# Nematic Liquid Crystal Composite Materials for DC and RF Switching
## 5 Performance Comparison among Different Composite Systems  
In this section, we compared various composite systems reviewed in this article. It is difficult to make a comparison between various published works on field-induced conductivity modulation due to the lack of standardization. There are lots of parameters that can affect the conductivity behavior such as dimensions and types of nanoparticles, doping concentrations, cell or electrodes structures,  
Figure 17: (**a**) The conductivity of a 1% GNP in HAT6 as a function of temperature; (**b**) the I-V sweep of a thin film of 5% GNP in HAT6 in the isotropic phase at 105 \({}^{\circ}\)C [93]. Figure 18: The self-assembly and reorientation of quantum dots (QD) in a nematic LC matrix by electric field. (The LC molecules are colored blue, and the CdS QDs are sphere with purple color.) frequency of applied field, operational temperature, purity liquid crystal host materials, etc. Different research group choose different combinations of these parameters in their experiments. Considering this difficulty, we tried to make a comparison among various reported works as summarized in Table 1.  
As discussed in Section 3.1, the concentration of CNTs has a strong effect of the conductivity of LC composites. After a threshold concentration, the conductivity increases rapidly (Figure 4) [31]. However, we cannot increase the CNT concentration indefinitely, as the CNT aggregation (in a large bundle) rate increases with doping concentrations. Moreover, after a certain threshold, the CNT concentration also changes the phase transition behavior [44]. As shown in Figure 8, in the range of 0.1-0.2 wt.% of CNTs, \(\mathrm{T}_{\mathrm{NI}}\) increases significantly for an E7/MWCNT composite. A similar scenario is also expected for LC/SWCNT composites. Fortunately, in all the reported works in this article, the CNT doping concentrations are well below this critical range (Table 1). Although an LC/MWCNT composite shows a better field-induced conductivity modulation than LC/SWCNT as demonstrated by [PERSON] et al. for E7 host material [15], the problem with MWCNT is that they aggregate in large bundles as compared to SWCNT. If the length of the CNTs or CNT bundles are larger than the electrode spacing, then a single CNT can create a high current path. In this case, the conductivity might be very large but local heating is very prominent, which leads the system to an isotropic phase rapidly [53]. Local heating can also occur even if the CNT length is much smaller than the electrode spacing [20], but in this case, it may be less severe, as a loosely connected network has a higher resistance that a single CNT bundle. For the E7/CNT composite, the ON/OFF conductivity ratio is nearly four orders of magnitude, which is pretty good, but this cannot operate in the isotropic phase (in case local heating derives the system in the isotropic phase) [20]. For a 5 CB/CNT composite, [PERSON] et al. demonstrated a 5 \(\times\) 10\({}^{3}\) order of magnitudes conductivity switching in an isotropic phase [57]. Although both of the systems had a comparable performance, the additional benefits of 5 CB was that it had a room temperature nematic phase (in temperature range 21-34.5 \({}^{\circ}\)C) and it showed a field-induced switching both in the nematic and isotropic phases [53; 57]. Dual frequency switching has an excellent performance in terms of switching speed, but the ON/OFF ratio demonstrated is only 170, which is much lower than that of the 5 CB/CNT composite [84]. HAT6/GNP shows a similar ON/OFF ratio to that of the E7/CNT composite, but the operational temperature (105 \({}^{\circ}\)C) is much higher than room temperature [93]. Moreover, this field-induced switching is completely irreversible. The 5 CB/Ag NP composite shows two orders of magnitude switching at room temperature, but this is much lower than the LC/CNT composites reported here. According to the discussion above, we found that the 5 CB/CNT composite is best-suited for a switching application.