Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF CERAMIC OXIDE POWDERS

Abstract

This invention relates to an improved process for the preparation of a soi useful for the preparation of spherical. po]ycrystalline plasma grade ceramic oxide powders in various compositions, e.g. Al2O3, Al2O3-TiO2, Al2O3-SiO2, ZrO2-Y2O3 etc, via a sol-emulsion-gel route in single step. The process drastically minimizes the processing steps like preparation of a hydrous precipitate, peptization of the hydrous precipitate. It does not require any rare chemicals and the gelling agent used herein, i.e. triethylamine (TEA) is much less expensive and more readily available than the high molecular weight amines (e.g. primene JMT).

Full Text

This invention relates to an improved process for the prepararion of ceramic oxide powders. This invention partxcularly relates to an improved process for the preparation of a sol usefull for file preparation of spherical, polycrystafline plasma grade ceramic oxide powders in a single step. This invention more particulaify relates to an imroved process for the preparation of plasma s{Hrayable, sph^cal ceramic oxide powders of various compositions, e.g. Al2O3, Al2O3,TiO2, Al2O3-SiO2, ZrO2-Y2O2 etc, via a scl-emulsion-^d route. These powders are used as the feed materials for thermal spraying in ceramic coating applications. The spherical morphology of the powder helps to maintmn a constant flow rate flirough the plasma hopper and consequently the fonnation of a uniform plasma-sprayed coating on the metal substrate. The powders, pr^aration of which is desct^)edin Hie present inventicHi, are used mainly for in apphcsAon of plasma sprayed coatings for diterent purposes, e.g. (i) Theimal bmrier (ii) Wear resistance (iii) Oxidation and coiTosion resistance (tv) Electrical insulation etc. and the potential areas of utilization are : apiculture, chemicals/petroleum, electronics, textiles etc. The mvcntion thus broadly relates to preparation of substances for the protection of metal parts exposed to drastic environmeatal and operational conditions, e.g. exc^tsive heat-load, wear/erosion, ccnrodon etc. The prior art of the prspantfkm of plasma-sprayable cemmic oxide powders is .presented below: (i) Melting process; (ii) Spray drying process and (iii) Sol-gel process In the xndting process, the individual oxides (for multicomponent systems) are blended together followed by heating above the melting temperatures. The melt is then rapidly cooled to (or quraiched at) the ambient teanperature. The soHdified mass is crushed and ground followed by sieving to the dedred particle size range. Reference may be made to F.G. Sherif and L.J. Shyu, J. Am. Ceram. Soc., 74, 375-80 (1991) for details of this method. The above method is associated with the following disadvantages : (i) lack of homogeneity in the final product, (it) requirement of high temperature for mehisg the components (since the individual oxide components generally have high melting points), (in) impurity pick up fi-om the mehing pot as well as during crushing and sieving operrations, (iv) dust hazards, (v) loss of considerable amount of materials duraig the prq>aration of powders of desired size and size distribution, (vi) generation of angular shaped material, which often fails to exhibit the required flow characteristics. In the spray diying process dther a mixed aqueous solution of suitable salts (e.g. chlorides, mtrates etc.) or a mixed sluny or suspen^on of suitable stalling mat^ials in presence of a Mnder is spray dried above 150°C under confessed air. The anhydrous precursor thus obtained is heat-treatod at a considerabfy high temperature to obtain the corresponding oxide powder. For details reference may be made to SJ. Lukasiewicz J. Am. Ceram. Soc. 72,617-24 (1989). The above metfiod suffers from the following iitnitations : (i) It is always difficult to prepare powders with tailor made size and size distribution by spniy diyrng (ii) addition of a binder may sometimes cause detrimental effect on (he ultimate powders, e.g. excess amount of osganic landers ofteb gives rise to burnout probkms while insufficient quantity yields products of low green strength, which in turn, creates severe loss of material during handling. In the sal-gel process, several variants arc possible which are described below in brief. The metihod involves the preparation of a sol using a metal salt (chloride, nitrate, sulphate etc.) as the startmg material. A hydrous pred^ntate k generally prq>ared by the addition of an ammonia solution to fte metal salt solution stf ambient tenq>erature under stirring. The electrdytes are removed from the hdrous precipitate by repeated washing with deionized water followed by filtration imder vacuum. The washed hydrous precipitate is then peptized (i.e. chemically broken into smalbr particles of coUotdal dimensions) witii a mmimum amount of add (HNO3, HCt etc.) at 80°-90°C, under Mgorous stirring until a stable cofioidal sol is obt^ed. The sol is then used to obtain the oxide microspheres Ihrpi^ an intermediate step of gel microsphere formation. Preparation of gel microsphere is done by ^ following two metiiods : (I) The sol is dispersed as aqueous dros^ets in a latfgc volume (> 200 times of that of the sol) of a dehydrating alcohol, e.g. 2-ethyM- hexwcd or n-butanol in presence of a surface active agent e.g. soibitan monookate (commrax^iaHy available as Span 80). The gel microspheres thus obtained arc washed and caldned for obtaining the oxide microsplieres.The process has been desctifoed in (a) J.L. Woodhcad, J. Mater. Educ., 6, 887-925 (1984) and (b) J.G. Uu and D.L. Wilcox, Sr., J. Mater. Res., 10, 84-94 (1995) The mediod has ^e following dssadvanti^es: (i) the yield of the uitiniate powder is poor, (ii) too high vohime ratio of the alc (iii) the process is lime consuming, since it mvntves slow removal of water from the dispersed sol droplets, (iv) the gel microspheres prepared by this metiiod are veiy much prone to water absorption and often reversibly transform to the sol stage if kept in moisture. Storage of flie gel powd^ thus requires proper defflccaling cond^om. (2) Jn a variation of this medtod the sol, prepared usualfy by peptization, is dispersed as aqueous droplets in a water-immisciblc organic solvent, e.g. l,1,l-trichloroethane (TCE) in presence of a surface active agent under cottfimious sttrring. The dispersed sol droplets are subjected to gel formation with a gelling agent, e.g. primene JMT, a hi^ molecular weight ( ~350) primaiy amine. The gel microspheres thus obtained are was^ied and calcined at the rcqitired tcn^ieraturc to obtain die oxide microspheres. The method has been described in J. L. Woodhead, J.Mater. Educ, 6, 887-925 (1984). The main disadvantages associated with this method are as follows : (i) The peptization method for die preparation of the precuisor sol is not suitable for all the cations, since all hydrated precipitates arc not peptizable at ambient temperature. Hydroxides of Al3+ can be cited as example; baycrite, Al(OH)3 formed from Al3+ with NH4OH at ambient temperature is not peptizabte with mineral acids. On the other hand, only boehmite (AlOOH) an oxyhydroxide of Al3+ ,which can be prepared by the hydrolysis of Al3+ sah wim ammonia solution at 90°C, is peptizable. (is) High molecular weight amines, generally uaed as me gelling agents (e.g. primene JMT) are also expensive and are not amongst the readily available chemicals. fiii) The method involves several steps, viz. precipitation of the metal cation, peptization of the hydrated precipitate, dispersion of the sol-droplets in an immiscible organic solvent, gelation of the sol-droplet to obtain the gel microspheres, washing of the gel microspheres and finally the calcination of die gel to obtain the ceramic oxide powders. As a result, the processing tee becomes too long and me proceas requires careful monitoring at each step. In the process of the present invention, a single-step process has been provided for the preparation of spherical, plasma-spray grade ceramic oxide powders via a sol-cmulsion-gel method. The main object of the present invention is to provide an improved process for the preparation of ceramic oxide powders which obviates the drawbacks of the hitherto known processes. Another objective of the present invention is to provide a single-step process for the preparation of ceramic oxide powders. Yet another object is to provide an improved process for the preparation of spherical noncrystalline plasma grade ceramic oxide powders. Still another object is to provide an improved process for the preparation of ceramic oxide powders which does not require any rare and expensive chemicals, thereby making the process economical. Accordingly, the present invention provides an improved process for the preparation of ceramic oxide powders, which comprises; i) preparing an aqueous solution of one or more water soluble metal salts as herein described filtering the said solution to remove impurities, optionally adding, to the said solution a soluble salt of one or more guest cation, such as TiCl4 YINO3)3 ii) adding 25-50% a water immiscible organic solvent as herein described under stirring to the resultant solution obtained in step (i) to obtain an emulsion; iii) deionizing the emulsion obtained in step (ii) at ambient temperature using an organic extractant, such as triettylomine in organic solvent as defined above keeping the ratio of the salt solution and the extractamt 1:1 to 1:3 and allowmg the whole system to stand for 1.5hr. under sturing to obtain an emulsion containing a deionized sol; iv) adding a nonionic surfactant as herein described and remaining 50% of organic solvent containing 8-10 voI% of spamgo to the emulsion containing a deionized sol2 under stirring to obtain a stable emulsion, allowing to equiliboate for 15-20 minados. v) Gelling the stable emulsion obtained in step (iv) by using triethyl amine in organic solvent under stirring till the mixture attains an alkaline pH, of 8-9, keeping the gel microspheses under stirring for 10-15 minutes, vi) filtering the microsphres under vacuum, washing with solvent as herein described, air drying at temperature in the range of 100-110°C for a period of 1-2 hrs, vii) calcining the dried gel microsphares at 1000-1300°C for a period of 1-2 hr. to obtain ceramic oxide powders. In an embodiment of the present invention, an aqueous solution may be prepared using deionized water at room temperature. i' In another embodiment, the water soluble metaj salts used may be such as AICI3.6H2O, ZrOCl2.8H2O,Al2(SO4)3. 18H2Oetc. In yet another embodiment, the soluble salts of the guest cations used may be such as TiCU, Y(NO3)3 etc. In still another embodiment, the water-immiscible organic solvent used may be such as 1,1,1-trich]oroethane, xylene, heptane, hexane etc. Ill yet another embodiment, deionization may be effected at ambient temperature. In another etnbodimeitt, the organic extractant used for ddomzation may be such as trielhylamtne. In still another embodiment, the non-fonic surfactant used may be such as sodntan mbnooleate, sorbitan monostearate, etc. In another embodiment, the gelling may be effected using an organic amine such as triethylamine, diethylamine. In yet another embodiment, removal of mipurities may be done by known methods, such as filtration under vacuum, multiple washing with lower alcohols and ketones, such as methanol and acetone, diying at 100°-110°C for a period of 1-2 h. Yet in another embodiment, the calcination may be done at a temperature as per requisition, may be such as from 1000°-1300°C for a period of 1-2 h. In the process of the present invention, here is profvided an improved process for the preparation of ceramic oxide microspher^i of plasma-sprayable quality m a single step through in-situ gpneration of precursm' sols. Hie process broadly comprises the following Operations : (i) Dissolution of metal salts (soluble chloride, ratrate, jndphate, e.g. AlCl3.6H2O or ZrOClz . 8H2O ,Al2(SO)3, 18H2O) in deionizcd water at room temperature followed by filtration for obtaining a clear solution, the "parent solution". (ii) Addition of soluble salt of the guest catiom e.g. TiCl4, Y(NO3)3 if necessary to the parent solution. (in) Addition of 25-50% by volume of an watcr-anmisdble organic solvent e.g. 1,1,1,-trichloroefliane (TCE), xylene, hexane, heptiaie etc (required for emulsification) to (1) for facititattng the mechanical stirring of the system. (iv) Deanionization (removal of the amons) of ihc salt solution at ambietit temperature using an organic ^ttractant e.g. tricfli^amme (VBA) in any of &e above organic solvent, e.g. heptane under constant stirring. (v) Addition of the required amount of a non-ionic surface active agent (sofhitan monooleate) under the stirred condition aloc^ wifii the remaining 75-50% of the otganic gotvent, e.g. heptane to form a stable emulsion. (vi) Gelation of tfie sol-droplets to yield gel microspheres with the addition of an organic amine sohiiion (e.g. TEA in heptane) maintataining the sdning conditions. Addition of the amine is to be continued until the stined mixture attains an alka&ne pH. (v) Filtration of fte gel microspheres under vacumn and subsequent washings wifli acetone and mcflianoL (vi) Air drying tfie gel microspheres at 100°-110°C for a period of l-2h, (vn) Calcination of the dried gel micro^heres at 1000°-1300°C (depending on tfie case) for a period of 1-2h to obtain the oxide mirosperes alter a pfe-heat treatment at around 200"C. Acccnding to the embodiment of die present invention the alumina or zirconia based oxide microspheres were prepared in following gmeral way : Aluminium chloride (AlCl3. 6H2O) or asrconyl chloride (ZrOCl2.8H2O) was dissolved in deionized water so as to olHain a dear solution (iiltered if needed) of the metal salt,in a concentration range of 1.0 to 3.0 M, prcfisrabfy around 2.0 M. Soluble salts of the chosen or required guest cation, e..g. Ti4+, Di4+, Y3 etc. was added to the parent metal solution in requisite quantity to obtain the bicompcment solution of the chosen composition. To this solution was added an oiganic diluent imntiscible with water, e.g. heptane, hexane .xylene , TCE. The volume of the organic diluent was twice that of the volume of the aqueous metal solution. The mixture was then subjected to mechanical storing. The hydrolytic acid (hydrochloric acid in me present invention) concentration in the aqueous phase was then reduced i.e. dechlorination with the addition of TEA (extractant) solution in heptane in the concentration range of 1.2 M to 3.6M preferably in the range of 1.4 M to 1.8 M under mechanical stirring. An aqueous metal sol was thus generated in~situ in the aqueous/organic biphasic mixture with the gradual depletion of the chloride ion in the aqueous phase. The volume ratio of the initial aqueous metal solution to the TEA/heptane was in the range of 1:1 to \*3 and would vary with the varying TEA concentration in heptane. The whole mass was allowed to stand for 1 to 2 h. A solution of Span 80, me surfactant in heptane, was men added to the stirred mixture of the aqueous sol and the organic diluent now containing the extracted hydrolytic acid. The volume of heptane added at this stage was again twice that of the original aqueous metal ion solution and me amount of Span 80 was between 5 and 12 volume per cent preferably between 8 and 10 vol% of heptane. The emulsion thus resulted was allowed to equilibrate for 15 to 20 minutes; gel formation was men accomplished by the addition of TEA in heptane. Concentration of TEA in heptane used for gelation step was between IA M and 2.4 M The addition of TEA/heptane solution was continued till the pH of the resulting mixture reached a value between 8 and 9. The entire experiment was carried out at ambient temperature and under constant stirring. Stirring was continued for 10-15 minutes even after me formation of the gel spheres. The gel microspheres mus obtained were coflectcd by filtration under vacuum in a Bucbner system and was washed several times with acetone and finally with methanol. The washed microspheres were then dried in air at 100°-110°C for 1-2 h and thereafter preheated at 200°C to remove most of the volatiles. Calcination of the dried gel microspheres at 1000°C to 1300°C (as the case required) for 1-2 h yielded the oxide microspheres. The preferred experimental parameters for the present invention are as follows : (i) Concentration of aluminium salt (or zirconium salt) 1.5-2.5 M in the initial solution (ii) Amount of water-immiscible organic solvent required 3 to 5 limes of the for emulsion formation (except the amount added volume of the initial as diluent with TEA ) metal solution (preferably 4 times) (vi) Concentration of the extractant, TEA 1 2 to 3.6 M in the organic solvent (Preferably 1.4 tol.8M) (iv) Volume ratio of the salt solution and the 1:1 to 1:3 extractant for deanioni zation (v) Amount of the surfactant (Span 80) used 5-12 vol% of organic solvent (preferably 8-10 vol%) required for emulsification (vi) Time allowed for equilibration of 15 - 20 mm aqueous sol droplets in organic solvent (vii) Stirring time after gel sphere formation 10 - 15 min The invention is described herein in details in the following examples, which are cited by way of illustration only and therefor* should not be construed to limit the scope of the invention. Example 1 48.3 g of aluminium chloride hexahydrate (AICI3.6 H2O) was dissolved in 100 ml of deionized water to produce a solution of aluminium concentration [Al3+] of 2 M 25 ml of 2 M Al3+ solution was taken. To it was added 50 ml of heptane. Initial volume ratio of aqueous to organic phase was thus 1:2. The mixture was men subjected to a mechanical stirring with a speed of 150 rpm. 75 ml of 1.4 M TEA in heptane was then slowly added to the stirred mixutre keeping the stirring speed constant at 150 rpm. The entire material was allowed to settle for 1.5 h. Another 50 ml of heptane loaded with 10 ml of sorbitan monooleate (commercially available Span 80), the surfactant, was then introduced into the mixture with the resumption of the mechanical stirring at the stirring speed mentioned above. The emulsion thus formed was allowed to equilibrate for 20 minutes. Gel sphere formation was accomplished by adding 1.4M TEA in heptane untfl a pH of 8-9 was noted. Stirring was continued for further 15 minutes. Hie gel spheres were then separated by filtration under vacuum and washed with acetone, dried at 100°C, pre-heated at 200°C and finally calcined at 1000°O1100°C to obtain α-Al2O2 microspheres. The powders exhibited a particle size distribution of 4-30 micron. Example :2 48.3 g of aluminium chloride hexahydrate (AlCl3.6H2O) was dissolved in 50 ml of deionized water and filtered; to mis was added 26.46 g of an ammonia stabilized silica sol containing 15.12% (w/w) SiO2 as estimated and the volume was made up to 100 ml. The resulting solution was 2.0 M whn respect to [Al3+] and was equivalent to 3 Al2O3.2SiO2 by composition. 25 ml of this solution was taken and 50 ml of TCE was added. The initial volume ratio of aqueous to organic phase was thus 1:2. The mixture was then subjected to a mechanical stirring with a speed of 150 rpm. 25 ml oi 1-4 M TEA in TCE was then slowly added to the stirred mixture. The entire material wis allowed to stand for 1.5 h. Another 50 ml of TCE loaded with 10 ml Span 80 was then added to the mixture with the resumption of mechanical stirring at the stirring speed men#°ned above. The emulsion thus generated was allowed to equilibrate for 20 minutes. Gel sphere formation was then accomplished by adding 1.4 M TEAflTCE until a pH within 8-9 was reached. Stirring of the whole mass was continued for further 15 minutes. The gel spheres were collected by filtration under vacuum and washed with acetone and methanol dried at 100 C in air, preheated at 200°C and finally calcined at 1300°C to obtain the oxide microspheres with composition of 3Al2O3.2SiO2 i.e. mat of mullite. A particle size distribution of 5-35 micron was noted. Example 3 36.25 g of aluminium chloride hexahydrate (AlCl3.6H2O) was taken and to this was added 3.7 ml of 3.86 M titanium tetrachloride (TiCU) solution. The volume was men made to 100 ml. The resultant solution was thus 1.5 M with respect to [ Al3+ ] and was equivalent to Al2O3 - 13 wt% TiO2 composition. 50 ml. xylene was then added to 25 ml of the said solution. Initial volume ratio of the aqueous to organic pfrase was thus 1:2. The mixture was subjected to a mechanical stirring with a speed of 150 ipm. 37.5 ml of 2.4 M TEA in xylene was then slowly added to the stirred mixture. The entire material was allowed to settle for 1.5 b. Another 50 ml of xylene loaded with 10 ml of Span 80 (the surfactant) was then added to the mixture with the resumption of the tflechamcal stirring. The emulsion thus formed was allowed to equilibrate for 20 minutes. Gel sphere formation was then acomplishcd by adding 1.8 M TEA in xylene until a pH within 8-9 was attained. Stirring of the whole mass was continued for further 15 minutes. The gel spheres thus obtained were men collected by filtration under vacuum and washed with acetone and methanol, dried at 100°C for lh, pre-heated at 200°C and finally calcined at 1000°C to yield the oxide microsphere* of composition Al2O3 -13 wt% TiO2. The oxide powder contained a-Al203 and ruffle and had a particle size distribution of 5-30 micron. Example 4. 80.6 g of zirconium oxychloride octahydrate (Z1OCI2.8H2O) was dissolved in 100 ml. of deionized water to prepare a solution with Zr4+ concentration of 2.5 M. To mis solution 5.45 g of hydrous yttrium nitrate [Y(NO3)3] was added to make the solution equivalent to ZrO2-5 wt% Y2O3 by composition. The solution was then filtered and 25 ml of the filtered solution was taken. 50 ml of hexane was added to the aqueous metal ion solution to make the initial volume ratio of aqueous to organic phase 1:2 - The mixture was subjected to mechanical stirring with a speed of 150 rpm. 25 ml of 1-4 M TEA in hexane was then slowly added to the stirred mixture. The entire material was then aflowed to stand for 1.5 h. A further 50 ml of hexane loaded with 8 ml of Span 80 (&e surfactant) was added to the mixture with the resumption of stirring at the stirring speed mentioned above. The emulsion thus generated was allowed to equilibrate for 20 minutes. Gel sphere formation was men accomplished by adding 1.4 M TEA in hexane until the pH within 8-9 was noted. Stirring of the whole mass was continued for further 15 minutes. The gel spheres were then collected by filtration under vacuum, washed with acetone and methanol, dried at 100°C for 1 h, pre-heated at 200°C and was finally calcined at 1200°C to obtain oxide microspheres of composition ZrO2-5wt% Y2O3. The calcined powder was found to contain monoclinic ZrOz as the major phase (about 85%) and tetragonal ZrO2 as the minor phase (about 15%). A particle size distribution of 5-50 micron was recorded for this powder. The main advantages of the improved process for the present invention are: (i) It drastically minimizes the processing steps like preparation of a hydrous precipitate, peptization of the hydrous precipitate etc. (ii) It does not require any rare chemicals and the gelling agent used herein, i.e. triethylamine (TEA) is much less expensive and more readily available than the high molecular weight amines (e. g. primene JMT). (iii) By varying the experimental parameters, e.g. concentration of the metal ions, speed of agitation, volume ratio of the aqueous sol to the water-immiscible organic solvent, amount of the surfactant (Span 80) etc., the particle size distribution as well as the average particle size can easily be tailored according to the necessity. We Claim: '1. An improved process for the preparation of ceramic oxide powders, which comprises; i) preparing an aqueous solution of one or more water soluble metal salts as herein described, filtering the said solution to remove impurities, optionally adding, to the said solution a soluble salt of one or more guest cation, such as TiCl4, Y(NO3)3 ii) adding 25-50% a water immiscible organic solvent as herein described under stirring to the resultant solution obtained in step (i) to obtain an emulsion; iii) deionizing the emulsion obtained m step (ii) at ambiant temperature using an organic extractant such as triehtylamine in organic solvent as defined above keeping the ratio of the salt solution and the extractamt 1:1 to 1:3 and allowing the whole system to stand for 1.5hr under stirring to obtain an emulsion containing a deionized sol; iv) adding a nonionic surfactant as herein described and remaining 50% of organic solvent containing 8-10 voJ% of span 80 to the emulsion containing a deionized sol under stirring to obtain a stable emulsion, allowing to equilibrate for 15-20 minutes, v) gelling the stable emulsion obtained in step (iv) by using triethyl amine in organic solvent under stirring till the mixture attains an alkaline pH of 8-9, keeping the gel microspheres under stirring for 10-15 minutes, vi) filtering the microsphres under vacuum, washing with solvent as herein described, air drying at temperature in the range of 100-110°C for a period of 1 -2 hrs, vii) calcining the dried gel microsphares at 1000-1300°C for a period of 1-2 hr. to obtain ceramic oxide powders. 2. An improved process as claimed in claims 1 wherein an aqueous solution is prepared using deionized water at room temperature. 3. An improved process as claimed in claims 1 and 2 wherein the water soulubie metal salt is dissolved in deionized water to obtain a clear solution having a molarity of 1.5 - 2.5 M to obtain a parent solution. 4. An improved process as claimed in claims 1 to 3 wherein water-immiscible organic solvent used in step (ii) is selected from 1,1, 1-trichIoroethane (TCE), xylene, hexane, heptane 5. An improved process as claimed in claims 1 to 4 wherein the solvent used for washing in step (vi) is selected from acetone and methanol. 6. An improved process for the preparation of ceramic oxide powders substantially as herein described with reference to examples.

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