THERMALLY ADDRESSED SMECTIC LIQUID CRYSTAL DEVICE

Abstract: A visual display device using a thermal electro-optic effect to provide a dark image upon a lighter background, and which can be multiplexed to a large number of rows, comprising a liquid crystal material, such as a cholesteric liquid crystal compound, mixed with a pleochroic dye of high order parameter. When the material experiences a rapid transition from a higher to a lower, smectic thermal phase, two different textures or light states are developed: a light absorbing state, and a transparent state which is developable by applying a sensitizing voltage to certain portions of the medium.

Description: THERMALLY ADDRESSED SMECTIC LIQUID CRYSTAL DEVICE Technical Field The invention pertains to thermally addressable liquid crystal display devices, and more particularly to such devices based on cholesteric-smectic liquid crystal compounds and which use a light absorption technique to provide a dark image upon a lighter background. Background Art Heretofore, the ability to fabricate large scale multiplexed liquid crystal displays was very difficult. This difficulty was primarily due to "cross-talk" effects, and the necessity to quickly refresh the slow responding liquid crystal medium. Large scale multiplexed displays notoriously have had problems with "cross-talk", i.e. the unwanted sensitizing of partially selected display elements. This problem results from the small root mean square voltage ratio between the "on" and "off" elements achievable in a large scale multiplexed liquid crystal 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 Hz (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 poor (below commercial standards) optical performance as the X-Y matrix gets larger and larger. These two problems have severely limited the ability to provide large scale multiplexed displays, and to date, no one has produced a device which has high contrast, wide viewing angle, 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 hasresolved the aforementioned problems, while providing a new liquid crystal device having many aavantages over the prior art. While the present invention is concerned primarily with large scale, thermally addressed multiplexed devices, its new light absorbing method 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. The invention features certain classes of smectic liquid crystal hosts that have a nematic 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 mixture is light absorbing due to the dye which strongly absorbs incoming light. The "offn elements and the background have homeotropic smectic A texture, where the dye exhibits minimumabsorption. The concept of pleochroic dye switching as the Guest Host effect in nematic liquid crystals, was first suggested in an article to: 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 twist 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 contrast of the image. Dyes of high order parameter in a cholesteric liquid crystal host were first suggested in an article to: D. L. White, G. N. Taylor, J. of App. Phys. 45 4718 (1974). Displays using this liquid crystal medium have high contrast and do not require external polarizers. These displays have high brightness and a wide viewing angle not available with the field effect twist nematic liquid crystal displays. Such devices use a cholesteric to nematic transition effect with liquid 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 absorption. In the on condition, the dye is in a homeotropic 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 addition, 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 contrast 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 non-anoate. 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 parameter and light stable dyes have become available. Devices using these dyes will provide viable displays for many applications. However, they have two major drawbacks which may restrict their application to simple displays of very low information content only. These dye displays are very difficult to multiplex. Even a few rows represent a state of the art development. Large size matrix addressing has only been achieved by adding external non-linear elements to each display element. For non-emissive (reflective) 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 complication increases the complexity and cost. In 1978 C. Tani and T. Ueno discussed the application of pleochroic dyes to certain smectic liquid crystals in a scientific paper (Appl. Phys. Lett. Vol. 33 No. 4, 15 Aug. 1978). The authors, however, specifically teach against the use of the smectic "A" phase as having utility in the pleochroic dye system: they indicate that it has application only in scattering applications such as in laser addressed light 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 that 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 heating as distinct from the laser heating as described in other art. Further it utilizes rapid cooling of the element with liquid crystals 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. Also 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 SMECTIC LIQUID CRYSTAL DISPLAY: M. Hareng, S. Le Berre, R. Hehlen, and J. N. Perbet, Thomson-CSF Laboratoire Central de Rechereches. Society of Information Display, Conf. Record of 1980 Biennial Display Res. Conf. "Post deadline paper". Such a system does not use dyes, and employs a scattering light technique, rather than a light absorption technique as described by this invention. In addition, the system described is embodied in a very different device than detailed in this invention. Because of the crucial difference of the light scattering as compared to light absorption, the device can only be vieWed through a projection optical system that results ln a very bulky, power intensive system. While the prior art teaches the use of pleochroic dyes of high order parameter for use in liquid crystal media, it should also be noted that these dyes are used primarily to enhance the light affects produced by the thermal phase transition of the media. The invention by contrast, relies upon the dye to do most of the light absorption for the crystal medium; the medium acting as a vehicle for orientating the dye to develop a light absorbing stance. Disclosure of 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 liquid crystal compound, preferably a 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 lower thermal phase is a smectic phase, and the upper thermal phase is preferably a cholesteric phase. The medium develops two textures in the smectic phase: a light absorbing texture and a homeotropic texture. The homeotropic texture is developed in portions of the medium by sensitizing the medium as it passes rapidly from an upper thermal phase to the lower, smectic phase. The light absorbing texture develops in the unsensitized portions of the medium as it goes through the transition to the smectic phase. The medium is sensitized by applying a voltage to those portions of the medium to be addressed. The addressed portions develop a substantially transparent light state, while the unaddressed portions develop a substantially light absorbing state. The coloring agent or dye which is locked within the liquid crystal medium as it develops its light 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 at right angles to each other, and in different planes. In order to obtain a pleasing direct viewable display device, 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 enhancing light absorption. The liquid crystal medium will generally contain an octyl cyano biphenyl impound and will have two thermal transitions: between an isotropic and either a nematic or cholesteric phase, and between the nematic or cholesteric phase and a smectic phase, in particular a smectic "A" phase. More particularly, the liquid crystal will be comprised of a mixture of cyano biphenyl compounds of the following formulas: EMI7.1 In one type of mixture, the octyl cyano biphenyl can be represented in the mixture in a weight percentage from approximately 35 to 65, and the deca cyano biphenyl can be represented in a weight percentage from approximately 30 to 60 In another type of mixture, there can he added to the above materials cyano alkyl oxy biphenyl compounds of the following formulas: EMI7.2 The octyl cyano oxy biphenyls can be represented in the mixture in a weight percentage from approximately 15 to 30 and the deca cyano oxy biphenyl in a weight percentage from approximately 11 to 26 while the octyl cyano biphenyl is approximately in the range 20 to 35 and the deca cyano biphenyl is approximately in the range 24 to 39 by weight. The pleochroic dye can be: 4(4'-N=N-Dimethylamine phenylazo) azobenzene, and can have a weight percent range from 0.5 to 3.0 of the total composition. More particularly, the octyl and deca cyano biphenyls may be mixed by weight 55.6 and 44.4 percent, respectively, and the 4(4'-N=N-Dimethylamino phenylazo) azobenzene can have 1 to 1.5 weight percent of the total composition. Also, the octyl, deca, octyl oxy, and deca oxy cyano biphenyls can be mixed by weight percentage 27.5, 31.5, 22.5, and 18.5, respectively. The dye will be mixed with the above in 0.75% to 1.75% weight percent of the total, respectively. A variety of cholesteric liquid crystal compounds may be used in a preferred embodiment, as many liquid crystal compounds with optically active terminal groups exhibit cholesteric phase. Some of them also exhibit one or more smectic 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. Liq. Cryst.,V. 27 pp. 417, 1973,lists 40 schiff base compounds with the general structure as: EMI8.1 where: R = OCO-(CH2)n - CH3 and C - N. Many compounds with high value of m and n exhibit a cholesteric phase followed by smectic phases upon cooling. To cite a few examples, we have: EMI8.2 EMI8.3 <tb> Crystal <SEP> 46.3 C <SEP> Smectic <SEP> <SEP> II <SEP> 74.5 C <SEP> Smectic <SEP> I <SEP> 77 C <SEP> Cholesteric <SEP> 83.6 C <SEP> Isotiopic <SEP> <tb> EMI8.4 EMI8.5 <tb> <SEP> 66 C <tb> Crystal <SEP> 40 C <SEP> Smectic <SEP> 48 C <SEP> Cholesteric <SEP> # <SEP> <SEP> Isotropic <tb> 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 anisotropy should be strongly positive. This is usually obtained by using liquid drystal compounds having C # N as one of the terminal 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 15 of Z; respectively, where: EMI9.1 More particularly, the aforementioned mixture can comprise: EMI9.2 by weight EMI9.3 Which has a phase transition as follows: EMI9.4 <tb> 34.5 C <SEP> 40.7 C <tb> Crystal <SEP> # <SEP> Smectic <SEP> # <SEP> <SEP> Cholesteric <SEP> # <SEP> Isotropic <SEP> <tb> 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 necessarily have to be narrow. To the host material, 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 composition. More particularly, about 1% by weight of a purple dye having the formula: EMI10.1 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 through its thermal transition from its upper thermal phase, preferably cholesteric phase, to its lower thermal, smectic phase. 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 nematic or cholesteric phase rapidly. This greatly enhances the optical effect and results in greater contrast. Certain portions of the media are sensitized. These portions define the background of the medium. These sensitized portions develop the substantially light transparent state when the medium passes into the smectic thermal phase. The remaining unsensitized portions of the medium develop a light absorbing statue. When light (generally ambient) is passed through the medium, the unsensitized portions absorb the light to provide a dark image upon the lighter sensitized background. The addressed portions of the medium may be sensitized in a chronological sequence. In a preferred embodiment based on cholesteric liquid crystal compounds, when the liquid crystal material cools down either from the isotropic state through the choesteric state to the smectic state, or from cholesteric to smectic state, the texture obtained in the smectic 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 Km 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 5000C/min) we have two cases: (1) Cooling from the isotropic phase through a narrow (approximately 100C or less) cholesteric phase: a clear homeotropic texture is obtained; (2) Cooling from the cholesteric phase to smectic phase: a scattering SA texture is obtained if the cooling rate is up to 100" C/min. region. With slower cooling rate, the clear homeotropic texture is obtained. With fast cooling rate corresponding to the actual display operation (up to 250,0000C/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 scattering state becomes a light absorbing state. Because of its finer structure, the color is very deep. Brief Description of Drawings 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 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 liquid crystal medium of the device of Figure 1, when the medium passes into its smectic phase; Figure 4a depicts a homeotropic, substantially light transparent texture, and Figure 4b illustrates a substantially light absorbing texture. Detailed Description Generally speaking, this invention relates to new methods, compositions, and visual devices utilizing the thermal addressing of liquid crystal media. The visual devices of this invention feature a 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 incorporate pleochroic dyes of high order parameter into a smectic A liquid crystal material that has either a nematic or cholesteric phase upon heating. By using a thermal electric addressing technique described hereinafter, this display has major advantages over the previously known dye switching displays. Smectic A phase is one of the most common liquid crystal phases encountered. Some materials having smectic A phase also exhibit a nematic phase upon heating. One example is the octyl cyano biphenyl: EMI13.1 which has a phase transition as follows: EMI13.2 <tb> <SEP> 20 C <SEP> 32 C <SEP> 40 C <tb> crystal <SEP> # <SEP> Smectic <SEP> A <SEP> # <SEP> Nematic <SEP> # <SEP> Isotropic <SEP> <tb> Alternatively, one may utilize a cholesteric licuid crystal with positive dielectric anisotropy, which can develop a homeotropic texture under the influence of an electric field. A 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 transition, one can create at his will, either a colored state or a non-colored state. Once these states are formed, they are stable until erased by heating into the isotropic or cholesteric phase again. Although the above description assumes that the material is heated into the isotropic state, it is noted that this is not 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 address the display elements. The other type is x y matrix addressed. The row electrodes are heated sequentially 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 affected. In other words, only the dots where the liquid crystal material is rapidly cooling to the smectic state respond to the writing pulses on the column electrodes. As the liquid crystal material cools rapidly through the nematic or cholesteric phase to the smectic phase, it can form two different textures. With a voltage applied on the column, the liquid crystal material is switched to a homeotropic state during the nematic or cholesteric phase and assumes the homeotropic smectic A texture after cooling is completed. Without the applied voltage, a light absorbing texture is developed instead. Thus, the dots associated with a rapidly cooling row electrode can be written into transparent state or a light absorbing state by applying or not applying voltages on the columns. The smectic or cholesteric-smectic material used in the inventive display device has positive dielectric anisotropy. 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 nematic or cholesteric phase rapidly. This greatly enhances the optical effect and results in greater contrast. Table 1 below illustrates this effect: TABLE 1 Impact of Rapid Cool Rapid Cool (1) Slow Cool (2) Test CR Brightness CR Brightness 1 3.1 28% 1.1 29% 2 5.24 34% 1.0 33% 3 6.6 29% 1.2 29% 4 8.4 28% 1.2 29% 5 8.7 32% 1.2 31% (1) Typical rapid cool; greater than 500"C per sec. (2) Typical slow cool; less than 5000C per sec. The present invention, however, must be carefully distinguished from other similar systems wherein a scattering texture rather than a light 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. The optical contrast developed by a scattering texture against a transparent texture is similar to those obtained with the dynamic scattering effect. Under many commonly encountered illumination conditions, it will not give a pleasing legible, high contrast image. The situation becomes quite different, however, when a pleochroic dye of high order parameter is introduced into the smectic A material. The dye becomes locked into the liquid crystal, and assumes the orientation of the liquid crystal molecules. The dye molecules in the scattering texture of the host absorb light strongly, transforming the normal scattering texture into a light absorbing texture, either deeply colored or dark, as shown in Figure 4b. Alternately, when a randomly oriented texture is produced due to rapid cooling through a nematic phase to the smectic phase, the dye molecules in the host absorb light strongly, transforming it into a light absorbing texture, either deeply colored or black. In fact, the randomly oriented texture may not scatter light as it usually does, if liquid crystal materials of very low birefringence are used. It will, however, strongly absorb the light due to the orientation of some of the dye molecules having their lightabsorbing axis partially transverse to the light path. In the homeotropic smectic texture, the dye molecules have minimum absorption, since they do not absorb light incident upon the edge of their molecular structure. This texture, therefore, develops a clear textured, i.e., transparent, background. This results in a high contrast display that is suitable for direct viewing. No external polarizers are required. The addressing technique is substantially 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 liquid crystal medium 11, preferably including a cholesteric-smectic compound together with 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 24, C1, C2, C3, etc., which make up one half of the x y matrix for addressing the liquid crystal material 11. The column electrodes 24 are made of electrically conductive, light transmitting 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 26, rl, r2, r3, etc., which make up the remaining half of the x y matrix. The row electrodes 26 are electrically conductive and are made diffusely reflective with material such as silver or aluminum, which can be deposited on the glass plate 13. The diffuse reflecting row electrodes provides a good display image with a wide viewing angle. The liquid crystal medium 11 is generally sealed between the two substrate plates 12 and 13 with the electrodes in contact 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 illustrated with a simple example of a 5 X 7 matrix displaying a character "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 that the liquid crystal material over the row 1 electrode rl is in the isotropic or cholesteric state. In time zone 1, row 2 electrode r2 is heated. In the meantime, row 1 rapidly cools down and the dots associated with it are written by applying electric voltage on the column electrodes. In this example, electrodes C1 and C5 have voltage applied such that the dots rlcl and rlc5 will be in the clear or transparent state. C2, C3, C4 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 rapidly 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 "A", is shown in Figure 2. The colored light absorbing texture associated with the "on" dots is metastable and has long relaxation time of the order of at least months. This texture can be automatically erased by heating the row during rewriting of the display, or can be erased by applying a voltage on the columns substantially higher than the writing voltage. Once the colored light absorbing texture is formed, it will not be affected by the writing or sensitizing voltage. 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 very fast. Generally, less than a 10cup second writing time can be achieved. If the display is refreshed at f, r times per second, the total number of rows that can be multiplexed will be: fr x Tw where Tw = the time required to write the row. With fr = 30 hertz, which is similar to the rate of a conventional CRT, and Tw = lOOU sec., we have n = 333 rows. Thus, the display can be multiplexed up to a rather large number of rows. In practical display driving, the heating pulse can be applied over several time zones before the cooling and writing cycle. This lowers the voltage requirement for the heating pulses. However, the heating pulse should be short enough to avoid heat spreading to the neighboring rows and to minimize glass heating that then prevents the rapid cooling desirable for good optical images. 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 reflective nature of the heating 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. However, in the light absorbing phase, the dye molecules 22 are locked in the crystal molecules 21 in a randomly angled pattern, 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 of at least one octyl cyano biphenyl compound. More particularly, the liquid crystal will be comprised of a mixture of cyano biphenyl compounds of the following formulas: EMI19.1 In one type of mixture, the octyl cyano biphenyl can be represented in the mixture in a weight percentage from approximately 35 to 55, and the decyl cyano biphenyl can be represented in a weight percentage from approximately 30 to 60. In another type of mixture, the undecyl cyano biphenyl can be added to the above, and can have an approximate weight percent range from 15 to 35. The pleochroic dye can be: 4(4'-N=N-Dimethylamino phenylazo) azobenzene, and can have a weight percent range from 0.5 to 3.0 of the total composition. More particularly, the octyl and decyl cyano biphenyls may be mixed by weight 55.6 and 44.4 percent, respectively, and the 4(4'-N=N-Dimethylamino phenylazo) azobenzene can have 1 to 1.5 weight percent of the total. Also the octyl decyl, and undecyl cyano biphenyls can be mixed by weight percentage: 44.4; 31.3; and 24.5, respectively. The dye will be mixed with the above in 1 to 1.5 weight percent of the total composition. Other compositions of the liquid crystal medium can be: EMI19.2 4-Cyano-4'-n-decyl biphenyl 458 by weight EMI20.1 4-Cyano-4'-octyl biphenyl 55%; or 4-Cyano-4'-octyl biphenyl 27.5% by wt. 4-Cyano-4'-decyl biphenyl 31.5% 4-Cyano-4'-octoxy biphenyl 22.5% 4-Cyano-4' -decyloxy b iphenyl 18.5%; or 4' -Ethyl-trans, trans-bicyclohexyl-4-carbonitrile . To the above liquid crystal compositions, any one of the following coloring agents may be added: EMI20.2 by weight; or EMI20.3 a dye mixture 1/4% by weight of: EMI20.4 EMI21.1 In a preferred embodiment utilizing cholesteric liquid crystal compounds, the crystal liquid medium 11 can be comprised of at least one alkyl alkyl cyano biphenyl compound. In such an embodiment, more particularly, the liquid crystal will be comprised of a mixture of cyano biphenyl compounds of the following formulas: EMI21.2 50.5% by weight EMI21.3 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 15 of Z; respectively, where: EMI21.4 More particularly, the aforementioned mixture can comprise: EMI22.1 50.5% by weight EMI22.2 Which has a phase transition as follows: EMI22.3 <tb> 34.5 C <SEP> 40.7 C <tb> Crystal <SEP> # <SEP> Smectic <SEP> # <SEP> Cholesteric <SEP> # <SEP> Isotropic <tb> 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 necessarily have to be narrow. To the host material, 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 composition. More particularly, about 1% by weight of a purple dye having the formula: EMI22.4 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: EMI23.1 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:

Claims: Clams 1. An electro-thermally addressed visual display device utilizing a medium containing at least one liquid crystal compound mixed with at least one pleochroic dye, said medium having at least two thermal phases, one of said phases being a smectic phase and including means for heating said medium to cause a transition to another phase, thereby altering the optical properties of the device, characterized by a) said medium comprising a liquid crystal compound having a type A smectic phase and a piochroic dye of high order parameter and having dielectric an isotropy, the high order parameter and dielectric an isotropy of the dye providing in combination with the smectic A phase of the liquid crystal compound an electrically orderable texture in which the mixture is substantially transparent, while normally, in the absence of such electrical ordering, the mixture has a substantially light absorbing texture, and b) means for appwing a sensitizing voltage to address portions of said medium when said medium passes rapidly into said smectic A phase to develop said substantially transparent ordered texture while the remaining portions develop said substantially light absorbing texture, thus providing a dark image against a lighter background. 2. A device according to claim 1, characterized by said liquid crystal compound comprising an alkyl cyano biphenyl. 3. A device according to claim 1, characterized by means for rapidly heating said medium to said another phase without substantially heating the surroundings such that upon cessation of said heating pulses the medium rapidly cools to said smectic A phase, including at least one electrode adjacent said medium for heating said medium to said another phase. 4. A device according to claim 3, further characterized by said heating electrode being disposed adjacent said medium for diffusely reflecting light passing through the medium back through the medium to thus provide a dark image observable from the same side as a light source passing light toward said device. 5. A device according to claim 1 characterized by at least one electrode for appling a sensitizing voltage to said addressed portions of said medium. 6. A device according to claim 1, further characterized by a matrix of electrodes disposed about said medium, a number of said matrix electrodes defining rows and a number of said matrix electrodes defining columns, said row electrodes for heating said medium to cause said medium to pass through a thermal transition between said smectic A phase and said another phase, and said column electrodes for applying sequential sensitizing voltages to address portions of said medium to develop said substantially transparent texture when said medium rapidly cools and passes rapidly into said smectic phase, the remaining unaddressed portions of said medium developing said substantially light absorbing texture. 7. A method of using the device of claim 1 to display a dark image against a lighter background, characterized by a) passing said medium through a rapid thermal transition from an upper thermal phase to a lower thermal, smectic phase; b) sensitizing certain portions of said medium defining a background to develop a substantially light transparent state in said background when said medium passes rapidly into said lower thermal, smectic phase, the remaining unsensitized portions of said medium developing a light absorbing state; c) passing light through said medium; and d) absorbing light in said unsensitized portions of said medium to provide a dark image upon said lighter sensitized background. 8. A method according to claim 7, wherein said dark image is observable from the same side as the light passing through said medium, and characterized by the further step of: e) diffusely reflecting the light passing through said medium back through said medium. 9. A method according to claim 7 characterized by said medium having dielectric anisotropy and by the further step of: e) appling a voltage to said certain portions of said medium in order to sensitize said certain portions into developing said light transparent state. 10. A device according to claim 1, characterized by said medium comprising a mixture of approximately from 35 to 65 weight percent of "x" and approximately from 30 to 60 weight percent of "y", wherein "x" and "y" are represented by the formulas: EMI27.1 at least one pleochroic dye of high order parameter approximately from 0.5 to 3 weight percent based upon the total composition mixture. 11. A device according to claim 1, characterized by said pleochroic dye comprising 4(4'-N=N-Dimethylamino phenylazo) azobenzene. 12. A device according to claim 1, characterized by said liquid crystal medium comprising a liquid crystal cholesteric compound.

Inventors: BUU-VINH CHUNG, DAVID, LU, SUN
Applicants: MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1982
Main classification: G02F1
IPC codes: G02F1