Browsing by Author "Vassilev V."
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Item Compositional dependence of the optical properties of vacuum evaporated thin GeSe2-GeTe-PbTe films(2013-01-01) Petkov K.; Tasseva J.; Vassilev V.; Aljihmani L.Chalcogenide glasses from the GeSe2-GeTe-PbTe system were synthesized by taking preliminary prepared GeSe2, GeTe and PbTe in their molecular percentages and melting them in an evacuated quartz ampoule. Thin films were deposited on optical glass substrates BK-7 and Si-wafer substrates by thermal evaporation. Using X-ray microanalysis it was found that the film composition differs to a certain degree from the bulk composition. The changes in the optical properties of thin GeSe2-GeTe-PbTe films were studied as a function of their composition, exposure to light and annealing. It is shown by XRD analysis that the thin films are amorphous up to layers with composition of Ge27Se51Te14Pb8. The optical constants (refractive index, n and absorption coefficient, ), the thickness, d as well as the optical band gap, Eg, depending on the film composition were determined by spectrophotometric measurements in the region 450-2500 nm applying the Swanepoel's envelop method and Tauc's procedure. With the increase of the Te content in the layers, the absorption edge is shifted to the higher wavelengths, the refractive index increases from 2.54 for Ge 31Se66Te3 up to 3.37 for Ge32Se 55Te13 while the optical band gap decreases from 1.86 eV for to 1.30 eV for the same compositions. Similar dependence was observed with the increase of the PbTe content in the layers. After annealing of thin films, the values of n decrease, the optical band gap increases and a shift of the absorption edge to the shorter wavelengths is observed. © 2013 The Authors. Published by Elsevier B.V.Item Dry etching of thin chalcogenide films(2010-01-01) Petkov K.; Vassilev G.; Vassilev V.Fluorocarbon plasmas (pure and mixtures with Ar) were used to investigate the changes in the etching rate depending on the chalcogenide glasses composition and light exposure. The experiments were performed on modified commercial HZM-4 vacuum equipment in a diode electrode configuration. The surface microstructure of thin chalcogenide layers and its change after etching in CCl2F2 and CF4 plasmas were studied by SEM. The dependence of the composition of As-S-Ge, As-Se and multicomponent Ge-Se-Sb-Ag-I layers on the etching rate was discussed. The selective etching of some glasses observed after light exposure opens opportunities for deep structure processing applications. © 2010 IOP Publishing Ltd.Item Electrical conductivity, photoconductivity and gas sensitivity of Ge-Se-Te thin films(2012-01-01) Nesheva D.; Levi Z.; Raptis Y.S.; Raptis C.; Petkov K.; Vassilev V.Films of GexSeyTez (x≈33) with a thickness of 0.8 μm and four different compositions were prepared by thermal evaporation of preliminary synthesized glasses. The glass and film compositions were determined by Energy-Dispersive X-ray Spectroscopy. X-ray diffraction measurements have confirmed the amorphous structure of both bulk and thin film samples. The temperature dependence of the dark conductivity has been measured in the range 20 - 110 °C. No significant change of the room temperature conductivity (around 5x10-7 S/cm) and the dark current activation energy (around 0.7 eV) have been observed with increasing z. Spectral photocurrent measurements have revealed that the increase of Te content results in: (i) a photoconductivity decrease and (ii) a red shift of the low-energy side of the spectrum which indicates an optical band gap decrease. Resistive chemical sensing tests carried out at room temperature with ammonia, acetone, ethanol or water vapours in air as carrying gas have shown that the Ge32Se 55Te13 films are sensitive to ammonia but not to water vapour and hence they are suitable for selective ammonia sensors operating at room temperature. The observed sensitivity is related to the specific lattice structure of the region nearest to the film surface.Item Glass formation in the As2Se3-As2Te 3-Sb2Te3 system(2008-01-01) Hristova-Vasileva T.; Vassilev V.; Aljihmani L.; Boycheva S.Chalcogenide glasses from the As2Se3-As 2Te3-Sb2Te3 system were synthesized for the first time. The glass-forming region was determined by X-ray diffraction and electron microscopic analyses. The basic physicochemical parameters such as density (d), microhardness (HV) and temperatures of phase transformations (glass transition Tg, crystallization Tcr and melting Tm) were measured. Compactness and some thermomechanical characteristics such as volume (Vh) and formation energy (Eh) of micro-voids in the glassy network as well as the elasticity module (E) were calculated. The glassforming ability was evaluated according to Hruby's criteria (K G). The correlation between composition and properties of the (As2Se3)x(As2Te3) y(Sb2Te3)z glasses was established and comprehensively discussed. © 2008 Elsevier Ltd.Item Industrial waste as a source for fabrication of composite ceramics-glass with a controlled porosity(2008-12-01) Adziski R.; Fidancevska E.; Anguseva B.; Milosevski D.; Vassilev V.; Milosevski M.Metallurgical slag with granulation (-0.125+0.063mm) and 20 wt% waste TV glass were used for obtaining a glass ceramic composite with a controlled porosity. This material obtained by sintering at 950°C/2h possessed thermal stability, integral porosity of 43.6% and E-modulus and bending strength of 12 GPa and 39 MPa, respectively. The composite was characterized with a permeability of 0.47 Da and generation of air bubbles with size of 1-4 mm in a water medium.Item New chalcogenide glasses in the Ag2Te-As2Se3-CdTe system(2008-07-01) Vassilev V.; Karadashka I.; Parvanov S.Chalcogenide glasses from the Ag2Te-As2Se3-CdTe system were synthesized. The basic physicochemical parameters such as density (d), microhardness (HV) and the temperatures of phase transformations (the glass transition Tg, crystallization Tcr and melting Tm) were measured. Compactness and some thermomechanical characteristics such as volume (Vh) and formation energy (Eh) of micro-voids in the glassy network, as well as the module of elasticity E were calculated. The overall mean bond energy 〈E〉, the mean coordination number Z, the mean bond energy of the average cross-linking/atom and the average bond energy per atom of the ``remaining matrix``-over(E, -)c and over(E, -)rm, as well as the average heteropolar bond energy Ehb, the degree of ``cross-linking/atom`` Pp and the radial bond strength were determined. The correlation between the composition and properties of the Ag2Te-As2Se3-CdTe glasses was established and comprehensively discussed. © 2008 Elsevier Ltd. All rights reserved.Item Obtaining porous quartz sand-glass composite for production of diffusers for water aeration(2008-01-01) Adžiski R.; Fidančevska E.; Angjuševa B.; Vassilev V.; Miloševski M.Quartz sand with granulation 0.125 ÷ 0.063 mm and 30 wt% waste TV glass were used for preparation of glass ceramic composites with controlled porosity of 24.4 ± 1.5 %, and interconnected pores with size of 250-400 μm. The production of this composite was realized at 950 °C/2h. This composite possesses E-modulus and bending strength of 12.0 ± 1.2 GPa and 16.1 ± 1.4 MPa, respectively. The permeability and form coefficient of this porous composite were K0 = 3.22 Da and C0 = 3.92·106m-1, respectively. This porous composite represents a potential material for production of air diffusers for water aeration.Item Photo- and thermo-induced changes in optical constants and structure of thin films from GeSe2-GeTe-ZnTe system(2013-01-01) Petkov K.; Todorov R.; Vassilev V.; Aljihmani L.We examined the condition of preparation of thin films from GeSe 2-GeTe-ZnTe system by thermal evaporation and changes in their optical properties after exposure to light and thermal annealing. The results for composition analysis of thin films showed absence of Zn independently of the composition of the bulk glass. By X-ray diffraction (XRD) analysis it was found that a reduction of ZnTe in ZnSe in bulk materials takes of place during the film deposition. A residual from ZnSe was observed in the boat after thin film deposition. Optical constants (refractive index, n and absorption coefficient,α) and thickness, d as well as the optical band gap, E g, depending of the content of Te in ternary Ge-Se-Te system are determined from specrophotometric measurements in the spectral range 400-2500 nm applying the Swanepoel's envelope method and Tauc's procedure. With the increase of Te content in the layers the absorption edge is shifted to the longer wavelengths, refractive index increases while the optical band gap decreases from 2.02 eV for GeSe2 to 1.26 eV for Ge 34Se42Te24. The values of the refractive index decrease after annealing of all composition and Eg increase, respectively. Thin films with composition of Ge27Se 47Te9Zn17 and Ge28Se 49Te10Zn13 were prepared by co-evaporation of (GeSe2)78(GeTe)22 and Zn from a boat and a crucible and their optical properties, surface morphology and structure were investigated. The existence of a correlation between the optical band gap and the copostion of thin films from the system studied was demonstrated. © 2013 The Authors.Item Polymer composites containing waste dust from power production: II Strength characteristics of composites based on UPR/HIPS polymer system(2008-01-01) Koleva M.; Vassilev V.; Vassilev G.Polymer composite material based on ``unsaturated polyester resin/ high impact polystyrene`` polymer system as matrix and waste dust from power production as filler was studied. The waste was mechanically activated at dry conditions at room temperature for 30 minutes. Strength characteristics - impact, bending and compressive strength - dependance on the waste concentration was determined. The optimal results were achieved at waste concentration of 10 w.%. Morphology observations of filled systems were performed to clarify the influence of the structure on composite's properties.