This invention relates to ferroelectric liquid crystal devices and ferroelectric liquid crystal mixtures.

Liquid crystal devices commonly comprise a thin layer of a liquid crystal material contained between two glass slides. Optically transparent electrodes are formed on the inner surface of both slides. When an electric voltage is applied to these electrodes the resulting electric field changes the molecular alignment of the liquid crystal molecules. The changes in molecular alignment are readily observable and form the basis for many types of liquid crystal device.

In ferroelectric liquid crystal devices the molecules switch between different alignment directions depending on the polarity of an applied electric field. These devices often exhibit bistability where the molecules tend to remain in one of two states until switched to the other switched state. This allows the multiplex addressing of quite large and complex devices.

One common multiplex display has display elements, ie pixels, arranged in an x, y matrix format for the display of eg, alpha numeric characters. The matrix format is provided by forming the electrodes on one slide as a series of column electrodes, and the electrodes on the other slide as a series of row electrodes. The intersections between each column and row form addressable elements or pixels. Other matrix layouts are known, eg seven bar numeric displays.

There are many different multiplex addressing schemes. A common feature involves the application of a voltage, called a strobe voltage to each row or line in sequence. Coincidentally with the strobe applied at each row, appropriate voltages, called data voltages, are applied to all column electrodes. The differences between the different schemes lies in the shape of the strobe and data voltage waveforms.

Other addressing schemes are described in GB-2,l ⁱ .6,473~A; GB-2.173.336-A; GB-2.173.337-A: GB-2.173.629-A; WO 89/05025; Harada et al 1985 S.I.D. Paper 8 Λ pp 131-134; Lagerwall et al 1985 I.D.R.C pp 213-221 and P Maltese et al in Proc 1988 IDRC p 90-101 Fast Addressing for Ferro Electric LC Display Panels.

The material may be switched between its two states by two strobe pulses of opposite sign, in conjunction with a data waveform. Alternatively, a blanking pulse may be used to switch the material into one of its states. Periodically the sign of the blanking and the strobe pulses may be alternated to maintain a net d.c. value.

These blanking pulses are normally greater in amplitude and length of application than the strobe pulses so that the material switches irrespective of which of the two data waveforms is applied to any one intersection. Blanking pulses may be applied on a line by line basis ahead of the strobe, or the whole display may be blanked at one time, or a group of lines may be simultaneously blanked.

It is well known in the field of ferroelectric liquid crystal device technology that in order to achieve the highest performance from devices, it is important to use mixtures of compounds which give materials possessing the most suitable ferroelectric smectic characteristics for particular types of device.

Devices can be assessed for speed by consideration of the response time vs pulse voltage curve. This relationship may show a minimum in the switching time (t _min ) at a particular applied voltage (V _nin ). At voltages higher or lower than Vmm. the switching ^σ time is long°er than taii.n. It is well understood that devices having such a minimum in their response time vs voltage curve can be multiplex driven at high duty ratio with higher contrast than other ferroelectric liquid crystal devices. It is preferred that the said minimum in the response time vs voltage curve should occur at low applied voltage and at short pulse length respectively to allow the device to be driven using a low voltage source and fast frame address refresh rate.

Typical known materials (where materials are a mixture of compounds having suitable liquid crystal characteristics) which do not allow such a minimum when included in a ferroelectric device include the commercially available materials known as SCE13 and ZLI-3654 (both supplied by Merck UK Ltd, Poole, Dorset) . A device which does show such a minimum may be constructed according to PCT GB 88/01004 and utilising materials such as eg commercially available SCE8 (Merck UK Ltd.). Other examples of prior art materials are exemplified by PCT/GB/86/00040, PCT/GB87/00441 and UK 2232416B.

It is the aim of this invention to provide devices having a shorter switching time and/or a lower voltage than previously achieved.

According to this invention a ferroelectric liquid crystal device (eg multiplex addressed) comprises two spaced cell walls each bearing electrode structures and treated on at least one facing surface with an alignment layer, a layer of a smectic liquid crystal material enclosed between the cell walls, a minimum in its response time versus voltage curve, characterised in that the liquid crystal material consists of three components; A, B and C, where the three components are given by:

Component A being present in the range of 0.1-50 wt % and is one or more optically active compounds capable of imparting a spontaneous polarisation to the material.

Component B being present in the range of 10-90 wt % and selected as one or more compounds having formula B:

R. and R ₂ independently represent straight or branched chain C ₃ -C ₁₂ alkyl or alkoxy; a, b and c are independently 0, 1 or 2 and the total of a+b+c is not greater than 4.

Component C is present in the range sufficient to enable A+B+C = 100 wt?. , and is one or more compounds selected from:

wherein a, b and c are independently 0, 1, 2, 3; R _χ and R ₂ are C ₁ _ ₁₅ alkyl or alkoxy, preferably C^-C.-,.

Preferably Component A is present in the range 1-15 wt % , even more preferably 1-5 wt% . Preferably Component B is present in the range 30-60 vt% .

Preferably the material contains one or more compounds of the formulae:

R. / 0 V θ -COO

Preferably the material contains optically active dopants of the formula below as described in PCT/GB88/01111;

CN R ₁ -[CH=CH—(-CH ₂ ) _r -0-] _n -X-(0CH ₂ ) _m C0 ₂ CH-R ₂

including wherein R. is selected from hydrogen, C._ ₁₂ alkyl, alkoxy, perfluoroalkyl and perfluoroalkoxy and may be straight chain or branched chain; R ₂ is alkyl, which may be C._ ₈ straight chain, C._ ₁₅ branched chain or cyclic, r is an integer 1-10, n and m are independently 0 or 1; X is a group of general formula:

where (F) indicates that the relevant phenyl ring may carry 1 or 2 fluorine substituents, p is 0 or 1, Z is a single bond when p is 0 and COO or a single bond when p is 1.

The CN group may be substituted by CH ₃ , CF ₃ , halogen, preferably F or Cl. The C0 ₂ group may be reversed. The chiral unit CH(CN)R ₂ may be replaced by a chiral epoxide group. One or more of the phenyl groups may be replaced by a cyclohexyl group.

Preferably *" the device has a Vmm. less than 4 volts and/or a tmi.n less than lOOμs.

The invention will now be described by way of example only with reference to the accompanying drawings of which:

Figure 1 is a diagrammatic view of a time multiplex addressed x, y matrix;

Figure 2 is a cross section of part of the display of Figure 1 to an enlarged scale;

Figure 3 is a representation of a turnaround in a (t _Bin /V _nin ) feature.

Figures 4-6 are graphs of t _min versus V _nin at 30°C for mixtures 1,2 and 3. respectively, (see Table 1 for mixtures).

The display 1 shown in Figures 1, 2 comprises two glass walls 2, 3 spaced about l-6μm apart by a spacer ring 4 and/or distributed spacers.

Electrode structures 5. 6 of transparent tin oxide are formed on the inner face of both walls. These electrodes are shown as rows and columns forming an X, Y matrix but may be of other forms. For example, of segments formed into a digital seven bar display.

A layer ^~ of liquid crystal material is contained between the walls 2, 3 and spacer ring 4.

Polarisers 8, 9 are arranged in front of and behind the cell 1. Row 10 and column 11 drivers apply voltage signals to the cell. Two sets of waveforms are generated for supplying the row and column drivers 10, 11. A strobe wave form generator 12 supplies row waveforms, and a data waveform generator 13 supplies ON and OFF waveforms to the column drivers 11. Overall control of timing and display format is controlled by a contrast logic unit 14.

Prior to assembly the walls 2, 3 are surface treated for example by spinning on a thin layer of polya ide or polyimide, drying and where appropriate curing; then buffing with a soft clean cloth (eg rayon) in a single direction Rl, R2. This known treatment provides a surface alignment for liquid crystal molecules. In the absence of an applied electric field the molecules align themselves along the rubbing direction; Rl and R2 are parallel (+/- 30°) in the same or opposite directions. When suitable unidirectional voltages are applied the molecular director aligns along one of two directors Dl, D2 which are at an angle of about 45° to each other.

The device includes means of discriminating the states optically, eg 1 or more polarisers. It may operate in a transmissive or reflective mode. In the former light passing through the device eg from a tungsten bulb is selectively transmitted or blocked to form the desired display. In the reflective mode a mirror is placed behind the second polariser 9 to reflect ambient light back through the cell 1 and two polarisers. By making the mirror partly reflecting the device may be operated both in a transmissive and reflective mode.

Pleochroic dyes may be added to the material 7- In this case, only one polariser is needed and the layer thickness may be 4-10 μm.

For a typical thickness of 2 μm the material at 22 ^β C is switched by a dc pulse of + or - 50 volts for 100 μs. The two switched states Dl, D2 may be arbitrarily defined as ON after receiving a positive pulse and OFF after receiving a negative pulse of sufficient magnitude. Polarisers 8, 9 are arranged with their polarisation axes perpendicular to one another and with one of the axes parallel to the director in one of the switched states.

In operation, strobe waveforms are applied to each row in turn whilst appropriate ON or OFF data waveforms are applied to each column electrode. This provides a desired display pattern formed by some x, y intersection in an ON state and another in an OFF state. Such addressing is termed multiplex addressing.

The following compounds and mixtures are examples illustrative of the invention.

^C 8 ^H 17 o CO,C ^* H(CN)CH(CH 3'2

R' J_

C R0-< 0 K 0 X 0 >-*R*

£3 C _g H^O-^^CO.^ ^-C ₈ H.,

Table 1: Mixtures containing component B

T _A _ _C = Smectic A - Smectic C phase transition temperature.

Q = Cone Angle.

Ps = Spontaneous Polarisation.

E = Electric field. cell thickness (d/μm) for the three different mixtures 1.2 and 3 is

2.3. 1-9 and 1.65 respectively.

Table 2: Mixtures containing no component B.

* Does not show a minimum in V-t measurement. ~ Minimum only measured. Cell thickness is 1.6μm.

When analysing the data presented in Tables 1 and 2 it is perhaps most instructive to consider the column headed E_mi.n/'Ps and tm.n x Ps ² .

It appears from Tables 1 and 2 that the introduction of a compound described by component B results in a lower V _min . Most of the mixtures which did not contain a material described by component B did not possess a V _min . It also appears from Table 1 that the choice of compound described by component C has an effect on the response time. For example the introduction of C. results in an improvement in the response time, compare mixtures 1 and 3 in Table 1 taking Ps into account.

Tables 3-7 list Ps and Θ data at different temperatures for Mixtures 1,2. _,5.£.

Table 3: Mixture 1

Table 4: Mixture 2

Table 5: Mixture 4

Table 6: Mixture 5

Table 7: Mixture 6