GB1569690A - Crossover smectic cell - Google Patents

Description

(54) CROSSOVER SMECTIC CELL (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company, of 190 Strand, London WC2R lDU England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to liquid crystal display cells, and in particular to such cells filled with a smectic material.

According to the present invention there is provided an internally electroded liquid-crystal display cell having a layer of a smectic material that exhibits a crossover frequency beneath which the material exhibits positive dielectric anisotropy and above which it exhibits negative dielectric anisotropy, which layer is sandwiched between two electroded plates having electrodes that overlap at least in part, at least one of which plates is transparent.

In a typical liquid crystal display cell filled with a nematic material the application of an electric potential between a pair of electrodes is used to cause the portions of nematic material lying between the electrodes to switch from a first state to a second state presenting a different appearance from that of the first state.

When the applied signal is removed, the switched portions revert back into the first state again. In the case of a liquid crystal display cell filled with a smectic material there is however the possibility of a storage facility being provided in which the portions switched to the second state remain in the second state after removal'of the applied signal or relax into a third state also presenting a different appearance from that of the first state. For such cells exhibiting this storage facility a way needs to be found of switching the switched portions back to the first state again.

In the specifications accompanying our earlier Patent Applications Nos.

46819/77, 46820/77 and 46821/77 Serial No. 1,569,686, 1,569,687, 1,569,688, to which attention is directed, there are described three types of smectic display cell in which switching from a first state to a second is effected electrically by the application of an electric potential between the electrodes of the display cell, and in which the cell is switched back to the first state again by thermal cycling. The present invention discloses how with a suitable choice of smectic material electrical switching in both directions may be achieved.

In the published literature certain nematic materials have been observed to have the property of exhibiting a cross-over frequency effect in which the material exhibits positive dielectric anisotropy at low frequencies beneath the cross-over frequency, and negative dielectric anisotropy at higher frequencies above the cross-over frequency. In the case of a nematic cell exhibiting no storage facility switching in both directions is not required since the cell automatically reverts to the first state on removal of the switching signal. We have found however that in some of these materials the cross-over frequency effect is maintained through a phase change from nematic to smectic.In the smectic phase its positive dielectric anisotropy can be made use of electrically to switch the cell in one-direction while use of its negative dielectric anisotropy is later made electrically to switch the cell back to its original state. An example of a material exhibiting this cross-over frequency effect in a smectic phase given by 4-n-pentylphenyl 2'-chloro-4'-(6'-n- hexyl-2-naphthoyloxy) benzoate

a monotropic liquid crystal with the following phase transition temperatures, C-N, 68.60C; [SA-N, 53.50C1; N-I, 178.90C. The following table shows how, for a 20 micron thick liquid crystal layer, the cross-over frequency varied with temperature in a region near the SA-N phase transition temperature.The table also shows the voltage used in each instance to effect switching.

TABLE Temperature Cross-Over Switching Frequency Voltage Applied 77"C 29 KHz 22V 72" 20 KHz 22V 67" 13KHz 22V 65" 11KHz 22V 63" 9.8KHz 18V N 61" 7.2KHz 22V 59" 6.2KHz 21.1V 57" 5.5 KHz 20.8 55 4.8 KHz 20.2 '54.5 4.6 KHz 20.4 54.0 4.6KHz 20.1 SA N 53.5 4.2 KHz 78V 53.2 4.2KHz 112V 52.0 4.2KHz 182V SA 50.0 3.9 KHz 204V There follows a description of a smectic liquid crystal display cell embodying the invention in a preferred form. The description relates to the accompanying drawing depicting a schematic perspective view of the cell.

Two glass sheets 1 2 are secured together with a perimeter seal 3 to form an envelope for a layer 5 of liquid crystal medium to be hermetically sealed within the cell. The cell is filled via an aperture formed by an interruption in the perimeter of the seal 3, and, after the cell has been filled, this aperture is sealed off with a plug 4 for instance of indium. Alternatively solder is used, the aperture having been previously metallised.

Before they are secured together the inwardly facing surfaces of the two sheets are provided with transparent electrodes (not shown) of appropriate layout for the required display to enable an electric field to be applied across the thickness of at least selected portions of the liquid crystal layer. For this purpose portions of the electrodes extend beyond the region of the seal to permit external connection. Optionally, in the region within the perimeter seal the electroded surfaces are covered with an insulating layer so that the cell is not damaged by the flow of direct current that might otherwise occur if the electrodes were accidentally connected with a source of unidirectional electric potential. This layer may be for instance a silica layer deposited by reacting silane with oxygen at atmospheric pressure.It is preferred to make the perimeter seal 3 by fusing glass frit because, with the appropriate choice of glass frit the liquid crystal filling is less liable to contamination by material leached from the seal than is generally the case of seals made of certain other materials such as epoxy resins. Typically the depth of frit is chosen to give a liquid crystal layer thickness of about 20 microns.

Whilst maintaining the completed cell at a temperature of 52"C it can be switched into a homeotropically aligned state by the application of an alternating electric potential of about 210 volts at a frequency beneath 4.2 KHz. In this state the switched portions of the cell appear black between crossed polarisers. These portions remain in this state after removal of the alternating signal, but are converted to a focal-conic light scattering state by the re-establishment of the signal at a frequency above 4.2 KHz. If a pleochroic dye is dispersed through the liquid crystal layer the appearance of the homeotropic and focal conic states are distinguishable without the aid of polarisers.In the homeotropic state the display portions appears substantially clear because the pleochroic molecules are aligned substantially homeotropically by their smectic host molecules. In the focal conic state the display portions appear coloured and scattering. In order to make a red display a pleochroic azo dye may be used. For certain applications where a blue display is desired the azo dye may be replaced with an anthraquinone dye such as the blue dichroic dye 1 -(4'-butyloxyaniline)-4-hydroxyanthraquinone. A typical filling used approximately 1.3 ó of this anthraquinone together with 0.05% of Waxoline Yellow A. The Waxoline Yellow, which is an isotropic dye, is added to compensate the residual blue of the display in its homeotropic state caused by the ordering within the smectic host being less than 100%.The presence of this yellow converts this residual blue to a substantially neutral grey. The optical density of the grey is relatively slight so that by eye its appearance is scarcely distinguishable from transparent.

It will be noted that the basic construction of the above described cell is distinguished from those described in the three specifications previously referred to by the absence of any surface treatment designed to produce specific surface induced alignment states. It is to be understood however that a smectic material exhibiting the cross-over frequency effect can also be used with cells that do have those sorts of surface alignment inducing surface treatments.

In the case of a cell having a homeotropic surface alignment inducing surface treatment such as described in the specification of Patent Application No. 46820/77 (Serial No. 1,569,687) the surface alignment treatment confers the possibility of switching the whole cell into homeotropic alignment by thermal cycling, and not merely those portions lying between opposed electrodes. In the case of a cell having a parallel pseudo-homogeneous surface alignment inducing surface treatment, such as described in the specification of Patent Application No.

46821/77 (Serial No. 1,569,688), the surface alignment provides the possibility of setting the cell into a different alignment state.

Parallel pseudo-homogeneous alignment may be provided by the surface treatment consisting of an oblique evaporation of a material that will produce parallel homogeneous alignment of a nematic phase. Typically silicon monoxide is evaporated at an angle of about 25 to the surface of the substrate. At this angle the evaporation produces homogeneous alignment of the nematic with substantially zero tilt angle. If a cell with this alignment treatment is thermally cycled in the absence of an applied electric field, and this cycling takes the cell into the nematic phase and then back into the smectic phase again. Then superficially there is not much change in the liquid crystal alignment on passing through the phase change from nematic to smectic.However close examination of the alignment reveals that in the smectic phase the material has in fact assumed a focal-conic state with relatively long slender cones that, instead of being randomly oriented, are aligned in a preferred direction. This is why this smectic alignment has been termed parallel pseudo-homogeneous. Typically the aspect ratio of the aligned cones of the pseudo-homogeneous state is approximately 10 to 1.

The parallel pseudo-homogeneous state is produced by cooling into the smectic phase a nematic phase existing in parallel homogeneous alignment, but is not normally produced by switching within the smectic phase. Therefore with a smectic filling exhibiting the cross-over frequency effect the display can be switched from parallel pseudo-homogeneous to homeotropic by a signal beneath the cross-over frequency, but it will be switched to scattering non-aligned focalconic alignment by a signal above the cross-over frequency. If the cell incorporates a pleochroic dye with the smectic filling the appearance of these three states are respectively substantially coloured non-scattering, uncoloured non-scattering, and coloured scattering. Alternatively in the absence of a pleochroic dye, by choice of a suitable liquid crystal layer thickness having regard to its birefringence, the three states will, between crossed polarisers appear respectively clear, black, and scattering. A polariser may also be used in conjunction with the pleochroic dye oriented so as to maximise the depth of colour presented by the cell in its coloured non-scattering state.

Since the cell is switched into the parallel pseudo-homogeneous state by the action of heat, selective switching into this state can be achieved by the use of localised heating, such as the provided by the intensity modulation of an imaged laser beam as it is scanned over the surface of the cell. For this purpose the wavelength of the laser is chosen so that it is absorbed either by the liquid crystal layer and its added dye, or by other material added to the layer, or by material adjacent that layer such as one of the electrode layers.

In view of the claims of Applications Nos. 46819/77 and 46820/77 (Serial Nos.

1,569,686 and 1,569,687) we make herein no claim to any internally electroded liquid crystal display cell having a layer of smectic material that exhibits a crossover frequency beneath which the material exhibits positive dielectric anisotropy and above which it exhibits negative dielectric anisotropy which layer is sandwiched between two electroded plates having electrodes that overlap at least in part, at least one of which plates is transparent, wherein the surfaces of the plates are such that, when the layer is taken into a smectic phase from a less-ordered non-smectic phase by cooling in the absence of an applied electric field, the layer is caused either to assume parallel non-homeotropic alignment with too large a tilt angle for the formation of focal-conic domains observable by optical microscopy in polarised light, or to assume substantially homeotropic alignment.

Subject to the foregoing disclaimer, WHAT WE CLAIM IS: 1. An internally electroded liquid crystal display cell having a layer of smectic material that exhibits a cross-over frequency beneath which the material exhibits positive dielectric anisotropy and above which it exhibits negative dielectric anisotropy, which layer is sandwiched between two electroded plates having electrodes that overlap at least in part, at least one of which plates is transparent.

2. A display cell as claimed in claim 1 wherein a pleochroic dye is dispersed through the layer of smectic material.

3. A display cell as claimed in claim 1 or 2 wherein the surfaces of the plates are such that when the layer is taken into a smectic phase from a less-ordered nonsmectic phase by cooling in the absence of an applied electric field the layer is caused to assume 'parallel pseudo-homogeneous' (as hereinbefore defined) alignment.

4. A display cell as claimed in claim 3 wherein said surfaces of the plates that cause said assumption of parallel pseudo-homogeneous alignment are provided by material deposited by oblique evaporation.

5. A display cell as claimed in claim 4 wherein said material deposited by oblique evaporation is silicon monoxide.

6. A display cell as claimed in claim 4 or 5 wherein said oblique evaporation is performed at an angle providing parallel pseudo-homogeneous alignment with substantially zero tilt angle.

7. A display cell as claimed in any preceding claim wherein the layer of smectic material consists of or includes 4-n-pentylphenyl 2'-chloro-4'-(6-n-hexyl-2-naphthoyloxy) benzoate.

8. A display cell as claimed in any preceding claim wherein both plates of the cell are transparent and the liquid crystal layer is located between crossed polarisers.

9. An internally electroded liquid crystal display cell substantially as hereinbefore described with reference to the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. oriented so as to maximise the depth of colour presented by the cell in its coloured non-scattering state. Since the cell is switched into the parallel pseudo-homogeneous state by the action of heat, selective switching into this state can be achieved by the use of localised heating, such as the provided by the intensity modulation of an imaged laser beam as it is scanned over the surface of the cell. For this purpose the wavelength of the laser is chosen so that it is absorbed either by the liquid crystal layer and its added dye, or by other material added to the layer, or by material adjacent that layer such as one of the electrode layers. In view of the claims of Applications Nos. 46819/77 and 46820/77 (Serial Nos. 1,569,686 and 1,569,687) we make herein no claim to any internally electroded liquid crystal display cell having a layer of smectic material that exhibits a crossover frequency beneath which the material exhibits positive dielectric anisotropy and above which it exhibits negative dielectric anisotropy which layer is sandwiched between two electroded plates having electrodes that overlap at least in part, at least one of which plates is transparent, wherein the surfaces of the plates are such that, when the layer is taken into a smectic phase from a less-ordered non-smectic phase by cooling in the absence of an applied electric field, the layer is caused either to assume parallel non-homeotropic alignment with too large a tilt angle for the formation of focal-conic domains observable by optical microscopy in polarised light, or to assume substantially homeotropic alignment. Subject to the foregoing disclaimer, WHAT WE CLAIM IS:

1. An internally electroded liquid crystal display cell having a layer of smectic material that exhibits a cross-over frequency beneath which the material exhibits positive dielectric anisotropy and above which it exhibits negative dielectric anisotropy, which layer is sandwiched between two electroded plates having electrodes that overlap at least in part, at least one of which plates is transparent.

2. A display cell as claimed in claim 1 wherein a pleochroic dye is dispersed through the layer of smectic material.

3. A display cell as claimed in claim 1 or 2 wherein the surfaces of the plates are such that when the layer is taken into a smectic phase from a less-ordered nonsmectic phase by cooling in the absence of an applied electric field the layer is caused to assume 'parallel pseudo-homogeneous' (as hereinbefore defined) alignment.

4. A display cell as claimed in claim 3 wherein said surfaces of the plates that cause said assumption of parallel pseudo-homogeneous alignment are provided by material deposited by oblique evaporation.

5. A display cell as claimed in claim 4 wherein said material deposited by oblique evaporation is silicon monoxide.

6. A display cell as claimed in claim 4 or 5 wherein said oblique evaporation is performed at an angle providing parallel pseudo-homogeneous alignment with substantially zero tilt angle.

7. A display cell as claimed in any preceding claim wherein the layer of smectic material consists of or includes 4-n-pentylphenyl 2'-chloro-4'-(6-n-hexyl-2-naphthoyloxy) benzoate.

8. A display cell as claimed in any preceding claim wherein both plates of the cell are transparent and the liquid crystal layer is located between crossed polarisers.

9. An internally electroded liquid crystal display cell substantially as hereinbefore described with reference to the accompanying drawing.