NO841709L - ANISOTROPICAL VASKECELLE - Google Patents

Description

The present invention relates to a liquid crystal cell for the optical display of electrical signals with a cholesteric liquid crystal between two carrier surfaces equipped with control electrodes.

Liquid crystal cells with cholesteric liquid crystals are known. These cells make use of the fact that cholesteric liquid crystals exist in several, that is, as a rule, two optically different state forms, between which they can be connected here and there by applying suitable voltages.

In a cell without an applied electric voltage, a form of state is generally stable, in which the molecules lie essentially parallel to the carrier rafts, and thus the cholesteric helix axis is essentially perpendicular to the substrate plane. Forms of state that have such a molecular arrangement are mostly called Grandjean or planar textures in the literature. As these terms however on

on the one hand it stands for a theoretical ideal form of the corresponding texture, but on the other hand in practice it is often also used to indicate more or less deformed textures,

and therefore may give rise to misunderstandings, they are avoided in the present description. Instead, the direction of the helix axis is sometimes used to describe a texture.

In the case of the second of the structures of interest here, the helix axis lies essentially parallel to the carrier plates. This structure is located, due to the unfavorable adaptation conditions in the edge zones are energetically higher than those with a plate-normal helix axis. Therefore, at a small ratio of cell thickness d to cholesteric screw height P, this state is formed without an applied field again in the more stable texture with plate-normal helix axis. However, at large values for ^ (typically greater than 10), the influence of the edge zone is so small that the texture with plate-parallel helix axis is not restored over several days either. In these cases, even the question of greater stability for one of the two textures seems to be open. The optical properties for the two states are very different. The texture with plate-normal helix axis reflects either left- or right-rotating circularly polarized light with wavelengths X with the value n.P, where n is the average refractive index. For a higher reflection coefficient, it must also apply that d.^n is greater than the wavelength, under which. Ain is the anisotropy of the refractive index. The second, non-reflected circularly polarized component of the incident light passes through the texture essentially undisturbed. In front of an absorbing background, one sees for n.P in the visible wavelength range the typical cholesteric reflection colors.

The liquid crystal in a state with plate-parallel helix axis, on the other hand, allows light passes unreflected, during which, however, forward scattering occurs in a narrow angle range. In front of an absorbing background, the liquid crystal layer in this state therefore appears dark.

The two states show a clear contrast in front of a well-absorbing background and on good mirror-gloss surfaces. Also in transmission, a good contrast can be achieved when little dispersed light is passed through the aperture in such a way that scattered light in a state with a plate-parallel helix axis can no longer pass through the apertures. In this mode of operation, the reflection property of the state with the plate normal is not required. helix axis.

The two aforementioned defined textures and their optical properties were already described at the beginning of this century. An application of the thereby determined optical properties was, however, fraught with the disadvantage that it was not possible to simply transfer the two states into each other in both directions, as in the case of electric fields.

From US patent no. 3,642,348 it is known that a cholesteric liquid crystal which is in a state of rest in the Grandjean texture can be transformed into the focal conic texture by applying a direct voltage or a low-frequency alternating current field. After reducing or switching off the field, - the crystals relax back to their resting state, the Grandjean texture. This recovery can take place in fractions of seconds, but can also extend over hours. It can be accelerated by mechanical influences or by heating.

From German Offenlegungsschrift 25.38.212 it is known that the textural transformation of Grandjean in the focalconic state occurs under the influence of an electrohydrodynamic effect, which sets itself up in dielectric negative nematic material at sufficiently low frequencies.

From US patent no. 3,680,950 it is known to switch a liquid crystal with negative dielectric anisotropy through the orienting effect of a higher frequency electric alternating current field from focal conic to Grandjean state.

Underneath, all these effects, when you combine them into a vice cell that can be connected in both directions, have a significant disadvantage: as a result of the current flow, which induces the electro-hydrodynamic turbulences, the liquid crystal slowly but continuously splits. Its lifespan is short.

It must be pointed out that, by this interesting effect and those. The previously mentioned known effects relate to textural change within the cholesteric phase and not to fundamentally different phase change effects.

In the case of phase change effects, the liquid crystal only has a cholesteric structure in one of the two coupling states, while in the case of an applied holding field it is homeotropic nematic.

In German explanatory document no. 25.42.189, it is e.g. described such a cell, which contains a cholesteric liquid-crystal mixture which has an energetically stable focalconic structure when no electric field is applied, and which can be switched from this state into a homeotropic nematic structure,

in which it remains when a suitable containment field is imposed. After the holding field is switched off, the liquid crystal runs again

into the stable focalconic state.

Apart from the fact that this phase change effect is fundamentally different from the texture change considered here, phase change also has such significant disadvantages that it has not yet found any technical application, even though it has been known for a long time. In addition, a very high control voltage is required for phase change and for the maintenance of the non-stable state, a high maintenance voltage.

The task of the present invention is to provide a liquid-crystal cell which, through the orienting effect of electric fields, can be switched between two stable, optically different states, and in which no current needs to flow in the liquid crystal, so that well-insulating liquid- crystal material can be used to avoid degradation phenomena.

According to the invention, this is achieved in that a cell of the type mentioned at the outset, the liquid crystal exists in two optically different stable textures and has a dielectric anisotropy that is positive at frequencies below a threshold value, whereby the liquid crystal upon application of an alternating current with such a low frequency, assumes one of the two stable textures, and at frequencies above the threshold value is negative, whereby the liquid crystal when an alternating current with such a higher frequency is applied passes into the other stable state.

The cholesteric liquid crystal is preferably a mixture of nematic liquid crystals with cholesteric additives.

The two stable state forms in which the liquid crystal is located are the texture with a plate-parallel helix axis, which the liquid crystal assumes when a lower frequency voltage is applied, and the texture with a plate-perpendicular helix axis, which the liquid crystal passes through when a high frequency voltage.

The surface of the upper plate which faces the liquid crystal, i.e. the plate through which the incident light takes place, preferably has a homogeneous wall orientation. However, the desired electro-optical effects also occur in cells that have no homeotropic or hydride wall orientations.

The invention is described in the following through the embodiments of the drawings. Fig. 1 schematically shows a cell with a cholesteric liquid crystal with plate-parallel helix axis, Fig. 2 schematically shows a cell with a liquid crystal

with.plate normal helix axis,

Fig. 3 shows a schematic curve of the course of dielectric anisotropy depending on the frequency of an electric field applied to the liquid crystal,

Fig. 4 schematically shows a matrix indicator to be obtained

with the new effect, and

Fig. 5 schematically shows the signal form for controlling a matrix indicator according to fig. 4. Fig. 1 shows a schematic cross section through part of a liquid crystal cell. The cell normally consists of two carrier plates 1, 2 arranged at a distance from each other, between which a cholesteric liquid crystal layer 3' is arranged.

The upper carrier plate 1 is the one through which the incident light takes place, and on whose side the observer is in the case of a reflective display. The surface of the carrier plate 1 facing the liquid crystal is treated in such a way that it orients the adjacent liquid crystal molecules homogeneously. This so-called homogeneous wall orientation can be achieved with the usual methods, that means e.g. by rubbing, oblique steam application, etc.

The homogeneous wall orientation of the carrier plate 1 is not absolutely necessary for the function of the display part. However, it serves to achieve a better optical homogeneity, especially in the state in which the helix axis of the liquid crystal is vertical to the carrier plate.

Both carrier plates 1, 2 are on the sides facing the liquid crystal equipped with electrodes, through which the control takes place. These electrodes consist in a known manner of thin, mostly evaporated layers of indium oxide, etc.'.

As the display is due to the optical difference between the two textures of the cholesteric liquid crystal, the liquid crystal must be visible through the upper carrier plate 1. This means that the carrier plate must consist of glass, transparent plastic, etc. The upper electrode 4 must also be transparent.

The lower carrier plate must absorb light, which is generally achieved by the carrier plate also being translucent and an absorbing layer 6 being placed on its outer side. Of course, other configurations are also conceivable, e.g. absorbent design of the electrode 5 or of the carrier plate 2 itself.

The electrodes 4, 5 are connected to a control electronics, which for this description is only shown schematically by two alternating current sources 7, 8 with different frequencies and a switch 9. For a concrete and detailed design of the control electronics, reference is made to the extensive common literature which is known to the person skilled in the art.-

The liquid crystal layer 3 consists of a so-called two-frequency mixture as described in Appl. Phys. Easy. _41, 697(1982), which can be mixed with suitable chiral molecules, so that the two-frequency property is maintained on the one hand, and on the other

■ on the other hand, the relevant cholesteric properties are induced in the mixture, in this case the reflectivity in the visible area.

A particularly well-suited liquid-crystal mixture has e.g. following composition: the nematic two-frequency mixture consists of the following components in the indicated weight ratios:

This between -6°C and +79°C nematic effect has at 22°C a transition frequency fc from approx. 1.4 kHz. At frequencies less than fc the dielectric anisotropy is positive, for f greater than fc negative.

The following chiral additives (in weight%) are added to this nematic mixture to achieve a good visible color effect in the green spectral range.

With the texture shown in fig. 1, the liquid crystal has the helically twisted molecular arrangement typical of the cholesteric phase, under which the axis of the screw lies more or less parallel to the plate surfaces. This is indicated schematically in fig. 1 with a number of molecules projected onto the drawing plane.

If the switch 9 is briefly connected to the voltage source 7,

an impulse with a frequency of more than 1.4 kHz, preferably approx. 10 kHz on liquid crystal 3, whereupon this passes into the state with plate-normal helix axis. This condition is shown in fig. 2. By it. schematic representation in fig. 2, it concerns the same cell as in fig. 1. Only the liquid crystal 3 now has a different texture, which is distinguished by the fact that the designed screw windings are vertical with their axis on the carrier plates 1, 2. The molecules are thus oriented in this state essentially parallel to the plate surface.

The texture with plate-normal helix axis also remains unchanged without external energy supply for a long time. This is indicated by the fact that also in fig. 2, the switch 9 is open. If the switch 9 is switched so that the voltage source 9 is connected to the electrodes, i.e. a voltage pulse is supplied with a frequency of less than 1.4 kHz, that is preferably approx. 100 Hz, the liquid crystal reverts to its texture with plate-parallel helix axis.

The display cell thus has two truly stable states and can be switched from one state to another by applying alternating current pulses of different frequencies. In order to maintain the two states, no maintenance voltage is required. As the experiments show, the two states remain unchanged and stable for several weeks without applied voltage.

In experimental cells, the switching operations were carried out using right-angle signals of 60 Volt RMS. For a 10 um thick cell with cholesteric filling with a step height P of P=0.38 um, it led to switching times from approx. 250 ms for connection in the state with plate-normal helix axis and in approx. 50 ms for the switchback process. At lower voltages, the switching times were longer, and the texture changes hardly took place below certain threshold values. When imposing low frequencies, at approx. double voltage, ie at 120 V, the cholesteric nematic phase transition place. However, you are then already in the area with a high risk of rebirth. By varying the voltage and modifying the two-frequency mixes, improvements in the switching times should also be achieved.

The necessary optical contrast for a display consists in the fact that the liquid crystal in the texture with a plate-parallel helix axis is fairly well permeable to light and thus the plate on the back is visible, which looks dark due to its absorbent property. In the state with plate-normal helix axis, the mentioned frequency-selective reflection of the light then takes place. This means that only certain parts of the white light are scattered back and the liquid crystal is therefore seen as strongly coloured. The individual colors depend on the height of the cholesteric liquid crystal lens. As the walking height of most cholesteric liquid crystals is temperature dependent, the color changes easily with changing temperature. However, it is known to the person skilled in the art how this can be compensated for by corresponding mixing ratios of the components.

Figures 4 and 5 show the application of the new effect in a matrix display. In the section shown in fig. 4 of a matrix indicator must e.g. the surface Z2/S2 is brought into a state with a plate-parallel helix axis, and the surface Z2/S4 into a state with a plate-normal helix axis. All other elements must be retained unchanged in their condition.

The information to be displayed can be entered line by line. Underneath, high- and low-frequency voltages follow alternately. On the selected line Z2, for each frequency, there is always a voltage of two amplitude units, while the other lines remain without voltage.

There is always an amplitude unit on the slots. Tor the elements, which must remain unchanged, i.e. in the slots Sl and S3 they are in phase with the line voltage. When the state of an element is to be redefined, i.e. in slots S2 and S4, the voltage is in opposite phase to the corresponding line voltage. There are therefore three voltage units on the elements to be switched, while one unit is on all the others. The times during which a frequency remains on are determined by the fact that when three voltage units occur, a clear definition of the state occurs, while during one unit in the change of high and low frequency, both state possibilities remain unchanged. These timesTNF, TThF depend on the material and cell parameters and must be optimized for each individual case.

Claims (7)

1. Liquid crystal cell with a cholesteric liquid crystal between two carrier plates equipped with control electrodes, characterized in that the liquid crystal exists in two optically different stable textures and has a dielectric anisotropy, which at frequencies below a threshold value is positive, whereby the liquid crystal upon application of an alternating current with such a low frequency assumes one of the two stable textures, at frequencies above the threshold value is negative, whereby the liquid crystal upon application of an alternating voltage with a higher frequency assumes the other stable texture. 2. Liquid crystal cell according to claim 1, characterized in that the carrier plate located on the incident side of the light on its surface facing the liquid crystal has a property that orients the liquid crystal molecules homogeneously. 3. Liquid crystal cell according to krawl, characterized in that the carrier plate located on the incident side of the light on its surface facing the liquid crystal has a property that orients the liquid crystal molecules homeotropically. 4. Liquid crystal cell according to one of the preceding claims, characterized in that the liquid crystal in one of the two stable textures has a helix axis that lies parallel to the carrier plates and in the other stable texture a helix axis that is perpendicular to the carrier plates. 5. Liquid crystal cell according to claims 1-4, characterized in that the control electrodes on one carrier plate are divided into line conductors and on the other carrier plate into slot conductors and in this way form a matrix detector. 6. Liquid crystal cell according to claim 5, characterized in that the line and slot conductors are alternately supplied with a low frequency and a high frequency signal. 7. Liquid crystal cell according to claim 6, characterized in that for the reading of a line, a signal with a specific amplitude is present on the line conductor that is aimed at and on all other line conductors no signal, while at the same time all slot conductors are supplied with a signal if amplitude is half as large as that of the line signal and for the crossing points to be switched in phase with the line signal, and for all other crossing points in phase with the line signal.