JPS63226624A - Liquid crystal element - Google Patents

Abstract

PURPOSE:To obtain the wholly strong contrast of the titled element by optically detecting at a temperature range lower than the temperature corresponding to the intermediate value of each max. tilt angles in the upper/lower phase transition points at the time of descending the temperature of a smectic liquid crystal. CONSTITUTION:The smectic liquid crystal layer 42 is enclosed between orientation control films, etc. mounted on transparent electrode substrates 41a, 41b respectively so as to be perpendicular to the surfaces of the substrates 41a, 41b, and the liquid crystal molecule 43 has dipole moment 44 in the direction of intersecting orthogonally with the molecule. At the time of impressing the voltage of more than the prescribed threshold between the electrodes mounted on the substrates 41a, 41b respectively, the spiral structure of the liquid crystal molecule 43 is released, and the moment of the molecule 44 all turns towards the direction of the electric field. The molecule 43 displays the anisotropy of the refractive index in major axial and minor axial directions of the molecule, respectively. Therefore, the most dark (the most bright) state is produced at the temperature range lower than the temperature corresponding to the intermediate value between the max. tilt angles thetaA, at the upper phase transition point and that thetaB at the lower phase transition point at the time of descending the temperature of the liquid crystal. The transmission axis (the absorption axis) of a polarizer and an analyzer are arranged in the position of producing max. dark state (the max. bright state), thereby enabling to change the optical characteristics of the titled element by polarity of the applied voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a liquid crystal element used in a liquid crystal display element, a liquid crystal-optical shutter, and the like.

(Prior Art) Conventionally, a liquid crystal display has a scanning electrode group and a signal electrode group configured in a matrix, and a layer of liquid crystal compound is filled between the electrodes to form a large number of pixels, and displays image lines or information. Display elements are well known.

In particular, ferroelectric smectic liquid crystal devices that produce monostable, bistable, or multistable states are known, as disclosed by Clark et al. in US Pat. No. 4,367,924 and US Pat. No. 4,563,059.

In this liquid crystal element, the distance between the pair of substrates is set to be thin enough to eliminate the helical structure unique to chiral smectic liquid crystal even in the absence of an electric field, thereby imparting a memory effect. has been done.

(Problem to be Solved by the Invention) The above-mentioned ferroelectric liquid crystal element has a tilt angle θ that fluctuates depending on temperature changes, as shown in FIG. There was a problem in that the tilt angle θ became small in a high temperature region.

The tilt angle θ shown in FIG. 3 is the tilt angle when a voltage exceeding the threshold voltage of the ferroelectric liquid crystal is applied to the ferroelectric liquid crystal layer, and is expressed as the maximum tilt angle θ.

Furthermore, when the aforementioned ferroelectric liquid crystal element was used in a display panel, the transmittance of the display panel fluctuated in response to changes in external temperature, resulting in a low-brightness display in high-temperature areas. .

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a novel liquid crystal device that solves the problems of conventional liquid crystal devices or liquid crystal light-shutters as described above.

(Means for Solving the Problems) That is, the present invention provides a liquid crystal cell in which a ferroelectric smectic liquid crystal is arranged between first and second substrates having electrode portions facing each other, and is in a cross Nicol relationship. In a liquid crystal element having an optical detection means having a polarizer and an analyzer, the maximum tilt angle θA at the upper limit phase transition point and the maximum tilt angle θB at the lower limit phase transition point of the ferroelectric smectic liquid crystal under decreasing temperature. The darkest state (or the brightest state! a
), the transmission axes (or absorption axes) of the polarizer and the analyzer are arranged at positions where the polarizer and the analyzer produce the following effects.

By configuring the liquid crystal element as described above, even if the tilt angle of the liquid crystal molecules changes due to temperature changes during use of the element, maximum contrast can be achieved in the low temperature range, and large transmission can be achieved within the operating temperature range. The light intensity can be maintained and a large contrast can be obtained overall.

Next, the present invention will be explained in more detail.

As the smectic liquid crystal having ferroelectricity used in the liquid crystal element of the present invention, chiral smectic C phase (S
ee”), H phase (SmH”), F phase (Sa+F”
), I-phase (Sm1''), J-phase (SmJ''), G-phase (SmG') or 2-phase (S+aK'') liquid crystals are suitable.

These ferroelectric liquid crystals are
E PH-YSIOUE LETTERS″36 (L
-69) +975, Ferroelect-ric
Liquid Crystals ;＾pplie
d Physics Let-ters'' 3B
(11) 1980 Submicro 5ec
ondBistableElectrooptic S
witching in Liquid Crysta
ls ; “Solid State Physics” 16 (141) 1981 r Liquid Crystals”, and any of the ferroelectric liquid crystals disclosed therein can be used in the present invention.

The example shown in the fourth figure schematically shows one example of the ferroelectric liquid crystal element of the present invention, and 41a and 41b in the figure are In2
O,, SnO2 or ITO (Indium-TtnO
A substrate (e.g., a glass plate) coated with a transparent electrode (e.g. A liquid crystal layer 42 made of smectic liquid crystal as described above is arranged between the substrates 41a and 41b.
The liquid crystal layer 42 is encapsulated as a liquid crystal layer 42 of 5IICs phase or the like, which is oriented perpendicular to the plane.

A thick line 43 represents a liquid crystal molecule, and this liquid crystal molecule 43 has a dipole moment (on P) 44 in a direction perpendicular to the molecule.

When a voltage higher than a certain threshold is applied between the electrodes on the substrates 41a and 41b of such a ferroelectric liquid crystal element, the helical structure of the liquid crystal molecules 43 is unraveled, and the dipole moment (P) 4
The orientation direction of the liquid crystal molecules 43 can be changed so that all of the liquid crystal molecules 43 are oriented in the direction of the electric field.

The liquid crystal molecules 43 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction.
By placing an optical detection means having a polarizer and an analyzer disposed in a crossed nicol position above and below the surfaces of the substrates 41a and 41b, it is easy to create a liquid crystal optical modulation element whose optical characteristics change depending on the polarity of applied voltage. be understood.

More preferably, when the thickness of the liquid crystal element is made sufficiently thin (for example, 10 μm or less), the helical structure of the liquid crystal molecules is unraveled (non-helical structure) even when no electric field is applied, as shown in FIG. ), its dipole moment P or P' is either upward (44a) or downward (44b).

When an electric field E or E' with a different polarity of a certain threshold value or more is applied to such an element for a predetermined time using an arbitrary voltage applying means (not shown) as shown in FIG. 5, the dipole moment P
Alternatively, P' changes its direction to be upward 44a or downward 44b in response to the electric field E or the electric field vector of E', and accordingly, the liquid crystal molecules 43 are either in the first orientation state Fi 45a or the second orientation state 45b. Orient to one side.

As mentioned above, there are two advantages to using such a ferroelectric liquid crystal element as an optical modulation element.

Firstly, the response speed is extremely fast, and secondly, the alignment of liquid crystal molecules has bistability. The second point will be explained with reference to FIG. 5, for example. When the electric field E is applied, the liquid crystal molecules are aligned in the first alignment state 45a, and in this state they are stable even when the electric field is turned off. Further, when an electric field E' in the opposite direction is applied, the liquid crystal molecules are aligned to the second alignment state 45b and the orientation of the molecules is changed, but they remain in this state even after the electric field is turned off. Further, as long as the applied electric field E does not exceed a certain threshold value, each orientation state is maintained. In order to effectively realize such fast response speed and bistability, it is preferable that the element be as thin as possible, generally 0.5 to 20 μm, particularly 1 to 20 μm.
5μm is passing through.

In addition, the first orientation state j9i45a and the second orientation state 45
The angle formed by b is set to twice the value of the tilt angle θ, and the maximum transmittance is obtained under crossed Nicol conditions when the tilt angle θ is 22.5°. Further, the value of 172 of the cone angle of the triangular pyramid shown in FIG. 4 corresponds to the maximum tilt angle.

A ferroelectric liquid crystal element having a matrix electrode structure using this kind of ferroelectric liquid crystal is proposed by Clark and Ragaval in US Pat. No. 4,367,924, for example.

In the liquid crystal device as described above, the present inventors investigated in more detail the arrangement state of the molecules of the smectic A phase having ferroelectricity and the optimization of the liquid crystal device by the combination of a polarizer and an analyzer in a crossed Nicol relationship. As a result, as shown in Figure 1, by aligning the absorption axis of the polarizer in the low temperature region with the molecular axis of the liquid crystal molecules when the ferroelectric liquid crystal is oriented in one of at least two stable states. However, if the darkest state is displayed, the contrast will decrease as the temperature changes from that temperature to a high temperature region.

On the contrary, it was found that the intensity of transmitted light increased, resulting in relatively high brightness.

Therefore, it was found that a high intensity of transmitted light can be maintained at all times in the normal operating temperature range (-20°C to 80°C), and that a large overall contrast can be obtained.

This effect is particularly noticeable when the temperature dependence of the tilt angle is large.

The present invention will be explained in more detail below based on Examples.

As a ferroelectric liquid crystal material, rcs+014 manufactured by Chisso Corporation
(Product name)" was used. The phase transition point of this liquid crystal material was as follows.

80.5℃ Ch+Is.

(Cry: crystalline phase, Sac'': chiral smectic C phase, SmA': smectic A phase, Ch: cholesteric phase, Isa: isotonic phase) The above-mentioned phase transition temperature was measured using a DSCT model manufactured by Perkin-Emul. It is based on (product name).

Polyimide l is placed on the glass substrate on which the ITO electrode is formed.
1K (Product name: Sem1cofine
) was coated with a thickness of 400. Next, the substrate was rubbed in one direction using a rotating fur brush. A cell was created by bonding two such substrates facing each other with a spacer interposed therebetween. This cell thickness is 3μ
It was m.

The liquid crystal material described above is applied to the cell at a temperature (10
0° C.) to construct a liquid crystal cell.

When the liquid crystal cell is slowly cooled at a rate of 5°C/hour while controlling the temperature of the liquid crystal cell, the liquid crystal in the liquid crystal cell passes through the Ch phase (cholesteric phase), the SmA' phase (smectic A phase), and then the SmC'' phase (chiral smectic C phase). phase).

These phase changes could be identified by placing a polarizer and analyzer on both sides of the liquid crystal cell. SmA
In the phase, the long sleeves of the liquid crystal molecules were aligned in the rubbing direction. That is, the liquid crystal molecule layer in the smectic phase was perpendicular to the rubbing direction and the substrate surface.

When the temperature is further lowered from the SmA'' phase, the SmC' phase becomes the SmC' phase.

Since this liquid crystal layer is sufficiently thin, the helical structure dissolves into a non-helical structure, and there are two stable states depending on the combination of a crossed Nicol polarizer and an analyzer that are orthogonal to each other. I was able to confirm that. The details are as follows.

FIG. 6 is a layout diagram of the light source/polarizer/liquid crystal cell/analyzer as seen with the eye placed on the optical path.

The axis O is the rubbing direction (uniaxial direction), and p,
, p2 and A, , A2 indicate the transmission axis (or absorption axis) of the polarizer and analyzer, respectively.

As mentioned above, two stable states can be observed in the SmC phase obtained by slow cooling from the iso-ro phase. When observing them, the axis 0 of the liquid crystal cell and
It can be seen that when the angle formed by the liquid crystal cell is set appropriately, the liquid crystal cell is usually divided into two types of domains with a mottled pattern.

For example, when the temperature of the liquid crystal cell is 5°C, as shown in Figure 6, the angle between the axis O and the absorption axis P1 of the polarizer is the angle between the transmission axis (or absorption axis) p+ of the polarizer and the transmission axis of the analyzer. When the angle formed with the axis (or absorption axis) AI is 90°, the angle at which the greatest gint last, that is, the darkest state is exhibited, corresponds to the tilt angle θ.

Also, set the transmission axis (or absorption axis) of the polarizer/analyzer to a position symmetrical to the above (P2/A2) as shown in Figure 6 with respect to axis 0.
), the domains are reversed, i.e. domains that were dark (dark green) in the (PI/A2) configuration become light (yellow), while domains that were bright (yellow) and dull become dark (deep green).

Also, set the transmission axis (or absorption axis) of the polarizer and analyzer to Pl
and A1 are arranged at 90 degrees, and when a DC voltage pulse of a specific polarity (for example, 10) is applied between the two opposing substrates, the entire surface turns dark (deep green). Then reverse polarity (
–), the whole becomes bright (yellow) when a DC voltage pulse of
Changes to

From the above, we think that the two stable states are a state in which the projection axis of the liquid crystal molecule axis onto the substrate surface is oriented in the direction of axis P on average, and a state in which it is oriented in the direction of axis P2. be able to. Furthermore, since these states are transferred by DC voltage pulses of opposite polarity, each of these two states is finite in the direction perpendicular to the substrate surface and has an average electric dipole moment of opposite polarity. I know that there is.

Next, the temperature of the liquid crystal cell was varied within a range of 25 to 50°C. As the temperature changed, the tilt angle of the liquid crystal also changed, and the transmitted light intensity and contrast also changed. Figure 3 shows the measurement results of the temperature dependence of the tilt angle. Tilt angle θ shown in Figure 3
is measured as the maximum tilt angle mentioned above.

In addition, the tilt angle θ at the upper limit phase transition point of a liquid crystal phase exhibiting ferroelectricity, especially the chiral smectic C phase, is 0°, and the tilt angle θB at the lower limit phase transition point is calculated by the following formula (1). It was calculated using approximate values.

Tilt angle θB = (TA - Ta - TC) l/2◆
In the C formula: Proportionality constant TA: Upper limit phase transition point (1) 'r8: Lower limit phase transition point ('C) TA-Tc: Origin temperature of the square power curve ('e) C: Origin tilt of the % power curve Angle (°) According to FIG. 3, the tilt angle θ6 at the lower limit phase transition point is about 26@. Therefore, in the present invention.

Since the temperature when the tilt angle θ is 13° is about 50°C, the temperature range is below 50°C, preferably below the temperature at the midpoint between the upper limit phase transition point and the lower limit phase transition point. The absorption axes (or transmission axes) of the polarizer and analyzer are set at a position where the liquid crystal element produces its darkest state (or brightest state), particularly in a temperature range of 25° C. or lower, more preferably 20° C. or lower. be able to.

As mentioned above, in the present invention, the absorption axes (or transmission axes) of the polarizer and analyzer are set at the position where the liquid crystal element produces its darkest state (or brightest state) in a low temperature region. The tilt angle θ at this time may be based on the maximum tilt angle, but more preferably the darkest ( The absorption axes (or absorption axes) of the polarizer and analyzer can be set at positions where the polarizer and analyzer are in the brightest or brightest state.

For example, when the temperature of the liquid crystal cell is 50°C, the tilt angle θ when a voltage higher than the threshold voltage of the ferroelectric liquid crystal is applied to the liquid crystal cell, for example, a pulse voltage of 50 Iisec at 20 V is applied to the liquid crystal cell.
is approximately 23° as shown in Fig. 3. Therefore, when the angle between the axis O shown in Fig. 6 and the absorption axis PI of the polarizer is set to 23°, the darkest state is reached under pulsed voltage application. can be exhibited. Further, in a preferred specific example of this example, an applied voltage to the liquid crystal cell that is equal to or lower than the threshold voltage of the smectic liquid crystal,
For example, when applying a pulse voltage of 5V for 50μsec, the tilt angle is approximately 15@, so the axis 0 shown in FIG.
The angle between the polarizer and the absorption axis P1 of the polarizer can be set to 15 degrees.

In the present invention, the tilt angle θ is measured based on the following method.

(1) By applying a positive polarity voltage higher than the threshold voltage to the liquid crystal cell, the ferroelectric liquid crystal is aligned to one stable alignment state, and the darkest state is found using a polarizer and an analyzer with a 90° crossed Nicols angle. .

(2) Next, by applying a negative polarity voltage higher than the threshold voltage to the liquid crystal cell, the liquid crystal cell is reversed from the darkest state to the bright state, and then the polarizer and analyzer are placed at an angle of 90” crossed Nicols. The rotation angle at this time was set at 172 as the tilt angle θ.

FIG. 1 clarifies the temperature dependence of contrast and transmittance when using the liquid crystal element of this example. At this time, the absorption axes of the polarizer and analyzer of the liquid crystal element were set at positions that would produce the darkest state at a temperature of 5° C. and when no voltage was applied. According to FIG. 1, high transmittance occurs over a wide temperature range. Therefore, when applied to a display panel, it is possible to provide a high brightness screen in a wide temperature range.

Furthermore, in this example, it is possible to provide a display screen with high contrast on the low temperature side.

Figure 2 shows the temperature dependence of transmittance when the absorption axes of the polarizer and analyzer used in the liquid crystal element are set at the position where the darkest state occurs when the temperature is 50°C and no voltage is applied. It is something. According to FIG. 2, the transmittance is lower than that of this example, and the contrast is low.

Further, in the present invention, at least one surface of the substrate is subjected to an alignment treatment having a uniaxial direction, and the uniaxial direction is defined as axis 0, and the first state of the two types of stable states is set. The projection axis of the liquid crystal molecule axis near the first substrate surface onto the substrate surface in the first state is defined as axis A, and the projection axis of the liquid crystal molecule axis near the second substrate surface in the first state onto the substrate surface is defined as axis A'. , and the projection axis of the liquid crystal molecule axis near the first substrate surface in the second state onto the substrate surface is axis B, and the projection of the liquid crystal molecule axis near the second substrate surface onto the substrate surface in the second state When the axis is axis B', in the case of a ferroelectric element in a splay alignment state where axes A and A' and axes B and B' do not coincide, the transmission axis (or absorption axis) of the polarizer is The absorption axis (or transmission axis) of the analyzer can be made to substantially coincide with axis A'' (or axis B'').

In other words, in this example, by making the transmission axis (or absorption axis) PI (or P2) of the polarizer approximately coincide with the axis AI (or A2), even if the tilt angle θ changes due to temperature changes, the operating temperature can be maintained. Overall high contrast and transmitted light intensity could be achieved within the range, with maximum contrast at the lowest temperatures.

(Operations/Effects) According to the present invention as described above, in the configuration of a liquid crystal element using a ferroelectric liquid crystal having two types of stable states, one of the stable states in the low temperature range of the operating temperature range of the liquid crystal element. By arranging the polarizer so that it displays approximately the darkest state, maximum contrast is achieved on the low temperature side even when the tilt angle changes due to temperature changes, and large transmitted light intensity is achieved within the operating temperature range. can be maintained, and a large contrast can be obtained overall.

[Brief explanation of the drawing]

FIGS. 1 and 2 show the temperature dependence of contrast and transmittance when the darkest state is set at temperatures of 5° C. and 50° C., respectively. FIG. 3 shows the change in tilt angle with temperature. 4 and 5 are perspective views schematically showing the liquid crystal element of the present invention. FIG. 6 is an explanatory diagram showing an aspect when a polarizer and an analyzer are orthogonal to each other. PI, P2; transmission axis (or absorption axis) of polarizer A+, A
2 ; Transmission axis (or absorption axis) of polarizer 0 Direction of knee axis Figure 1 Figure 3 Figure 6

Claims (12)

[Claims] (1) An optical detection means having a liquid crystal cell in which a ferroelectric smectic liquid crystal is arranged between first and second substrates having electrode portions facing each other, and a polarizer and an analyzer in a crossed nicol relationship. In the liquid crystal element having the above, the maximum tilt angle θ_A at the upper limit phase transition point and the maximum tilt angle θ_B at the lower limit phase transition point of the ferroelectric smectic liquid crystal as the temperature decreases corresponds to the intermediate value (θ_B - θ_A)/2. The transmission axes (or absorption axes) of the polarizer and analyzer are located at the position where the darkest state (or brightest state) occurs in a temperature range lower than the temperature
A liquid crystal element characterized by arranging. (2) The low temperature region is a temperature region lower than the intermediate temperature between the upper limit phase transition point and the lower limit phase transition point (
The liquid crystal element described in item 1). (3) The liquid crystal element according to claim (1), wherein the low temperature region is a temperature region of 25° C. or lower. (4) The liquid crystal element according to claim (1), wherein the low temperature region is a temperature region of 20° C. or lower. (5) The liquid crystal element according to claim (1), wherein the transmission axes (or absorption axes) of the polarizer and the analyzer, which are in a crossed nicol relationship, cross at an angle of 90°. (6) Claim (1) in which a display is produced by causing a phase transition between two stable molecular arrangement states by selectively applying voltages of opposite polarity to the opposing electrodes. The liquid crystal element described in . (7) At least one surface of the substrate is subjected to an alignment treatment having a uniaxial direction, and the uniaxial direction is aligned with the axis O.
Of the two stable states, the axis A is the projection axis of the liquid crystal molecule axis near the first substrate surface in the first state onto the substrate surface, and the axis A is the projection axis of the liquid crystal molecule axis near the first substrate surface in the first state, and The projection axis of the liquid crystal molecule axis onto the substrate surface is defined as axis A', the projection axis of the liquid crystal molecule axis near the first substrate surface in the second state onto the substrate surface is defined as axis B, and When the projection axis of the liquid crystal molecule axis near the substrate surface in step 2 onto the substrate surface is axis B', the transmission axis (or absorption axis) of the polarizer is approximately aligned with axis A (or axis B), and the analyzer is The liquid crystal element according to claim (6), wherein the absorption axis (or transmission axis) of the liquid crystal element substantially coincides with axis A' (or axis B'). (8) The liquid crystal element according to claim (7), wherein the angle between the axis O and the axis A is substantially equal to the angle between the axis O and the axis B'. (9) The liquid crystal element according to claim (8), wherein the angle between the axis O and the axis A' is approximately equal to the angle between the axis O and the axis B. (10) The liquid crystal element according to claim (1), wherein the ferroelectric smectic liquid crystal is a chiral smectic liquid crystal. (11) Chiral smectic liquid crystal has C phase, H phase,
The liquid crystal element according to claim 10, which is an F phase, an I phase, a J phase, a G phase, or a K phase. (12) The liquid crystal according to claim (10), wherein the thickness of the chiral smectic liquid crystal is set to be sufficiently thin to eliminate the helical structure inherent in the chiral smectic liquid crystal in the absence of an electric field. element.