Estudo das propriedades mecânicas e químicas de vidros fosfatos com adição de óxido de nióbio

Abstract

In this work, niobium phosphate glasses were synthesized by melt-quenching in the binary system (100-x)KPO3-xNb2O5 in which x varied from 20% to 50%. A transparent glass-ceramic was also obtained by heat-treatment from the glass composition 50KPO3-50Nb2O5. Previous studies by the research group on glass and glass-ceramics at the Federal University of Alfenas-UNIFAL point out this vitreous system as promising for applications in optics and photonics, given its low cost and strategic interest of niobium in the national scenario. Thermal analysis was carried out to determine the characteristic temperatures of each composition, and the increase in the glass transition temperature was observed with the addition of niobium oxide. Chemical stability was evaluated in acidic, neutral and basic media, showing that the addition of Nb2O5 significantly increases the chemical resistance of these glassy compositions. Raman spectroscopy and 31P NMR measurements allowed the identification of structural variations as a function of the Nb2O5 content and crystallinity state responsible for the thermal and chemical properties. In particular, structural investigations indicate that NbO6 units are inserted into phosphate chains, increasing the connectivity of the vitreous network. For higher Nb2O5 contents, three-dimensional aggregates of NbO6 units are formed between the mixed niobium phosphate chains, which act as crystallization nuclei for the precipitation of niobate phases. Other physical investigations also identified an increase in density, refractive index and atomic packing density for higher Nb2O5 contents. The investigation of the mechanical properties highlighted that the incorporation of Nb2O5 from 20% to 50% drastically modifies the mechanical behavior of the glasses with a significant increase in the elastic modulus, indentation fracture toughness, hardness nano-indentation and Vickers hardness. In the case of the transparent glass-ceramic obtained from the 50KPO3-50Nb2O5 glass composition, scanning electron microscopy identified the formation of crystals on a nanometric scale with an average diameter of 66±14 nm, while the x-ray diffraction results allowed to identify the presence of crystalline Nb2O5 and another unidentified phase in the sample treated for 12h at 771ºC. These experimental results represent a contribution to the studies of niobium-based glass and glass-ceramics.