在平行導電玻璃夾層中流體介電質之放電圖形研究

Abstract

摘要 將具有特殊形狀電極的Hele-Shaw樣品連接至高直流電壓源¸於是¸劇烈的放電崩潰發生並造成許多有趣的燒焦軌跡且永久地保留在玻璃表面上。儘管這些燒焦軌跡經常是很複雜的¸但它們也許潛藏著許多物理方面的資訊。 在這次的研究¸我們首先試著確認一些影響放電圖形結構的因素。我們得到的結果顯示這些影響因素包括電極材料、導電膜厚度、電極形狀、樣品溫度、以及介電質種類等等。雖然有如此多的因素對整體的放電圖形造成影響¸但是似乎所有的放電圖形都是由兩種基本代表性的結構所組成。第一種是放射對稱狀結構¸這種結構經常發生在較高電壓放電的初期階段。第二種是密集彎曲結構¸這種結構經常發生在較低電壓放電或是放電反應快結束的末階段。我們也發現液體與空氣樣品在放電圖形的微觀結構上有顯著的不同。長方形電極得到的放電圖形有一個特徵是圓形電極放電圖形所難以看到的。這個特徵顯示出朝正極方向發展的放電軌跡比朝負極方向的軌跡有較為彎曲的結構。 放電延遲現象大多發生在以固定電壓方式進行放電的5CB液晶樣品。藉由正交偏光裝置的觀察¸我們發現放電延遲並不是起因於液晶分子改變方向所需的時間。當放電反應開始時¸方向波出現在放電起始點附近且向著四方傳遞。連通電極引導放電圖形的分枝朝它們的位置進行。從攝影機的連續影像中可以看到放電火花的速度在放電初期是較快速的¸當放電反應快結束時火花速度減少。 活性行走模型模擬出三種實驗所得到的放電圖形。這三種圖形分別為密集彎曲、放射與彎曲混合、以及單純的放射結構。模擬與實驗的圖形分別以質量維度法與回轉半徑法進行碎形維度的分析。此外¸藉由放電圖形的觀察與比較¸我們也討論了放電可能的機制。

Abstract When a high dc voltage is applied to the Hele-Shaw cell † with specific-shaped electrodes, some interesting burnt tracks are generated and left behind permanently on the glass surfaces after violent dielectric breakdown (DB). These patterns of burnt tracks probably contain much information in physics, although they generally appear in a complicated form. In this study we first try to identify the factors affecting the structure of DB patterns. We conclude that these factors include the materials of electrode, the thickness of conducting film, the shapes of electrode, the temperature of cell, and the dielectric materials etc. Although there are so many factors affecting on the global DB patterns, all patterns are almost composed of two basic typical structures. The first is the radial and symmetrical structure, which usually occurs at initial DB stage while applying with higher voltage. The second is the dense and winding structure, which usually appears while applying with lower voltage or at final stage when DB reaction is almost finished. We also find that the microscopic structures of DB patterns are obviously different between liquid and air cells. In rectangular patterned ITO (Indium Tin Oxide) cells the burnt tracks possess a unique feature, which hardly can be find in circular patterned ITO cells. This feature exhibits that the tracks developing towards the positive pole are more wiggling than those towards the negative pole. Time-delay phenomenon mostly exists in 5CB cells discharging with fixed-voltage method. With cross-polarizers observation we find that the time-delay is not the cause of the time spent on the reorientation of liquid crystal molecules. In the beginning of DB reaction, a reorientational wave appears near the starting point and then propagates outward in radial. The linkages of electrode lead the branching tracks developing towards them. The sequential frames recorded with CCD camera show that the developing streaks of DB reaction propagate fast at initial stage and slow down when DB reaction almost finished. With Active Walker Model (AWM) simulation three typical structures of DB patterns can be found and compared with experimental results. In global view, these three types represent dense winding, mixed of radial and winding, and pure radial structures, respectively. The fractal dimension analyses of generated patterns by simulation and experiment are done with two methods, mass dimension method and radius of gyration method, respectively. In addition, we will discuss the possible DB mechanisms by means of observation and comparison of DB patterns in our experiment. † Note: Hele-Shaw cell: it was invented by an English engineer in 1898 for facilitating the study of the physical properties of flowing liquids. The cell is composed of two parallel plates separated by a narrow gap. (Ref: H. S. Hele-Shaw, Nature, 58, 34, 1898.)