Ultrasonic measurement technique to study thermometric effects in glass

Ultrasonic measurements of shear wave propagation in alkali containing glasses reveal temporal instabilities and thermal after-effects which parallel thermometric effects, the secular rise and zero point depression in the ice point of glass thermometers. The ultrasonic method of observing these effects has advantages over the thermometric technique. Sound velocity is a dynamic quantity proportional to the elastic moduli and density which can be measured on an absolute basis and over a frequency range if desired. Thus in measuring instabilities in the elastic properties of glass the ultrasonic method provides additional information over the use of the thermometric technique whose measured changes relate only to density changes in the glass and only on a relative basis. In addition sound velocity can be measured over as wide a temperature range as desired extending from cryogenic to above glass transition range temperatures. Also no fixed reference points are required as with thermometers. Sample preparation is relatively simple and the availability of ‘packaged’ ultrasonic measurement facilities makes this method of measuring very small changes (ppm) in elastic properties very attractive.Data as a function of time and temperature are presented for a series of alkali-lead-silicates incorporating separately Li, Na, K, Rb, Cs, and for a lead silicate containing the alkaline earth Ba. The results indicate the reversible nature of the instabilities and after-effects and express their magnitudes and sensitivities to the temperature and temperature interval of measurement. They are also related to the thermal history of the glass, and the concentration and species of the incorporated alkali.