Title of Invention	A METHOD OF PREPARATION OF A NOVEL SOL FOR DEPOSITION OF ELECTROCHROMIC TUNGSTEN OXIDE THIN FILMS
Abstract	A method of preparation of a novel sol for deposition of electrochromic tungsten oxide thin films by dissolving sodium tungstate in water free from organic and inorganic contaminants under stirring in the ratio of 0.5-1.0 gm of sodium tungstate : 40-120 ml of water at ambient temperature, treating the solution so obtained with mineral acid of concentration in the range of 3N-6N to obtain a precipitate, separating the precipitate from the mother liquor by filtration, washing the precipitate with water free from organic and inorganic contaminants till complete removal of metal cation of the tungstate and anion of the acid, •4 dissolving the washed precipitate in a mixture of 20-30% aqueous solution of Q hydrogen peroxide with glacial acetic acid in a ratio in the range of H2O2 O :H2COOH = 3:1, 1:3 and intermediates (v/v) under stirring, drying the resultant solution at a temperature in the range of 40°-60°C to obtain a solid mass, dissolving the solid mass so obtained in alcohol in a ratio in the range of 0.2-0.65 gm of solid mass : 10-20 ml of alcohol at a temperature in the range of 40-60°C under stirring and filtering the solution to obtain a novel sol.

Title of Invention

A METHOD OF PREPARATION OF A NOVEL SOL FOR DEPOSITION OF ELECTROCHROMIC TUNGSTEN OXIDE THIN FILMS

Abstract

A method of preparation of a novel sol for deposition of electrochromic tungsten oxide thin films by dissolving sodium tungstate in water free from organic and inorganic contaminants under stirring in the ratio of 0.5-1.0 gm of sodium tungstate : 40-120 ml of water at ambient temperature, treating the solution so obtained with mineral acid of concentration in the range of 3N-6N to obtain a precipitate, separating the precipitate from the mother liquor by filtration, washing the precipitate with water free from organic and inorganic contaminants till complete removal of metal cation of the tungstate and anion of the acid, •4 dissolving the washed precipitate in a mixture of 20-30% aqueous solution of Q hydrogen peroxide with glacial acetic acid in a ratio in the range of H2O2 O :H2COOH = 3:1, 1:3 and intermediates (v/v) under stirring, drying the resultant solution at a temperature in the range of 40°-60°C to obtain a solid mass, dissolving the solid mass so obtained in alcohol in a ratio in the range of 0.2-0.65 gm of solid mass : 10-20 ml of alcohol at a temperature in the range of 40-60°C under stirring and filtering the solution to obtain a novel sol.

Full Text	The present invention relates to a method of preparation of a novel sol for deposition of electrochromic tungsten oxide thin films. The present invention particularly relates to a method of preparation of a modified sol for deposition of electrochromic tungsten oxide thin films and electrochromic tungsten oxide thin films made thereof. These are used in electrochromic devices which are known to change the transmission of light passing through them in a reversible manner from a bleached state to a colored one and back to the original as a function of a suitable back and forth voltage sweep through a given range. Such devices are now set for commercial use in smart windows such as architectural glazing, automobile and building sun roofs and rear view mirrors, displays for light regulation and energy saving. A present day method of making the precursor sols for deposition of electro chromic tungsten oxide thin film comprises use of alkoxides such as tungsten hexaethoxide and tungsten oxo-tetra-n-butoxide, reference to which may be made to H. Unuma, K. Tonooka, Y. Suzuki, T. Furusaki, K. Kodaira and T. Matusshia in Jour. Mat. Sci. Lett., 5, 1248-50, 1986 and Z.A.E.P. Vroon and C.I.M.A. Spee in Jour. Non-Cryst. Solids, 218, 189-95, 1997 wherein alkoxide, namely, tungsten hexaethoxide was used for preparing the precursor sols for tungsten oxide coating. Reference may also be made to K.D. Lee in Solar Energy. Mat. Solar Cells 57, 21-30, 1999 wherein a similar process was followed using tungsten oxo-tetra-n-butoxide. Reference may be made to J.P. Cronin, S.R. Kennedy, A. Agarwal, T.J. Gudgel, Y.J. Yao, J.C.L. Tonazzi and D.R. Uhlmannn in Ceram. Trans. 92, Amer. Ceram. Soc., Ohio, USA, pp. 175-93, 1999; J.P. Cronin, A. Agrawal, D.J. Tarico and J.C.L. Tonazzi. in U.S. Patent 5, 2277, 986, January 1994; J.P. Cronin, D.J. Tarico, A. Agrawal and R.L. Zhang in U.S. Patent 5,457,218, October 1995; J.P. Cronin, A. Agrawal, D. J. Tarico and J.C.L.Tonazzi in U.S. Patent 5, 525, 264, June 1996; N. Ozer and N, Dogan in Proc. SPBE 3424, 106-113, 1998; N. Ozer and C. M. Lamport in "Thin Solid Films 349,205-211, 1999; B. Munro, S. Kramer, P. Zapp, H. Krug and H. Schmidt in Jour. Non-Cryst Solids, 218, 185-88, 1997, wherein fine tungsten metal particles (around 1 micron in size) were dissolved in excess HaOa (generally 30 wt% solution) in presence of organic acids. The excess H2O2as in some cases decomposed by using a platinum mesh catalyst. The product was mixed with ethanol and acetic acid, or dried to obtain a peroxotungstic acid/ester; the former was ready for glass coating and the latter, obtained as a solid, was dissolved in ethanol to form a sol. Reference may further be made to A. Chemseddine, F. Babonneau and J. Livage in Jour. Non-Cryst. Solids 91, 271-78, 1987; P. Judeinstein and J. Livage in SPIE1328, 344-51, 1990; T. Nanba , Y. Nishiyama and I. Yasui in Jour. Mater . Res. 6, 1324-33, 1991, wherein an aqueous solution of sodium tungstate is passed through a cation exchange resin column to effect Na+ - H+ replacement (acidification step). The acidified colloidal tungstic acid solution is used for glass coating. The drawbacks of the above processes are : 1. Tungsten alkoxides are rare and expensive as also highly hydrolysable, and must be very carefully insulated from the atmospheric moisture to avoid degradation. 2. Fine tungsten powders are susceptible to the attack by moisture / oxygen from the atmosphere, leading to surface oxidation on storage. This causes a loss of reactivity of the powder, leading to long dissolution time and loss of raw materials due to the undissolved oxidised part. When the powder is fresh, the reaction causes violent evolution of heat, leading to frothing and upwards creeping of the reactants; a recommended procedure is to use a low temperature (e.g. 0°±1°C), again requiring a long time (24 h) for completion of reaction for small quantities of the powder 3. Colloidal solutions obtained from ion-exchange of sodium tungstate solutions are unstable, become turbid and turn to a gel after a few hours. Further, gelation can also take place in the resin column, leading to fouling of the column and requiring frequent replacement or cleaning of the resin. The main object of the present invention is to provide a method of preparation of a novel sol for deposition of electrochromic tungsten oxide thin films which obviates the drawbacks as detailed above. Another object of the present invention is to discard the use of highly hydrolysable starting chemicals in the formation of a sol for deposition of electrochromic tungsten oxide thin films. Still another object of the present invention is to discard the use of highly expensive fine sized tungsten powders which when oxidised due to storage lead to incomplete reactions and when fresh, lead to violent exothermic reactions releasing heat. Yet another object of the present invention is to use an aqueous solution of sodium tungstate and precipitate a solid matter by dilute acidic solution for further processing, so as to discard the use of cation exchage resin column which leads to quick gelation of the coating sol. Still another object is to provide electrochromic tungsten oxide thin films using the modified sol of the present invention. Accordingly the present invention provides a method of preparation of a novel sol for deposition of electrochromic tungsten oxide thin films which comprises dissolving , sodium tungstate in water free from organic and inorganic contaminants under stirring in the ratio of 0.5 - 1.0 gm of sodium tungstate : 40 - 120 ml of water at ambient temperature, treating the solution so obtained with mineral acid of concentration in the range of 3N-6N to obtain a precipitate, separating the precipitate from the mother liquor by filtration, washing the precipitate with water free from organic Accordingly, the present invention provides a method of preparation of a novel sol for deposition of electrochromic tungsten oxide thin films which comprises dissolving sodium tungstate in water free from organic and inorganic contaminants under stirring in the ratio of 0.5-1.0 gm of sodium tungstate : 40-120 ml of water at ambient temperature, treating the solution so obtained with mineral acid such as herein described of concentration in the range of 3N-6N to obtain a precipitate, separating the precipitate from the mother liquor by filtration, washing the precipitate with water free from organic and inorganic contaminants till complete removal of metal cation of the tungstate and anion of the acid, dissolving the washed precipitate in a mixture of 20-30% aqueous solution of hydrogen peroxide with glacial acetic acid in a ratio in the range of H2O2 :H2COOH = 3:1, 1:3 and intermediates (v/v) under stirring, drying the resultant solution at a temperature in the range of 40°-60°C to obtain a solid mass, dissolving the solid mass so obtained in alcohol in a ratio in the range of 0.2-0.65 gm of solid mass : 10-20 ml of alcohol at a temperature in the range of 40-60°C under stirring and filtering the solution to obtain a novel sol. In an embodiment of the present invention mineral acids used may be HCI, HNO3, H2SO4 or mixture thereof. In another embodiment of the present invention the alcohols used may be methanol, ethanol, propanol, butanol or combinations thereof. Accordingly, the present invention provides electrochromic tungsten oxide thin films, which comprises coating the novel sol, of the present invention, onto a pre-cleaned transparent conduction oxide coated glass by known methods. The details of the process of the present invention are given below : (i) Sodium tungstate in powder form was dissolved in water free from organic and inorganic contaminants under stirring in the ratio 0.5 - 1.0 gm. Sodium tungstate : 40-120 ml water at ambient temperature, (ii) The solution in step (i) above was treated with mineral acid (3N-6N), such as hydrochloric acid, sulphuric acid, nitric acid or mixture thereof to obtain a light yellow precipitate, (iii) The precipitate in step (ii) above was separated from the mother liquor by filtration, and the residue washed repeatedly in water free from organic and inorganic contaminants to completely remove sodium and the anion of the acid. (iv) The washed precipitate of step (Hi) above was then dissolved in 20 -30% aqueous solution of hydrogen peroxide under stirring in presence of glacial acetic acid in the ratio H2O2: CH3COOH = 3:1 to 1:3 (v/v). (v) The solution obtained in step (iv) was dried at 40° - 60°C to obtain a solid matter. (vi) The solid matter obtained in step (v) above was dissolved in methanol, ethanol, propanol, butanol or combinations thereof in the ratio of 0.2 - 0.65 gm.: 10 - 20 ml. at 40° - 60°C under stirring; the solution was filtered to remove undissolved solid matter to obtain the modified sol. (vii) The modified sol obtained in step (vi) above was used for coating transparent conducting oxide coated glass by known methods to obtain electrochromic tunsten oxide thin film. The heart of an electrochromic device is an electrochromic thin film on transparent conducting oxide coated glass (TCO) ; the film is generally amorphous or crystalline tungsten oxide (WO3nH2O). This electrochromic coated glass is put into a device containing an ion storage coating on TCO coated glass (as above) and an electrolyte. The transparent conducting oxide layers are connected to a voltage source for changing the voltage as necessary for light regulation. Deposition of such electrochromic films is made by different means, though sol-gel methods are preferable because of the low cost of chemicals and deposition equipment. Any sol-gel method for electrochromic films starts with a sol suitable for deposition of films on glass. Thus, the utility of any invention pertaining to the development of a sol from inexpensive chemicals by simple means lies in providing a means for fabricating films easily and at a low cost. Various methods can be used for preparation of tungsten oxide films, such as vacuum evaporation deposition, sputtering, chemical vapour deposition , but such methods either involve expensive equipment and precursors or are otherwise unsuitable for fabricating large area films. Sol-gel methods have been found to be relatively suitable in this respect. There are, however, a large number of variants of this method in respect of the precursor sol for thin film Formation of W03.nH2O. The present invention does not use any highly hydrolysable chemicals for sol Preparation. Preparation of an aqueous solution of sodium tungstate and obtaining a precipitate from the solution by acidification can be performed under ambient conditions instead of near 0°C, as m some cases in the prior art. Dissolution of the washed precipitate in hydrogen peroxide-acetic acid takes place under ambient conditions requiring no refluxing, leading to the formation of peroxotungstic acid. No ion-exchange step, leading to quick gelation of the sol, is involved. Thus, the novelty of the present process lies in obviating undesirable hydrolysis, uncontrolled reactivity, uncontrolled gelation by utilising the principle of dissolution of sodium tungstate with the formation of peroxotungstic acid containing modified sol. The inventive step, accordingly, is the precipitation of solid tungsten compound and dissolution of sodium tungstate by controlled formation of peroxylinkages followed by sol formation mostly in ambient condition. The following examples are given by way of illustrations and therefore should not be construed to limit the scope of the present invention. Example 1 0.5 gm. of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 40 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 5ml of 3 M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed The precipitate was left for 0.5h to dry up. It was now almost white in color. 10 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate., carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution. The solution thus obtained was filtered through a Millipore system to remove fine uodissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 15 ml of dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50° C. 0.45 gm. of the powder obtained in the previous step was added to dehydrated ethanol in small lots under constant stirring condition. Stirring was continued for 9 hrs. The solution was then filtered through a Millipore system. The filtrate was pale yellow in color. This solution was preserved under refrigeration for at least a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min"1 and then withdrawn at the same rate. After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then cured in air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt. Example 2 5.0 gm.of sodium tungstate, dihydrate powder was taken in a 1000 ml beaker loaded with a stirring paddle. 400 ml. of deiornised, double distilled water was added and the contents stirred till complete dissolution took place. To this solution 30ml of 3M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 2 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 60 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution. The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 15 ml of dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50° C. 0.45 gm. of the powder which was a part of the same obtained in the previous step was added to dehydrated ethanol in small lots under constant stirring condition. Stirring was continued for 9 hrs. The solution was then filtered through a Millipore system. The filtrate was pctfe yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate. After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then cured in air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt. Example 3 10.0 gm.of sodium tungstate, dihydrate powder was taken in a 1000 ml beaker loaded with a stirring paddle. 800 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 60 ml of 3M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 2.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 60 ml. (50:50) mixture of 30 wt% hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution. The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 15 ml of dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50° C. 0.45 gm. of the powder which was a part of the same obtained in the previous step was added to dehydrated ethanol in small lots under constant stirring condition. Stirring was continued for 9 hrs. The solution was then filtered through a Millipore system. The filtrate was pale yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate. After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then cured in air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt. Example 4 0.5 gm.of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 40 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 5ml of 3 M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 10 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution. The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 10 ml. of methanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50°C. 0.20 gm. of the powder was added to methanol in small lots under constant stirring condition. Stirring was continued for 15 hrs. The solution was then filtered through a Mitlipore system. The filtrate was very pale yellow in color. The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate. After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then cured in air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt. Example 5 0.5 gm.of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 40 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 5ml of 3 M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 10 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution. The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 10 ml. 50:50 mixture of methanol and dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50°C. 0.20 gm. of the powder was added to the mixture in small lots under constant stirring condition. Stirring was continued for 15.5 hrs. The solution was then filtered through a Millipore system. The filtrate was very light yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip- coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate. After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then curedin air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt. Example 6 0.5 gm.of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 40 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 5ml of 3 M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till ail chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 10 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution. The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 10 ml of n-propanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50°C, 0,20 gm. of the powder was added to n-propanol in small lots under constant stirring condition. Stirring was continued for 13.5 hrs. The solution was then filtered through a Millipore system. The filtrate was very very light yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1and then withdrawn at the same rate. After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then curedin air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt. Example 7 0.5 gm.of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 40 ml. of deionised, double distilled water was added and the contents stirred till complete dissolution took place. To this solution 5ml of 3 M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yeDow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 10 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 10 ml. of 2-butanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50°C. 0.20 gm. of the powder was added to 2-butanol in small lots under constant stirring condition. Stirring was continued for 13.5 hrs. The solution was then filtered through a Millipore system. The filtrate was almost colorless.This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate. After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then curedin air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt. Example 8 1.0 gm. of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 80 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 6 ml of 3M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 20 ml. (1:2) mixture of 30 % hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution. The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 5 ml of dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50° C. 0.20 gm. of the powder which was a part of the same obtained in the previous step was added to dehydrated ethanol in small lots under constant stirring condition. Stirring was continued for 9 hrs. The solution was then filtered through a Millipore system. The filtrate was pale yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate. After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate The coating thus prepared was then curedin air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt. Example 9 1.0 gm. of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle 120 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 6 ml of 3M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow solid matter occurred The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 20 ml. (2:1) mixture of 30 % hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution. The solution thus obtained was filtered through a Millipore system to remove fine undissolvcd matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 5 ml of dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50° C. 0.20 gm. of the powder which was a part of the same obtained in the previous step was added to dehydrated ethanol in small lots under constant stirring condition. Stirring was continued for 9 hrs. The solution was then filtered through a Millipore system. The filtrate was pale yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate. After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then curedin air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt. The main advantages of the present invention are: (i) Replacement of highly hydrolysable starting chemicals such as alkoxides and tungsten powders by nonhydrolysable raw materials such as metal tungstate in the formation of a sol for deposition of electrochromic tungsten oxide thin films. (ii) A very cheap and easily available source of tungsten has been used. (iii) All other raw materials are, similarly, inexpensive and easily available. (iv) All operations in the process of preparing the sol can be carried out at room temperature to 50° - 60°C, i.e. unlike in prior art, no step involves operation at 0°C or close to it. (v) The sols prepared from metal tungstate, as embodied in this document, have much longer shelf life than the same made by resin treatment in prior art using the same chemical, i.e. sodium tungstate. We claim : 1. A method of preparation of a novel sol for deposition of electro chromic tungsten oxide thin films which comprises dissolving sodium tungstate in water free from organic and inorganic contaminants under stirring in the ratio of 0.5-1.0 gm of sodium tungstate : 40-120 ml of water at ambient temperature, treating the solution so obtained with mineral acid such as herein described of concentration in the range of 3N-6N to obtain a precipitate, separating the precipitate from the mother liquor by filtration, washing the precipitate with water free from organic and inorganic contaminants till complete removal of metal cation of the tungstate and anion of the acid, dissolving the washed precipitate in a mixture of 20-30% aqueous solution of hydrogen peroxide with glacial acetic acid in a ratio in the range of H2O2 :CH3COOH = 3:1, 1:3 and intermediates (v/v) under stirring, drying the resultant solution at a temperature in the range of 40°-60°C to obtain a solid mass, dissolving the solid mass so obtained in alcohol in a ratio in the range of 0.2-0.65 gm of solid mass : 10-20 ml of alcohol at a temperature in the range of 40-60°C under stirring and filtering the solution to obtain a novel sol. 2. A method as claimed in claim 1 wherein mineral acids used is HCI, HNO3 H2SO4 or mixture thereof. 3. A method as claimed in claims 1-2 wherein the alcohol used is methanol, ethanol, propanol, butanol or combinations thereof. 4. A method as claimed in claims 1-3 wherein the novel sol is coated on to a pre cleaned transparent conducting oxide coated glass. 5. A method of preparation of a novel sol for deposition of electro chromic tungsten oxide thin films substantially as herein described with reference to the examples.

Full Text

The present invention relates to a method of preparation of a novel sol for deposition of electrochromic tungsten oxide thin films.
The present invention particularly relates to a method of preparation of a modified sol for
deposition of electrochromic tungsten oxide thin films and electrochromic tungsten oxide
thin films made thereof. These are used in electrochromic devices which are known to
change the transmission of light passing through them in a reversible manner from a
bleached state to a colored one and back to the original as a function of a suitable back
and forth voltage sweep through a given range. Such devices are now set for
commercial use in smart windows such as architectural glazing, automobile and building
sun roofs and rear view mirrors, displays for light regulation and energy saving.
A present day method of making the precursor sols for deposition of electro chromic
tungsten oxide thin film comprises use of alkoxides such as tungsten hexaethoxide and
tungsten oxo-tetra-n-butoxide, reference to which may be made to H. Unuma, K.
Tonooka, Y. Suzuki, T. Furusaki, K. Kodaira and T. Matusshia in Jour. Mat. Sci. Lett., 5,
1248-50, 1986 and Z.A.E.P. Vroon and C.I.M.A. Spee in Jour. Non-Cryst. Solids, 218,
189-95, 1997 wherein alkoxide, namely, tungsten hexaethoxide was used for preparing
the precursor sols for tungsten oxide coating. Reference may also be made to K.D. Lee
in Solar Energy. Mat. Solar Cells 57, 21-30, 1999 wherein a similar process was
followed using tungsten oxo-tetra-n-butoxide. Reference may be made to J.P. Cronin,
S.R. Kennedy, A. Agarwal, T.J. Gudgel, Y.J. Yao, J.C.L. Tonazzi and D.R. Uhlmannn in
Ceram. Trans. 92, Amer. Ceram. Soc., Ohio, USA, pp. 175-93, 1999; J.P. Cronin, A.
Agrawal, D.J. Tarico and J.C.L. Tonazzi. in U.S. Patent 5, 2277, 986,
January 1994; J.P. Cronin, D.J. Tarico, A. Agrawal and R.L. Zhang
in U.S. Patent 5,457,218, October 1995; J.P. Cronin, A. Agrawal,
D. J. Tarico and J.C.L.Tonazzi in U.S. Patent 5, 525, 264, June 1996; N. Ozer and N, Dogan in Proc. SPBE 3424, 106-113, 1998; N. Ozer and C. M. Lamport in "Thin Solid Films 349,205-211, 1999; B. Munro, S. Kramer, P. Zapp, H. Krug and H. Schmidt in Jour. Non-Cryst Solids, 218, 185-88, 1997, wherein fine tungsten metal particles (around 1 micron in size) were dissolved in excess HaOa (generally 30 wt% solution) in presence of organic acids. The excess H2O2as in some cases decomposed by using a platinum mesh catalyst. The product was mixed with ethanol and acetic acid, or dried to obtain a peroxotungstic acid/ester; the former was ready for glass coating and the latter, obtained as a solid, was dissolved in ethanol to form a sol. Reference may further be made to A. Chemseddine, F. Babonneau and J. Livage in Jour. Non-Cryst. Solids 91, 271-78, 1987; P. Judeinstein and J. Livage in SPIE1328, 344-51, 1990; T. Nanba , Y. Nishiyama and I. Yasui in Jour. Mater . Res. 6, 1324-33, 1991, wherein an aqueous solution of sodium tungstate is passed through a cation exchange resin column to effect Na+ - H+ replacement (acidification step). The acidified colloidal tungstic acid solution is used for glass coating.
The drawbacks of the above processes are :
1. Tungsten alkoxides are rare and expensive as also highly hydrolysable, and must be very carefully
insulated from the atmospheric moisture to avoid degradation.
2. Fine tungsten powders are susceptible to the attack by moisture / oxygen from the atmosphere,
leading to surface oxidation on storage. This causes a loss of reactivity of the powder, leading to
long dissolution time and loss of raw materials due to the undissolved oxidised part. When the
powder is fresh, the reaction causes violent evolution of heat, leading to frothing and upwards
creeping of the reactants; a recommended procedure is to use a low temperature (e.g. 0°±1°C),
again requiring a long time (24 h) for completion of reaction for small quantities of the powder
3. Colloidal solutions obtained from ion-exchange of sodium tungstate solutions are unstable,
become turbid and turn to a gel after a few hours. Further, gelation can also take place in the resin
column, leading to fouling of the column and requiring frequent replacement or cleaning of the
resin.
The main object of the present invention is to provide a method of preparation of a novel sol for
deposition of electrochromic tungsten oxide thin films which obviates the drawbacks as detailed
above.
Another object of the present invention is to discard the use of highly hydrolysable starting
chemicals in the formation of a sol for deposition of electrochromic tungsten oxide thin films. Still
another object of the present invention is to discard the use of highly expensive fine sized tungsten
powders which when oxidised due to storage lead to incomplete reactions and when fresh, lead to
violent exothermic reactions releasing heat.
Yet another object of the present invention is to use an aqueous solution of sodium tungstate and
precipitate a solid matter by dilute acidic solution for further processing, so as to discard the use of
cation exchage resin column which leads to quick gelation of the coating sol.
Still another object is to provide electrochromic tungsten oxide thin films using the modified sol of
the present invention.
Accordingly the present invention provides a method of preparation of a novel sol for deposition of
electrochromic tungsten oxide thin films which comprises dissolving , sodium tungstate in water
free from organic and inorganic contaminants under stirring in the ratio of 0.5 - 1.0 gm of sodium
tungstate : 40 - 120 ml of water at ambient temperature, treating the solution so obtained with
mineral acid of concentration in the range of 3N-6N to obtain a precipitate, separating the
precipitate from the mother liquor by filtration, washing the precipitate with water free from organic
Accordingly, the present invention provides a method of preparation of a novel sol for deposition of electrochromic tungsten oxide thin films which comprises dissolving sodium tungstate in water free from organic and inorganic contaminants under stirring in the ratio of 0.5-1.0 gm of sodium tungstate : 40-120 ml of water at ambient temperature, treating the solution so obtained with mineral acid such as herein described of concentration in the range of 3N-6N to obtain a precipitate, separating the precipitate from the mother liquor by filtration, washing the precipitate with water free from organic and inorganic contaminants till complete removal of metal cation of the tungstate and anion of the acid, dissolving the washed precipitate in a mixture of 20-30% aqueous solution of hydrogen peroxide with glacial acetic acid in a ratio in the range of H2O2 :H2COOH = 3:1, 1:3 and intermediates (v/v) under stirring, drying the resultant solution at a temperature in the range of 40°-60°C to obtain a solid mass, dissolving the solid mass so obtained in alcohol in a ratio in the range of 0.2-0.65 gm of solid mass : 10-20 ml of alcohol at a temperature in the range of 40-60°C under stirring and filtering the solution to obtain a novel sol.
In an embodiment of the present invention mineral acids used may be HCI, HNO3, H2SO4 or mixture thereof.
In another embodiment of the present invention the alcohols used may be methanol, ethanol, propanol, butanol or combinations thereof.
Accordingly, the present invention provides electrochromic tungsten oxide thin films, which comprises coating the novel sol, of the present invention, onto a pre-cleaned transparent conduction oxide coated glass by known methods. The details of the process of the present invention are given below :
(i) Sodium tungstate in powder form was dissolved in water free from organic
and inorganic contaminants under stirring in the ratio 0.5 - 1.0 gm. Sodium
tungstate : 40-120 ml water at ambient temperature, (ii) The solution in step (i) above was treated with mineral acid (3N-6N), such as
hydrochloric acid, sulphuric acid, nitric acid or mixture thereof to obtain a light
yellow precipitate, (iii) The precipitate in step (ii) above was separated from the mother liquor by
filtration, and the residue washed repeatedly in water free from organic and
inorganic contaminants to completely remove sodium and the anion of the
acid.
(iv) The washed precipitate of step (Hi) above was then dissolved in 20 -30% aqueous solution of
hydrogen peroxide under stirring in presence of glacial acetic acid in the ratio H2O2: CH3COOH =
3:1 to 1:3 (v/v).
(v) The solution obtained in step (iv) was dried at 40° - 60°C to obtain a solid matter.
(vi) The solid matter obtained in step (v) above was dissolved in methanol, ethanol, propanol,
butanol or combinations thereof in the ratio of 0.2 - 0.65 gm.: 10 - 20 ml. at 40° - 60°C under
stirring; the solution was filtered to remove undissolved solid matter to obtain the modified sol.
(vii) The modified sol obtained in step (vi) above was used for coating transparent conducting oxide
coated glass by known methods to obtain electrochromic tunsten oxide thin film.
The heart of an electrochromic device is an electrochromic thin film on transparent conducting oxide coated glass (TCO) ; the film is generally amorphous or crystalline tungsten oxide (WO3nH2O). This electrochromic coated glass is put into a device containing an ion storage coating on TCO coated glass (as above) and an electrolyte. The transparent conducting oxide layers are connected to a voltage source for changing the voltage as necessary for light regulation. Deposition of such electrochromic films is made by different means, though sol-gel methods are preferable because of the low cost of chemicals and deposition equipment. Any sol-gel method for electrochromic films starts with a sol suitable for deposition of films on glass. Thus, the utility of any invention pertaining to the development of a sol from inexpensive chemicals by simple means lies in providing a means for fabricating films easily and at a low cost. Various methods can be used for preparation of tungsten oxide films, such as vacuum evaporation deposition, sputtering, chemical vapour deposition , but such methods either involve expensive equipment and precursors or are otherwise unsuitable for fabricating large area films.
Sol-gel methods have been found to be relatively suitable in this respect. There are, however, a large number of variants of this method in respect of the precursor sol for thin film Formation of W03.nH2O.
The present invention does not use any highly hydrolysable chemicals for sol Preparation. Preparation of an aqueous solution of sodium tungstate and obtaining a precipitate from the solution by acidification can be performed under ambient conditions instead of near 0°C, as m some cases in the prior art. Dissolution of the washed precipitate in hydrogen peroxide-acetic acid takes place under ambient conditions requiring no refluxing, leading to the formation of peroxotungstic acid. No ion-exchange step, leading to quick gelation of the sol, is involved. Thus, the novelty of the present process lies in obviating undesirable hydrolysis, uncontrolled reactivity, uncontrolled gelation by utilising the principle of dissolution of sodium tungstate with the formation of peroxotungstic acid containing modified sol. The inventive step, accordingly, is the precipitation of solid tungsten compound and dissolution of sodium tungstate by controlled formation of peroxylinkages followed by sol formation mostly in ambient condition.
The following examples are given by way of illustrations and therefore should not be construed to limit the scope of the present invention.
Example 1
0.5 gm. of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 40 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 5ml of 3 M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed
repeatedly with deionised double distilled water till all chloride ions were removed The precipitate was left for 0.5h to dry up. It was now almost white in color. 10 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate., carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution.
The solution thus obtained was filtered through a Millipore system to remove fine uodissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 15 ml of dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50° C. 0.45 gm. of the powder obtained in the previous step was added to dehydrated ethanol in small lots under constant stirring condition. Stirring was continued for 9 hrs. The solution was then filtered through a Millipore system. The filtrate was pale yellow in color. This solution was preserved under refrigeration for at least a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate.
In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min"1 and then withdrawn at the same rate.
After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then cured in air
oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt.
Example 2
5.0 gm.of sodium tungstate, dihydrate powder was taken in a 1000 ml beaker loaded with a stirring paddle. 400 ml. of deiornised, double distilled water was added and the contents stirred till complete dissolution took place. To this solution 30ml of 3M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 2 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 60 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution.
The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 15 ml of dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around
50° C. 0.45 gm. of the powder which was a part of the same obtained in the previous step was added to dehydrated ethanol in small lots under constant stirring condition. Stirring was continued for 9 hrs. The solution was then filtered through a Millipore system. The filtrate was pctfe yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate.
After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then cured in air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt.
Example 3
10.0 gm.of sodium tungstate, dihydrate powder was taken in a 1000 ml beaker loaded with a stirring paddle. 800 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 60 ml of 3M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 2.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and
washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 60 ml. (50:50) mixture of 30 wt% hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution.
The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 15 ml of dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50° C. 0.45 gm. of the powder which was a part of the same obtained in the previous step was added to dehydrated ethanol in small lots under constant stirring condition. Stirring was continued for 9 hrs. The solution was then filtered through a Millipore system. The filtrate was pale yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate.
After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then cured in air
oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt.
Example 4
0.5 gm.of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 40 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 5ml of 3 M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 10 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution. The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 10 ml. of methanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50°C. 0.20 gm. of the powder was added to methanol in small lots under constant stirring condition. Stirring was continued for 15 hrs. The solution was then

filtered through a Mitlipore system. The filtrate was very pale yellow in color. The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate.
In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate.
After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then cured in air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt.
Example 5
0.5 gm.of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 40 ml. of deionised , double distilled water was added and the contents stirred till complete
dissolution took place. To this solution 5ml of 3 M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 10 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution. The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 10 ml. 50:50 mixture of methanol and dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50°C. 0.20 gm. of the powder was added to the mixture in small lots under constant stirring condition. Stirring was continued for 15.5 hrs. The solution was then filtered through a Millipore system. The filtrate was very light yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate.
In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-
coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate.
After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then curedin air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt.
Example 6
0.5 gm.of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 40 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 5ml of 3 M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till ail chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 10 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution. The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the
solution and drying of the residue produced a pale yellow shining fine powder. 10 ml of n-propanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50°C, 0,20 gm. of the powder was added to n-propanol in small lots under constant stirring condition. Stirring was continued for 13.5 hrs. The solution was then filtered through a Millipore system. The filtrate was very very light yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1and then withdrawn at the same rate.
After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then curedin air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt.
Example 7
0.5 gm.of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 40 ml. of deionised, double distilled water was added and the contents stirred till complete
dissolution took place. To this solution 5ml of 3 M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yeDow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 10 ml. (50:50) mixture of 30 hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 10 ml. of 2-butanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50°C. 0.20 gm. of the powder was added to 2-butanol in small lots under constant stirring condition. Stirring was continued for 13.5 hrs. The solution was then filtered through a Millipore system. The filtrate was almost colorless.This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate.
In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate.
After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then curedin air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt.
Example 8
1.0 gm. of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle. 80 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 6 ml of 3M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow solid matter occurred. The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 20 ml. (1:2) mixture of 30 % hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution.
The solution thus obtained was filtered through a Millipore system to remove fine undissolved matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 5 ml of dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50° C.
0.20 gm. of the powder which was a part of the same obtained in the previous step was added to dehydrated ethanol in small lots under constant stirring condition. Stirring was continued for 9 hrs. The solution was then filtered through a Millipore system. The filtrate was pale yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate.
After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate The coating thus prepared was then curedin air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt.
Example 9
1.0 gm. of sodium tungstate, dihydrate powder was taken in a 100 ml beaker loaded with a stirring paddle 120 ml. of deionised , double distilled water was added and the contents stirred till complete dissolution took place. To this solution 6 ml of 3M hydrochloric acid (HC1) was added under constant stirring condition. Stirring continued for 1-1.5 h till complete precipitation of a pale yellow
solid matter occurred The precipitate was filtered through Whatman 1 filter paper and washed repeatedly with deionised double distilled water till all chloride ions were removed. The precipitate was left for 0.5h to dry up. It was now almost white in color. 20 ml. (2:1) mixture of 30 % hydrogen peroxide and glacial acetic acid was taken in a 100 ml. beaker loaded with a stirring paddle. The precipitate, carefully scraped out from the filter paper, was transferred to the hydrogen peroxide - acetic acid mixture and the suspension stirred for 0.25 h at room temperature. The solid matter dissolved completely, giving a clear solution.
The solution thus obtained was filtered through a Millipore system to remove fine undissolvcd matter or other impurities, if any. Next, it was spread out in a glass petri dish and placed in an air oven at a temperature around 50°C. Evaporation of the solution and drying of the residue produced a pale yellow shining fine powder. 5 ml of dehydrated ethanol was taken in a 100 ml. capacity round bottom flask. The flask was placed in an oil bath with the temperature maintained at around 50° C. 0.20 gm. of the powder which was a part of the same obtained in the previous step was added to dehydrated ethanol in small lots under constant stirring condition. Stirring was continued for 9 hrs. The solution was then filtered through a Millipore system. The filtrate was pale yellow in color. This solution was preserved under refrigeration for a few weeks without precipitation. Using this solution, films were coated onto fluorine doped tin oxide (FTO) glass substrate. In order to promote good adhesion, substrates were ultrasonically cleaned in acetone, rinsed with deionised double distilled water, washed with isopropanol to remove any surface contaminants, followed by drying in air oven at around 50°C. The coatings were made by dip coating in a dip-coater. The preserved or freshly prepared sol was placed in a sol tank and the substrate was dipped into the sol at a rate of 23 cm min-1 and then withdrawn at the same rate.
After coating, the substrate was dried at room temperature for 5 mins. Further two coating operations were performed at the same pulling rate. The coating thus prepared was then curedin air oven at 200°C for 1 hr. The film thus prepared was transparent. The coloration - bleaching was confirmed by using the coated substrate as an electrode in a cyclic voltammetric (cv) analysis by subjecting it to a voltage sweep from -1.0 to 0.75 volt.
The main advantages of the present invention are:
(i) Replacement of highly hydrolysable starting chemicals such as alkoxides and tungsten powders
by nonhydrolysable raw materials such as metal tungstate in the formation of a sol for deposition of
electrochromic tungsten oxide thin films.
(ii) A very cheap and easily available source of tungsten has been used.
(iii) All other raw materials are, similarly, inexpensive and easily available.
(iv) All operations in the process of preparing the sol can be carried out at room temperature to 50°
- 60°C, i.e. unlike in prior art, no step involves operation at 0°C or close to it.
(v) The sols prepared from metal tungstate, as embodied in this document, have much longer shelf
life than the same made by resin treatment in prior art using the same chemical, i.e. sodium
tungstate.

We claim :
1. A method of preparation of a novel sol for deposition of electro chromic
tungsten oxide thin films which comprises dissolving sodium tungstate
in water free from organic and inorganic contaminants under stirring in
the ratio of 0.5-1.0 gm of sodium tungstate : 40-120 ml of water at
ambient temperature, treating the solution so obtained with mineral
acid such as herein described of concentration in the range of 3N-6N
to obtain a precipitate, separating the precipitate from the mother liquor
by filtration, washing the precipitate with water free from organic and
inorganic contaminants till complete removal of metal cation of the
tungstate and anion of the acid, dissolving the washed precipitate in a
mixture of 20-30% aqueous solution of hydrogen peroxide with glacial
acetic acid in a ratio in the range of H2O2 :CH3COOH = 3:1, 1:3 and
intermediates (v/v) under stirring, drying the resultant solution at a
temperature in the range of 40°-60°C to obtain a solid mass, dissolving
the solid mass so obtained in alcohol in a ratio in the range of 0.2-0.65
gm of solid mass : 10-20 ml of alcohol at a temperature in the range of
40-60°C under stirring and filtering the solution to obtain a novel sol.
2. A method as claimed in claim 1 wherein mineral acids used is HCI,
HNO3 H2SO4 or mixture thereof.
3. A method as claimed in claims 1-2 wherein the alcohol used is
methanol, ethanol, propanol, butanol or combinations thereof.
4. A method as claimed in claims 1-3 wherein the novel sol is coated on
to a pre cleaned transparent conducting oxide coated glass.
5. A method of preparation of a novel sol for deposition of electro chromic
tungsten oxide thin films substantially as herein described with
reference to the examples.

Documents:

905-del-2000-abstract.pdf

905-del-2000-claims.pdf

905-del-2000-correspondence-others.pdf

905-del-2000-correspondence-po.pdf

905-del-2000-description (complete).pdf

905-del-2000-form-1.pdf

905-del-2000-form-19.pdf

905-del-2000-form-2.pdf

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Patent Number

242183

Indian Patent Application Number

905/DEL/2000

PG Journal Number

34/2010

Publication Date

20-Aug-2010

Grant Date

18-Aug-2010

Date of Filing

06-Oct-2000

Name of Patentee

COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	DIBYENDU GANGULI	CENTRAL GLASS AND CERAMIC RESEARCH INSTITUTE, CALCUTTA 700032, INDIA.
2	AMITAVA PATRA	CENTRAL GLASS AND CERAMIC RESEARCH INSTITUTE, CALCUTTA 700032, INDIA.
3	KAUSICK AUDDY	CENTRAL GLASS AND CERAMIC RESEARCH INSTITUTE, CALCUTTA 700032, INDIA.
4	PRASANTA KUMAR BISWAS	CENTRAL GLASS AND CERAMIC RESEARCH INSTITUTE, CALCUTTA 700032, INDIA.

PCT International Classification Number

B05D 5/12

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA