PROCESS FOR PREPARING A PRECURSOR OF ZIRCONIUM DIOXIDE

Abstract: A precursor of zirconium dioxide in form of submicronic particles is obtained by: ▪ hydrolysing a zirconium alkoxide by bringing said zirconium alkoxide into contact with a microemulsion formed by an external oil phase, an internal aqueous phase, an emulsifier and a co-emulsifier; ▪ discontinuing the reaction of hydrolysis in order to control the growth of the particles; and ▪ recovering the precursor of zirconium dioxide from the reaction mixture. Such a precursor can be submitted to treatments of calcination in order to prepare zirconium dioxide for ceramic uses.

Description: The present invention relates to a process for preparing a precursor of zirconium dioxide in submicronic particle form, suitable for being converted into zirconium dioxide for ceramic uses. Zirconium dioxide is a material widely used in the art. For example, its catalytic characteristics in the processes of pyrolysis of hydrocarbons and in several organic reactions are known. Owing to its high melting temperature (of about 2,700°C), zirconium dioxide has been used for long time in the manufacture of ceramic and refractory materials. In particular, glass industry makes large use of it, due to its good mechanical characteristics and its inertness towards concentrated acids and alkalies. With these characteristics, those combine, hich relate to the high refractive index, to the low coefficient of thermal expansion and to the high electrical resistance of this product. Zirconium dioxide has recently found an use in the manufacture of ceramic components for electronic uses, both in the sector of semiconductors, and in the sector of superconductors, for example in alloy with alluminum and yttrium oxides. In these uses, zirconium dioxide is required in the form of submicronic particles, which should be as uniform and regular as possible, for the purpose of obtaining a ceramic material homogeneous in all of its points, endowed with high mechanical characteristics, and capable of generating highly reliable end products for use in electronics. Various processes have been proposed in the art, in order to obtain zirconium dioxide in submicronic form. Such processes are mainly based on the controlled hydrolysis of zirconium alkoxides: more particularly, hydrolysis of Zr alkoxides and precipitation in alcoholic media are described by B.Fegley et al, Bull.Am.Cer.Soc., 63 , 374 (1983) and by H.K.Bowen et al in Advances in Ceramics, Am.Cer.Soc.Press, Columbus, Ohio, (1984), Vol.9, Page 140. Hydrolysis and precipitation from emulsions is disclosed by A.Bagley in MIT Report N°54, Jan.1985. Moreover, JP-A-63185802 discloses a process for preparing ultrafine particles of ZrO₂ by adding a metal alkoxide to a 4-component W/O-type microemulsion of water, a nonpolar organic solvent, an emulsifier and an alkanol. The processes based on hydrolysis of Zr alkoxides require an accurate control of the hydrolysis kinetics to reproducibly obtain hydrolysate particles of the desired size and are difficult to control the nucleation process even in dilute systems, and this invention has the objective of overcoming the drawbacks outlined above. It has been ascertained that the hydrolysis kinetics of Zr alkoxides can be easily controlled whenever the hydrolyzing water is the internal phase of a water-in-oil microemulsion. A water-in-oil-microemulsion is a transparent, thermodynamically stable colloidal dispersion composed of an external oily phase, an internal aqueous phase, an emulsifier and a coemulsifier, wherein the particle size of the internal phase is lower than one fourth of the wavelength of the visible light. In particular, the present Applicant found, according to the present invention, that the hydrolysis of zirconium alkoxides, in such a microemulsion, proceeds rapidly until particles of a precursor of zirconium dioxide are formed, which are of a size similar to the size of the dispersed phase, without any substantial further particle growth. In this way, controllably and reproducibly obtaining a precursor which can be transformed by calcination into ceramic-grade zirconium dioxide results possible. Furthermore, inasmuch as said microemulsion is stable also when it contains substantially large concentrations of the dispersed phase, said precursor can be produced with good productivity values. In accordance therewith, the present invention relates to a process for preparing a submicronically particulated zirconium dioxide precursor by contacting a zirconium alkoxide with a microemulsion consisting of an external oily phase, an internal aqueous phase. An emulsifier and a co-emulsifer, comprising the steps of: (a) preparing a microemulsion in which the internal aqueous phase particle-size is from 5 nm to 20 nm, the amount of water being equal to, or up to 20% larger than, the amount which is stoichiometrically required for hydrolyzing the zirconium alkoxide; (b) hydrolyzing the zirconium alkoxide concerned in an acidic environment, at room temperature, and for a time of from 15 s to 2 min, and (c) discontinuing the hydrolysis reaction, to stop the growth of the solid zirconium dioxide precursor particles, by adding an aliphatic alcohol to the reaction mixture. The hydrolysis is carried out in an acidic environment, and in particular, at acidic pH values comprised within the range down to 1. It was found that an acidic medium can favour the homogeneousness of the particles of the hydrolysate, and the obtainment of particles having an even shape. Zirconium alkoxides useful for the purposes of the present invention are those alkoxides which contain from 2 to 4 carbon atoms in the alkyl-oxy moiety of their molecule, such as, e.g., zirconium tetraethoxide, zirconium tetra-n-propoxide, zirconium tetra-n-butoxide and zirconium tetra-sec-butoxide. The oil phase, which constitutes the external phase of the microemulsion, should necessarily be inert towards zirconium alkoxides, and water should be insoluble, or substantially insoluble, in said oil phase. In accordance therewith, aliphatic hydrocarbons and aromatic hydrocarbons liquid under room conditions are useful constituents of the oil phase. For the intended purpose, toluene, nonane, decane, undecane and relevant mixtures are preferably used. The emulsifier used in the microemulsion according to the present invention is an oil-soluble surfactant of non-ionic or anionic type. In order to meet the requirements of oil-solubility, such a surfactant should have an HLB (hydrophile-lipophile balance) lower than 10. The co-emulsifier used in the microemulsion according to the present invention is a liquid aliphatic alcohol containing from 4 to 6 carbon atoms in its molecule. Preferred co-emulsifier is butanol. The above disclosed microemulsion is spontaneously formed by means of the simple mixing of the constituents thereof. One could proceed by dissolving the emulsifier in the oil and then adding the other two constituents to the solution. The so obtained microemulsion is transparent and thermodynamically stable. The particles of the internal phase show a maximal size of the order of 20 nm (200 Å), and generally comprised within the range of from 5 nm to 20 nm (50 to 200 Å). The microemulsion maintains its characteristics of stability within a wide range of concentrations of its constituents. More particularly, the oil phase can be present in an amount comprised within the range of from 35 to 80% by weight, the aqueous phase within the range of from 0,5 to 10% by weight, the emulsifier within the range of from 1 to 20% by weight, and the co-emulsifier within the range of from 1 to 20% by weight. According to a form of practical embodiment of the process of the present invention, the process is carried out in a neutral medium, and zirconium alkoxide is fed and brought into contact with the preformed microemulsion, by operating at room temperature (20-25°C), or at temperature values close to room temperature. According to the invention, the hydrolysis is carried out under acidic pH conditions, using a mineral acid, such as, e.g., hydrochloric acid, as the hydrolysis catalyst and in this case, the preferred procedure consists in placing a first mixture containing a zirconium alkoxide, oil, emulsifier and co-emulsifier, into contact with a second mixture containing the aqueous solution of mineral acid, oil, emulsifier and co-emulsifier, still operating within the global composition limits as reported hereinabove. In any case, water contained in the microemulsion shall be in an amount equal to, or slightly larger than (e.g., up to 20% larger) than the stoichiometrically amount required by the reaction of hydrolysis of zirconium alkoxide. Under the above pointed out conditions, a fast hydrolysis of zirconium alkoxide is obtained at room temperature (20-25°C), within reaction times comprised within the range of from about 15s to about 2 min, and generally of the order of from 1 to 2 min, with a suspension of solid particles of the precursor of zirconium dioxide being formed. The reaction is then discontinued by means of the addition of an aliphatic alcohol, e.g., butanol. The solid particles are finally separated from the reaction medium, e.g., by centrifugation, and are washed. The precursor of zirconium dioxide is thus obtained in the form of submicronic particles, with a cubic prismatic crystal habit. The size of these particles is smaller than 0,4 µm, and is generally comprised within the range of from 0,2 to 0,3 µm. Such a precursor can be submitted to the usual treatments of calcination at high temperatures in order to remove the hydration water and any possible organic residues, and obtain a ceramic powder suitable for being processed in the sinterization processes. The following experimental examples are given in order to better explain the invention. Example 1 A microemulsion is prepared, which has the following composition, as expressed as % by weight: ▪ toluene 52,25% ▪ n-butanol 25,23% ▪ sodium bis(2-ethylhexyl)sulfosuccinate 21,02% ▪ water 1,50% In particular, in the preparation of said microemulsion, sodium bis(2-ethylhexyl)sulfosuccinate is first dissolved in toluene, and to the so obtained solution n-butanol and water are then added, with water being preliminarily bidistilled and filtered on 0.2 µm-Millipore filter. The operations of formation of the microemulsion are carried out at room temperature. The microemulsion is then filtered on an 0.45 µm-Teflon® membrane, so as to remove any residual solid particles. To 33,3 g of this microemulsion, 0,42 g of zirconium tetra-n-propoxide is added. Zirconium tetra-n-propoxide is added by means of a pipette with a Teflon tip kept dipped inside the microemulsion, while this latter is being kept stirred inside a cylindrical container under a nitrogen blanketing atmosphere. By operating at room temperature, the mass is kept stirred for a 2-minutes time. At the end of the reaction time, the hydrolysis is stopped by adding n-butananol in an amount of 10:1 by volume, relatively to the volume of the reaction mixture, and the solid is recovered by centrifugation and is washed with alcohol and water. At the analysis, the so obtained precursor of zirconium dioxide appears to be in the farm of particles of size smaller than 1 µm. Example 2 The process is carried out in the same way as in Example 1, with a microemulsion on being prepared, which has the following composition, as expressed as % by weight: ▪ toluene 52,23% ▪ n-butanol 25,25% ▪ sodium bis(2-ethylhexyl)sulfosuccinate 21,02% ▪ water 1,50% To 33,3 g of this microemulsion, 0,21 g of zirconium tetra-n-propoxide is added. By proceeding in the same way as disclosed in Example 1, a precursor of zirconium dioxide is obtained in the form of particles of size smaller than 1 µm. Example 3 A mixture is prepared, which contains toluene (53% by weight), n-butanol (25,6% by weight), and sodium bis(2-ethylhexyl)sulfosuccinate (21,34% by weight). This mixture is subdivided into two portions. To the first portion, zirconium tetraoxy-n-propoxide is added, and to the second portion 2 N aqueous hydrochloric acid is added. Said portions are combined, and the overall composition of the mixture, as % by weight, is as follows: ▪ toluene 47,10% ▪ n-butanol 22,6 % ▪ sodium bis(2-ethylhexyl)sulfosuccinate 18,9 % ▪ zirconium tetra-n-propoxide 0,43%, and ▪ 2 N aqueous hydrochloric acid 10,9 % The hydrolysis is carried out in the same way as disclosed in the preceding examples. A precursor of zirconium dioxide is obtained, in the form of homogeneous particles of size smaller than 1 µm. Example 4 The process is carried out in the same way as in Example 3, with a composition being prepared, with contains, as expressed as % by weight: ▪ toluene 46,0 % ▪ n-butanol 22,2 % ▪ sodium bis(2-ethylhexyl)sulfosuccinate 18,5 % ▪ zirconium tetra-n-propoxide 2,2 %, and ▪ 2 N aqueous hydrochloric acid 11,1 % The hydrolysis is carried out in the same way as disclosed in the preceding examples. A precursor of zirconium dioxide is obtained, in the form of homogenous particles of size of the order of 1 µm.

Claims: A process for preparing a submicronically particulated zirconium dioxide precursor by contacting a zirconium alkoxide with a microemulsion consisting of an external oily phase, an internal aqueous phase, an emulsifier and a co-emulsifer, comprising the steps of: (a) preparing a microemulsion in which the internal aqueous phase particle-size is from 5 nm to 20 nm, the amount of water being equal to, or up to 20% larger than, the amount which is stoichiometrically required for hydrolyzing the zirconium alkoxide; (b) hydrolyzing the zirconium alkoxide concerned in an acidic environment, at room temperature, and for a time of from 15 s to 2 min, and (c) discontinuing the hydrolysis reaction, to stop the growth of the solid zirconium dioxide precursor particles, by adding an aliphatic alcohol to the reaction mixture. Process according to Claim 1, wherein step (b) is carried out at a pH of from 7 to 1. Process according to Claim 1, wherein the zirconium alkoxide contains from 2 to 4 carbon atoms in the alkyl-oxy moiety of its molecule. Process according to Claim 3, wherein the zirconium alkoxide is selected from zirconium tetraethoxide, zirconium tetra-n.propoxide, zirconium tetra n.butoxide and zirconium tetra sec.butoxide. Process according to Claim 1, wherein the external oily phase consists of an aliphatic or an aromatic hydrocarbon which is liquid at room temperature. Process according to Claim 5, wherein said hydrocarbon is selected from toluene, nonane, decane, undecane and mixtures of any of the foregoing. Process according to Claim 1, wherein the emulsifier is an oil-soluble nonionic or anionic surfactant having an HLB (Hydrophile-Lipophile Balance) lower than 10. Process according to Claim 1, wherein the co-emulsifier is a C4-C6 aliphatic alcohol which is liquid at room temperature. Process according to Claim 8, wherein said alcohol is n.butanol. Process according to Claim 1, wherein the microemulsion contains from 35% to 80% by weight of the oily phase, from 0,5 to 10% by weight of the aqueous phase, from 1% to 20% by weight of the emulsifier, and from 1% to 20% by weight of the co-emulsifier. Process acording to Claim 1, wherein the hydrolysis time is from 1 min to 2 min.

Inventors: CASTELLANO, MAURIZIO
Applicants: ENIRICERCHE S.P.A.
Application year: 1989
Main classification: C01G25
IPC codes: C01G25, C01B13