# Nematic Liquid Crystal Composite Materials for DC and RF Switching
## 3 Electrical Conductivity of Liquid Crystal and Carbon Nanotube Composites
### Field-Induced Reorientation of CNT in LC Matrices and Conductivity Modulation  
An intensive study has been done on orienting particles in ferronematic fluids [PERSON] et al. (2016). A theoretical analysis using continuum fluid models predicts that it is possible to align anisometric particles in a bulk nematic crystal [PERSON] et al. (2016); [PERSON] et al. (2016). [PERSON] et al. also did an accurate analysis for Single-Walled Carbon Nanotubes (SWCNTs) considering the molecular interaction [PERSON] et al. (2016). [PERSON] showed the evidence of CNT alignment in 5 CB liquid crystal solution, but the degrees of order and CNT orientation have yet to be investigated [PERSON] et al. (2016). [PERSON] et al. demonstrated the short- and long-range CNT ordering on a membrane surface [PERSON] et al. (2016). They also showed that, in a 5 CB nematic phase LC, CNTs were oriented parallel to the surface microgrooves with an orientational order of 0.7-0.9 (Figure 6). Furthermore, it was possible to control the alignment in the film by using an electric field. They deposited two copper pads separated by 20 \(\upmu\)m onto the membrane. By applying an in-plane electric field of about 1.8 V\(\upmu\)m\({}^{-1}\) perpendicular to the grooves (along LC director), they found a significant reorientation of the LC director. With a sufficiently high coverage of SWCNT, a continuous conduction path formed between the two electrodes, resulting in a resistivity of 10\({}^{1}\)-10\({}^{2}\) m\(\Omega\) along the tube axis [PERSON] et al. (2016). [PERSON] et al. demonstrated electrically driven OFF-ON and ON-OFF switches based on liquid crystal-nanotube dispersion [PERSON] et al. (2016). They made an LC-CNT mixture by sonication and introduced it in a typical LC cell with a cell gap of 6 \(\upmu\)m. They used two commercially available LC solutions: E7 and ZLI-2806 with positive and negative dielectric anisotropies respectively. The authors determined the orientation order of MWCNT (bundle length 20 \(\upmu\)m and width 3 \(\upmu\)m) in E7 dispersion. It is evident that the MWCNT are aligned along the director of the LC with an order parameter S = 0.9. This is also reported by [PERSON] et al. for MWCNT in a 5 CB solution with S = 0.7-0.9 [PERSON] et al. (2016). Such an ordering happens for SWNT also but cannot be analyzed by optical microcopy because of their small size (on the order of nm). Figure 6: Controlling the orientation of a Multiwalled Carbon Nanotube (MWCNT) by an electric field: The two dashed lines are the copper electrodes to apply the electric field perpendicular to the surface microgrooves (for an initial alignment parallel to them). When the electric filed is above 1.8 V\(\upmu\)m\({}^{-1}\), the CNTs reorient along the field direction [PERSON] et al. (2016). Furthermore, they reoriented the CNTs by applying an AC electric field of 2 KHz to the sandwiched cell. The E7 cell goes from a planer to homeotropic orientation as the voltage exceeded the threshold value of Freedericksz transition as illustrated in Figure 7. This induced a large increase in conductance for the E7-SWCNT system compared to only slight increses for the pure LC system. This is also reported by [PERSON] et al. for the same composite system, when the applied voltage exceeded the transition value of 2.3 V and the conductance increased dramatically by approx. 4 orders of magnitude [20]. This provides evidence that a CNT follows the field = induced reorientation of the LC director. CNTs have an anisotropic conductivity, i.e., highly conductive along the tube axis but insulating across the tube. In the reorientation process, CNT bundles create a loosely connected network between the electrodes, forming a conducting path along its axis. A similar voltage dependent behavior was also found for E7-MWCNT as reported by [PERSON] et al. in their previous work [15]. However, in this case, the change was much higher. A longer size of the MWCNT might be the reason behind this. The opposite situation occurred for ZLI-2806-CNT dispersion.