# Nematic Liquid Crystal Composite Materials for DC and RF Switching
## 3 Electrical Conductivity of Liquid Crystal and Carbon Nanotube Composites
### Local Heating and Phase Transition during Switching  
Nanoparticles can significantly alter the nematic to isotropic phase transition behavior of liquid crystals. This phenomenon is very important in device applications as it impacts the LC composite properties, i.e., electrical, mechanical, etc. Metal-nanoparticle doping lowers the nematic to isotropic phase transition temperature (T\({}_{\text{NI}}\)) [49; 50; 51]. On the contrary, [PERSON] et al. found that CNT doping can increase the T\({}_{\text{NI}}\). For this case, the transition temperature increased for E7 and the MWCNT mixture over a narrow range of nanotube concentrations for 0.1-0.2% (Figure 8) [52]. This is attributed to the anisotropic mutual alignment of LC molecules on MWCNT, stabilizing the nematic phase. They did the same experiment with fullerene to confirm that a high aspect ratio (anisotropic structure) of CNT can affect the phase stabilizaiton of the LC composite. Since fullerene is isotropic, there was no discernible change in T\({}_{\text{NI}}\). Such phase diagrams (Figure 8) help to select the suitable concentration of CNTs for different applications because the transition behavior will also affect the electrical conductivity characteristics of liquid crystal composites. Figure 7: The electric-field-induced reorientation processes of CNTs in LC-CNT composites: (**a**) From a planar to homeotropic transition for a dielectrically positive LC-CNT dispersion and (**b**) from a homeotropic to planar transition for a dielectrically negative LC-CNT dispersion. Besides increases in T\({}_{\text{NI}}\), there are some other side effects of enhancing an LC composite's electric field-induced conductivity modulation by CNT doping. As field-induced conductivity increases by creating a high current path between electrodes, Joule heating will increase a local temperature which propagates to the entire cell with time. [PERSON] et al. demonstrated an electric-field-controlled nematic to isotropic (N-I) phase transition in a 5 CB (T\({}_{\text{NI}}\) = 42 \({}^{\circ}\)C, nematic range 21-34.5 \({}^{\circ}\)C) and annealed CNT mixture [53]. They filled an LC cell of an electrode separation of 5 \(\upmu\)m with an LC-CNT mixture without any alignment layer. At room temperature, they applied a 1 kHz square wave with an amplitude 20 V\({}_{\text{PP}}\) on the cell. After a few tens of seconds, an isotropic domain started to form and eventually spread all over the cell within 100 s in a curtain-like fashion. Upon removal of the electric field, a nematic phase started to appear again after a few seconds. The entire cell came back to a nematic phase within 19 s. Such an I-N transition is also overserved if the sample is heated externally. Most of the CNTs (5-60 \(\upmu\)m of length) were longer than the cell height. Therefore, when the applied electric field is larger than [PERSON]'s transition threshold, LC molecules rotated the CNTs. In this reorientation process, longer CNTs shortened the electrodes, making a high current path. This created local Joule heating around the CNTs. This, in turn, disturbed the nematic orders which propagate across the cell by a diffusion process. By the phase contrast mode of an optical microscope, they observed a field-induced isotropic bubble formation around the CNT bundles within a second. This indicates that faster switching is possible for nano- or microscale devices because an active switching volume is much lower compared to large devices (sized in mm range). Also, they observed a linear decrease in the switching time with an increasing CNT concentration in the range of 0.04 wt.% to 0.08 wt.%. Below the threshold concentration, no complete transition occurs; only local isotropic droplets were formed. This observation confirmed that a low concentration of doping did not affect the LC property much. Other groups also found a similar observation. [PERSON] et al.