Effect of melting time on the properties of Sn-P-O-Cl glasses

Abstract

Phosphate glasses with low melting temperature, high thermal expansion coefficient (α), low glass transition temperature (Tg) and softening temperature (Td) are of increasing interest and desirable for many applications, e.g. the molding of optical elements, glass to metal seals, thick film paste and low temperature enamels for metals [1–5]. Unfortunately, their poor chemical durability makes them generally unsuitable for practical applications [5, 6]. However, many authors have reported the ultra-low Tg glasses based on the stannous fluorophosphate (Sn–P–O–F) glasses have improved chemical durability [7–10]. It was suggested that the addition of SnO, PbO, and PbF2 results in the formation of Sn–O–P and Pb–O–P bonds, which replace the P–O–P bonds, and leads to dramatic improvement in the chemical durability of the stannous fluorophosphate glasses [8–10]. Although stannous fluorophosphate glasses offer possibility for the commercial applications where low melting point is required, they have some potential problems. During melting, it was found that there was considerable change in composition due to volatilization losses. This was found to result in variation in property [5]. Hu and Jiang have indicated that stannous chlorophosphate (Sn–P–O–Cl) glasses exhibit higher chemical durability than that of fluorophosphate system [11]. The results offer a new glass system with low melting points and good relative chemical durability for commercial development. The effects of additives on the properties and structure of Sn–P–O–Cl glasses also were studied [12–13]. However, the volatilization effects during melting on the properties of Sn–P–O–Cl glasses have not been studied in detail previously [11– 13]. The paper was undertaken to examine the effect of melting time and the volatilization of chlorine on the properties, including thermal expansion coefficient, Tg, Td and chemical durability of Sn–P–O–Cl glasses. Glasses with composition 40P2O5–60SnCl2 were prepared from reagent grade ammonium dihydrogen phosphate (NH4H2PO4) and stannous chloride (SnCl2 · 2H2O) by a two-step process. First, NH4H2PO4 were preheated slowly to 500 ◦C in alumina crucible for 30 min. Then the crucible was removed from the furnace and stannous chloride was added. The crucible was then transferred in to a furnace which was keeping at 500 ◦C and melted for 5–40 min. The melt was cast and annealed for 1 h immediately for stress relieving. Powder X-ray diffraction (XRD) analysis of the asquenched melt was used to verify the amorphous state. The thermal expansion coefficient (α) and dilatometric softening temperature (Td) were determined by a dilatometer, using a heating rate of 5 ◦C/min. Td was defined as the temperature of maximum thermal expansion. Glass transition temperature (Tg) was determined by the differential thermal analyses (DTA; Perkin Elmer DTA7 Analyzer) at a heating rate of 10 ◦C/min. Relative chemical durability was determined by measuring the weight loss of the polished glass sample after immersion in deionized water at 30 ◦C for 24 h. All sample were polished to 1000 grit finish prior to test. The dissolution rate was defined as weight loss per unit surface area per unit time (g/cm2-min). The chlorine content of the glass was measured at 25 ◦C by an ion analyzer (model 3045; Jenway/U.K.) with a Cl− ion-selective electrode (ISE). The sample solutions were prepared by dissolving small quantity of glass sample in 0.5 N HF solutions. About 2 parts of ISA (ionic strength adjuster) solution (5 mol/l NaNO3) was added to 100 parts of the sample solution for adjustment of total ionic strength. The electromotive force of this solution was measured and the chlorine content was determined using a calibration curve that was made by using a NaCl solution (Orion Research, Inc., USA) as chloride standard. The glassy state was confirmed by XRD and all glasses are transparent. The properties and chlorine content of the glasses are listed in Table I. During melting, volatilization occurs and the chlorine content in the glass decreases with increasing the melting time (see Table I and Fig. 1). First, the chlorine content decreases rapidly with melting time and then decreases slowly to about 2.6–3.2 wt% as the melting time is greater than 20 min. The effect of melting time on Tg, Td, thermal expansion coefficient, and dissolution rate of the investigated stannous chlorophosphate glasses are shown in Figs 2 and 3, respectively. Both Tg and Td of the glass increase with melting time. Tg increases