Dielectric studies of dispersions of carbon nanotubes in liquid crystals 5CB

Abstract

The frequency dependences of the imaginary ε″ and real ε′ parts of complex dielectric permittivity inherent to planarly aligned layers of nematic liquid crystals 5CB doped with multiwalled carbon nanotubes (CNT) were investigated in a wide range of frequencies (f = 10-2-10 6 Hz) and CNT concentrations (c = 0-0.25 wt.%). It has been shown that the studied frequency range can be divided in three parts according to behavior of ε′ (f) and ε″ (f) curves. The low-frequency range (10-2 < f < 10 1 Hz) reflects the near-electrode processes in the cell. The ratio ε″/ε′ used to characterize these processes sharply grows if the concentration of CNT exceeds 0.05 wt.%. The moderate frequency range (10 1 < f < 10 5 Hz) corresponds to the alternating current conductivity, σ АС. At the nanotubes concentration less than 0.025 wt.%, σ АС does not depend on the frequency that implies its ionic origin. In its turn, at the c ≥ 0.025 wt.%, σ АС is a power function of the frequency that is typical for electronic hopping mechanism. The transition from the ionic to electronic conductivity can be explained by the percolation theory with the critical concentration of nanotubes 0.031 wt.% and percolation parameter 2.5. The high-frequency range (10 5 < f < 10 6) is mainly attributed to dipole volume polarization. For c < 0.05 wt.% such polarization is well described by the Debye equation. The time of dielectric relaxation in this frequency range increases with nanotubes content. This is explained by effective interaction of nanotubes with 5CB molecules.

Liquid Crystal Dispersions of Carbon Nanotubes: Dielectric, Electro-Optical and Structural Peculiarities

“…For CNTs with very high aspect ratio, typically, r500-1000, this theoretical estimation results in extremely low values of the percolation threshold, c c <0.1 wt %, which is in full correspondence with available experimental data (Lisunova et al, 2007;Mamunya et al, 2008;Lebovka et al, 2009). The percolation behavior of electrical conductivity in different LC-CNTs composites was recently reported (Lisetski et al, 2007;Lebovka et al, 2008;Koval'chuk et al, 2008;Lisetski et al, 2009;Goncharuk et al, 2009). Fig.…”

“…The interest to the electrophysical behavior of LC composites doped with carbon nanotubes is continuously growing (Lebovka et al, 2008;Lu & Chien, 2008;Koval'chuk et al, 2008). Dispersed nanotubes essentially influence the concentration and spatial distribution of charges in LC cells and thus determine the actual electric field applied to the composites and their electro-optic response (Lee et al, 2004.…”

“…These composites demonstrate huge changes in electrical conductivity  with a small change of CNT concentration c (c<0.5 wt %) (Lisetski et al, 2007;Dierking et al, 2008;Lisetski et al, 2009;Zhao et al, 2009). Besides, the  vs. c curve shows clear percolation behavior (Lebovka et al, 2008;Koval'chuk et al, 2008;Goncharuk et al, 2009). The huge conductivity increase may result in dielectric breakdown and local heating effects (Jayalakshmi & Prasad, 2009).…”

Electrooptic and Dielectric Studies in Cadmium Sulphide Nanorods/Ferroelectric Liquid Crystal Mixtures

“…Advances in Condensed Matter Physics The frequency dependence on ac conductivity (σ ac ) is given by the relationship [41][42][43] σ ac (ω) = ε o ε 2π f ,…”

Electrical Double Layers and their Effect on the Low-Frequency Dielectric Permittivity of 4-n-Pentyl-4'-Cyanobiphenyl (5СВ)

“…В рамках представленной модели были рассчитаны некоторые характеристики двойных слоев: их толщина (δ) и емкость (С ДЭС ). Для расчета этих характеристик использовались значения диэлектрической проницаемости ЖК для ячеек с гомеотропной (ε ║ eff ) и планарной ориентациями (ε eff ), имеющими толщины порядка 15 мкм, полученные методом диэлектрической спектроскопии [11,[14][15][16][17][18][19][20][21][22] при амплитуде измерительного напряжения U изм = 0,1 В и нормальном атмосферном давлении 10 5 Ра. Площадь поверхности принималась равной 1 см 2 (рис.…”