Browsing by Author "Lilova V."
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Item 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.Item Effect of InP/ZnS quantum dots aggregation on the kinetics of birefringence recorded in thin azopolymer composite films(2024-09-01) Stoilova A.; Dimov D.; Trifonova Y.; Mateev G.; Lilova V.; Nazarova D.; Nedelchev L.The article describes the preparation of new thin film composite materials based on the azopolymer (poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt]) doped with three different concentration of InP/ZnS quantum dots with size of 7.5 nm. Clusters of aggregated InP/ZnS were microscopically observed in the fabricated composite samples. Birefringence (Δn) was induced in the films at two different wavelengths of the pump laser (355 nm and 444 nm) and the obtained higher values of Δnmax for the samples doped with quantum dots up to 2 wt% in comparison to the non-doped film and the sample with the highest nanocrystals concentration are discussed, based on the measured fluorescence spectra, in terms of possible local energy transfer between the azobenzene chromophores and the InP/ZnS quantum dots.Item 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.Item ENHANCING SURFACE CHARACTERISTICS OF AA2024-T3 AIRCRAFT ALLOY THROUGH SYNERGISTIC ANODIZATION AND CERIUM CONVERSION COATING. PART I: PERFORMANCE IN MODEL CORROSIVE MEDIUM(2024-01-01) Portolesi S.; Girginov C.; Kozhukharov S.; Lilova V.; Petkov P.The present study is devoted to the monitoring of the performance of AA2024-T3 specimens, after the formation of Anodic Aluminum Oxide (AAO) layer and/or deposition of a Ceruim Conversion Coating (CeCC), obtained at 20°C and 50°C, respectively. Although both methods are well described in the literature, there is no sufficient information regarding the effect of their combination on their behaviour in corrosive media. The performance of the respective specimens was elucidated after 24 h of exposure to 3.5 % NaCl model corrosive medium (MCM) by means of Electrochemical Impedance Spectroscopy (EIS) and acquisition of Potentiodynamic Scanning (PDS) curves. Since the combined AAO/CeCC coating primers revealed superior barrier properties, the respective specimens were subjected to long-term corrosion tests of up to 672 h of exposure to the MCM to evaluate their durability. The results revealed the synergistic effect of the combination of the surface treatment procedures on effective corrosion mitigation.Item 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.Item 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.Item OPTICAL PROPERTIES OF ELECTROSPRAY DEPOSITED PAZO POLYMER FILMS DOPED WITH GeTe4-Cu CHALCOGENIDE PARTICLES(2022-01-01) Stoilova A.; Lilova V.; Ivanova V.; Trifonova Y.; Dimov D.Composite films based on the azopolymer poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt] doped with GeTe4-Cu chalcogenide particles have been prepared through electrospray deposition. Polarization microscopy was applied to study the films morphology. The transmittance coefficient (T), the reflectance coefficient (R), the refractive index (n), the extinction coefficient (k), the optical absorption coefficient (α) and the optical band gap (Formula presented) of the composite films and of the pure azo polymer film were determined. A significant change in the optical properties of the composite films in comparison to the non-doped azo polymer film has been observed.Item Optical properties of novel azo multi-functional dye thin films(2019-01-01) Stoilova A.; Trifonova Y.; Lilova V.By evaluation the practical applicability of new synthesized compounds, starting point represents the thin film optical properties analysis, which provides rich information about chemical bonding, layer thickness, electronic structure, etc. Here, we present optical properties determination of thin nanocomposite films of three novel N-phthalimide azoazomethine dyes containing OH-group at ortho-position in respect to the -CH=N- chromophore group and designed with different electron donating and electron withdrawing substituents. The structure of these previously synthesized by us dyes is shown below. With the aim to explore potential practical application of these novel azo multi-functional dyes, we prepared thin nanocomposite films via vacuum thermal evaporation and low temperature spray technique, and determined the transmittance coefficient (T), the reflectance coefficient (R), the refractive index (n), the extinction coefficient (k), the optical absorption coefficient (a) and the optical band gap (Eg) of the fabricated thin films in the spectral range between 340 and 800 nm. The thin films prepared through vacuum thermal evaporation show higher transmittance, higher reflectance, higher refractive index (n=1.5÷2.1) and higher values of the optical band gap (Eg=2.93÷3.53, eV). The effect of the substitutions and of the fabrication method on the optical performance of the nanocomposite thin films has been discussed.Item OPTICAL PROPERTIES OF PAZO POLYMER COMPOSITE FILMS DOPED WITH PARTICLES OF A NOVEL COPPER HYDANTOIN COMPLEX(2021-01-01) Lilova V.; Trifonova Y.; Stoilova A.; Georgieva S.; Todorov P.In this work the transmittance coefficient (T), the reflectance coefficient (R), the refractive index (n), the extinction coefficient (k), the optical absorption coefficient (a) and the optical band gap (Eof thin (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]), shortly PAZO, azopolymer films with embedded particles of Cu(II) 3-amino-5,5‘-dimethylhydantoin at three different concentrations have been determined. The influence of the dopants concentration on the optical performance of the composite films has been discussed.Item Photoinduced birefringence in pazo polymer nanocomposite films with embedded particles of biologically active metal complexes(2019-01-01) Mateev G.; Stoilova A.; Nazarova D.; Nedelchev L.; Todorov P.; Georgieva S.; Trifonova Y.; Lilova V.The composite materials based on azo polymers doped with metal particles attract considerable attention due to their advantageous electrical and optical properties. The present communication reports results from measuring the photoinduced birefringence in composite films of the azopolymer (poly[1-[4-(3-carboxy-4-hydroxyphenylazo) benzenesulfonamido]-1,2-ethanediyl, sodium salt]), shortly PAZO, doped with metal complexes of Cu(II) 3-amino-5,5'-dimethylhydantoin (CLP) and Ni(II) 3-amino-5,5'-dimethylhydantoin (NLP) of different concentrations. The thin film materials are prepared via spin coating. The thickness of the samples is in the range between 500 nm and 800 nm. The photoinduced birefringence is measured by a classical polarimetric setup, where a vertically polarized He-Cd laser emitting at ? = 442 nm is used as a pump laser, while a DPSS laser emitting at = 635 nm is used as a probe laser. The influence of the dopants concentration on the maximal induced birefringence and the time of response are discussed.Item Physico-chemical characterization of new tellurium based chalcogenide materials(2018-01-01) Trifonova Y.; Ivanova V.; Lilova V.; Vasileva D.; Boteva T.Bulk samples of Ge-Te-Cu system are synthesized by melt-quenched technique into a water/ice mixture. The synthesis is carried out at a constant heating rate of 5.10-2 K s-1 reaching the final temperature of 1373 K. Some physico-chemical properties of the synthesized bulk samples are defined. The density of the latter undergoes various changes with Cu content increase most likely due to the gradual substitution of tellurium atoms with copper leading to structural changes in the samples. The dependencies of the bulk samples compactness, molar volume and free volume on their composition follow that of the density. A change in the structure of GeTe4-Cu leading to extreme physico-chemical properties is observed in accordance with the theory of Thorphe for transitions in the structure of covalent chalcogenide glasses at an average coordination number 2,4. It proceeds at Zglass = 2,46 corresponding to a content of 10 mol % of Cu.Item Se-BASED CHALCOGENIDE GLASSES AS HOLOGRAPHIC MEDIA(2020-01-01) Trifonova Y.; Stoilova A.; Ivanova V.; Lilova V.; Petkov P.The present paper reports on the effect of the addition of IIIA metal (Ga or In) to Se-based chalcogenide thin films on the diffraction efficiency. The holographic scalar recording is accomplished using standard interferometric configuration with argon laser with a wavelength of 448 nm, spatial frequency of 2,2.10s m−1 and diameter of the spot 2 mm. The diffraction efficiency relation to the glassy system composition and the time of illumination at constant intensity of a laser beam is investigated. The results are interpreted in terms of the electronic structure in disordered systems.Item 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.Item Structural investigation of tellurium based thin films(2018-01-01) Ivanova V.; Trifonova Y.; Lilova V.; Mikli V.; Stoyanova-Ivanova A.Tellurium based materials possess a low temperature phase change transition. Sometimes they cannot be obtained in an amorphous state under normal conditions. Thus they can be used as an active element in phase change memory devices. Our study is dedicated to the synthesis of tellurium based materials using the systems (GeTe3)100-xInx, x = 5 mol %, 10 mol %, 15 mol % and 20 mol % and (GeTe4)100-xInx, x = 5 mol %, 10 mol %, 15 mol % and 20 mol %, the deposition of thin films on their ground and the investigation of their morphology, topology and structure aiming to obtain new knowledge for evaluation of the possibility for application of these materials in phase change memory devices.