Browsing by Author "Petkov P."
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Item Batio3 Structure as a Function of the Preparation Method(2020-01-01) Nicheva D.; Harizanova R.; Ilcheva V.; Mihailova I.; Petkova T.; Petkov P.Barium titanate is synthesized by sol-gel and hydrothermal methods. The sol-gel technique allows the preparation of amorphous powders which are subsequently subjected to appropriate time-temperature programs and barium titanate is precipitated. The allotropic modification of the barium titanate from the obtained sol-gel powder depends on the temperature applied. It is cubic for heat treatment at 900 °C for 4 h, while in case of crystallization at 1100 °C for 3 h the tetragonal modification occurs as witnessed by X-ray diffraction analyses. Barium titanate is also successfully prepared by the hydrothermal method which results in the crystallization of the cubic allotrope as observed by X-ray diffraction. The microstructure of the formed barium titanate powders is imaged by scanning electron microscopy and reveals the formation of large fraction of crystals which are polydispersed for the samples prepared via sol-gel method and tend to agglomerate in case of hydrothermal synthesis. The infrared spectroscopy investigation of the powders obtained by both synthesis methods shows the presence of the absorption peak characteristic for the barium titanate phase in the range 540–580 cm−1.Item Compositional dependence of the optical properties of silver containing As2Se3 thin films(2012-01-01) Ilcheva V.; Petkov P.; Petkova T.; Boev V.; Monchev B.We obtained amorphous thin films by evaporation and condensation in three-component systems based on As, Se and Ag. The aim was to investigate the influence of the third component on the thin film structure and optical properties. The refractive index and the film thickness were determined from the upper and lower envelopes of the optical transmission spectra measured in the spectral range 400 - 2500 nm. The absorption coefficient (α) was determined after extrapolating the values of n in the region of strong absorption (where α ≥ 104 cm-1); its spectral distribution is discussed. The dispersion of the refractive index was analyzed in terms of the single-oscillator Wemple-DiDomenico model. The shift of the optical absorption edge was described using the non-direct transition model proposed by Tauc. The optical band gap was calculated from the Tauc plot ahv B(Egopt - hv)2; the compositional dependence of the optical gap is also discussed. © Published under licence by IOP Publishing Ltd.Item EXPLORING THE STRUCTURAL AND ELECTRONIC CHARACTERISTICS OF AMORPHOUS Ge – Te - In MATERIAL THROUGH AB INITIO METHODS(2024-01-01) Zaidan A.; Ivanova V.; Petkov P.; Bancheva-Koleva P.This study employs density functional theory (DFT) and molecular dynamics to meticulously investigate the structural and electronic properties of ternary chalcogenide compounds, specifically (GeTe4)1-x Inx, and (GeTe5)1-x Inx across a range of compositions ( x = 0, 5, 10, 15, 20 at %). Utilizing the local density approximation within the framework of first-principles calculations, we comprehensively analyze the pair correlation function, static structural factor, electronic density of states, and electronic band gap energy. Our results reveal a notable decrease in the energy band gap of Germanium-Tellurium with the incorporation of Indium atoms. The structural changes observed in the Ge-Te matrix with Indium doping, as evidenced by the changes in the pair correlation function and static structure factor, are consistent with and supportive of the observed decrease in the band gap energy. This phenomenon is primarily attributed to the significant contribution of Indium atoms to the conduction band edge, offering new insights into the material’s electronic behaviour.Item 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.Item Influence of the preparation method on the As-Se-AgI thin films behaviour(2008-05-01) Hineva T.; Petkova T.; Petkov P.; Mikli V.; Socol G.; Mihailescu C.; Mihailescu I.Bulk glasses of the (As2Se3)1-x (AgI) x and (AsSe)1-x(AgI)x systems, where x 5, 10, 15 up to 35 mol.% have been prepared by the melt-quenched technique. The thin films have been deposited by means of vacuum thermal evaporation (VTE) and pulsed laser deposition (PLD). The XRD investigation reveals a generally amorphous structure; small peaks are only observed in the samples with the highest AgI. The film compositions have been determined by EDS (energy dispersive X-ray microanalysis). WDS (wavelength dispersive spectroscopy) studies have shown that the films do not contain oxygen within the accuracy of the method (1 %). The films are dense with smooth surface as revealed by using scanning electron microscopy (SEM) and atomic force microscopy (AFM). © 2008 IOP Publishing Ltd.Item Mechanical behaviour of thin As-S-AgI films for sensor applications(2008-05-01) Kolev K.; Petkova T.; Popov C.; Petkov P.The mechanical behaviour of chalcogenide thin films is of great importance for a variety of applications. The requirements for homogeneity, uniformity, surface smoothness, excellent adhesive strength and minimal residual stress are decisive for the technological performance of thin films in the field of optical and sensor devices. Thin films from the system have been prepared by vacuum thermal evaporation from the corresponding bulk glasses. The film structure and surface morphology have been studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The investigated chalcogenide films are amorphous as revealed by XRD, featureless and with smooth surfaces viewed from the AFM pictures. The stress measurements of the thin films deposited on silicon cantilevers have been carried out by cantilever bending technique. The results obtained have been discussed with respect to the film composition and structure. The change of the stress from tensile to compressive registered in the films with AgI content higher than 15 mol. % could be explained with structural transformations. © 2008 IOP Publishing Ltd.Item Molybdenum-Doped ZnO Thin Films Obtained by Spray Pyrolysis(2024-05-01) Bancheva-Koleva P.; Zhelev V.; Petkov P.; Petkova T.A batch of ZnO thin films, pure and doped with molybdenum (up to 2 mol %), were prepared using the spray pyrolysis technique on glass and silicon substrates. The effect of molybdenum concentration on the morphology, structure and optical properties of the films was investigated. X-ray diffraction (XRD) results show a wurtzite polycrystalline crystal structure. The average crystallite size increases from 30 to 80 nm with increasing molybdenum content. Scanning electron microscopy (SEM) images demonstrate a smooth and homogeneous surface with densely spaced nanocrystalline grains. The number of nuclei increases, growing over the entire surface of the substrate with uniform grains, when the molybdenum concentration is increased to 2 mol %. The estimated root mean square (RMS) roughness values for the undoped and doped with 1 mol % and 2 mol % of ZnO thin films, defined by atomic force microscopy (AFM), are 6.12, 23.54 and 23.83 nm, respectively. The increase in Mo concentration contributes to the increase in film transmittance.Item Optical behavior of (GeS1.5)1-x(AgI)x glasses(2013-01-01) Monchev B.; Petkova T.; Petkov P.; Ilcheva V.In this research we report deposition of thin films from the already synthesized chalcohalide (GeS1.5)1-x(AgI)x glasses, where x=5, 10, 20 mol.%. The bulk samples used and the corresponding layers are checked by XRD analysis which reveals only a diluted halo on the respective diffractograms. Some basic optical parameters of the investigated Ge-SAgI thin films are estimated and their relation with materials' composition is determined. The influence of the third component on the optical absorption is discussed. The optical energy gap (Eg) is determined from the Tauc plot h =B(EgTauc - h )2 and Eg 04 is found from the relationship a=f(hv ). The Eg values calculated by both methods reveal slight increase with silver iodide addition probably due to structural changes.© 2013 The Authors. Published by Elsevier B.V.Item Optical properties of chalcogenide Ge-Te-In thin films(2012-01-01) Zaidan A.; Ivanova V.; Petkov P.Thin films of the chalcogenide (GeTe4)1-xIn x with various compositions (x 0, 5, 10, 15, 20 at %) were deposited under vacuum on glass substrates by thermal evaporation. The optical transmission and reflection spectra of the films at normal incidence were investigated in the spectral range from 800 to 2600 nm. Using the transmission spectra, the optical constants (refractive index (n) and extinction coefficient (k)) were calculated based on Swanepoel's method. The optical band gap (E gopt) was also estimated using Tauc's extrapolation procedure. © Published under licence by IOP Publishing Ltd.Item Optical properties of thermally evaporated (As2Se 3)100-xAgx thin films(2013-01-01) Ilcheva V.; Petkov P.; Boev V.; Petkova T.We obtained amorphous thin films by evaporation and condensation in three component systems based on As, Se and Ag. The aim was to investigate the influence of the third component on the thin film structure and optical properties. The refractive index and the film thickness were determined from the upper and lower envelopes of the optical transmission spectra, measured in the spectral range 400 - 2500 nm. The absorption coefficient (α) was determined after extrapolation the values of n in the region of strong absorption (whereα ≥104 cm-1) and its spectral distribution was discussed. The dispersion of the refractive index was analyzed in terms of single-oscillator Wemple-DiDomenico model. The shift of the optical absorption edge was described using the non-direct transition model, proposed by Tauc. Optical band gap was calculated from the Tauc plot αhν =B(EgTauc - hν )2 and the compositional dependence of the optical gap was discussed. In order to realize photoinduced changes in the films, they were illuminated by He-Ne laser and their transmission spectra were measured before and after illumination. The shift of the absorption edge of as - deposited and illuminated films was investigated and discussed as a function of the films composition. © 2013 The Authors.Item Photoinduced changes in amorphous gallium doped GeTe4 chalcogenides(2008-05-01) Ilchev P.; Petkov P.; Sharlandjiev P.; Koserkova A.; Ilcheva V.Chalcogenide thin films from the system Ge-Te-Ga were deposited by thermal vacuum evaporation on glass substrates from initial bulk synthesized materials. Optical measurements on as-deposited and irradiated thin films were performed. The optical constants - refractive index (n) and extinction coefficient (k) were evaluated from the transmission and reflectance spectra. Irradiation of the films for 3 hours through an interference filter with pass-band centered at 1,06 μm led to changes in the transmission spectra of the samples. A shift of the absorption edge was observed depending on the gallium content. The most significant change in the absorption edge was obtained in films with composition (GeTe4)95Ga5 with estimated coordination number 2.43. According to the covalent network model in glasses with coordination number close to 2.43, a floppy-rigid transition occurs. The spectral distribution of the refractive index change showed a sign inversion - from negative to positive. Changes in the physical thickness of the samples investigated due to the irradiation were not established. © 2008 IOP Publishing Ltd.Item Photoinduced changes in As-Se-Ag amorphous films(2008-05-01) Ilcheva V.; Boev V.; Roussev D.; Petkov P.; Petkova T.; Sharlandjiev P.; Nasarova D.Thin films from the As-Se-Ag system have been prepared on glass and silicon substrates by thermal vacuum evaporation from previously fabricated bulk glassy samples. The amorphous state of the samples has been proved by X-ray diffraction. Some optical properties of the amorphous As-Se-Ag thin films have been studied in relation to the Ag concentration in the sample. In order to investigate the photoinduced changes due to irradiation by with He-Ne laser light, transmission spectra of the thin films have been measured before and after irradiation. The optical characteristics as a function of the composition have been studied. © 2008 IOP Publishing Ltd.Item Physical Vapor Deposition of Indium-Doped GeTe: Analyzing the Evaporation Process and Kinetics(2024-08-01) Zaidan A.; Ivanova V.; Petkov P.Chalcogenide glasses have broad applications in the mid-infrared optoelectronics field and as phase-change materials (PCMs) due to their unique properties. Chalcogenide glasses can have crystalline and amorphous phases, making them suitable as PCMs for reversible optical or electrical recording. This study provides an in-depth analysis of the evaporation kinetics of indium-doped chalcogenides, GeTe4 and GeTe5, using the physical vapor deposition technique on glass substrates. Our approach involved a detailed examination of the evaporation process under controlled temperature conditions, allowing precise measurement of rate changes and energy dynamics. This study revealed a significant and exponential increase in the evaporation rate of GeTe4 and GeTe5 with the introduction of indium, which was particularly noticeable at higher temperatures. This increase in evaporation rate with indium doping suggests a more complex interplay of materials at the molecular level than previously understood. Furthermore, our findings indicate that the addition of indium affects the evaporation rate and elevates the energy requirements for the evaporation process, providing new insights into the thermal dynamics of these materials. This study’s outcomes contribute significantly to understanding deposition processes, paving the way for optimized manufacturing techniques that could lead to more efficient and higher-performing optoelectronic devices and memory storage solutions.Item Preface(2018-01-01) Petkov P.; Tsiulyanu D.; Popov C.; Kulisch W.Item Preface(2018-01-01) Petkov P.; Tsiulyanu D.; Popov C.; Kulisch W.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 Structure of GeSe4-In and GeSe5-In glasses(2010-10-13) Kaban I.; Jóvári P.; Petkova T.; Petkov P.; Stoilova A.; Hoyer W.; Beuneu B.(Ge0.2Se0.8)100-xInx and (Ge0.17Se0.83)100-xInx (x = 0, 5, 10, 15 at.%) chalcogenide glasses have been studied with high-energy x-ray diffraction, neutron diffraction and extended x-ray absorption spectroscopy at Ge, Se and In K-edges. The experimental data were modelled simultaneously with the reverse Monte Carlo simulation method. GeSe4/2 tetrahedra are shown to be the main structural units in the binary and ternary glasses investigated. Indium bonds to the excess Se atoms in the ternary Ge-Se-In glasses. While the majority of In atoms have three Se neighbours, some In atoms may be tetrahedrally coordinated by Se. © 2010 IOP Publishing Ltd.Item STRUCTURE, THERMAL, AND PHYSIC-CHEMICAL PROPERTIES OF SOME CHALCOGENIDE ALLOYS(2023-01-01) Adam A.M.; Abdel-Rehim W.M.F.; Petkov P.; El-Qahtani Z.M.H.; Alqannas H.S.; Alruqi A.B.; Hakamy A.; Ataalla M.Bulk products of crystalline Bi2Se3-xTex alloys (x =0.0, 0.1, 0.3, 0.5) were pre-pared using simple melting synthesis. Crystalline features, microstructure, and surface morphologies of the synthesized samples were examined via X-ray dif-fraction, scanning electron microscope, and energy dispersive X-ray spectrome-ter. Elemental distribution was studied by energy dispersive analysis of X-ray spectroscopy. Polycrystalline of rhombohedral crystal structure was observed for the concerned samples. Perfect crystallinity and micro-scalability of the prepared were also reflected by the physic-chemical properties of each sample. Thermal behavior was studied throughout differential scanning calorimetry and thermo-gravimetric analysis showing that the samples are of high stability over high temperature range. Physic-chemical properties were determined in terms of ex-perimental density. These properties were compactness value, molar volume and the percentage of free volume. Density of Bi2Se3 alloy was obtained at 7.37 gm/cm3. The Te doping enhanced the density of the Bi2Se3-xTex system. The most Te doped alloy showed density of 9.018 gm/cm3. All other physic-chemical prop-erties showed strong dependence on the Tea amounts in the system.