Liquid coating composition for forming transparent conductive films and a coating process for using said composition

A liquid coating composition for formation of a transparent conductive film, which comprises a solution of indium nitrate in a .beta.-diketone or a mixture of a .beta.-diketone and another organic solvent or a reaction product of indium nitrate with a .beta.-diketone, an activator and an organic solvent other than a .beta.-diketone, is disclosed. When this coating composition is coated on a substrate and the coated substrate is heat-treated at a temperature higher than about 350.degree. C., there can be obtained a transparent conductive film having excellent transparency, electrical conductivity and mechanical strength.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a liquid coating composition for the 
formation of a transparent conductive film and a process for forming a 
transparent conductive film on a substrate by using this liquid coating 
composition. 
(2) Description of the Prior Art 
Transparent conductive films have been broadly used for image pick-up 
tubes, de-icing materials for automobiles and airplanes, electric screens, 
electrodes of fluorescent indicator tubes, liquid crystal display devices 
and the like. These transparent conductive materials are required to have 
high transparency, good electrical conductivity and high mechanical 
strength. As materials providing transparent films meeting such 
requirements, there have been used materials composed mainly of metal 
oxides such as tin oxide and indium oxide. Indium oxide type materials are 
ordinarily inferior to tin oxide type materials with respect to thermal 
stability. However, since films formed from indium oxide type materials 
can easily be etched by a hydrochloric acid solution, these materials are 
frequently used for formation of transparent electrodes on which a 
delicate pattern should be formed, such as those of liquid crystal display 
devices. As glass substrates for formation of liquid crystal display 
devices, sodium glass sheets are mainly used for the purpose of reducing 
manufacturing costs. Since the melting point of sodium glass is low, it is 
necessary to adopt a temperature as low as possible for the coating 
operation, and, simultaneously, it is necessary to impart good electrical 
conductivity and high mechanical strength to the resulting films. 
A vacuum evaporation coating process has heretofore been mainly adopted for 
formation of conductive films of indium oxide type materials. Although 
this process is satisfactory with respect to the quality of the resulting 
films, since the melting point of starting indium oxide is high, 
high-temperature heating is indispensable and the equipment is inevitably 
expensive and since the process is carried out batchwise, mass production 
is very difficult. Accordingly, attempts have been made to develop 
processes in which the coating operation can be facilitated and the 
foregoing disadvantages can be eliminated. For example, Japanese Patent 
Publication No. 3282/56 proposes a process comprising coating a substrate 
with a varnish comprising basic indium trifluoroacetate and heating the 
coated substrate at a temperature higher than 600.degree. C. to form a 
conductive film of the indium oxyfluoride (InOF) type. In this process, no 
good conductivity can be obtained unless the heat treatment temperature is 
elevated to about 700.degree. C. Accordingly, if a substrate having a low 
melting point, such as a sodium glass substrate, is employed, no good 
results can be obtained. Further, Japanese Patent Application Laid-Open 
Specification No. 37763/77 discloses a process comprising coating a 
substrate with a coating solution formed by dissolving an inorganic salt 
of indium in an organic solvent and heating the coated substrate. This 
process, however, is disadvantageous in that the used indium salt is 
precipitated in the formed film and the film becomes opaque. Moreover, 
Japanese Patent Application Laid-Open Specification No. 75991/76 discloses 
a process for forming conductive films free of such opaqueness, which 
comprises spray-coating a substrate with a liquid coating composition 
formed by adding hydrogen fluoride to a solution of indium chloride in 
water or an alcohol and heating the coated substrate at 420.degree. C. to 
form an indium fluoride type film. Still further, Japanese Patent 
Application Laid-Open Specification No. 1497/77 teaches a process for 
forming an indium oxide type conductive film, which comprises coating a 
substrate with an organic solvent solution of indium naphthenate and 
diethyldiethoxyindium and exposing the coated substrate to a high 
temperature. In this process using organic indium compounds, a 
disadvantage that the film is rendered opaque by precipitation of crystals 
is eliminated, but at the sintering step, decomposition of the organic 
compounds occurs, and the resulting film is rendered porous and carbon is 
readily left in the film. As a result, the conductivity is reduced the 
mechanical strength is lowered and there is a further disadvantage in that 
the film is readily scratched or damaged. 
BRIEF SUMMARY OF THE INVENTION 
Research has been conducted with a view to overcoming the foregoing defects 
and disadvantages involved in conventional techniques. As a result of 
investigations made on the combined use of an indium salt and an organic 
compound, we previously developed a process in which indium chloride and a 
.beta.-diketone are used in combination (though this process was not 
publicly reported). It was found, however, that in this process there is 
involved a risk that glass as a substrate is corroded by chlorine 
contained in indium chloride or at a heat treatment conducted at about 
500.degree. C., indium chloride is sublimated or evaporated, and it was 
also found that this process disadvantageous in that the mechanical 
strength of the resulting film is insufficient. Accordingly, it was 
confirmed that this process cannot be commercialized. 
We furthered our research and found that when indium nitrate is employed, 
even if the heat treatment is conducted at a relatively low temperature, 
there can be obtained a conductive film having a high transparency, good 
electrical conductivity and high mechanical strength. We have now 
completed the present invention based on this finding. 
More specifically, in accordance with one fundamental aspect of the present 
invention, there is provided a liquid coating composition for formation of 
a transparent conductive film, which comprises a solution of indium 
nitrate in a .beta.-diketone or a mixture of a .beta.-diketone and another 
organic solvent or a reaction product of indium nitrate with a 
.beta.-diketone, an activator and an organic solvent other than a 
.beta.-diketone. 
In accordance with another fundamental aspect of the present invention, 
there is provided a process for formation of transparent conductive films 
on substrates, which comprises coating a substrate with a liquid coating 
composition comprising a solution of indium nitrate in a .beta.-diketone 
or a mixture of a .beta.-diketone and another organic solvent or a 
reaction product of indium nitrate with a .beta.-diketone, an activator 
and an organic solvent other than a .beta.-diketone, and heat-treating the 
coated substrate at a temperature higher than about 350.degree. C. 
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Indium nitrate that is used in the present invention is insoluble in an 
ordinary organic solvent, but it is very soluble in .beta.-diketones and 
it readily reacts with .beta.-diketones at room temperature. As the 
reaction advances between indium nitrate and a .beta.-diketone, the color 
of the solution changes. Namely, the solution is first colorless and 
transparent, but as the reaction advances, it becomes yellow, yellowish 
brown and brown. When the reaction is conducted for a long time, the 
solution becomes blackish brown color. If the .beta.-diketone is used in 
an amount of about 2 to about 3 moles per mole of indium nitrate, the 
reaction is accelerated. This reaction can take place even at room 
temperature and the reaction rate can be accelerated by heating. 
Accordingly, from the viewpoint of facilitating the operation, it is 
preferred that the solution be heated at a temperature of from about 
40.degree. to about 100.degree. C. In this case, the reaction advances to 
a desirable degree in about 1 to about 8 hours. It is construed that the 
solution which has thus been reacted contains a complex compound formed 
from indium nitrate and the .beta.-diketone and the unreacted 
.beta.-diketone. The degree of advance of the reaction can be controlled 
and determined by colorimetry using an infrared absorption spectrum or 
color analyzer or by ultraviolet absorption analysis. When the reaction 
advances to a desirable degree, the temperature is lowered to room 
temperature and according to need, an organic solvent other than the 
.beta.-diketone, preferably an organic solvent having a hydroxyl group, is 
added to the solution to stop the reaction. 
In the present invention, a brownish solution of a reaction product of 
indium nitrate and a .beta.-diketone, in which the above reaction is 
slightly advanced, can also be used. In this case, a film having highly 
improved electrical conductivity and mechanical strength can be obtained. 
However, if a dark brown solution is employed, which results from the 
reaction having been conducted for a long period of time, the amount of 
the complex compound present in the solution is increased causing 
decomposition of the organic substance during the heat-treating step to 
produce the coated substrate. Accordingly, the resulting film often 
becomes porous and there is observed a disadvantage that the mechanical 
strength is reduced. 
When the .beta.-diketone used is solid at room temperature, it is necessary 
to dissolve the .beta.-diketone in another organic solvent and incorporate 
indium nitrate into the resulting solution. 
The .beta.-diketone that is used in the present invention is a compound 
represented by the following general formula (I): 
##STR1## 
wherein R.sub.1 and R.sub.2, R.sub.3 and R.sub.4, which may be the same or 
different, are a hydrogen atom, an alkyl, an alkoxy, an aryl or a 
heterocyclic group, and at least one of R.sub.1, R.sub.2, R.sub.3 and 
R.sub.4 being not a hydrogen atom may be substituted by a halogen atom. It 
is also possible that the structure of formula I be such that R.sub.1 and 
R.sub.4, which may be the same or different, are an alkyl group, an alkoxy 
group, an aryl group, or a heterocyclic group, and at least one of R.sub.1 
and R.sub.4 may be substituted by a halogen atom, and R.sub.2 and R.sub.3, 
which may be the same or different, are a hydrogen atom, an alkyl group, 
an alkoxy group, an aryl group or a heterocyclic group and, at least one 
of R.sub.2 and R.sub.3, being not a hydrogen atom, may be substituted by a 
halogen atom. 
As the .beta.-diketone represented by the above general formula (I), there 
are preferably employed acetylacetone, C-methylacetylacetone [CH.sub.3 
COCH(CH.sub.3)COCH.sub.3 ], ethyl acetoacetate, acetylmethylethylketone, 
trifluoroacetylacetone, diethyl malonate, benzoylacetone, 
dibenzoylmethane, benzoyltrifluoroacetone, furoylacetone, 
2-furoylbenzoylmethane, 2-thenoylacetone, 2-thenoyltrifluoroacetone, 
malonaldehyde, 2-thenoylbenzoylmethane, bis(thenoyl)methane, 
2-furoyltrifluoroacetone, hexafluoroacetylacetone, 
.beta.-naphthoyltrifluoroacetone and mixtures thereof. 
By the term "activator" is meant a component incorporated in a minute 
amount so as to impart a high electrical conductivity to the resulting 
film. For example, compounds of tin, titanium and zinc can be mentioned as 
effective activators. Tin compounds such as tin halides, tin nitrate and 
tin acetate are especially preferred. Either divalent tin compounds or 
tetravalent tin compounds can be effectively used. As specific examples of 
the tin halide, there can be mentioned stannous chloride, stannic 
chloride, stannous bromide, stannic bromide, stannous iodide and stannic 
iodide. 
.beta.-Diketones that are used in the present invention are ordinarily poor 
in the wetting properties with regard to the substrate. Some 
.beta.-diketones are solid at room temperature. In order to improve the 
wetting property of the coating composition of the present invention so 
that it can be uniformly coated on the substrate, or in order to mix 
indium nitrate homogeneously with the .beta.-diketone, an organic solvent 
is used for the coating composition of the present invention. Any of the 
organic solvents that are compatible with the reaction solution and can be 
coated on the substrate without repelling may be used in the present 
invention. For example, there can be used alcohols, esters, ethers, 
ketones and mixtures thereof. As suitable examples of the alcohol, there 
can be mentioned methyl alcohol, ethyl alcohol, propyl alcohol, butyl 
alcohol and ethylene glycol. As suitable examples of ethers, there can be 
mentioned ethyl ether, isopropyl ether, dioxane and tetrahydrofuran. As 
suitable examples of the ester, there can be mentioned methyl acetate, 
ethyl acetate, propyl acetate and butyl acetate, and as suitable examples 
of the ketone, there can be mentioned methylethylketone, methylbutylketone 
and cyclohexanone. 
The mixing ratios of the ingredients of the liquid coating composition of 
the present invention will now be described. It is ordinarily preferred 
that the total solid content of unreacted indium nitrate and the indium 
nitrate-.beta.-diketone complex be about from 2 to about 20% by weight 
based on the total composition. Further, it is preferred that the 
concentration of the activator be from about 1 to about 30% by weight, 
particularly from about 3 to about 15% by weight based on indium 
(calculated as IN.sub.2 O.sub.3). 
The liquid coating composition of the present invention can be prepared 
according to various embodiments in addition to the above-mentioned 
method. For example, there can be mentioned a method comprising adding a 
necessary amount of indium nitrate to an organic solvent solution 
containing the activator and a necessary amount of the .beta.-diketone, a 
method comprising adding a necessary amount of the .beta.-diketone to an 
organic solvent solution containing indium nitrate and the activator, and 
similar methods. 
The process for formation of a transparent conductive film on a substrate 
will now be described. 
As the substrate that can be used in the present invention, there can be 
mentioned, for example, glass, ceramics and mica. Any materials that have 
heretofore been used as substrates for formation of transparent conductive 
films can be used in the present invention. Since a relatively low 
temperature can be adopted for the heat treatment when the liquid coating 
composition of the present invention is used, it is possible to use a 
heat-resistant plastic material, for example, a polyimide, as the 
substrate as well as the foregoing conventional materials. 
Any known coating methods can be adopted for application of the liquid 
coating composition of the present invention. For example, there can be 
adopted a spinner coating method, a spray coating method, a brush coating 
method and a dipping draw-out method. The present invention is 
advantageous in that the size of the substrate is not particularly 
critical. When scores of substrates having a size larger than that of 20 
cm.times.20 cm are coated at one time, the dipping draw-out method is 
preferably employed. The amount of the liquid coating composition employed 
is varied depending on the concentration of the coating composition and 
the desired thickness, but, in general, the liquid coating composition is 
coated in such an amount that the thickness of the resulting film is in 
the range of from several hundred angstroms to several thousand angstroms. 
After coating the substrate with the liquid coating composition, the heat 
treatment is carried out. The heating atmosphere is not particularly 
critical and the heat treatment may be conducted in oxygen or air. 
However, in order to obtain a film having a further improved electrical 
conductivity, it is preferred that the heat treatment be carried out in an 
atmosphere of an inert gas such as nitrogen. The present invention is 
advantageous over conventional processes in that a conductive film can be 
formed at a relatively low temperature, for example, about 350.degree. C. 
However, as the heat treatment temperature becomes higher, the 
conductivity of the resulting film is improved. Accordingly, it is 
preferred to carry out the heat treatment at a temperature as high as 
possible within a range limited by the heat resistance of the substrate 
used. A longer heat treatment time is preferred, but from the viewpoint of 
facilitating the operation, it is preferred to conduct the heat treatment 
for about 10 to about 60 minutes at a temperature in the range of from 
about 350.degree. C. to about 1100.degree. C. 
When the coating is carried out by using the liquid coating composition of 
the present invention, according to the above-mentioned process of the 
present invention, a transparent, conductive and continuous film free of 
pin holes, having a thickness of from about 0.03 to about 0.3 micron, can 
be formed on various substrates. Since such film can be prepared very 
easily at low cost, the present invention is very advantageous from the 
commercial viewpoint. The present invention is especially characterized in 
that a transparent conductive film having good quality can be formed, even 
if the treatment temperature is much lower than the treatment temperature 
adopted in the conventional process, the present invention is especially 
advantageous in the field where the use of sodium glass as the substrate 
is indispensable. Accordingly, the present invention makes various 
valuable contributions to the art.

The present invention will now be described by reference to the following 
Examples that by no means limit the scope of the invention. 
EXAMPLE 1 
In 50 g of acetylacetone was dissolved 50 g of indium nitrate, and the 
reaction was conducted for 4 hours under heating at 70.degree. C. The 
solution which had been colorless and transparent was converted to a 
yellowish brown solution. The temperature was lowered to room temperature, 
and 400 g of ethyl alcohol was added to the reacted solution, and 5 g of 
stannic chloride was added thereto to obtain a coating solution. The 
coating solution was applied to a smooth sodium glass sheet having a 
thickness of 1 mm, which was usually used as a window pane, and the 
coating operation was carried out by using a spinner rotated at a rate of 
3,000 rpm. 
The so prepared samples were divided into groups A and B and were subjected 
to the following treatment. 
[Group A] 
The coated glass sheet was placed in a nitrogen atmosphere and maintained 
at a temperature of 500.degree. C. for 30 minutes to form a first coating. 
The above-mentioned coating solution was coated on the first coating again 
in the same manner as described above, and the above heat treatment was 
carried out again to form a second coating. Further, a third coating was 
formed on this second coating in the same manner as described above. 
Properties of the so obtained coatings are shown in Table 1. Tests for each 
coating were conducted prior to applying the next coating, i.e. the first 
coating was formed and tested prior to applying the second coating, etc. 
TABLE 1 
______________________________________ 
Second Second 
First Coating 
Coating Coating 
______________________________________ 
Transmittance (wave 
85% 80% 85% 
length = 400-700.OMEGA.m) 
Sheet Resistance 
1.0% 0.6% 0.2% 
(K.OMEGA./.quadrature.), i.e. 
kiloohm/square 
Thickness (A) 400 700 1000 
Etching Rate (18% 
hydrochloric acid 
15 seconds 25 seconds 
35 seconds 
solution, 45.degree. C.) 
Scratch Strength (g) 
50 50 50 
______________________________________ 
Note 
The scratch strength was determined by using a crosscut tester 
manufactured by Kamishima Seisakusho. 
For comparison, the scratch strength of an indium oxide film formed by the 
vacuum evaporation coating was measured. It was found that the scratch 
strength of this film was 45 g. 
The above described heat treatment was repeated again in the same manner 
except that the temperature was changed as shown in Table 2, and the sheet 
resistance of the first coating was measured to obtain results shown in 
Table 2, from which it will readily be understood that elevation of the 
heat treatment temperature results in reduction of the sheet resistance. 
TABLE 2 
______________________________________ 
Heat Treatment Temperature 
Sheet Resistance 
(.degree.C.) (K.OMEGA./.quadrature.) 
______________________________________ 
350 100 
400 15 
450 4 
475 1.5 
500 1.0 
600* 0.5 
______________________________________ 
Note 
*a borosilicate glass sheet was used instead of the sodium glass sheet 
[Group B] 
The coated glass sheet was heat-treated at 500.degree. C. for 30 minutes in 
an oxygen atmosphere. 
Properties of the resulting coatings were not different from those of the 
coatings obtained in the heat treatment of the group A, except the sheet 
resistance due to the use of an oxygen atmosphere. Namely, the sheet 
resistances of the coatings obtained in the group B are as follows: 
First coating: 2.0 K.OMEGA./.quadrature. 
Second coating: 1.3 K.OMEGA./.quadrature. 
Third coating: 0.5 K.OMEGA./.quadrature. 
The heat treatment temperature was changed as indicated in Table 3, and the 
sheet resistance of the first coating as described above, was measured to 
obtain results shown in Table 3. 
TABLE 3 
______________________________________ 
Heat Treatment Temperature 
Sheet Resistance 
(.degree.C.) (K.OMEGA./.quadrature.) 
______________________________________ 
350 200 
400 30 
450 8 
475 3 
500 2 
600* 1 
______________________________________ 
Note 
*a borosilicate glass sheet was used instead of the sodium glass sheet 
EXAMPLE 2 
In 50 g of ethyl acetoacetate was dissolved 50 g of indium nitrate, and the 
reaction was conducted at 25.degree. C. for 8 hours. The color of the 
solution was changed to yellowish brown. Then, 250 g of methyl alcohol, 
100 g of acetone and 50 g of ethyl acetate were added to the reacted 
solution, and 5 g of tin nitrate was further added thereto to form a 
coating solution. A borosilicate glass (Pyrex glass) sheet having a 
thickness of 1 mm was dipped in the coating solution and drawn up at a 
rate of 30 cm/min to form the coating. The coated glass sheet was 
heat-treated at 600.degree. C. for 30 minutes in a nitrogen atmosphere to 
obtain a coating film having the following properties: 
Transmittance (wave length=400-700 nm): 87% 
Sheet resistance: 0.5 K.OMEGA./.quadrature. 
Etching rate (18% hydrochloric acid, 45.degree. C.): 18 seconds 
Thickness: 395 A 
Scratch strength: 100 g 
EXAMPLE 3 
In 50 g of trifluoroacetylacetone was dissolved 50 g of indium nitrate, and 
the reaction was conducted at 90.degree. C. for 1 hour. The color of the 
solution changed to yellowish brown. The temperature was lowered to room 
temperature, and 400 g of n-propyl alcohol was added to the reacted 
solution and 5 g of tin bromide was further added thereto to form a 
coating solution. The coating solution was applied to a ceramic substrate 
having a thickness of 1.5 mm and coating was conducted by using a spinner 
rotated at 3,000 rpm. The coated substrate was then heat-treated at 
500.degree. C. for 1 hour to obtain a coating film having the following 
properties: 
______________________________________ 
First Coating 
Second Coating 
______________________________________ 
Sheet resistance 
20 K.OMEGA./.quadrature. 
2.0 K.OMEGA./.quadrature. 
Etching rate (18% hydro- 
chloric acid, 45.degree. C. 
10 seconds 
Thickness 400 A 
Scratch strength 
30 g 
______________________________________ 
When the heat treatment was carried out at 1000.degree. C. for 1 hour, the 
sheet resistance of the resulting coating film was reduced to 1 
K.OMEGA./.quadrature.. 
EXAMPLE 4 
In 50 g of acetone were dissolved 50 g of indium nitrate and 50 g of 
benzoylacetone, and the solution was treated in the same manner as 
described in Example 1 to obtain a coating solution. The coating solution 
was applied to a polyimide film (Capton film manufactured by du Pont) 
having a thickness of 0.2 mm and the coating operation was carried out by 
using a spinner rotated at 3000 rpm. The coated substrate was heat-treated 
at 400.degree. C. for 30 minutes in a nitrogen atmosphere to obtain a 
coating film having a sheet resistance of 40 K.OMEGA./.quadrature..