Abstract
We’ve observed that ionic compounds melt at a temperature lower than their normal transition temperature during microwave heating because of a resonant cavity with an opening on the top in use, and the intensity of reflective signal from the cavity becomes larger vividly at the same time. The reason is a tremendous change in electric properties after ionic compounds are molten which results in shifting of the resonant frequency in the cavity, and raising the intensity of reflective signal since mismatching with the input microwave frequency. According to the temperature of the heated sample and the variance in reflective intensity, we’ve designed an experiment to measure the transition point of ionic compounds in a microwave resonant cavity and found that the melting point reductions are varied with different materials. For the 9 kinds of alkali halides that we’ve measured, a general trend has been found that the larger the cation, the more the reduction is, but opposite for the anion. We’ve also discussed the effect of microwave field on thermodynamic properties, especially for the entropy and enthalpy change at phase transition under microwave field. Since the input microwave frequency and thermal-vibrational frequency are much different in the order of magnitude, microwave makes almost no contributions on the entropy change at phase transition. On the other hand, we’ve tried to explain the effect on the enthalpy change and melting point reduction based on the microwave effect on the internal energy, and ensured the research interests in the future because of a good result in numerical calculation.