Hot Like Hell: Visco-Elasticity Measurements of Silica Glass Melt at Extremely High Temperatures

The most common glass types for tableware, laboratory glass or construction glass are borosilicate, soda-lime, lead-alkali or aluminosilicate compositions. Some glass applications demand specific optical or physical properties, corrosion-resistance or thermal stability.

For high temperature processes, SiO2 glass offers competitive temperature stability and chemical resistance. However, the high thermal stability of SiO2 glass stems from its high glass transition temperature of 1202 °C and a melting temperature of 1723 °C. On the flip side, melting and processing of SiO2 glass is energy-intensive and it is very difficult to determine the flow behavior at such high temperatures from a measurement technology point of view. To top it all, SiO2 melt has extremely high viscosities around 106.5 Pa·s (also known as the softening point) at 1760 °C, thus above the melting temperature of crystalline SiO2.

This study presents viscosity measurements of SiO2 melt, performed with an FRS 1800 – high temperature rheometer. Visco-elasticity measurements are not reported in literature due to the analytical difficulties at these extremely high temperatures. This study presents the first such data up to 1760 °C, the highest temperature ever reached with an oscillatory rheometer.

Samples and Methods

Few viscosity measurements on SiO2 melt exist, and the values provided by Urbain et al. (1982) are usually cited as reference values. The high-temperature thermo-mechanical analysis with concentric cylinder geometry presented here is an established and commonly-used method to characterize the visco-elastic behavior of polymers. However, this study presents the first quantitative visco-elasticity measurements with a rotational rheometer at extremely high temperatures relevant for glass melts, thus providing a completely new way to characterize substances with extremely high melting points, like SiO2 (Figure 1).

The measurements were performed with a development model of the furnace rheometer system (FRS 1800) combined with an air-bearing DSR 502 measuring head at Anton Paar headquarters in Graz, Austria (Figure 2). The rotational and oscillatory measurements were verified with reference values for viscosity (DGG 1) and visco-elasticity (polydimethylsiloxane, PDMS, Wacker AK 1000000). The measuring system used was an Al2O3 concentric cylinder with a 30 mm cup inner diameter and 19 mm bob outer diameter.

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