JPS6218522A - Liquid crystal element - Google Patents

Abstract

PURPOSE:To realize a particularly excellent bistable state by intersecting uniaxial orientation axes so as to have intersection angles with each other. CONSTITUTION:The molecules of a smectic phase A orient along an axis 14 having theta/2 corresponding to 1/2 the intersection angle theta when a liquid crystal compd. between two sheets of substrates of having the uniaxial orientation axes 11, 12 intersected at the intersection angle theta is slowly cooled to transform the phase from the isotropic phase to the smectic phase A and chiral smectic phase C. Moreover, the molecules in the chiral smectic phase C orient to either the 1st orientation state or the 2nd orientation state having the tilt angle specific to the liquid crystal material around the axis 14 when the smectic phase A in such orientation state is slowly cooled to generate the chiral smectic phase C. The bistability having the 1st orientation state and the 2nd orientation state in this state is improved as compared with the bistability formed under the parallel uniaxial orientation axes having no intersection angle between the upper and lower substrates.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal element that can be used in display panels, optical shutter arrays, etc., and more particularly to a ferroelectric liquid crystal element that achieves a good bistable state.

[Conventional technology]

As a conventional liquid crystal element, for example, M-Ken Shut (M,
5chadt) and W. Helfrich (W,
He1frich) “Applied Physics Letters”
tters”) Volume 18, No. 4 (February 15, 1971
"Voltage Dependent Optical - 7 Activity Observations - Twisted Nematic Liquid Conflict Crystal"
ical Activity of a Twis
ted Nematic LiquidCryst
Twisted ◆ Nematic (tw
There are known devices using liquid crystals. The number of pixels in this TN liquid crystal was limited because of the problem of crosstalk occurring when driving by segmentation using a matrix electrode structure with high pixel density.

Furthermore, a display element is known in which a switching element using a thin film transistor is connected to each pixel, and each pixel is switched. However, the process of forming the thin film transistor on the substrate is extremely complicated, and it is difficult to use a display element with a large area. There are some problems that make it difficult to create.

To improve the drawbacks of conventional liquid crystal devices, the use of bistable liquid crystal devices is proposed by Clark (C1
ark) and Lagauer (Lage rwa l 1)
) (Japanese Unexamined Patent Publication No. 56-107216, U.S. Pat. No. 4,367,924, etc.), liquid crystals having bistability generally include chiral smectic C phase (SmC") or H phase (SmH A ferroelectric liquid crystal with a
It has a bistable state consisting of an optically stable state of The liquid crystal is aligned in an optically stable state, and the liquid crystal is aligned in a second optically stable state with respect to the other electric field vector. In addition, this type of liquid crystal has a property (memory effect) of very quickly taking one of the above two stable states in response to an applied electric field and maintaining that state when no electric field is applied. By utilizing such properties, many of the problems of the conventional TN type device described above can be significantly improved.

[Problem that the invention seeks to solve]

However, conventional ferroelectric liquid crystal elements have had a problem in that the stabilization energy levels in the first alignment state and the second alignment state under an electric field vector are not the same. That is, the magnitude of the electric field required to change from the first orientation state to the second orientation state is different from the magnitude of the electric field required to change from the second orientation state to the first orientation state. There's a problem. Furthermore, also under the electric field vector the first
A phenomenon in which the orientation changes from an orientation state to a second orientation state, but when the electric field is removed, the original orientation state returns from the second orientation state without exhibiting the original memory effect. It also had the problem that (return phenomenon) occurred.

These two phenomena are considered to be due to the bistable state in which the first orientation is more stable than the other stable orientation state. Such a state will be referred to as "two asymmetric stable states" in this specification.

When such "two asymmetric stable states" exist, a difference occurs in the threshold voltage when switching is performed by an electric field between the two stable orientation states, which causes problems in driving.

Furthermore, pixels that should originally form a uniform domain in the second orientation state are mixed with another domain based on the first orientation state, resulting in a drawback that the light transmittance or light shielding rate decreases. .

[Means for Solving the Problems] and [Operations] An object of the present invention is to provide a liquid crystal element that eliminates the above-mentioned problems or drawbacks, and in particular to provide a liquid crystal element that achieves an excellent bistable state. An object of the present invention is to provide a transparent liquid crystal element.

That is, the present invention provides a liquid crystal element in which a ferroelectric liquid crystal is arranged between two substrates having uniaxial alignment axes, in which the uniaxial alignment axes intersect with each other at an intersection angle (θ). It has characteristics.

[Example] FIG. 1 is a plan view schematically showing a liquid crystal element of the present invention. 11 in the figure represents the uniaxial orientation axis 512 of the upper substrate and the uniaxial orientation axis of the lower substrate, and θ represents the intersection angle of the uniaxial orientation axes 11 and 12. In addition, 13 represents a molecule in a uniaxial anisotropic phase such as smectic A phase on the higher temperature side than the ferroelectric liquid crystal, and one molecule 13 is an angle θ corresponding to 1/2 of the intersection angle 0.
/2 along the axis 14.

This was completely unexpected for the present inventors, but by slowly cooling a liquid crystal compound between two substrates whose uniaxial alignment axes intersect at the above-mentioned crossing angle θ, it changes from an isotropic phase to a smectic A phase. When phase transition is made to chiral smectic C phase, the smectic A phase becomes 1/1 of the intersection angle of 0.
It was confirmed that the orientation was along the axis 14 forming θ/2, which corresponds to 2. Moreover, when the smectic A phase in such an oriented state is slowly cooled to generate a chiral smectic C phase, the molecules in the chiral smectic C phase have a tilt angle peculiar to the liquid crystal material about the axis 14, and the molecules in the chiral smectic C phase have a tilt angle peculiar to the liquid crystal substance, as described below.
In this case, the bistability with the first orientation state and the second orientation state means that the upper and lower substrates are parallel to each other with no intersecting angle. This is an improvement compared to the bistability formed under the normal orientation axis.

Further, in the present invention, the above-mentioned crossing angle θ is set in the range of 5'' to 85°, but preferably in the range of crossing 1'' x angle of 0200 to 70'. In the most preferred embodiment, the crossing angle θ is between 30° and 50'
can be set to a range of .

The results shown in FIG. 1 are interesting regarding the relationship between the direction of the uniaxial alignment axis and the actual alignment direction of liquid crystal molecules, but the details of the cause of this phenomenon are unclear.

The liquid crystal used in the present invention has ferroelectricity, and specifically, chiral smectic C phase (SmC"),
H phase (SmH'), I phase (SmI''), J phase (SmJ
”), Ni phase (Snx powder), G phase (SmG”) or F phase (
A liquid crystal having SmF8) can be used. Specific liquid crystal compounds include DOBAMBC; decyloxybenzylidene-P'-amino-2-methylbutylcinnamate, HOBACPCB hexyloxybenzylidene-P;
Various compounds such as '-amino-2-chloroprobyl cinnamate can be used.

FIG. 2 schematically depicts an example of a cell for explaining the operation of a ferroelectric liquid crystal.

21a and 21b are substrates (glass plates) covered with transparent electrodes made of thin films such as In203, Sn02 or ITO (indium tin oxide);
In between, liquid crystal of SmC" phase or SmH" phase, which is oriented such that the liquid crystal molecular layer 22 is perpendicular to the glass surface, is sealed. A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment (P) 24 in a direction perpendicular to the molecule. Substrates 21a and 21b
When a voltage higher than a certain threshold is applied between the upper electrodes, the helical structure of the liquid crystal molecules 23 is unraveled, and the dipole moment (P
Above) The alignment direction of the liquid crystal molecules 23 can be changed so that all of the liquid crystal molecules 24 are oriented in the direction of the electric field. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, the polarity of the applied voltage changes. What makes a liquid crystal optical modulation element whose optical properties change?

easily understood.

In particular, in the present invention, an alternating current bias can be applied to the liquid crystal layer in order to impart bistability to this type of liquid crystal element. At this time, a liquid crystal having negative dielectric anisotropy is used as the liquid crystal.

The liquid crystal cell preferably used in the liquid crystal element of the present invention can have a sufficiently thin thickness (for example, 10 μl or less). As the liquid crystal layer becomes thinner, the third
As shown in the figure, even when no electric field is applied, the helical structure of the liquid crystal molecules unwinds and assumes a non-helical structure, and its dipole moment (Pa or pb) is either upward (34a) or downward (34b). Take. When an electric field Ea or Eb of different polarity above a certain threshold value is applied to such a cell by the voltage applying means 31a and 31b as shown in FIG. 3, the dipole moment corresponds to the electric field vector of the electric field Ea or Eb. Then, the direction is changed to upward direction 34a or downward direction 34b, and accordingly, the liquid crystal molecules are aligned in either the first stable alignment state 33a or the second stable alignment state 7133b.

As mentioned earlier, there are two advantages to using such ferroelectricity as an optical modulation element.

The first is that the response speed is extremely fast, and the second is that the alignment of liquid crystal molecules has bistability. To further explain the second point, for example with reference to FIG. 3, the electric field Ea
When the voltage is applied, the liquid crystal molecules are aligned in the first stable alignment state 33a, and this state remains stable even when the electric field is turned off.

Further, when an electric field Eb in the opposite direction is applied, the liquid crystal molecules are aligned to the second stable alignment state 33b and change their orientation, but they remain in this state even after the electric field is turned off. or,
Each orientation state is maintained as long as the applied electric field Ea does not exceed a certain threshold value. In order to effectively achieve such high response speed and bistability, it is preferable that the cell be as thin as possible.

FIGS. 4(A) and 4(B) show an embodiment of the liquid crystal element of the present invention. FIG. 4(A) is a plan view of the liquid crystal element of the present invention, and FIG. 4(B) is a sectional view taken along the line AA'°.

In the cell structure 100 shown in FIG. 4, a pair of substrates 101 and 101' made of glass or plastic plates are held at a predetermined distance by a spacer 104, and an adhesive 106 is used to seal the pair of substrates. It has a bonded cell structure, and furthermore, an electrode group (for example, an electrode group for applying a scanning voltage in a matrix electrode structure) consisting of a plurality of transparent electrodes 102 is arranged on a substrate 101 in a predetermined pattern such as a strip pattern. It is formed of. Substrate 101'
An electrode group (for example, a signal voltage application electrode group of the matrix electrode structure) is formed on the transparent electrode 102 and the plurality of transparent electrodes i02' intersecting with each other.

The substrate 101' provided with such a transparent electrode 102' has
For example - inorganic insulating materials such as silicon oxide, silicon dioxide, aluminum oxide, zirconia, magnesium fluoride, cerium Mide, cerium fluoride, silicon nitride, silicon carbide, boron nitride, polyvinyl alcohol, polyimide, polyamideimide, polyester. Orientation control films 105 and 105 formed using an organic insulating material such as imide, polyparaxylerin, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride, polyamide, polystyrene, cellulose resin, melamine resin, urea resin, or acrylic resin. ' can be provided.

The alignment control films 105 and 105' are provided with a uniaxial alignment axis formed by a rubbing process or the like.

In another preferred embodiment of the present invention, the orientation controlled JII I O5 and 105 are provided with a uniaxial orientation axis by forming a film of an inorganic insulating material such as SiO or 5i02 on the substrate 101' by oblique vapor deposition. ′ can be obtained.

It is preferable that the above-mentioned alignment control films 105 and 105' also function as insulating films at the same time, and for this reason, the thickness of the alignment control films 105 and 105' is generally 100 to 100%.
1p, preferably in the range of 500 people. This insulating film also has the advantage of being able to prevent the generation of current caused by trace amounts of impurities contained in the liquid crystal layer 103, and therefore does not deteriorate the liquid crystal compound even if the operation is repeated.

The liquid crystal layer 103 in the cell structure 100 shown in FIG.
SmC”, SmH”, SmF end.

It can be a chiral smectic phase such as SmI'', SmG''. The liquid crystal layer 103 exhibiting this chiral smectic phase is slowly cooled to change from tojoytic phase to cholesteric phase-S.
mA (smectic A phase) −+SmC”, Tokichi phase +Sm
A-+SmC”, Toyoshi phase → cholesteric phase → SmC”
It is preferable to use SmC'' which is generated by the phase transition from the Tokichi phase to the SmC''.

Further, the liquid crystal element of the present invention has a pair of substrates 101 and 101'.
A plurality of spacer members 201 are arranged between them. This spacer member 201 is made of, for example, SiO, 5i02.
Inorganic compounds such as A203゜rio2, polyvinyl alcohol, polyimide, polyamideimide, polyesterimide, polyparaxylylene, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride, polyvinyl acetate, polyamide, polystyrene,
It can be obtained by forming a film using a suitable method using a resin such as cellulose resin, melamine resin, urea resin, acrylic resin, or photoresist resin, and then etching the spacer member 201 so that it is placed at a predetermined position.

Furthermore, in the present invention, alumina particles, glass fibers, etc. can also be used as the spacer member 201.

Such a cell structure 100 includes polarizers 107 in a crossed Nicol state on both sides of the substrates 101 and 101'. and 10
8 are arranged respectively to produce optical modulation when a voltage is applied between the electrodes 102 and 102'.

The liquid crystal element of the present invention can also be of a guest-host type in which a dichroic dye such as an anthraquinone dye, an azo dye, or a cyanine dye is dissolved in the liquid crystal. At this time, only one polarizer as used above is sufficient, and the opening/closing contrast of the pixel shutter can be made to a sufficiently large value.

The present invention will be described below with reference to specific examples.

Example 1 In this example, a liquid crystal element shown in FIG. 4 was created.

In the liquid crystal element 100 shown in FIG. 4, first, ITO electrodes 102 and 1 are placed on a pair of glass substrates lot and 101'.
02' was patterned into a stripe shape with a thickness of 1000 mm, and then, as alignment control films 105 and 105',
A polyimide film was formed to a thickness of 1000 mm by spinner coating and cured, and then a spacer 201 was formed of polyimide to a height of Ig, patterned by photoetching, and cured. Next, the surfaces of both substrates were subjected to rubbing treatment in the rubbing axial direction as described later.
Next, the two substrates were combined so that the electrodes intersected planarly, and the upper and lower substrates were attached.

Thereafter, DOBAMBC was sealed as a ferroelectric liquid crystal having a chiral smectic phase, and this was used as a liquid crystal layer 103), and the four sides were sealed to form a liquid crystal element 100. The temperature of the liquid crystal layer 103 of this liquid crystal element was raised until it was in an isotonic phase state, and then it was slowly cooled at 0.5° C./hour to orient it.

The liquid crystal element 100 produced as described above has a substrate 10
Polarizers 107 and 108 in a crossed Nicol state are arranged on both sides of electrodes 102 and 101', respectively.
Optical modulation occurs when a voltage is applied between ', and ° switching was observed under this condition.

In this example, when the angle at which the rubbing axes of the upper and lower substrates intersect is θ, as a comparative example, an element in which the directions of the rubbing axes of the upper and lower substrates are the same (θ:=O°) and the rubbing treatment of the upper substrate The element is bonded with the rubbing axis of the lower substrate intersecting the axis at an angle of 30° (θ=30
”) were created and 1. Experiments were conducted on the switching characteristics of each under the conditions described below.The measurement temperature was 70°C.
Met.

As a result, in a liquid crystal element with θ=0°, the voltage nozzle width is 1
When switching between bistable states in m5ec,
The threshold voltage from a more stable orientation state to an unstable orientation state is 21.8 V, whereas the threshold voltage from a more unstable orientation state to a stable orientation state is 30 V. It was .2V. Furthermore, when switching from a more stable orientation state to an unstable orientation state, a "return phenomenon" was observed in which the orientation state returned to the more stable orientation state 1 to 2 seconds after switching.

On the other hand, in the liquid crystal element with θ = 30°, the threshold voltage between two stable alignment states is 20.5 V for both, regardless of the direction of change in the alignment state, which appears when θ = 0°. No "return phenomenon" was observed.

From the above results, a ferroelectric liquid crystal with better bistability can be obtained by intersecting the two rubbing axes formed on the upper and lower substrates at a certain angle.

Examples 2 to 4 In this example, in the liquid crystal element manufactured in Example 1,
A liquid crystal element manufactured in the same manner as in Example 1 was used except for the type of liquid crystal sealed and the intersection angle θ of the rubbing axis directions of the upper and lower substrates as shown below.

First, the chiral smectic path crystal used in this example was a mixture of the following compounds in the weight ratio shown.

In addition, the temperature range of the SmC phase of this mixed liquid crystal is 3°C to 3°C.
The temperature was 5°C.

Furthermore, in addition to the element with θ = O'' as a comparative example, three elements with θ of 30°, 45°, and 90'' (referred to as Examples 2.3 and 4, respectively) were created, and The results obtained by measuring in exactly the same manner as in 1 are summarized in Table 1. Note that the measurement temperature was 28°C.

In Table 1, state A is #:=辷; more stable orientation state than the one with lower threshold voltage (1.N side), and state B is the other more unstable orientation state with higher threshold voltage. It refers to the orientation state.

From the above results, good bistability can be achieved by shifting the rubbing axis by a certain angle, preferably 30'' to 50 degrees.

Example 5 In this example, in the liquid crystal element created in Example 1,
Instead of the Bo1J imide film used as the orientation control film,
A liquid crystal element was used that was prepared in exactly the same manner as in Example 1, except that 5t02 obliquely deposited Ill shown below was used.

That is, IT is placed on a glass substrate as described in Example 1.
The substrate on which the O electrode has been patterned is set in an oblique evaporation device, and the following 1. A crystal of 5i02 was placed on the substrate using a predetermined method, and 800 people were Obliquely deposited films were created for each of the upper and lower substrates.

After this, DOBAMBC was sealed as a liquid crystal using the same method as in Example 1, and the crossing angles of the uniaxial alignment axes formed on the upper and lower substrates were changed to θ=0° (comparative example) and θ=3°.
Two devices of "O" (Example) were prepared, and experiments on switching characteristics were conducted in the same manner as in Example 1. The measurement temperature was 70°C.

As a result, for a liquid crystal element with θ=0°, the voltage pulse width is 1 m5.
When switching between bistable states with ec, the threshold voltage from a more stable orientation state to an unstable orientation state is 23
．． OV, whereas the vIA value voltage from a more unstable alignment state to a stable alignment state was 2 B, 5 V. A ``return phenomenon'' was also observed.

On the other hand, in the liquid crystal element with θ = 30°, the threshold voltage between two stable alignment states is 23.5 V for both, regardless of the direction of change in the alignment state, and no "return phenomenon" was observed. Ta.

From the results of Examples 1 to 5 above, it is clear that superior bistableness can be achieved by using upper and lower substrates having crossed uniaxial orientation axes, regardless of whether the orientation treatment method is the rubbing method or the 5i02 oblique evaporation method. You can see that you can realize your sexuality.

[Brief explanation of drawings]

FIG. 1 is a plan view schematically showing a liquid crystal element of the present invention. FIGS. 2 and 3 are perspective views schematically showing a ferroelectric liquid crystal element used in the present invention. FIG. 4(A) is a plan view of a liquid crystal element used in the present invention, and FIG. 4(B) is a sectional view taken along the line AN.

Claims (11)

[Claims] (1) A liquid crystal element in which a ferroelectric liquid crystal is arranged between two substrates having uniaxial alignment axes, wherein the uniaxial alignment axes intersect with each other at a crossing angle (θ). . (2) The liquid crystal according to claim 1, wherein molecules in a uniaxial anisotropic phase on a higher temperature side than the ferroelectric liquid crystal are oriented at an angle of θ/2 of the intersection angle (θ) of the uniaxial alignment axes. element. (3) The liquid crystal element according to claim 1, wherein the intersection angle (θ) is an angle of 5° to 85°. (4) The liquid crystal element according to claim 1, wherein the intersection angle (θ) is an angle of 20° to 70°. (5) The liquid crystal element according to claim 1, wherein the intersection angle (θ) is an angle of 30° to 50°. (6) The liquid crystal element according to claim 2, wherein the uniaxial anisotropic phase is a smectic A phase. (7) The liquid crystal device according to claim 1, wherein the ferroelectric liquid crystal is a chiral smectic liquid crystal. (8) The liquid crystal element according to claim 1, wherein the uniaxial alignment axis is a rubbing treatment axis. (9) The liquid crystal element according to claim 1, wherein the uniaxial alignment axis is formed on an alignment control film provided on a substrate. (10) The liquid crystal element according to claim 9, wherein the alignment control film is formed of an insulating organic resin film. (11) The liquid crystal element according to claim 9, wherein the alignment control film is formed of an inorganic insulating film.