关键词: 相变, 热力学性质, 测量, 相变蓄热材料, 声学特性, 原位.

Abstract: The traditional method of measuring phase change process of materials is off-line characterization by instruments such as differential scanning calorimetry (DSC) and differential thermal analysis (DTA). In practical applications of many phase change materials, in-situ measurements are more conducive to exploring the phase change process. During the phase change of materials, there is a heat exchange between themselves and the outside world, and their internal molecules and macrostructures also undergo great changes. Ultrasound has unique advantages in revealing the internal structural information of materials, and the theory and technology of ultrasonic measurement have also developed a lot. Based on the strong penetration and non-contact properties of ultrasound, the thermodynamic and acoustic properties of paraffin in this work were explored. In order to study the internal structure and state of paraffin wax during the phase change process, a pulsed ultrasonic wave with a center frequency of 5 MHz was used, and an online measuring device which was capable of controlling the rate of temperature change was designed to dynamically measure the phase change process of paraffin. The variations of ultrasonic velocity and attenuation signal at different temperature variation rates were analyzed and discussed. The results were also compared with the thermodynamic properties measured by DSC, and the recorded images were used to be an auxiliary observation. These methods helped to explore the features and principles in dynamic processes. The results showed that both methods obtained an initial condensation point at about 50℃, and the phase change reflected by the signals of the two methods were consistent. The ultrasonic attenuation and sound velocity can embody the acoustic characteristics of paraffin wax during the phase change. The ultrasonic measurement was suitable for measurement, and analysis in open environments and may form a new in-situ measurement method of wax properties and provide an effective mean of monitoring the practical application of phase change materials.

Key words: phase change, thermodynamic properties, measurement, thermal storage material, acoustic characteristics, in-situ.