外加電壓及表面電傾效應對FLC配向之影響研究

The Study of Electric Field and Surface Electro-clinic Effect on the Alignment of the Ferroelectric Liquid Crystal Cell with N*-SmC* Phase Transition

李季芬(Chi-Fen Lee)

指導教授 ： 陳殿榮 ； 吳俊傑

國立臺北科技大學/電資學院/光電工程系研究所/碩士(2007年)

摘要

具有N*-SmC*相轉移順序的液晶，在形成SmC*相時，假設上下配向膜的條件相同，液晶分子會有兩種極化方向的可能性。同時由於表面電傾效應的影響，不同極化方向的表面液晶分子會以rubbing方向為中心，往相反方向夾一偏離的角度α。因此造成兩種不同軸向的區塊進而產生缺陷。為了解決配向不均勻的問題，我們利用在N*-SmC*相轉移的過程，外加一電場(直流和交流)，經由改變電場的振幅，分析偏向角α與配向的關係，並提出兩個均勻配向的模型。
在外加直流電場實驗中，當直流電場大於一臨界電壓時，液晶分子可以感應相同極性的Ps方向而形成單一區塊的排列。並且，隨著直流電場的增加，偏向角α也會跟著增加。另外，當改變電場極性時，偏向角的方向和液晶分子的Ps方向也會跟著增加。根據直流電場實驗，當在N*-SmC*相轉移的過程施加交流電場，液晶分子感應兩個極性的電壓，因此會產生兩個相反方向的偏向角且由於Ps的方向相反，液晶層的傾斜方向相反，造成缺陷的產生。然而，從實驗中發現，當外加電壓振幅足夠大，使得偏離角α等於1/2的圓錐角，層的正切方向會與rubbing方向平行，因此缺陷消失形成均勻配向。

關鍵字

表面電傾效應 ； 配向 ； 強誘電性液晶 ； 相轉移

並列摘要

In the ferroelectric liquid crystal(FLC) material with N*-SmC* phase sequence, two types of domain with opposite polarization direction randomly formed in the SmC* phase. The director of the molecules in each domain deviate a fixed angle in opposite side from the rubbing direction due to the surface electroclinic effect. Therefore, horizontal chevron defects form and then, the alignment in this kind of material is not uniform. In this paper, we investigated the relationship between the alignment and the deviation angle in the ferroelectric liquid crystal cell by applying an electric field (direct current or alternating current) during N*-SmC* phase transition. The deviation angle is denoted as the angle between the molecular axis and the rubbing direction.
In the case of the direct current(DC) electric field applied to the FLC cell during N*-SmC* phase transition, when the amplitude of the electric field is sufficiently larger then a threshold voltage, the molecules can be induced the same polarity and make the same deviation angle from the rubbing direction. As a result, the FLC cell can form a mono-domain alignment. By increasing the amplitude, the angle between molecular axis and rubbing direction become larger. When the polarity of the DC electric field changes the direction of the deviation angle and the polarity of the molecules will be changed.
According to the result of the condition of DC electric field, while applying alternating current(AC) electric field with a fixed frequency to the FLC cell during N*-SmC* phase transition, two types of domain with opposite deviation angle and polarization sign form in the SmC* phase. Because two types of domain coexist in the cell, the layer normal of two domains tilt in opposite direction with respect to the rubbing direction and form horizontal chevron defects. However, when the deviation angle is equal to half of the apparent angle of the FLC material, the layer normals of the two domains are parallel to the rubbing direction and a uniform dark state can be observed under crossed polarizers.

並列關鍵字

surface electro-clinic effect ； alignment ； ferroelectric liquid crystal ； N*-SmC*

參考文獻

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