GB2359304A - Preparation of a solid precursor material useful for tungsten oxide based electrochromic coatings - Google Patents

Abstract

A process for the preparation of a highly stable solid precursor material useful for tungsten oxide based electrochromic coatings comprises: <SL> <LI>a) preparing an aqueous solution of sodium tungstate in deionized water; <LI>b) passing the aqueous solution prepared in step a) through a vertical column containing acidic ion exchange resin to give an elute; <LI>c) collecting the elute from step b) directly into a peroxide as colloidal polytungstate sol with a pH not exceeding 2; <LI>d) stirring the mixture obtained in c) for a period ranging between 30 to 120 minutes to form a peroxypolytungstate solution; <LI>e) mixing an organic acid to said peroxypolytungstate solution as obtained in step d) under reflux conditions at a temperature of 55{C for a time period in the range of 1-2 hours to form a peroxypolytungstate acid derivative; <LI>f) converting said peroxypolytungstate acid derivative to a highly stable solid precursor material by fast vacuum drying; </SL>

Description

<B>PROCESS FOR THE PREPARATION OF A SOLID PRECURSOR MATERIAL</B> <B>USEFUL FOR TUNGSTEN OXIDE BASED</B> ELECTROCHROMIC <B>COATINGS</B> <B>Field of the invention</B> The present invention relates to a process for the preparation of a solid precursor material useful for tungsten oxide based electrochromic coatings. <B>Background of the invention</B> Electrochromic (EC) coatings undergo reversible coloration induced by an electric field or current. Possible transmission l reflection modulation when EC coatings are incorporated into electrochromic devices (ECDs) allow their use in solar control windows, anti-dazzling mirrors for automobiles and information display devices. ECDs are multilayer devices like batteries, supercapacitors, etc. of which EC coatings form one layer. Depending on the applications, the EC coatings may differ in their area, thickness etc.

Both inorganic and organic materials exhibit electrochromism. Inorganic EC materials of interest are all oxides and can be broadly classified into those that color cathodically due to double injection of electrons and cations (group VI B oxides such as W03, M003) and those group Vill oxides that color anodically (such as Ir02, Rb203, NiO and<B>COO).</B>

EC oxide coatings can be prepared by numerous methods. Thermal evaporation and sputtering are the most commonly used vacuum techniques to deposit EC materials. Non vacuum techniques involve anodization or solution deposition. The prime objective in preparation of all oxides by solution deposition is to obtain initially a deposition solution from a suitable precursor material. Using the deposition solution coatings can be deposited by dip coating, spray coating or spin coating. Film deposition is followed by drying l annealing to convert into EC oxide coatings. Conversion to oxide occurs preferably by hydrolysis but alternately by chemical reaction or thermal or oxidative decomposition. These techniques are advantageous being less capital extensive. and thus less expensive.

There are a number of different types of precursor materials that can be used. The precursor materials can be solids or liquids. If in the solid form they should be soluble in organic solvents to make deposition solution. The precursor materials in the liquid form may be used as deposition solutions with or without any further modifications. Reproducibility of the oxide coatings prepared by solution deposition can be ensured if the precursor materials and the deposition solutions made thereof are stable, i.e. they do not change their properties over a long period. Deposition solutions if unstable not only would result in coatings without reproducibility but also if turned unsuitable e.g. highly viscous, for depositing coatings would add to the cost of the process of making EC coating. Thus stability of the precursor material and that of the deposition solution play a very important role towards the reproducibility of the coatings and the cost of the process of making them.

Many other factors such as the cost of the starting materials, the number of process steps, temperature, pressure and duration of individual steps contribute towards the cost effectiveness of the process of making EC oxide coatings. Inexpensive and easily available starting materials, less number of steps involving normal temperature and pressure over short duration and more importantly highly stable precursor materials and the deposition solutions are thus most preferred. The carrier solvent used for preparing the deposition solution is another very important factor. The deposition solution needs to have good wetting properties with the substrates. The adhesion of the coatings to the substrates and as a result the quality of the coatings is controlled by the carrier solvent. Thus good wetting, which is important, makes ethanol the most preferred solvent. The coatings prepared with water or a mixture of water and alcohol as carrier solvents in this regard do not compare favorably with those made with alcohol based deposition solutions. As a result solubility of the solid precursor materials in alcohol or liquid precursor materials based purely on alcohol are the most preferred.

Alkoxides like Si(OC2H5)4 and Ti(OC2H5)4 that are easily hydrolizable into corresponding hydroxide or oxide are the generally used precursor materials. Partially hydrolyzed solution form the deposition solution and using any one of the above mentioned three techniques coatings can be prepared. Complete hydrolysis and condensation during post deposition drying I annealing treatments forms an oxide coating.

Metal salts like SnC12.2H20, Ce(NH4)3(N03)s provide a viable alternative provided they are easily converted into the oxide by thermal or oxidative decomposition and are preferably soluble in organic solvents. Sol-gel preparations involving metal salts are usually more complex than those with only alkoxides.

For group VI B oxide EC coatings like tungsten oxide (W03), tungsten alkoxide based solutions have been used as precursor materials. But their cost has limited their usage. Colloidal sots like peroxotungstic acid and its derivatives are the other reported precursor materials. Colloidal sots are easy to prepare and follow ion exchange route. Aqueous solution of hydrated salts of tungsten like hydrated tungstates of sodium or ammonium, when acidified by passing through an ion exchange column at room temperature gives tungstic acid - the colloidal sot. The tungstic acid derived using Na2W04 .2H20, has been reported by E. Richardson in J. Inorganic. Nucl. Chemistry 12,79 (1959) to be unstable, decomposing to form white tungstic acid. This acid was shown not to be metatungstic acid H6[H2W,204o] but probably hexatungstic acid H3[H3W602,]. For the tungstic acid obtained this way, gelation occurs at room temperature in a few hours and after 3 or 4 days it precipitates to yellow hydrated oxide of tungsten as reported by J. Lemerle and J. Lefebvre in <I>Can. J.</I> Chem; 55, 3758 (1977). In aqueous solutions, the acid was shown to be formed by tetramers of W60,92- units, which give a gel after association. X-ray analysis of tungstic acid in the solid state demonstrated the absence of crystallinity and in hydroorganic medium like Dimethylsulfoxide (DMSO) the acid was shown to stabilize, existing in equilibrium with Y tungstic and hexatungstic anions.

Electrochromism in coatings obtained by depositing colloidal tungsten oxide was first reported by A. Chemseddine, R. Morineau, J. Livage [Solid State tonics 9 & 10, 357 (1983)]. The colloidal solution obtained by ion exchange deposited onto a conductive transparent electrode placed into an electrochemical cell containing a liquid electrolyte such as LiC104 in propylene carbonate showed colouration - bleaching. The coloured film was shown to have reduced W(V) ions. These authors have mentioned enhanced stability of the tungstic acid by addition of organic solvents but there is no mention about any specific solvent used. Properties of both the xerogel - tungstic acid dried at room temperature and the coatings heat treated at different temperatures have been investigated.

Following this, there have been a few publications till 1993 by various groups [G. Xu and L. Chen, Solid State Tonics 28-30, 1726 (1988); J. Judeinstein, J. Livage, A. Zarudiansky, R. Rose; Solid State tonics 28-30 1722 (1988); M. Habib and David Glueck, Solar Energy Materials 18, 127 (1989); T. Nanba, Y. Nishiyama, 1. Yasui, J. Mater. Res. 6, 1324 (1991); S. Badilescu, N. Minh-Ha, G. Bader, P. V. Ashrit, F. E. Girouard, Vo-Van Truong, J. Mol. Structure 297, 393 (1993)] depositing the coatings using colloidal tungstic sot and characterizing. However, the efforts of none of these groups were directed towards deposition of reproducible, large area uniform coatings on a large scale. Rather these were the preliminary attempts that showed feasibility of using the colloidal tungstic sot for.electrochromic applications if coatings are prepared using the sot prior to formation of viscous gel.

Preparation of large area EC coatings on large scale and with good reproducibility demands good stability of the deposition solution without affecting their EC response. While low cost, easily available starting materials and their room temperature acidification to colloidal sot are the advantages of this route, short gelation time is the greatest drawback. Reproducibility of the deposited coatings as a result is difficult to ensure. Further, coatings can not be prepared once sot gets converted into gel.

US Patent Nos. 5, 772, 978 and 5, 911, 965 disclose a process for producing a tungsten oxide .precursor solution that includes converting a peroxypolytungstate solution to a stable oxide polytungstate solution. The precursor solution is subsequently converted by heat treatment to tungsten oxide having electrochromic properties. The process comprises steps of 1) vacuum drying a peroxypolytungstate solution to form a powder 2) dissolving or dispersing solid powder in a solvent comprising an alcohol to form an alcoholic solution 3) heating said alcoholic solution to a stable oxide polytungstate solution. This solution has been claimed to have stability of the order of 50 days at room temperature to several months under refrigerated storage conditions (at about 5 to 10 C).

Films of thickness 3000 A, deposited using the solution by dip coating method on ITO substrates exhibited an average integrated transmission of about <B>79-81%.</B> When tested with a three electrode cell (an AglAgCl2 reference electrode and a Pt auxiliary electrode) in a solution of 0.1 N LiCF3S03 or LiN(CF3S02)2 in acetonitrile with charging and discharging voltage of -1 and +1 volts respectively, under the inserted charges of 20-30 mClcm2 exhibited an average integrated transmission of about 14-20 %.

Dried peroxypolytungstate solution obtained by step 1) above invariably does not show complete solubility in alcohol alone but needs some amount of water (more than 50 % or substantially free of water as is claimed). Presence of water, behaving simultaneously as a solvent and a complexing agent, is expected to affect the wetting property of the depositing solution and modify the structural characteristics of sots.

Inexpensive and readily available starting materials processed for less than five hours lead to a stable polytungstate solution in this method. The process thus gives a precursor in the form of a solution (and not solid powder) containing peroxypolytungstate species partially converted to oxide in a medium substantially free of water. This does not warranty complete absence of water. The presence of water however small in amount in deposition solution affects its wetting properties to the substrates and as a result the quality of the coatings. Acidified ammonium metatungstate forms the preferred initial polytungstate solution to give a stable oxide polytungstate. However, if the initial polytungstate solution is a product of acidification of an alkali tungstate such as Na2W0a then the amount of peroxide needs adjustment to ensure formation of a peroxy product that will dissolve. Possible insolubility of the peroxy product as a result of inappropriate amount of peroxide does not allow further process steps to follow.

US Patent No. 5, 252, 354 discloses a transition metal peroxy acid product obtained by reacting a transition metal with a mixture of hydrogen peroxide and an organic acid. By reacting with lower carbon alcohols and subsequent drying under low pressure the resulting transition metal peroxy acid product is converted into a peroxyester transition metal derivative. This derivative in solution in lower carbon alcohols provides a working solution to make the coatings. Drying and externally firing these coatings in an oven complete the necessary reactions to yield electrochromic oxide coating having exceptional electrochromic properties. When a coating of thickness 3000 A obtained this way was coloured in a cell containing sulfuric acid (0.1 Normal) and a platinum counter electrode by applying negative 1.8 volts with reference to the counter electrode, the coating coloured from 85% transmission to 10% transmission in 100 seconds at 550 nanometers.

The starting material in this case is the tungsten metal, not as inexpensive as the salts used as starting materials in the ion exchange route. The reaction of tungsten metal powder with hydrogen peroxide being exothermic needs to be carried out at 0 C. Further completion of this reaction needs duration in the range between 16 to 26 hours. .

Objects of the invention The main object of the present invention is to provide a process for the preparation of a highly stable solid precursor material useful for tungsten oxide based electrochromic coatings, which obviates the drawbacks as mentioned above.

Another object of the present invention is to have a process which will result in a precursor material in a solid form with a long term stability of at least one year and even more with the additives under refrigerated storage conditions (at about 5 to 10 C).

Another object of the present invention is to prepare a solid precursor material with good solubility in alcohol.

Yet another object of the present invention is to prepare a precursor material by a process preferably not involving steps requiring very low or very high temperature maintained for more than 4 to 5 hours.

Another object of the present invention is to use low cost and easily available starting materials.

Summary of the invention Accordingly the present invention provides a process for the preparation of a highly stable solid precursor material useful for the tungsten oxide based electrochromic coatings, which comprises: a) preparing an aqueous solution of sodium tungstate in deionized water; b) passing the aqueous solution prepared in step a) through a vertical column containing acidic ion exchange resin to give an elute; c) collecting the elute from step b) directly into a peroxide as colloidal polytungstate sol with a pH not exceeding 2; d) stirring the mixture obtained in c) for a period ranging between 30 to 120 minutes to form a peroxypolytungstate solution; e) mixing an organic acid to said peroxypolytungstate solution as obtained in step d) under reflux conditions at a temperature of 55 C for a time period in the range of 1-2 hours to form a peroxypolytungstate acid derivative; f) converting said peroxypolytungstate acid derivative to a highly stable solid precursor material by fast vacuum drying.

In an embodiment of the present invention the strength of the sodium tungstate solution is in the. range of 0.3 M to 0.7M.

In another embodiment of the present invention the peroxide used is hydrogen peroxide in the range of 12 to 15 vol. %.

In still another embodiment of the present invention organic acid used may be selected from glacial acetic acid and propionic acid.

In yet another embodiment of the present invention peroxypolytungstate acid derivative devoid of free peroxide is fast vacuum dried at pressure less than 1 Torr to recover flaky solid precursor material.

In an embodiment of the present invention, an initial colloidal polytungstate sol is treated with a peroxide to form a peroxypolytungstate solution followed by mixing it with an organic acid under reflux conditions to convert into peroxypolytungstate acid derivative which on further fast vacuum drying forms a highly stable solid precursor material. Detailed description of the invention The initial colloidal polytungstate solution is preferably. in the form of an aqueous solution and may be prepared according to a variety of methods including acidification of soluble tungstate salts e.g. ammonium meta tungstate or hydrolysis of tungsten alkoxides.

A particularly preferred initial colloidal polytungstate sol is an aqueous solution of "acidified sodium tungstate derivative" (ASTD). ASTD is a clear, yellowish colored sol. It may be prepared at ambient temperature by eluting an aqueous solution of sodium tungstate dehydrate through an acidified cation exchange column or by mixing sodium tungstate solution with an acidified cation exchange resin followed by removing the resin by filtering, decanting or a comparable method. Acidification by direct addition of an acid such as HCi followed by dialysis may also be used. The strength of starting sodium tungstate solution may be between 0.3 to 0.7 M. The cation exchange column offers the advantage of removing unwanted cations without need for further purification. Examples of suitable acidified cation exchange media are commercially available and include DOWEX acidic ion exchange resin from Aldrich Chemical Co. The acidification reaction is concluded such that the product solution has a pH no greater than 2.

Stability of the colloidal polytungstate so] is over a time scale controlled by the initial concentration of sodium tungstate solution. The next step is to treat the colloidal polytungstate sol with a peroxide to form a peroxypolytungstate solution. The reaction kinetics for the formation of said peroxypolytungstate solution is enhanced by stirring the mixture for an appropriate duration. The preferred peroxide is hydrogen peroxide. The amount of peroxide added is adequate enough not to allow gelation of said colloidal polytungstate sol and is with decrease in color intensity I decoloration of original said colloidal polytungstate sol. The amount of hydrogen peroxide added to the said colloidal polytungstate sol to yield a clear, pale-yellow solution or almost colorless peroxypolytungstate solution typically ranges from 8-20% of the volume of said colloidal polytungstate sol and more preferably about 12 -15 % of the said colloidal polytungstate sol.

The said peroxypolytungstate solution is next mixed with an organic acid to form a solution of peroxypolytungstate acid derivative. Acids with low carbon content such as glacial acetic acid, propionic acid etc may be utilized - preferably, glacial acetic acid is employed for this step. The amount of glacial acetic acid incorporated into the said peroxypolytungstate solution typically ranges from 8 20% of the volume of said colloidal polytungstate sol and more preferably about 12 -15% of the said colloidal polytungstate sol.

The reaction mixture is then refluxed for an adequate duration of time as prolonged refluxing time periods result in the formation of highly viscous and deep yellow colored solutions or coagulation of colloidal particles leading to gelation or precipitation of insoluble residues thus yielding decomposed products insoluble in alcohol l water. In contrast, too short a refluxing time results in incomplete conversion of peroxypolytungstate solution into its corresponding peroxypolytungstate acid derivative which would render the final product insoluble in alcohol.

Typical reflux periods are of the order of 1 - 2 hours at 55 C for about 30 50 cc of initial colloidal polytungstate sol. The resulting peroxypolytungstate acid derivative is then fast dried under vacuum at a pressure less than 1 Torr at a temperature not exceeding 55 C to yield the solid precursor material (a tungsten peroxy acid product) for tungsten oxide based electrochromic coatings. This precursor material has a stability of about one year if stored under refrigerated conditions and dissolves in polar solvents like alcohol and water. The tungsten content as determined by gravimetric method ranges between 0.75 - 0.85 gm per gm of the precursor material.

The stability of the solid precursor material can further be enhanced by esterification. This involves dissolving the solid precursor material in a lower carbon alcohol such as ethanol as for making the deposition solution. Fast vacuum drying at a pressure less than 1 Torr at a temperature not exceeding 55 C results in isolating the ester derivative. This tungsten peroxyester derivative readily dissolves in alcohol at room temperature and can be used to make the deposition solution at room temperature. The deposition solutions are stable for a period of about two weeks at room temperature and for several months when stored at temperatures below 10 C. Although esterification incorporates an additional step it yields a precursor material with better solubility in alcohol and enhanced stability.

For depositing the electrochromic coatings using the precursor material, the deposition solution is made by dissolving the solid precursor material in alcohol to form a clear solution on heating the mixture near the boiling point of alcohol for a short duration and filtration by Whatman 42 filter paper.

Using the deposition solution, coatings are then deposited on electrically conducting substrates using any one of the spraying dipping or spinning techniques. Subsequent heat treatment removes any solvent if remaining and converts the film into tungsten oxide based EC coatings. Heating is conducted in air in two stages at temperature of about 100 C in the first step and at temperature ranging between 150 - 350 C in the second step. The temperature of heat treatment in both the steps is reached gradually from room temperature at a rate of More preferred heating in the second step is at temperature between 200 - 250 C. The duration of heating in each step ranging between 45 - 70 minutes, preferably for about 50 to 60 minutes.

The . following examples illustrate the preparation of preferred solid precursor material and the electrochromic coating made thereof and should not be construed to limit the scope of the present invention.

Example 1 Preparation of precursor material 0.5 M solution (pH 9) of sodium tungstate (Na2W04. 21120) [Merck India] was prepared in distilled deionized water. A cylindrical column of 1.7 cm inner diameter and 25 cm length was filled with DOWEX 50W-X8(H) standard grade acidic ion exchange resin [BDH laboratory England]. About 65 cc of the above aqueous solution of sodium tungstate was then passed through the column. The eluate was collected at the rate of about 2.0 cc l min. When the pH of the eluate acquired a value less than 2, the collection started and continued till eluate with this value of pH was obtained. About 50 cc of the eluate could thus be collected. This clear, yellow eluate, was the colloidal polytungstate sol that was collected directly into about 7.5 cc of 30 % hydrogen .peroxide solution [Merck, India]. The mixture of the colloidal polytungstate sol and hydrogen peroxide was stirred for 1 hr at room temperature. To this .solution then glacial acetic acid [99.8%, E- Merck India] was added in amount equal to that of hydrogen peroxide followed by refluxing for a period of 2 hrs at 55 C to form peroxypolytungstate acid derivative-acetylated peroxypolytungstate solution. The combined stirring and refluxing yielded clear, light yellow solution. Next this acetylated peroxypolytungstate solution was fast vacuum dried at a pressure less than 1 Torr at 550C to give a flaky solid precursor material which was white - light yellow in color and was stable over a period of about one year when stored under refrigerated conditions (5 C).

Preparation of Electrochromic coating using the precursor material Electrochromic coatings were prepared by using the solid precursor material by heating a mixture of 9 gm of solid precursor material and 30 cc of dry alcohol below the boiling point of alcohol for a period of 3 minutes. After this the solution is filtered through Whatman 42filter paper, after cooling to room temperature and subsequently spin coating the solution on electrically conducting substrates. After the spin coating the substrates were gradually heated in air from room temperature to 100 C at a rate 5 C l min and maintaining at 100 C for a period of 45 minutes followed by cooling to room temperature. Next re-heating is done of the coating in air from room temperature to 250 C at a rate of 50C/min and maintaining at 250 C for a period of 45 minutes followed by cooling to room temperature.

The coatings prepared as above with thickness 3500 A were tested for their electrochromic properties in a test cell containing a test electrolyte of 1 M lithium perchlorate in butyrolactone and a platinum counter electrode. The coatings were examined for the electrochromic properties by probing the center of the coatings by a laser beam (X, = 632.8 nm) in conjunction with a photodiode. The coatings under the injected charge of about 30mCIcm2 change transmission from original 77% to 7%.

Example-2 Preparation of precursor material 0.3 M solution (pH - 9) of sodium tungstate (Na2W04.2H20) [Merck India] was prepared in distilled deionized water. A cylindrical column of 1.7 cm inner diameter and 25 cm length was filled with DOWEX SOW-X8(H) standard grade acidic ion exchange resin [BDH laboratory England]. About 65 cc of the above aqueous solution of sodium tungstate was then passed through the column. The eluate was collected at the rate of about 1.5 cc/min. When the pH of the eluate acquired a value less than 2, the collection started and continued till eluate with this value of pH was obtained. About 50 cc of the eluate could thus be collected. This clear, yellow eluate, was the colloidal polytungstate sol that was collected directly into about 7.5 cc of 30 % hydrogen peroxide solution [Merck. India]. The mixture of the colloidal polytungstate sol and hydrogen peroxide was stirred for 1 hr at room temperature. To this solution then glacial acetic acid [99.8%, E-Merck India] was added in amount equal to that of hydrogen peroxide followed by refluxing for a period of 2 hrs at 55 C to form peroxypolytungstate acid derivative acetylated peroxypolytungstate solution. The combined stirring and refluxing yielded clear, light yellow solution. Next this acetylated peroxypolytungstate solution was fast vacuum dried at a pressure less than 1 Torr at 55 C to give a flaky solid precursor material which was white - light yellow in color and was stable over a period of about one year when stored under refrigerated conditions (10 C). Preparation of Electrochromic coating using the precursor material Next 5 gms of this solid precursor material was dissolved in 9 cc of alcohol- dry ethanol and the mixture was heated at 55 C for 3 minutes. The resulting clear yellow solution after filtering through Whatman 42 was used to prepare coatings by spin coating technique.

To prepare the electrochromic coatings from the precursor material the solution as prepared above was spin coated onto electrically conducting substrates at a spin speed of 2500 rpm for 30 sec. The coatings prepared this way with thickness 3500 A were heated in air at 100 C for 1 hr, cooled to room temperature and further heated in air at 250 C for 1 hr. The coatings were tested for their electrochromic properties in a test cell containing a test electrolyte of 1 M lithium perchlorate in butyrolactone and a platinum counter electrode and probing the center of the coatings by a laser beam (?, = 632.8nm) in conjunction with a photodiode. The coatings under the injected charge of about 30 mClcm2 change transmission from original 77% to 32%.

Example-3 Preparation of precursor material 0.3 M solution (pH-9) of sodium tungstate (Na2W04.2H20) [Merck India] was prepared in distilled deionized water. A cylindrical column of 1.7 cm inner diameter and 25 cm length was filled with DOWEX 50W-X8(H) standard grade acidic ion exchange resin [BDH laboratory England]. About 65 cc of the above aqueous solution of sodium tungstate was then passed through the column. The eluate was collected at the rate of about 2.0 cc/min. When the pH of the eluate acquired a value less than 2, the collection started and continued till eluate with this value of pH was obtained. About 50 cc of the eluate could thus be collected. This clear, yellow eluate, was the colloidal polytungstate sol that was collected directly into about 7.5 cc of 30 % hydrogen peroxide solution [Merck, India]. The mixture of the colloidal polytungstate sol and hydrogen peroxide was stirred for 1 hr at room temperature. To this solution then glacial acetic acid [99.8%, E-Merck India] was added in amount equal to that of hydrogen peroxide followed by refluxing for a period of 2 hrs at 55 C to form peroxypolytungstate acid derivative- acetylated peroxypolytungstate solution. The combined stirring and refluxing yielded clear, light yellow solution. Next this acetylated peroxypolytungstate solution was fast vacuum dried at a pressure less than 1 Torr at 55 C to give a flaky solid precursor material which was white - light yellow in color and was stable over a period of about one year when stored under refrigerated conditions (5 C).

Preparation of Electrochromic coating using the precursor material Next 5 gms of this solid precursor material was dissolved in 9 cc of propanol and the mixture was heated at about 55 C for 3 minutes. The resulting clear yellow solution after filtering through Whatman 42 was used to deposit coatings by spin coating technique.

To prepare the electrochromic coatings from the precursor material the solution as prepared above was spin coated onto electrically conducting substrates at a spin speed of 2500 rpm for 30 sec. The coatings prepared this way with thickness 3500 A were heated in air at 100 C for 1 hr, cooled to room temperature and further heated in air at 250 C for 1 hr. The coatings were examined for their electrochromic properties in a test cell containing a test electrolyte of 1 M lithium perchlorate in butyrolactone and a platinum counter electrode and by probing the center of the coatings by a laser beam (X=632.8 nm) i ,onjunction with a photodiode. The coatings under the injected charge of about 30 mClcm2 change transmission from original 63% to 12%.

Example-4 Preparation of precursor material 0.3 M solution (pH-9) of sodium tungstate (Na2W04.2H20) [Merck India] was prepared in distilled deionized water. A cylindrical column of 1.7 cm inner diameter and 25 cm length was filled with DOWEX 50W-X8(H) standard grade acidic ion exchange resin [BDH laboratory England]. About 65 cc of the above aqueous solution of sodium tungstate was then passed through the column. The eluate was collected at the rate of about 2.0 cc/min. When the pH of the eluate acquired a value less than 2, the collection started and continued till eluate with this value of pH was obtained. About 50 cc of the eluate could thus be collected. This clear, yellow eluate, was the colloidal polytungstate sol that was collected directly into 7.5 cc of 30 %. hydrogen peroxide solution [Merck, India]. The mixture of the colloidal polytungstate sol and hydrogen peroxide was stirred for 1 hr at room temperature. To this solution then propionic acid [99.8%, E-Merck India] was added in amount equal to that of hydrogen peroxide followed by refluxing for a period of 2 hrs at 55 C to form peroxypolytungstate acid derivative. The combined stirring and refluxing yielded clear, light yellow solution. Next this peroxypolytungstate acid derivative was fast vacuum. dried at a pressure less than 1 Torr to give a flaky solid precursor material which was white - light yellow in color and was stable over a period of about one year when stored under refrigerated conditions (10 C). Preparation of Electrochromic coating using the precursor material Next 5 gms of this solid precursor material was dissolved in 9 cc of alcohol- dry ethanol and the mixture was heated at about 55 C for 3 minutes. The resulting clear yellow solution after filtering through Whatman 42 was used to deposit coatings by spin coating technique.

To prepare the electrochromic coatings from the precursor material the solution as prepared above was spin coated onto electrically conducting substrates at a spin speed of 2500 rpm for 30 sec. The coatings prepared this way with thickness 3500 A were heated in air at 100 C for 1 hr, cooled to room temperature and further heated in air at 250 C for 1 hr. The coatings were examined for their electrochromic properties in a test cell containing a test electrolyte of 1 M lithium perchlorate in butyrolactone and a platinum counter electrode and by probing the center of the coatings by a laser beam (% = 632.8 nm) in conjunction with a photodiode. The coatings under the injected charge of about 30 mClcm2 change transmission from original 70% to 10%.

The novelty of the present invention is that the process for the preparation of the precursor material provides the material with a stability of at least one year, having excellent solubility in alcohol to yield electrochromic coatings.

The inventive step is in the use of an organic additive to peroxypolytungstate sol, which helps in giving the precursor material with high stability.

The main advantages of the present invention are: 1. The invention uses inexpensive and readily available starting materials. 2. The processing is carried out at ambient and moderate temperatures. 3. The precursor material is obtained in a solid form. 4. The solid precursor material obtained has stability over a period of about one-year.

5. The whole process of preparation of the solid precursor material takes less than 5 hours.

6. The coatings prepared using the precursor material according to the present invention exhibit excellent electrochromic properties.

7. Desired volumes of deposition solution can be prepared as required.

Claims (7)

1. We claim: 1. A process for the preparation of a highly stable solid precursor material useful for tungsten oxide based electrochromic coatings which comprises: a) preparing an aqueous solution of sodium tungstate in deionized water; b) passing the aqueous solution prepared in step a) through a vertical column containing acidic ion exchange resin to give an elute; c) collecting the elute from step b) directly into a peroxide as colloidal polytungstate sol with a pH not exceeding 2; d) stirring the mixture obtained in c) for a period ranging between 30 to 120 minutes to form a peroxypolytungstate solution; e) mixing an organic acid to said peroxypolytungstate solution as obtained in step d) under reflux conditions at a temperature of 55 C for a time period in the range of 1-
2. 2 hours to form a peroxypolytungstate acid derivative; f) converting said peroxypolytungstate acid .derivative to a highly stable solid precursor material by fast vacuum drying; 2. A process as claimed in claim 1 wherein the strength of the sodium tungstate solution used ranges between 0.3M - 0.7 M.
3. 3. A process as claimed in claim 1 wherein the peroxide used is hydrogen peroxide of about 12 to about 15 % by volume of said colloidal polytungstate sol.
4. 4. A process as claimed in claim 1 wherein organic acid used is selected from . glacial acetic acid and propionic acid.
5. 5. A process as claimed in claim 1 wherein the organic acid used is glacial acetic acid of about 12 to about vol. 15
6. 6. A process as claimed in claim 1 wherein peroxypolytungstate acid derivative devoid free peroxide is fast vacuum dried at a pressure less than 1 Torr.
7. 7. A precursor material made by the process of present invention as claimed in claims 1-6, said precursor material being: i. highly soluble in alcohol. ii. highly stable.