Electrochromic coating and method for making same

Transition metal nitrates such as nickel nitrate are dissolved in monoalcohols having from 1 to 5 carbon atoms. The coatings are applied to a substrate material and fired to convert the coating to an electrochromically active state.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for producing electrochromic 
films on glass or equivalent substrates, such as ceramics. 
Electrochromic films undergo reversible coloration induced by an applied 
electric field or current. These solid-state inorganic electrochromic 
layers can be broadly classified into those that color cathodically due to 
dual injection of electrons and ions (typically group VI-b oxides, such as 
WO.sub.3 and MoO.sub.3) and those that color anodically due to dual 
ejection of electron and cation or electron ejection/anion injection 
(typically group VIII oxides, such as IrO.sub.2, Rh.sub.2 O.sub.3, NiO and 
CoO). Such coatings are used in information display devices, solar control 
windows, and light modulators. 
Vacuum techniques, typically evaporation and sputtering, are widely used to 
deposit electrochromic thin films. Nonvacuum techniques, such as 
anodization and atmospheric chemical vapor deposition are also reported. 
Evaporation deposition and sputter coating require a high vacuum. The 
necessary equipment is expensive, making such processes capital intensive. 
However, they have been commonly used to produce electrochromic coatings. 
Three similar nonvacuum coating techniques which have not been used to any 
significant degree for electrochromic coatings are dip coating, spray 
coating and spin coating. Dip coating is commonly used to coat glass with 
SiO.sub.2. It involves lowering a glass substrate into a solution 
containing an appropriate precursor of the desired oxide..sup.1 This is 
typically a solution of a partially hydrolyzed alkoxide dispersed in a 
suitable organic solvent, e.g., tetraethylorthosilicate dissolved in an 
ethanolic solution and partially hydrolyzed. After dipping the substrate 
into the solution, the glass is withdrawn at a controlled rate. As the 
glass is withdrawn (typically at a rate of several centimeters per 
minute), the solution coats the surface. The coating is then dried and 
fired in an oven to complete hydrolysis and condensation and to densify 
the newly formed oxide coating. 
FNT Spin coating and spray coating are similar except that instead of dipping 
the glass, the precursor solution is applied to the glass, which is spun 
to spread the coating out, or is sprayed onto heated glass. 
However, the alkoxylates of some of the important electrochromic metal 
oxides do not afford satisfactory coating results if directly dissolved in 
typical solvents. For example, unsatisfactory results have been obtained 
attempting to dip coat a glass substrate in a solution formed by 
dissolving tungsten butyrate in alcohol solution. Hence, an important low 
cost approach to creating electrochromic coatings appears to be 
impractical based on present technology. 
French Pat. No. 2,527,219 discloses dipping glass substrates in a colloidal 
polymetallic acid of a transition metal, preferably in aqueous medium. 
However, such suspensions are reported to be very unstable, having a 
useful life of 24 hours or less. 
Nakatani et al U.S. Pat. No. 4,420,500 discloses the deposition of 
transparent conducting films onto glass and ceramic substrates from a 
coating composition containing indium compounds and alkyl tin nitrate 
compounds. 
Washo U.S. Pat. No. 4,347,265 discloses forming an electrochromic coating 
gel by dissolving tungsten chloride or molybdenum chloride in an alcohol 
or other organic solvent, such as xylene. A glass or indium-tin oxide 
(ITO) coated glass substrate is coated with this coating and then fired to 
yield a tungsten oxide coating. However, that process is inoperable with a 
very important electrochromic material, nickel. Mere dissolution of nickel 
chloride in alcohol does not result in effective electrochromic coating 
solutions or resultant effective fired coatings. 
SUMMARY OF THE INVENTION 
In the present invention, electrochromic coating solutions are prepared by 
dissolving a transition metal nitrate with lower carbon alcohols. A glass 
or equivalent substrate, typically having a conductive coating thereon, is 
coated with the resulting solution, preferably by dipping, to give a 
coating of the desired thickness. The coating is then dried and fired in 
an oven to yield an oxide coating having exceptional electrochromic 
properties. 
These and other objects, aspects and advantages of the invention will be 
more fully understood and appreciated by reference to the written 
specification.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Successful electrochromic coating requires that: 
1. The coating solution must have an adequate shelf life, preferably two 
weeks or longer. It must not gel or particulate, and it should be stable 
within the expected usage lifetime, preferably two weeks or longer. 
2. The solution must coat the substrate easily. It must wet the surface and 
fire to a clear oxide with good cosmetics. 
3. Once fired, the coating must be functional. It must have the desired 
hardness, adhesion, optical properties, electrical properties, etc. 
These objectives are met in the present invention where, in the preferred 
embodiment, the hydrated transition metal nitrate is refluxed with a lower 
carbon alcohol or a mixture of two or more lower carbon alcohols. The 
reaction vessel is fitted with a stirrer, a thermometer and a reflux 
condenser to minimize alcohol loss. 
Basically, the chemistry involves the formation of a solution of the 
transition metal nitrate. When a conducting substrate is dipped into this 
solution, a thin layer of transition metal nitrate is formed and heating 
the coated substrate to 100.degree. to 400.degree. C. gives an 
electrochromic coating that has a remarkable change in optical 
transmission at 550 nm. The coating formed on heating the coated substrate 
is an oxide coating of the transition metal nitrate used. 
For electrochromic coatings, the transition metal must be 
electrochromically active. Representative examples include nickel, cobalt, 
manganese, chromium, yttrium, copper and iron. Surprisingly, particularly 
exceptional results are obtained using solutions of nickel nitrate. Nickel 
nitrate is the most preferred transition metal nitrate used in the present 
invention. 
The alcohols which are employed in forming solutions comprising transition 
metal nitrates in accordance with the invention are monoalcohols having 
from one to five carbon atoms, with alcohols having from one to three 
carbon atoms being currently preferred. Thus, the most preferred alcohols 
are methanol, ethanol, propanol, isopropanol and mixtures thereof. The 
butyl and pentyl alcohols are broadly operable, but the higher alcohols 
having six or more carbon atoms, and sometimes even the butyl and pentyl 
alcohols, will sometimes result in transition metal precipitates. At 
times, solubility can be enhanced and other advantages achieved by using 
mixtures of alcohols. For example, the use of a methanol and ethanol 
mixture can be more effective in solubilizing the transition metal 
nitrates. 
Preferably, the transition metal nitrate is slowly added to the alcohol. 
However, variations of this procedure are operable as is illustrated by 
the example set forth below. Reflux for a period of time can be used to 
enhance solubility but appears to be an unnecessary step. 
The ratio of transition metal nitrate to alcohol will be varied by the 
formulator as a function of the thickness of the coating desired, the 
intended substrate withdrawal rate and the solubility of the nitrate in 
the solution. If the solution concentration and withdrawal rate result in 
too heavy a coating, the coating may crack during drying. Typical 
solutions will comprise from 5 to 20 weight/volume percent transition 
metal nitrate to alcohol for withdrawal rates of 8 to 50 centimeters per 
minute. The withdrawal rate will be a function of the thickness desired. 
With a 5 weight/volume percent metal nitrate to alcohol, the withdrawal 
rate of 8 to 50 centimeters per minute will yield coatings having a 
thickness in the range from 500 to 2000 angstroms. 
Coating thickness can generally be determined by the following formula: 
##EQU1## 
where: 
t=coating thickness 
v.sub.s =withdrawal rate 
n=viscosity 
d=coating density 
g=gravitational constant 
The relationship between final coating thickness and withdrawal rate for a 
specific dipped and fired nickel oxide film, at a specific constant 
concentration and under specific constant firing conditions (275.degree. 
C. for one hour), is illustrated in FIG. 1. The procedure outlined in 
Example 1 was followed, with only the withdrawal rate varied as indicated 
on the abscissa of the graph of FIG. 1. The coating thickness can be 
varied as indicated over a wide range merely by regulating withdrawal 
rate. Since thicker coatings are typically desired, it is particularly 
advantageous that thicker films result using faster withdrawal rates. 
Where coatings thicker than 2,000 angstroms are desired, one or more repeat 
dips are performed. The coating of a first dip is allowed to dry, and then 
the second dip is conducted. Typically, the thickest coating that can be 
obtained in a single dip is about 2,000 angstrom units. Hence, a triple 
dip would be required in order to get a coating 6,000 angstroms thick. 
Surprisingly, the introduction of a wetting agent into the dip solution 
enhances the quality of the coatings after firing. Normally, the coatings 
have poor cosmetics, especially the thicker coatings, i.e., in excess of 
2,000 angstroms. They are spotty or appear runny. By adding 0.5% to 20% by 
volume, preferably 1% to 15% by volume, of a nonionic wetting agent to the 
dip solution, preferably after the alcohol-transition metal nitrate mixing 
is completed, one can obtain thick coatings of excellent cosmetic quality. 
Surprisingly, the most preferred nonionic wetting agents by far are the 
alkyl polyoxyalkylene ethers. These are usually 100% active. The precise 
most preferred amount to be used in any given solution may vary, but can 
be empirically determined by those skilled in the art. The most preferred 
of these is Mazawet DF.TM., a product of Mazer Chemicals, Inc. 
The use of a small amount of ethyl acetate in the solution also improves 
the uniformity of the coatings obtained. Preferably, ethyl acetate 
comprises from about 5 to about 60% by volume of the solution. 
Further upon dip coating, it is preferable to modify the transition metal 
nitrate solution by mixing it with an equal amount of an alcohol 
solvent/silica coating solution containing from about 5% to about 10% by 
weight/volume partially hydrolyzed tetraethyl-o-silicate. This results in 
a coating with superior hardness and superior adherence to an underlying 
glass or ceramic substrate. The ratio of the partially hydrolyzed 
tetraethyl-o-silicate solution to the metal nitrate solution is 5:95 to 
75:25, preferably 50:50, to provide an amount of tetraethyl-ortho silicate 
in the range from 0.25 to 7.5 weight percent, based on total volume of 
solution. 
Hydrolysis and condensation of the resulting coating is then completed by 
exposure to the atmosphere and firing at temperatures of about 100.degree. 
C. to about 400.degree. C. Firing is allowed to proceed for from about 15 
to about 120 minutes. Different metal nitrate coatings will require 
different firing conditions, as will be appreciated by reference to the 
examples herein. 
Firing also has an important impact on coating density and electrochromic 
coloring efficiency. When the coatings are applied at the same withdrawal 
rate from the same solution, thinner coatings are obtained after firing at 
higher temperatures. Increased firing temperature increases film density. 
This makes the film tougher and more resistant to scratching and the like. 
The relationship between coating thickness and firing temperature is 
illustrated in FIG. 2. The procedure followed for the FIG. 2 data points 
is as outlined in Example 7, at a withdrawal rate of 20 cm/minute. All 
test pieces were fired for one hour. 
However, the desirability of a tougher film must be balanced against the 
coloring efficiency of the film. Coloring efficiency is a measure of color 
produced (or depth of coloring) in a film of unit area (e.g., cm.sup.2) 
when it stores a unit amount of charge (e.g., coulomb). As illustrated in 
FIG. 3, coloring efficiency is decreased as firing temperatures are 
increased. The firing is done in air. The FIG. 3 data points were 
generated using the same coatings as were used for the data points in FIG. 
2. 
In order to optimize these competing values, density versus coloring 
efficiency, the firing temperature is preferably between 150.degree. C. 
and 350.degree. C., and most preferably between 200.degree. C. and 
300.degree. C. 
The optical transmission of typical coatings made in accordance with the 
present invention decreases from in excess of 80% to less than 50% in a 
matter of seconds, giving an easily visible and rapid color change. FIGS. 
4 and 5 were generated using the coating solution of Example 7. Glass 
samples coated with indium-tin oxide were dipped, removed at 20 cm/minute 
and fired at 250.degree. C. for an hour. The glass sample was then made 
into an electrochromic cell with 1 N potassium hydroxide as the 
electrolyte and with a platinum counterelectrode. It was alternatively 
subjected to 1.8 volts positive and negative to effect coloring and 
bleaching. A typical result is shown in FIGS. 4 and 5 for a nickel oxide 
coating which is repeatedly cycled from colored to clear. FIG. 4 shows the 
change in optical transmission when cycled every 15 seconds. FIG. 5 shows 
the effect of cycling the same coating every 5 seconds. 
One advantage to the solutions of the present invention is that solutions 
of nitrates of differing transition metals can be combined to achieve a 
mixed oxide coating. Thus, for example, a solution of nickel nitrate and 
cobalt nitrate can be blended together. An indium-tin oxide coated glass 
substrate can be dipped into the combined solution and fired as described 
above. The resulting coating will be a nickel oxide-cobalt oxide coating. 
In this manner, mixed oxide coatings can easily be obtained which may have 
interesting properties. 
EXAMPLES 
Preparation of the coating solutions in accordance with the present 
invention is illustrated by the following examples: 
Example 1 
44 g of nickel nitrate hexahydrate were placed in a 500 ml round bottomed 
flask fitted with a thermometer, stir bar and reflux condenser. 20 ml 
methanol and 150 ml anhydrous denatured alcohol were added to the flask. 
The solution was refluxed for 41/2 hours. It was then cooled to 35.degree. 
C. and filtered. The filter was washed with 35 ml ethyl acetate to wash 
residual nickel nitrate out of the filter medium and in addition to the 
wash, 25 ml ethyl acetate was added to the solution. Another 75 ml ethyl 
acetate was added to 150 ml of the filtered solution. 
To the filtered solution there was then added 0.5 ml alkylpolyalkylene 
ether nonionic wetting agent. An electrically conductive indium-tin oxide 
(90:10 ratio) coated glass was dipped into the clear green solution and 
removed at a rate of 25 cm per minute. The cosmetic appearance of the 
coating was excellent. Fired at 260.degree. C. for one hour it showed a 
reversible change in optical transmission (at 530 nm) of 34% (94-60%) 
which increased to 45% (85-40%) on recycling several times, when made into 
an electrochromic cell with 1 N potassium hydroxide and a platinum 
counterelectrode and subjected to a potential of 2 volts. 
Example 2 
To 22 g cobalt nitrate hexahydrate in a 500 ml flask fitted with a 
thermometer, stir bar and reflux condenser, 150 ml anhydrous denatured 
alcohol was added. The solution was refluxed for 5 hours and then allowed 
to stir overnight (16 hours). The solution was then filtered at 20.degree. 
C. and the filter washed with 15 ml anhydrous denatured alcohol and with 
40 ml ethyl acetate. Then 110 ml additional ethyl acetate was added. The 
solution was then divided into 2 parts of 155 ml each. The solution was 
clear and red in color. 
0.5 ml alkylpolyalkylene ether wetting agent was added to 155 ml of the 
cobalt nitrate solution. It was necessary to dry the dipped glass in a hot 
oven immediately after dipping to get a good coating. Electrically 
conductive indium-tin oxide (90:10 ratio) coated glass was dipped into the 
clear red solution and removed at a rate of 45 cm per minute. The cosmetic 
appearance of the coating was excellent. The glass was dried immediately 
in a hot oven and fired at 200.degree. C. for one hour. The glass was then 
made into an electrochromic cell using 1 N potassium hydroxide and a 
platinum counterelectrode. The cobalt oxide coating showed a reversible 8% 
change (53% to 45%) in optical transmission at 530 nm when subjected to a 
potential of 2 volts. 
Example 3 
To 80 ml of anhydrous denatured alcohol were added 36 g of 50% manganese 
nitrate (0.1 mol). The mixture was stirred for one hour and diluted with 
an additional 40 ml of anhydrous denatured alcohol. The solution was clear 
and colorless. 3 ml of alkylpolyalkylene ether anionic wetting agent was 
added to the solution. An electrically conductive indium-tin oxide (90:10 
ratio) coated glass was dipped into the anionic wetting agent-containing 
coating solution and removed at a rate of 25 cm per minute. The coating 
was of uniform thickness and otherwise cosmetically excellent. The coated 
substrate was heated to 300.degree. C. for one hour to dry and convert the 
coating to a manganese oxide coating. The manganese oxide coating had an 
optical transmission of 40% at 550 nm which changed to 30% when subjected 
to a potential of 2 volts when made into an electrochromic cell with 1 N 
potassium hydroxide electrolyte and a platinum counter-electrode. 
Example 4 
120 ml methanol and 900 ml anhydrous denatured alcohol was added to 264 g 
nickel nitrate hexahydrate in a 2 liter round bottomed flask, equipped 
with a thermometer, stir bar and reflux condenser. The nickel nitrate went 
into solution quickly and the solution was refluxed (76.degree.-77.degree. 
C.) for 4.5 hours. The solution was filtered into a 4 liter plastic 
container. The filter was washed with 100 ml ethyl acetate and 800 ml more 
ethyl acetate was added. The solution totaled 2000 ml. To the solution was 
added 200 ml of a 7% solids solution of partially hydrolyzed 
tetraethyl-o-silicate and 50 ml of an alkyl polyalkylene ether type of 
nonionic surfactant. The silicate solution added to the hardness of the 
coating and the surfactant helped the solution wet the substrate better. 
Electrically conductive, indium-tin oxide (90:10 ratio) coated glass was 
dipped into the clear green solution and removed at a rate of 28 cm per 
minute. The coated glass was fired at 300.degree. C. for an hour. When 
made into an electrochromic cell with 1 N potassium hydroxide as the 
electrolyte and a platinum counterelectrode and subjected to a potential 
of 1.8 volts, it showed a reversible change in optical transmission (at 
550 nm) cf 23.3% (89.8 to 66.5%). The coating thickness was 685 .ANG.. The 
coloring efficiency at 550 nm was 32 sq. cm per coulomb. 
A similar coating was made at a removal rate of 45 cm per minute and fired 
at 300.degree. C. When made into an electrochromic cell as above, it 
showed a reversible change in optical transmission at 550 nm of 24.3% 
(85.7 to 61.4%). The coating thickness was 1553 .ANG. and the coloring 
efficiency 25.2 sq. cm per coulomb. 
A similar coating was made at a removal rate of 14 cm per minute and fired 
at 300.degree. C. When made into an electrochromic cell as above, it 
showed a reversible change in optical transmission of 16.3% (88.1 to 
71.8%). The coating thickness was 478 .ANG. and the coloring efficiency 
27.8 sq. cm per coulomb. 
The above solution was also sprayed as a fine mist onto electrically 
conductive glass that had been preheated to 350.degree. C. The solution 
coated the glass with a good cosmetic appearance and showed good 
electrochromic performance when tested as above. The reversible change in 
optical transmission was 5.9% (92.4 to 86.5%). The coating thickness was 
about 1200 .ANG. and the coloring efficiency at 550 nm was 18.4 sq. cm per 
coulomb. 
1.5 ml of the above solution was placed on an 8 centimeter square of 
conductive coated glass which was spun at 2000 rpm. The coating wetted the 
surface. The coated glass was then heated to 300.degree. C. for 30 
minutes. The coating showed good electrochromic properties, reversibly 
changing 16% (91.2 to 75.2%) in optical transmission when tested as above. 
The coating thickness was about 1150 .ANG. and the coloring efficiency at 
550 nm was 30.1 sq. cm per coulomb. 
Example 5 
170 ml methanol was added to 44 g nickel nitrate hexahydrate in a 500 ml 
flask equipped with a thermometer, reflux condenser and stir bar. 
Agitation was started and the mixture was heated on an electric hot plate 
to reflux temperature (65.degree. to 67.degree. C.) for 4.5 hours. The 
mixture was cooled, filtered and the filter washed with 50 ml ethyl 
acetate. 150 ml more ethyl acetate was added to the filtered solution. 20 
ml of 2,4-pentanedione and 14 ml of a nonionic surfactant, especially one 
of the alkylpolyoxyalkylene ether type, was added. 
The indium-tin oxide coated glass was dipped into this solution and removed 
at a withdrawal rate of 28 cm per minute. The glass was fired at once to 
270.degree. C. for an hour. The test piece was made into an electrochromic 
cell using 1 N potassium hydroxide as the electrolyte and a platinum 
counterelectrode. When subjected to a potential of 1.8 volts, the nickel 
oxide coated glass had a reversible change in optical transmission at 550 
nm of 26% (93% to 67%). The coating had a coloring efficiency of 23 sq. cm 
per coulomb and a thickness of 403 .ANG.. 
Example 6 
250 ml isopropyl alcohol was added to 66 g nickel nitrate hexahydrate in a 
one liter flask equipped with a thermometer, reflux condenser and magnetic 
stir bar. The agitation was turned on and the mixture heated to reflux 
(82.degree. to 83.degree. C.) for five hours. The solution was filtered to 
ensure clarity. 100 ml ethyl acetate was added. The solution volume was 
400 ml. 1.8 ml of a nonionic surfactant, one of the alkylpolyoxyalkylene 
ether type, was added to 200 ml of the above solution. 
A piece of electrically conductive indium-tin oxide coated glass was dipped 
into the solution and removed at a rate of 28 cm per minute. The glass was 
fired at 300.degree. C. in an electric furnace. This was made into an 
electrochromic cell with 1 N potassium hydroxide and a platinum 
counterelectrode. When the cell was subjected to a potential of 1.8 volts, 
it had a reversible change in optical transmission at 550 nm of 60% (95% 
to 35%). 
Example 7 
250 ml of n-propyl alcohol was added to 66 g nickel nitrate hexahydrate in 
a flask equipped as in Example 6. The agitation was started and the 
mixture heated to reflux (93.degree. C.) for five hours. It was cooled and 
filtered to ensure clarity and 100 ml of ethyl acetate added. Nine ml of a 
nonionic surfactant, one of the alkylpolyoxyalkylene ether type, was added 
to the filtered solution. 
A piece of electrically conductive indium-tin oxide coated glass was dipped 
into the solution and removed at the rate of 28 cm per minute. The glass 
was fired at 300.degree. C. in an electric furnace. When this was made 
into an electrochromic cell with 1 N potassium hydroxide and a platinum 
counterelectrode and subjected to a potential of .+-.1.8 volts, it had a 
reversible change in optical transmission at 550 nm of 56% (89% to 33%). 
The thickness of the nickel oxide coating was 2000.ANG. and the coloring 
efficiency 33 sq. cm per coulomb. 
Example 8 
150 ml of n-butyl alcohol and 20 ml of methanol was added to 44 g nickel 
nitrate hexahydrate in a 500 ml flask equipped as in Example 6. The 
agitation was started and the flask heated to reflux (95.degree. C.) for 
4.5 hours. It was cooled to 22.degree. and filtered and washed with 50 ml 
ethyl acetate. 150 ml more ethyl acetate was added to the solution. Nine 
ml of a nonionic surfactant, one of the alkylpolyoxyalkylene ether type, 
was added to the solution. 
A piece of electrically conducting indium-tin oxide coated glass was dipped 
into the solution and removed at a rate of 28 ml per minute. The glass was 
fired in an electric furnace at 275.degree. C. 
This was made into an electrochromic cell with 1 N potassium hydroxide and 
a platinum counterelectrode. When subjected to a potential of 1.8 volts, 
it showed a change in optical transmission of 28% (85% to 57%) at 550 nm. 
The nickel oxide coating thickness was 545 .ANG. and the coloring 
efficiency 31.4 sq. cm per coulomb. 
Example 9 
To 44 g nickel nitrate hexahydrate, 20 ml methanol and 150 ml anhydrous 
denatured ethanol was added in a 500 ml flask equipped with a thermometer, 
reflux condenser and magnetic bar stirrer. The mixture was agitated at 
room temperature (20.degree. to 24.degree. C.) for 4.5 hours. All went 
into solution quickly. The mixture was filtered through a fine filter 
paper and washed with 25 ml of ethyl acetate. Then 175 ml ethyl acetate 
was added to the solution. 19 ml of an anionic surfactant of the 
alkylpolyalkylene ether type was added and an additional 115 ml of ethyl 
acetate. The product had good wetting properties. 
Electrically conductive indium-tin oxide coated glass was dipped into this 
solution and removed at a rate of 28 cm per minute. The glass test piece 
was then fired at 275.degree. C. for one hour. When the test piece was 
made into an electrochromic cell with 1 N potassium hydroxide as the 
electrolyte and a platinum counterelectrode and subjected to a potential 
of .+-.1.8 volts, the coating gave a reversible change in optical 
transmission of 26% (92% to 66%) at 550 nm. The coloring efficiency was 
28.3 sq. cm per coulomb and the coating thickness 339 .ANG.. 
Example 10 
To 60.5 g chromic nitrate nonahydrate in a 500 ml round-bottomed flask 
equipped with a thermometer, stir bar and reflux condenser, 180 ml 
anhydrous denatured alcohol was added. The mixture was stirred at room 
temperature for five hours. The solution was filtered through a fine 
filter paper to ensure clarity. The filter was washed with 40 ml ethyl 
acetate and 160 ml more ethyl acetate was added to the filtered solution. 
Four ml of an alkylpolyoxyalkylene ether type of nonionic surfactant was 
then added to the solution. 
Indium-tin oxide coated conductive glass was dipped into the solution and 
removed at a withdrawal rate of 28 cm per minute. It dried quickly at room 
temperature and was fired at once to 300.degree. C. for an hour. A test 
piece thus formed was made into an electrochromic cell using 1 N potassium 
hydroxide as the electrolyte and a platinum counterelectrode. When 
subjected to a potential of 1.8 volts, the coated glass had a reversible 
change in optical transmission at 550 nm of 6% (86% to 80%). The coating 
had a thickness of about 901 .ANG.. 
Example 11 
To 29 g yttrium nitrate hexahydrate in similar equipment to that used in 
Example 10, there was added 85 ml of anhydrous denatured alcohol. The 
mixture was heated to reflux (78.degree. C.) for 4.5 hours. To ensure 
clarity the cooled solution was filtered and the filter washed with 20 ml 
ethyl acetate. 80 ml ethyl acetate was added to the solution. Three ml of 
a nonionic surfactant of the alkylpolyalkylene ether type and 50 ml more 
ethyl acetate were then added. 
Indium-tin oxide coated conductive glass was dipped into the final solution 
and removed at a withdrawal rate of 28 cm/minute. It dried quickly at room 
temperature and was fired at once to 300.degree. C. for an hour. A test 
piece thus formed was made into an electrochromic cell using 1 N sulfuric 
acid as the electrolyte and a platinum counterelectrode. When subjected to 
a potential of 1.8 volts, the coated glass had a reversible change in 
optical transmission of about 8.5% (93.5% to 85%) at 550 nm. The coating 
had a thickness of about 1642 .ANG.. 
Example 12 
To 45 g cupric nitrate hexahydrate in similar equipment to that used in 
Example 10 was added 20 ml methanol and 150 ml anhydrous denatured 
alcohol. The mixture was heated to reflux (76.degree.-77.degree. C.) for 
4.5 hours. After allowing the solution to cool to room temperature, the 
solution was filtered through fine paper to ensure clarity. The filter was 
washed with 50 ml ethyl acetate and 150 ml more ethyl acetate was added to 
the solution. Four ml of a nonionic surfactant of the alkylpolyalkylene 
ether type was then added. 
Indium-tin oxide coated conductive glass was dipped into the solution and 
removed at a withdrawal rate of 28 cm/minute. It dried quickly at room 
temperature and was fired at once to 300.degree. C. for an hour. A test 
piece thus formed was made into an electrochromic cell using 1 N potassium 
hydroxide as the electrolyte and a platinum counterelectrode. When 
subjected to a potential of 1.8 volts, the coated glass had a reversible 
change in optical transmission of 7% (85% to 78%) at 550 nm. The coating 
had a thickness of about 400 .ANG.. 
Example 13 
To 61 g of ferric nitrate nonahydrate in similar equipment to that used in 
Example 10, there was added 20 ml methanol and 150 ml anhydrous denatured 
ethanol. The mixture was stirred at room temperature for 40 hours. The 
cooled solution was filtered to ensure clarity and the filter washed with 
50 ml ethyl acetate. Then, 150 ml ethyl acetate was added. One ml of a 
nonionic surfactant of the alkylpolyalkylene ether type was then added. 
Indium-tin oxide coated conductive glass was dipped into the solution and 
removed at a withdrawal rate of 28 cm/minute. It dried quickly at room 
temperature and was fired at once to 300.degree. C. for an hour. A test 
piece thus formed was made into an electrochromic cell using 1 N potassium 
hydroxide as the electrolyte and a platinum counterelectrode. When 
subjected to a potential of 1.8 volts, the coated glass had a reversible 
change in optical transmission of 3.5% (53% to 49.5%) at 550 nm. The 
coating had a thickness of about 935 .ANG.. In a similar cell using a 1 N 
sulfuric acid electrolyte, the change was 2% (50% to 48%) in optical 
transmission at 550 nm. 
Example 14 
It is easy to make mixtures of metal oxide coatings either by mixing the 
nitrates on any basis that may be desired and preparing the solution of 
the mixture or by making coating solutions of each metal nitrate and then 
mixing them on an exact basis. It is also possible to combine both methods 
as illustrated in Example 15. 
To 32 g of nickel nitrate (molecular weight 291) and 32 g of cobalt nitrate 
(molecular weight 291) contained in a 500 ml round bottomed flask equipped 
with a thermometer, magnetic stir bar and reflux condenser, 30 ml methanol 
and 225 ml anhydrous denatured alcohol was added. All dissolved readily at 
room temperature (21.degree. C.) with agitation. After stirring the 
solution for three and a half hours, the solution was filtered and diluted 
with 225 ml ethyl acetate. To improve the wetting of the solution for 
electrically conducting indium-tin oxide coated glass, 5 ml of Mazawet DF 
(a nonionic surfactant of the alkyl polyoxyalkylene ether type) was added 
to the 500 ml of solution. This solution is a 50:50 mol percent mixture of 
nickel and cobalt nitrates. 
After dipping the indium-tin oxide coated glass in the solution and 
removing it at a rate of 14 cm per minute, it was dried by holding it over 
a hot plate which gave a clear coating over most of the glass. The coated 
glass was made into an electrochromic cell with 1 N potassium hydroxide as 
the electrolyte and a platinum counter-electrode, the cell showed a 
reversible change in optical transmittance of 8.2% (56.5 to 48.3%) when 
subjected to a voltage of .+-.0.5 volts. The coloring efficiency at 550 nm 
was 18.3 sq. cm per coulomb. 
Example 15 
As an example of mixing solutions, a 90% nickel nitrate to 10% cobalt 
nitrate solution was made by mixing 0 ml of the 50:50% solution of nickel 
nitrate-cobalt nitrate as prepared in Example 14 with 80 ml of nickel 
nitrate solution containing about 3 ml of Mazawet DF (as in Example 9). 
This solution coated indium-tin oxide (90:10) coated conductive glass 
well. 
A sample of such coated glass was dipped into the 90% nickel nitrate: 10% 
cobalt nitrate solution and removed at a rate of 28 cm per minute and then 
fired at 300.degree. C. for an hour. When a sample was made into an 
electrochromic cell with 1 N potassium hydroxide solution as the 
electrolyte and a platinum counterelectrode, it showed good electrochromic 
properties with a 12.9% reversible change in optical trans 
mission (84.5 to 71.6%) when subjected to .+-.0.5 volts. The film thickness 
was 1096 .ANG. and the coloring efficiency at 550 nm was 30.4 sq. cm per 
coulomb. 
Example 16 
In a similar way, a 90% cobalt nitrate: 10% nickel nitrate solution was 
prepared by mixing 10 ml of the 50:50% mixture of nickel nitrate: cobalt 
nitrate of Example 14 with 80 ml of the cobalt nitrate solution of Example 
2. To the mixture, 0.5 ml of Mazawet DF was added to improve wetting of 
the solution for glass and indium-tin oxide (90:10%) coated glass. 
A sample of indium-tin oxide coated conductive glass was dipped into the 
90% cobalt nitrate: 10% nickel nitrate solution and removed at a rate of 
28 cm per minute. It was fired at 300.degree. C. for an hour. 
A sample of that glass was made into an electrochromic cell with 1 N 
potassium hydroxide as the electrolyte and with a platinum 
counterelectrode. It showed a reversible electrochromic change in optical 
transmission of 3.8% (58.5 to 54.7%) when subjected to .+-.0.5 volts. The 
film thickness was 1475 .ANG. and the coloring efficiency at 550 nm was 
13.7 sq. cm per coulomb. 
The process of the present invention is very amenable to coating 
electrochromic oxides. This compares favorably to vacuum techniques where 
refractory oxide deposition typically requires electron beam evaporation, 
reactive sputter deposition (dc or rf) or rf sputter deposition from a 
pressed oxide target. 
The present method is a nonvacuum technique. Equipment is thus relatively 
inexpensive. The process is easy to scale up and it is amenable to coating 
very large substrates. The technique can be used to commercially coat 
glass panes of several square meters in area. 
It is economical to coat thick oxide films. Using the preferred dip coating 
technique, the faster the withdrawal rate, the thicker the film. This 
compares very favorably to other techniques where, typically, a 5000 
angstrom WO.sub.3 coating takes roughly 10 times longer than a 500 
angstrom coating. 
While dip coating is preferred, spray or spin coating can, in the broader 
aspects of the invention, be used instead of dip coating. 
Finally, substrate coated in accordance with the present invention can be 
coated on both sides simultaneously if desired. 
Of course, it is understood that the above is merely a preferred embodiment 
of the invention and that various changes and alterations can be made 
without departing from the spirit and broader aspects thereof as set forth 
in the appended claims.