Positive dielectric anisotropy materials

Aromatic liquid crystals, such as compounds 106-109 collated in Table 3.10, have been investigated to a much lesser extent since they were perceived to be intrinsically more viscous than the analogous cyclohexane derivatives. This is indeed often the case. It was hoped that the incorporation of a fluorine atom in a lateral position of a polar compound, such as the ester (106), would create a nematic material, such as the esters (107 and 108), of positive dielectric anisotropy and low ratio of and Ae/sj.. Although this was partially... 

In a liquid crystal cell based on the TN mode, a homogeneously aligned layer of a nematic liquid crystalline material with positive dielectric anisotropy (Ac), helically twisted by 90°, is placed in an ITO-lined glass cell between crossed polarizers... 

Scheme 4.11 Examples of typical super-fluorinated materials (SFM) used in the current generation of active matrix LCD. The liquid crystals 7-13 have positive dielectric anisotropy, compounds 14 and 15 have negative dielectric anisotropy. The approximate orientation of the molecular dipole moment for the two classes of material is indicated by arrows.

In the first ZBD prototypes the pulse voltages required were quite high but after about 15 years of dedicated work in display analysis and material parameter optimization they are now more reasonable, about 20 V, still somewhat higher than for mainstream displays. The first ZBD displays used standard positive dielectric anisotropy nematics like E7 from Merck the most recent ones use materials in which the material parameters including the dielectric anisotropy and flexoelectric coefficients have been tuned. Today s ZBD displays can be driven by standard STN-drivers and have recently found the way to the market. The power consumption is extremely low in static ZBD displays showing the same information for years as the device only consumes power during updates. The power consumption is, however, about 10 times higher for ZBD displays that are refreshed every minute compared to reflective-mode TN displays. ... 

Considering data on the correlation between display performance parameters and material constants, we have classified those LCs into three categories, N (strongly positive dielectric anisotropy nematic LC), N (weakly positive dielectric anisotropy nematic LC), and N +Np (binary mixed system of negative and positive dielectric anisotropy nematic LC). The definitions of symbols are tabulated in Table 2. 

To some extent, development of smectic materials has been slow and usually come as an off shoot of work on nematics. For example, the binary mixture shown in (38) exhibits a room temperature smectic A, a short range nematic, and is of positive dielectric anisotropy.Such properties may be used in thermally addressed displays where a transition from a scattering to a clear state forms the optical effect. This transition may also be effected by an electric field. Obviously, more work on useful smectics is required although their inherent high viscosity is a problem. 

FIGURE 12 Mechanisms for inducing transitions between scattering (focal conic) and clear (homeotropic) textures in a smectic A device. The material is assumed to have positive dielectric anisotropy. 

An optically isotropic liquid crystal (LC) refers to a composite material system whose refractive index is isotropic macroscopically, yet its dielectric constant remains anisotropic microscopically [1]. When such a material is subject to an external electric field, induced birefringence takes place along the electric field direction if the employed LC host has a positive dielectric anisotropy (Ae). This optically isotropic medium is different from a polar Uquid crystal in an isotropic state, such as 5CB (clearing point = 35.4°C) at 50 C. The latter is not switchable because its dielectric anisotropy and optical anisotropy (birefringence) both vanish in the isotropic phase. Blue phase, which exists between cholesteric and isotropic phases, is an example of optically isotropic media. 

Several cases of dielectric, hydrodynamic, and flexoelectric instabilities and domain structures have been observed and extensively studied in CLCs. Their appearance depends on the initial orientation of molecules, the physical parameters of the material, and the applied electric field. In CLCs with positive dielectric anisotropy Ae > 0, an electric field applied along the helix axis of a planar (Grandjean) texture can induce a two-dimensional spatially periodic deformation which has the form of a square grid [96], The period and threshold voltage of this field-induced instability depend on the elastie constants, the dieleetric anisotropy, and the sample thickness [97],... 

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