Obtenção e caracterização de novos materiais vítreos e vitrocerâmicos no sistema NaPO3-MoO3-V2O5

Abstract

Studies on the NaPO3-MoO3 and NaPO3-V2O5 systems have been widely reported in the literature, but significant discrepancies regarding the structures formed in these systems have been reported. In the present work, glasses in both systems were explored using thermal analysis techniques and short- and long-range structural analysis, X-ray diffraction, and vibrational spectroscopy, in order to observe which existing studies in the literature might more accurately indicate the structures found with glass formation. In this context, the NaPO3-MoO3-V2O5 ternary system is a novel system in the literature, and a structural study is of great interest due to the potential for observing electrical conductivity properties. Thus, this glass system and some compositions of its glass-ceramics were prepared and investigated using thermal analysis techniques (DSC/DTA) and structural techniques such as X-ray diffraction and Raman and infrared spectroscopy. The glasses were obtained by the fusion method and rapid cooling of the melt, and compositions were later selected to be annealed and thermally treated, as well as evaluated for electrical conductivity and exposure to the atmosphere, since phosphate-based glasses are widely known for their hygroscopic property. A broad range of glass and glassceramic compositions were prepared in the NaPO3-MoO3-V2O5 system, and structural characterizations carried out by Raman and infrared spectroscopy showed a pattern in the binary systems NaPO3-MoO3 and NaPO3-V2O5 and subsequently in the ternary system NaPO3-MoO3- V2O5. The incorporation of low amounts of transition metals produced new Mo-O-P and V-OP bonds, increasing the glass network and the glass transition temperatures (Tg). This increase in the glass network was found to be limited to certain fractions of the transition metal oxides, and for values above these, a decrease in the Tg values was observed in the binary systems. This may have occurred due to the breakdown of the phosphate-based network, generating Q0 and Q1 species isolated in the glass matrix bound to the transition metals, which do not contribute to the formation of larger glass networks, and the formation of structures in the glass matrix containing bonds between species of the transition metals themselves, Mo-O-Mo and VO-V, thus forming clusters that act as network modifiers. Specifically, for the ternary system, in addition to these observations, the increase in V2O5 content caused a decrease in Tg values compared to the introduction of MoO3, as the incorporation of vanadium oxide demonstrated a more efficient formation of these structural clusters. On the other hand, the introduction of molybdenum caused the network formed by Mo-O-P bonds to form at higher MoO3 contents before the transition metal ions converted into network-modifying clusters. The ternary system did not present Mo-O-V bonds in the glass network, as these depend on the presence of phosphate species to form, throughout the studied range. The glass-ceramics prepared exhibited the growth of crystalline phases that reflect the ordering of the respective glass structure, confirming the structures identified by the spectroscopic techniques. These crystalline structures within the glass matrix showed transition metal ions in different geometries and possible different oxidation states. VIV and VV ions are present as species in penta- and hexacoordinated geometries, while MoIV, MoIV, and MoVI ions were also reported, potentially present as octahedral species in the investigated glass-ceramics. The crystalline phase Na0.76V6O15 (COD ID 9000884) is noteworthy due to its presence in glass-ceramics with different compositions and thermally treated in various ways. The evaluation of conductivity of the selected glasses and glass-ceramics showed a relationship with the observed structure, where samples with lower MoO3 contents compared to V2O5 exhibited lower resistance at 1V, which may be related to the presence of molybdenum ions in various oxidation states compared to vanadium ions, as well as possibly being related to a greater connectivity of the glass network, as observed from the Tg values. Crystal growth in the glass matrix contributed to higher conductivity at each stage of thermal treatment. The stability evaluation of the glasses upon exposure to atmospheric moisture showed that glasses with high vanadium content have better stability than glasses with high molybdenum content.