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    Study of the hydrothermal crystallization process of barium titanate by means of X-ray mass attenuation coefficient measurements at an energy of 59.54 keV
    (Elsevier Science Inc, 2017) Akbulut Özen, Songül; Ozen, Murat; Sahin, Mehmet; Mertens, Myrjam
    In this work, the X-ray mass attenuation coefficients of hydrothermally synthesized barium titanate (BaTiO3) samples were calculated with the purpose of determining the crystallization sequence of BaTiO3. Hydrothermally synthesized samples prepared at 100 degrees C and 200 degrees C, and reacted for varying reaction times between 15 min up to 120 h were studied. Attenuation coefficient measurements were done with a coaxial HPGe gamma detector (Ortec, GEM55P4-95) with a working range in the X-ray energy region. The samples were made into pellets and were exposed to Am-241 radioisotopes at an energy of 59.54 keV for 300 s. Additionally, FT-Raman and XRD measurements were done to support the X-ray mass attenuation measurements. It was found that secondary barium titanate (BT) phases (BaTi2O5 and Ba2TiO4) were formed from the precursor material at the early stages of the hydrothermal reaction and that phase pure BaTiO3 was formed at longer reaction times. The sequence of barium titanate crystallization was determined as follows: BaTi2O5; BaTi2O5 and BaTiO3; BaTi2O5, Ba2TiO4 and BaTiO3: and phase pure BaTiO3.
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    Template-free aqueous tape casting of hydrothermally synthesized barium titanate powder and the fabrication of highly {001}-{100} textured tapes
    (Elsevier Sci Ltd, 2018) Özen, Murat; Mertens, Myrjam; Snijkers, Frans; Cool, Pegie
    Aqueous slurries of hydrothermally prepared barium titanate (BaTiO3) powders were tape casted without the use of large anisotropic template materials. The tapes were sintered at 1400 degrees C or 1500 degrees C, and at different furnace parameters. The texture of the sintered tapes was evaluated by the semi-quantitative XRD Lotgering method and the microstructure was studied by SEM. The Lotgering factor (f) of the sintered tapes was positively influenced by two main variables. First, directly heating to the sintering temperature resulted in a 22-30% increase. Second, sintering in the liquid-phase (Ti-excess) temperature region and at a high enough temperature (1500 degrees C) drastically improved the relative intensity of the {100} and {001} lines by 72%, at the expense of the {101}- and {111}-lines. The average volume weighted grain-size of the tape, which was directly heated to the sintering temperature (i.e., skipping the calcination step), was decreased by 77%.