Microstructure research of glasses by impulse excitation technique (IET)

The stiffness and internal friction (Q−1) of a CaO–Al2O3 SiO2 glass and an Na2O–CaO–SiO2 glass have been measured from room temperature to their respective glass transition temperatures (Tg) using the impulse excitation technique (IET). Thermomechanical analysis (TMA) is used to confirm the analysis results for the CaO–Al2O3–SiO2 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–Al2O3–SiO2 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 Ca2+ and Na+ ions from the structure units of [AlO4]Ca[AlO4] and [AlO4]Na. In the case of the Na2O–CaO–SiO2 glass, Ca2+ 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–Al2O3–SiO2 glass, the Al3+ ions within the structure units of [AlO4]Ca[AlO4] and [AlO4]Na start to anelastically relax at temperatures well above the Q−1 peak (such as 550°C). For the Na2O–CaO–SiO2 glass, the [SiO4] 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–Al2O3–SiO2 glass microstructure are reversible. On the other hand, even below the temperature of 500°C the microstructure of the Na2O–CaO–SiO2 glass is not stable. IET is shown to be a very effective method to investigate these subtle changes of the microstructure of glasses.