ULTRASONIC RELAXATION AND COMPLEX HEAT-CAPACITY

The similar origin of the sound absorption due to thermal relaxation p rocesses in ultrasonic experiments and the frequency-dependent complex heat capacity measured by temperature-modulated calorimetric (TMC) ex periments is reviewed. Furthermore, the similarities and limitations o f the two experimental methods for investigations of the glass relaxat ion and the relaxation of composition fluctuations near a second-order critical point are discussed. The theories for the ultrasonic attenua tion near a second-order phase transition, which include the descripti on of the complex frequency-dependent heat capacity, are referred to a nd illustrated with some examples. It has been shown that, for those r elaxation processes the ultrasonic spectroscopy can be considered as a high-frequency extension of the TMC. (C) 1997 Elsevier Science B.V.