pH = 2; 80°S) on the deposition kinetics, morphology, component and phase composition of Ni-Co-P coatings was studied. When only one of the studied additives is present in the solution, the P content in the coatings reaches maximum of 15 % and their structure is amorphous. The P and Ni incorporation in the coatings is facilitated when the electrolytes with prevailing concentrations of H3PO3 are used. The maximal P content (over 30 %) in the coatings was reached when both additives were present in the electrolyte. These coatings possess mixed structure with nano-dimensional amorphous phase and crystal hexagonal phase of solid solutions Ni2P and Ni2P-Co2P. The increasing of the current density causes P content decrement and the resulting crystal phase is tetragonal, composed by solid solutions of Ni3P and Ni3P-Co3P.Item Effect of the electrode position conditions on the morphology and corrosion behavior of Ni-Co alloys part 2: Phase composition and corrosion behavior of Ni-Co alloys, electrodeposited from citrate electrolyte(2016-01-01) Ignatova K.; Avdeev G.The changes in the phase composition (through X-Ray analysis) and the corrosion behaviors (through potentiodynamic polarization studies in 4 % NaCl) of Ni-Co alloys, electrodeposited from a citrate electrolyte by changing the Ni/Coratio, content of organic additive (saccharine) and the content of sodium citrate in the solution, are studied. It is found that the increase of Ni/Co ratio from 1 to 5 increases the content of Ni in the Ni-Co alloy to 48 mass %. The proportion between the phases of Co and Ni with cubic face-centered crystal lattice is in approximate compliance with the proportion of the two metals in the alloy. The peaks in the difractograms are weak and stretched, which corresponds to the nano-sized structure of the alloy. The Ni-Co alloys with highest content of Ni have a higher corrosion resistance compared to those containing a higher percentage of Co. It is also shown that the Ni-Co coatings obtained in the presence of addition of saccharine show a much higher corrosion resistance than the coatings obtained in absence of additive. Ni-Co alloys, deposited from electrolyte with more sodium citrate show only a slight growth of Ni content and the effect of sodium citrate on corrosion behaviors of the alloy is insignificant.Item Electron microscopy investigation of the microstructure and elemental composition of barium titanate precipitated in oxide glasses(2018-01-01) Harizanova R.; Vladislavova L.; Avdeev G.; Bocker C.; Gugov I.; Rüssel C.The present work reports on the synthesis of glasses for two different ratios of alumina to sodium oxide in compositions of the system Na2O/Al2O3/BaO/TiO2/B2O3/SiO2 with addition of Fe2O3. Barium titanate-based glass-ceramics of different average crystallite sizes, depending on the ratio Na/Al-oxides, are precipitated after appropriate thermal treatment of the glasses prepared. The phase composition analyses performed by X-ray diffraction show the formation of cubic barium titanate, BaTiO3 as a main phase and also, some BaTi0.75Fe0.25O2.888. Additional crystalline phases, such as nepheline, NaAlSiO4 appear in the course of thermal treatments at higher temperatures and longer annealing times. The scanning electron micrographs of the prepared glass-ceramics verify the presence of interconnected spherulitic structures, which correspond to barium titanate crystals and which start to grow with crystallization time increase. The morphology of the spherulitic crystals is elucidated by means of transmission electron microscopy. The elemental analysis carried out by TEM verifies the precipitation of two main crystalline phases - barium titanate and barium titanium iron oxide.Item Evaluation of the electrochemical performance of Ag containing aao layers after extended exposure to a model corrosive medium(2020-01-01) Kozhukharov S.; Girginov C.; Kiradzhiyska D.; Tsanev A.; Avdeev G.The coating procedure appears to be an indispensable finishing stage in the production of Al based industrial products, engineering facilities and equipment. For this reason, there is an ever-increasing interest towards the elaboration of reliable corrosion protective layers with apparent coverage, adhesion, and barrier properties. In this sense, both the formation of anodized aluminum oxide (AAO) layer and its further modification with silver enable the elaboration of advanced (Al-O-Ag) films with extended beneficial characteristics. The present research activities are aimed at the determination of the corrosion protective properties of electrochemically synthesized Al-O-Ag layers on the technically pure AA1050 alloy. The structures and compositions of the obtained Al-O-Ag layers were characterized by X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). The research activities were accomplished by means of two independent electrochemical characterization methods: electrochemical impedance spectroscopy (EIS) and potentiodynamic scan-ning (PDS). The electrochemical measurements were performed after 24, 168 and 672 hours of exposure to 3.5 % NaCl solution used as a model corrosive medium (MCM), in order to determine the barrier properties and durability of the elaborated Al-O-Ag layers. The analysis of the obtained results has undoubtedly shown that the proposed electrochemical Al-O-Ag layer formation can successfully be used for the creation of self-standing layers with apparent corrosion protective properties. Besides, Al-O-Ag system can be used as a basis for development of efficient protective layers suitable for application in biologically contami-nated media.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 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 Synthesis of new complex [Bi6O6(OH)2](ClC6H4SO3)4and investigation of its thermal decomposition(2023-05-01) Kaloyanov N.; Zahariev A.; Parvanova V.; Avdeev G.; Girginov C.A new [Bi6O6(OH)2](ClC6H4SO3)4complex is synthesized through a reaction of Bi2O3 with aqueous 4-chlorobenzenesulfonic acid at elevated temperature. The product composition is elucidated by elemental analysis, ICP-OES and FTIR techniques. By means of XRD analysis, the compound s indexation, monoclinic P2(3) space group and unit cell parameters: A=31.25(3) A, b=19.78(1) A, c=8.795(8) A, γ = 94.33(4)o as well as a cell volume = 5424.21 A3 are determined. Based on DTA-TG technique, three specific temperatures aimed at studying product s thermal decomposition are determined. The compositions of solid residues after isothermal heating are proved following the XRD and FTIR data.Item Two dimensional polymerization of graphene oxide: Bottom-up approach(2015-08-01) Atanasov V.; Russev S.; Lyutov L.; Zagraniarsky Y.; Dimitrova I.; Avdeev G.; Avramova I.; Vulcheva E.; Kirilov K.; Tzonev A.; Abrashev M.; Tsutsumanova G.We demonstrate a bottom-up synthesis of structures similar to graphene oxide via a two dimensional polymerization. Experimental evidence and discussion are conveyed as well as a general framework for this two dimensional polymerization. The proposed morphologies and lattice structures of these graphene oxides are derived from aldol condensation of alternating three nucleophilic and three electrophilic centers of benzenetriol.