Effect of nitridation on the properties of Na–Zn–P–O glasses

Abstract

Phosphate glasses generally have a low melting temperature, a low glass transition temperature (Tg) and a high thermal expansion coefficient (α) [1–6]. These characteristics make them potential candidates for low temperature applications, such as the molding of optical elements [7] and sealing to high expansion metals [8]. However, the applications of phosphate glasses are greatly limited in practice by their poor chemical durability [6, 9]. It has been shown that the chemical durability of phosphate glasses could be significantly improved by nitridation and/or addition of appropriate additives, due to the increase of crosslink density and bonding strength in the glass network [9–18]. The effect of nitridation on the glass properties results in the two-coordinated oxygen atom being replaced by the three-coordinated nitrogen atom [19–22]. Either doping the melt with metal nitrides or remelting the glass in an ammonia atmosphere can lead to the incorporation of nitrogen atoms into the glass network [23–25]. Higher nitrogen contents can be obtained by the remelting method than by doping with metal nitrides [10, 24]. It has been shown that the nitrogen content of nitrided glasses would increase with increased remelting temperature and remelting time, but this may make melt volatilization more likely [10–12]. The purpose of this work is to improve the chemical durability of sodium-zinc phosphate (Na Zn P O) glasses by remelting the base glasses at a lower temperature and a shorter holding time under an ammonia (NH3) atmosphere. A base glass with composition 50P2O5–20Na2O– 30ZnO (mol%) was investigated in this study. The base glasses were prepared from mixtures of reagent grade sodium dihydrogen phosphate (NaH2PO4), zinc oxide (ZnO), and ammonium dihydrogen phosphate (NH4H2PO4). Well-mixed powders were melted in an alumina crucible in air at 850 ◦C for 1 h. The melts were quenched and crushed to frits. Then the crushed base glasses were nitrided in alumina boat crucibles under a flowing anhydrous ammonia atmosphere at 600 ◦C for different durations, from 0.5–5 h. The ammonia flow rate was kept at 300 cm3/min. After selected periods of time, the glass samples were removed and annealed in a preheated furnace under an air atmosphere. Then the oxynitride glass samples were cooled overnight to room temperature. Density measurements were carried out at room temperature, using the Archimedes method with deionized water as the immersion fluid. A micro-Vickers tester was used to measure the hardness with a 300 g load on polished glass samples. Glass powders were used for the differential thermal analysis (DTA) at a heating rate of 10 ◦C/min to determine the glass transition temperature (Tg). The relative chemical durability was estimated by measuring the dissolution rate of the polished glasses, which were immersed in deionized water for 24 h at 30 ◦C. The nitrogen content in the glasses was analyzed by an inert-gas fusion method, using an oxygen-nitrogen analyzer. X-ray photoelectron spectroscopy (XPS) spectra of N 1s core levels were recorded by a V. G. Scientific EscaLab 210 XPS spectrometer. The X-ray source was Mg Kα (hν = 1253.6 eV). The nitrogen contents of the nitrided glasses with various remelting times are shown in Fig. 1. The nitrogen content of the glasses increases with increasing remelting time, reaching a maximum value of 1.81wt% (2.9 at.%) for a remelting time of 5 h. The dissolution rate and concentration of nitrogen in the glass network are affected by glass composition and the remelting conditions. It was suggested that higher temperatures significantly accelerate nitrogen dissolution [11, 12, 26]. Alkali phosphorus oxynitride (Na P O N and Li P O N) and sodium alkalineearth metaphosphorus oxynitride glasses have been studied [10, 25, 26]. Lithium phosphorus oxynitride glasses that were prepared at 700 ◦C for 48 h had a maximum nitrogen content of 9.2 wt%, whereas those glasses that were prepared at 800 ◦C had a value of 12.6 wt% [12]. The nitrogen solubility in sodium alkaline-earth metaphosphorus oxynitride glasses decreased with increasing concentration of alkaline-earth oxides [26]. It was suggested that the divalent cations