RU2092883C1 - Liquid-crystal seignette-electric display unit - Google Patents

Abstract

FIELD: electric optics, in particular, information displays, image processing devices, light shutters. SUBSTANCE: device has two transparent plates with transparent current-conducting covers which are connected to alternating-sign power supply. Space between plates is filled with Seignette-electric liquid crystal which alters its optical anisotropy when sign of voltage is changed. This results in alternation of light-passing state of unit and light which is passed through unit is modulated. When voltage is switched off, unit keeps either state of maximal/minimal light pass (two-stable states) or keeps arbitrary intermediate state of light pass (multiple-stable states) depending on molecular structure of Seignette-electric liquid crystal. Said states do not depend on chemical composition and structure current-conducting covers without electric training because depth of Seignette-electric liquid crystal layer is greater than 10 mcm, this layer is optically active with spiral step of more than 1 mcm and Seignette-electric liquid crystal consists of non-chiral smectic mix which includes displaced diphenylpyrimidine and phenyl-benzoate compounds and chiral doping which includes optically active derivatives of terphenyldicarbon acid. Mix components selection and their weight ratio conform to condition of two stable states and lack of chevron deformation of layer of Seignette-electric liquid crystal, if value of spontaneous polarization of Seignette-electric liquid crystal is greater than 10 nC/sq.cm. Multiple stable states are achieved if value of spontaneous polarization of Seignette-electric liquid crystal is greater than 50 nC/sq.cm. EFFECT: increased functional capabilities. 3 cl, 2 tbl, 5 dwg

Description

 The invention relates to electro-optical devices and can be used to create information displays, light shutters, image processing devices.

 A known liquid crystal ferroelectric display cell.

In FIG. 1 explains the general principle of its structure and structure; the numbers and letters in the drawings and further in the text denote: 1 flat transparent plates, 2 transparent conductive layers, on the surface of which R is formed, providing a uniform orientation of the liquid crystal molecules, 3 planes of smectic liquid crystal layers, perpendicular to the plate surfaces 1, 4 source of alternating voltage, connected to the layers 2, E is an electric field vector located in the plane of the smectic layer, N is a vector showing the orientation direction of the long axes molecules in smectic layers, a ferroelectric liquid crystal (SLC) F _with a vector of spontaneous polarization SLC, L normal to the smectic layers, z coordinate axis which coincides with the direction of vector L, x coordinate axis parallel to the plates 1, θ _o angle of inclination of the long axes of the molecules with respect to the vector L (the angle between the vectors N and L), Φ is the angle in the XY plane between the normal to the plates 1 and the vector P _c , P, A of the directions of the transmission axes of the polarizer and analyzer deposited on the outer surface of the plates 1, b is the angle between field passing axes izatora and analyzer, I ₀ the intensity of light incident on the cell, I cell intensity modulated light.

Figure 2 illustrates the fundamental property of FLC at E 0: the periodic dependence of the azimuthal angle v on the z coordinate, which is called a helicoidal swirl; in the drawing and further in the text: P ₀ is the pitch of the helical twist spiral. The value of P ₀ is the distance along the z coordinate between the planes of the smectic layers, in which the vector P _c has the same orientation and the angle v is the same value. Depending on the chemical structure of the FFA, the value of P ₀ varies from 0.16 micrometers to infinity. If P _o → ∞, then Φ (z) const, that is, the orientation of the vector P _{c is the same} in all smectic layers. This case is shown in FIG. 1. If P _{o is a} finite value, for example, 0.16 10 micrometers, then vf (z) is a periodic function with a period P _o . If E ≠ 0 and E> E _c , where E _{c is the} critical value of the electric field, depending on the structure and thickness of the FLC layer, then the helicoid is untwisted by the electric field, that is, the situation v (z) const corresponding to FIG. one.

где
Δn величина двулучепреломления слоя СЖК;
d его толщина;
λ длины волны света,

угол между векторами N и L. Максимально возможное светопропускание ячейки Т I/I_о 1 достигается, согласно (1), если:

Таким образом, при полярности электрического напряжения источника происходит изменение направления электрического поля в образце СЖК от +Е к -Е, что приводит к изменению светопропускания ячейки от минимального значения до максимального, а при изменении от -Е к +Е светопропускание изменяется от максимального значения. Описанный процесс происходит, если Е > Е_с.The modulation of light by a cell is as follows. Natural non-polarized light is incident on the cell, the intensity of which is I _o (Fig. 1). Passing through the polarizer P, the light becomes polarized in the direction of the transmission axis of the polaroid and, passing through layers 1 and 2, falls on the FLC layer. The propagation of polarized light through the FLC depends on the relative position of the vector N and the transmission axis of the polarizer P. The direction of the vector N in the cell depends on the sign of the voltage of source 4, that is, on the direction of the field E. The angle between the vectors N (+ E) and N (-E) is 2θ _o (Fig. 1). If the FFA is in the + E field and the polaroid is applied so that its axis is parallel to the N (+ E) vector, then the light propagates along the main optical axis of the FLC and therefore does not experience birefringence, and at β = π / 2 the cell does not transmit light. If the field direction changes by -E, the light will propagate at an angle of 2θ _o to the main optical axis of the FLC and therefore undergo birefringence, as a result of which the polarization of light from linear to elliptical is converted. In this case, at β = π / 2, the cell transmits light. A change in the light transmission of the cell occurs due to a change in the anisotropy of the FLC with a change in the direction of the electric field vector. The intensity of transmitted light I is determined by the ratio:

Where
Δn is the birefringence of the FLC layer;
d its thickness;
λ wavelength of light,

the angle between the vectors N and L. The maximum possible light transmission of the cell T I / I _about 1 is achieved, according to (1), if:

Thus, when the source voltage is polarity, the direction of the electric field in the FLC sample changes from + E to -E, which leads to a change in the light transmission of the cell from the minimum to maximum, and when changing from -E to + E, the light transmission changes from the maximum value. The described process occurs if E> E _s .

After changing the polarity of the electric voltage of source 4 and its subsequent shutdown, that is, at E 0, it is possible to save for a time from 10 ^{- 3} s to 10 ⁶ s, called the memory time, two states of light transmission of the cell minimum (opaque state) and maximum. This phenomenon is called bistability.

 The phenomenon of multistability is also known, which is observed in liquid crystal antiferroelectric display cells [3]. Its essence lies in the fact that at E 0 it is possible to preserve not only two states of light transmission of the cell maximum, but also any intermediate state. The indicated effect can be used to obtain grayness in information displays. However, in the liquid crystal ferroelectric display cells, the effect of multistability has not yet been observed.

It is known that to date, bistability has been achieved due to the interaction of FLC with surfaces 2. Bistable liquid crystal ferroelectric display cells of this type are called surface-stabilized structures. They are characterized by the following relationship between the helix pitch and the thickness of the FLC layer: P _o ≥ d. These structures have significant drawbacks. Firstly, the stability of bistability strongly depends on the uniformity of surfaces 2 and their identity, which is difficult to achieve technologically. Secondly, in order to achieve bistability, a surface-stabilized structure must first be subjected to a special electrical action [2] called electrical training of the sample. Some time after the electrical training, the sample loses its bistability and must be trained again. Thirdly, surface-stabilized structures operate with an FLC layer thickness of d 1.5 2 micrometers, which is difficult to ensure in the manufacture of cell samples. Fourth, faults are observed in smectic layers, called chevron defects or chevrons (Fig. 3a). The presence of chevrons limits the memory time and leads to light scattering in the cell, which degrades its quality and reduces the contrast of light transmission.

Closest to the claimed invention (prototype) is a bistable liquid crystal display cell stabilized by a helicoid. This type of cell is called helical-stabilized structures and is characterized by the fact that P _o ≅ d. Helicoid-stabilized structures do not need sample training. However, as in the case of surface-stabilized structures, there is a dependence of the stability of bistability on surface parameters, although it is much weaker than for surface-stabilized structures. In addition, gelcoid-stabilized structures, as well as surface-stabilized ones, operate at d 1.5 2 micrometers.

 This invention solves the problem of creating a liquid crystal display cell, in which there are no chevron defects and which has both bistability and multi-stability without electrical training with an FLC layer thickness of more than 10 micrometers, regardless of the chemical structure and surface structure 2.

фенилбезоанты вида:

оптически активные производные терфенилдикарбоновой кислоты вида:

или вида:

или R $\genfrac{}{}{0ex}{}{\text{*}}{\text{1}}$ , R $\genfrac{}{}{0ex}{}{\text{*}}{\text{2}}$ те же радикалы, что и в структуре VI.This is achieved by choosing combinations of the chemical structures of the molecules that make up the FFA. To achieve the goal, it is necessary that the composition of the FFA simultaneously include chemical compounds belonging to the following chemical classes:
substituted phenylpyrimides of the form:

phenylbesoants type:

optically active derivatives of terphenyldicarboxylic acid of the form:

or type:

or R $\genfrac{}{}{0ex}{}{\text{*}}{\text{one}}$ , R $\genfrac{}{}{0ex}{}{\text{*}}{\text{2}}$ the same radicals as in structure VI.

The combination of these structures in certain ratios, namely: the weight ratios of phenylpyrimidines I III and phenylbenzoates IV-V can vary from 4 1 to 1 4, the optically active derivatives of terphenyl dicarboxylic acid VI and VII amount to 10 40% by weight of FFA, - this gives rise to bistability in FLC layer with a thickness of more than 10 μm, if structures VI and VII are selected in such a way that P _about > 1 μm. The bistability in this case is the result of intermolecular interactions in the FLC, and not the result of the interaction of the FLC with the surface, as in surface stabilized structures, therefore there is no dependence of the stability of bistability and memory time on the parameters of the layers 2.

If the concentration of structures VI and VII in FFA exceeds 25 wt. then the chevron defects disappear (Fig. 3b), since bending of the smectic layers is not allowed by the rigid terphenyl cores of the molecules of optically active derivatives of terphenyl dicarboxylic acid. In this case, the memory time reaches 10 ⁸ sec or more with a layer thickness of FLC 10 100 micrometers.

If structures VI and VII, the concentration of which in the composition of FFA exceeds 25 wt. so selected that P _o > 1 μm, P _c > 50 nl / cm ₂ and the weight ratios of phenylpyrimidines I III and phenyl benzoates IV V vary from 4 1 to 1 4, then the liquid crystal ferroelectric display cell is multistable at d> 10 μm. The optical manifestations of multistability are illustrated by the micrograph of FIG. 4b, in which a periodic sequence of white and black stripes is visible. The period of this structure is 14 micrometers.

 It is known that ferroelectrics, both solid and liquid crystalline, are characterized by the presence of a hysteresis loop. A typical view of the hysteresis loop of the light intensity transmitted through a multistable liquid crystal display cell is shown in FIG. 4a. At point A of the curve (Fig. 4a), the voltage source was turned on, the cell remembered the sequence of white (transparent) and black (opaque) bands (Fig. 4b). Multistable cells differ in that they store any point on the curve of the hysteresis loop at which the voltage source 4 is turned off. Moreover, the transmittance of the cell is determined by the ratio of the areas of white and black bands.

The physical reason for the appearance of the periodic structure under consideration is the Sengnelectric domains in FLC. These domains occur at P _c > 50 nl / cm ² . Their period D is inversely proportional to the square of the spontaneous polarization of the FFA: D P _s ^{- 2} . The presence of domains is manifested in the periodic modulation of the angle Φ along the z coordinates (Fig. 5) and is associated only with the material parameters of the FLC. When changing the magnitude of the voltage of the source 4, those regions of the cell at which v is maximum are switched first, and other regions are not switched.

 This leads to the appearance of periodic modulation of the light transmission of the cell (Fig. 4b), and the storage of such a periodic lattice at any point of the hysteresis loop is ensured by a combination of the above chemical structures. Multistability, as well as bistability, is characterized by the time of memory, the time for which the periodic lattice is remembered. The steriochemical conditions for obtaining bistability and multistability coincide in many respects, and all multistable cells have, in particular, bistability.

 The device of a liquid crystal ferroelectric display cell in statistics is completely described by the circuit shown in FIG. 1. The structural difference from the previously proposed solutions lies only in the fact that the thickness of the liquid crystal layer is more than 10 micrometers, and not 1.5 2 micrometers.

 The light modulation of the proposed cell is carried out in the same way as the known ones. However, to obtain bistability and multistability, there is no need to select the chemical structure and structure of surfaces 2 and preliminary electrical training of the sample.

 The bistable mode of operation of the liquid crystal ferroelectric display cell and the presence of chevrons in it, depending on the molecular structure of the FLC, are illustrated by the examples given in Table. 1, and multistable in the table. 2.

Claims (3)

1. A liquid crystal ferroelectric display cell containing two flat transparent plates located parallel to one another, on which one side polaroids are deposited, and on the other side transparent conductive coatings connected to a source of alternating electric voltage, on the surface of which a marked direction is specified to ensure uniform orientation liquid crystal molecules, a ferroelectric liquid crystal (FLC) located in the space between transparent conductive coated plates that changes its optical anisotropy by an electric field composed of achiral smectic C liquid crystal mixture of 60 to 90% by weight of free fatty acids and chiral additive, of 10 to 40% by weight of free fatty acids having a spontaneous polarization P _with more than 10 nC / cm ₂ and a smectic tilt angle of more than 10 ^o , characterized in that the conductive coatings of the flat transparent plates are spaced from each other at a distance of more than 10 μm, and the non-chiral smectic C liquid crystal mixture includes includes a combination of substituted phenylpyrimidines and phenylbenzoates in a ratio of 4 1 to 1 4, moreover, as phenylpyrimidines

one of the structures of I III acts, either all together, or in any combination, but as phenyl benzoates

either one of structures IV, V, or all together, or in any combination, or as a chiral additive, the composition of FFA includes either one of structures VI VII

Where

or

Where

or R $\genfrac{}{}{0ex}{}{\text{*}}{\text{one}}$ , R $\genfrac{}{}{0ex}{}{\text{*}}{\text{2}}$ - the same radicals as in structure VI, or all structures, or in any combination, so that the pitch of the helix P _{about the} helicoid in FFA is greater than 1 μm.  2. The cell according to claim 1, characterized in that the non-chiral smectic C liquid crystal mixture comprises 60 75% of the total weight of the FFA, and the chiral additive is 25 40% of the total weight of the FFA. 3. The cell according to paragraphs. 1 and 2, characterized in that the chiral additive is selected so that P _with more than 50 nC / cm ² .

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