DE2364641C3 - Electro-optical device comprising a layer of a nematic liquid crystal - Google Patents

Description

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Adf the Slül / .plattc 1 are four electrical attachments 4,5,6 and 7 attached (Fig. 1 and 2), which consist of a thin transparent Z'mnoxidschicht. The electrodes are arranged in groups of seven, each forming an eight. In a known way, digits being represented. On the support plate 2, an electrode H is attached, which is also made of a thin There is a transparent tin oxide layer. A voltage difference between one or more electrodes uuf the Stlllzplalle 1 and the electrode 8 on the support plate 2 creates an electric field perpendicular to the The lying crystal layer 3 emerges.

The support splits 1 and 2 are processed by rubbing in such a way that the elongated molecules, which are located in a layer adjacent to the support, have a preferred orientation. In the case of the support plate 1, this preferred orientation is parallel to the plane of the drawing and, in the case of the support plate 2, is perpendicular to the plane of the drawing. This is indicated symbolically in FIG. The orientation is passed by the molecules to adjacent molecules shifted whereby the orientation of the molecules of the Stutzplntte 2 to the support plate 1 over 90 ° <- ~ is. Filling light 9 passes a polarizer 10 with a direction of polarization perpendicular to the plane of the drawing. The direction of polarization of the light, which the layer of a nemutic liquid crystal 3 »wipe two non-excited electrodes, z. B. 8 and 4, happens, is shifted by the so-called "twist" (rotation) in the liquid crystal over 90 °. The light is then transmitted by a polarizer It with a direction of polarization parallel to the plane of the drawing.

However, there is a voltage difference between the electrodes 8 and 5 or 8 and 7. Through this the elongated molecules are aligned in such a way that they correspond to the field lines of the electric field are parallel because at the applied frequency ε || greater than elist

This has the consequence that the direction of polarization of the light is not between the relevant electrodes is rotated, whereby the light can not pass the polarizer 11. The electrodes 5 and 7 rise so dark against a light background. Because the polarization directions of the polarizers 9 and 11 are both chosen perpendicular to the plane of the drawing, the excited electrodes are highlighted against a dark background. It is also possible not to have a "twist" in the liquid crystal layer 3 cause, but z. B. all molecules in the unexcited state one to the plane of the drawing to give parallel orientation. The liquid crystal layer then acts as a birefringent one in a known manner Element and can, if placed between two polarizers, in a known manner for Image reproduction purposes are used.

When switching on, the rise time is from 10% 90% of the maximum contrast approx. 15 msec with an alternating voltage of 25 V, 500 Hz. After switching off of the voltage, the decrease from 90% to 10% of the maximum contrast is about 500 msec. This Fall time is by nature for image reproduction purposes

way too long. In a device according to the invention, an alternating voltage of 25 V, 70 kHz is used in order to shorten the dropping time. The decay time is thereby reduced to about 15 msec. This is due to the fact that at 70 kHz el is greater than ε \\ , whereby the elongated molecules are aligned perpendicular to the field lines of the electric field. As a result, they are exposed to an additional force which drives them back into the unexcited state. Without alternating voltage, the molecules reach the unexcited state because the molecules in the layers adjoining the support plates 1 and 2 pass on their preferred orientation. This phenomenon propagates to the middle of the layer 3, which takes a longer time the thicker the layer 3 and the greater its viscosity

Another example of one in accordance with the invention Applicable liquid crystal device is a mixture of the following three substances:

.,) - C = N

where R ₁ = Ii-C ₄ H ₉ O-. R ₂ = η-QH ₁₃ O- and

is this mixture will
Physics «, Issue 56, No. 14,
1494-1497.

in "Journal of Chemical February 15, 1972, p.

For this 1 blue drawing

Claims (1)

Claim: Electro-optical device with a layer of a nematic liquid crystal between a first and a second support plate, in which at least one of these support plates is transparent, and the support plates are provided with electrodes for generating an electric field perpendicular to the liquid crystal layer, in the absence of said electrical field Field, the major axis of the molecules of the liquid crystal has an orientation parallel to said supporting plates, the orientation in a layer adjoining the first supporting plate being parallel to a first direction and in a layer adjoining the second supporting plate being parallel to a second direction which is further is provided with means with the aid of which electrical voltages are supplied to the electrodes mentioned, and in which the liquid crystal has a dielectric anisotropy Δε = ε || - Has εΐ, which is positive below a certain critical frequency and negative above said critical frequency, characterized in that said electrical voltages have a frequency below said critical frequency for exciting the device and a frequency above said critical frequency for accelerated shutdown of the device. 35 The invention relates to an electro-optical device with a layer of a nematic liquid crystal between a first and a second support plate, in which at least one of these support plates is transparent, and the support plates are provided with electrodes for generating an electric field perpendicular to the liquid crystal layer which in the absence of said electric field the major axis of the molecules of the liquid crystal has an orientation parallel to said support plates, the orientation in a layer adjoining the first support plate to a first direction and in a layer adjoining the second support plate to a second Direction is parallel, which is further provided with means by means of which electrical voltages are supplied to the electrodes mentioned, and in which the liquid crystal has a dielectric anisotropy Δε = ε || - has si, which is positive below a certain critical frequency and negative above the mentioned critical frequency. Here, under ε || the one on the big axis of the molecules parallel electric field measured dielectric constant, and under El the Dielectric constant measured for an electric field perpendicular to the major axis of the molecules. The orientation of the major axis of the molecules can be such that the said first and the said second direction are mutually perpendicular. 6s The device is then sometimes referred to as a "Twistor" and is known from the article "Voltage dependent optical activity of a twisted nematic liquid crystal ", Appl. Phys. Lett. 18 (1971), pp. 127 and 128. Die The orientation of the molecules shifts over 90 ° from the first support plate to the second Support plate. The polarization direction of linearly polarized light is of such a device in shifted over 90 ° in the unexcited state. If a DC voltage or a If a low frequency voltage is applied, the orientation of the molecules at that point will be in connection with their positive static dieelectric anisotropy of parallel to the Support plates changed to perpendicular to the support plates. This has the consequence that the polarization direction of the incident light is no longer shifted. When the device is between crosswise Polarizers with polarization directions parallel to said first or to said second direction is located, this has the consequence that excited parts of the device are dark against a light Stand out background. If the device is between parallel polarizers with to that mentioned first polarization directions or polarization directions parallel to said second direction is located, this has to The result is that excited parts of the device stand out brightly against a dark background. The orientation of the major axis of the molecules can also be such that the said first and the said second direction are parallel to each other. The device then acts in the de-energized state as birefringent cell. In this case, too, a low-frequency electric field has the consequence that the Orientation of the molecules changed from parallel to the supporting plates to perpendicular to the supporting plates The use of cross-wise polarizers then again prevents the passage of light in the excited state. A disadvantage of the devices described is that the de-energized state is slow is reached after switching off the voltage. This is due to the fact that after switching off the tension is the preferred orientation of the molecules in the layers adjoining the support plates must propagate to the middle of the layer, the thickness of the layer and the viscosity of the Liquid crystals play a role. On the other hand, it is switched on quickly because all of the molecules are involved at the same time exposed to the influence of the electric field. The invention aims to provide means with which Help the switch-off time of the device is shortened. According to the invention, this object is achieved by that the said electrical voltages have a frequency below the said critical frequency to excite the device and a frequency above said critical frequency for have accelerated shutdown of the device. If the dielectric anisotropy is negative, i. H, that el is greater than e || is, the molecules are aligned by an electric field in such a way that their large Axis is perpendicular to the direction of the electric field. In the present case, this means that the Molecules are exposed to an external force that leads them to an orientation parallel to the support plates For this purpose, a voltage of high frequency and a r.eir.atischer liquid crystal used, the dielectric anisotropy of which changes its sign with increasing frequency.