JP2762191B2 - Driving method of optical element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element used for a light valve, a display device, a dimming window and the like, and more particularly to an element for controlling light transmittance and reflectance by applying a voltage. Things.

[0002]

2. Description of the Related Art The principle of an optical element that controls light transmittance and reflectance by dispersing anisotropic particles in a medium and changing the orientation of the particles by applying a voltage thereto has been known for a long time. (See, for example, US Pat. No. 1,955,923). A typical optical element 1 will be described with reference to FIG. 1. In a cell 4 formed by joining two opposing transparent walls (support 2) with a sealing material 3,
A suspension 7 in which anisotropic particles 5 are dispersed in a dispersion medium 6 is sealed, and an AC voltage is applied from a power supply 9 through a transparent electrode 8 provided on the inner surface of the cell 4 to form an anisotropic particle. The orientation of the conductive particles 5 is changed. At this time, at least one electrode and the cell wall need to be transparent, and an oxide conductor is usually used as the transparent electrode.

In such an optical element, when no voltage is applied, the particles are distributed in random directions, and when a voltage is applied, the particles are oriented in the direction of the electric field and light is easily transmitted. Use that thing. Further, since not only the transmittance but also the reflectance and the refractive index change, there is also an application as a reflection mirror. Further, there is an electrophoretic display device as an element similar to the above optical element. The electrophoretic display device is a device represented by FIG. 1 in which spherical particles are dispersed in a medium instead of anisotropic particles, and a DC voltage is applied instead of an AC voltage. As a result, the particles are attracted to the electrode on one side, and the color tone is changed by changing the particle density. Therefore, the particles can be used not only as a display device but also as a mirror.

[0004]

However, a voltage is applied to the above-described optical element for a long time,
When -off is repeated, particles adhere to the electrodes and aggregation of a spot pattern occurs. This aggregation is irreversible,
Once this occurs, the light control performance is significantly reduced. Aggregation is
Since the particles start to be generated due to the particles colliding with and adhering to the electrodes, it is easy to imagine that if the adhesive force between the particles and the electrodes is reduced, a coagulation preventing effect can be obtained. In order to prevent this, coating of an electrode or the like has been conventionally attempted as a method for preventing this, but the effect is not sufficient, and an optical element for effectively preventing aggregation has not yet been put to practical use. In particular, in the electrophoretic display device, improvements such as coating of electrodes (for example, see Japanese Patent Application Laid-Open No. 55-4067) and a method of applying a voltage (see Japanese Patent Publication Nos. 57-506 and 57-507) have been improved. Although it has been attempted, it is not enough for practical use, and other effective concrete methods have not been found yet.

[0005] The inventors of the present invention have conducted intensive studies to find a solution to the aggregation, and as a result, have found that the crystal orientation of the oxide used as an electrode has a remarkable influence on the aggregation. Is preferentially oriented so as to be parallel to the electrode surface, thereby making it possible to effectively prevent aggregation. Furthermore, subsequent studies have found that, not only using electrodes in which the densest crystal plane is preferentially oriented, but also driving the device at a high frequency makes the effect of preventing aggregation more remarkable. A method using a high frequency when driving an optical element has already been disclosed in
No. 4464, which states that when a voltage having a frequency of 400 kHz or more is applied, aggregation is significantly reduced. Then, in the description of the example, aggregation occurs within 10 minutes at a frequency of 100 kHz or less. Therefore, conventionally, when the optical element is applied with an AC voltage, the high frequency is indispensable as described above, and when the optical element is driven at a lower frequency, the aggregation preventing effect is sufficiently obtained. Was thought not to be.

However, such a high frequency (400 kHz)
When the above voltage is applied, the amount of current required to drive the optical element becomes large, a large-capacity power source is required, and the power consumption is also considerably increased. There was a problem. However, the applicant has noted that the combination of the above-described optical element (the electrode of the preferential orientation) and the frequency of the applied voltage has a very synergistic effect on the prevention of aggregation, and this optical element is used at a lower frequency. It has been found for the first time that coagulation is effectively prevented even when driven. As a result, the present invention has been completed, and it has become possible to use an optical element having a high aggregation-preventing effect at an economically practical level.

[0007]

That is, according to the method of driving an optical element of the present invention, at least the closest crystal plane of the crystal on the surface side of the electrode is preferentially oriented parallel to the electrode surface.
An optical element in which a suspension in which anisotropic particles are dispersed in a solvent is sealed in a cell having this electrode, at a frequency of 300 Hz or more and 10 kHz.
It is characterized in that it is driven by applying an AC voltage having a frequency of z or less.

[0008] Examples of the particles in the present invention include polarizing particles and electrophoretic particles. The point is that the orientation is changed by applying a voltage.
Also, the shape and size of these particles are not particularly limited, and examples of the shape include rod-like, needle-like, platelet-like, and spherical shapes. As the liquid for dispersing the particles, an organic liquid or the like capable of stably dispersing the particles is selected. If necessary, a surfactant or the like is added for the purpose of improving the dispersibility of the particles. As an electrode formed on the cell wall, Au, Pd or the like can be used in addition to oxides such as indium tin oxide (ITO), tin oxide, and indium oxide. These electrodes are usually formed by vapor deposition, sputtering, or the like, but the forming method is not limited to these methods. The crystal constituting the electrode is oriented according to its type such that the closest crystal plane is parallel to the electrode surface at least on the surface side. Therefore, it is not necessary that all of the crystals constituting the electrode are preferentially oriented.

[0009]

The operation of the present invention will be described with reference to ITO, which is the most common transparent conductive glass, as an example. Generally, in order to form an ITO film on glass, a sol-gel method is used in addition to a thin film method such as vapor deposition and sputtering. In either method, ITO usually becomes a polycrystal and has a random crystal orientation. Typical X-ray diffraction peaks found in a normal ITO film are as shown in FIG. However, if the conditions such as the ITO forming temperature, the atmospheric pressure, and the atmospheric gas change, a so-called oriented crystal in which a specific crystal plane is preferentially oriented on the surface and not a randomly oriented ITO is obtained. The type of the orientation plane and the degree of orientation vary depending on the conditions.

Here, as a measure of the degree of orientation, (400)
Using the X-ray diffraction intensity ratio between the crystal plane and the (222) crystal plane, it can be easily displayed as follows. Degree of orientation = (P ₄₀₀ / P ₂₂₂ ) − (P ₄₀₀ / P ₂₂₂ ) _R where (P ₄₀₀ / P ₂₂₂ ) is the X-ray of the (400) crystal plane and the (222) crystal plane of the preferentially oriented oxide film. It is a diffraction intensity ratio. (P ₄₀₀ / P ₂₂₂ ) _R is the X-ray diffraction intensity ratio between the (400) crystal plane and the (222) crystal plane of the oxide film in the random crystal, and is, for example, an ASTM file 6-4.
16. The inventors have shown in FIG.
It was found that ITO having a line diffraction peak and preferentially oriented in the (400) plane was effective for agglomeration of particles. In the ITO film electrode, the closest crystal plane is a (400) crystal plane, but in an electrode made of another material, the closest crystal plane determined according to the crystal structure of the substance is preferentially oriented. It has been confirmed that the same effect can be obtained by using And I think the mechanism is as follows.

Since the closest surface has the maximum surface energy, components such as a surfactant and a polymer in the dispersion are strongly adsorbed to cover the electrode surface in an attempt to reduce the surface energy. Therefore, the particles hardly collide directly with the electrode, and the steric hindrance effect is enhanced. Further, it has been found by tunneling microscope observation that when the preferred orientation occurs, a large number of minute projections are formed on the electrode film surface. Even if the particles collide with the electrode and try to adhere to it, the contact area becomes very small due to the projections, and when the voltage is applied in the opposite direction, the particles are easily separated from the electrode, so that aggregation does not eventually occur.
It is presumed that these two phenomena are effectively acting on aggregation prevention. And the degree of orientation represented by the above formula is
If it is less than 1, the effect of preventing aggregation is hardly expected.
Driving the optical element using the crystal electrode with an AC voltage of 300 Hz or more and 10 kHz or less further improves the effect of preventing aggregation. In order to express the degree of difficulty of the occurrence of aggregation, it is common to turn on and off the voltage at regular time intervals and compare the number of repetitions until aggregation. When the electrodes are ITO with random orientation, the number of repetitions is at most 200 times even if the frequency is increased to 10 kHz. In contrast, the preferentially oriented I
When TO is used for the electrode, a life of more than 100,000 times can be obtained at 1 kHz, for example.

In the present invention, the frequency range is set to 300 Hz.
Is limited to 10 kHz for the following reason.
In other words, if the frequency is less than 300 Hz, a synergistic effect with the crystal orientation of the electrode cannot be expected so much, and thus the material does not exhibit properties that can be used practically. As a result of studying various conditions, the inventors have confirmed that there is an effect of preventing aggregation at a frequency of 300 Hz or more. On the other hand, when the frequency increases, a large-capacity power supply or the like is required as described above. To limit such economic disadvantages to an acceptable range,
An upper limit of 0 kHz was set.

[0013]

Embodiments of the present invention will be described below. (Example 1) 100 g of a suspension in which 0.3 g of iodide crystal of cinchonidine sulfate corresponding to particles and 20 g of an acrylate polymer as a dispersant were dispersed in a fluororesin was prepared, and the suspension was prepared in a 100 mm x 100 mm glass cell. Enclosed. On the inner surface of the opposing glass, a 200 nm thick IT
An O film is formed by sputtering, and a lead wire is attached to the ITO film so that electricity can be supplied. At this time, the cell gap is 0.1 mm. When the ITO film is formed by sputtering, the substrate temperature is raised from room temperature to 40 ° C.
Two types of ITO films having different degrees of crystal orientation were produced by changing the temperature to 0 ° C. and setting the oxygen partial pressure to minimize the resistance value at each substrate temperature. When energizing the optical element,
At an alternating current of 100 V, the frequency was changed variously between 50 Hz and 10 kHz, and the on-off was repeated at intervals of 5 seconds.

Since it is difficult to rationally define the degree of agglomeration, in this test, the time at which the state of aggregation is about the same as that of the sample is defined as the aggregation time, based on the limit sample prepared in advance. Then, the degree of aggregation was determined.
The degree of orientation of the closest crystal plane was expressed by the degree of orientation defined above. Specifically, the diffraction intensity P _{400 on} the (400) crystal plane and the diffraction intensity P ₄₀₀ on the (222) crystal plane
₂₂₂ was measured to give the ratio (P ₄₀₀ / P _222). Further, referring to the diffraction intensity of the (400) crystal plane and the (222) crystal plane in the random crystal of ITO given by the ASTM file 6-416, the ratio (P ₄₀₀ / P ₂₂₂ ) _R
And the difference between the respective ratios is defined as the degree of orientation as shown below. Orientation degree _{= (P 400 / P 222)} - (P 400 / P 222) ITO and 3.6 IT of the _R orientation degree -0.2 (Comparative material 1)
The number of repetitions up to aggregation was compared for two types of electrodes of O (inventive material). As shown in FIG. 4, the result of the agglutination test shows that the device (comparative material) having a degree of orientation of −0.2 was 5 even at a high frequency.
An element with a lifetime of only 0 times but a degree of orientation of 3.6 (inventive material)
In this case, the number reached 500,000 times at 1 kHz, which was excellent in the effect of preventing aggregation.

(Example 2) Next, a test was conducted in the same manner as in Example 1 except that indium oxide was used as the transparent electrode. The results are also summarized in FIG. The device having an orientation degree of 0.4 (comparative material 2) had a life of 100 times or less even at a high frequency, but the device having an orientation degree of 3.5 (invention material) far exceeded 100,000 times at 1 kHz.

[0016]

As described above, according to the driving method of the optical element of the present invention, the orientation of the closest crystal plane of the crystal constituting the electrode is controlled, and the AC voltage of a predetermined range of frequency is applied. Drive, it is possible to prevent particles from adhering to the electrode surface and irreversibly agglomerate, and to obtain an optical element whose performance does not deteriorate for a long time.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a typical structure of an anisotropic particle dispersion type optical element.

FIG. 2 is a representative X-ray diffraction pattern of indium tin oxide having a degree of orientation of less than 1.

FIG. 3 is a typical X-ray diffraction pattern of indium tin oxide having a degree of orientation of 1 or more.

FIG. 4 is a graph showing the influence of frequency and the degree of crystal orientation of an electrode on the number of repetitions until the occurrence of aggregation.

[Explanation of symbols]

Reference Signs List 1 optical element 2 support 3
Seal material 4 Cell 5 Anisotropic particle 6
Dispersion medium 7 Suspension 8 Transparent electrode 9
Power supply

──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Junichi Tsujita 1-3-3 Takanodai, Yokkaido, Chiba Pref. Japan Steel Works Co., Ltd. (72) Inventor Toru Mashita 1-3-3 Takanodai, Yotsukaido City, Chiba Pref. Japan Steel Works Co., Ltd. (72) Inventor Hiroshi Inaba 1510 Oguchi-cho, Matsusaka City, Mie Prefecture, Japan Inside Central Glass Co., Ltd. .Cl. ⁶ , DB name) G02F 1/19

Claims (2)

(57) [Claims] At least the closest crystal plane of at least the surface side crystal of the electrode is preferentially oriented parallel to the electrode surface, and a cell having this electrode is filled with a suspension of anisotropic particles dispersed in a solvent. For driving the optical element by applying an AC voltage having a frequency of 300 Hz or more and 10 kHz or less. 2. The electrode according to claim 1, wherein the electrode is formed of an indium oxide film or an indium tin oxide film, and the oxide film has an orientation degree given by the following formula of 1 or more. Driving method of the described optical element Orientation degree = (P ₄₀₀ / P ₂₂₂ ) − (P ₄₀₀ / P ₂₂₂ ) _R (P ₄₀₀ / P ₂₂₂ ) is the (400) crystal plane of the oxide film preferentially oriented. (P ₄₀₀ / P ₂₂₂ ) _R is the X-ray diffraction intensity ratio between the (400) crystal plane and the (222) crystal plane of the oxide film in the random crystal.
X-ray diffraction intensity ratio