Estudo de cristalização em vidros de fosfatos alcalinos contendo óxido de nióbio

Abstract

The present work aimed to synthesize and characterize vitreous samples based on alkali phosphate containing niobium oxide, studying different precursors of alkaline phosphate. Glass ceramics were obtained through the controlled heat treatment of alkaline niobium phosphate glasses of the system (50 – x) NaPO3 – (x) KPO3 – 50 Nb2O5, where x =0, 25 and 50. They were synthesized by the classical melting/cooling method, visually clear glasses with up to 50 mol % Nb2O5. Differential scanning calorimetry (DSC) analyzes were performed to determine the characteristic temperatures and the thermal stability parameter against crystallization of the obtained glasses. Through these DSC curves it was also possible to establish heat treatment parameters for the growth of crystalline phases to obtain transparent glass-ceramics from the precursor glasses. The samples with 50 mol % of Nb2O5 (50 NaPO3 – 50 Nb2O5; 50 KPO3 – 50 Nb2O5; 25 NaPO3 – 25 KPO3 – 50 Nb2O5) showed two clear and separate crystallization peaks, the first being related to the formation of niobium-rich regions (containing NbO6 species) that when precipitate, can form crystalline phases of niobates, which are of great technological interest in optics. The glass-ceramics were obtained from heat treatment for 12 hours at the initial crystallization temperatures (Tx1) of the samples with 50 mol % of Nb2O5. X-ray diffraction (XRD) analysis confirmed the vitreous character of the synthesized samples and identified the precipitated crystalline phases (Nb2O5, NaNbO3 and Na2Nb8O21) in the glass-ceramics obtained. Spectroscopic analysis in the ultraviolet-visible (UV-Vis-Nir) and Fourier transform infrared (FTIR) region allowed us to determine the transparency window of the samples obtained. Raman spectroscopy indicated that niobium was incorporated into the phosphate matrix, confirming its intermediate character in the formation of the vitreous network, in the form of NbO6 octahedra. It was also observed that these octahedral units also precipitate as part of the crystalline phase in glass-ceramics. Finally, transparent glass-ceramics with niobate crystalline phases were obtained for the samples of composition 50 KPO3 – 50 Nb2O5 and 25 NaPO3 – 25 KPO3 – 50 Nb2O5 and translucent for the sample of composition 50 NaPO3 – 50 Nb2O5.