JPH0422492B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid crystal device applied to display devices, liquid crystal-optical shutter arrays, etc., and more particularly to a liquid crystal device with improved display characteristics and drive characteristics.

[Description of prior art]

Utilizing the refractive index anisotropy of ferroelectric liquid crystal molecules,
A type of display element that controls transmitted light in combination with a polarizing element has been proposed by Clark and Lagerwall (Japanese Unexamined Patent Publication No. 107216/1983, US Pat. No. 4,367,924, etc.). This ferroelectric liquid crystal generally exhibits a chiral smectic C phase (SmC*) or H phase (SmH*) in a specific temperature range.
Under a specific layer thickness condition of this liquid crystal layer, a first optically stable state (first stable orientation) and a second optically stable state (second stable orientation) are created in response to an applied electric field. ), and maintains that state when no electric field is applied, that is, it has bistability, and it also responds quickly to changes in the electric field, making it suitable as a high-speed and memory-type display element. It is expected to be widely used.

On the other hand, by applying a high-frequency alternating current electric field to a ferroelectric smectic liquid crystal with negative dielectric constant anisotropy (Δε<0), the ferroelectric liquid crystal has a twin structure consisting of a first stable orientation and a second stable orientation. It is conceivable to provide a stable orientation state. This method has the advantage that it is not necessary to set the liquid crystal layer to a specific layer thickness.

[Problem that the invention seeks to solve]

However, according to experiments conducted by the present inventors, the bistable orientation state of the ferroelectric smectic liquid crystal imparted in the presence of the above-mentioned high-frequency alternating current electric field has the problem of disappearing due to temperature fluctuations. It has been found.

Therefore, an object of the present invention is to solve the above-mentioned problems, namely, to provide a liquid crystal device using a ferroelectric smectic liquid crystal in a bistable alignment state that is independent of temperature fluctuations under a high-frequency alternating electric field. It is in.

[Means and actions for solving problems]

The present invention provides: a) a liquid crystal cell in which a ferroelectric liquid crystal having negative dielectric constant anisotropy is arranged between a pair of electrodes; and b) a liquid crystal cell oriented in either a first stable orientation or a second stable orientation. means for applying an alternating current voltage between the pair of electrodes so as to maintain the alignment state of the liquid crystal molecules; c means for setting the voltage value of the alternating voltage to a higher value in accordance with a decrease in temperature; d. a first voltage of one polarity sufficient to transition liquid crystal molecules in one stable orientation to a second stable orientation;
means for selectively applying a second voltage of the other polarity between the pair of electrodes, which is sufficient to transfer the liquid crystal molecules in the second stable orientation to the first stable orientation, depending on input information; , e A liquid crystal device comprising means for creating an optical difference between a light beam passing through liquid crystal molecules aligned in a first stable orientation and a light beam passing through liquid crystal molecules aligned in a second stable alignment. It is a feature.

〔Example〕

Hereinafter, the present invention will be described in further detail with reference to the drawings as necessary.

A particularly suitable liquid crystal material for use in the present invention is a chiral smectic liquid crystal having ferroelectric properties. Specifically, chiral smectic C phase (SmC ^* ), chiral smectic G phase (SmG ^* ), chiral smectic F phase (SmF ^* ), and chiral smectic I phase (SmI ^* )
Alternatively, a chiral smectic H phase (SmH ^* ) liquid crystal can be used.

For more information on ferroelectric liquid crystals, see for example “Le Géneurard de Feuisique L’Etre”.
(“LE JOURNAL DE PHYSIQUE
LETTRE”) 36 (L-69) 1975 “Ferroelectric Liquid Grystals”; “Applied Physics Letters” 36 (11) 1980 “Submicro Submicro Second Bistable Electro-Optical Switching in Liquid Crystals
"Bistable Electrooptic Switching in Liquid Crystals");"Solid State Physics" 16 (141) 1981 "Liquid Crystals", etc., and the present invention uses strong electrooptic switching with negative dielectric anisotropy. Dielectric liquid crystals can be used.

In particular, as a preferable ferroelectric liquid crystal, one that shows a cholesteric phase at higher temperatures can be used, for example, a phenyl ester liquid crystal that shows a phase transition temperature listed in the examples below can be used. .

When constructing an element using these materials, the element can be supported by a copper block or the like in which a heater is embedded, if necessary, in order to maintain the temperature at which the liquid crystal compound forms a desired phase.

Figure 1 is for explaining the operation of ferroelectric liquid crystal.
This is a schematic drawing of an example of a cell. Hereinafter, explanation will be given using SmC ^* as an example of the desired phase.

11a and 11b are In ₂ O ₃ , SnO ₂ or ITO
A substrate (glass plate) coated with a transparent electrode made of a thin film of (Indium-Tin Oxide), etc., between which SmC ^* phase liquid crystal with liquid crystal molecular layer 12 oriented perpendicular to the glass surface is sealed. There is. A thick line 13 represents a liquid crystal molecule, and the liquid crystal molecule 13 continuously forms a helical structure in the plane direction of the substrate. The angle formed between the central axis 15 of this helical structure and the axial direction of the liquid crystal molecules 13 is expressed as a tilt angle. This liquid crystal molecule 13 has a dipole moment 14 in a direction perpendicular to the molecule. A terminal 16a is connected between the electrodes on the substrates 11a and 11b.
When a voltage Ea above a certain threshold value is applied from
The alignment direction of the liquid crystal molecules 13 can be uniformly changed in the direction of the liquid crystal molecules 13a. Conversely, when a voltage Eb is applied, the helical structure of the liquid crystal molecules 13 is similarly unraveled, but the liquid crystal molecules 13 are oriented so that all the dipole moments 14 are directed in the opposite direction of the electric field.
The orientation direction can be uniformly changed in the direction of .
Moreover, the orientation state oriented by voltage Ea or Eb is
When the voltage Ea or Eb is released, the liquid crystal molecules return to the original helical structure orientation state. The liquid crystal molecules 13 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, voltage can be applied. It is easily understood that the liquid crystal optical element is a liquid crystal optical element whose optical properties change depending on the polarity.

FIGS. 2A and 2B show an embodiment of the liquid crystal element of the present invention. FIG. 2A is a plan view of the liquid crystal element of the present invention, and FIG. 2B is a sectional view taken along line A-A'.

The cell structure 100 shown in FIG. 2 includes a pair of substrates 101 made of glass plates, plastic plates, etc.
It has a cell structure in which a and 101b are held at a predetermined distance by a spacer 104 and bonded with an adhesive 106 to seal the pair of substrates.
Furthermore, a plurality of transparent electrodes 10 are provided on the substrate 101a.
A group of two electrodes (for example, a group of electrodes for applying a scanning voltage in a matrix electrode structure) is formed in a predetermined pattern such as a strip pattern. On the substrate 101b, an electrode group (for example, a signal voltage application electrode group in a matrix electrode structure) is formed, which is made up of a plurality of transparent electrodes 102b intersecting with the transparent electrode 102 described above.

A substrate 10 provided with such a transparent electrode 102b
1b includes, for example, inorganic insulating materials such as silicon monoxide, silicon dioxide, aluminum oxide, zirconia, magnesium fluoride, cerium oxide, cerium fluoride, silicon nitride, silicon carbide, and boron nitride, polyvinyl alcohol, polyimide,
Orientation control film formed using organic insulating materials such as polyamideimide, polyesterimide, polyparaxylylene, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride, polyamide polystyrene, cellulose resin, melamine resin, urea resin, and acrylic resin. 105 can be provided.

This orientation control film 105 is obtained by forming a film of an inorganic insulating material or an organic insulating material as described above, and then rubbing the surface in one direction with velvet, cloth, or paper.

In another preferred embodiment of the invention, SiO or SiO ₂
The alignment control film 105 can be obtained by forming a film of an inorganic insulating material such as the above on the substrate 101b by an oblique vapor deposition method.

The above-mentioned orientation control film 105 preferably functions as an insulating film at the same time, and for this purpose, the thickness of the orientation control film 105 can be generally set in the range of 100 Å to 1 μ, preferably 500 Å to 5000 Å. 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. do not have.

Furthermore, in the liquid crystal element of the present invention, a film similar to the above-described alignment control film 105 can be provided on the other substrate 101a.

Liquid crystal layer 1 in cell structure 100 shown in FIG.
03 can be, for example, SmC ^* . or,
Although the thickness of the liquid crystal layer 103 is not particularly limited, it is generally preferably about 3 μm to 15 μm.

Such a cell structure 100 includes a substrate 101a
Polarizers 107 and 108 in a crossed Nicol state are arranged on both sides of electrodes 102a and 101b, respectively.
Optical modulation when a voltage is applied between 2b can be detected.

Now, by applying a high frequency alternating current electric field to the liquid crystal cell described above in parallel to the liquid crystal molecular layer 12, the electric field
It will be oriented in either the orientation state in the direction in which it is oriented when Ea or Eb is applied. The orientation states under application of a high-frequency AC electric field are referred to as a first stable orientation state and a second stable orientation state. Such a phenomenon occurs because the liquid crystal material has negative dielectric anisotropy. The frequency of this alternating electric field must be set to a sufficiently high value so that the liquid crystal cannot respond at a predetermined voltage value.

The AC voltage has a frequency of 400 Hz or more and 100 KHz or less, and triangular waves, sine waves, rectangular waves, etc. can be used as waveforms, and depending on the thickness of the liquid crystal layer, Vpp (peak-to
-peak) is preferably about 20V to 100V.

According to the experiments of the present inventors, the bistability under the condition where an alternating current electric field of a constant voltage value is applied as described above tends to disappear when the temperature decreases to a certain extent; It was found that bistability could be maintained as is by setting the voltage value high. The amount of change in frequency in this case varies depending on the liquid crystal material used, cell thickness, and various conditions, but generally, when the temperature fluctuates by 10°C, about 1°C to 20°C is suitable.

An example of a driving method used in the liquid crystal optical element of the present invention is shown below. FIG. 3A shows a displayable matrix electrode structure, in which 31 is used as a scanning electrode and 32 is used as an information signal electrode. 3B, a and b represent the signals applied to the scanning electrodes, with FIG. 3a representing the scanning selection signal and FIG. 3B and b representing the scanning non-selection signal. FIGS. 3c and 3d show information signals applied to the information signal electrodes, which are selectively applied to the information signal electrodes according to write input information, respectively. Figure 3c is the third
It represents a voltage waveform when applied to a pixel using the electrical signal shown in Figure B. Figure 3c, a and b are
The voltage waveforms applied to the pixels on the scan line to which the scan selection signal is applied, and FIG. 3C, c, and d correspond to the pixels to which the scan selection signal is not applied, respectively. The voltage applied to these pixels is set below a threshold value. Therefore, writing can be performed sequentially for each scanning line by selectively applying the writing signals shown in FIG. , the voltage applied to the pixel is shown in Fig. 3 C, c
As shown in and d, each is set below the threshold value, so that the writing state on the scanning line is memorized for one frame or one field.

FIG. 4A shows a time-series signal when such a signal is applied to perform the display shown in FIG. 3A. In FIG. 4A, the high frequency alternating current component added by superimposing it on the information signal is omitted for the sake of simplicity. Figure 4B shows that to prevent crosstalk,
This is an embodiment in which an auxiliary signal corresponding to an information signal is given at a phase of Δt.

Furthermore, as another embodiment, it is also possible to apply a high frequency AC component to the scanning electrode side. moreover,
By applying phase matching to both the scanning electrode side and the signal electrode side, it is also possible to reduce the required breakdown voltage of the terminal driver ICs on the scanning electrode side and the signal electrode side.

FIG. 5 shows yet another embodiment.

In this embodiment, the AC signal is provided as a scanning non-selection signal. The scan selection signal provides the same waveform as that in FIG. 3B, a. High frequency when selected
By turning off, the liquid crystal molecules move more easily, making switching easier, and also by avoiding the superposition of the selection signal (a: low frequency component) and non-selection signal (b: high frequency component), which improves scanning. This has the advantage of reducing the required breakdown voltage of the side driver IC.

FIG. 6 is a block diagram showing an example of a drive circuit for displaying an MXN dot liquid crystal display panel 601 using the liquid crystal element of the present invention. 60 in the diagram
2 is an AC generating circuit (its output X is input to the AC superimposing circuit 607 on the data signal side);
is the information input, 604 is its clock input, 605
is the scan line timing input, and 606 is its clock input.

The AC superimposition circuit 607 on the data signal input side uses symbols to add voltages, and I′, I″, and X are voltage values, I ₁ ′=I ₁ ″X I ₂ ′=I ₂ ″X 〓 〓 AC superimposition circuit 6 on the scanning signal input side has the function of I _N ′=I _N ″X (N: number of data lines)
In 08, when S′, S″, and y are voltage values, S ₁ ′=S ₁ ″y S ₂ ′=S ₂ ″y 〓 〓 〓 S _M =S _M ″y (M: number of scanning lines) It has the following functions. On the data signal side, the signal from the information input 603 is sequentially input to a shift register 609, a latch 610 controlled by a clock 604 and a counter 611, the aforementioned AC superimposition circuit 607 and a drive circuit 612, and the signal is transmitted to N data lines. Sent. On the scan signal side, the scan line timing input 605 and clock input 606 are input to a shift register 613, and the signals are sequentially input to the aforementioned AC
The scanning signal is input to the superimposition circuit 608 and the drive circuit 614, and is input to M scanning lines.

Furthermore, the temperature sensor 615 is connected to the liquid crystal display section 60.
1 and inputs the signal to the phase converter 602. Therefore, by controlling the phase of the AC signal superimposed on the data line side and the scanning line side based on this temperature, the voltage of the AC signal that is effectively applied is changed. This allows bistability to be maintained even if temperature fluctuations occur.

Hereinafter, the present invention will be explained by giving specific examples.

Example 1 A liquid crystal element shown in FIG. 3 was used. At this time, a glass substrate was used as the substrate, a transparent 1000 Å stripe electrode was used as the electrode, and a polyamide film with a surface rubbing treatment was formed on the glass substrate on which the electrode was formed. These two glass substrates
The distance was maintained by a spacer made of 10 μm polyamide. Further, the peripheries of these two glass substrates were sealed with epoxy resin to create a cell.

A mixed ferroelectric liquid crystal having negative dielectric constant anisotropy shown below was injected into this cell in an isotropic phase.

mixed lcd This mixed liquid crystal causes the phase transition shown below.

The phase transition point at this time is a value measured by DSC (differential scanning calorimeter).

Next, after sealing the injection port, 0.5
It was gradually cooled down to SmC ^* at a rate of °C/h. When this liquid crystal element was observed under a microscope under crossed Nicols, the formation of monodomains was confirmed over a wide area, and
Pitches of SmC ^* were observed.

The drive circuit shown in FIG. 6 was incorporated into the liquid crystal element thus prepared to perform display. At this time, a high frequency AC electric field of 10 KHz and 60 Vpp was used, so that the switching pulse had the waveform shown in FIG. 3B (however, a and b in the same figure also showed high frequency AC waveforms). Specifically, t ₁ + t ₂ (t ₁ = t ₂ ) =
2 msec, V = 21.5 volts was used. The phase converter was controlled to increase the AC voltage at a rate of 1% for a temperature drop of 1°C with 30°C as the standard.

As a result, it was found that a good display condition could be obtained even if the temperature varied between 10°C and 30°C. In other words, this shows that a sufficient bistable state is maintained even if the temperature changes between 10°C and 30°C.

On the other hand, when the experiment was repeated in exactly the same way except that the use of the phase converter used above was omitted, sufficient display characteristics were obtained on the low temperature side for temperature fluctuations of 10°C to 30°C. I couldn't help it. This indicates that no bistable alignment state is formed at low temperatures.

Example 2 10μ used when creating the liquid crystal element of Example 1
A liquid crystal element was produced in exactly the same manner as in Example 1, except that the 7 μm polyimide spacer was used instead of the 7 μm polyimide spacer. In this liquid crystal element, monodomain formation was observed in a wide area, and unlike in Example 1, no helical pitch of SmC ^* was observed (the helical structure was unraveled), but it was bistable when no electric field was applied. No condition was observed (however,
(The voltage value of AC voltage was set to 42Vpp).

When this liquid crystal element was incorporated into a drive circuit and displayed in the same manner as in Example 1, it was found that, as in Example 1, a stable bistable alignment state was maintained against temperature fluctuations.

〔effect〕

According to the present invention, a stable bistable alignment state can be maintained even when external temperature fluctuations occur, and as a result, there is an advantage that good display characteristics that are independent of temperature fluctuations can be obtained.

[Brief explanation of drawings]

FIG. 1 is a perspective view schematically showing a ferroelectric liquid crystal element of the present invention. FIG. 2A is a plan view of the liquid crystal element of the present invention, and FIG. 2B is a sectional view taken along line A-A'. FIG. 3A is a plan view showing the matrix electrode structure of the present invention, FIG. 3B is an explanatory diagram showing the waveforms of signals applied to the scanning line and data line, and FIG. 3C is a plan view showing the matrix electrode structure of the present invention. FIG. 3 is an explanatory diagram showing a waveform of an applied voltage signal. Figure 4A is an explanatory diagram showing the signals used in Figure 3 in time series, and Figure 4B
is an explanatory diagram showing another time-series signal. Fifth
The figure is an explanatory diagram showing signal waveforms applied to scanning lines and data lines. FIG. 6 is a block diagram showing one example of a drive circuit used in the liquid crystal element of the present invention.

Claims (1)

[Scope of Claims] 1 a. A liquid crystal cell in which a ferroelectric liquid crystal with negative dielectric anisotropy is arranged between a pair of electrodes, and b. Either one of a first stable orientation and a second stable orientation. (c) means for applying an alternating current voltage between the pair of electrodes so as to maintain the alignment state of the liquid crystal molecules oriented in the same direction; c) means for setting the voltage value of the alternating voltage to a higher value in accordance with a decrease in temperature; , d a first voltage of one polarity sufficient to transition the liquid crystal molecules in the first stable orientation to the second stable orientation, and a first voltage of one polarity sufficient to transition the liquid crystal molecules in the second stable orientation to the first stable orientation. means for selectively applying a sufficient second voltage of the other polarity between the pair of electrodes according to input information; 2. A liquid crystal device comprising means for producing an optical difference between light beams passing through liquid crystal molecules aligned in two stable orientations.