離子化合物在微波場中的熔點下降現象

由於我們使用了一個上方設有開口的單模微波共振腔進行微波加熱，得以在微波加熱過程中經由肉眼觀察到離子化合物在低於正常熔點的溫度下熔化，與此同時來自共振腔的反射訊號強度在離子化合物熔化時有明顯增加的現象。當離子化合物從固態熔化到液態後，電性上從非導體變成導體的劇烈變化，改變了共振腔之中的介電性質上的空間分布，造成共振腔的共振頻率有所偏移，與原來輸入的微波頻率不匹配，進而提高了來自共振腔的反射率與反射訊號強度。所以，藉由離子化合物的溫度上升與當下的反射訊號的變化程度作為依據，我們設計了一個可以在微波共振腔中測量離子化合物熔點的實驗，並且發現熔點下降的程度會隨著離子化合物種類不同而有所差異。我們總共測量了九種鹼土金屬鹵化物，從化合物的組成元素來看，熔點下降的程度有著隨著陽離子半徑增加而增加，但是隨著陰離子半徑增加而減少的趨勢。　　我們也從熱力學性質的方向進行了理論分析，討論了微波場在材料相轉變時的對亂度和熱焓的貢獻：由於輸入的微波頻段與熱振動頻率有數量級上的差異，對亂度的影響甚小，不足以造成與實驗結果相同的熔點下降現象；另一方面，我們從內能在微波場下的變化，進一步來解釋熱焓在微波場下的變化，以及所觀察到的離子化合物熔點下降現象，在數值計算上得到了相同的數量級，更加地確立了往後研究可行的方向。

關鍵字

微波；共振腔；熔點；離子化合物；相轉變

並列摘要

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.

並列關鍵字

microwave ； resonant cavity ； melting point ； ionic compound ； phase transition

參考文獻

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[10] J. H. Booske, R. F. Cooper, I. Dobson, and L. McCaughan, ‘‘Model of Nonthermal Effects on Ionic Mobility During Microwave Processing of Crystalline Solids,’’ Ceram.Trans., 21, pp.185–192, 1991.

[11] J. H. Booske, R. F. Cooper, L. McCaughan, S. Freeman, and M. Binshen, ‘‘Studies of Nonthermal Effect During Intense Microwave Heating of Crystalline Solids,’’ Mater. Res. Soc. Proc., 269, pp.137–143, 1992.

[13] K. I. Rybakov and V. E. Semenov, ‘‘Possibility of Plastic Deformation of an Ionic Crystal Due to the Nonthermal Influence of a High-Frequency Electric Field,’’ Phys. Rev. B, 49 [1],pp.64–68, 1994.

[14] R. Wroe and A. T. Rowley, "Evidence for a non-thermal microwave effect in the sintering of partially stabilized zirconia," Journal of Materials Science, Volume 31, Number 8, pp.2019-2026, 1996

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離子化合物在微波場中的熔點下降現象

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