Title of Invention	"AN IMPROVED PROCESS FOR THE PREPARATION OF A PRECURSOR SOL USEFUL FOR THE PREPARATION OF CRYSTALLINE FIBERS OF P URE ORDOPED ALUMINIUM OXIDE"
Abstract	An improved process for the preparation of a sol useful for the preparation of crystalline fibers of pure or doped aluminum oxide which comprises; i) dissolving hydrated aluminum chloride such as herein described, in water and obtaining a clear solution; ii)reacting the said AlCl3 solution with aluminum powder added in small amounts (molar ratio 1.7 to 2.1) under reflux conditions such as herein described, to obtain a precursor sol of predetermined aluminum content, iii) repeated reflux of the resultant followed for a period upto 8 hours by intermittent ageing at room temperature, iv) mixing dopant cations such as herein described, in the form of water soluble salts or sols with the resultant of step(iii) after reflux, v) heating the resultant sol at 40°-60°C for attaining proper fiberizability and spinnability.

Title of Invention

"AN IMPROVED PROCESS FOR THE PREPARATION OF A PRECURSOR SOL USEFUL FOR THE PREPARATION OF CRYSTALLINE FIBERS OF P URE ORDOPED ALUMINIUM OXIDE"

Abstract

An improved process for the preparation of a sol useful for the preparation of crystalline fibers of pure or doped aluminum oxide which comprises; i) dissolving hydrated aluminum chloride such as herein described, in water and obtaining a clear solution; ii)reacting the said AlCl3 solution with aluminum powder added in small amounts (molar ratio 1.7 to 2.1) under reflux conditions such as herein described, to obtain a precursor sol of predetermined aluminum content, iii) repeated reflux of the resultant followed for a period upto 8 hours by intermittent ageing at room temperature, iv) mixing dopant cations such as herein described, in the form of water soluble salts or sols with the resultant of step(iii) after reflux, v) heating the resultant sol at 40°-60°C for attaining proper fiberizability and spinnability.

Full Text	This invention relates to an improved process for the preparation of a precursor sol useful as the starting material for the preparation of gel fibres from the said sol and polycrystalline pure or doped aluminium oxide fibres containing at least 80-100% of A1203 prepared thereof. The polycrystalline fibres obtained from the sol, the preparation of which is described in the present invention, are used mainly in the following areas : (i) as insulating materials in high temperature furnaces after processing them into mats, boards etc. (ii) as reinforcement materials in composites with metals, ceramics, cement and other matrices. The invention thus broadly relates to high temperature furnaces, ceramics and metal matrix composites and cement industries. Starting materials for the preparation of aluminium oxide (or briefly, "alumina") or high-alumina fibres are known to have been prepared by the following major techniques : (a) Preparation based on hydrolysis of aluminium alkoxide : In this method, a solution is prepared of aluminium isopropoxide (an alkoxide of aluminium), citric acid and water. Through hydrolysis and other chemical reactions, a powdery solid is obtained, which is then dissolved in water. The solution is concentrated at 100°C to obtain a highly viscous sol which is spun to obtain precursor fibres. These fibres are calcined to obtain alumina fibres. In a variant of the above method, aluminium chloride hexahydrate, A1C13.6H20 is also added to the starting solution. The above invention is described by T. Nishio and Y. Fujiki (J. Ceram. Soc. Japan, vol. 98, pp. 1223-30, 1990). The above process had the following disadvantages : (i) Relatively high cost of alkoxide as starting material, (ii) Generation of inhomogeneous fibres in absence of AlCl3.6H2O and critical amount of citric acid, thus requiring strict compositional control. (iii) Essentiality of the presence of a volatile organic material in the system, i.e. citric acid which is expected to break down on heating to other volatile fractions (whose escape is likely to damage the fibres), and to carbon particles on the surface of the fibres. (iv) Aluminium alkoxides are highly prone to uncontrolled attack by water molecules, thus requiring slow and careful addition of water for homogeneous reaction. (b) Preparation based on slurries : In this method, fine particles containing at least 80% by weight of alumina are mixed in an aqueous phase in which an alumina precursor is dissolved. The solid particles are required to provide 13-97% by weight of the total available oxide. The slurry is extruded through a spinneret to obtain fibres which are prefired at 300-400°C and finally heated at high temperatures (1000-1800°C) to obtain the final product. The invention has been described in US Patent nos. 3,808,015 and 3,953,561. The above process has the following limitations: (i) Slurries are in general difficult to spin into fibres compared to sols or similiar liquid rich precursors, (ii) A prefiring step is required in the process, which means additional time and increase in energy expenditure, (iii) The final firing temperature is relatively high. (c) Preparation based on aluminium oxychloride : In this method, several variants are possible, which are described below in brief. (c.l) Aluminium oxychloride (also called basic aluminium chloride) is dissolved in water to obtain a solution. Gamma alumina powder of extremely small size (0.02 micron) is mixed with this solution, and an aqueous solution of polyethylene oxide is added to it and the mixture intimately blended. The resulting mixture is concentrated to the desired viscosity and spun through a spinneret to obtain fibres. The fibres are prefired at 300°-800°C and finally fired at 1500°C to obtain alpha alumina. The details are described by K. Koba et el. in U.S. Patent no. 4,812,271. The limitations are: (i) A prefiring stage is required. (ii) An organic 'component has to be added, which causes problems during prefiring (and final firing) as discussed above in the prior art (a). (c.2) An aluminium oxychloride solution is prepared by dissolving aluminium trichloride in water and digesting the required quantity of aluminium foil in it under reflux at 100°C. An aluminium formoacetate solution is prepared separately by digesting aluminium foil in formic acid, acetic acid and water in required proportions under reflux at 100°C. Another solution containing polyvinyl pyrollidone (PVP) dissolved in water is also prepared. The three solutions are mixed and lactic acid and nitric acid are also added to this solution mixture. An yttria sol (containing yttrium ions) is added to obtain alumina-yttria fibres. This step, however, is not essential for fiberizability of the precursor. In this and other inventions, liquid sources of various cations have been added to diversify the product range and product properties. The precursor is spun in a spinneret to obtain continuous fibres, which are heated in nitrogen and air atmospheres up to about 1000°C. This invention is described by Budd and Wilson in U.S. Patent no. 5,217,933. The major drawback of the process is the addition of a variety of organic materials which are liable to decompose to fugitive components and carbon (specially because a heating in nitrogen atmosphere is involved) at different temperatures and cause defects like cracks, pores etc. leading to degradation of fibre quality. Wood, Wilson and Sowman, in European Patent EP 0 294 208 A2 have used a nitroformoacetate solution of aluminium as one of the constituents of the precursor. (c.3) In a similar but simpler method, aluminium oxychloride is prepared by dissolving aluminium in hydrochloric acid. An aqueous solution of polyvinyl alcohol (PVA) is added to it to obtain the precursor for fibre spinning. The fibres thus obtained are calcined to 1250°C with a hold at 300°C. The invention has been described by Hosoi, Endo, Ando and Shoji in U.S. Patent no. 4,931,239. (c.4) In a similar method, alumina-phosphorus oxide fibres are prepared from concentrated solutions of aluminium oxychloride (made from aluminium chloride and aluminium metal), a phosphorus salt and PVP or aluminium formoacetate or both. Carbon deposition has been observed in most heated fibres. In absence of PVP, the fibres were either black (due to carbon) or friable and weak, depending upon the temperature of calcination. The invention is embodied in Indian Patent no. 169482 by Sowman and Tai-Thi Than. The above accounts of available information' on the preparation of pure and doped (with other oxides like 5' Y2O3, etc) alumina fibres reveal that: (a) The liquid-rich precursors contain (i) fine alumina particles in the form of a slurry or (ii) aluminium oxychloride solutions along with or without organic salts of aluminium, e.g. aluminium formacetate. (b) In addition the precursors also contain, as a matter of rule, other organic polymers like polyvinyl alcohol, polyvinyl pyrollidone, etc., presumably to achieve fiberizability of precursor. The deleteriosu effect of organic salts or polymers on the process of oxide fibre formation from precursor bibres by heat-treatment to about 600°-1000°C has been indicated earlier in this document, and is also revealed in some of the patented or published accounts. Considering the above points, a process has been invented for the preparation fo a precursor sol suitable for obtaining pure or doped alumina fibres. The process of preparation of the precursor is characterized by the superiority over the prior art that no organic component was necessary for obtaining fiberizability or spinnability of the precursor, thereby avoiding depostion of carbon and formation of pores and cracks during the calcination stage. Accordingly an improved process for the preparation of a sol useful for the preparation of crystalline fibers of pure or doped aluminum oxide which comprises; i) dissolving hydrated aluminum chloride in deionized water to obtain a clear solution having 1-1.5 MA1C13, ii) reacting the said AlCl3 solution with aluminum powder added in small amounts (molar ratio 1.7 to 2.1) under reflux conditions such as herein described, to obtain a precursor sol of predetermined aluminum content, iii) repeated reflux of the resultant followed for a period upto 8 hours by intermittent ageing at room temperature, iv) mixing dopant cations such as herein described, in the form of water soluble salts or sols with the resultant of step(iii) after reflux, v) heating the resultant sol at 40°-60°C for attaining proper fiberizability and spinnability Thus according to the present invention there is provided an improved process for the preparation of sols useful for the preparation of polycrystalline pure and doped alumina fibres which comprises: i) dissolving aluminum chloride (AlCl3. 6H2O) in water at ambient temperature for obtaining a clear solution; ii) heating the said aluminum chloride solution to 100°C; iii) adding fine aluminum (Al) powder to the said heated solution under reflux in incremental doses so as to obtain a clear solution after each dose and continuing this process for 6-8 hrs; iv) allowing an ageing period of 12-18 hrs at room temperature in between the 6-8 hrs steps of addition of aluminum powder as in (iii) above; v) allowing the solution, obtained after complete addition of Al powder, a further reflux of 4-5 hrs. vi) adding as an option solutions or sols containing one or more of the following cat ions : Si4+ , Cr3+'Fe3+,Mg2+etc; vii) further ageing of the sol thus obtained at the temperature of 40°-60°C until spin ability is obtained. According to an embodiment of the present invention gel fibres are spun in bunches through a spinneret system with 100-200 holes of 0.4 mm diameter, using the sol ready for spinning; heating the resulting gel fibers at a temperature in the range 900°-1200°C for a period in the range 0.5-1 h for obtaining the oxide fibers. In the following, the above process is described in some more detail. Aluminum chloride is dissolved in deionized water to obtain a clear solution, preferably in a concentration of 1-1.5 M. It is then heated under reflux conditions. As soon as the addition of aluminium (Al) powder starts, the temperature of the heating system is reduced to 70°-80°C; the exothermicity of the reaction due to the above addition assures maintenance of the reflux conditions. The rate of addition of Al powder is dependent on the volume of the A1C13 solution, increasing proportionately with increase in volume of AlCl3 solution. On addition of Al powder, the solution assumes a relatively basic character and the Al cation forms a hydrated species and finally, polymeric particles are obtained. A sol is thus formed. As an option, solutions or sols of a variety of cations are added to the above sol under stirring. The additions are in the form of colloidal silica sol or nitrates or chlorides of Zr, Cr, Mg etc. The sol thus prepared is aged at 40°-60°C until proper viscosity for fibre spinning is attained. The durations of ageing at the corresponding temperatures are important because at shorter durations than that prescribed under given conditions, fibres seem to be obtainable when a glass rod is dipped in the sol and taken out gradually. However, such sols with low ageing period cannot be spun and hand drawn fibres are thick and often recede back to the glass rod. Gel fibres may be obtained by spinning the sol prepared as described above. The spun gel fibres are dried at a temperature in the range of 100°-200°C after collection from the spinneret system. They are then calcined for crystallisation at 950° (delta -phase) to 1300°C (alpha -phase) for 1.0-0.5h. The preferred conditions for carrying out of the present invention are : (i) Concentration of aluminium chloride = 1.0 - 1.5 M in the initial solution (ii) A1/C1 molar ratio after addition of = 1.7-2.1 Al powder (iii) Temperature during Al powder = 100°C addition (iv) Time period of each phase of Al = 6 - 8 h. powder addition (v) Ageing period at room temperature = 15 - 17 h. between two phases of addition of Al powder (vi) Ageing period at reflux temperature = 4 - 5 h. after complete addition of Al powder (vii) Viscosity of a spinnable sol = 300-500 P. The preferred conditions for processing of the spun gel fibres are : (i) Temperature for gel fibre drying = 100°-200°C (ii) Soaking period at the drying ' = 1 - 3 h. temperature (iii) Calcination (crystallization) = 900°-1300°C temperature (iv) Soaking period of calcination = 0.5 - 1.0 h. The invention is described in detail in the following examples which are given by way of illustration only and therefore should not be construed to limit the scope of the invention. EXAMPLE 1 60 ml of 1 M A1C13 solution was prepared by dissolving 14.5 g of AlClq.SH2O powder in deionized water until a clear solution was obtained. The 1 M AlClq solution thus prepared was heated to 100°C followed by the addition of fine aluminium powder. 8.1 g of Al powder was added under reflux conditions in order to obtain a final sol composition of Al : Cl = 2 : 1 after reflux. The addition and dissolution of Al powder was continued for a period of 7 h followed by room temperature ageing for a period of 15 h. After the last addition of Al powder, the reflux was continued for an additional 5 h. The total time of reflux was 21 h and the ageing time was 30 h. After completion of the reflux, a clear sol having a viscosity of 24 cps and pH 3.2 was obtained. Equivalent Al2O3 content of this sol was 32%. The refluxed solution was then heated at 40°-60°C till a viscosity of around 450 P was reached. This viscous sol was then poured in a centrifugal spinning apparatus having a spinneret cup with 140 holes of 0.4 mm diameter. Fibres in mat form having a diameter of 10-12 micron were obtained. These fibres had very good flexibility and handleability. The fibres were transparent to opaque depending upon the conditions of spinning and sticking between the fibres in the wet condition was negligible. These gel fibres were then calcined in air according to the following schedule : (a) room temperature to 200°C with 2 h hold, 200°-500°C at 60°C/h and holding for 1 h, 500°-950°C at 250°C/h and holding for 1h. (b) room temperature to 200°C with 2 h hold, 200°-500°C at 60°C/h and holding for 1 h, 500°-1200°C at 250°C/h and holding for 0.5 h. Fibres calcined at 950°C were yellowish white to white in colour, flexible and reasonably strong. Microscopic examination revealed a smooth fibre surface. The phase obtained by X-ray diffraction was primarily delta-alumina. Fibres calcined at 1200°C were white in colour, weaker and less flexible than those calcined at 950°C. The primary crystal phase as identified by X-ray diffraction was alpha-alumina. EXAMPLE 2 500 ml of 1 M A1C13 solution was prepared by dissolving 120.1 g of AlClq.6H2O in deionized water until a clear solution was obtained. Reflux was carried out in the same manner as described in Example 1. 67.5 g of Al powder was added in order to get a A1:C1 ratio of 2:1 in the final sol. A period of 8 h aluminium addition was followed by room temperature ageing for 15 h. The total time of reflux was 20 h and ageing time was 32 h. After completion of reflux a sol having a viscosity of 27 cps and pH 3.4 was obtained. Equivalent A1703 content in the sol was 34%. This sol was then heated at 40°-60°C to a viscosity of about 500 P. This viscous sol was poured in a centrifugal spinneret and fibres in mat form having a diameter of 10-12 micron were obtained. The gel fibres obtained were then calcined following the two heating schedules described in Example 1 at 950° and 1200°C. The fibres after calcination had diameters of 3-5 micron. Fibers calcined at 950°C had a delta-alumina structure whereas those calcined at 1200°C had an alpha-alumina structure. Fibres calcined at 950°C were smooth and yellowish white to white in colour. They were strong and flexible. Fibres calcined at 1200°C were weaker and more friable as compared to the fibres calcined at 950C. EXAMPLE 3 100 ml of 1 M AlCl3 solution was prepared by dissolving 24.41 g of AlCl3.6H2O in deionized water until a clear solution was obtained. Reflux was carried out in the same manner as described in Example 1. 9.4 g Al powder was added in order to get an A1:C1 ratio of 1.7:1 in the final sol. Total time of reflux was 21 h and ageing time was 30 h. The sol after reflux had a pH of 3.1 and a viscosity of 22 cps and equivalent A1203 content was 30%. This sol was heated at 40°-60°C until a viscosity of about 500 P was reached. This viscous sol was poured in a centrifugal spinneret and gel fibres having a diameter of 10-12 micron were obtained. The gel fibres obtained were calcined following the schedules described in Example 1 and final calcined fibres having diameter of 3-5 micron were obtained. Fibres calcined at 950°C had a delta-alumina structure whereas fibres calcined at 1200°C had an alpha-alumina structure. Fibres calcined at 950°C were strong and yellowish white to white in colour. They were smooth and flexible as indicated by observations using optical and scanning electron microscopy. Fibres calcined at 1200°C were white, weak, friable and had a rough fibre surface as compared to the fibres calcined at 950°C. EXAMPLE 4 100 ml of 1.5 M AlCl3 solution was prepared by dissolving 36.21 g of AlCl3.6H2O in deionized water until a clear solution was obtained. Reflux of this solution by addition of Al powder was carried out as described in Examples 1-3. 15.39 g of Al powder was added in order to get an A1:C1 ratio of 1.7:1. The total time of reflux was 21 h and ageing time was 30 h. Final sol had a viscosity of 30 cps and pH of 3.5. This sol was then heated at 40°-60°C till the viscosity of the sol reached about 500 P. Fibres were spun from this sol as described in Example 1. Fibres obtained were then calcined at 950°C and 1200°C following the heating schedules as described in Example 1. The fibres calcined at 950°C were yellowish white to white in colour, flexible, strong and had delta-alumina crystalline form. The fibres calcined at 1200°C were white, weak and friable as compared to the fibres calcined at 950°C and had alpha-alumina crystalline form. Diameter of the gel fibres was 10-12 micron while that of the calcined fibres was 3-5 micron. EXAMPLE 5 100 ml of 1.0 M AlCl3 solution was prepared in the same manner as described in Example 3. 13.5 g of Al powder was then added slowly in order to obtain an A1:C1 ratio of 2 : 1 in the final sol after reflux. The total time of reflux was 21 h and the ageing time was 30 h. To this sol was added 3.9 g of colloidal silica sol containing 33 % Si02 so as to obtain a SiOo equivalent of 4%, w/w in the final oxide product. The sol was then heated at 40°-60°C until a viscosity of about 400 P was reached. The sol was then centrifugally spun, as described in Example 1, in order to obtain gel fibres having a composition of Al2O3-4wt% SiO2o. Fibres were calcined at 950°C and 1200°C following the heating schedules described in Example 1. Fibres calcined at 950°C were yellowish white to white in colour, strong, flexible and had delta-alumina crystalline form. Fibres calcined at 1200°C were white, strong but less flexible than those calcined at 950°C and had alpha-alumina crystalline form. Diameter of the gel fibres was 10-12 micron while that of the calcined fibres was 3-5 micron. EXAMPLE 6 100 ml of 1.0 M AlCl3 solution was prepared as described in Example 3 and then 13.5 g of Al powder was added under reflux at 100°C. The AlrCl ratio of the final sol after reflux was 2:1. The total time of reflux was 21 h and the ageing time was 30 h. To this sol was added 6.48 g of MgCl2-6H20 to yield fibres of composition of Al203-4wt%MgO. Fibres were spun from this sol in the same manner as described in Example 1. The gel fibres obtained were then calcined at 950°C and 1200°C following the same heating schedules mentioned in Example 1. Fibres calcined at 950°C were yellowish white to white, strong and flexible and had delta-alumina crystalline form. Fibres calcined at 1200°C were white, moderately strong but less flexible than those calcined at 950°C and had an alpha-alumina crystalline form. Diameter of the gel fibres was 10-12 micron while that of the calcined fibres was 3-5 micron. EXAMPLE 7 To 100 ml AlCl3 solution (prepared as described in Example 3), 13.5 g of Al powder was added under reflux to yield a final sol with an A1:C1 ratio of 2:1. The total time of reflux was 21 h and the ageing time was 30 h. To this solution was added 4.16 g of ZrOClo.SHoO to give an ultimate oxide product of composition Al2O3-5wt . %ZrO2. Fibres were then spun from this sol as described in Example 1 and crystallised at 950°C and 1200°C following the same heating schedules as described in Example 1. Fibres calcined at 950°C were yellowish white to white in color, strong, flexible and had a delta-alumina crystalline form. Fibres calcined at 1200°C were white, moderately strong and had an alpha-alumina crystalline form. Diameter of the gel fibres was between 10 and 12 micron while that of the calcined fibres was between 3 and 5 micron. EXAMPLE 8 A spinnable alumina sol was prepared in the same manner as described in Example 7. 3 ml of chromia sol obtained by dechlorinating a 3.5 M Cr solution having a Cl~ /Cr+0 ratio of 2.45 with the help of solvent extraction technique, was then added to the alumina sol. The mixed sol was then heated as in earlier examples and spun to produce gel fibres as described in Example 1. The final composition of the oxide fibres was Al203-4wt.% Cr2O3. The gel fibres were then calcined at 950°C and 1200°C following the same heating schedules as described in Example 1 . Fibres calcined at 950°C werC we green in colour, weak, friable and had delta - alumina crystalline form. Fibres calcined at 1200° were green in colour, weak, friable and had alpha - alumina crystalline form. Dimer of the gel fibre was 10-12 micron whereas that of the calcined fibres was 3 -5 micron

Full Text

This invention relates to an improved process for the preparation of a precursor sol useful as the starting material for the preparation of gel fibres from the said sol and polycrystalline pure or doped aluminium oxide fibres containing at least 80-100% of A1203 prepared thereof. The polycrystalline fibres obtained from the sol, the preparation of which is described in the present invention, are used mainly in the following areas :
(i) as insulating materials in high temperature furnaces after processing them into mats, boards etc.
(ii) as reinforcement materials in composites with metals, ceramics, cement and other matrices.
The invention thus broadly relates to high temperature furnaces, ceramics and metal matrix composites and cement industries.
Starting materials for the preparation of aluminium oxide (or briefly, "alumina") or high-alumina fibres are known to have been prepared by the following major techniques :
(a) Preparation based on hydrolysis of aluminium alkoxide :
In this method, a solution is prepared of aluminium isopropoxide (an alkoxide of aluminium), citric acid and
water. Through hydrolysis and other chemical reactions, a powdery solid is obtained, which is then dissolved in water. The solution is concentrated at 100°C to obtain a highly viscous sol which is spun to obtain precursor fibres. These fibres are calcined to obtain alumina fibres. In a variant of the above method, aluminium chloride hexahydrate, A1C13.6H20 is also added to the starting solution. The above invention is described by T. Nishio and Y. Fujiki (J. Ceram. Soc. Japan, vol. 98, pp. 1223-30, 1990).
The above process had the following disadvantages : (i) Relatively high cost of alkoxide as starting material, (ii) Generation of inhomogeneous fibres in absence of AlCl3.6H2O and critical amount of citric acid, thus requiring strict compositional control.
(iii) Essentiality of the presence of a volatile organic material in the system, i.e. citric acid which is expected to break down on heating to other volatile fractions (whose escape is likely to damage the fibres), and to carbon particles on the surface of the fibres.
(iv) Aluminium alkoxides are highly prone to uncontrolled attack by water molecules, thus requiring slow and careful addition of water for homogeneous reaction.
(b) Preparation based on slurries :
In this method, fine particles containing at least 80% by weight of alumina are mixed in an aqueous phase in which an alumina precursor is dissolved. The solid particles
are required to provide 13-97% by weight of the total available oxide. The slurry is extruded through a spinneret to obtain fibres which are prefired at 300-400°C and finally heated at high temperatures (1000-1800°C) to obtain the final product. The invention has been described in US Patent nos. 3,808,015 and 3,953,561.
The above process has the following limitations: (i) Slurries are in general difficult to spin into fibres compared to sols or similiar liquid rich precursors, (ii) A prefiring step is required in the process, which means additional time and increase in energy expenditure, (iii) The final firing temperature is relatively high.
(c) Preparation based on aluminium oxychloride :
In this method, several variants are possible, which are described below in brief.
(c.l) Aluminium oxychloride (also called basic aluminium chloride) is dissolved in water to obtain a solution. Gamma alumina powder of extremely small size (0.02 micron) is mixed with this solution, and an aqueous solution of polyethylene oxide is added to it and the mixture intimately blended. The resulting mixture is concentrated to the desired viscosity and spun through a spinneret to obtain fibres. The fibres are prefired at 300°-800°C and finally fired at 1500°C to obtain alpha alumina. The details are described by K. Koba et el. in U.S. Patent no. 4,812,271.
The limitations are: (i) A prefiring stage is required.
(ii) An organic 'component has to be added, which causes problems during prefiring (and final firing) as discussed above in the prior art (a).
(c.2) An aluminium oxychloride solution is prepared by dissolving aluminium trichloride in water and digesting the required quantity of aluminium foil in it under reflux at 100°C. An aluminium formoacetate solution is prepared separately by digesting aluminium foil in formic acid, acetic acid and water in required proportions under reflux at 100°C. Another solution containing polyvinyl pyrollidone (PVP) dissolved in water is also prepared.
The three solutions are mixed and lactic acid and nitric acid are also added to this solution mixture. An yttria sol (containing yttrium ions) is added to obtain alumina-yttria fibres. This step, however, is not essential for fiberizability of the precursor. In this and other inventions, liquid sources of various cations have been added to diversify the product range and product properties. The precursor is spun in a spinneret to obtain continuous fibres, which are heated in nitrogen and air atmospheres up to about 1000°C. This invention is described by Budd and Wilson in U.S. Patent no. 5,217,933.
The major drawback of the process is the addition of a variety of organic materials which are liable to decompose
to fugitive components and carbon (specially because a heating in nitrogen atmosphere is involved) at different temperatures and cause defects like cracks, pores etc. leading to degradation of fibre quality. Wood, Wilson and Sowman, in European Patent EP 0 294 208 A2 have used a nitroformoacetate solution of aluminium as one of the constituents of the precursor.
(c.3) In a similar but simpler method, aluminium oxychloride is prepared by dissolving aluminium in hydrochloric acid. An aqueous solution of polyvinyl alcohol (PVA) is added to it to obtain the precursor for fibre spinning. The fibres thus obtained are calcined to 1250°C with a hold at 300°C. The invention has been described by Hosoi, Endo, Ando and Shoji in U.S. Patent no. 4,931,239.
(c.4) In a similar method, alumina-phosphorus oxide fibres are prepared from concentrated solutions of aluminium oxychloride (made from aluminium chloride and aluminium metal), a phosphorus salt and PVP or aluminium formoacetate or both. Carbon deposition has been observed in most heated fibres. In absence of PVP, the fibres were either black (due to carbon) or friable and weak, depending upon the temperature of calcination. The invention is embodied in Indian Patent no. 169482 by Sowman and Tai-Thi Than.
The above accounts of available information' on the preparation of pure and doped (with other oxides like 5'
Y2O3, etc) alumina fibres reveal that:
(a) The liquid-rich precursors contain (i) fine alumina particles in the form of a slurry or (ii)
aluminium oxychloride solutions along with or without organic salts of aluminium, e.g.
aluminium formacetate.
(b) In addition the precursors also contain, as a matter of rule, other organic polymers like
polyvinyl alcohol, polyvinyl pyrollidone, etc., presumably to achieve fiberizability of
precursor.
The deleteriosu effect of organic salts or polymers on the process of oxide fibre formation from precursor bibres by heat-treatment to about 600°-1000°C has been indicated earlier in this document, and is also revealed in some of the patented or published accounts.
Considering the above points, a process has been invented for the preparation fo a precursor sol suitable for obtaining pure or doped alumina fibres. The process of preparation of the precursor is characterized by the superiority over the prior art that no organic component was necessary for obtaining fiberizability or spinnability of the precursor, thereby avoiding depostion of carbon and formation of pores and cracks during the calcination stage.
Accordingly an improved process for the preparation of a sol useful for the preparation of crystalline fibers of pure or doped aluminum oxide which comprises; i) dissolving hydrated aluminum chloride in deionized water to obtain a clear solution having
1-1.5 MA1C13, ii) reacting the said AlCl3 solution with aluminum powder added in small amounts (molar
ratio 1.7 to 2.1) under reflux conditions such as herein described, to obtain a precursor sol of
predetermined aluminum content, iii) repeated reflux of the resultant followed for a period upto 8 hours by intermittent ageing at
room temperature, iv) mixing dopant cations such as herein described, in the form of water soluble salts or sols
with the resultant of step(iii) after reflux, v) heating the resultant sol at 40°-60°C for attaining proper fiberizability and spinnability
Thus according to the present invention there is provided an improved process for the preparation of sols useful for the preparation of polycrystalline pure and doped alumina fibres which comprises: i) dissolving aluminum chloride (AlCl3. 6H2O) in water at ambient temperature for obtaining a
clear solution;
ii) heating the said aluminum chloride solution to 100°C; iii) adding fine aluminum (Al) powder to the said heated solution under reflux in incremental doses
so as to obtain a clear solution after each dose and continuing this process for 6-8 hrs; iv) allowing an ageing period of 12-18 hrs at room temperature in between the 6-8 hrs steps of
addition of aluminum powder as in (iii) above;
v) allowing the solution, obtained after complete addition of Al powder, a further reflux of 4-5 hrs. vi) adding as an option solutions or sols containing one or more of the following cat ions : Si4+ ,
Cr3+'Fe3+,Mg2+etc; vii) further ageing of the sol thus obtained at the temperature of 40°-60°C until spin ability is
obtained.
According to an embodiment of the present invention gel fibres are spun in bunches through a spinneret system with 100-200 holes of 0.4 mm diameter, using the sol ready for spinning; heating the resulting gel fibers at a temperature in the range 900°-1200°C for a period in the range 0.5-1 h for obtaining the oxide fibers.
In the following, the above process is described in some more detail. Aluminum chloride is dissolved in deionized water to
obtain a clear solution, preferably in a concentration of 1-1.5 M. It is then heated under reflux conditions. As soon as the addition of aluminium (Al) powder starts, the temperature of the heating system is reduced to 70°-80°C; the exothermicity of the reaction due to the above addition assures maintenance of the reflux conditions. The rate of addition of Al powder is dependent on the volume of the A1C13 solution, increasing proportionately with increase in volume of AlCl3 solution.
On addition of Al powder, the solution assumes a relatively basic character and the Al cation forms a hydrated species and finally, polymeric particles are obtained. A sol is thus formed.
As an option, solutions or sols of a variety of cations are added to the above sol under stirring. The additions are in the form of colloidal silica sol or nitrates or chlorides of Zr, Cr, Mg etc.
The sol thus prepared is aged at 40°-60°C until proper viscosity for fibre spinning is attained. The durations of ageing at the corresponding temperatures are important because at shorter durations than that prescribed under given conditions, fibres seem to be obtainable when a glass rod is dipped in the sol and taken out gradually. However, such sols with low ageing period cannot be spun and hand drawn fibres are thick and often recede back to the glass rod.
Gel fibres may be obtained by spinning the sol prepared as described above. The spun gel fibres are dried at a temperature in the range of 100°-200°C after collection from the spinneret system. They are then calcined for crystallisation at 950° (delta -phase) to 1300°C (alpha -phase) for 1.0-0.5h.
The preferred conditions for carrying out of the present invention are :
(i) Concentration of aluminium chloride = 1.0 - 1.5 M
in the initial solution
(ii) A1/C1 molar ratio after addition of = 1.7-2.1
Al powder
(iii) Temperature during Al powder = 100°C
addition
(iv) Time period of each phase of Al = 6 - 8 h.
powder addition
(v) Ageing period at room temperature = 15 - 17 h. between two phases of addition of
Al powder
(vi) Ageing period at reflux temperature = 4 - 5 h. after complete addition of Al
powder
(vii) Viscosity of a spinnable sol = 300-500 P.
The preferred conditions for processing of the spun gel fibres are :
(i) Temperature for gel fibre drying = 100°-200°C
(ii) Soaking period at the drying ' = 1 - 3 h.
temperature
(iii) Calcination (crystallization) = 900°-1300°C
temperature
(iv) Soaking period of calcination = 0.5 - 1.0 h.
The invention is described in detail in the following examples which are given by way of illustration only and therefore should not be construed to limit the scope of the invention.
EXAMPLE 1
60 ml of 1 M A1C13 solution was prepared by dissolving 14.5 g of AlClq.SH2O powder in deionized water
until a clear solution was obtained. The 1 M AlClq solution
thus prepared was heated to 100°C followed by the addition of fine aluminium powder. 8.1 g of Al powder was added under reflux conditions in order to obtain a final sol composition of Al : Cl = 2 : 1 after reflux. The addition and dissolution of Al powder was continued for a period of 7 h followed by room temperature ageing for a period of 15 h. After the last addition of Al powder, the reflux was continued for an additional 5 h. The total time of reflux was 21 h and the ageing time was 30 h.
After completion of the reflux, a clear sol having a viscosity of 24 cps and pH 3.2 was obtained. Equivalent Al2O3 content of this sol was 32%.
The refluxed solution was then heated at 40°-60°C till a viscosity of around 450 P was reached. This viscous sol was then poured in a centrifugal spinning apparatus having a spinneret cup with 140 holes of 0.4 mm diameter. Fibres in
mat form having a diameter of 10-12 micron were obtained. These fibres had very good flexibility and handleability. The fibres were transparent to opaque depending upon the conditions of spinning and sticking between the fibres in the wet condition was negligible. These gel fibres were then calcined in air according to the following schedule :
(a) room temperature to 200°C with 2 h hold, 200°-500°C at
60°C/h and holding for 1 h, 500°-950°C at 250°C/h and holding
for 1h.
(b) room temperature to 200°C with 2 h hold, 200°-500°C at
60°C/h and holding for 1 h, 500°-1200°C at 250°C/h and holding
for 0.5 h.
Fibres calcined at 950°C were yellowish white to white in colour, flexible and reasonably strong. Microscopic examination revealed a smooth fibre surface. The phase obtained by X-ray diffraction was primarily delta-alumina. Fibres calcined at 1200°C were white in colour, weaker and less flexible than those calcined at 950°C. The primary crystal phase as identified by X-ray diffraction was alpha-alumina.
EXAMPLE 2
500 ml of 1 M A1C13 solution was prepared by dissolving 120.1 g of AlClq.6H2O in deionized water until a
clear solution was obtained. Reflux was carried out in the same manner as described in Example 1. 67.5 g of Al powder

was added in order to get a A1:C1 ratio of 2:1 in the final
sol. A period of 8 h aluminium addition was followed by room
temperature ageing for 15 h. The total time of reflux was 20
h and ageing time was 32 h.
After completion of reflux a sol having a viscosity of 27 cps and pH 3.4 was obtained. Equivalent A1703 content in the sol was 34%.
This sol was then heated at 40°-60°C to a viscosity of about 500 P. This viscous sol was poured in a centrifugal spinneret and fibres in mat form having a diameter of 10-12 micron were obtained.
The gel fibres obtained were then calcined following the two heating schedules described in Example 1 at 950° and 1200°C. The fibres after calcination had diameters of 3-5 micron. Fibers calcined at 950°C had a delta-alumina structure whereas those calcined at 1200°C had an alpha-alumina structure. Fibres calcined at 950°C were smooth and yellowish white to white in colour. They were strong and flexible. Fibres calcined at 1200°C were weaker and more friable as compared to the fibres calcined at 950C.
EXAMPLE 3
100 ml of 1 M AlCl3 solution was prepared by dissolving 24.41 g of AlCl3.6H2O in deionized water until a clear solution was obtained. Reflux was carried out in the
same manner as described in Example 1. 9.4 g Al powder was added in order to get an A1:C1 ratio of 1.7:1 in the final sol. Total time of reflux was 21 h and ageing time was 30 h.
The sol after reflux had a pH of 3.1 and a viscosity of 22 cps and equivalent A1203 content was 30%. This sol was heated at 40°-60°C until a viscosity of about 500 P was reached. This viscous sol was poured in a centrifugal spinneret and gel fibres having a diameter of 10-12 micron were obtained.
The gel fibres obtained were calcined following the schedules described in Example 1 and final calcined fibres having diameter of 3-5 micron were obtained.
Fibres calcined at 950°C had a delta-alumina structure whereas fibres calcined at 1200°C had an alpha-alumina structure. Fibres calcined at 950°C were strong and yellowish white to white in colour. They were smooth and flexible as indicated by observations using optical and scanning electron microscopy. Fibres calcined at 1200°C were white, weak, friable and had a rough fibre surface as compared to the fibres calcined at 950°C.
EXAMPLE 4
100 ml of 1.5 M AlCl3 solution was prepared by dissolving 36.21 g of AlCl3.6H2O in deionized water until a
clear solution was obtained. Reflux of this solution by addition of Al powder was carried out as described in Examples
1-3. 15.39 g of Al powder was added in order to get an A1:C1 ratio of 1.7:1. The total time of reflux was 21 h and ageing time was 30 h. Final sol had a viscosity of 30 cps and pH of 3.5. This sol was then heated at 40°-60°C till the viscosity of the sol reached about 500 P. Fibres were spun from this sol as described in Example 1. Fibres obtained were then calcined at 950°C and 1200°C following the heating schedules as described in Example 1. The fibres calcined at 950°C were yellowish white to white in colour, flexible, strong and had delta-alumina crystalline form. The fibres calcined at 1200°C were white, weak and friable as compared to the fibres calcined at 950°C and had alpha-alumina crystalline form. Diameter of the gel fibres was 10-12 micron while that of the calcined fibres was 3-5 micron.
EXAMPLE 5
100 ml of 1.0 M AlCl3 solution was prepared in the same manner as described in Example 3. 13.5 g of Al powder was then added slowly in order to obtain an A1:C1 ratio of 2 : 1 in the final sol after reflux. The total time of reflux was 21 h and the ageing time was 30 h. To this sol was added 3.9 g of colloidal silica sol containing 33 % Si02 so as to obtain
a SiOo equivalent of 4%, w/w in the final oxide product. The sol was then heated at 40°-60°C until a viscosity of about 400 P was reached. The sol was then centrifugally spun, as described in Example 1, in order to obtain gel fibres having a composition of Al2O3-4wt% SiO2o. Fibres were calcined at 950°C
and 1200°C following the heating schedules described in Example 1. Fibres calcined at 950°C were yellowish white to white in colour, strong, flexible and had delta-alumina crystalline form. Fibres calcined at 1200°C were white, strong but less flexible than those calcined at 950°C and had alpha-alumina crystalline form. Diameter of the gel fibres was 10-12 micron while that of the calcined fibres was 3-5 micron.
EXAMPLE 6
100 ml of 1.0 M AlCl3 solution was prepared as described in Example 3 and then 13.5 g of Al powder was added under reflux at 100°C. The AlrCl ratio of the final sol after reflux was 2:1. The total time of reflux was 21 h and the ageing time was 30 h. To this sol was added 6.48 g of MgCl2-6H20 to yield fibres of composition of Al203-4wt%MgO. Fibres were spun from this sol in the same manner as described in Example 1. The gel fibres obtained were then calcined at 950°C and 1200°C following the same heating schedules mentioned in Example 1. Fibres calcined at 950°C were yellowish white to white, strong and flexible and had delta-alumina crystalline form. Fibres calcined at 1200°C were white, moderately strong but less flexible than those calcined at 950°C and had an alpha-alumina crystalline form. Diameter of the gel fibres was 10-12 micron while that of the calcined fibres was 3-5 micron.
EXAMPLE 7
To 100 ml AlCl3 solution (prepared as described in Example 3), 13.5 g of Al powder was added under reflux to yield a final sol with an A1:C1 ratio of 2:1. The total time of reflux was 21 h and the ageing time was 30 h. To this solution was added 4.16 g of ZrOClo.SHoO to give an ultimate
oxide product of composition Al2O3-5wt . %ZrO2. Fibres were then spun from this sol as described in Example 1 and crystallised at 950°C and 1200°C following the same heating schedules as described in Example 1. Fibres calcined at 950°C were yellowish white to white in color, strong, flexible and had a delta-alumina crystalline form. Fibres calcined at 1200°C were white, moderately strong and had an alpha-alumina crystalline form. Diameter of the gel fibres was between 10 and 12 micron while that of the calcined fibres was between 3 and 5 micron.
EXAMPLE 8
A spinnable alumina sol was prepared in the same manner as described in Example 7. 3 ml of chromia sol obtained by dechlorinating a 3.5 M Cr solution having a Cl~ /Cr+0 ratio of 2.45 with the help of solvent extraction technique, was then added to the alumina sol. The mixed sol was then heated as in earlier examples and spun to produce gel fibres as described in Example 1. The final composition of the oxide fibres was Al203-4wt.% Cr2O3. The gel fibres were then calcined at 950°C and 1200°C following the same heating

schedules as described in Example 1 . Fibres calcined at 950°C werC we green in colour, weak, friable and had delta - alumina crystalline form. Fibres calcined at 1200° were green in colour, weak, friable and had alpha - alumina crystalline form. Dimer of the gel fibre was 10-12 micron whereas that of the calcined fibres was 3 -5 micron

Documents:

689-del-1996-abstract.pdf

689-del-1996-claims.pdf

689-del-1996-correspondence-others.pdf

689-del-1996-correspondence-po.pdf

689-del-1996-description (complete).pdf

689-del-1996-form-1.pdf

689-del-1996-form-2.pdf

689-del-1996-form-3.pdf

689-del-1996-form-4.pdf

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Patent Number

195801

Indian Patent Application Number

689/DEL/1996

PG Journal Number

31/2009

Publication Date

31-Jul-2009

Grant Date

07-Jul-2006

Date of Filing

29-Mar-1996

Name of Patentee

COUNCTL OF SCIENTIFC AND INDUSTRIAL, RESEARCH,

Applicant Address

RAFI MARG, NEW DELHI-110001, INDIA

Inventors:

#	Inventor's Name	Inventor's Address
1	RAMANAN VENKATESH	C.G.C.R.I. CALCUTTA, INDIA.
2	SILADITYA BANDYOPADHYAY	C.G.C.R.I. CALCUTTA
3	MINATI CHATTERJEE	C.G.C.R.I. CALCUTTA
4	PRASANTA KUMAR CHAKRABARTY	C.G.C.R.I. CALCUTTA
5	DIBYENDU GANGULI	C.G.C.R.I. CALCUTTA

PCT International Classification Number

B23B 17/04

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA