# Nematic Liquid Crystal Composite Materials for DC and RF Switching
## 7 Discussion  
In this paper, we reviewed the effect of NP doping (focusing on CNT doping) on an LC composite's electrical conductivity. We focused on electric-field-induced conductivity modulation, considering possible applications of the composite for DC and RF switching. We also described issues associated with field-induced conductivity modulation. Some of these issues need to be overcome to realize an efficient DC and RF switching device. After a brief discussion on liquid crystals, we discussed electric-field-driven conductivity modulation in pure liquid crystals. It is reported that the conductivity change of pure LC materials is very low, which is unacceptable in DC and RF switching application. To enhance the conductivity, one possible way is to dope the LC with a small amount of CNTs. At first, we reviewed the seminal work of [PERSON] et al. [15; 19], which described how the field-induced reorientation of LC molecules modulate the conductivity of the LC-CNT composites through elastic interactions. Then, we discussed how the doping concentration and temperature alters the conductivity and switching behavior. There is a solubility limit for CNT doping in LC matrices. CNT tends to aggregate into large bundles in LC media as concentration increases. In addition to that, in a small window of CNT doping concentrations, the nematic to isotropic transition temperature (\(T_{\text{NI}}\)) change a lot. During field-induced switching, Joule heating in the conductive state can drive the system to an isotropic phase from the nematic phase [20]. Besides in the nematic phase, conductivity modulation can also be achieved in the isotropic phase for some LC composites. In this case, LC molecules anchor on the CNT wall and form a pseudonematic domain; these PNDs interact with an external electric field. One of the major issues with switching is irreversibility, also described as memory effect. The memory \begin{table}
\begin{tabular}{c c c c c} \hline \hline \(R_{\text{ON}}\) (\(\Omega\)) & \(C_{\text{OFF}}\) (\(p\)F) & \begin{tabular}{c} **Insertion loss at 10** \\ **GHz (dB)** \\ \end{tabular} & \
\begin{tabular}{c} **Isolation at 10** \\ **GHz (dB)** \\ \end{tabular} &
\begin{tabular}{c} **Return loss at 10** \\ **GHz (dB)** \\ \end{tabular} \\ \hline
50 & 0.5 & 5.7 & 6 & 31 \\
20 & 0.5 & 3 & No data & 38.6 \\
5 & 0.5 & 0.96 & No data & 60.9 \\
5 & 0.05 & 0.95 & 30 & 61 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The RF performance parameters of the device. Figure 22: The transmission (return loss denoted by \(S_{11}\)) characteristic of the proposed RF device. (**a**) For on state resistance, \(R_{\text{on}}=50\)\(\Omega\) and of state capacitance, \(C_{\text{off}}=0.5\) pF; (**b**) For on state resistance, \(R_{\text{on}}=5\)\(\Omega\) and of state capacitance, \(C_{\text{off}}=0.05\) pF. effect increases upon doping with CNTs and other nanoparticles. Although out-of-plane devices are prominent, there are works on in-plane switching with a very simple microfabrication technique. An in-plane cross-electrode configuration could be a viable solution for reversible switching [70]. For device application, switching speed is a very important performance parameter. Most of the NLC devices suffer from a longer switching time. A faster switching is achievable through dual frequency nematic liquid crystal materials [84]. After the discussion of LC-CNTs' conductivity behavior, we also discussed field-controlled conductivity modulation in an LC/metal-nanoparticle composite to compare their performances with LC-CNT composites. Finally, we proposed a simple model for an RF switching device using LC composite materials. With this simple analytical model, we simulated and analyzed some RF performance parameters. Although the application of LC composites in the field of RF switching fields are not explored yet, our simulation suggests that LC-based devices could open a new avenue. However, to make an efficient microswitch, some issues or challenges need to be considered, which are summarized below. 1.