Microstructure research of glasses by impulse excitation technique (IET)

The stiffness and internal friction ( Q −1) of a CaO–Al 2O 3 SiO 2 glass and an Na 2O–CaO–SiO 2 glass have been measured from room temperature to their respective glass transition temperatures ( T g) using the impulse excitation technique (IET). Thermomechanical analysis (TMA) is used to confirm the analysis results for the CaO–Al 2O 3–SiO 2 glass. IET samples of both glasses have a resonance frequency near 10 kHz. At this frequency and in the measured temperature range, the glasses are characterized by one Q −1 peak superimposed on an exponential background. The internal friction peak of the CaO–Al 2O 3–SiO 2 glass occurs at a temperature of about 320°C. From the reversibility of the changes of stiffness, it is concluded that the peak is due to anelastic relaxation, i.e. that of Ca 2+ and Na + ions from the structure units of [AlO 4]Ca[AlO 4] and [AlO 4]Na. In the case of the Na 2O–CaO–SiO 2 glass, Ca 2+ and Na + ions induce a Q −1 peak as well, but at about 230°C. In this glass, the cations diffuse through the network holes causing irreversible stiffness changes. For the CaO–Al 2O 3–SiO 2 glass, the Al 3+ ions within the structure units of [AlO 4]Ca[AlO 4] and [AlO 4]Na start to anelastically relax at temperatures well above the Q −1 peak (such as 550°C). For the Na 2O–CaO–SiO 2 glass, the [SiO 4] structure units begin to move slightly and rearrange at about 450°C. This corresponds to the beginning of the glass transition. If the temperature is lower than 600°C, the changes of the CaO–Al 2O 3–SiO 2 glass microstructure are reversible. On the other hand, even below the temperature of 500°C the microstructure of the Na 2O–CaO–SiO 2 glass is not stable. IET is shown to be a very effective method to investigate these subtle changes of the microstructure of glasses.