Liquid crystal electro-optic device with two materials different in refractive index dispersed in the liquid crystal

A liquid crystal device comprising a liquid crystal electro-optic layer supported between a pair of substrates furnished with electrodes, provided that at least one of said substrates is transparent, characterized by that said liquid crystal electro-optic layer comprises a liquid crystal material, a first transparent material, and a second transparent substance whose refractive index is different from those of the liquid crystal material and the first transparent material is disclosed. The liquid crystal electro-optic layer may otherwise comprise a liquid crystal material and a mixed transparent substances provided that the refractive index thereof is controlled to be about the same as that of the liquid crystal material. The device has improved display contrast and is capable of an increase light transmittance to 4 times as large as that of a conventional one.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electro-optic device, more 
particularly, it relates to a device comprising a liquid crystal layer 
having a large display area, which is capable of providing an extended 
visual field and an animation. The liquid crystal device according to the 
present invention may be used for electric machines for office automation 
(OA), such as laptop personal computers. 
2. Description of the Prior Art 
Liquid crystal display devices heretofore put into practice include those 
using singly nematic liquid crystals, i.e., the TN (twisted nematic) mode 
or the STN (super-twisted nematic) mode liquid crystals. There is also 
proposed a device using a ferroelectric liquid crystal alone. However, the 
liquid crystal display devices mentioned hereinbefore use a polarizer 
sheet on the surface and the back of the device. Furthermore, those 
devices are provided with molecule-orienting films (orientation control 
films) on the electrodes of the devices. The molecule-orienting films are 
formed by an orientation treatment. On the other hand, there is also known 
a method of producing liquid crystal devices having high contrast without 
applying such molecular orientation treatments. The method comprises 
dispersing liquid crystals or liquid crystal granules in a polymer by 
granulating the liquid crystal, and then making a film of the resulting 
polymer. As the substrances for granulating, proposed are gelatin, gum 
arabic, poly vinyl alcohol, and the like as proposed in unexamined 
published Japanese patent application No. 58-501631 and U.S. Pat. No. 
4,435,047. In this method, the granulated liquid crystal molecules arrange 
themselves along the electric field when an electric voltage is applied, 
and the display itself turns transparent if the refractive index of the 
liquid crystal agrees with that of the polymer. When the electric field is 
removed, the liquid crystal molecules recover the random orientation, and 
the display becomes opaque white because the light is dispersed at the 
boundary between the liquid crystal and the polymer due to the difference 
in refractive index between them. 
However, since the liquid crystal device described above does not use the 
conventionally known polarizer sheets, the achievement of a sufficiently 
high contrast has been a great problem to be solved. 
SUMMARY OF THE PRESENT INVENTION 
An object of the present invention is to provide a liquid crystal device 
which overcomes the problem set forth hereinbefore. 
The object of the present invention has been achieved by a device 
comprising a liquid crystal electro-optic layer (a composite of a liquid 
crystal and a transparent substance) being supported between a pair of 
substrates provided with electrodes thereon, wherein said liquid crystal 
electro-optic layer (referred to simply as `electro-optic layer`, 
hereinafter) comprises the liquid crystal and the transparent substance in 
a mixed state, provided that either of them is dispersed in the layer 
(that is, they constitute a disperse system). Furthermore, the present 
invention provides a liquid crystal device which provides a high contrast 
by increasing light transmittance, since it comprises a transparent 
substance composed of a composite of materials differed in refractive 
index, to thereby adjust the refractive index of the transparent substance 
to be about the same as that of the liquid crystal along the light 
transmitting direction upon applying an electric field to the liquid 
crystal. That is, the light transmittance is increased by making an 
apparent refractive index as composition of the refractive index of one 
component of the transparent materials and the refractive index of the 
other component of the transparent materials substantially equal to the 
refractive index of the liquid crystals. For this purpose, one component 
of the transparent materials has a refractive index different from that of 
the other component of the transparent materials and that of the liquid 
crystal material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The transparent substance for use in the present invention is a highly 
transparent resin, and in general, an organic resin is used. The organic 
resin may be an ultraviolet (UV)-curable resin, such as poly methyl 
methacrylate (PMMA). It is also effective to add a fluorine-containing 
compound which is a ferroelectric material, for example, polyvinylidene 
fluoride (1-100%), in the transparent substance to thereby adjust the 
refractive index of the transparent resin in such a way that it may 
coincide with that of the liquid crystal material upon applying an 
electric field. 
The refractive index of the transparent substance can be adjusted to be 
substantially the same as that of the liquid crystal material by mixing 
transparent substances, as mentioned earlier, or by laminating a plurality 
of different transparent substances to obtain a synthesized refractive 
index which correspond to that of the liquid crystal. By this 
configuration, since the transparent substances (the different transparent 
substances) in combination have substantially the same effect upon light 
refraction as the liquid crystal material, light transmittance through the 
liquid crystal electro-optic layer is increased. 
The composite transparent substance is mixed with the liquid crystal at the 
former to the latter ratio in the range of from 20/80 to 80/20 in weight 
ratio. The present invention can then be realized by applying an AC 
electric field to the electro-optic layer obtained in this way. 
The present invention is now described in further detail by making 
reference to illustrative embodiments, but it should be understood that 
the present invention is not to be construed as being limited thereto. 
Referring to FIGS. 1(A) and 1(B), a liquid crystal electro-optic device in 
accordance with a first preferred embodiment of the present invention is 
explained below. A liquid crystal electro-optic layer was prepared using 
PMMA as the first transparent substance, a carbon film as the second 
transparent substance, and a liquid crystal as the liquid crystal 
material. The properties of the liquid crystal are described hereinafter. 
Minute particles 4 of PMMA from 1000 .ANG. to 5 .mu.m in size, preferably 
about 1 .mu.m in size were prepared, and were coated with a carbon film 5 
about 200 .ANG. in thickness by a known plasma deposition process. The 
carbon-film 5 coated transparent substance thus obtained was then mixed 
with the liquid crystal 2, and after stirring, ultrasonics was applied 
thereto to obtain a mixture having the transparent substance uniformly 
dispersed therein. It was found that a more effective process comprises 
heating the electro-optic layer material to obtain an isotropic phase 
(liquid phase) liquid crystal, stirring, and applying thereto ultrasonics 
to disperse uniformly the carbon-film coated transparent substance in the 
liquid crystal, and cooling thereafter to a temperature at which the 
liquid crystal electro-optic layer exhibits liquid crystal 
characteristics. 
A transparent glass substrate 1 provided with one of the electrodes thereon 
was then coated with the mixture of the liquid crystal and the transparent 
substance, and was laminated with the other substrate provided with the 
other electrode thereon with an alumina spacer (5 .mu.m in diameter) 
having inserted therebetween. In FIGS. 1(A) and 1(B), the electrodes are 
omitted. 
Instead of using the alumina spacers, a part of the first transparent 
substance may be substituted by particles having a diameter larger than 
that of the transparent substance. Such particles are surface-coated with 
an adhesive material as the second transparent substance, and the 
substrates are then laminated with the particles sandwiched therebetween 
to obtain a cell structure in which the particles function as a spacer. 
The liquid crystal device thus fabricated comprises a randomly oriented 
liquid crystal if no voltage is applied between the electrodes provided on 
the upper and lower substrates as shown in FIG. 1(A); thus, light 6 is 
scattered in this case. Once a voltage is applied to the electrodes as 
shown in FIG. 1(B), the liquid crystal takes a particular arrangement 
oriented to one direction according to the direction of the electric 
field, and generates an electro-optic effect. Thus, the light is 
transmitted in this case. Under an electric field, as is shown in FIG. 
1(B), if the refractive index of the liquid crystal 2 along the direction 
of light transmittance becomes equal to that of the transparent substance, 
a maximum amount of light can be transmitted. 
The liquid crystal material used in the first preferred embodiment 
comprises at least an ester type ferroelectric liquid crystal material, 
having a phase transition series of Iso-SmA-SmC*-Cry. The transition 
temperatures thereof are given below: 
##EQU1## 
This liquid crystal has a positive dielectric anisotropy with an optical 
anisotropy, i.e., an anisotropy of the refractive index, .DELTA.n, of 
about 0.2. 
Referring now to FIG. 2, a liquid crystal electro-optic device in 
accordance with a second preferred embodiment of the present invention is 
illustrated therein. The device comprises a pair of substrates 11, a 
liquid crystal 13, an organic resin 14, and a material 12 covering the 
liquid crystal 13 and having a refractive index different from that of the 
liquid crystal 13 and that of the organic resin 14. 
There is no particular restriction with respect to the liquid crystal to be 
used in the present invention; TN mode liquid crystals, nematic liquid 
crystals, smectic liquid crystals, and cholesteric liquid crystals as well 
as the ferroelectric liquid crystals may be used, and other liquid 
crystals as well. A dye may be added to the liquid crystal. The present 
invention provides an liquid crystal device which yields an increased 
light transmittance 4 times as large as that of the conventional TN mode 
liquid crystal devices using polarizer sheets, since no polarizer sheets 
are used in the liquid crystal device in accordance with the present 
invention. 
Furthermore, the liquid crystal device according to the present invention 
provides a favorable display without using a back light, since the 
contrast of light transmittance and the non-transmittance is improved. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.