JPS6086521A - Phase transition type liquid crystal display element - Google Patents

Abstract

PURPOSE:To reduce power consumption and to prevent deterioration of an electrode by using a compensation type liquid crystals made of a mixture of cholesteric liquid crystals having a dextro-rotatory spiral structure and those having a levo-rotatory structure, and lowing phase transition temp. CONSTITUTION:Cholesteric liquid crystals having a dextro-rotatory spiral structure, such as cholesteryl chloride are mixed with those having a levo-rotatory structure, such as cholesteryl myristate, to prepare compensation type liquid crystals. An objective phase transition type liquid crystal display element capable of writing by using heat or an electric field is obtained by using said compensation type liquid crystals. As a result, since the compensation point temp. of the compensation type liquid crystals using such a mixture of cholesteryl chloride and cholesteryl myristate in a 1.75:1 ratio is as low as 44 deg.C, the power consumption which has been needed so far can be lowered.

Description

DETAILED DESCRIPTION OF THE INVENTION +a+ Technical Field of the Invention The present invention relates to a liquid crystal display element, and more particularly to a phase change type liquid crystal display element that utilizes an accumulation effect based on a phase transition.

...) Technology Background Liquid crystal display elements can be used to construct flat-panel display devices with low voltage and low power consumption.Also, because they are light-receiving display elements, their brightness depends on external illumination light, and they are self-luminous. It has the design and manufacturing advantage that there is no need to make a compromise between brightness and operating characteristics, as is the case with display elements, and in particular, this method uses the difference in optical reflectance between selected points and non-selected points. In this case, the advantage is that high contrast can be obtained in a bright operating environment, and these characteristics are expected to be applied to display devices for office automation (08) equipment.

ic) Prior Art and Problems Display elements using nematic liquid crystals have already been used in display devices with relatively small display capacities, such as electronic desktop calculators and personal computers. However, since nematic liquid crystals do not change sharply in their optical properties (light transmittance, reflectance, contrast ratio, etc.) near the dark value voltage of the display, crosstalk due to half-selective voltage is likely to occur. When displaying characters in don't matrix, it is theoretically impossible to display more than four lines (64 scanning lines). Furthermore, when the number of scanning lines increases, liquid crystal molecules at non-writing points are oriented by repeated application of a high-frequency half-selective voltage.
It becomes too narrow to be of practical use.

In response, a phase transition type liquid crystal display system has been proposed that utilizes the phase transition of smectic liquid crystal from a smectic phase to an isotropic liquid due to heat, and performs writing using heat and an electric field (Display magazine). , 1981 October issue, SID; 5ociety of Informa
tion Dis-play Symposium Proceedings 238-239rJ, and pages 252-253].

In this method, a liquid crystal that exhibits a smectic phase at room temperature is used, and when the liquid crystal is heated to a temperature that allows the above-mentioned phase transition to become an isotropic liquid and is cooled, an electric field is applied and the liquid crystal enters a bomeotropic state (transparent). ), and on the other hand, it becomes a focal conic state (opaque) when no electric field is applied, and each state is frozen unless heat is applied to reach the phase transition temperature again. Since the written information is stored as described above, there is no need for refresh scanning, and since cooled smectic liquid crystals are usually solid and do not undergo phase transitions due to electric fields, crosstalk does not occur. It has the advantage that there are few possibilities and there is no limit on the display capacity (number of scanning lines) in principle.

However, the phase transition temperature of smectic liquid crystals is as high as 100 to 200° C., which has the disadvantage that the amount of heat added is extremely large. By the way, 250
When performing matrix display of x 250 dots, approximately 10 watts of power is required for heat. This value is not only extremely large for a liquid crystal display element, but it also means that a current of up to 1 ampere, for example, is passed through the transparent resistive electrode or reflective resistive electrode, which also functions as a heating element, which reduces the deterioration of the resistive electrode. In addition, in practical use, a dedicated large current capacity I is required for the driver of the drive circuit.
There was also the drawback that development of C was essential.

(d1 Purpose of the Invention) The present invention aims to reduce the required heat generation of resistive electrodes by lowering the phase transition temperature in a phase transition type liquid crystal display element that performs writing using heat and electric field. It is an object of the present invention to provide a liquid crystal display element that can display a large capacity with low power consumption and high contrast ratio by reducing the amount and power consumption.

te1 Structure of the Invention The present invention provides that in a compensated liquid crystal consisting of a mixture of a cholesteric liquid crystal having a dextrorotatory helical structure and a cholesteric liquid crystal having a levorotatory wI helical structure, the phase from the liquid crystal state to the isotropic liquid state is Taking advantage of the relatively low transition temperature,
It is characterized in that thermal scanning is performed using relatively low power to reach the phase transition temperature of the compensation type liquid crystal, and writing is performed by applying an electric field to the writing point in synchronization with the thermal scanning.

(Example of f1 invention Embodiments of the present invention will be described below with reference to the drawings.

Compensated cholesteric liquid crystal is composed of a mixture of cholesteric liquid crystal with a dextrorotatory helical structure and cholesteric liquid crystal with a levorotatory WLL structure, and is composed of a cholesteric liquid crystal with a winding direction opposite to the helical axis as described above. The link liquid crystal is a liquid crystal whose spiral pitch is extremely long by compensating for each vl twist. Il! in the compensated liquid crystal. The ll' rotational pitch changes depending on the liquid crystal material, its mixing ratio, and temperature.

For example, cholesteryl chloride (C
C), cholesteryl myristate (C
In the case of a compensated liquid crystal consisting of M), it has been reported that the helical pitch changes as shown in Figure 1 (J
our, Chem, phys, 51+No, 8+pI+
3213-3219 (1969)). As shown in the figure, the compensation type liquid crystal has an infinite spiral pitch near 42° C., that is, it becomes a nematic phase, and at lower temperatures, it assumes a levorotatory helical structure, and the helical pinch decreases as the temperature decreases. On the other hand, at temperatures higher than 42° C., a dextrorotary spiral structure is adopted, and the helical pitch decreases as the temperature increases. In other words, the compensation type liquid crystal undergoes a phase transition from a cholesteric phase to a nematic phase at around 42°C. If the changes in optical properties caused by the application of an electric field during the cooling process of this nematic phase liquid crystal are frozen as they are, the above-mentioned smectic liquid crystal with a high phase transition temperature (10
It is possible to obtain a phase change type liquid crystal display element that can write with a much lower amount of heat than a conventional phase change type liquid crystal display element that utilizes a temperature of 0 to 200°C.

The structure of the phase change type liquid crystal display element according to the present invention is exactly the same as that of a conventional phase change type liquid crystal display element that utilizes heat and electric field effects. That is, in Figure 2,
On the surfaces of the glass substrates 1 and 2, strip electrodes 3 and 4 made of, for example, an indium tin oxide thin film are formed, and the strip electrodes 3 and 4 are arranged to intersect with each other to form a minute gap (for example, 20 μm). They are facing each other. The peripheries of the glass substrates 1 and 2 are hermetically sealed, and the small gap therein is filled with the compensated cholesteric liquid crystal.

One of the strip electrodes 3 and 4, for example strip electrode 3, is given a suitable resistance value (for example, IKΩ), and both ends thereof are connected to a heating power source 6 via a suitable switch circuit 5. There is. On the other hand, the strip electrode 4 is connected to a write signal type IP,7.

Now, the switch circuit 5 is switched to sequentially apply current to the strip electrodes 3 to generate heat. As a result, along the strip electrode 3, the nearby compensated cholesterin liquid crystals are successively heated to their phase transition temperature. That is, the surface of the compensated cholesteric liquid crystal layer is thermally scanned. A strip electrode 3
Immediately after energization is carried out and the cooling process begins, a write signal is applied to the in+ strip electrode 4 which intersects the strip electrode 3 and whose intersection becomes the display selection point. The strip electrodes 3 are sequentially switched, and a write signal is applied to a predetermined strip electrode 4 in the same manner as described above. In this way, the compensated cholesteric liquid crystal layer is thermally scanned over the entire surface, and a writing signal is applied to the predetermined strip electrode 4 in synchronization with the nuclear thermal scanning, thereby writing on the entire display surface. Complete.

How can we explain the above process in terms of physical properties? The ili-compensated cholesterin liquid crystal transitions into a nematic phase temporarily by thermal scanning, ie during a period of time when it is heated near its phase transition temperature. The liquid crystal in the nematic phase has electric field responsiveness to a relatively low electric field, and is oriented so that its molecular dipoles are aligned in the direction of the electric field. The surfaces of glass substrates I and 2 are subjected to vertical alignment treatment in advance (processing to make it easier for the long axes of molecules to orient perpendicularly to the glass substrates).
is applied. Now, if we consider the case where liquid crystal molecules whose dipole moments are oriented in the long axis direction of the molecules are near the phase transition temperature, in the absence of an electric field, the liquid crystal molecules that are in contact with the glass substrate 1 have their long axes aligned with the glass substrate 1. The liquid crystal molecules inside the layer are oriented perpendicularly to the plane, and the liquid crystal molecules inside the layer have a spiral structure.
There is. When an electric field is applied, all liquid crystal molecules are rotated so that their long axes are perpendicular to the glass substrate surface. As a result, in the cooled portion in the presence of an electric field, ie, at the display selection point, the compensating cholesteric liquid crystal loses its turbulent structure.

This state in which all liquid crystal molecules are aligned in the same direction is called a homeotropic structure, and the liquid crystal layer is transparent.

On the other hand, in a portion cooled in the absence of an electric field, that is, at a non-selected point, the liquid crystal molecules return to the cholesteric phase by rapid cooling, and the helical axis becomes parallel or nearly parallel to the glass substrate. This structure is called a focal conic structure, exhibits light scattering properties, and appears cloudy.

In the above, if we consider the case where the dipole moment anisotropy of the liquid crystal molecules is perpendicular to the long axis of the molecules, due to the vertical alignment treatment applied to the surfaces of the glass substrates 1 and 2, the dipole moment anisotropy is near the phase transition temperature. The liquid crystal molecules in a liquid crystal layer in a certain nematic phase have their long axes oriented perpendicular to the glass substrate surface, as described above, but in the part where an electric field is applied, the helical structure is dissolved and the liquid crystal molecules move. The molecular arrangement is disturbed by the effect of unidirectional scattering (US). As a result, the liquid crystal molecules cooled in the presence of an electric field become randomly oriented with respect to the glass substrate, resulting in a cloudy appearance. On the other hand, in a liquid crystal layer cooled in the absence of an electric field, its helical axis is aligned perpendicular to the glass substrate. Therefore, when such a compensated liquid crystal is used, the presence or absence of an electric field and the transparent and cloudy states are opposite to the aforementioned compensated liquid crystal in which the dipole moment is oriented in the direction of the long axis of the molecule. It becomes a relationship.

As mentioned above, the compensated cholesterlink liquid crystal, which is once heated near the phase transition temperature and is in the nematic phase, takes on two different states depending on the presence or absence of an electric field during the cooling process, exhibiting transparency and light scattering properties, respectively. . That is,
Homeotropic state (transparent) and focal conic state! /3 (cloudy), or focal conic state (cloudy) and planar (or grandshuan) state (transparent)
Therefore, for example, by applying an electric field to a selected point and making it transition to a transparent (or cloudy) state, and on the other hand, making a non-selected point transition to a cloudy (or transparent) state under no electric field. , a display is performed. Each state is maintained continuously unless the compensated cholesterin liquid crystal is again heated to near its phase transition temperature and brought into a homeotropic state.

As mentioned above, a display element using compensated cholesteric liquid crystal has a memory function, and after one screen's worth of information has been written, it cannot be scanned for refreshing or applied voltage to maintain the display. do not need.

Further, the crosstalk in the compensated cholesteric liquid crystal is caused by the fact that the applied voltage Vc for causing the transition between the homeotropic state or the Brehner state and the focal conic state without heating causes the phase transition. This does not occur if the applied voltage vh is greater than the voltage vh required for each transition in the cooling process of the nematic phase when it is heated near the temperature. The ratio of Vc and vh is about 10:1, and the current value of the write signal is extremely small, so the voltage drop due to the resistance of the strip electrode functioning as a heating element is small. Therefore, in principle, there is no possibility of crosstalk occurring, and there is no restriction on the number of display electrodes.
It is possible to fabricate an element with any display capacitance.

Since it is not necessary to use a polarizing plate in the display element using the compensated cholesteric liquid crystal of the present invention, there is no viewing angle dependence like in other phase change type liquid crystal display elements, and 1
A viewing range of 80 degrees can be provided.

Next, a specific manufacturing example of a display element using the compensated cholesterin liquid crystal according to the present invention will be described.

As shown in FIG. 2, the surfaces of the glass substrates 1 and 2 on which 5×7 strip electrodes 3 and 4 made of indium tin oxide thin films are formed are vertically aligned using a silane cuff pulling agent. processed.

The strip electrodes 3 and 4 connect these with a minute gap (16 μm
) so that the glass substrates 1
After sealing the surroundings of and 2 with adhesive and evacuating the inside, cholesteryl coolide (CC): cholesteryl myristate (CM) = 1.75: 1.
A compensated type 6 Lesterlink liquid crystal having a composition of 0.00 was filled. The compensation point temperature of the compensation type cholesteric liquid crystal is 44℃
Met.

The above liquid crystal display element requires heating power of several tens to hundreds of
II11, the write signal could be driven with a rectangular wave voltage of 20 V and a width of 30 m5ec. Compared to a phase change type liquid crystal display element using conventional smectic liquid crystal, the write signal voltage is the same, but the heating power is about 20 times lower than that of a conventional phase change type liquid crystal display element with the same display capacity. It has been reduced to 1.

The above power consumption and storage effect results suggest that a high capacity dot matrix liquid crystal display can be realized using compensated cholesteric liquid crystals.

(Effects of the Invention According to the present invention, a phase change type liquid crystal display element with low power consumption can be realized. As a result, deterioration of electrodes can be prevented and long-term reliability of the display element can be improved. This has the effect of making it possible to reduce the cost of the drive circuit and promoting the practical use of liquid crystal display devices with a large display capacity.

[Brief explanation of the drawing]

FIG. 1 shows an example of the relationship between the temperature and helical pitch of a compensated cholesterlink liquid crystal made of a mixture of cholesteryl chloride and cholesteryl myristate, and FIG. 2 shows a display element using the compensated cholesterlink liquid crystal. FIG. 2 is a diagram showing an outline of the structure. In the figure, 1 and 2 are glass substrates, 3 and 4 are strip electrodes, 5 is a switch circuit, 6 is a heating power source, and 7 is a write signal power source. 1st A65 60 j550 45 474 35 30 2
5 Shi Nagi no Koma (C)

Claims (1)

[Claims] +11 In a phase change type liquid crystal display element that performs writing by superimposing heat and electric field, a mixture of a cholesteric liquid crystal having a dextrorotatory helical structure and a cholesteric liquid crystal having a levorotatory IIJ helical structure. A phase change type liquid crystal display element characterized in that it uses a compensation type liquid crystal consisting of. (2) A phase change type liquid crystal display element according to claim 1, characterized in that a compensation type liquid crystal made of a mixture of cholesteryl chloride and cholesteryl myristate is used. (3) The phase change type liquid crystal display element according to claim 2, wherein the mixing ratio of cholesteryl chloride and cholesteryl myristate is 1.75:1.00.