Liquid crystal electro-optical element and process for preparation thereof

In an electro-optical element comprising a liquid crystal, such as a smectic liquid crystal, sealed between two substrates arranged to confront each other with a space of an order of .mu.m, if spacer particles for regulating the space between the substrates and phenol type curing agent-incorporated epoxy resin adhesive particles for moderating stresses generated by distortions or warps of the substrates are arranged in the space between the substrates, a minute uniform space, especially a uniform space of 1 to 3 .mu.m effective for a ferroelectric liquid crystal, can be maintained between the substrates. In this structure, alignmental films are not disturbed at all. Furthermore, even if a nematic or super-twist liquid crystal is used, a uniform space can be stably maintained between the substrates.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electro-optical element comprising a 
liquid crystal, such as a smectic liquid crystal or nematic liquid 
crystal, which is inserted and gripped between two substrates. More 
particularly, the present invention relates to an electro-optical element 
having a uniform space of an order of .mu.m and a process for the 
preparation thereof. Furthermore, the present invention relates to a 
technique of securing two substrates, between which a liquid crystal is 
sealed, by a spot adhesive. 
2. Description of the Related Art 
A liquid crystal electro-optical element comprises two glass substrates 
having formed on the surfaces thereof a driving transparent electrode film 
and an oriented film for arranging molecules of a liquid crystal, which 
are arranged to confront each other with a certain space therebetween, and 
a liquid crystal sealed in the space. 
A liquid crystal electro-optical element utilizing a ferroelectric liquid 
crystal showing a chiral smectic C phase has recently been developed (see, 
for example, Japanese Unexamined Patent Publication No. 56-107216). More 
specifically, a liquid crystal having a chiral smectic C phase, such as 
p-desiloxy-benzylidene-p'-amino-2-methylbutyl cinnamate or 
p-hexyloxybenzylidene-p'-amino-2-chloropropyl cinnamate, has a liquid 
crystal molecule arrangement having a spiral layer structure. When the 
liquid crystal is injected between two substrates arranged to confront 
each other with a space narrower than the spiral period, the liquid 
crystal molecules lose the spiral structure and bistable states are 
produced by influences of the oriented film. By utilizing the 
ferroelectric characteristic of the liquid crystal molecules, the bistable 
states are changed over to each other at a high speed by application of a 
voltage to drive the element. When the voltage is removed, the liquid 
crystal molecules retain one of the bistable states. In short, the liquid 
crystal has a memory characteristic. 
As another known technique, there can be mentioned a technique of bonding 
and securing glass sheets by using an encapsulated adhesive, as disclosed 
in Japanese Unexamined Patent Publication No. 57-29031. However, this 
technique is not preferred because the bonding force of the adhesive is 
low and the adhesive has bad influences on a liquid crystal. 
SUMMARY OF THE INVENTION 
In order to realize bistable states in a liquid crystal substance having a 
chiral smectic C phase, it is an indispensable condition that two 
substrates should be held while a uniform space smaller than several .mu.m 
is maintained therebetween. However, since distortions or warps are 
present in the substrates per se, it is difficult to decrease the length 
of the space between the substrates. 
For example, in a conventional structure shown in FIG. 3, spacer particles 
2 having a diameter equal to the intended space length are scattered on 
the surface of one substrate 1 and another substrate 3 having convexities 
and concavities owing to warps is piled and bonded onto the substrate 1 by 
using a sealing material 4, as shown in FIG. 3-(A). However, after heat 
bonding under pressure, as shown in FIG. 3-(B), the spacer particles are 
destroyed in the convexities 5 of the substrate 3 while the spacer 
particles are separate from the substrate 3 in the concavities 6. 
Therefore, realization of a uniform space between substrates is very 
difficult. 
Under this background, it is a primary object of the present invention to 
provide a cell structure in which two substrates are arranged in parallel 
to each other with a space as narrow as possible therebetween 
Another object of the present invention is to provide a process for the 
preparation of the above-mentioned cell structure. 
In accordance with the present invention, these objects can be attained by 
(1) a liquid crystal electro-optical element comprising a liquid crystal, 
two substrates secured by a sealing material to confront each other and 
hold the liquid crystal therebetween, spacer particles dispersed and 
arranged between the two substrates to maintain a certain space between 
the substrates, oriented films present in interfaces between the liquid 
crystal and the substrates to line up molecules of the liquid crystal and 
driving means for applying a voltage to the molecules of the liquid 
crystal, wherein the substrates are spot-bonded to each other through 
adhesive particles comprising as the main component an epoxy resin having 
a latent curing agent incorporated therein, which are dispersed and 
arranged in the space between the substrates, and (2) a process for the 
preparation of a liquid crystal electro-optical element comprising a 
liquid crystal, two substrates secured by a sealing material to confront 
each other with a certain space and hold the liquid crystal therebetween, 
oriented films present in interfaces between the liquid crystal and the 
substrates to line up molecules of the liquid crystal and driving means 
for applying a voltage to the molecules of the liquid crystal, said 
process comprising the steps of arranging the sealing material on one 
substrate having an electrode and oriented film formed on the surface 
thereof in a peripheral edge portion thereof on the oriented film side, 
dispersing and arranging spacer particles having a diameter equal to an 
intended space length and adhesive particles comprising as the main 
component an epoxy resin having a latent curing agent incorporated 
therein, which have a particle size larger than the diameter of the spacer 
particles, on the surface of one substrate, bonding the two substrates by 
heat-pressing the substrates in the piled state, and sealing the liquid 
crystal in the space between the substrates.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described in detail with reference to the 
accompanying drawings. 
FIG. 1 is a partially cut-out perspective view showing the basic structure 
of the present invention. Each of reference numerals 1 and 3 represents a 
glass substrate having a transparent electrode (not shown) and an 
alignmental film 7, which are formed on the surface thereof. Spherical or 
polygonal fine particles 2 (hereinafter referred to as "spacer particles") 
composed of a heat-resistant material are uniformly dispersed to define 
the length of a space between the substrates 1 and 3, and these substrates 
are heat-bonded in the state attracted to each other by a sealing material 
4 arranged in the peripheral edge portion of the substrate 1 and adhesive 
particles 8 composed of a latent curing agent-containing epoxy resin, 
whereby a cell is constructed. 
The epoxy resin adhesive particles are crushed, and they act as a cushion 
for preventing the spacer particles from being destroyed by the 
convexities or the substrates and serve to attract the concavities of the 
substrate toward the confronting substrate by bonding, whereby a uniform 
space is realized between the two substrates. 
When a phenolic curing agent, especially an adduct of a bisphenol glycidyl 
ether or a condensation product thereof and a polyhydric phenol compound, 
particularly a bisphenol, is used as the latent curing agent, the latent 
curing agent is sufficiently compatible with the epoxy resin and a high 
bonding force can be attained, and contamination or destruction of the 
alignmental film can be effectively prevented. 
Preferably, the epoxy resin adhesive particles have a spherical shape. In 
general, where the particles are used in an amount of 0.1 to 50 mg per 100 
cm.sup.2 of the substrates, a strong adhesion is achieved and their 
existance is imperceptible in the image of the liquid crystal 
electro-optical element. 
When a liquid crystal substance having a chiral smectic C phase is injected 
into the cell having the above-mentioned structure, the liquid crystal 
flows into gaps defined by the spacer particles 2 and the epoxy resin 
adhesive particles 8 and the space is filled with the liquid crystal. 
Accordingly, even if an external force is applied to the cell, the uniform 
space is maintained between the substrates by the spacer particles 2 and 
epoxy resin adhesive particles 8, and since these particles act as 
hindering members, flow of the liquid crystal substance is blocked. Of 
course, the spacer particles 2 and epoxy resin adhesive particles 8 are 
composed of materials having no bad influences on the behavior of the 
liquid crystal and the distribution densities of the particles are small, 
and therefore, bad influences are not imposed on displayed pictures by the 
presence of these particles. 
The process for the preparation of the above-mentioned cell will now be 
described with reference to FIG. 2. 
A glass substrate 1 having a transparent electrode 9 and an alignmental 
film 7, formed thereon, is horizontally arranged so that the front surface 
is on the oriented film side, and a heat-melt-bondable sealing material 4 
is coated in a certain thickness larger than the cell thickness in the 
peripheral edge portion of the front surface of the substrate 1. Spacer 
particles 2 of aluminum oxide having a diameter equal to the intended cell 
thickness and adhesive particles 8' of a latent curing agent-containing 
epoxy resin of the B stage, which have a diameter larger than the intended 
cell thickness and almost equal to the seal thickness, are dispersed on 
the surface of the substrate 1 in a region surrounded by the sealing 
material 4 [see FIG. 2-(A)]. 
Another substrate 3 is piled on the substrate 1 so that the side of the 
alignmental film 7 is located below, and the two substrates 1 and 2 are 
arranged in parallel with a certain space therebetween through the sealing 
material 4 of the lower substrate 1 and the epoxy resin adhesive particles 
8' [see FIG. 2-(B)]. 
In this state, a pressure P is applied to the upper and lower two 
substrates 1 and 3 and the assembly is heated at a temperature softening 
the sealing material 4 and the particles 8' of the latent curing 
agent-containing epoxy resin of the B stage, that is, the semi-cured 
stage. At this point, the sealing material 4 and the epoxy resin adhesive 
particles 8' begin to soften. The epoxy resin adhesive particles 8' 
uniformly receive the pressure p and are crushed flat while they are 
fusion-bonded to the glass substrate 1 and 3. When the upper glass 
substrate 3 thus abuts against the spacer particles 2, the two glass 
substrates 1 and 3 are supported by the spacer particles 1 and 3 and the 
movement of the substrates 1 and 3 is stopped in such a state that the 
substrates 1 and 3 are arranged in parallel to each other and a space 
corresponding to the diameter of the spacer particles 2 is held between 
the substrates [see FIG. 2-(C)]. 
In the above-mentioned structure, even if there are present certain 
convexities and concavities (ordinarily about 20 to about 30 .mu.m) on the 
substrates 1 and 3, by effecting bonding by pressing under heating, a 
certain distance can be maintained between the substrates. Namely, the 
convexities and concavities of about 20 to about 30 .mu.m can be 
corrected. 
If heating is continued in this state, the adhesive particles 8 of the 
epoxy resin are crushed flat and are cured in the state fusion-bonded to 
the two substrates 1 and 3. 
The two substrates 1 and 3 are secured while they receive a force of 
attracting them to each other by the sealing material 4 and the epoxy 
resin adhesive particles 8 but the inward movement is regulated by the 
spacer particles 2, whereby a cell is formed. 
The epoxy resin adhesive particles 8 exert a cushioning action at the 
press-bonding step and prevent the spacer particles 8 from being crushed 
and broken by convexities of the undulated substrates. 
Furthermore, since the epoxy resin adhesive particles 8 comprises a latent 
curing agent, a contaminant reaction gas is not generated at the curing 
reaction and hence, the oriented film is not deteriorated. Accordingly, an 
electro-optical element having a good contrast ratio can be obtained. 
Moreover, since the epoxy resin adhesive particles 8 are chemically 
stable, the liquid crystal is not modified or deteriorated even if the 
liquid crystal is used for a long time, and the durability characteristic 
is highly improved. 
In the present invention, it is preferred that the liquid crystal be a 
smectic liquid crystal, because the response speed to the voltage is high 
and the picture image is clear. In order to attain the objects of the 
present invention, it is preferred that the smectic liquid crystal be a 
ferroelectric liquid crystal having a spiral molecular arrangement 
structure. In case of a ferroelectric liquid crystal, it is most preferred 
that the space between the substrates be 1 to 3 .mu.m. If the liquid 
crystal is a cholesteric, super-twist or nematic crystal, the space 
between the substrates is 3 to 20 .mu.m. 
For the liquid crystal substrate, compounds of the following formula and 
combinations thereof are also useful. As the chiral smectic C, in addition 
to the compounds as described on page 2, lines 6 and 7, the following 
compounds may be advantageously used alone or as a mixture of two or more 
thereof. These compounds are disclosed in "Technical Reports in the 
Television Society", Feb. 3, 1986, ED 917, IPD 104-1. 
##STR1## 
R.sub.1 and R.sub.2 are as listed in the Table below. In the Table, the 
asterisked carbon atoms are asymmetric carbon atoms. 
TABLE 
______________________________________ 
No. R.sub.1 R.sub.2 
______________________________________ 
##STR2## n-C.sub.8 H.sub.17 
2 n-C.sub. 8 H.sub.17 
##STR3## 
3 
##STR4## 
##STR5## 
4 
##STR6## n-C.sub.8 H.sub.17 
5 
##STR7## n-C.sub.8 H.sub.17 O 
6 
##STR8## n-C.sub.8 H.sub.17 O 
7 
##STR9## 
##STR10## 
8 n-C.sub. 8 H.sub.17 O 
##STR11## 
9 n-C.sub.11 H.sub.23 O 
##STR12## 
10 
##STR13## 
##STR14## 
11 
##STR15## n-C.sub.11 H.sub.23 O 
12 
##STR16## n-C.sub.8 H.sub.17 O 
13 
##STR17## n-C.sub.11 H.sub.23 O 
14 
##STR18## n-C.sub.8 H.sub.17 O 
15 
##STR19## n-C.sub.11 H.sub.23 O 
16 
##STR20## n-C.sub.8 H.sub.17 O 
17 
##STR21## n-C.sub.11 H.sub.23 O 
18 
##STR22## n-C.sub.8 H.sub.17 O 
______________________________________ 
In the present invention, it is preferred that the epoxy resin adhesive 
particles should have a deformed spherical shape pressed by the pressure 
given by the two substrates, because the oriented film or 
electroconductive film is not damaged. 
In the present invention, the substrate is preferably a glass sheet, 
because the glass sheet is excellent in the transparency and hardness 
Furthermore, the substrate may be a plastic sheet, and the plastic sheet 
is safe and light. Plastics excellent in the transparency, such as 
polymethyl methacrylate and polycarbonate, are preferred. In order to 
improve the abrasion resistance, it is preferred that a hard coat layer of 
silica, epoxysilane, organic polysiloxane or crosslinked polyacrylate be 
formed on the surface of the plastic sheet to be exposed to the outer air. 
Furthermore, a silica-containing layer is preferably formed to impart a 
reflection-preventing property to the outermost layer to be contacted with 
the outer atmosphere. Of course, a reflection-preventing hard coat layer 
may be formed. 
In the present invention, an unoriented or monoaxially oriented film may be 
used at least as the upper substrate. The reason is that in the case where 
the surface of the panel (display) is convexly curved, an iridescent 
pattern is not found even if seen obliquely. The direction of the 
monoaxial orientation may be longitudinal or lateral. The orientation 
degree is such that the draw ratio is about 1.5 to about 7, preferably 5 
to 6. 
An acetate film is preferred as the unoriented film, and any of known drawn 
resin films can be used but a polyethylene terephthalate film is preferred 
as the uniaxially oriented film. The polyethylene terephthalate film has a 
high melting point and is stable to liquid crystals and can be used for a 
long time. Furthermore, the polyethylene terephthalate film is 
advantageous in that the cost is cheap. The thickness of the film is not 
particularly critical, so far as the thickness is enough to form a panel. 
It is indispensable that an electroconductive layer should be formed on the 
film so as to give charges to the liquid crystals. Any of known 
electroconductive layers can be used, but an electroconductive layer 
composed of indium oxide and tin oxide is preferred. This 
electroconductive layer can be formed by vacuum evaporation deposition, 
sputtering or ion plating (inclusive of the ion assist method) of metals 
in an oxidizing atmosphere. 
As is apparent from the foregoing description, according to the present 
invention, since two substrates are secured and bonded by adhesive 
particles of a latent curing agent-containing epoxy resin in the state 
where spacer particles having a diameter equal to an intended space are 
dispersed between the two substrates, the space can be defined at many 
points by the spacer particles in the state where inwardly attracting 
forces are given to the substrates by many epoxy resin adhesive particles, 
and inherent distortions of the substrates can be corrected and a parallel 
cell structure can be formed. Accordingly, even if an external force is 
applied, the uniform space can be maintained and downward flow of the 
liquid crystal can be prevented. Moreover, since the space between the 
substrates is regulated by the spacer particles and epoxy resin adhesive 
particles, a minute uniform space can be maintained irrespectively of the 
areas of the substrates and a liquid crystal panel having a large area can 
be realized. Moreover, since a phenolic curing agent is used, no 
contaminant gas is generated and the orientation of the liquid crystal is 
not disturbed. 
The present invention will now be described in detail with reference to the 
following examples that by no means limit the scope of the invention. 
EXAMPLE 1 
An epoxy resin adhesive liquid was coated in a thickness of about 7 .mu.m 
on the surface of the peripheral region of a glass substrate having a 
transparent electrode coat layer and a rubbed or unrubbed polyimide coat 
layer as an alignmental film to form a sealing portion. An epoxy type 
spherical particulate adhesive (having a composition described below) 
having a diameter of 5.5 .mu.m and fine particles of alumina having a 
diameter of 2 .mu.m were scattered at predetermined densities (for 
example, 200 particles per mm.sup.2) on the inner region surrounded by the 
sealing portion. Another glass substrate was piled on the glass substrate, 
and the assembly was heated at an elevated temperature (for example, 80 to 
200.degree. C.) under a pressure (for example, 0.3 to 5 Kg/cm.sup.2). A 
cell structure comprising the glass substrates secured in parallel with a 
space of 2 .mu.m was thus obtained. 
A ferroelectric chiral smectic liquid crystal (for example, 
p-desiloxybenzylidene-p'-amino-2-methylbutyl cinnamate as described above) 
was injected into the formed cell, and the cell was driven and the ratio 
of the contrast at the time of transmission of light to the contrast at 
the time of interception of light was measured. The contrast ratio was 5.5 
to 6.5. Accordingly, it was confirmed that the contrast ratio was 
sufficiently high and the orientation state of the liquid crystal was not 
disturbed. 
Specific examples of the epoxy type spherical particulate adhesive are 
described below. 
ADHESIVE 1 
A polyethylene cup having a capacity of 300 cc was charged with 20 g of 
Epikote 828 and 20 g of Epikote 1001 as the epoxy resin, each having a 
commercially available bisphenol A diglycidyl ether type epoxy resin 
supplied by Yuka-Shell Epoxy, and 4 g of Emulsit 9 (supplied by Diichi 
Kogyo Seiyaku), which is a polyoxyethylene nonylphenyl ether having an HLB 
value of 16.2, as the surface active agent was added and 4 g (about 0.12 
equivalent) of Epicure 171N (supplied by Yuka-Shell Epoxy), which is an 
adduct of condensed bisphenol A diglycidyl ether and bisphenol A, as the 
latent curing agent was added. The entire mixture was heated at 95.degree. 
C. and promptly stirred to form a transparent compatible liquid. 
A stirrer having a Teflon plate vane attached to the top end thereof was 
set in the cup, and the liquid was stirred at 800 rpm at a temperature 
maintained at 50.degree. C. Then, 6 cc of water contained in a syringe and 
maintained at 50.degree. C. was added and the mixture was stirred for 40 
seconds. This operation was repeated 4 times. Thus, the mixture of the 
epoxy resin and Epicure 171 was emulsified by 24 cc of water as a whole. 
A curing liquid formed by diluting 0.44 equivalent of piperazine with 32 cc 
of water was added to the emulsion and the mixture was gently stirred to 
uniformalize the emulsion. 
The emulsion was allowed to stand still at 25.degree. C. for 6 days to 
obtain spherical particles having an average particle size of about 6 
.mu.m. 
Water classification (elutriation) was carried out to obtain such a 
particle size distribution that particles having a size of 5.5.+-.2 .mu.m 
occupied 95% by weight of total particles. 
Silica sol (Snowtex N, solid content of 40% by weight) was added in an 
amount of 2.5% by weight based on the particles to the classified particle 
suspension, and the mixture was stirred for 30 minutes to make silica 
adsorbed on the particles. 
The particles were recovered by suction filtration and dried at normal 
temperature under reduced pressure. 
To measure a tear bonding strength, 0.5 mg of the particles were uniformly 
scattered on a square region having a side of 15 mm on a slide glass and 
the scattered region was covered by a slide glass having the same size as 
that of the above-mentioned glass. The side glasses were secured by clips 
and were subjected to the curing treatment for 2 hours in a hot air drier 
maintained at 170.degree. C. It was found that the tear bonding strength 
was 40 kg/15 mm. 
ADHESIVE 2 
A polyethylene cup having a capacity of 300 cc was charged with 40 g of 
Epikote 828, 12 g (about 0.26 equivalent) of Epicure 171N as the latent 
curing agent and 4 g of Noigen EA137 (supplied by Daiichi Kogyo Seiyaku), 
which is a commercially available polyoxyethylene phenol substituted ether 
type surface active agent having an HLB value of 13, as the surface active 
agent, and they were heat-mixed at 95.degree. C. to obtain a transparent 
compatible liquid. The liquid was emulsified in the same manner as 
described in Adhesive 1 except that the emulsifying temperature was normal 
temperature. 
A curing liquid formed by diluting 0.3 equivalent of piperazine with 32 cc 
of water was added to the emulsion and the mixture was gently stirred to 
uniformalize the emulsion. 
The emulsion was allowed to stand still at 25.degree. C. with gentle 
stirring at about 1 to about 3 rpm for 4 days to obtain spherical 
particles having an average particle size of 6.5 .mu.m. 
In the same manner as described in Adhesive 1, water classification was 
carried out so that particles having a size of 5.5.+-.2 .mu.m occupied 95% 
by weight of total particles, and in the same manner as described in 
Adhesive 1, 1% by weight of silical was adsorbed on the particles. 
After drying under reduced pressure, the particles had a tear bonding 
strength of 35 Kg/15 mm. 
ADHESIVE 3 
The following curing agents were used as the phenolic curing agent to be 
contained in the epoxy type spherical particulate adhesive. In each case, 
a good contrast ratio was obtained. 
Methylon 75/08 supplied by G-E 
Resimene P97 supplied by Monsanto 
Varcom 1281B supplied by Varcom 
Super-Beckacite supplied by Japanese Reichhold 
Hitanol 4010 and Hitanol 4020 supplied by Hitachi Kasei 
##STR23## 
EXAMPLE 2 
On one surface of a monoaxially oriented polyethylene terephthalate film 
(draw ratio of 5.5) having a thickness of 100 microns, vacuum evaporation 
deposition was carried out by resistance-heating an evaporation source 
comprising metallic indium and metallic tin (metallic tin content of 12% 
by weight), which was charged in a tungsten boat, under a high vacuum 
(2.times.10.sup.-2 Torr) in an oxygen atmosphere. The thickness of the 
obtained electroconductive coat layer was 850 .ANG.. Then, the film was 
subjected to an oxidizing heat treatment at 150.degree. C. for 20 minutes 
to obtain a transparent film having a sheet resistivity of 50 .OMEGA.. 
Then, an alignmental film was formed on the electroconductive coat layer. 
By using the so-obtained electroconductive film as the top plate, cured 
epoxy resin particles (having an average particle diameter of 2 .mu.m) as 
the spacer and the epoxy resin spherical particulate adhesive as obtained 
in Adhesive 1 of Example 1 as the particulate adhesive, a liquid crystal 
display cell as shown in FIGS. 1 and 2 was prepared with a space of 2 
.mu.m by carrying out cure-bonding at 150.degree. C. The electroconductive 
layer or the film was not damaged at all, and even if the surface was 
curved, no iridescent pattern was formed. In short, a good cell heretofore 
not obtainable could be obtained. 
REFERENTIAL EXAMPLE 
For comparison, a ferroelectric liquid crystal electro-optical element was 
prepared by using epoxy resin particles of the B stage containing an amine 
type latent curing agent, and the contrast ratio was measured. It was 
found that the contrast ratio was reduced to 3.0 to 4.0, and it was 
confirmed that the orientation of the liquid crystal was disturbed. 
Indeterminate particles formed by heating an epoxy resin (Stractbond 
X-7479-50 supplied by Mitsui-Toatsu) and amine type latent curing agent at 
90.degree. C. for 30 minutes to convert it to a resin of the B stage, 
pulverizing the resin and classifying the particles to adjust the particle 
size to about 7 .mu.m were used as the particles of the amine type latent 
curing agent-containing epoxy resin of the B stage. 
In the foregoing examples, the spacer particles and epoxy resin adhesive 
particles having a spherical shape were used. Needless to say, however, 
similar effects can be attained even if ellipsoidal or polygonal particles 
are used. 
In Example 1, glass substrates were used. Similar effects can be attained 
by using rigid heat-resistant polymeric resin plates as the substrates. 
Moreover, a chiral smectic liquid crystal was used in Example 1. However, 
other liquid crystal substances such as a smectic A liquid crystal and a 
nematic liquid crystal can be similarly used.