Title of Invention

"AN IMPROVED PROCESS FOR THE PREPARATION OF BARIUM TITANYL OXALATE"

Abstract

This invention relates to an improved process for the preparation of barium titanyl oxalate having formula Ba TiO (C2O4)2.4H2O (BTO). Barium titanyl oxalate is useful for the preparation of active barium titanate powders having formula BaTiO3 by pyrolysis of BTO in air. The process steps are: reacting a solution of titanium tetrabutoxide in an organic solvent with a solution of oxalic acid in an organic solvent at room temperature to obtain a solution of an intermediate precursor oxalotitanic acid [H2TiO(C2O4)2 xH2O] (HTO) containing TiO(C2O4)2-2 species, adding to the said precursor barium hydroxide in solid form directly or dispersed in an organic solvent in the oxalotitanic acid solution under constant stirring, followed by adding distilled water to initiate the acid-base reaction to obtain the barium titanyl oxalate.

Full Text

This invention relates to an improved process for the preparation of barium titanyl oxalate having formula BaTiO(C2O4)2.4H2O (BTO). Barium titanyl oxalate is useful for the preparation of active barium titanate powders having formula BaTiO3 by pyrolysis of BTO in air. BaTiO3 powders have various applications. BT powders are useful to produce ceramics, which has numerous applications because of useful electronic properties. These applications include capacitors, piezoelectric materials for transducers and sensors. Undoped BaTiO3 ceramics are widely used for various types of capacitors i.e. multilayer, chip. disc, grain boundary layer etc. These are important because they have very high dielectric constant and low dielectric loss. These advanced active BaTiO3 powders with sub-micron particle size will allow improved volumetric efficiency of capacitor. In the prior art. BT powders are prepared by classical solid state reaction between BaCO:, and TiO2 at 1100oC in air. The BT powders thus obtained are inhomogeneous. coarser, and less reactive. Therefore to reduce the formation temperature of BaTiO3. molecular precursors such as barium titanyl oxalate. barium titanyl citrate are synthesized. These molecular precursors produce BaTiO3 powders at lower temperature i.e. 750°C. which possess better physical and chemical characteristics and reactivity. Barium titanyl oxalate is one of the most important molecular precursor which is commercially exploited for the production of BaTiO3 powders. The barium titanyl oxalate is prepared by two methods: 1 .Clabaugh's synthesis method and 2. Exchange reaction method According to a method reported by Clabaugh [J. Res. Natl. Bur. Stds.. 1956. p-289] barium titanyl oxalate is precipitated by adding aqueous solutions of barium chloride and titanyl chloride to 10% excess oxalic acid at 80°C. The titanyl chlcride solution was prepared by adding the titanium tetrachloride to ice cooled distilled water. Although barium titanyl oxalate is commercially produced by this method there are some problems, which has origin in chemistry occurring in aqueous medium and suffers from following drawbacks, i. Starting reagentTiCl4 has vapor pressure at room temperature and it is prone to hydrolysis. ii. Existence of heterogeneous equilibria i.e.Ba-H2O-C2O4. Ti-H2O-C2O4 and H2C2O4-H2O governs the kinetics of BTO formation. Therefore the conditions to obtain the single phase BTO becomes very critical. iii. The BTO obtained requires washing step to avoid Cl" contamination, iv. Use of 10-20% excess of oxalic acid than the stiochiometric requirement in BTO is a must v. To aid in controlling Ba:Ti ratio in BTO the estimation of Ti in TiCl1 from batch to batch is essential. Another method, which has been tried to precipitate BTO. is by making use of exchange reaction principles in aqueous media. In this method equimolar aqueous solution of Na/ K titanyl oxalate is reacted with equimolar aqueous solution of barium salts like barium nitrate/ barium acetate /barium chloride at room temperature. This method suffers from foliowintg disadvantages: i. Dissolution of Na / K titanyl oxalate in aqueous medium produces anionic species i.e. TiO(C2O4)2-2 which are stable in the pH range of 2.5-3.1. Therefore to get single phase BTO by adding equimolar barium salt solution it is necessary to maintain the pH in the range 2.5-3.1 during exchange. There is no barium precursor available, which has pH in this range. ii. The BTO obtained by this exchange reaction is contaminated with Na' or K' ions. iii. The byproduct obtained during the exchange reaction is removed by washing with distilled water i.e. washing step is necessary. iv. Depending on the extent of destabilization of anionic TiO(C2O4)2-2 species. BTO is always associated with barium oxalate and titanyl oxalate. Due to this barium titanate is obtained at higher calcination temperature i.e. T > 900°C as compared to the pure BTO which gives BT at T=750°C. All the above problems faced during the synthesis of BTO in method 1 and 2 are also discussed in a publication entitled "Factors affecting the synthesis of barium titanyl oxalate" by Fang et.al [J. Am. Ceram. Sac. 72 (10) (1989) 1899]. The present invention is an attempt to overcome these problems and provide a simple and easier procedure for the preparation of barium titanyl oxalate. Another method, which is known as the Pechini or liquid mix process [US Patent 3330697 July 1967] is similar to oxalate process. Tetrabutyl titanate, citric acid and ethylene glycol are used to obtain an aqueous solution. Barium carbonate dissolved in the formic acid and water is then added to the titanium precursor solution. After adjusting the pH a 1:1 mixed crystalline citrate BaTi(C6H6O7)3 6H2O is obtained. Heat treatment of this precursor then results in the formation BaTiO3 through solid state reactions between intermediate species formed during the decomposition. The main disadvantages are high weight losses during the calcination stage as compared to the oxalate process and carbon contamination. Still in another approach [US patent 5032375 Jul 16. 1991] an amorphous powder of titanium oxide and of barium oxide is precipitated by the hydrolysis of titanium alcoholate in the presence of barium oxide/hydroxide and acid organic compound containing more than six carbon atoms. The amorphous powder is heated with stream of gas containing water vapor, ammonia or CO2 to crystallize barium titanate and distilled off acid organic. The experimental procedure is not simple and BaTiO3 phase is crystallized below 1000oC. which is comparatively higher than the oxalate route. The main object of present investigation is to provide an improved process for the preparation of the barium titanyl oxalate (BTO) with higher yield. Another objective is to provide a process wherein BTO is precipitated at room temperature rather than 60 to 80oC for the known methods. Still another object is to provide process, which produces barium titanyl oxalate without contamination of unwanted species. To achieve these objectives, the present invention provides a process using cheaper & easily available titanium tetrabutoxide. oxalic acid and barium hydroxide. Accordingly the present invention provides an improved process for the preparation of barium titanyl oxalate which comprises of reacting at room temperature a solution of titanium tetrabutoxide in organic solvent, with a solution of oxalic acid in an organic solvent to obtain a solution of an intermediate precursor oxalotitanic acid [H2TiO(C2O4)2.xH2O] (HTO) containing TiO(C2O4)2-2 species, adding to the said precur a solid barium hydroxide directly or in an organic solvent, into the oxalotitanic acid solution under constant stirring and finally followed by adding distilled water to initiate the acid-base reaction to obtain barium titanyl oxalate. In one the embodiment of the present investigation the organic solvent used may be methanol, ethanol, isopropanol exemplified by methanol. ethanol, isopropanol. In another embodiment the molar ratio of titanium tetrabutoxide and oxalic acid may be in the range asl :2 to 1:4. In yet another embodiment the concentration of the barium hydroxide dispersed in organic solvent is equimolar or higher. In still another embodiment the quantity of distilled water may be in the range of 500 to 1500 ml. In a feature of the present investigation, the semi aqueous media allow free barium ions to be generated in a controlled manner depending upon the solubility of barium hydroxide so that the spontaneous precipitation of BTO is controlled. In other words controlled release of Ba2^ ions not only helps to get pure BTO but also local pH variations are avoided. The reaction is kinetically controlled therefore optimum time span of 3 to 5 days is required to drive the precipitation reaction to its completion. In yet another feature, the as-dried BTO may be calcined at 750°C for 4 hr to obtain cubic BaTiO3 powders. In still yet another feature, the BaTiO3 powders thus obtained may be processed in the form of pellets and then sintered at 1300°C for 4 hr in air to obtain dense ceramics exhibiting tetragonal phase. The process of the present invention is described herein below with examples, which are illustrative only should be construed to limit the scope of the present invention in any marine i. Example 1 This example describes the preparation of barium titanyl oxalate (BTO) leading to barium titanate (BT) powders. Titanium tetrabutoxide 17ml (0.05M) was dissolved in 180ml of IPA at room temperature. 0.1M solution of oxalic acid was made, by dissolving 12.607g of oxalic acid in 350ml of IPA. This solution was kept in 31iter capacity beaker under constant stirring. 0.05M solution of titanium tetrabutoxide in IPA was added to 0.1M solution of oxalic acid in 1PA under constant stirring condition to obtain a clear solution of 0.05M oxalotitanic acid [H2TiO(C2O4)2] in IPA. 15.774g of Ba(OH)2 (0.05M) dispersed in IPA was added to 0.05M solution of oxalotitanic acid with continuous stirring. 500ml of distilled water was added to start the precipitation reaction and the reaction mixture was kept for 5 days for the precipitation reaction to go to its completion. The precipitate thus obtained in step 5 was filtered and air-dried at room temperature. The yield of BTO was > 99 % (22.5gm). The as-dried BTO was calcined at 750oC for 4h to obtain cubic BaTiO-, powders. The BaTiO} powders obtained in step 7 are processed in the form of pellets and then sintered at 1300°C for 4h in air to obtain dense ceramic exhibiting tetragonal phase. DTA/TGA data and powder characteristics of BTO/BT are given in tables 1 to 4 herein below. Table 5 gives comparative evaluation data on dielectric properties of BT ceramics obtained viz. a viz that reported in the literature. Table 1. Microanalysis data on as-dried precursor BTO.4H2O (Table Removed)Table 2. Chemical analysis data on as-dried precursor BTO.4H2O (Table Removed)* Water content determined from the microanalvsis data Table 3. DTA/TGA data on as-dried precursor BTO.4H2O(Table Removed)Table 4. Comparative data of BTO/BT powders(Table Removed) Table 5. Comparative data on ceramic BT compacts (Table Removed)*The standard data in table 5 has been complied from the references given below: 1 Jaffe B. Cook (Jr) W.R & Jaffe H. In piezoelectric ceramics (Academic press. New York). 1971. 2. Landolf-Bornstein Series (New) (Springer-Verlag. Berlin). Vol III /169. 1981. 3. Subbarao E.G. in Solid state chemistry, edited by C.N.R. Rao (Marscel Dekker Inc.. New York). 1974. 4. Subbarao E.G.. Prasad V.C.S.. Rao Veerbhadra K. in Preparation and charcterizattiuon of materials edited by J.M. Honig. C.N.R. Rao (Academic press. New York) 1981. Example 2 Titanium tetrabutoxide 34ml (0.1M) was dissolved in 360 ml of IPA at room iemperature. 0.2M solution of oxalic acid was made by dissolving 25.214 gm of oxalic acid in 750 ml of IPA. This solution was kept in 3 lit. capacity beaker under constant stirring. 0.1M solution of titanium tetrabutoxide in IPA was added to 0.2M solution of oxalic acid in IPA under constant stirring condition to obtain a clear solution of 0.1 M oxalotitanic acid [H2TiO(C2O4)2] in IPA. 31.548 gmof Ba(OH)2 (0.1M ) dispersed in 400 ml IPA was added to 0.1 M solution of oxalotitanic acid with continuous stirring. 1000 ml of distilled water was added to start precipitation reaction and the reaction mixture was kept for 4 days for the precipitation reaction to go to its completion. The precipitate thus obtained in step 5 was filtered and air-dried at room temperature. The yield of BTO was > 99 % (45 gm). The as-dried BTO was calcined at 750"C for 4h to obtain cubic BaTiO3 powders. The BaTiO3 powders obtained are processed in the form of pellets and then sintered at 1300°C for 4h in air to obtain dense ceramic exhibiting tetragonal phase. Example 3 Titanium tetrabutoxide 17ml (0.05M) was dissolved in 200 ml of water free dry ethanol at room temperature. 0.1M solution of oxalic acid was made, by dissolving 12.607g of oxalic acid in 400 ml of ethanol. This solution was kept in 31iter capacity beaker under constant stirring. 0.05M solution of titanium tetrabutoxide in ethanol was added to 0. 1M solution of oxalic acid in ethanol under constant stirring condition to obtain a clear solution of 0.05M oxalotitanic acid [H2TiO(C2O4)2] in ethanol. 15.774g of Ba(OH)2 (0.05M) dispersed in ethanol was added to 0.05M solution of oxalotitanic acid with continuous stirring. 500ml of distilled water was added to start the precipitation reaction and the reaction mixture was kept for 5 days for the precipitation reaction to go to its completion. The precipitate thus obtained was filtered and air-dried at room temperature. The yield of BTO was > 99 % (22.5 gm). The as-dried BTO was calcined at 750°C for 4h to obtain cubic BaTiO3 powders. The BaTiO3 powders obtained are processed in the form of pellets and then sintered at 1300°C for 4h in air to obtain dense ceramic exhibiting tetragonal phase. Example 4 Titanium tetrabutoxide 34ml (0.1M) was dissolved in 360 ml of IPA at room temperature. 0.2M solution of oxalic acid was made by dissolving 25.214 gm of oxalic acid in 750 ml of IPA. This solution was kept in 3liter capacity beaker under constant stirring. 0.1M solution of titanium tetrabutoxide in 1PA was added to 0.2M solution of oxalic acid in IPA under constant stirring condition to obtain a clear solution of 0.1M oxalotitanic acid [H2TiO(C2O4)2] in IPA. 31.548 gm of Ba(OH)2 (0.1M) was directly added to 0.1M solution of oxalotitanic acid with continuous stirring. 800 ml of distilled water was added to start precipitation reaction and the reaction mixture was kept for 3-days for the precipitation reaction to go to its completion. The precipitate thus obtained was filtered and air-dried at room temperature. The yield of BTO was > 99 % (45gm). The as-dried BTO was calcined at 750°C for 4 hr to obtain cubic BaTiO3 powders The BaTiO3 powders obtained are processed in the form of pellets and then sintered at 1300°C for 4h in air to obtain dense ceramic exhibiting tetragonal phase. Advantages of presently described route over the reported methods described earlier: 1. Simple acid-base reaction produced single-phase BTO with yield > 99% and reproducibility. 2. A self-sustaining acid-base reaction in semi-aqueous media at ambient condition enabled us to obtain BTO without contamination with impurities like Cl". Na" and k+. 3. Tbe disadvantages mentioned for the method 1 (i to v) and method 2 (i to iv) are overcome in the presently described route. 4. Stiochiometric. active, sub-micron sized BT powders are obtained by the pyrolysis of BTO at 750°C /4hr in air. 5. These BT powders gave dense compacts with 95% density after sintering at 1300°C for 4hr in air and showed dielectric properties comparable to the reported data in the literature (table 5). We Claim: a i. An improved process lor the preparation of barium titanyl oxalate (BTO) which comprises of reacting at room temperature a solution of titanium tctabutoxide in an organic solvent with a solution of oxalic acid in an organic solvent, to obtain i solution of an intermediate precursor oxalotitanic acid [H2TiO(C2O4)2: xH2O] (HTO) containing TiO(C2O4)2-2 species. adding to the said precurserbarium hydroxide in solid form directly or dispersed in an oragnic solvents in the oxalotitanic acid solution under constant stirring, followed by adding distilled water to initiate the acid-base reaction to obtain the barium titanyl oxalate. 2. A process as claimed in claim 1. wherein the organic solvent used arc melhanol. ethanol. isopropanol exemplified by melhanol. ethanol. isopropanol. 3. An improved process as claimed in claims 1 and 2. wherein the molar ratio of titanium tetrabutoxide and oxalic acid ranges from 1:2 to 1:4. 4. An improved process as claimed in claims 1 to 3. wherein the concentration of solid barium hydroxide dispersed in organic solvent is added equimolar or higher. 5. An improved process as claimed in claims 1 to 4. u herein barium ntanyi oxalate is used for the preparation of active barium titanate. 6. A process as claimed in claim i -5 wherein the barium many! oxalate (BTO)- having following characteristic.-,. Table 1. Microanalvsis data on as-dried precursor BTC).4H>() (Table Removed) Table 2. Chemical analysis data on as-dried precursor BTO.4H:O (Table Removed)* Watercontent determined from the microanaivsis data 7. An improved process for the preparation of barium titanyl oxalate as fully described herein before with reference to examples 1 to.34

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