Two-step electrical percolation in nematic liquid crystals filled with multiwalled carbon nanotubes

Abstract

Percolation of carbon nanotubes (CNTs) in liquid crystals (LCs) opens the way for a unique class of anisotropic hybrid materials with a complex dielectric constant widely controlled by CNT concentration. Percolation in such systems is commonly described as a one-step process starting at a very low loading of CNTs. In the present study the two-step percolation was observed in the samples of thickness 250 μm obtained by pressing the suspension between two substrates. The first threshold concentration, C(n)(p(1))∼10(-4) wt.%, was sensitive to temperature and phase state of LC, while the second one, C(n)(p(2))∼10(-1) wt.%, remained practically unchanged in the temperature tests. The two-stage nature of percolation was explained on a base of mean-field theory assuming core-shell structure of CNTs.

Effective dielectric response of dispersions of graded particles

“…Second, this approach can be extended to dispersions of particles with complex permittivities. In particular, it has already been shown to be efficient for the description of electric percolation phenomena in composites of core-shell particles [54], two-step electrical percolation in nematic liquid crystals filled with multiwalled carbon nanotubes [55], and the effective structure parameters of suspensions of nanosized insulating particles [59]. …”

“…(22), but with different φ(c, δ), also holds in the case where the interphase shells can be treated as fully penetrable (freely-overlapping) [54] (see also [55]). For such systems, the scaled-particle estimate [56] gives…”

On applicability of differential mixing rules for statistically homogeneous and isotropic dispersions

“…The validity of the approach was demonstrated in [34] by contrasting its results with existing rigorous analytic results and computer simulations for dispersions of hard dielectric spheres with power-law permittivity profiles, and by processing experimental data on the effective dielectric response of nonconducting polymer-ceramic composites. Earlier, the CGA was efficiently applied to dispersions of particles with complex permittivities to describe electric percolation phenomena in composites of core-shell particles [5], two-step electrical percolation in nematic liquid crystals filled with multiwalled carbon nanotubes [35], and effective parameters of suspensions of nanosized insulating particles [36]. Finally, the idea of compact groups was also used by Sushko to evaluate the effects of multiple short-range reemissions between particles on the mean free path and the transport mean free path of photons in concentrated suspensions [37] and to discover the 1.5 molecular light scattering in fluids near the critical point [38,39]; the results were supported by extensive experimental data.…”

Nano-Objects and Ions in Liquid Crystals: Ion Trapping Effect and Related Phenomena

“…Table 2 provides a short summary of the reported effects. As can be seen, both decrease [118,119,[121][122][123][124][125][126][127][143][144][145][146][147][148][149][150][151][152] and increase [105,120,[128][129][130][131][132][133][134][135][136][137][138][139][140][141][142][150][151][152][153] in the electrical conductivity of liquid crystals doped with carbon-based nanomaterials were reported.…”

“…The electrical properties of liquid crystals doped with carbon nanotubes (CNT) were explored in numerous papers [105,116,. The majority of these manuscripts [105,116,117,[128][129][130][131][132][133][134][135][136][137][138][139][140][141][142] report the enhancement of the electrical conductivity  of liquid crystals doped with carbon nanotubes. The increase in  is associated with the percolation phenomenon and high intrinsic electrical conductivity of carbon nanotubes: a sharp transition from the ionic conductivity to the dominating charge hopping conductivity occurs at a certain concentration called the percolation concentration [105,116].…”