Browsing by Author "Lilov E."

Now showing 1 - 6 of 6
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    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.
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    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.
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    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.
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    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.
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    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.
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    Solution conductivity as a factor determining the photocatalytic reaction rate
    (2024-06-01) Lilov E.; Nedev S.; Lilova V.; Kozhukharov S.; Girginov C.
    A rather simple model was developed, which predicted a hyperbolic relation between the photocatalytic reaction rate and solution conductivity. Experimental validation was performed using methyl orange as a model pollutant and titanium dioxide as a photocatalyst. The experiments, conducted with three different salts confirm the hyperbolic correlation between the photocatalytic degradation rate and solution conductivity. The results have evinced this correlation, albeit the model ignores crucial factors, such as the role of one of the two types of charges generated during light absorption and the chemical decomposition pathway. In addition, the experimentally derived hyperbolic function displays a positive shift along the ordinate.