Browsing by Author "Bocker C."

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    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.
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    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.