JPS5913725B2 - How a liquid crystal display cell works - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a dynamic scattering liquid crystal display cell using a smectic liquid crystal with positive dielectric anisotropy.

A nematic liquid crystal cell is generally used as a dynamic scattering liquid crystal display cell.

This nematic liquid crystal display cell uses a nematic material with negative dielectric anisotropy, and when no energy is applied, it exhibits a homeotropic alignment and is transparent, but under an electric field, the liquid crystal molecules rotate due to the electric field. Dynamic scattering results in a cloudy state. In the present invention, a smectic material is used as the liquid crystal, and a smectic material having positive dielectric anisotropy due to the dielectric constant distribution is used. For example, the use of smectic liquid crystal in display cells was introduced in Japanese Patent Application Laid-open No. 47 (1973).
It is described in the publication No.-36886.

As described on page 2 of the same publication, smectic liquid crystals have hardly been used in the past. This is because the smectic liquid crystal has a high viscosity and its molecules do not easily move or rotate under the influence of an electric field. In the display cell described in the publication, dynamic scattering is generated by applying a strong electric field, and this state persists even after the electric field is removed, but the dynamic scattering is electrically eliminated. Since this is not possible, erasing is performed by heating the liquid crystal. In a normal nematic liquid crystal display cell, even if it is put into a dynamic scattering state by an electric field, it returns to its original transparent state when the electric field is removed, so it cannot maintain the display state, but display and erasure can be electrically controlled. is possible.

However, when the dynamic scattering state generated by the electric field is maintained even after the electric field is removed, as in the display cell described in the above publication, there is no known means to erase it electrically, so heating is used. There is no other way to delete it. The present invention is based on the discovery of a new phenomenon in smectic liquid crystals, and is based on the discovery of a new phenomenon in smectic liquid crystals, which enables display, or writing, and erasing to be performed entirely electrically in a liquid crystal display cell in which the display, or scattering state, continues even after the electric field is removed. This is what I did.

That is, the present inventor discovered that when a display cell made of a smectic liquid crystal having homeotropic alignment and positive dielectric anisotropy in a state in which no energy is applied is applied with an alternating potential above a certain threshold, liquid crystal molecules A spiral dynamic scattering is generated by the rotation of the alternating potential, and the scattering state is maintained even after the alternating potential is removed, and in this state, a second alternating potential is generated which differs in amplitude, frequency, or both from the first alternating potential. It was discovered that by supplying

Such a phenomenon was completely unknown in the past. The present invention utilizes this phenomenon to write (
The present invention provides a liquid crystal display cell that can maintain a dynamic scattering state for a long time even after the potential is removed, and can electrically control writing and erasing (transparent state) by supplying different alternating potentials. In the display cell of the present invention, when the alternating potential is removed, the dynamic scattering of the swirl pattern becomes a focal cone (FOc).
alcOnic) scattering mode, and the scattering state is maintained as it is, so the written state is preserved as is.

The living liquid crystal display cell of the present invention is equipped with a medium that induces homeotropic alignment, thereby further promoting the above-mentioned effects.

The medium inducing homeotropic alignment is dispersed in the smectic liquid crystal or coated as a layer on the inner surface of the cell.
Below, a dynamic scattering, smectic liquid crystal display cell embodying the present invention in a preferred manner is described.

The two glass plates 1, 2 shown in FIG. 1 are provided with transparent electrodes 3, 4, for example made of indium-tin oxide, as is customary in nematic display cells with dynamic scattering.
The surfaces of both electrode plates were then coated with Hexadecyltrimethlammonium oxide, a polar surfactant commonly used to provide homeotropic alignment in nematic cells.
It is desirable to coat with niumbrOmide).
This coating is carried out by repeatedly dipping the electrode surface into a solution of the surfactant. Next, the two electrode plates are fixed with an epoxy end seal 5, and the inner 6
is filled with 4-cyan-4'-n-octyl biphenyl, which is a smectic material (4) having positive dielectric anisotropy (ε=11.ε1-5).

The thickness of the resulting liquid crystal layer 6 is determined by the thickness of the end seal 5 and is typically approximately 20 μm thick. Selection of the appropriate thickness of the liquid crystal layer depends on several factors.

For example, the switching threshold voltage (Switchingthreth
(OldVOltage) and the switching speed both decrease as the thickness of the liquid crystal layer decreases. On the other hand, such a change has the disadvantage that the relative proportion of scattering decreases and the strength of the switching field increases. The latter serves to bring the strength of the switching field closer to the dielectric breakdown value of the liquid. It has thus been found that, for example, with the above-mentioned smectic material it is possible to switch cells with a layer thickness of 10 μm. However, the cell irreversibly breaks down if the voltage is too high. It has been found that cells with a layer thickness of 61 tm break down before reaching the switching jump. In an alternative cell structure, a polar surfactant suitable for liquid crystals, such as Versamide (trade name)-
A homeotropic arrangement is obtained by adding a low viscosity polyamide resin commercially available under 100%. It has been found that a suitable concentration of this surfactant is 0.1%. The advantage of adding a surfactant to the liquid crystal over adding it directly to the cell walls is that the cell can be made first. This allows the use of glass frit end seals. In the first described construction, an epoxy seal was used because the surfactant could not withstand the firing temperatures of typical glass frits. A third structure of cells does not use surfactants, but instead relies on a deposited layer to induce homeotropic alignment. In the past, tilted deposition was known to produce parallel and uniform alignment of liquid crystals. However, it is known that the deposition of suitable materials, such as silicon monoxide, silicon dioxide, alumina or titania, with incidence perpendicular to the surface of the plate results in homeotropic alignment. In the case of alumina or titania deposition, a 30° tilt to the plane also results in a homeotropic alignment. These deposited layers that provide homeotropic alignment can withstand typical glass frit firing temperatures and therefore
The epoxy end seals can be replaced with glass frit end seals if necessary. Typically, when one of these cells is first made, the liquid crystals are almost entirely homeotropically aligned, but there are many small scattering centers.

These are small conical domains (SmallfOcal-COni) localized in small defects on the two defining major surfaces.
This is thought to be due to Q formation (cdOmain).
They can be eliminated by applying a suitable low frequency alternating potential between the opposing electrodes 3 and 4. This state can be switched to a dynamic scattering state by application of a suitable low frequency alternating potential, but this is not a ready state since this switch is very slow and typically takes about an hour. Thereafter, when the cell conditioning is complete, the cell can be switched from homeotropic alignment to dynamic scattering in less than 1 second. The three curves on the left side of Figure 2 show the threshold voltage (ThreshOldVOltag) that results in the transition from homeotropic alignment to dynamic scattering.
The relationship between e) and frequency is shown.

Since this idle value is also a function of layer thickness, 59Itm
, 31 pm and 21 ttm thick layers are shown. It can be seen from this figure that the frequency used to promote the transition should be 100 Hz or less. The reason for this is that as the frequency increases, the leap voltage clearly increases rapidly to the asymptotic limit in the 1000 Hz region. The cell is ready by applying a potential equal to or greater than the alternating leap potential and holding it until the entire region between opposing portions of the electrodes has switched to the dynamic scattering state.

Dynamic scattering tends to nucleate at points along the circumference of the electrodes, and then gradually expands the area of scattering to cover the entire distance between the electrodes. Furthermore, scattering nuclei may initially be generated at localized points on the electrode rather than around the electrode, but this is thought to be caused by local defect points. The time required to spread the scattering over the entire area of the overlap depends at least on the shape and dimensions of the overlap.
It was found that in a cell whose area is 2×2c1n, complete switching occurs after about 45 minutes. The scattering of light through the cell is accompanied by turbulence that occurs in the molecular rotation in a spiral pattern with an average swirl diameter of approximately 0.4 mm. After the cell is fully conditioned, the applied voltage is uniformly and gradually removed (or reduced to a value below the switching voltage), typically over a period of about 1 to several seconds.
The aforementioned disordered (or random) scattering disappears and the cell returns to its transparent state. However, when the voltage is quickly removed by the operation of a common snap switch, the dynamic scattering of the vortex turbulence transforms into a static scattering state brought about by the formation of multiple focal conical small domains. The production display is apparently of infinite persistence, since in the absence of external stimuli its pyroconical domains have no tendency to convert into lamellar arrangements (or the reverse process). The transition from a dynamic scattering state to a static scattering state is generally accompanied by a slight decrease in scattering power. Therefore,
Dynamic scattering in a typical cell reduces the transmission of light normal to the cell by a factor of 16, but removing the voltage from dynamic scattering to replace static focal cone scattering reduces the transmission by a factor of 16. It was found that it can be reduced by a factor of 10. The cell was then inspected from time to time over a period of over 1000 hours and no change in scattering intensity was observed over the entire period.
Elimination of the pyroconic scattering region can be achieved by thermal cycling.

This requires heating the cell enough to eliminate the smectic phase of the liquid crystal and then cooling the cell material sufficiently to return to the smectic phase. The disadvantage of this method is that the cells need to be readjusted after erasing. Apart from this method, the focal cone scattering region can be eliminated by applying a suitable alternating potential between the electrodes.

The curve on the right side of FIG. 2 shows the relationship between threshold voltage and frequency that results in this erasure. This erasure threshold is also the curve on the left (
(display) and is a function of the thickness of the liquid crystal layer. Therefore, curves for three thicknesses (59 μm, 31 μm, and 21 μm) are shown. It can be seen from FIG. 2 that there is some overlap between the left and right threshold curves, especially for thin cells. Therefore, for thinner cells, the right side (erased)
There is a frequency range in which the same frequency can be used for the curve on the left (displayed). However, it can be seen that an increase in frequency generally lowers the cancellation threshold. However, - in general, frequencies that are too high for cancellation are undesirable. The reason for this is that at frequencies above a certain limit, for example in the 5 KHz range, erasure is complete and cells must be readjusted. Below this limit frequency, erasing is followed by a display process that typically lasts less than 1 second. It has been found that both the writing speed and the erasing speed are significantly dependent on voltage.

Therefore, when testing a cell with a liquid crystal layer approximately 20 μm thick, a 50 Hz 1200 V signal (R.m.s...
・The display time (root mean square) is approximately 80 msec, but
If the signal strength is halved, the display time will be 450 msees.
and rapidly increasing. In the case of erasing, the effect is even more remarkable, i.e. 1KHz 1100V (R.m.s...) in the same cell.
) signal, it can be erased in about 90 msec, but if the signal strength is halved, the erasing time increases dramatically to about 3.2 seconds. The aforementioned results regarding display and erase threshold voltages and response times were obtained from Optical Coatings Laboratory, Inc.
tingSLab0rat0riesInc. ) was obtained using a cell using transparent electrodes manufactured by

Surprisingly, it was found that when cells were fabricated using transparent electrodes manufactured by Balzers, their threshold switching characteristics were significantly different compared to cells using electrodes manufactured by 0CLI as described above. These characteristics are shown in FIG. 3, in which the solid line represents the cell characteristics using the 0CLI electrode and the broken line represents the cell characteristics using the Balzers electrode. The reason for the difference between the two is not well understood, but it may be due to electrochemical differences at the liquid crystal electrode interface or surface texture (Tex
It is believed that this is at least partially due to the difference in . If so, the properties can be significantly altered by the choice of method of obtaining the homeotropic sequence. The foregoing results were obtained in a cell providing a homeotropic arrangement with a hexadecyl trimethyl ammonium bromide surfactant. Eutectic mixture of octyl and decyl biphenyl (58 (weight)% octyl: 42
It has been found that replacing the octyl biphenyl liquid crystal material (% decyl by weight) widens the temperature range of the smectic (A) phase from 5°C to 40°C, but this is accompanied by an increase in the switching threshold voltage. These smectic cells are called cholesteric cells (ChOlest-Eric
-Nematic) can be modified to display color contrast in the same way that cells that rely on phase changes are modified.

This improvement consists of dispersing the appropriate pleochroic dye throughout the liquid, relying on Guest-Host interactions. In general, smecticks can be expected to be good hosts for pleochroic staining, because typical smecticks have higher order parameters (0.7) than those of typical nematics or cholesterics (~0.7). .8), and therefore the dye molecules are arranged well and the contrast is improved.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of a liquid crystal display cell according to the invention; FIGS. 2 and 3 are graphs showing the relationship between switching threshold voltage (expressed as the root mean square) and frequency. 1, 2... Glass plate, 3, 4... Transparent electrode, 5... Edge seal, 6... Liquid crystal layer.

Claims (1)

[Scope of Claims] 1. A dynamic scattering liquid crystal display cell comprising opposed internal electrodes, filled with a smectic liquid having positive dielectric anisotropy, and equipped with a medium that induces homeotropic alignment in the smectic liquid. In the method of operation, dynamic scattering is obtained by the creation of a vortex state of smectic liquid crystal molecules, and writing and erasing of the display is achieved by applying first and second alternating potentials between the electrodes of the cell, respectively. Dynamic scattering is controlled entirely electrically, and the characteristics (at least one of voltage and frequency) of the second alternating potential are different from the characteristics of the first alternating potential. How a dynamic scattering liquid crystal display cell works. 2. The smectic liquid in the region between the opposing electrodes is driven into the dynamic scattering state (writing state) by the application of a first alternating potential, then converted into the focused conical scattering state by the removal of said first alternating potential, and then 2. A method as claimed in claim 1, characterized in that the smectic liquid is returned to a substantially scatter-free state (erased state) by applying a second alternating potential. 3. The method according to claim 1 or 2, wherein the second alternating potential has a higher frequency than the first alternating potential. 4. The method according to any one of claims 1 to 3, wherein the medium for inducing homeotropic alignment is a polar surfactant. 5. The method of claim 4, wherein the polar surfactant is dispersed throughout the smectic liquid. 6. The method according to claim 4 or 5, wherein the polar surfactant is present as a layer covering the inner main surface of the cell.