向列型液晶盒中離子傳輸之電動力學

Electrodynamic Investigations of Ion Charge Transport in Liquid-Crystal Cell

陳惠玉(Hui-Yu Chen)

指導教授 ： 李偉

中原大學/理學院/應用物理研究所/博士(2007年)

https://doi.org/10.6840/cycu200700304

摘要

本論文為首度揭示了在外加電場反轉作用下，通過向列液晶盒之暫態電流具有雙峰的特性曲線之研究。在此，提出了一藉由分析有效電場、液晶分子轉角之理論模型，解釋暫態電流的雙峰現象與其相關影響參數。經由同時偵測通過液晶盒之暫態穿透光及電流訊號於外電場反轉瞬間以及改變樣品溫度由液晶相至均相態，可以獲知造成雙峰的原因。在向列型液晶樣品中，暫態電流的訊號可以來自兩個不同的產生原因：一為因液晶盒中離子效應所導致液晶分子再轉向造成之電位移電流訊號；另一為因離子流動所造成之電流訊號。在外加電場極性反轉前，液晶盒中之正負離子因受外加電場之作用，會從液晶材料、盒中雜質等解離出，並往相反電性之方向運動並被配向層上之陷位捕獲或吸附於其上；同時，若外加電場之振幅大於閾值電場，液晶分子則會受電場之驅動，長軸方向會平行外加電場方向。由液晶盒內兩側吸附離子所造成之內電場，會與外加電場的方向相反，並形成相互抵消之效應，使得橫跨液晶盒之有效電場減弱(甚至小於閾值電場)，造成液晶導軸方向會趨於平行基板。所以在外加電場之極性反轉瞬間，內外電場之方向將在極短的時間內同向；此時，有效電場將會被提升，使得液晶導軸發生再度平行於電場之轉向效應，並誘發產生一因液晶分子再轉向行為之暫態電流峰訊號。在理論模型與實驗結果中，發現了極性反轉電場下之暫態電流訊號或液晶分子之再轉向效應，主要是受到離子遷移率、離子濃度以及離子在配向層上覆蓋率所掌控。論文中，利用變換波型的方式，將提出相關之實驗及理論模擬之結果；這些結果將會有效地幫助解釋，當碳奈米管加入液晶後，對離子效應的影響。另外，本論文也提供了其它碳奈米管對影響液晶材料特性之實驗結果與討論。分析之結果可知，碳奈米管的加入，會使液晶分子的排列在有外加電場之條件下，產生較差的秩序性；進而造成混合物之旋轉黏滯度與介電異方性的下降。論文中，有關於離子效應之研究、液晶/碳奈米管混合物之物性討論，都有助於解釋此混合物在電光實驗中所觀察到的現象。

關鍵字

奈米碳管 ； 暫態電流 ； 離子效應 ； 液晶

並列摘要

Two current bumps are shown on the curve of the transient current after reversing the polarity of an external field in this thesis firstly. To explain this two-peak phenomenon of the transient current, which was not observed in previous studies, a modification theoretical model has been explored. By detecting the transient transmission of the light intensity and the response of the transient current at the same time at the onset of flipping the polarity of the external field and with raising the temperature around the liquid crystal cell from nematic to isotropic phase, these two current peaks can be distinguished. The experimental evidence exhibits that they are not only originated from ion transport, but also originated from reorientation of the liquid-crystal molecules, which gathers a dramatic change in the effective dielectric constant as well as the phase retardation. This reorientation phenomenon of the liquid-crystal director is caused by ion screening effect in the nematic liquid-crystal cell. Initially, the ions are free and distribute uniformly in the liquid-crystal layer and the orientation of the liquid-crystal director is dominated by the alignment layer. When an external field (i.e. prefield) is applied on the cell and it is greater than the threshold electric field, the liquid-crystal director tends to be parallel with the direction of the effective electric field across the cell. Meanwhile, the positive and negative ions move to the electrodes with appositive electric properties and then are trapped (adsorbed) on the interface of the alignment layers. As time goes, more and more ions accumulate on there and form an internal field to reduce the effective electric field across the cell. Hence, the orientation of the liquid-crystal director relaxes back to parallel with the substrates again when the effective voltage is reduced to smaller than the threshold voltage. When the polarity of the external field is reversed, at the onset, the effective electric field across the cell is enhanced, due to coincidence of the internal and external fields. Meanwhile, the liquid-crystal director reorients to parallel with the field again, and then it induces an obvious current peak on the curve of the transient current. From the theoretical model developed in this thesis, the behavior of the transient current as well as reorientation of the liquid-crystal director is dominated by the mobility, density and coverage of ions. Moreover, the theoretical model and the experimental technique help for discussing the influence of doping carbon nanotube into the nematic mixture on the ion effect. Compared the behaviors of the transient current of carbon-nanotube-doped cell and pristine cell in a bipolar field, the density and/or coverage of ions are/is suppressed by doping carbon nanotubes into liquid-crystal host. The influence of carbon nanotube on the physical parameters of liquid-crystal mixtures also has been discussed in this thesis. We find that, due to decrease in order parameter, the rotational viscosity and even dielectric anisotropy are reduced in the doped cell. These experimental evidences are helpful for explaining the electro-optical characterize in the carbon-nanotube-doped cell.

並列關鍵字

transient current ； ion effect ； carbon nanotubes ； liquid crystals

參考文獻

Chapter 1

[1] P. G. de Gennes and J. Prost, The Physics of Liquid Crystals, Second edition (Oxford University Press, New York, 1993).

[2] S. Chandrasekhar, Liquid Crystals (Cambridge University Press, New York, 1980).

[3] I. Dierking, Textures of Liquid Crystals (Wiley-VCH Verlag, Germany, 2003).

[4] D. Demus and L. Richter, Textures of Liquid Crystals (Verlag Chemie, New York, 1978).