摻雜奈米石墨烯於4-正辛基-4’-氰基聯苯液晶之介電特性研究

Abstract

本論文研究奈米材料/液晶膠體結構，利用介電頻譜術以及偏光顯微鏡的觀察，主要探討以奈米石墨稀片作為添加物對於4-正辛基-4’-氰基聯苯液晶8CB之相轉變以及離子行為的影響。我們量測待測液晶盒在頻率為10 kHz時的實部介電常數隨溫度變化的曲線圖並對溫度做一階微分的方式，說明石墨烯/8CB液晶膠體在不同濃度下的相轉變順序與溫度。更進一步地，基於本實驗室針對液晶盒中離子行為的相關研究成果，我們量測待測液晶盒在低頻段(頻率小於 1 kHz）的介電頻譜，透過數據擬合得到離子濃度與擴散係數，闡述在不同相態時液晶盒中的離子行為。 實驗結果顯示，奈米石墨烯片具有改變8CB液晶相轉變溫度點的效果。特別是在晶相與層列A相間的相變溫度，隨著摻雜濃度由0.3 增加至0.7 wt%，該相變溫度會往高溫位移約5 °C。值得一提的是，透過偏光顯微鏡觀察可發現，純8CB和石墨烯/8CB液晶在結晶相的光學紋理分別呈現單一以及多重區塊。吾人認為，石墨烯與液晶分子間的交互作用使排列秩序度產生梯度變化，因此成為形成結晶相的成核點。針對液晶盒中的離子行為，我們發現，懸浮於8CB液晶中的石墨烯可有效的吸附液晶盒中的離子雜質，使得離子濃度下降。

In this thesis, we consider several colloids of the 4-n-octyl-4’-cyanobiphenyl liquid crystal (LC) 8CB doped with graphene nanoplatelets (GNPs) and investigate the phase and ionic behaviors by means of temperature-dependent dielectric spectroscopy and observations of microscopic textures. The phase sequence and transition temperatures between two adjacent phases of both pure and GNP-doped 8CB are clarified according to the data of the first derivative of the real-part dielectric permittivity with respect to the temperature. Moreover, based on our previous studies of the ionic effect in LC cells, the dielectric spectra in the low-frequency regime (f ≤ 1 kHz) characterized by space-charge polarization, allow one to investigate the impurity-ion properties of GNP-doped LC cells in comparison with their neat counterpart. Experimental results indicate that GNPs are responsible for modifying the phase transition temperatures of the LC 8CB. Especially for the transition temperature between the crystalline and the SmA phases in the heating cooling process, it is found that the value is approximately increased by 5 °C as the content of GNPs is increased from 0.3 to 0.7 wt%. Noticeably, the pure 8CB in the crystal phase exhibits regular a single-domain texture whereas it becomes random poly-domains in the GNP-doped 8CB cells. It is suggested that the interaction between the LC molecules and GNPs leads to the gradient change in molecular ordering in the bulk of the cell; thus, the GNPs suspended in the 8CB cells can be regarded as nuclei during the crystallization. In regard to the ionic behavior in the cells, our results based on dielectric analysis show that the ionic concentrations in GNP-doped 8CB cells are lower than that in the pure counterpart. This suggests that a minute amount of GNPs as an ion-suppressor can effectively trap impurity ions in 8CB cells.