# Nematic Liquid Crystal Composite Materials for DC and RF Switching
## 3 Electrical Conductivity of Liquid Crystal and Carbon Nanotube Composites
### Field Induced Conductivity Modulation in Isotropic Phase: \(\pi\)-\(\pi\) Staking
As there is no elastic interaction of CNTs with the LC medium in an isotropic phase, the insulator-to-conductor transition is irreversible. There is no restoring force to revert back the PNDs to break the CNT wire networks connecting the two electrodes of the LC cell. [PERSON] et al. indeed observed no discernible change in resistivity even after one week of the insulator-to-conductor transition. This experiment verifies a nonvolatile electromechanical memory effect of the LC-CNT composites [57]. Figure 10: The field-induced resistivity modulation in 3.3 \(\upmu\)m cells for (i) pure 5 CB in the isotropic phase at T = 40 \({}^{\circ}\)C (**the blue circle**); (ii) CNT (0.005 wt. %) in ethanol at room temperature (**the green triangle**), and (iii) 5 CB/CNT (0.005 wt. %) in the isotropic phase at T = 40 \({}^{\circ}\)C (**red square**) [57]. Figure 9: (**a**) Psudonematic domain (PND) formation around a CNT in an LC matrix; (**b**) PND in the isotropic phase; and (**c**) PND alignment along the electric field. As mentioned earlier, PND formation can also occur in the case of a graphene/liquid crystal (LC-GP) composite [60; 61]. [PERSON] et al. demonstrated a field-induced conductivity modulation in the LC-GP composites [62]. They incorporate monolayer graphene flakes (average thickness of 0.35 nm and an average lateral size of 550 nm) in MLC-1500-100, which shows an N-I transition at 101 \({}^{\circ}\)C. The concentration of graphene ranges from 0.786 \(\times\) 10\({}^{4}\) to 5.77 \(\times\) 10\({}^{4}\) wt.%. The incorporation of graphene (GP) stabilizes the nematic phase, and even in the isotropic phase, the composite shows dielectric anisotropy. It is well-known that graphene flakes act as a conductor in the in-plane directions, whereas it acts as insulator in the out-of-plane direction. The composite shows a significant increase in conductivity compared to pure LC upon the application of electric fields as shown in Figure 11. The possible explanation is the same as in the case of the CNT/5 CB composite described above [57]. However, the increase in conductivity is only three times, which is much lower than the CNT/5 CB composites. One very important property for switching application is reversibility. In contrast to pure LC, an LC composite is known to show an irreversible electro-optic response. This phenomenon is called the electro-optic memory effect. The effect is characterized by an increasing optical transmittance with switching operation cycles. An electro-optic memory effect is observed in the LC-CNT composite by several groups [57; 59; 63; 64]. [PERSON] et al. demonstrated irreversible memory effects on suspensions of aerosil particles in a 5 CB liquid crystal [63]. They observed that the memory effect strongly depends on the LC media, filling concentration, and applied electric-field parameter (frequency and amplitude). [PERSON] et al. showed that an EBBA/MWCNT composite shows a similar electro-optical response (Figure 12) [64]. When the CNT concentration is below 0.01 wt.%, the response is reversible with a low conductivity. However, above 0.01 wt.%, it starts to show irreversible effects and maximizes at c = 0.02-0.05 wt.%. Besides this, [PERSON] et al. observed a memory effect in the 5 CB-0.005 wt.% MWCNT composite in the isotropic phase [65]. They found an irreversible change in the dielectric properties of the composite. They also explained the internal mechanism behind this effect by the formation of the pseudonematic domain as described above (Section 3.4). Figure 11: The electrical conductance, \(\sigma\), as a function of the applied rms (root mean square) field E (f = 1000 Hz) in the nematic phase (T = 50 \({}^{\circ}\)C) for the pure LC and LC/graphene (GP) composite (GP concentration 2.29 \(\times\) 10\({}^{4}\) wt. %.) The reorientation of the graphene flakes with the nematic director is shown in the inset schematics [62].