Browsing by Author "Parvanov S."

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    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.
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    Synthesis of doped bismuth titanate ceramics with Nd2O3 and SiO2 and their electrical properties
    (2013-05-21) Slavov S.; Krapchanska M.; Kashchieva E.; Parvanov S.; Dimitriev Y.
    Bismuth-titanate ceramics containing SiO2 and Nd2O3 as additives are synthesized by melt quenching method in the system Bi2O3-TiO2-Nd2O3-SiO2 at temperature range 1260 - 1500oC. The phase composition of the obtained materials is determined by X-ray diffraction analysis and energy dispersive spectroscopy. Using scanning electron microscopy different microstructures are observed in the samples depending on the composition. The addition of SiO2 and Nd2O3 allows controlling the crystallization, glass formation ability, melting temperature and Curie temperature. Different values of conductivity, dielectric losses and relative permittivity are obtained depending on the composition. Measurements of the electrical conductivity, capacitance and dielectric losses of selected samples are performed by DC resistible bridge and digital capacity meter (with frequency of 1 kHz) using two-terminal method and a suitable sample holder with graphite electrodes. It is established that all investigated samples are dielectric materials with conductivity between 10-9 and 10-13 (O cm)-1 at room temperature, dielectric permittivity from 1000 to 3000 and dielectric losses tgd between 0,0002 and 0,1.
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    Synthesis Of Znteo3 by crystallization of glasses in TeO2-Bi2O3-ZnO-Nb2O5 system
    (2018-01-01) Ganev S.; Bachvarova-Nedelcheva A.; Iordanova R.; Ranguelov B.; Parvanov S.
    The aim of the present work is to verify the synthesis of zinc tellurite by crystallization of glasses in TeO2 - Bi2O3 - ZnO - Nb2O5 system. The investigation refers to TeO2 - based glasses containing up to 10 mol % of Nb2O5 and Bi2O3 and ZnO content raging from 5 mol % to 10 mol %. Several glasses are selected and heat treated for 6h at 500°C. The XRD analysis detects formation of several ZnTeO3, TeO2 and ZnO crystalline phases. Additional information for the formation of ZnTeO3 crystalline phase is obtained by infrared spectroscopy (IR). There is a difference in the IR spectra of the crystalline sample of 50TeO2.50ZnO (ZnTeO3) and those of the multicomponent glasses. The spectra analysis shows that the glass network consists mainly of TeO4 (tbp) units, while TeO3 (tp) are the main structural units in the crystalline ZnTeO3 phase. The microstructure of the selected crystallized samples is examined by SEM analysis.
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    The effect of Nd substitution on the glass forming ability and thermal properties in the system Bi2O3-TiO2-SiO2
    (2015-01-01) Slavov S.; Kashchieva E.; Parvanov S.; Dimitriev Y.
    The paper reports data on glass formation in wider concentration ranges. The samples amorphous nature is characterized by x-ray diffraction analysis, their thermal characteristics and stability are determined by differential thermal analysis (DTA), while the transmission and absorption characteristics outlined are investigated by ultraviolet-visible spectroscopy (UV-Vis). SiO2 content increase leads to a shift of the absorption border to lower wavelengths. The experimental results show that it is possible to obtain low-melting glasses containing 5 mol % - 20 mol % SiO2 and 30 mol % - 63 mol % Bi2O3 with the synthesis technique applied. Substitution of Bi2O3 with Nd2O3 up to 10 mol% leads to the preparation of a complex multicomponent glasses and glass-crystalline materials of high thermal stability.