CA1187692A - Thermally addressed cholesteric-smectic liquid crystal device - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A visual display device is featured which uses a new thermal addressing technique to provide a dark image upon a lighter background. The display is capable of being mutliplexed to a large number of rows The device comprises a cholesteric-smectic liquid crystal material mixed with a pleochroic dye of high order parameter. When the material experiences a transition from a higher to a lower, smectic thermal phase, two different textures or light states are developed; a transparent state and a light absorbing state. The transparent state is developed by applying a sensitizing voltage to certain portions of the medium.
The pleochroic dye absorbs the light passing through the unsensitized portions of medium to provide the dark image. The row electrodes are made diffusely reflective so as to provide a double light pass through the medium to improve contrast.

Description

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This is a divisional application of Canadian patent application Serial No. 400,415 filed April 2, 1982 and assigned to Minnesota Mining and Manufacturing Company.
FIELD OF INVENTION
The invention pertains to thermally addressable ~hoL~steric-smeetic li~uid crystal display devices, and more par~L~ularly to thermally addressed visual display devices which use a licJht absorption technique to provide a dark image upon a lighter background.
1~ sACKGRO~ID OF T~IE INVENTION
Heretofore, the ability to fabricate large scale multi-plaxed liquid crystal displays was very difficult. The difficulty was primarily due to "cross-talk" effects, and the necessity to quicky refresh the slowly responding liquid crystal medium. Large seale multiplexed displays notoriously have had problems with "cross-talk", i.e., the unwanted sensitizing of partially seleeted display elements. This problem results ~rom the small root mean square voltage ratio between the "on" and "off" elements aehievable in a large scale multiplexed liquid cxystal display.
As displays become larger, a new problem appears. Most device effects do not have intrinsic storage. The display must therefore be repeatedly scanned to update; this is often with typical display effects done at 60 ~z (per frame). The result for large area matrices is a small duty cycle for each individual row or column. Most display media only partially respond to small duty cycle voltage information and the resulting effect is only a fraction of the dc equivalent voltage. The result is low contrast or brightness. As the display matrix gets larger, the duty cycle gets less and less and optical performance gets poorer and poorer. The result is a very poo.r (below commercial standards) optical performance as the x-Y matrix gets larger and larger.
These two problems have severly limited the ability to provide large scale multiplexed displays, and to -la-~ ~2~
date, no one has produced a device which has high contrast, wide viewing angle, which is easy -to fabricate, easy to operate, and which has low cost.
The present invention has developed a low cost, large scale multiplexed, visual display device that has resolved -the aforemen-tioned problems, while providing a new lLquid crystal device having many advantages over -the prior art.
Whi:Le the presen-t invention is concerned prlmarily wi-th large scale, thermally addressed multiplexed devices, its new ligh-t absorbing rnethod is easily applicable to devices which are not large scale, and which do not utilize multiplexing. The subject invention is believed to have wide application in the field of thermally addressed liquid crystal displays, and is not considered as being limited to any particular device or system.
DISCUSSION OF RELATED ART
The invention features certain classes of smectic liquid crystal hosts tha-t have a cholesteric phase upon heating. A small percentage of pleochroic dye is added to the material. The display is addressed in a thermal electric mode. For the "on" elements, the liquid crystal texture is light absorbing due to the dye which strongly absorbs incoming light. The "off" elements and the background have homeotropic smectic A texture, where the dye exhibits minimum absorption.
The concept of pleochroic dye switching as the Guest Host effect in nematic liquid crystals, was first suggested in an article toO G.H. Heilmeier, J.A.
Castellano, and L.A. Zanoni, Mol. Crystals and Liquid Crystals 8, 293 (1969).
Others have suggested that -the liquid crystal structure can be twisted nematic, homogeneous, or homeotropic. Most of these devices using pleochroic dyes mixed with the liquid crystal material have generally required external devices such as polarizers or wave plates to improve the contras-t of the image.

Dyes of high order parameter in a choles-teric liquid crystal host were firs-t suggested in an ar-cicle to:
D.L. White, G.N. Taylor, J. of App Phys. 45 4718 (1974).
Displays using this liquid crystal medium have high con-trast and do not require external polarizers.
These displays have high brigh-tness and a wave viewing an~le not available with -the field effect twis-t nematic :LLq~lid crystal d:isplays. Such devices use a cholesteric to l~tnat:ic transition effec-t with liquid crystal displays.
tO Such clevices ~lse a cholesteric to nematic transition effect w:i-th l:iquid crystals of positive dielectric anisotropy.
In the no field (off) mode, the dye molecules follow the helical structure of the host and exhibit strong light absorp-tion. In the on condition, the dye is in a homeo-tropic nematic host and the absorption is minimized.
Thus, the display presents a white image against a dark (or colored) background. A white image against a dark background is, however, generally not desirable. In addi-tion, it has been well reported that such a cholesteric to nematic transition effect cannot be multiplexed above approximately 5-10 lines and give commercial performance.
This is due to the change in the slope of the contras-t versus voltage relationship that causes "cross-talk".
Recently, a paper was presented in the 8th International Liquid Crystal Conference at Kyoto, Japan by Professor A. Sasaki et al., entitled "Laser Addressed Liquid Crystal Multifunction Light Valve"; in which he described a laser addressed projection display utilizing a liquid crystal of 90:10 mixture of p-p' cyano-octyl biphenyl and cholesteryl nonanoate. The mixture should have a cholesteric phase followed by a smectic A phase upon cooling. However, the display is a projection device -that derives its image contrast purely through scattering. The thermal addressing is by a scanning laser beam. No dyes are used in his material.
Recently, high order parame-ter and light stable dyes have become available. Devices using these dyes will . ~. .

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provide viab]e displays for many applications. However, they have two major drawbacks which may restrict their application to simple displays of very low information content only.
S These dye displays are very difficul-t to mul-tiplex. Even a few rows represen-t a s-tate of the art development. Large size matrix addressing has been achieved only by adding external non-linear elemen-ts -to each display element.
E~'or non-emissive (reflec-tive) displays, a white image against a dark background is formed. This is esthetically undesirable and of limited commercial utility.
Techniques to reverse the image contrast to a more pleasing dark against a light background are available, but the added complica-tion increases the complexity and cost.
In 1978 C. Tani and I. Ueno discussed the application of pleochroic dyes to certain smectic liquid crystals in a scientific paper (Appl. Phys. Let-t. Vol. 33 No. ~, 15 Aug 1978). The authors, however, specifically teach against the use of the smectic "A" phase as having utility in the pleochoric dye system: they indicate that i-t has application only in scattering applica-tions such as in laser addressed ligh-t valves. They concluded that only materials having smectic H or possibly B phase structure have useful properties in combination with pleochroic dyes.
Further, they discuss the utilization of slow cooling as having utility with pleochroic dyes and tha-t rapid cooling of the elements is only applicable to light scattering devices.
The present invention utilizes pleochroic dyes to produce an absorbing state rather than a scattering state and uses thermal XY local hea-ting as distinct from the laser heating as described in other art. Further it u-tillzes rapid cooling of the element wi-th liquid cyrstals preferentially of the smectic "A" phase. The last factor is directly against the teaching of Tani and Ueno, but has been found to be most effective in this application.

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~5--~ lso, recently, a system has been reported in -the French Literature, which uses a thermally addressed smectic "A" crystal medium~ Such a system is described in an article entitled: MATRIX ADDRESSED S~ECTIC LIQUID CRYSTAL
DISPLAY: M. Hareng, S. Le Berre, R. Hehlen, and JoN~
:Perbet, Thomson-CSF Laboratoire Central de Recherches.
Proceedirlgs Erom Society of Information Display 1980 ~on~erence, :L,ate News Paper.
Such a system does not use dyes, and employs a l~ scatterincJ light technique, ra-ther than a light absorption technique as described by this invention.
In addition, the system described is embodied in a very different device than de-tailed in this invention.
Because of the crucial difference of the light scattering as compared to light absorption, the device can be viewed only through a projection optical system that results in a very bulky, power in-tensive system.
While the prior art teaches the use of pleochroic dyes of high order parameter for use in liquid crystal meclia, i-t should also be noted -that these dyes are used primarily to enhance the light effects produced by the thermal phase transition of the media. The invention by con-trast, relies upon the dye to do most of the light absorp-tion for the crystal medium, the medium acting as a vehicle for orienting the dye to develop a light absorbing stance.
BRIEF DESCRIPTION OF THE INVENTION
The invention relates to a thermally addressed visual device which provides a dark image against a lighter background. The device comprises a liquid crystal medium including at least one cholesteric liquid crystal compound mixed with at least one coloring agent, generally a pleochroic dye of high order parameter. The medium has positive dielectric anisotropy. The medium is thermally sensitive and has a transition between at least two thermal phases: the upper thermal phase is a cholesteric phase and a lower thermal phase is a smectic phase. The .. . ~ .

medium develops two textures in the smectic phase: a light absorbing tex-ture and a homeo-tropic texture. The homeotropic texture is developed in portions of the medium by sensitizing the medium as it passes rapidly from the upper cholesteric phase to the lower, smec-tic phase. The light absorbing -texture develops in the unsensi-tized portions of the medium as i-t goes -through -the transition to the smectic phase.
The medium is sensi-tized by applying a voltage to :L0 those portions of the medium -to be addressed. The acldressed portions develop a substan-tially transparen-t light state, while -the unaddressed por-tions develop a substantially light absorbing state. The coloring agent or dye which is locked within the liquid crystal medium as it develops its ligh-t absorbing texture in the smectic phase will absorb most of -the light passing through the medium;
the liquid crystal acting as a vehicle to orient -the dye molecules into a light absorbing position. Electrodes are provided to sensitize the medium. They are disposed adjacent the medium. Heating electrodes are also provided to heat -the medium to an upper thermal phase. In a multiplexed device, these electrodes define a matrix of columns and rows disposed substantially a-t righ-t angles to each other, and in different planes.
In order to obtain a pleasing direct viewable display, -the row electrodes are made diffusely reflective to provide high contrast as well as wide viewing angle.
The reflective electrodes provide for a double pass of light through the cell enchancing light absorbing.
The liquid crystal medium will generally contain an alkyl cyano biphenyl compound and will generally have two thermal transitions: between an isotropic and choles-teric phase, and between the cholesteric and a smectic "A" phase.
Many liquid crystal compounds with optically active terminal groups exhibit cholesteric phase. Some of them also exhibit one or more smec-tic phases when the compounds are cooled down from the cholesteric phase. For example, a paper published by Joseph A. Castellano, C.S.
Oh, M.T. McCaffray, Mol. Crystal l,iq. Cryst., V.?7 pp 417, 1973, lists 40 Schiff base compounds with -the general structure:

3 - CH = N - ~ - R

where: R = Oco-(cH2)n - CH3 and C--N
Many compounds wi-th high value of m and n exhibit a cholesteric phase followed by smec-tric phases upon cooling. To cite a few examples, we have:
3 - CH = N - ~ -OCO-(CH2)4-CH3 46.3C 74.5C 77C
Crystal -~ ~ Smec-tic II~ -_ ~ Smec-tic I ~ _~ Cholesteric 83.6oc I so troplc and C2H5 - CH - (CH2)3 O _ ~ - CH = N - ~ - C = N

Crystal ---~ Smectic-------~ Cholesteric - ~ Istropic Although these compounds have the desirable phase transitions for device application, Schiff bases are generally not very stable. Also, there are other requirements that a practical material should have. Thus, typical working materials are formalized with stable compounds at suitable composition.
One of the requirements for the host liquid crystal is -that its dielectric anis-tropy should be strongly ~' 3'7~

posi-tive. This is usually obtained by using liquid crys-tal compounds having C - N as one of the termi.nal groups.
One example of a workable cholesteric liquid crystal comprises a mixture of X, Y and Z materials, each having a percentage by weight in an approximate range of:
40 to 60 of X; 30 to 50 of Y; and 5 to :L5 o:E Z;
~espect:ively, where:

- ~ L7 ~ ~ ~ C --N;
10 21 ~ C = N; and

2 5 ICH CH2 ~ C - N.
C~13 More particularly, the aforemen-tioned mixture can comprise:
X jS rC8H17 - ~ ~ CN 50.5% by weigh-t Y iS ~ C10H21 ~ ~ CN 41.4%
Z iS ~C2H5 - ICH - CH2 ~ CN 8.1%

Which has a phase transition as follows:
34.5C 40.7C
Crystal -~ Smectic~____ Cholesteric -~.Isotropic In a liquid crystal which has a smectic phase followed by a nematic phase, good display performance requires the temperature range of the nematic phase to be narrow. With cholesteric materials, however, the temper-ature range of the cholesteric phase does not necessarily have to be narrow.
To the host material, a high order parameter pleochroic dye or dye mixture is added in a range of approx-imately 0.5 to 3 percent by weight of the total compositin.
More particularly, about 1% by weight of a purpledye have the Eormula:

N ~ - N = N - ~ N = N - ~ - N = N ~ N

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g is added -to the above cholesteric liquid crystal medium.
This dye is sold by E.M. Laboratories, Elmsford, N.Y.
In operating the device, -the liquid crystal medium is passed rapidly through i-ts thermal transi-tion from its upper cholesteric phase -to its lower thermal, smectic phase. The transition must be accomplished reasonably rapidly, hence rapid thermal pulses are used l.hat heat the liquid crystal locally but do not significantly heat the surrounding glass. Hence -the natural cooling :L0 per:iod lmmediately following the passage of the heat pulse is also rapid and hence the liquid crys-tal medium passes through -the nematic phase rapidly. This greatly enhances the optical eEfect and results in greater contrast.
Certain portions of the medium are sensitized.
These portions define the background of the medium. These sensitized portions develop the substantially light trans-paren-t state when the medium passes in to the smectic thermal phase. The remaining unsensitized portions of -the medium develop a light absorbing state. When light (generally ambien-t) is passed through the medium, the unsensi-tized portions absorb the light to provide a dark image upon -the ligh-ter sensi-tzed background. The addressed portions of the medium may be sensitized in a chronological sequency.
When the liquid crystal material cools down either from the isotropic state through the cholesteric state to the smectic state, or from cholesteric to smectic state, the texture obtained in the smec-tic state depends on the cooling rate, surface alignment, the pitch of the cholesteric molecules and some other factors. The materials best suitable for this new device have molecular pitch in the 1- 3 ~m region. Most display devices have perpendicular alignment on both glass surfaces. This type of alignment is not absolutely necessary for this new display.
When the cooling rate is slow (for example less than 500C/min) we have two cases:
(1) Cooling from the isotropic phase through a narrow (approximately 10C or less) choles-teric phase: a clear homeotropic texture is '7~

obtained.
(2) Cooling from the cholesteric phase to smectic phase: a scattering SA texture is obtained if the cooling rate is up to 100C/min. region with slower cooling rate, the clear homeotropic te~ture is obtained.
With fast cooling rate corresponding to the actual displ~ operation (up to 250,000C/min.), scattering textures are always obtained.
The scattering state obtained with fast cooling in the cholesteric to smectic system has finer structure as compared to those obtained with nematic to smectic system. When a pleochroic dye is added to the material, the scatteri~g state becomes a light absorbing state. Because of its finer structure, the color is very deep.
The present invention and that of copending application Serial No. 400,415 will now be described in greater detail with reference to the accompanying drawings in which:
Figure 1 is a perspective, exploded, schematic view of a visual device made in accordance with the invention;
Figure 2 is a plan schematic view of the device shown in Figure 1, illustrating how an image can be formed in the cholesteric-smectic liquid crystal medium by a multiplexing technique;
Figure 3 is a graphical illustration of the chronological sequencing of the row and column electric waveforms of the device depicted in Figure 1.

Figures 4a and 4b show a schematic view of two different light modulating textures developed in the cholesteric liquid crystal medium of the device of Figure 1, when the medium passes rapidly into its smectic phase Figure 4a depicts a homeotropic, s~tbstantially light transparent texture, and Figure 4b illustrates a ~ub~tantially light abso.rbing texture.
_t'~ ED D~SCRI-PTION OF THE INVENTION
Generally speaking, thîs invention relates to new methods, compositions, and visual devices utilizing the thermal addressing of cholesteric smectic liquid crystal media. The visual devices of this invention feature a -lOa---ll--highly contrasted dark image on a lighter background.
Where the devices of the invention are multiplexed, they are capable of being multiplexed up to a large number of rows.
This invention provides new displays that incor-porate pleochroic dyes of high order parameter lnto a smectic A liquid crystal material tha-t has a cholesteric phase upon heating. By us:ing a thermal electric addressing techn:ique described hereinafter, this display has m~jor advantages over the previously known dye switching displays.
A cholesteric liquid cr~stal with positive dielectric aniso-tropy can develop a homeotropic texture under the influence of an electric field. ~ homeotropic smectic A phase is formed, if the material is rapidly cooled through the phase transition. The homeotropic SA
phase is clear or transparent and shows very little color (colorless) with dissolved pleochroic dye. Without an electric field, a light absorbing -texture is formed in the medium. Thus, by controlling -the electric field across the liquid crystal layer during the cholesteric to smectic A
phase transi-tion, one can create at his will, either a colored sta-te or a non-colored state. Once these states are formed, they are stable until erased by heating into the istropic or coholesteric phase again.
Although the above description assumes that the material is heated into -the isotropic state, it is noted that this is no-t absolutely necessary. In reality, only heating to the cholesteric state is needed. Also, due to the different physical mechanisms of forming the colored scattering state, the temperature range of the cholesteric state does not necessarily have to be narrow to ensure a good display performance.
The smectic A phase can be aligned homeotropically, as shown in Figure 4a, if the surface of the display is treated with materials such as Lecithin. In -this structure, the material is transparent.
There are two forms of thermally addressed smectic A displays. One type uses a scanning laser beam to i~
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address the display elements. The o-ther -type is x, y ma-trix addressed. The row electrodes are heated sequen-tially with electric current and the display is written by applying voltages on the columns. During the writing process, only the dots associated -to the row where the heating current has just been removed are aEfec-ted. In oLher words, only -the dots where the liquid crystal mal:t.~l.Lcl.L is cooling to -the smec-tic state respond to -the wrLting pu.Lses on the column electrodes.
:lO As the :liquid crystal material cools rapidly throuyh -t:he cholesteric phase -to the smectic phase, i-t can Eorm two difEerent textures. Wi-th a voltage applied on the column, the liquid crystal material is switched -to a homeo-tropic state during the cholesteric phase and assumes -the transparent homeotropic smectic A texture after cooling is completed. Without the applied voltage, a light absorbing -texture is developed instead. Thus, the dots associa-ted with a cooling row electrode can be written into a -txansparent state or a light absorbing sta-te by applying or not applying voltages on the columns. The choles-teric-smec-tic ma-terial used in the invention display device has positive dielectric aniso-tropy. The transition must be accomplished reasonably rapidly, hence rapid thermal pulses are used that heat the liquid crystal locally but do not significantly heat the surrounding glass. Hence the natural cooling period immediately following the passage of the heat pulse is also rapid and hence the liquid crystal medium passes through the cholesteric phase rapidly. This greatly enhances the optical efEect and results in greater contrast.
The present invention, however, must be carefully distinguished from other similar systems wherein a scattering texture rather than a ligh-t absorbing texture is developed in the smectic material. Displays developing the scattering texture are generally not suitable for direct viewing, and are often used only in projection systems.
Tlle optical contrast developed by a scattering ~L~7~

texture against a transparent texture is simiular to those obtained with -the dynamic scattering effect. Under many commonly encoun-tered illumination conditions, it will not give a pleasing, high contrast image.
The situation becomes quite difEerent, however, when a pleochroic dye of high order parameter is introduced :ln to the smectic A material. The dye becomes locked into the l.iquid crys-tal, and assumes the orientation of the :L:iquid crystal molecules. The dye molecules in the 1~ scatkering texture of the host absorb light strongly, I:ransEorming the normal scat-tering tex-ture into a light absorbing texture, either deeply colored or dark, as shown in Figure ~b. In the homeotropic smectic texture, the dye molecules have minimum absorption, since they do not absorb ligh-t incident upon the edge of their molecular structure.
This texture, therefore, develops a transparent background.
This results in a high contrast display that is suitable for direct viewing. No external polarizers are required.
The addressing technique is substan-tially the same as smectic displays without the dye.
Now referring to Figure 1, an exploded view of a -typical multiplexed, visual display device 10, is illustrated. The device comprises a cholesteric-smectic liquid crystal medium 11 containing the pleochroic dye, which material is disposed between two glass substrate plates 12 and 13, respectively. The top substrate plate 12 supports a plurality of column electrodes Cl, C2, C3, etc~, which make up one half of the x y matrix for addressing the liquid crystal material 11. The column electrodes are made of electrically conductive, light transparent material such as indium tin oxide~ which can be vacuum deposited on the glass plate 12.
The bottom plate 13 supports a plurality of row electrodes rl, r2, r3, etc., which make up -the remaining half of the x y matrix. The row electrodes are electrically conductive and are made diffusely reflective with material such as silver or aluminum. The row .~

electrodes are designed to be diffusely reElec-tive in order to provide good display image with wide viewing angle.
The liquid crystal medium 11 is generally sealed between the two substra-te plates 12 and 13 wi-th the electrodes in contac-t on either side. Light (generally ambient) is passed -through (arrow 18) the glass composite, as shown.
The physical operation of -this display 10 can best be illustra-ted wi-th a simple example of a 5 x 7 matrix displaying a charac-ter "A", as shown in Figure 2. The rows of the matrix are -tied together at one end to the common 16 and are sequentially heated by applying electric pulses to the other ends 17. In time zone 0, (see Figure 3) row 1 is heated such -tha-t the liquid crystal material over the row 1 electrode rl is in the isotropic or cholesteric sta-te. In time zone 1, row 2 elec-trode r2 is hea-ted. In -the meantime, row 1 rapidly cools down and the dots associated with it are written by applying electric vol-tage on the column electrodes. In this example, electrodes C
and C5 have voltage applied such that the do-ts rlcl and rlc5 will be in the transparent state. C2, C3, C~ have no voltage applied, and the dots rlc2, rlc3, rlc4 have a colored light absorbing texture. During time zone 2, row

3, electrode r3 is heated and row 2 cools down, and the voltage on the columns assume the values corresponding to the "on" and "off" pattern of dots associated to row 2.
The entire waveform for displaying a character l'A", is shown in Figure 2~
The colored light absorbing texture associated to the "on" dots is metastable and has long relaxation time generally over a few months. This texture can be automatically erased by hea-ting the row during rewriting of the display. The light absorbing texture is not affected by the writing voltage applied on the column electrodes.
This assures that "cross-talk" will not be a problem, and makes possible a large scale matrix display.
The erase-writing process for this display is .~

z very fast. Generally, less than a 100 ~ second writing -time can be achieved. If the display is refreshed at fR
-times per second, the total number of rows that can be multiplexed will be M =
R T
where ~r - the tlme required -to write the row.
W.ith :ER = 30 her-tz, which is similar to the rate oE a conventional CRT, and T = 100 ~ sec., we have n = 333 rows. Thus, the display can be multiplexed up -to a rather large number of rows.
In prac-tical display driving, the heating pulse can be applied over several time zones before the cooling and wri-ting cycle. This lowers the voltage requiremen-t for -the heating pulses. However, the heating pulse should be IS short enough to avoid heat spreading to the neighboring rows and to minimize glass heating that inhibits rapid cooling.
A high contrast is achieved for the colored or black image due to -the light absorbing character of the dye material vis-a-vis the transparent background.
The contrast is further improved by the diffusely reElective nature of the row electrodes, which provide a double light pass back through (arrow 15) the medium 11, wherein the unaddressed dye molecules in the light absorbing state (image) can absorb more light as compared to the addressed transparent background.
The medium 11 is depicted in the sensitized (addressed) homeotropic phase in Figure 4a, and is shown in the unaddressed light absorbing phase in Figure 4b. Light (arrow 20) entering the homeotropic material of Figure 4a, passes between the liquid crystal molecules 21. The dye molecules 22 are not light absorbing in this phase, because they are locked in the crystal to confront the light rays upon their edge, as shown.

.j .~

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However, in the light absorbing phae, -the dye molecules 22 are locked in the crystal molecules 21 in a randomly angled pat-texn, as shown in Figure 4b. In this phase, the dye molecules 22 will strongly absorb the impinging light rays 20 to produce an intensely colored or dark image.
The crystal liquid medium 11 can be comprised o:E
at least one al.kyl cyano biphenyl compound.
More particularly, the liquid crystal will be comprised of a mixture of cyano biphenyl compounds of the :Eollowing formulas:

X is C8H17 ~ ~ CN 50.5~ by weight Y is CloH21 _ ~ ~ CN 41.4 Z is C2H5 - ICH CH2 ~ CN 8.1 One example of a workable cholesteric liquid crystal comprises a mixture of X, Y, and Z materials, each having a percentage by weight in an approximate ran~e of:
40 to 60 of X; 30 to 50 of Y; and 5 to 15 of Z;
respectively, where:

X is C8H17 ~ - C - N;
Y is CloH21 * ~ C N; and Z is C2H5 - tCH ~ ~ - C a N.

More particularly, the aforementioned mixture can comprise:

X is C8H17 ~ ~ CN 50.5~ by weight Y is C10H21 - ~ CN 41.4%
Z is C2H5 - ~CH - CH2 ~ CN 8.1 7~;~2 Which has a phase transition as follows:
34.5C 40.7C
Crystal - ~ Smectic ~ Cholesteric~ -~ Istropic In a liquid crystal which has a smectic phase followed by a nematic phase, good display performance, requires the temperature range of the nematic phase to be narrow. With cholesteric materials, however, the temperature range of the cholesteric phase does not necessar:ily have to be narrow.
To the host ma-terial, a high order parameter pleochroic dye or dye mixture is added in a range of approximately 0.5 to 3 percent by weight of the total composi-tion.
More particularly, about 1% by weight of a purple dye having the formula:

N ~ - N = N ~ N = N ~ N = N ~ N

is added to the above cholesteric liquid crystal medium.
This dye is sold by E.M. Laboratories, Elmsford, N.Y.
While the medium generally features pleochroic dyes of high order parameter, it is also contemplated that other coloring agents such as:

C4Hg ~ N = N ~ N = N ~ N

may also provide reasonable image contrast.
Having thus described the invention, what is desired to be protected by Letters Patent is presented by the following appended claims:

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Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A liquid crystal composition which can be thermally addressed and which can provide a dark image upon a lighter background, comprising a mixture comprising approximately from 40 to 60 weight percent of "x" and approximately from 30 to 50 weight percent of "y", and approximately 5 to 15 weight percent of "z" wherein x, y, and z are represented by the formulas:
x.
y. z.

at least one pleochroic dye of high order parameter approximately from 0.5 to 3 weight percent based upon the total composition mixture.

2. The liquid crystal composition of claim 1, wherein said pleochroic dye comprises:

3. The liquid crystal composition of claim 1, comprising approximately 50.5 percent weight of x, 41.4 percent weight of y, and 8.1 percent weight of z.

4. The liquid crystal composition of claim 3 wherein said pleochroic dye comprises:

5. The liquid crystal composition of claim 4, wherein said dye is approximately 1 weight percent of the total composition mixtue.