電磁波於二聚體間激發的極化電荷強化現象和微波燒結中的非熱效應

Abstract

介電質的微波特性多年來一直被廣泛研究，工業上也有相當多的應用。尤其是微波對介電質加熱所具備的非接觸性、高均勻度、高加熱速率等優點，使其在多個領域大放異彩：化學分析、食物製程、害蟲控制、空氣淨化等，皆有大量的論文研究，探討其適用的範圍。然而，許多重要的微波加熱介電質物理卻鮮少有人討論，像是極化電荷屏蔽效應、二聚體的電場相互增強、微波共振與多個物理效應之間的競爭關係。本篇論文將以數學解析解、電磁模擬與實驗結果交叉論證，用簡單的物理模型解釋上述現象。 首先，我將對介電質在微波照射下的物理做簡單介紹，幫助讀者在後續文章中理解；其此，以數學解析解分析均勻靜電場中與平面波照射下的介電質圓球，分別了解極化電荷屏蔽效應和微波共振，並以準靜態近似將兩種現象在不同大小尺度下的介電質做銜接；接著，介紹二聚體的電場相互強化，以兩個不同頻率微波（無磁場的低頻段27 MHz和強烈微波共振的高頻段2.45 GHz）的電磁模擬與實驗結果，展示其強化效果的適用性；最後，將上述的三種現象帶入到介電質材料的微波燒結中，以電磁模擬與簡單物理模型解釋在目前燒結中的非熱效應，如微波燒結溫度較傳統方式低溫、燒結後材料結構改變等。

Microwave phenomena in dielectrics have been researched for decades. In particular, microwave is extensively used in industrial applications, especially as a unique heating source. The advantages of microwave heating include its non-contact nature, great penetration depth, and high heating rate, as compared to the conventional methods. These advantages lead to numerous, multi-disciplinary research activities in, for example, chemical synthesis, plasma generation, food processing, pest control, and cultural heritage preservation. However, there are also a large number of problems, notably heating non-uniformity, which have hindered further advances of this method. Some of these problems are obvious (such as standing wave pattern in a resonator), some have been understood (such as thermal runaways), and some are physics issue which have not yet been realized or fully understood (such as the occurrence of sparks and the non-thermal effects in some applications). This dissertation addresses a few of such issues, all originating from the induced polarization charges and some complicated by electromagnetic resonances in the heated body. Based on a dimer model, analytic theories, electromagnetic simulations, and experiments are carried out for the understanding of polarization-charge shielding/enhancement of the applied electric field and a potential non-thermal effect due to the attractive force between polarization charges. We begin with a review of relevant basic theory on DC and AC field interactions with a dielectric object. Analytic expressions are presented to demonstrate two important microwave effects of dielectric spheres: the polarization charge shielding effect in a static, uniform, external electric field and the internal microwave resonance induced by a plane wave. Next, we consider a decades-long, popular-science puzzle in the form of sparks ignited by the electromagnetic wave in the gap of two grapes in a household microwave oven. The latest and generally-accepted physical explanation models the two grapes by two water spheres (referred to as a dimer). It attributes the sparks to the merging of electromagnetic resonances from each sphere of the dimer into a hot spot. We point out the flaws of the “electromagnetic origin” of the hot spot, then propose a new explanation of electric nature. The electric field in the gap of the dimer due to mutual enhancement of the induced polarization charges on the gap surfaces is shown in simulation to be high enough to cause an air breakdown, hence the sparks. This electric origin is verified in experiments in two ways. First, the sparks are shown to occur at 27 MHz, at which no electromagnetic resonance in the dimer is possible. Second, the electromagnetic origin would result in a repulsive force between the two spheres, while the electric origin would lead to an attractive force. Our experiment at 2.45 GHz clearly demonstrates the attractive motion of the two spheres. The validation of a strong gap electric field between the two spheres either closely-spaced or in contact suggests the existence of a significant attractive force between the two spheres. On the other hand, in microwave sintering, it is well known that, to reach the same degree of density, the required temperature is well below that in a conventional oven. Reasons for this advantage are still under investigation although several theories have been proposed. We thus end the dissertation with a rigorous analytical analysis to put the attractive force in closed form. The evaluation of the forces under common sintering conditions yields rather large values to allow us to propose a new non-thermal mechanism for the interpretation of the lower temperature in microwave sintering. This mechanism explains an early microwave sintering experiment, while further experimental evidences are needed to assess its significance in a more conclusive manner.