DSpace Repository :: Browsing by Author "Kozhukharov S."

Browsing by Author "Kozhukharov S."

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Addition of transition elements to the phosphate sealing of cerium oxide protective primer deposited on AA2024-T3 alloy
(2016-01-01) Girginov C.; Avramova I.; Kozhukharov S.
A concise investigation was undertaken to determine the impact of transitional elements on the phosphate sealing of cerium oxide primer layers deposited on AA2024-T3 aircraft alloy. The cerium oxide primer layers were deposited electrochemically on preliminary treated alloy specimens under potentiostatic conditions. The subsequent phosphate sealing layer deposition was performed in equimolar solutions of NaMoO4, NaVO3 or ZrO(NO3)2 in (NH4)2HPO4 medium for 30 min at 90°C. The already coated and sealed specimens were submitted to comparative measurements via electrochemical impedance spectroscopy and linear voltammetry after 168 h of exposure to 3.5% NaCl model corrosive medium. The obtained bi-layer coatings were characterized by systematic analyses using scanning electron microscopy, energy dispersion X-ray spectroscopy and X-ray photoelectron spectroscopy. The latter analysis evinced that cerium present in a cerium phosphate coating was predominantly in Ce(III) state, while doping by Mo, V and Zr caused oxidation to Ce(IV) species. The results obtained reveal successful inclusion of these elements inside the phosphate sealing layer.
Alternative technological approach for synthesis of ceramic pigments by waste materials recycling
(2016-01-01) Doynov M.; Dimitrov T.; Kozhukharov S.
Alternative technological approach is proposed enabling utilization of raw materials from an oil refinery, such as waste guard layers from reactors. Reagent grade and purified MgO, Cr2O3, Fe2O3, and nitric acid (HNO3), were used as additional precursors. The homogeneous mixtures obtained were formed into pellets and sintered at different temperatures. The main phase was proved by X-ray phase analysis (XRD) and compared to ICPDS database. The main phase in the ceramics synthesized was solid solution of spinel MgAl2O4 and magnesiochromite. These minerals are classified as chromspinelide MgCr1.2Al0.4Fe0.4O4 and alumochromite MgCr1.6Al0.4O4. Additional SEM observations, combined with EDX analysis were performed, evincing agglomeration at lower temperatures, followed by agglomerate crumbling, at elevated calcination temperature. The complete transformation of initial precursors into the final ceramic compounds was found to occur at 800°C - 1 h. The ceramic samples synthesized had high density of 1.72-1.93 g/cm3 and large absorption area - 32.93% which is probably due to the high porosity of the sample.
Assessment of Nd2-xSrxNIO4-d as a cathodic material for solid oxide fuel cell applications
(2013-05-21) Simeonov S.; Kozhukharov S.; Grenier J.; Machkova M.; Kozhukharov V.
The features and performance of each device and equipment is directly related to its composing materials. The efficiency and durability of the Solid Oxide Fuel Cells (SOFC) also depend on their composing elements. Other important factors for the SOFC behavior are the working conditions (compositions, and humidity of the fuel and oxidant gaseous mixtures, temperature, pressure, etc.). These facts predetermine the necessity for detailed investigation of the behavior of each composing layer of SOFC. For this reason, the electrical characteristics of Nd2-xSrxNiO4-d as SOFC cathodic material were evaluated by high temperature electrochemical methods, durability tests, and posterior XRD and SEM characterization. As a result, the optimal composition and working conditions for the investigated material in assembled SOFC were determined. The obtained material reveals excellent durability and compatibility with the rest SOFC components.
CHANGE OF THE ANTIMONY ANODIZING KINETICS BY COATING THE ELECTRODE WITH A THIN FILM
(2023-01-01) Lilov E.; Lilova V.; Nedev S.; Kozhukharov S.; Adam A.M.; Girginov C.
The present research work is dedicated on the processes kinetics description of the antimony anodization in potassium phosphate (K3PO4) aqueous solutions. The data have shown that the slope of the kinetic curves is dependent on the phosphate solution concentration. Besides, their mathematic approximation has revealed occurrence of a critical phosphate solution concentration of 0.11 mol dm-3. At this concentration, the slope of the kinetic curve tends to zero. Above this critical concentration, induction periods occur during the anodization. Additional experiments were performed with thin Sb2O3 layers preliminary deposited in vacuum. This approach has enabled to define the impact of such layers on the kinetic behavior during the anodization process. The results have shown that various stages of the process could be skipped. The final data analysis has confirmed the suggestion that the induction periods and voltage variations during the anodization process are caused by the formation of a thin Sb2O3 film, whose dielectric properties change with the film thickness increment.
Characterization of anodized Al 1050 with electrochemically deposited Cu, Ni and Cu/Ni and their behavior in a model corrosive medium
(2021-01-01) Girginov C.; Kozhukharov S.; Tsanev A.; Dishliev A.
The specific benefits of the modified films formed on preliminary anodized aluminum, including the versatility of their potential applications impose the need for evaluation of the exploitation reliability of these films. In this aspect, the dura-bility of Cu and Ni modified anodized aluminum oxide (AAO) films on the low-doped AA1050 alloy was assessed through extended exposure to a 3.5% NaCl model corrosive medium. The electrochemical measurements by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) after 24 and 720 hours of exposure have revealed that the obtained films do not change their obvious barrier properties. In addition, supplemental analyses of the coatings were performed, in order to elucidate the impact of the AC-deposition of Cu and Ni inside the pores. The scanning electron microscopy (SEM) images have shown that the surface topology is not affected and resembles the typical surface of an etched metal. The subsequent energy dispersive X-ray spectroscopy (EDX) tests have revealed a predominance of Cu in the combined AAO-Cu/Ni layers, whereas additional X-ray photoelectron (XPS) analyses showed that both metals form oxides with different oxidation states due to alterations in the deposition conditions, promoted by the application of AC-polarization of the samples.
Characterization of anodized Al 1050 with electrochemically deposited Cu, Ni and Cu/Ni and their behavior in a model corrosive medium
(2021-01-01) Girginov C.; Kozhukharov S.; Tsanev A.; Dishliev A.
The specific benefits of the modified films formed on preliminary anodized aluminum, including the versatility of their potential applications impose the need for evaluation of the exploitation reliability of these films. In this aspect, the dura-bility of Cu and Ni modified anodized aluminum oxide (AAO) films on the low-doped AA1050 alloy was assessed through extended exposure to a 3.5% NaCl model corrosive medium. The electrochemical measurements by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) after 24 and 720 hours of exposure have revealed that the obtained films do not change their obvious barrier properties. In addition, supplemental analyses of the coatings were performed, in order to elucidate the impact of the AC-deposition of Cu and Ni inside the pores. The scanning electron microscopy (SEM) images have shown that the surface topology is not affected and resembles the typical surface of an etched metal. The subsequent energy dispersive X-ray spectroscopy (EDX) tests have revealed a predominance of Cu in the combined AAO-Cu/Ni layers, whereas additional X-ray photoelectron (XPS) analyses showed that both metals form oxides with different oxidation states due to alterations in the deposition conditions, promoted by the application of AC-polarization of the samples.
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.
Efficiency of the Galvanostatic Formation of Anodic Antimony Oxide in Oxalic Acid Solutions
(2022-03-12) Girginov C.; Lilov E.; Kozhukharov S.; Lilova V.
The formation of thick anodic oxide films on antimony in diluted solutions of oxalic acid (CO(OH)2) was studied under galvanostatic and isothermal conditions. The film formation was always accompanied by a dissolution process which strongly depended on the growth conditions. The formation efficiency, as determined by the dissolved metal amount, was affected by the CO(OH)2 concentration, the current density and the anodization time. The dissolved antimony amount increased with higher CO(OH)2 concentrations and anodization time, and with lower current densities. The analysis of the total current density suggests the occurrence of a formed film and a dissolving component. According to the calculations, the anodic oxides composition is close to Sb2O3. The growth of anodic Sb2O3 took place at high electric fields within the oxide film. The thicknesses of the formed films were calculated by taking into account their dissolution. The film formation efficiency was determined at various current densities.
Elaboration of advanced glass-ceramic glaze for anti-slip porcelain stoneware
(2017-01-01) Albert E.B.; Roures I.C.; Chiva D.F.; Núñez J.A.M.; Kozhukharov S.; Castelló J.B.C.
The present communication describes the synthesis of anti-slip enamel exhibiting glass-ceramic nature using new matte frits and raw materials. The glass-ceramic glazes obtained are characterized by various instrumental techniques (X-ray fluorescence (XRF), scanning electron microscopy (SEM), X-ray diffraction (XRD), mechanical profilometry and microhardness measurements) to elucidate the nature of the crystallized phases, their morphology, surface roughness and the finished tile microhardness. The quality of the glazed piece is evaluated by the regulations of chemical resistance, stain-resistance and slipperiness. The enamel obtained devitrifies in crystals of calcium and barium silicoaluminates. It complies with anti-slip and stain-resistance standards, because its surface roughness is similar to that of non-slip enamel.
Electrochemical assessment of the barrier ability of galvanic Zn coating primers electrodeposited on low-carbon steel
(2024-01-01) Girginov C.; Kozhukharov S.; Boshkova N.
Low-carbon steel is highly susceptible to corrosion, necessitating protective coatings capable of efficiently shielding its surface from corrosive environments. This brief research focuses on evaluating the protective capacity of electrochemically deposited Zn coatings, both with and without the addition of benzalacetone. Two independent electrochemical methods, Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Scanning (PDS), were employed after exposing the samples to a 0.01 M NaCl model corrosive medium (MCM) for 24 hours. The results demonstrate the discernible barrier performance of the Zn protective films compared to the bare steel substrates.
Elucidation of the contribution of modified titania films over the performance of thin film humidity sensors
(2013-05-21) Kozhukharov S.; Nenova Z.; Nenov T.; Ivanov S.; Machkova M.
The present research focuses on comparative measurements of the influence of complex additives on the sensitivity of humidity sensors, prepared by the sol-gel metrhod. Three different dopants were added to the primary mixture in order to improve the properties of the films, expressed in water uptake capability, as follows: Bismuth acetate (CH3COO)3Bi, Vanadium-2,4 pentadionate (C15H21O6V), and Sodium tert-butoxide (CH3)3CONa. The properties of the obtained sensors were evaluated by determination of their electrical resistances in a chamber with controlled humidity. As a result, it was experimentally established that the most significant positive effect over both the increase of sensitivity and the measurement range enlargement come from V(III)-2,4 pentadionate with Sodium tert-butoxide.
Enhancement of the cerium oxide primer layers deposited on AA2024-T3 aircraft alloy by preliminary anodization
(2018-01-01) Kozhukharov S.; Girginov C.
The possibility for combination between anodized aluminum oxide (AAO) and cerium oxide primer layer (CeOPL) for elaboration of efficient protective coatings for AA2024-T3 aircraft alloy is proposed in the present research. The combined AAO/CeOPL coating characterizations include electrochemical impedance spectroscopy (EIS) combined with linear voltammetry (LVA), for extended times (until 2520 hours) to a model corrosive medium (3.5 % NaCl). Topographical and cross-sectional (SEM and EDX) observations were performed in order to determine the AAO/CeOPL film thickness and composition. The AAO/CeOPL layer durability tests were confirmed by standard neutral salt spray (NSS). The data analysis from all the used measurement methods has undoubtedly shown that the presence of AAO film significantly improves the cerium oxide primer layer (CeOPL) protective properties and performance.
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.
Humidity Sensing Elements Based on Silica-Graphene Surface Layers
(2024-01-01) Nedev N.; Nenova Z.; Kozhukharov S.; Ivanov S.; Nenov T.
The paper presents humidity sensing elements based on silica-graphene surface layers, prepared via a sol-gel method. The samples were sintered at temperatures of 400 °C, 600 °C and 800 °C. Tetraethoxysilane was used as a precursor of SiO2 and as a dopant - graphene aggregates of nanoplatelets. A description of the sample preparation procedure is provided. Using a precise impedance analyzer and a calibrator for humidity and temperature, the changes in their resistance R were investigated with variations in relative humidity in the range of 30% to 90% at a temperature of 25 °C and at a frequency of 20 Hz. Morphological observations and map data analyses were carried out by Scanning Electron Microscopy (SEM) and Energy Dispersion Spectroscopy (EDX). The developed sensor elements have a good sensitivity to the humidity, and the resistance change reaches two orders of magnitude. Additions of graphene improve the sensitivity of the elements. The characteristics of the sensing elements at different sintering temperatures and different amounts of graphene dopant were modeled using an artificial neural network.
Impact of the final thermal sealing of combined zinc/cerium oxide protective coating primers formed on low carbon steel
(2022-01-01) Kozhukharov S.; Girginov C.; Boshkova N.; Tzanev A.
The final sealing possesses a proven beneficial effect on the protective properties of anodic oxide films on aluminum. In this sense, the present research is devoted to the evaluation of the impact of this procedure on the barrier ability of combined Zn/Ce oxide layers deposited on low carbon steel samples. For this purpose, four samples were submitted to galvanic zinc deposition, followed by spontaneous formation of cerium oxide primer layer (CeOPL). Afterwards, two of the samples underwent thermal sealing in boiling water in order to enhance their barrier ability. Its evaluation was performed by two electrochemical methods: electrochemical impedance spectroscopy (EIS) and potentiodynamic scanning (PDS) after 24 hours of exposure to a diluted model corrosive medium (MCM). Other instrumental methods were used in order to describe the effect of this final procedure on the color characteristics and hydrophobicity of the films. The results were collected from multiple tests, followed by statistical data treatment. In addition, the surfaces of the obtained films were submitted to direct observation by scanning electron microscopy (SEM), coupled with energy dispersion X-ray (EDX). Their composition was determined by means of X-ray Photoelectron Spectroscopy (XPS). The acquired data have revealed a detrimental effect of the final sealing in boiling water. It was expressed by the loss of the barrier properties of the Zn/CeOPL films, combined with additional decolorization and hydrophilization. Finally, the mechanism of this detrimental effect was determined by further SEM, EDX and XPS analyses.
Influence of ce(iii)/ce(iv)-supplements on the characteristics of humidity sensors with tio2 films prepared via a sol-gel method
(2013-03-01) Kozhukharov S.; Nenova Z.; Nenov T.; Machkova M.; Kozhukharov V.
Humidity sensors have been prepared via a sol-gel method to deposit TiO2 films with additions of Ce-compounds on alumina substrates, with interdigitated silver palladium electrodes. Observations by scanning electron microscopy (SEM) were performed in order to determine the surface morphology of the respective layers. Structural and compositional characterization was done by X-ray diffraction analysis (XRD), and energy dispersive X-ray spectroscopy (EDX) for investigation of the relation between the film structures and the parameters of the respective sensors. The influence of Ce-compounds on the electrical characteristics of the samples as humidity sensing elements has been evaluated by an impedance analyzer.
INFLUENCE OF THE FINAL THERMAL SEALING ON THE PROPERTIES OF COMBINED ANODIC ALUMINA/CERIUM CONVERSION COATINGS ON AA2024-T3 AIRCRAFT ALLOY
(2023-01-01) Girginov C.; Kozhukharov S.; Tsanev A.; Georgieva M.; Petrova M.; Lilov E.; Petkov P.
The current research presents results following corrosion tests, performed on combined anodic alumina/cerium conversion coatings (Al2O3/CeCC), thermally sealed for 15 min at 100°C either in boiling water or in hot-air medium. The coatings were formed on AA2024-T3 aircraft alloy substrates, at the optimum conditions of anodization and cerium conversion coating deposition, described in previous works. Prior to the corrosion tests, both the color characteristics and wetting ability of the combined films were evaluated. The corrosion protective properties of the sealed films were assessed by means of Electrochemical Impedance Spectroscopy (EIS) and Potentiodynamic Polarization (PDP). The measurements were performed after 672 hours of exposure to a 5 % NaCl model corrosive medium (MCM). Additional long term (up to 1344 hours of exposure) durability tests were then performed under the same conditions for the samples and superior performance was established. The electrochemical measurements were conducted regularly, once a week. Low and high-resolution Scanning Electron Microscopy (SEM) studies were performed on selected samples. The experimental results have shown that the sealing procedure in an aqueous environment enhances the corrosion protective ability and durability of the coating, probably due to formation of a hydrate layers, that suppresses the access of corrosive species to the surface of the substrate. This inference was additionally confirmed by the subsequent chemical analysis by means of X-Ray Photoelectron Spectroscopy (XPS). The results acquired have also shown that the incorporated Ce is predominantly in the form of Ce(IV)-oxides/hydroxides and after the thermal treatment almost entirely consists of cerium.
MODEL FOR PHOTODEGRADATION WITH A MODIFIED RATE CONSTANT. PART 1: USE OF ANODICALLY-PREPARED THIN ZnO FILMS FOR METHYL ORANGE PHOTODEGRADATION
(2022-01-01) Lilov E.; Lilova V.; Nedev S.; Kozhukharov S.; Girginov C.; Yancheva D.
Four sets of samples of anodically prepared ZnO thin films were tested as potential photocatalysts for degradation of methyl orange (MO). The results show that the best properties belong to the samples prepared by anodization in oxalic acid and subsequently annealed 3 hours at 341°C. The dependences of the rate of the reaction on the time of use of the photocatalyst, the temperature of the decomposition reaction, the pH of the solution and the illumination were investigated too. The activation energies of the reactions were derived from the dependence of the rate of the reaction on the temperature. The properties of the acquired ZnO anodic films, as well as their compositions and structures were investigated by various analytical techniques, as follows: Attenuated Total Reflection Fourier Transformation Infrared Spectroscopy (ATR-FTIR), X-ray Diffractometry (XRD), Atomic Force Microscopy (AFM), Inductively Coupled Plasma - Optical Emission Spectroscopy (ICP-OES), Scanning Electron Microscopy (SEM) and UV-VIS Spectrophotometry.
MODEL FOR PHOTODEGRADATION WITH A MODIFIED RATE CONSTANT. PART 2: МODEL DESCRIPTION
(2022-01-01) Lilov E.; Lilova V.; Nedev S.; Kozhukharov S.; Girginov C.
The dependence of the photodecomposition rate on the initial concentration of methyl orange (MO) as model pollutant was investigated. On the basis of relatively simple calculations, it was concluded that the role of the thermally exited charge carriers could not be neglected in case of high pollutant concentrations. It was found that the correlation between the reaction rate and the methyl orange concentration possesses a complicated N-shaped form. A model was proposed, on the basis of two reactions obeying the Langmuir-Hinshelwood mechanism. It describes the dependence of the reaction rate on the model pollutant concentration, on the basis of an equation, which comprises three parameters. Here, two of them were estimated by two, different ways and show excellent match, confirming that the model describes the dependence correctly.
Optical and color characteristics of porous alumina with electrochemically incorporated silver
(2018-01-01) Kiradzhiyska D.; Mantcheva R.; Girginov C.; Kozhukharov S.
Following the ever increasing interest towards the combination between aluminum anodization and incorporation of relatively affordable metals, a brief study was performed on the impact of the duration of AC-electrochemical incorporation of silver on the resulting optical properties and color characteristics. The experiments performed on technically pure aluminum included: anodization in galvanostatic regime; AC-electrochemical silver incorporation, characterization of the optical properties by light reflectance and refractive colorimetric measurements. This systematic study enabled to determine some correlation between the coloring process duration and the resulting optical and color features.

Browsing by Author "Kozhukharov S."

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