Liquid crystal materials and devices containing them
This invention relates to liquid crystal materials and devices containing them. 
     There are many known liquid crystal display devices. Typically they comprise a thin layer of a liquid crystal material contained between two electrode bearing glass slides. Application of a suitable electric voltage to the electrodes across the layer causes a visible change in optical property and this effect is used to display information. The electrodes may be arranged in numeric or matrix form to display e.g. time (on a watch display) or waveforms. 
     Liquid crystal materials may be smectic, cholesteric or nematic. This invention concerns nematic liquid crystal material or a mixture of nematic plus a small percentage of cholesteric material. 
     The voltage at which an effect is observed in the liquid crystal layer is often termed a threshold voltage. Normally displays are operated some way above threshold to ensure a clear display, e.g. the operating voltage may be about twice the threshold voltage. 
     An advantage of liquid crystal displays is their low power consumption which makes them eminently suitable for battery powered displays such as wrist watches and pocket calculators. It is therefore desirable that the threshold voltage is as low as possible so that a single low voltage battery (e.g. a standard 1.5 volt battery) may be used. 
     It is an object of this invention to provide a liquid crystal display having a low operating voltage. 
     As used herein a liquid crystal cell is defined as a layer of liquid crystal material contained between two substrates, at least one of which is transparent, with electrodes arranged on the substrate for applying an electric field across at least selected parts of the layer; the liquid crystal material may be a single compound or a mixture of compounds and may include dyes. 
     According to a first aspect of this invention a liquid crystal material comprises a mixture of at least three liquid crystal materials at least two of which are selected from the following compounds: ##STR2## or the above compounds wherein a cyclohexane ring is substituted for one of the benzene rings, where X is a short alkyl group C n  H 2n+1  and n is an even integer, or where X is a short alkoxy group C m  H 2m+1  and m is an odd integer. 
     The component materials may or may not exhibit a liquid crystalline phase by themselves but exhibit a liquid crystalline phase when incorporated into other liquid crystal materials. 
     The mixture may include two or more components from the same compound but with different n, m values or may be mixtures of the different compounds listed above. One or more components may be a compound not satisfying the n=even, m=odd integer and may be added to enhance the nematic temperature range of the mixture. Such a compound may be e.g. 5 CT or BB15 described below. 
     According to a second aspect of this invention a liquid crystal display device for operating at low voltages comprises a liquid crystal cell, as hereinbefore defined, wherein the liquid crystal material comprises the mixture defined in the first aspect of the invention. 
     Liquid crystal materials having the general form ##STR3## where X is an alkyl, alkoxy, alkanoyloxy, or alkenyl group and CN is a cyano group are described and claimed in U.K. Patent Specification No. 1,433,150, and are termed biphenyl materials. 
     It has been found that when n is an even integer lower threshold voltages are obtained than when n is an odd integer. Similarly when m is odd lower threshold voltages are obtained than when m is even. It has also been found that the lower values of n and m give lower operating voltages. The preferred values for low voltage operation are therefore: 
     n=2, 4, 6 
     m=1, 3, 5. 
     However materials with low n, m values tend to have melting points above room temperature and usually have a melting point above the isotropic temperature (clearing point), e.g. for biphenyls. 
     
                       TABLE 1                                                     
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Melting Point °C.                                                  
                    Clearing Point ° C.                            
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1      109              (45)                                              
2      75               (22)                                              
3      68               (28)                                              
4      46               (16)                                              
5      22               35                                                
6      13               27                                                
m                                                                         
1      104              (85)                                              
2      102              (90)                                              
3      72               (64)                                              
4      78               (76)                                              
5      48               68                                                
6      58               76                                                
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 The brackets () indicate a virtual monotropic transition.                
 
    
     The brackets () indicate a virtual monotropic transition. 
     Therefore the mixture may also contain some component materials with high n, m values (e.g. n=6, m=5) so the resulting mixture has an acceptable nematic liquid crystalline temperature range. The average value of n, m in such a mixture should be as low as possible, preferably n≦6 and m≦5. For mixtures containing alkyl and alkoxy groups the average value of n and m should still be as small as possible and preferably less than 6 or 5. 
    
    
     The invention will now be described by way of example only with reference to the accompanying drawings of which: 
     FIG. 1 is an exploded view of a twisted nematic display; 
     FIG. 2 is a front view of a watch having a twisted nematic liquid crystal display constructed as shown in FIG. 1; and 
     FIG. 3 is a sectional view of the display part of the watch in FIG. 2 but drawn to an enlarged scale. 
    
    
     As shown in FIG. 1 a display comprises a polariser 1 arranged with its polarisation axis 2 vertical. A liquid crystal cell 3 comprises two glass slides 4, 5 containing a layer of nematic liquid crystal 6. Electrodes 7, 8 e.g. of tin oxide are arranged on the inner faces of the slides 4, 5. Behind the cell 3 is an analyser or second polariser 11 arranged with its polarisation axis 12 horizontal. A brushed aluminium reflector 27 is behind the analyser 11. 
     Prior to assembling the cell the slides 4, 5 are coated on their inner faces with silicon monoxide or magnesium fluoride. This coating is formed by evaporating a stream of e.g. silicon monoxide onto the slide at an angle of about 5° to the surface as for example described in U.K. Patent Specification No. 1,454,296. On assembly the slides are arranged with the evaporation direction on the two slides 4, 5, at 90° to one another. With such a coating liquid crystal molecules at the coated surface lie in a single direction (parallel to the evaporation direction) and at an angle of about 25° to 35° typically about 30° to the slide surface. Alternatively the slides may be cleaned, dipped in a solution of water and 0.2% by weight of polyvinyl alcohol (P.V.A.), and then when dry rubbed in a single direction with a soft tissue. The slides are assembled with the rubbing direction orthogonal. In this cell the liquid crystal molecules lie at about 2° to the slide surface. As a result the liquid crystal molecules progressively twist from one slide 4 to the other slide 5. Thus the plane of plane polarised light is rotated through 90° on passing through the cell 3 providing the direction 13 of the molecules at the surface of the slide 4, 5 is parallel (or perpendicular) to the polarisers axis 2, 12 respectively. Thus when zero voltage is applied to the cell light passes through the analyser 1, cell 3 and analyser 11, and is then reflected back to an observer 15. 
     When a suitable voltage is applied between the electrodes 7, 8 the molecules between the electrodes are caused to align parallel to the applied field and light transmitted by the polariser is transmitted without rotation to the analyser 11. Since the polariser and analyser 11 are crossed no light is transmitted to the reflector and no light is reflected back to the observer at those parts of the cell 3 between electrodes 7, 8 which therefore appear dark on a lighter background. Thus by shaping each electrode 7, 8 into say seven discrete parts separately connectable to a voltage source the number 0 to 9 may be displayed. 
     In the watch display of FIGS. 2, 3 the electrodes 7, 8 are arranged to provide four seven bar numeric characters plus a period dot which may be arranged to pulse once per second. 
     The voltage required to align the molecules parallel to the applied field, ie the `ON` state, is dependent on layer 6 thickness, materials used, temperature and surface treatment of the slides 4, 5. A typical thickness of layer is 12 μm. The material used must be stable over the temperatures of use i.e. ambient temperatures which can be assumed constant for comparison purposes. A surface treatment i.e. evaporation of silicon monoxide at 5° giving large tilt e.g. 30° of molecules reduces the voltage necessary to turn the cell `ON` at the expense of a degraded contrast or speed. 
     In typical watch displays using twisted nematic liquid crystal displays a 1.5 volt battery is used to drive the oscillator and logic circuits of the watch and an up converter is used to raise the voltage to 3.0 or even 4.5 volts for operating the display. 
     Using nematic liquid crystal materials according to this invention the operating voltage require to drive the liquid crystal display may be reduced to e.g. 1.25 to 1.40 volts. 
     Small amounts of a cholesteric material may be added to the nematic liquid crystal to give a long pitch cholesteric material. To avoid a patchy appearance it is then preferable to match the cholesteric pitch to the relative alignment on the slides 4, 5 as taught in U.K. Pat. Ser. No. 1,472,247. 
     As an example about 1% of C.15 cholesteric cyano biphenyl (marketed by British Drug Houses) may be added to a nematic material. The material C.15 has the formula ##STR4## 
     The following table shows examples of nematic mixtures and operating voltage necessary for 10% transmission i.e. almost fully ON of normally incident light through a cell prepared and operated as follows: 
     (i) layer thickness 12 μm, slides coated with SiO 2 , drive voltage 1 KH 3  sine wave, layer temperature 22° C.; 
     (ii) layer thickness 7 μm, slides PVA coated and rubbed, drive voltage 100 H 3  square wave, layer temperature 22° C. 
     Percentages are by weight. 
     
                                           TABLE 2                                 
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                                    Operating                             
                                           Voltage                        
Mixture                             (i)    (ii)                           
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1     E.7[5CB(51%)/7CB(25%)/80CB(16%)/5CT(8%)]                            
                                    1.70   2.15                           
2     2CB(10%)/6CB(80%)/5CT(10%)    1.25   --                             
3     6CB(65%)/10CB(9%)/30CB(15%)5CT(11%)                                 
                                    1.35   1.77                           
4     2CB(10%)/6CB(60%)/70CB(20%/5CT(10%)                                 
                                    1.40   --                             
5     2CB(10%)/6CB(56%)/10CB(6%)/30CB(10%)/                               
                                    --     1.47                           
      50CB(10%)5CT(8%)                                                    
6     2CB(15%)/4CB(15%)/6CB(43%)/10CB(3%)/30CB(12%)/                      
                                    --     1.53                           
      5CT(7%)/BB15(5%)                                                    
where                                                                     
       ##STR5##                                                           
       ##STR6##                                                           
       ##STR7##                                                           
       ##STR8##                                                           
       ##STR9##                                                           
       ##STR10##                                                          
       ##STR11##                                                          
       ##STR12##                                                          
       ##STR13##                                                          
       ##STR14##                                                          
       ##STR15##                                                          
       ##STR16##                                                          
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     The display of FIGS. 1, 2, 3 may be modified by replacing the reflector 27 with a partial reflector and having a beta light or low power tungsten bult behind the partial reflector so that the display can be observed by light transmission (instead of light reflection) when ambient light is not available.