Browsing by Author "Ilcheva V."
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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 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 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.