Magnesium borate fibers by electrospinning

Reference Presenter Authors
14-017 Enrico Storti Storti, E.(TU Bergakademie Freiberg); Colombo, P.(Università degli Studi di Padova); Aneziris, C.G.(TU Bergakademie Freiberg); The electrospinning technique was applied for the synthesis of magnesium borate fibers, to be used as a reactive filter coating for the purification of steel melts. A solution of alkoxides (precursors for the ceramic phase) and one of polyvinyl-pyrrolidone (to generate the fibrous structure) were prepared in organic solvents. Scanning electron microscopy on the as-spun sample confirmed the presence of fibers with average diameters in the 500-700 range. The DSC/TG analysis showed the peak for the degradation of PVP at about 350°C, followed by the first crystallization of a ceramic phase at 660°C. A remarkable weight loss of 70% up to 1000°C was also detected. Heat treatments at 900°C in air formed crystalline and dense fibers with slightly smaller dimensions and the so-called necklace structure. The main phase detected by XRD analysis was Mg2B2O5, mainly in the triclinic form. Mg3B2O6 was also present as a secondary phase. The heat treatment at 800°C in reducing atmosphere delivered thin fibers with higher microporosity and no necklace structure. In this case part of the boron from the precursors was retained in a glassy phase, hence the main compound detected by X-ray diffraction was Mg3B2O6. The measured particle and calculated crystallite size in these fibers were both smaller than for the samples treated in air. Finally, the BET specific surface area was also over 10 times larger, according to nitrogen adsorption. Thanks to the electrical conductivity of carbon-bonded filters, it was possible to e-spin the fibers directly on one filter surface. However, the lack of a strong bond between fibers and filter substrate and their remarkable shrinkage during thermal treatment prevented any immersion test into molten steel. Direct heating of a green fiber mat in contact with carbon-bonded alumina and steel up to 1550°C in argon showed the expected shrinkage, followed by melting and coalescence of the magnesium borate fibers.
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