Nematic electrical characteristics

Electrical Characteristics of Nematic Liquid Crystal Displays 113... 

Electrical Characteristics of Nematic Liquid Crystal Displays 2.5.1. Characteristics of Liquid Crystal Displays... 

To respond to an applied electric field, the liquid crystal must exhibit dielectric anisotropy (Ae = ey — e ), defined as the difference between the dielectric constant parallel and perpendicular to the director (n) of the nematic phase. The relationship between Ae on a supramolecular level and the physical characteristics of the single molecules is described by the Maier-Meier formula (Eq. 1) ... 

Unlike thin film transistors, electroluminescence devices operate in a high electric field, and therefore homogeneity and low defect density of the thin films are more important than the carrier transport characteristics. In this sense, the approach to the use of nematic semiconductors by Kelly and O Neill should be reasonable. 

Nematic liquid crystals are also oriented with their long axes parallel, but they are not separated into layers, and they behave like toothpicks in a box, maintaining their orientation but free to move in any direction. Nematic substances can be aligned by electric and magnetic fields, resulting in a number of characteristics such as the ability in some cases to be electrically switched from clear to opaque. This property gives rise to many technical applications such as in display systems. 

All physical parameters mentioned above are material specific and temperature dependent (for a detailed discussion of the material properties of nematics, see for instance [4]). Nevertheless, some general trends are characteristic for most nematics. With the increase of temperature the absolute values of the anisotropies usually decrease, until they drop to zero at the nematic-isotropic phase transition. The viscosity coefficients decrease with increasing temperature as well, while the electrical conductivities increase. If the substance has a smectic phase at lower temperatures, some pre-transitional effects may be expected already in the nematic phase. One example has already been mentioned when discussing the sign of Ua- Another example is the divergence of the elastic modulus K2 close to the nematic-smecticA transition since the incipient smectic structure with an orientation of the layers perpendicular to n impedes twist deformations. 

Electrical field-driven pure nematic LCE actuators have not been developed because the characteristic rotation energy density is too low compared with the... 

On the other hand, miscibility is, in one sense, one of the characteristic properties in liquid crystals. Miscibility in liquid crystals is a well-known macroscopic property where one can see two mesogens exhibiting a thermodynamically identical liquid crystalline phase are mixed to show its phase at arbitrary component ratio. This has already been applied to liquid crystals for LCDs to control some properties such as temperature range of nematic phase. A diversity of functional properties such as temperature range, anisotropic electrical permittivity, viscosity etc. can be controlled in nematic blends and non-mesogenic molecules also can be a component which contributes to the resultant properties as they behave like a solute in liquid solution. However, charge transport property has not yet been well studied in terms of molecular blends with liquid crystalline materials, while thin film organic photovoltaics have been so extensively studied in recent years as molecular blends. 

Prior to the development of thin film transistors (TFT) and active matrix technology for liquid crystal displays, the maximum number of hnes or rows in any display with an acceptable contrast was severely limited by the shallow voltage-transmission characteristics of, for instance, the twisted nematic mode. This inspired work to develop a liquid crystal device that could remain in either of two states (ideally black and white) after the removal of the electric field used to switch the liquid crystal into the selected state. With a memory in the liquid crystal an unlimited number of hnes can be displayed using a simple passive matrix and the constraints are instead in the refreshment requirements. 

In the frame of Landau theoiy we can also consider the influence of the external magnetic or electric field on the N-I phase transition temperature. Imagine that we apply the electric field E along the director of a nematic and increase temperature. In the case of positive dielectric anisotropy , even a weak field changes the symmetry of both phases to cmfical (Coov)> and, strictly speaking, the phase transition vanishes. However, in the continuous temperature dependence of the order parameter, a characteristic inflection point appears that may be considered as an apparent N-I phase transition temperature 7).. The latter may be changed with an applied field. 

In both the cases considered, an optical contrast of the patterns observed in isotropic liquids is very small. Certainly, the anisotropy of Uquid crystals brings new features in. For instance, the anisotropy of (helectric or diamagnetic susceptibility causes the Fredericks transition in nematics and wave like instabilities in cholesterics (see next Section), and the flexoelectric polarizaticm results in the field-controllable domain patterns. In turn, the anisotropy of electric conductivity is responsible for instability in the form of rolls to be discussed below. All these instabilities are not observed in the isotropic liquids and have an electric field threshold controlled by the corresponding parameters of anisotropy. In addition, due to the optical anisotropy, the contrast of the patterns that are driven by isotropic mechanisms , i.e. only indirectly dependent on anisotropy parameters, increases dramatically. Thanks to this, one can easily study specific features and mechanisms of different instability modes, both isotropic and anisotropic. The characteristic pattern formation is a special branch of physics dealing with a nonlinear response of dissipative media to external fields, and liquid crystals are suitable model objects for investigation of the relevant phenomena [39]. 

Figure 2.7 shows the concentration dependence of the perpendicular component of the electrical conductivity for two nematic mixtures, A (based on azoxy-compounds) and the Schiff-based mixture B, doped with ionic impurities tetrabutylammonium picrat (TBAP), tetrabutylammonium bromide (TBAB), acceptor impurities tetracyanoethylene (TCE), 2,3-dichloro-5,6-dicyanobenxoquinone (DCDCBQ), tetracyanoquinone-dimethane (TC-QDM), and donor impurity p-phenylenediamine (PDA). Measurements are given for the ohmic part of the current-voltage curves (frequency 1 kHz and cell thickness 100 /xm). The characteristic dependence of a on concentration O oc where c = N/ up/M, predicted for the simple ionization-recombination process discussed, is only observed for ionic impurities. Here the relationship K /Kr can also be determined if we assign a value to the... 

The viscoelastic properties of liquid crystals are very important, and mainly determine the behavior of liquid crystals in external electric fields, defining such characteristics as controlling voltages, steepness of the transmission-voltage curve, response times, etc. Now only the phenomenological theory of the viscoelastic properties of nematic liquid crystals is essentially complete [18, 28]. 

The surface flexoelectric energy, which is found from (4.2) and (4.3). Attaining the minimum of the nematic free energy, (4.5) or (4.6), it is possible to derive the equilibrium director distribution in a static case. To find the response times, we have to solve the equations of nematodynamics in the electric field. The corresponding analysis shows that the director reorientation is always accompanied by the macroscopic flow, the so-called backflow [5]. (The only exclusion is the pure twist rotation of the director [1].) Backflow considerably affects the characteristic times of the electrooptical effects in uniform structures, especially in the case of strong deformations of the initial director orientation [3, 5]. 

This effect has been observed experimentally in comparatively thick cells (d 50 /xm) [113]. In cells with d 20 /xm, the final twisted state (in the field) proves to be insufficiently stable and the nematic liquid crystal layer is gradually transformed into a planar structure. The addition of small quantities of cholesteric liquid crystals to the initial nematic mixture enables a stable twisted structure to be achieved with the application of a field and improves the electrooptical characteristics of the device. The electrooptical response of electrically induced twist nematic cells includes intensity oscillations observed both in the switching on and switching off regimes [114]. These oscillations take place due to the variation of birefringence, which are not important in the usual twist effect. 

More detailed measurements of the dependences Uth f) in pure and doped MBBA at various temperatures (for sandwich cells) were performed [76, 109]. The results of these measurements are represented in Fig. 5.20. The threshold of the vortical motion was taken as the onset of the circular tion of the solid impurity particles in the electrode plane. The shape of the curves in Fig. 5.20 depends on the electrical conductivity. With a high electrical conductivity the curves have a plateau in the low-frequency region and a characteristic dependence I7th oc at frequencies above the critical frequency. At the transition point to the nematic phase the threshold voltage of the instability does not change. It is shown in [109] that the height of the low-frequency plateau is proportional to and at frequencies of u > 47r(j/e the threshold field does not depend on <7. Moreover, it does not depend on the thickness of the sample, i.e., on the separation between the electrodes. 

In this section we will consider another type of nonuniform liquid crystal structures in nematics. These structures are created by a spatially nonuniform electric field, and have nothing in common with the modulated orientational and electrohydrodynamic patterns discussed above which, in fact, were created as a result of self-organization. A spatially nonuniform electric field exists in an electrooptical cell in many important cases such as, photosensitive liquid crystal cells [152-154], spatial light modulators with matrix addressing [152], liquid crystal defectoscopy of surfaces [155], liquid crystal microlens [156], etc. By analyzing the liquid crystal electrooptical behavior in a nonuniform field we can estimate different characteristics of the layer, in particular, sensitivity (i.e., the intensity of the optical response at a given voltage), spatial resolution, etc. 

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