In-situ high temperature Raman and Brillouin light scattering studies of sodium silicate glasses

https://doi.org/10.1016/j.jnoncrysol.2012.04.034Get rights and content

Abstract

In-situ Raman and Brillouin light scattering experiments were carried out to study the structure and elastic properties of sodium silicate glasses xNa2O–(100  x)SiO2 (x = 0, 8, 10, 15, 20, 25, 30, and 40 mol%) from room temperature up to the respective glass transition temperature for each composition. Temperature independent shear modulus and Young's modulus were observed in 15Na2O–85SiO2 glass up to 400 °C, while the temperature independent bulk modulus occurred in glass with 15 to 20 mol% Na2O in the same temperature range. A simple correlation was established between the Raman spectrum of a sodium silicate glass at room temperature and the evolution of its elastic properties with increasing temperature. The silica network stiffens upon heating up due to the conformation change from α-like to β-like rings in silica-rich sodium silicate glasses, characterized by the main band at ~ 440 cm 1 in Raman spectra. When the 950 cm 1 band associated with the Q2 species becomes obvious after substantial amount of Na2O was added into the glass matrix, glasses behave like normal solids (softening upon heating up). If neither the main band at ~ 440 cm 1 nor the 950 cm 1 band is well defined, intermediate glasses emerge, whose elastic moduli do not change with temperature because the stiffening and softening effects become comparable.

Highlights

► Sodium silicate glasses were studied by in-situ high-temperature light scattering techniques. ► “Intermediate glasses” were identified from Brillouin light scattering. ► Structural difference between normal, abnormal and intermediate glasses was illustrated.

Introduction

Previous experimental and computational studies have shown that elastic properties, and their temperature- and pressure-dependence, of silicate glasses can be tuned by adding various amounts of network modifiers [1], [2], [3], [4], [5], [6], [7], [8]. The elastic moduli of anomalous glasses (e.g., silica and silica-rich glasses) increase with temperature and decrease with pressure, respectively, in contrast to the behaviors of normal glasses, such as window glass (soda-lime-silica) containing more network modifiers. In between, there are intermediate glasses whose elastic moduli are independent of temperature and/or pressure [9]. Alkali oxides like Li2O, Na2O and K2O have been shown to be quite effective network modifiers in tuning the elastic moduli of silicate glasses [4], [5]. The basic principles for chemical modification by alkali ions are based on the intriguing roles they play inside the three-dimensional silica network [10], [11]. The alkali modifiers transform some of the bridging oxygens (BOs) into non-bridging oxygens (NBOs), effectively decreasing the polymerization of the glass network structure [12]. At the same time, the alkali ions form new bonds with NBOs to maintain the local charge neutrality, as well as occupy the network interstices to compact the network structure [11]. As a result, the changes of elastic properties due to the addition of different kinds/amounts of modifiers are quite different due to these complicated factors.

Compared to lithium silicate and potassium silicate glasses, sodium silicate glass was shown to be a more likely candidate for an intermediate glass [3], [6]. Manghnani et al. [3] found that sodium silicate glasses with 10 to 20% Na2O have temperature independent shear modulus and bulk modulus between 80 and 480 K. However, the structural origin of the different response of elastic moduli to temperature in normal, anomalous and intermediate glasses had not yet been clarified. A structural understanding of normal, anomalous, and intermediate glasses will help the optimum design of durable glasses to be used under extreme conditions (e.g., large temperature range during service). In this work, we combined in-situ Raman and Brillouin light scattering experiments to investigate the structural characteristics and elastic properties of sodium silicate glasses from room temperature up to the glass transition temperature for each composition. From Brillouin light scattering results, we can clearly indentify the “intermediate” compositions that have temperature independent elastic moduli. Detailed analysis based on in-situ Raman measurements revealed the structural differences in response to temperature in normal, anomalous, and intermediate glasses.

Section snippets

Experimental techniques

All of the sodium silicate glass samples xNa2O–(100  x)SiO2 (x = 8, 10, 15, 20, 25, 30, and 40 mol%) were synthesized by the traditional powder melting method. Appropriate amount of Na2CO3 (Sigma-Aldrich, ≧ 99.0%) and SiO2 powder (Alfa Aesar, 99.5%) were weighted according to their nominal compositions and thoroughly mixed by using an agate pestle and mortar set. The mixed powder was then transferred into a 10% Rh–Pt crucible and melted in a high temperature furnace at ~ 1500 °C for 20, 30, and 40 

Results

For each composition of sodium silicate glasses xNa2O–(100  x)SiO2 (x = 0, 8, 10, 15, 20, 25, 30 and 40 mol%), five measurements were carried out to obtain the average density in Fig. 1, and the error bars are comparable to the symbol size. It shows that the density of sodium silicate glasses increases with the Na2O concentration. This is in good agreement with Manghnani's results [3], although our values are slightly lower than theirs.

For Brillouin light scattering experiments, five spots on each

Discussion

Sodium silicate glasses in the composition range between 2% and 20% of Na2O are known to have a propensity for phase separation [16], which might affect their elastic properties accordingly. To take a close look at the possible phase separation in glasses with low sodium concentration, we used AFM to check the phase homogeneity of three different samples of 10, 15 and 20 mol% Na2O–SiO2 glasses. For each composition, two samples were heat-treated at 200 °C and 400 °C for half hour, respectively,

Conclusion

In this work, we combined in-situ Raman and Brillouin light scattering to investigate the evolution of the structure and elastic properties as a function of temperature for sodium silicate glasses. Our study showed that 15Na2O–85SiO2 glass has temperature independent shear and Young's moduli from room temperature to 400 °C, while glass with Na2O concentration between 15 and 20 mol% would show a temperature independent bulk modulus. Below 15% and above 20% Na2O, glasses behave anomalously and

Acknowledgements

This work is supported by the National Science Foundation under Grant No. DMR-907076. We thank Professor M. Tomozawa at Rensselaer for helpful discussions and providing some of the samples.

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