## 2 Microwave Liquid Crystal Technology
### Next Generation of Microwave Liquid Crystals for Electronically Steerable Antennas  
Beside the strong dependents of the response times on the effective LC layer height \(h_{LC}\), i.e., on the chosen device's topology and biasing concept, it also depends very much on the elastic properties (the rotational viscosity \(\gamma_{\text{rot}}\) and the elastic constant \(K_{11}\) for the splay deformation) of the material itself as obvious from Figure 8, since the switch-off response time for the various LCs differ significantly. E.g., for the LC capacitor cell with a height \(h_{LC}\) of 1, 2, 4, 5 \(\upmu\)m filled with Merck's first and second-generation LCs, \(\tau_{\text{off}}\) is 3.1, 12.3, 49.1, 76.7 and 16.4, 65.5, 261.9, 409.2 ms for GT3-23001 and TUD-566, respectively, compared to 1.8, 7.2, 29.2, 45.6 ms for the benchmark LC K15. In general, Figure 8 implies relatively slow switch-off response times compared to a fast shift of the ion from and back to the lattice center of a 1 to 5 \(\upmu\)m thick ferroelectric capacitor, which is at least in the ns range. However, this soth of nematic LCs implies excellent linear behavior [184; 185; 186]. To enable faster relaxation of the LC molecules, special classes of LC materials have been investigated such as polymer-stabilized nematic LCs [72; 233; 234], porous PTFE membranes impregnated with LC [185; 235; 236], nematic LCs doped with nanoparticles [51; 237; 238; 239; 240], ferroelectric LCs [51; 241; 242], ferroelectric LC mixtures doped with carbon nanotubes [243], BaTiO\({}_{3}\) nanoparticles suspended in a ferroelectric LC mixture [244] and dual-frequency LCs [51; 62; 186; 245]. This can improve the response time by one up to two orders of magnitude compared to pure nematic LCs. E.g., the phase shift response time could be reduced from more than 1 s down to 33 ms for a microstrip line phase shifter with a 100 \(\upmu\)m thick membrane impregnated LC layer [235] or from about 20 s down to less than 0.4 s in an ITO glass cell and from 80 s down to 2 s in a CoPlanar Waveguide (CPW)-type phase shifter by using a polymer-stabilized nematic LC [72]. However, this improvement in response time is usually at the expense of much higher dielectric losses or lower anisotropy. Moreover, it often requires larger driving voltages for components with the same thickness. Therefore, despite a slower response time, research on nematic LCs is going on, since there is still room for improvements in the performance metrics in terms of tunability, the material's figure-of-merit, and response time, where the overall performance metric can be tailored to match the requirements of a specific application. Thus, some groups are developing LC materials of the third generation, focusing on a high tunability class, still with low losses. Thus, the systematic investigations in [50; 51; 53; 54] have been updated in [60] with newly developed LC compositions with large optical anisotropy and low melting point on the basis of synthesized quaterphenyl and quinquinphenyl LC compounds, containing lateral substituents. Some selected four-ring compounds have a low melting point of less than 70 \({}^{\circ}\)C and exhibit large anisotropy \(\Delta\varepsilon\approx\) 1.12 to 1.34, which is accompanied by low maximum loss tangent \(\max(\tan\delta)\approx\) 0.002 to 0.006 at 29 GHz. However, the elastic properties, and hence, the switch-off response times, are not given there. Merck too, developed LC materials of the third generation [58; 59]. These are subdivided into a low-loss material class and a high tunability class with a substantially improved tunability of more than 30%. An example is GT7-29001 in Table 1 with a high anisotropy \(\Delta\varepsilon\) = 1.07 and high tunability \(\tau_{LC}\) = 30.3%, but a moderate material's figure-of-merit \(\eta_{LC}\) = 26.1; however, it does exhibit a fast response time \(\tau_{\text{off}}\) = 2.1, 8.6, 34.3, 53.6 ms for \(h_{LC}\) = 1, 2, 4, 5 \(\upmu\)m height, respectively, similar to the standard LC K15 (see Figure 8).