GB2141859A

GB2141859A - A display device

Info

Publication number: GB2141859A
Application number: GB08414347A
Authority: GB
Inventors: Colin David Rickson
Original assignee: ADCO IND COMPONENTS
Current assignee: ADCO IND COMPONENTS
Priority date: 1983-06-20
Filing date: 1984-06-05
Publication date: 1985-01-03
Also published as: GB8414347D0

Abstract

A display device (1) comprises a layer (2) of a thermochromic liquid crystal composition, a resistive layer (7) electrically energisable to heat at least part of the layer (2) of thermochromic liquid crystal composition so that the latter undergoes a reversible phase change accompanied by a reversible change in optical properties and a sheet (11) of metallic material, e.g. aluminium foil, acting as a heat sink. The resistive layer (7) may comprise a plurality of isolated resistive strips for enabling selective parts of the display to be repeatedly switched on and off to provide a display with continually changing information. <IMAGE>

Description

SPECIFICATION A display device This invention relates to a display device incorporating a thermochromic liquid crystal composition layer.

Thermochromic liquid crystal compositions are well known and heretofore have been employed in a wide range of temperature sensing applications, e.g.

in thermometry. These known applications make use of the fact that a thermochromic liquid crystal composition can exist in a smectic phase, a cholesteric phase or an isotropic phase depending upon the temperature of the composition.

When in the smectic phase, a thermochromic liquid crystal composition is virtually transparent and reflects virtually no light. On transition from the smectic phase to the cholesteric phase the thermochromic liquid crystal composition undergoes a colour play, e.g. from red to blue. The temperature over which this colour play takes place is known as the colour play temperature range. At highertemperatures the thermochromic liquid crystal composition undergoes a transition from the cholesteric phase to the isotropic phase in which latter phase the composition reflects virtually no light. The temperature at which the liquid crystal composition undergoes the transition from the cholesteric phase to the isotropic phase is known as the 'clearing point'.

Display devices incorporating layers of thermochromic liquid crystal compositions are already known. For example U.S. Patent Specification No.

3,617,374 discloses a display device comprising an opaque, electrically non-conductive layer having, in contact with its upper surface, a layer of encapsulated cholesteric liquid crystals and, in contact with its lower surface, a layer of electrically conductive ink. In normal ambienttemperatures,the encapsulated cholesteric liquid crystals are transparent and no information is displayed. However, electrical energisation of the electrically conductive ink generates heat which produces reversible colour changes in the encapsulated liquid crystals and results in display of information when the device is viewed from above.However this known display device is not suitable where it is desired to have a rapid presentation and removal of information, since several seconds may elapse from the time that the electrical energisation of the electrically conductive ink layer ceases and the time that the temperature of the encapsulated cholesteric liquid crystal falls sufficientlyforthe latterto revert to their original colour or state. Accordingly the display fades, accompanied by a colour play, for a period after cessation of the electrical energisation of the ink layer. Furthermore it is difficult to control the amount of heat supplied by the electrically conductive ink and consequently the colour of the display may vary over a period of time.

Another known display device incorporating a layer of thermchromic liquid crystal composition is shown in U.S. Patent Specification No. 3,410,999. In this known display device a layer of cholesteric liquid crystalline material undergoes a reversible colour change in response to temperature changes generated by electrical energisation of electrically conductive strips. This known display device is provided with a relatively massive heat sink over which a liquid, e.g. water, flows to maintain the display device at a desired substantially constant temperature and to rapidly cool the cholesteric liquid crystalline material after cessation of the electrical energisation of the conductive strips.Although this known display device enables the rapid presentation of information on, and removal of information from, the display, this is only achieved with the aid of a relatively massive (e.g. 2 cm thick), liquid-cooled heat sink.

The present invention seeks to provide a display device incorporating a thermochromic liquid crystal composition layer responsive to variations in heat for presenting or removing display of information, which display device avoids at least some of the disadvantages of known devices.

According to the present invention there is provided a display device comprising a display layer of a thermochromic liquid crystal composition, resistive means for generation of heat upon electrical energisation to modify the temperature of at least a part of the layer of thermochromic liquid crystal composition so that the latter undergoes a reversible phase change accompanied by a reversible change in optical properties and a sheet of metallic material acting as a heat sink for dissipating heat from the thermochromic liquid crystal composition after cessation of energisation of the resistive means to cause the thermochromic liquid crystal composition to revert to its original phase.

The thermochromic liquid crystal composition of the display layer is thus arranged to undergo a phase transition between two phases, in one of which phases the liquid crystal composition reflects incident light and displays thermochromism and in the other of which phases the liquid crystal composition is substantially transparent and reflects little or no incident light.

Preferably the sheet of metallic material acting as a heat sink is made of aluminium. The aluminium sheet material is conveniently less than 3 mm thick and is typically much thinner, e.g. in the form of a foil.

Conveniently the resistive means is applied as a layer over the sheet of metallic material acting as a heat sink via the intermediary of a thin, electrically non-conductive material. Any suitable known type of low power resistive means, for example electrically conductive inks, may be employed.

In the case where the resistive means is opaque, e.g. black as in the case of a 'carbon ink', the thermochromic liquid crystal composition may be applied directly to the resistive means. Alternatively, however, the thermochromic liquid crystal composition is applied to an opaque, preferably black, substrate which itself is applied over the resistive means.

A pattern-defining layer acting as a screen is preferably arranged over the layer of the thermochromic liquid crystal composition to define, for example, an alphanumeric display. Conveniently the pattern-defining layer may be applied as an ink, e.g.

a black ink. Atransparent protective covering layer may be applied over the pattern-defining layer.

In the case where the display device includes a pattern-defining layer acting as a screen, the layer of the thermochromic liquid crystal composition need only be applied beneath those regions of the patterndefining layer which do not block the passage of light. Thus the liquid crystal composition layer may comprise a number of regions isolated from each other.

The resistive means may also comprise a number of separately energisable regions for heating separate parts of the layer of the thermochromic liquid crystal composition. In this manner the controlled energisation of the different energisable regions enables the information displayed to be altered.

The thermochromic liquid crystal composition may be selected so as to exist at ambient temperatures, or normal operating temperatures, either in a cholesteric phase or a smectic phase.

If the thermochromic liquid crystal composition exists in the cholesteric phase at normal ambient temperatures, the composition should be selected to display substantially the same colour over a wide range of ambient temperatures and to have a clearing point at a temperature, e.g. 40"C, well above normally experienced ambient temperaturess. In this case the colour of the pattern-defining layer should be matched to the colour exhibited by the thermochromic liquid crystal composition when in its cholesteric phase. In order to display information on the display device, energy is supplied to the resistive means to heat the thermochromic liquid crystal composition to a temperature above its clearing point.As the composition clears, the opaque, e.g. black, layer of the substrate (or resistive means) can be viewed through non-shielding parts of the pattern-defining layer. On cessation of the supply of energy to the resistive means, the metal layer acting as a heat sink rapidly dissipates heat from the thermochromic liquid crystal composition so that the temperature of the latter is rapidly brought below the clearing point. The thermochromic liquid crystal composition may comprise a mixture of liquid crystal compounds. For example, the composition may comprise a mixture of chiral nematic liquid crystal compounds mixed in proportions to adjust the clearing point of the composition and a nematic liquid crystal compound mixed in a proportion to adjust the colour of the composition in its cholesteric phase.Typically the chiral nematic liquid crystal compounds comprise TM74 and TM75 (available from BDH Chemicals Limited), and the nematic liquid crystal compound comprises ME35 (also available from BDH Chemicals Limited).

Methods of achieving mixtures with low temperature dependence on colour are described in an article by Dr. D.G. McDonnell entitled 'Thermochromic Liquid Crystals' published by BDH Chemicals Limited.

If the thermochromic liquid crystal composition exists in the smectic phase at normal ambient temperatures, the colour of the pattern-defining layer should be matched to the colour of the opaque substrate layer (or resistive means if this acts as the substrate layer). Normally the opaque colour will be black. In order to display information on the display device, electrical energy is supplied to the resistive means to cause the thermochromic liquid crystal composition to undergo a reversible phase change into its cholesteric phase. As a result of this phase change, the liquid crystal composition changes from being substantially transparent to exhibiting a colour (which is neither black nor colourless or transparent), e.g. blue.On cessation of the supply of electrical energy to the resistive means, the metal layer acting as a heat sink rapidly dissipates heat from the thermochromic liquid crystal composition so that the latter is rapidly transformed back into its smectic phase in which it is substantially transparent.

In the case where the thermochromic liquid crystal composition exists in the smectic phase at normal ambient temperatures (e.g. from 0 C to 300C), it is advantageous to control the temperature to which the liquid crystal composition is heated by the resistive means, e.g. to prevent, in the extreme case, the liquid crystal composition being heated past its clearing point. Even if the resistive means does not supply sufficient energy to heat the liquid crystal composition past its clearing point, it is still advantageous to control the temperature of the liquid crystal composition while the latter is in its cholesteric phase since the colour displayed by the composition whilst in the cholesteric phase is temperature dependent.Thus in order obtain a display whose colour does not change, it is important to be able to control the temperature of the thermochromic liquid crystal composition, whilst the latter is in its cholesteric phase, to within a predetermined temperature range of a few degrees, e.g. from 35"C 45"C. To this end the display device may include temperature sensing means connected to control means for controlling the electrical energisation of the resistive means. The temperature sensing means may comprise, for example, an element having a positive temperature coefficient of resistance.Preferably, however, the temperature sensing means comprises a heat-sensitive capacitive element having a pair of spaced apart electrodes, e.g. of aluminium, and a dielectric material therebetween comprising a thermochromic liquid crystal composition which undergoes a reversible, temperature dependent phase transition between its isotropic phase (or state) and its nematic or cholesteric phase (or state). A capacitive element of this kind is described in co-pending unpublished U.K. Patent Application No. 8307257 and makes use of the phenomenon that the dielectric constant of the liquid crystal composition is markedly different when the liquid crystal composition is in its isotropic state compared with when the liquid crystal composition is in its nematic or cholesteric state. The capacitive element thus acts as a temperature switch exhibiting different capacitance values dependent on whether the temperature of the liquid crystal composition is above or below the clearing point Ideally the sheet of metallic material acting as a heat sink constitutes one of the electrodes of the capacitive element.

Although the temperature to which the display layer of thermochromic liquid crystal composition is heated by the resistive means is not detected directly by the capacitive element, its temperature will, at least approximately be related to the temperature of the capacitive element which is heated by the resistive means.

The thermochromic liquid crystal composition of the display layer (and of the capacitive element if provided) is suitably produced as a liquid crystal ink with the thermochromic liquid crystal composition protected in a polymer by microencapsulation or dispersion.

Embodiments of the invention will now be discribed, by way of example, with reference to the accompanying drawing, in which: Figure 1 shows a schematic sectional view of one embodiment of a display device according to the invention, and Figure 2 shows a schematic sectional view of another embodiment of a display device according to the invention.

In Figure 1, reference numeral 1 designates one embodiment of a display device according to the invention. The display device 1 comprises a layer 2 of a thermochromic liquid crystal composition coated on an opaque, preferably black substrate 3 of plastics material. The thermochromic liquid crystal composition exists in a substantiallytransparent smectic phase at normal ambient temperatures (e.g.

from 0 C to 300C) and is prepared as an ink with the liquid crystal composition protected in a polymer by microencapsulation. A screen layer 4 having a colour, e.g. black, which matches the colour of the substrate 3 is applied over the layer 3. Suitably the screen layer 4 is applied as an ink with patterndefining apertures 5 in the layer 4. A protective, transparent top layer 6, e.g. a polymer film, is provided as a top layer of the devicel.

Beneath the substrate 3 is a low power electrically conductive, resistive layer 7, e.g. in the form of an electrically conductive ink, to the lower surface of which is applied a pair of spaced apart electrically conductive metal terminal strips 8 and 9. Athin, electrically insulating layer 10 is applied to cover the strips 8 and 9 and the part of the resistive layer 7 not covered by the strips 8 and 9. Against the layer 10 there is applied a thin sheet 11 of aluminium acting as a heat sink for the display device 1. The aluminium sheet 11 is preferably less than 3mm thick and is typically in the form of a foil.

In the embodiment shown in Figure 1, part of the aluminium sheet 11 comprises an electrode of a heat-sensitive capacitor generally designated 12.

The capacitor 12 further comprises another electrode 13 spaced from the sheet 11 by means of a layer 14 of dielectric material in the form of a thermochromic liquid crystal composition which undergoes a reversible, temperature-dependent phase transition between its isotropic state and its nematic or cholesteric state. Under normal ambient temperature conditions the thermochromic liquid crystal composition of the layer 14 exists in its nematic or cholesteric phase or state, the composition being protected in a polymer by microencapsulation.

The electrode 13 and the sheet 11, constituting the other electrode of the capacitor 12, are connected to an electronic control circuit 15. The control circuit is also connected to the terminal strips 8 and 9.

In use of the display device 1, no information is displayed at normal ambient temperatures when the resistive layer 7 is not energised. The reason for this is that the thermochromic liquid crystal composition of the layer 2 is in its smectic phase and hence the layer is substantially transparent. Thus the opaque substrate 3 is viewed through the apertures 5, the colour of the substrate 3 being the same, e.g. black, as that of the screen layer 4.

If electrical energy is now supplied, under the control of switching means (not shown), from the control circuit 15, via the terminal strips 8 and 9 to the resistive layer 7, the latter generates heat which in turn heats the thermochromic liquid crystal composition of the layer 2. The liquid crystal composition is rapidly heated so that it undergoes a reversible phase change as its temperature passes through a predetermined temperature (e.g. 30"C) which exceedsthe upper temperature of the normal ambient temperature range. The liquid crystal composition of the layer 2 is designed to be heated to a temperature (e.g. 40"C) which lies within a temperature range (e.g. 35"C - 45"C) at which the colour display is substantially constant.The capacitor 12 serves to maintain the liquid crystal composition of the layer 2 within the desired temperature range by sensing the temperature of the aluminium sheet 11. This sensed temperature is a function of the temperature of the layer 2 and, if the sheet 11 becomes too hot, the capacitance of the capacitor 12 changes as the liquid crystal composition of the layer 14 undergoes a phase transition from its nematic or cholesteric state to its isotropic state. The sudden change in value of the capacitor 12 is sensed in the control circuit 15 and energisation of the resistive layer 7 is ceased. As the temperature of the layer 2 falls as a result of having no heat supplied thereto from the deenergised resistive layer 7, the temperature of the layer 14 of the capacitor 12 also falls.As soon as the liquid crystal composition of the layer 14 passes back into its nematic or cholesteric state, the capacitance of the capacitor changes back to its previous value. This sudden change in value of the capacitor 12 is sensed by the control circuit 15 and the resistive layer 7 is re-energised so that the layer 2 is again heated. In this manner, by means of an on/off control of the electrical energisation of the resistive layer 7, the temperature of the layer 2 is maintained within the predetermined temperature range (e.g. 35"C 45"C) at which the thermochromic liquid crystal composition exhibits a colour which contrasts with the surrounds.

When it is desired to turn 'off' the display of information, the resistive layer 7is de-energised and the heat of the layer 2 is rapidly dissipated via the aluminium sheet 11.

Figure 2 shows another embodiment of a display device according to the invention generally designated by the reference numeral 20. The display device 20 is similar in many respects to the display device 1 shown in Figure 1 and, where possibie, similar parts of the two devices have been designated with the same reference numerals.

The display device 20 differs from the display devicel in that it is not provided with a capacitor (12) and in that it has a display layer 21 comprising a thermochromic liquid crystal composition which exists in a cholesteric phase at normal operating temperatures and is prepared as an ink with the liquid crystal composition protected in a polymer by microencapsulation. The colour e.g. green, displayed by the thermochromic liquid crystal composition at normal operating temperatures (e.g. at least within the temperature range 0 C to 300C) is matched to the colou r of the screen layer 4.

In use of the display device 20, no information is displayed when no electrical energy is supplied to the resistive layer 7. Howeverwhen electrical energy is supplied, under the control of switching means (not shown), from the control means 15, via the terminal strips 8 and 9, to the resistive layer 7, the latter generates heat which in turn heats the display layer 21. The thermochromic liquid crystal composition ofthe layer 21 thus undergoes a reversible phase transition from the cholesteric phase to the isotropic phase, the composition clearing and becoming transparent as the temperature of the composition passes through the clearing point (e.g.

30"C). When the liquid crystal composition exists in its transparent, isotropic phase, the opaque, e.g.

black, substrate 3 beneath the apertures 5 contrasts with the colour of the screen layer 4 so that a visible display is apparent to a viewer. On ceasing the supply of electrical energy to the resistive layer 7, heat is rapidly dissipated from the display device 20 to the ambient air via the aluminium sheet 11 and displayed information rapidly disappears as the thermochromic liquid crystal composition of the layer 21 undergoes a phase transition back to its original cholesteric phase.

Display devices according to the invention may be modified in many ways within the scope of the invention. For example the display layers 2 and 21 of the devices 1 and 20 respectively, are shown as continuous layers. However, in practice, each of these layers 2, 21 is only applied in regions immediately beneath the apertures 5. Furthermore the substrate 3 may be dispensed with if the resistive layer itself is opaque, e.g. a black, carbon resistive layer.

The resistive layer 7 has been shown as a continuous layer in the illustrated embodiments of the display devices. However, the resistive layer 7 may comprise a plurality of resistive strips isolated from each other, each resistive strip having terminal means connected thereto for enabling electrical energy to be selectively applied thereto. In this manner, selective parts of the display can be repe atedlyswitched on and off to provide a display with continually changing information.

In the embodiment of display device shown in Figure 1 ,the capacitor 12 may be replaced by other forms of temperature sensing means, e.g. an element having a positive temperature coefficient of resistance.

Claims

1. A display device comprising a display layer of a thermochromic liquid crystal composition, resistive means for generation of heat upon electrical energisation to modify the temperature of at least a part of the layer of thermochromic liquid crystal composition so that the latter undergoes a reversible phase change accompanied by a reversible change in optical properties and a sheet of metallic material acting as a heat sink for dissipating heat from the thermochromic liquid crystal composition after cessation of energisation of the resistive means to cause the thermochromic liquid crystal composition to revert to its original phase.

2. A display device according to claim 1, in which the said sheet of metallic material acting as a heat sink is made of aluminium.

3. A display deviceaccording to claim 2, in whichthe aluminium sheet material is less than 3mm thick.

4. A display device according to claim 2 or 3, in which the aluminium sheet material is in the form of a foil.

5. A display device according to any of the preceding claims, in which the resistive means is applied as a layer over the sheet of metallic material acting as a heat sink via the intermediary of a thin, electrically non-conductive material.

6. A display device according to any of the preceding claims, in which the resistive means comprises electrically conductive ink.

7. A display device according to any of the preceding claims, in which the resistive means comprises a number of separately energisable regions for heating separate parts of the layer of the thermochromic liquid crystal composition.

8. A display device according to any of the preceding claims, in which the resistive means is opaque and the thermochromic liquid crystal composition is applied directly to the resistive means.

9. A display device according to any of claims 1 to 7, in which the thermochromic liquid crystal composition is applied to an opaque substrate which itself is applied over the resistive means.

10. A display device according to any of the preceding claims, in which a pattern-defining layer acting as a scree is arranged over the layer of the thermochromic liquid crystal composition.

11. A display device according to claim 10, in which the pattern-defining layer is applied as an ink.

12. A display device according to claim 10 or 11, in which a transparent protective covering layer is applied over the pattern-defining layer.

13. A display device according to claim 10,11 or 12, in which the layer of thermochromic liquid crystal composition is only applied beneath those regions of the pattern-defining layer which do not blockthe passage of light.

14. A display device according to any of the preceding claims, in which the thermochromic liquid crystal composition is selected so as to exist at normal ambient temperatures (e.g. from 0'Cto 300C) or normal operating temperatures either in a cholesteric phase or a smectic phase.

15. A display device according to claim 14, in which the thermochromic liquid crystal composition exists in the cholesteric phase at normal ambient temperatures and is selected to display substantially the same colour over a wide range of ambient temperatures and to have a clearing point at a temperature, e.g. 40"C, well above normally experienced ambient temperatures.

16. A display device according to claim 15 when dependent upon any of claims 10 to 13, in which the colour of the pattern-defining layer is matched to the colour exhibited by the thermochromic liquid crystal composition when in its cholesteric phase.

17. A display device according to claim 14 when dependent upon any of claims 10 to 13, in which the thermochromic liquid crystal composition exists in the smectic phase at normal ambient temperatures, the colour of the pattern-defining layer being matched to the colour of the resistive means or, if provided, the opaque substrate layer.

18. A display device according to claim 14 or 17, including temperature sensing means connected to control means for controlling the electrical energisation of the resistive means.

19. A display device according to claim 18, in which the temperature sensing means comprises an element having a positive temperature coefficient of resistance.

20. A display device according to claim 18, in which the temperature sensing means comprises a heat-sensitive capacitive element having a pair of spaced apart electrodes and a dielectric material therebetween comprising a thermochromic liquid crystal composition which undergoes a reversible, temperature dependent phase transition between its isotropic phase or state and its nematic or cholesteric phase or state.

21. A display device according to claim 20, in which the sheet of metallic material acting as a heat sink constitutes one of the electrodes of the capacitive element.

22. A display device according to claim 20 or 21, in which the thermochromic liquid crystal composition of the capacitive element is produced as a liquid crystal ink with the thermochromic liquid crystal composition protected in a polymer by microencapsulation or dispersion

23. A display device according to any of the preceding claims, in which the thermochromic liquid crystal composition comprises a mixture of liquid crystal compounds.

24. A display device according to claim 23, in which the thermochromic liquid crystal composition comprises a mixture of chiral nematic liquid crystal compounds mixed in proportions to adjust the clearing point of the composition and a nematic liquid crystal compound mixed in a proportion to adjust the colour of the composition in its cholesteric phase.

25, A display device according to any of the preceding claims, in which the thermochromic liquid crystal composition of the display layer is produced as a liquid crystal ink with the thermochromic liquid crystal composition protected in a polymer by microencapsulation or dispersion.

26. A display device constructed and arranged substantially as herein described with reference to, and as illustrated in, Figure 1 or Figure 2 of the accompanying drawing.