Browsing by Author "Harizanova R."
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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 Crystallization behaviour of the systems Na2O/BaO/TiO2/SiO2/B2O3/Al2O3 and Na2O/BaO/TiO2/SiO2/B2O3/Fe2O3/Al2O3(2015-01-01) Harizanova R.; Mazhdrakova A.; Vladislavova L.; Avdeev G.; Bocker C.; Gugov I.; Rüssel C.This work reports data on the synthesis of glasses in three selected compositions of each of the systems Na2O/BaO/ TiO2/SiO2/B2O3/Al2O3 and Na2O/BaO/TiO2/SiO2/B2O3/Fe2O3/Al2O3 and the preparation of glass-ceramics by applying appropriate time-temperature regimes. Crystallization of a large volume fraction of cubic BaTiO3 is observed in both systems. The phase compositions of the obtained annealed specimens are investigated by x-ray diffraction, while the microstructure of the obtained glass-ceramics is studied by optical and scanning electron microscopy. The barium titanate crystals in the samples containing iron have a size varying from ca 100 nm to some hundreds of nm. They form interconnected structures which tend to grow further for all temperature-time schedules applied. The microstructure of the annealed samples containing no iron, as proved by optical and electron scanning microscopy, is characterized by barium titanate crystals of a size of some micrometers which are uniformly dispersed in the amorphous matrix. They start to grow independently and tend to merge upon longer annealing times.Item Electron microscopy investigation of the microstructure and elemental composition of barium titanate precipitated in oxide glasses(2018-01-01) Harizanova R.; Vladislavova L.; Avdeev G.; Bocker C.; Gugov I.; Rüssel C.The present work reports on the synthesis of glasses for two different ratios of alumina to sodium oxide in compositions of the system Na2O/Al2O3/BaO/TiO2/B2O3/SiO2 with addition of Fe2O3. Barium titanate-based glass-ceramics of different average crystallite sizes, depending on the ratio Na/Al-oxides, are precipitated after appropriate thermal treatment of the glasses prepared. The phase composition analyses performed by X-ray diffraction show the formation of cubic barium titanate, BaTiO3 as a main phase and also, some BaTi0.75Fe0.25O2.888. Additional crystalline phases, such as nepheline, NaAlSiO4 appear in the course of thermal treatments at higher temperatures and longer annealing times. The scanning electron micrographs of the prepared glass-ceramics verify the presence of interconnected spherulitic structures, which correspond to barium titanate crystals and which start to grow with crystallization time increase. The morphology of the spherulitic crystals is elucidated by means of transmission electron microscopy. The elemental analysis carried out by TEM verifies the precipitation of two main crystalline phases - barium titanate and barium titanium iron oxide.Item Glass–crystalline materials with high iron oxide concentration: Phase composition, redox ratio and magnetic properties Vitrocerámicas con alta concentración de óxido de hierro: Composición de fases, relación redox y propiedades magnéticas(2024-01-01) Harizanova R.; Mihailova I.; Cherkezova-Zheleva Z.; Paneva D.; Georgieva M.; Tzankov D.; Avdeev G.; Rüssel C.The synthesis of glass–crystalline materials in the system Na2O/CaO/SiO2/Fe2O3 with high concentrations of Fe2O3 (20, 25 and 30 mol%) by applying the melt-quenching technique is reported. The melts spontaneously crystallize during pouring and the formation of magnetite (Fe3O4), hematite (α-Fe2O3) and ɛ-Fe2O3, as identified by X-ray diffraction (XRD) is observed. The microstructure and the elemental composition of the prepared materials are further investigated by scanning electron microscopy (SEM) and two different morphological types of Fe-containing crystals – needle-like and dendrite-shaped are detected. Mössbauer spectroscopy showed the presence of Fe3+ and Fe2+, as well as the existence of iron ions both in tetrahedral and octahedral coordination and the precipitation of hematite, ɛ-Fe2O3 and magnetite. The magnetic measurements on the prepared samples reveal ferrimagnetic properties with well defined hysteresis curves, although due to relatively small volume fraction of the iron-rich crystalline phases, the net magnetic moment is quite low compared to the bulk values for magnetite.Item IR-SPECTROSCOPY, THERMOPHYSICAL AND MECHANICAL PROPERTIES OF GLASSES IN THE SYSTEM Na2O/BaO/TiO2/B2O3/SiO2/Al2O3(2023-01-01) Harizanova R.; Gaydarov V.; Gugov I.; Zamfirova G.; Mihailova I.; Pernikov M.; Rüssel C.Glasses with the compositions (23.1-x)Na2O/23.1BaO/23TiO2/9.8B2O3/21SiO2/xAl2O3 x = 3, 7, 11 and 15 mol% were melted using a traditional melt-quenching technique. The thermophysical properties of the obtained glasses, i.e., glass transition temperature, Tg, and crystallization peak maximum temperature, Tc, were determined by differential scanning calorimetry and showed that the increasing alumina concentration leads to an increase of both the Tg and Tc values, while the difference Tg-Tc, used as a glass stability criterium, changes in the opposite manner, being highest for the glass with 3 mol % Al2O3 and smallest for the composition with 15 mol % Al2O3. The structure of the prepared series of glasses was studied by infrared spectroscopy and showed that the main structural units present are isolated and interconnected SiO4 tetrahedra. Both BO4 and BO3 structural units, which number initially increases with the increasing alumina concentration and then starts to decrease, are also present. The conducted depth-sensing indentation on the obtained glasses allowed to determine mechanical properties such as universal (HMV) and indentation (Hit) hardness, elastic modulus and elastic part of indentation work. The change of the Na2O/Al2O3 ratio resulted in increasing HMV and Hit up to 11 mol% Al2O3 and then a slight decrease of the respective parameters was observed.Item PHASE COMPOSITION AND MICROSTRUCTURE CHARACTERIZATION OF STRONTIUM-MODIFIED BARIUM TITANATE GLASS-CERAMICS(2023-01-01) Harizanova R.; Pernikov M.; Mihailova I.; Tatchev D.; Avdeev G.; Avramova I.; Rüssel C.Dielectric glass-ceramic materials find various applications as parts of sensors, electronic components and even in biomedicine. The present work reports on the synthesis of glass-ceramic materials in the complex oxide system (23.1-z)Na2O/17.1BaO/6SrO/23TiO2/17.4SiO2/7.6B2O3/5.8Fe2O3/zAl2O3, z = 0 and 3 mol %. The melts show spontaneous crystallization during quenching. The phase composition is studied using X-ray diffraction and reveals crystallization of Ba1-xSrxTiO3 solid solutions with varying Ba/Sr ratio. The valence states and the local environments of the Ba, Ti and Fe ions of the prepared materials are investigated by utilizing X-ray photoelectron spectroscopy. It is shown that all the elements in the composition are present in their oxidized states and the formation of barium titanate based compounds is possible. The microstructure of the obtained samples is imaged by scanning electron microscopy and shows bright polygon-like crystalline structures embedded in a dark amorphous matrix. The further evaluation of the microstructure as average crystal size and volume fraction of the precipitated crystals is done by using microcomputed tomography. Thus, a relatively high crystalline volume fraction is observed for the samples without alumina. The samples with alumina possess comparable as size and volume fraction crystals.Item PHYSICOCHEMICAL AND STRUCTURAL CHARACTERIZATION OF SILICATE GLASSES CONTAINING IRON OXIDES(2023-01-01) Mihailova I.; Harizanova R.; Tasheva T.; Dimova N.; Rüssel C.The interest in glasses containing iron oxides is determined by the possibility for application in optics, sensors, nanoelectronics, biotechnology and medicine. Subject of the present investigation is the preparation of glasses in the system 16Na2O/10CaO/(74-x)SiO2/xFe2O3, x = 0, 5, 10 and 15 mol % by applying the traditional melt-quenching technique. X-ray diffraction analysis proves that the materials with up to 15 mol % Fe2O3 are amorphous while for higher concentrations, the crystallization of magnetite and hematite occurs during melt quenching. For the physicochemical characterization of the glasses, the density, molar volume and oxygen packing density have been determined. The Becke line method has been utilized for the refractive indices measurement. The structural evolution in dependence of the Fe2O3 concentration has been studied by Fourier Transformed Infrared Spectroscopy. The data obtained from the different methods correlates well and reveals a depolymerization tendency of the silicate network with the increasing Fe2O3 concentration, i.e. an increase in the number of SiO4 tetrahedra with more non-bridging oxygens as a result of the occurrence of Fe-containing polyhedra in the glass structure.Item Prof. Christian Rüssel - always one step ahead and in search of something new in glass science(2015-01-01) Harizanova R.; Gugov I.The 18th Conference on Glass and Ceramics is dedicated to Prof. Christian Rüssel for his outstanding contributions to glass science and technology. Due to his close connections to Bulgarian scientists from Bulgarian Academy of Sciences and University of Chemical Technology and Metallurgy he was awarded the title Dr. h. c. of the University of Chemical Technology and Metallurgy on September 29th, 2014, while on October 7th, 2014 he received the Golden Band Honorary Medal ``Prof. Marin Drinov`` from the Bulgarian Academy of Sciences. Christian Rüssel was born on the 1st of August, 1952 in Nürnberg. He obtained his diploma in Chemistry in 1980. He is married to his wife Angela, who is also an engineer in glass and ceramics. He has a daughter Kathrin and two grandchildren, Clara and Johann. Between 1980 and 1992 C. Rüssel gathered tremendous scientific experience by successively joining the Institute of Physical and Theoretical Chemistry, Erlangen University, the Fraunhofer Institute of Solar Energy, Freiburg and the Faculty of Engineering, Department of Materials Science, Erlangen. In 1992, he headed the Otto-Schott-Institut for Glass Chemistry, Jena where he organized four Otto Schott Colloquia. He was an active member of the management of the Friedrich Schiller University working as Dean of the Faculty of Chemistry and Geoscience (1996-1997) and Vice Rector of Research (2002-2004). His versatile investigations encompass novel and intriguing topics: electrochemistry of polyvalent metal ions in glass melts; energy storage oriented glass-ceramics by extrusion or electrochemically induced nucleation; nano-sized glass ceramics and self-constraint crystallization; electron backscatter diffraction studies of surface crystallization of glass; high temperature spectroscopy of redox pairs in glass; synthesis of nanocrystalline glass ceramics of advanced thermomechanical and optical properties.