WO1995007858A1 - Process for forming large silica spheres by low temperature nucleation - Google Patents
Process for forming large silica spheres by low temperature nucleation Download PDFInfo
- Publication number
- WO1995007858A1 WO1995007858A1 PCT/US1994/010155 US9410155W WO9507858A1 WO 1995007858 A1 WO1995007858 A1 WO 1995007858A1 US 9410155 W US9410155 W US 9410155W WO 9507858 A1 WO9507858 A1 WO 9507858A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- microspheres
- particles
- precursor
- solution
- silica
- Prior art date
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B33/00—Silicon; Compounds thereof
- C01B33/113—Silicon oxides; Hydrates thereof
- C01B33/12—Silica; Hydrates thereof, e.g. lepidoic silicic acid
- C01B33/18—Preparation of finely divided silica neither in sol nor in gel form; After-treatment thereof
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/01—Particle morphology depicted by an image
- C01P2004/03—Particle morphology depicted by an image obtained by SEM
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/30—Particle morphology extending in three dimensions
- C01P2004/32—Spheres
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/51—Particles with a specific particle size distribution
- C01P2004/52—Particles with a specific particle size distribution highly monodisperse size distribution
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/51—Particles with a specific particle size distribution
- C01P2004/53—Particles with a specific particle size distribution bimodal size distribution
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01P—INDEXING SCHEME RELATING TO STRUCTURAL AND PHYSICAL ASPECTS OF SOLID INORGANIC COMPOUNDS
- C01P2004/00—Particle morphology
- C01P2004/60—Particles characterised by their size
- C01P2004/61—Micrometer sized, i.e. from 1-100 micrometer
Definitions
- This invention relates to an improved process for preparing small spheres of silica having many applications, such as for catalyst supports and in high pressure liquid chro atography. Such spheres are small, but very uniformly sized.
- Particles in the range of 0.1 to 1.0 ⁇ m were prepared by Yuasa et al., as discussed in U.S. Patent 4,567,030.
- the patentees incorporated metals from Groups I, II, III, and IV of the Periodic Table with the hydrolyzable silicon compounds such as tetraalkyl silicates to form particles containing both silica and the selected metals as their oxides. They state that if the content of the metal oxide is 0.01 to 15 mol percent a true sphere with a uniform particle size is obtained.
- the patentees also observe that the amount of water affects the ability to produce spherical shape. Also, they s .ite that it is ".
- a non-porous spherical particle of only silica was made by a different method discussed in U.S. Patent 4,775,520 to Unger et al. Their particles were said to have mean particle diameters of 0.05 to 10 ⁇ m, although no particles larger than 3.1 ⁇ m were actually exemplified.
- a two-stage process was employed which was similar to one method of Yuasa et al. in that continuous addition of a tetraalkoxysilane was used to increase the size of the particles.
- the patentees defined their process as adding the silica precursor solution to a preformed sol of seed particles at a rate at which substantially no additional particles were formed, that is, the number of particles initially added determined the number of spheres produced.
- Unger et al. It was characteristic of the particles formed by Unger et al. that they were non-porous and highly uniform. They assumed that pores in the original seed particles are sealed by the secondary growth and that new pores do not form. Unger et al. recommend using reaction temperatures between 35* and 75'C, preferably between 40 * and 65'C. They state that around room temperature larger particles are formed but that a wider range of particle sizes was produced. Their examples indicate that the reaction mixture was controlled at 40*C.
- Shimizu et al. in U.S. 4,842,837 disclose a process for making fine silica spheres less than 0.1 ⁇ m used as a polish for semi conductor wafers. Hydrolysis of an alkoxysilane was carried out above 30*C and the patentees indicated that lower temperatures were not desirable since larger spheres were formed. Their examples indicate that a constant temperature was maintained throughout the reaction.
- a hydrolyzable silica precursor such as a tetraalkoxysilane, preferably tetraethoxysilane (TEOS) , is reacted with water in a solution containing ammonia and an alcohol, preferably ethanol, in proportions such that two liquid phases would form.
- TEOS tetraethoxysilane
- the seed particles are grown to microspheres of at least about 2.5 ⁇ m by addition of portions of the reactants while maintaining the temperature of the mixture at about 15* to 45*C, particularly 35*-40*C.
- the added portions are at ambient temperature.
- the largest microspheres may be recovered and separated from the fines by a multi-step procedure of settling and decanting the supernatant liquid containing the fines. Any agglomerated spheres tend to settle first and may be eliminated by discarding the bottom of the settled bed of spheres. Alternatively, other methods may be used, such as filtering and sieving and the like.
- the monodisperse particles are useful in applications where uniformity of size is important.
- Figure 1 is a photomicrograph of spheres produced in Example 2.
- Figure 2 is a photomicrograph of spheres produced in Example 3.
- Figure 3 is a photomicrograph of spheres produced in Example 4.
- Figure 4 is a photomicrograph of spheres produced in Example 5.
- Figure 5 is a photomicrograph of spheres produced in Example 6.
- Figure 6 is a particle size histogram of the spheres produced in Example 6.
- the process is similar to that of Barder et al. in that it combines a hydrolyzable silica precursor, an alcohol (or a mixture of alcohols) , ammonia, and water in proportions such that two phases would form.
- a hydrolyzable silica precursor particularly a tetraalkoxysilane, represented by formula Si(OR) 4 where R is a lower alkyl group.
- Tetraethoxysilane also known as tetraethylorthosilicate (TEOS)
- TEOS tetraethylorthosilicate
- an alcohol solvent typically it will be the same as is produced by hydrolysis of the silica precursor but this is not required.
- a mixture of alcohols may be used if desired and it should be understood that "an alcohol n as used herein is meant to include such mixtures.
- the hydrolyzable silica precursor such as tetraethoxysilane, reacts with water to decompose into silica, probably via intermediate compounds which subsequently react further to provide silica.
- tetraethoxysilane is the silica precursor, the reaction with water produces ethanol and silica or the intermediate compounds formed as the ethoxy moieties react with water.
- Alkoxysilanes which include alkyl groups such as the alkyltrialkoxysilanes may be included in the precursor solutions to provide silica spheres containing alkyl moieties. Examples of such alkylalkoxysilanes are methyl triethoxysilane, ethyltriethoxysilane, and the like.
- the second precursor solution is an aqueous ammonia solution, also optionally containing an alcohol.
- an alcohol consistent with the alcohol produced by hydrolysis of the tetraalkoxysilane typically is employed.
- ethanol would be used when the silica precursor is tetraethoxysilane.
- other alcohols could be used, provided that spherical silica particles are formed.
- at least one of the precursor solutions will contain an alcohol, preferably an alkanol corresponding to that produced by the hydrolysis of the silica precursor.
- composition of the precursor solutions will be determined by the desired composition of the reacting mixture.
- the composition of each precursor solution may be adjusted and the rate at which the solution is added also may be varied to provide the desired composition in the reacting mixture. It will be evident that considerable flexibility is available in the process since the composition of the precursor solutions can be varied as well as their relative rates of addition.
- composition of the combined solutions will be such that the reaction mixture initially forms two phases.
- such compositions may be 20 to 50 wt.% silica precursor, 5 to 30 wt.% alkanol, 40 to 70 wt.% water, and 5 to 10 wt.% ammonia.
- Preferred compositions would be within the range of 25 to 35 wt.% silica precursor, 5 to 10 wt.% alkanol, 50 to 60 wt.% water, and 5 to 15 wt.% ammonia.
- the hydrolysis reaction produces an alcohol by ⁇ product which adds to the alcohol in the precursor solutions and the two phases initially present become miscible and only a single phase remains.
- the silica precursor is tetraethoxysilane and ethanol is the solvent, the combined and reacting solutions appear to become single phase when the solution reaches about 45 wt. percent ethanol.
- the preferred silica precursor tetraethoxysilane will be considered as it reacts with water in the presence of ammonia and ethanol and produces additional ethanol and silica or the intermediate ethoxysilanols.
- a molecule of tetraethoxysilane reacts with a molecule of water, a molecule of ethanol is formed, leaving the intermediate compound triethoxysilanol, (EtO) 3 Si(OH) , which can be further reacted with another molecule of water or with another molecule of tetraethoxysilane or an intermediate thereof.
- the finished particles are substantially only silica, the composition of the reacting surface of any individual particle at any time during the process is unknown, but it seems likely that the particle will contain both silica and the intermediate products at the same time. It seems probable also that the presence of the by-product ethanol will influence the access of water molecules to the reacting silicon compounds, but the actual effect is not known. However, it is observed that when two phases are present in the reacting solution, which is vigorously agitated, reacting tetraethoxysilane forms a dispersed phase in an ethanol-water ammonia continuous phase. It is believed that the particles of tetraethoxysilane serve to saturate the surrounding solution at all times. Once enough ethanol has been produced, the tetraethoxysilane is completely dissolved and the amount of tetraethoxysilane available for reaction is determined by the addition rate.
- the particles can be grown by continuing to add the precursor solutions until the microspheres reach a size of about 2.5 ⁇ m at which time new seed particles begin to be formed to create a polymodal distribution of particles sizes.
- the precursor solutions will continue to be added at the same rates used during the two-phase reaction and the spheres grown at an accelerated rate.
- the addition rates can be increased or decreased as the spheres grow.
- the precursor solutions may be added continuously or periodically since both have been found to produce similar results.
- the microspheres have a relatively high surface area, but such porous spheres may be converted to low porosity spheres by recovering and redispersing the porous spheres in water for up to about 24 hours after addition of the precursor solutions has ceased. Also lengthy contact with the solution after the addition of the silica precursor has ceased will tend to produce low surface area spheres. There is no need to add the silica precursor at a low rate to close the pores as taught by Unger et al.
- the porosity may be maintained by harvesting the spheres at once and avoiding further contact with water, for example by storing them in ethanol. While the principal objective is to generate monodisperse (i.e.
- microspheres polydisperse microspheres may be desired in certain applications. Consequently, the process of the invention can be carried out so that either large monodisperse spheres of 2.5 ⁇ m diameter or greater are produced or alternatively polydisperse spheres having a maximum diameter of at least 2.5 ⁇ m up to 10 ⁇ m or even larger diameter if desired. There is no known upper limit to the sphere diameter which might be produced by the process, although practical considerations appear to make the process less attractive above about 10 ⁇ m.
- microspheres having a diameter of about 2.5-3 ⁇ m can be made without the presence of significant amounts of fines.
- the larger particles are made along with smaller ones (i.e. below 2.5 ⁇ m) and these polydisperse microspheres may be recovered together for appropriate uses where monodispersity is not required or where polydispersity is advantageous.
- a monodisperse group of large microspheres i.e. above 2.5-3 ⁇ m
- Fine particles remain suspended and are removed with the supernatant liquid; any agglomerates tend to settle first and can be eliminated by discarding the bottom portion of the settled bed.
- the feed solutions are premixed and placed in reservoirs where they are added with stirring to a reactor vessel stepwise using premeasured volumes over predetermined time intervals.
- the reservoir containing the tetraalkoxysilane is kept under a nitrogen purge to exclude moisture.
- a hydrolysis mixture containing 38.5 vol.% deionized water, 13.5 vol.% ethanol, and 48 vol % aqueous ammonia (29.5 wt.% NH 3 ) and then 100 mL of tetraethoxysilane (TEOS) , both of the liquids and the beaker having been previously cooled to 4.5'C.
- TEOS tetraethoxysilane
- Example 2 Preparation of 2.5 um Spheres The procedure of Example 1 was repeated except that the initial temperature was lowered to 1*C, the stirring speed was reduced to 150 rpm, and a total of 1100 mL of TEOS and 2200 L hydrolysis solution were added. The product again was found to be monodisperse, that is, to have a uniform diameter of 2.5 ⁇ m with only a small amount of agglomerated particles and fines, as may be seen in the photomicrograph of Figure 1.
- Example 2 The procedure of Example 2 was repeated for the nucleation step. Then, six portions each of 200 mL hydrolysis solution and 100 mL TEOS were added every five minutes (for a total of 1400 mL hydrolysis solution and 700 mL TEOS) while the temperature of the mixture rose to and was maintained at about 40 * C. The slurry mixture was transferred to a 4-liter beaker and stirring continued at 150 rpm. Nine additional portions of each of the two reacting solutions were added at five minute intervals until a total of 1600 mL TEOS and 3200 mL hydrolysis solution were in the beaker. A large number of fine particles were observed.
- Example 3 The second half of the slurry separated in Example 3 was placed in a 4-liter beaker and 2 liters of water was added with stirring. After the mixture was well dispersed, stirring was stopped and the particles were allowed to settle for 8 hours. The supernatant liquid was decanted and the remaining bed of particles was redispersed in 2 liters of water and the procedure repeated for two additional times, except that the period for settling was reduced from 8 hours to 2 hours. After the final settling period, the solids collected were dried at 70*C for at least 12 hours and then examined by SEM. The results shown in the photomicrograph of Figure 3 illustrate the effectiveness of the procedure in separating the large 3.5 ⁇ m spheres from the small ones.
- Example 2 The procedure of Example 2 was repeated except that a total of 2700 mL TEOS and 5400 mL hydrolysis solution were added.
- the spheres Were examined by an optical microscope and found to have a diameter of about 4 ⁇ m, with smaller particles present as before.
- the slurry was divided in half and additional 500 mL TEOS and 1000 mL hydrolysis solution added stepwise in 100 mL and 200 mL portions as before. Examination of the particles by SEM shows in Figure 4 that the large spheres had been grown to about 5 ⁇ m size. Many fine particles were also present.
- Example 6 Separation of 5 um Particles
- the product slurry of Example 5 was stirred with 3 liters of water in a 4-liter beaker until they were well dispersed and then allowed to settle for 8 hours. As before, the supernatant liquid was decanted and the procedure was repeated three times. The settled bed from the final decantation was dried at 70*C for at least 12 hours an then examined by SEM. The separated particles are shown in Figure 5 and compared with the particles prior to the separation process.
- the separated product was analyzed with a Coulter Multisizer II after first being dispersed in a solution of an aqueous electrolyte solution supplied by Coulter. The results are displayed in Figure 6, where it can be seen that the spheres are predominantly 5.1 ⁇ m in diameter.
- Example 7 (Comparative. The procedure of Example 1 was repeated except that the initial solutions were at room temperature (-23*C) rather than 4.5*C and a total of 1300 L of TEOS and 2400 L of hydrolysis solution were added, rather than the 900 mL TEOS and 1800 mL hydrolysis solution of Example 1. A sample was removed from the resulting slurry and examined under the scanning electron microscope. The microspheres were found to have a diameter of 1.3 ⁇ m.
- Example 1 made microspheres having a diameter of 2.5 ⁇ m, although much less TEOS was added. Thus, it is clear that many fewer, but larger, microspheres must have been formed using the process of the invention.
Abstract
Description
Claims
Priority Applications (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP7509263A JPH09502693A (en) | 1993-09-17 | 1994-09-13 | A method for forming large silica spheres by low temperature nucleation. |
EP94929162A EP0719237A1 (en) | 1993-09-17 | 1994-09-13 | Process for forming large silica spheres by low temperature nucleation |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US08/123,283 | 1993-09-17 | ||
US08/123,283 US5425930A (en) | 1993-09-17 | 1993-09-17 | Process for forming large silica spheres by low temperature nucleation |
Publications (1)
Publication Number | Publication Date |
---|---|
WO1995007858A1 true WO1995007858A1 (en) | 1995-03-23 |
Family
ID=22407765
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/US1994/010155 WO1995007858A1 (en) | 1993-09-17 | 1994-09-13 | Process for forming large silica spheres by low temperature nucleation |
Country Status (4)
Country | Link |
---|---|
US (1) | US5425930A (en) |
EP (1) | EP0719237A1 (en) |
JP (1) | JPH09502693A (en) |
WO (1) | WO1995007858A1 (en) |
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WO1998048453A1 (en) | 1997-04-23 | 1998-10-29 | Advanced Chemical Systems International, Inc. | Planarization compositions for cmp of interlayer dielectrics |
US6686035B2 (en) | 1999-02-05 | 2004-02-03 | Waters Investments Limited | Porous inorganic/organic hybrid particles for chromatographic separations and process for their preparation |
US6334880B1 (en) | 1999-12-07 | 2002-01-01 | Silbond Corporation | Abrasive media and aqueous slurries for chemical mechanical polishing and planarization |
US6528167B2 (en) | 2001-01-31 | 2003-03-04 | Waters Investments Limited | Porous hybrid particles with organic groups removed from the surface |
JP4216716B2 (en) * | 2001-08-09 | 2009-01-28 | ウォーターズ・インヴェストメンツ・リミテッド | Porous organic / inorganic hybrid monolithic material for chromatographic separation and method for producing the same |
GB2414993B (en) | 2002-10-30 | 2007-07-11 | Waters Investments Ltd | Porous inorganic/organic homogenous copolymeric hybrid materials for chromatographic separations and process for the preparation thereof |
WO2004058326A2 (en) * | 2002-12-20 | 2004-07-15 | Cardiac Inventions Unlimited, Inc. | Left ventricular pacing lead and implantation method |
US20070215547A1 (en) * | 2004-02-17 | 2007-09-20 | Waters Investments Limited | Porous Hybrid Monolith Materials With Organic Groups Removed From the Surface |
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US20060099130A1 (en) * | 2004-11-08 | 2006-05-11 | Rolando Roque-Malherbe | Silica mesoporous materials |
EP2101999A4 (en) | 2007-01-12 | 2017-12-27 | Waters Technologies Corporation | Porous carbon-heteroatom-silicon hybrid inorganic/organic materials for chromatographic separations and process for the preparation thereof |
AR066853A1 (en) * | 2007-06-04 | 2009-09-16 | Alltech Associates Inc | SILICON PARTICLES AND METHODS TO PREPARE AND USE THE SAME |
US11439977B2 (en) | 2009-06-01 | 2022-09-13 | Waters Technologies Corporation | Hybrid material for chromatographic separations comprising a superficially porous core and a surrounding material |
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US8197782B2 (en) * | 2010-02-08 | 2012-06-12 | Momentive Performance Materials | Method for making high purity metal oxide particles and materials made thereof |
US9249028B2 (en) | 2010-02-08 | 2016-02-02 | Momentive Performance Materials Inc. | Method for making high purity metal oxide particles and materials made thereof |
CN101857675B (en) * | 2010-06-24 | 2013-01-16 | 常州嘉众新材料科技有限公司 | Preparation method of high-purity spherical full-pore silica gel particles |
EP3834927A1 (en) | 2010-07-26 | 2021-06-16 | Waters Technologies Corporation | Superficially porous materials comprising a substantially nonporous core having narrow particle size distribution; process for the preparation thereof; and use thereof for chromatographic separations |
US10442899B2 (en) | 2014-11-17 | 2019-10-15 | Silbond Corporation | Stable ethylsilicate polymers and method of making the same |
CN105801659B (en) * | 2014-12-30 | 2018-12-18 | 广西梧州制药(集团)股份有限公司 | Purposes of the monodisperse polymer silica gel in ginsenoside Re and Rd purification |
CN105801655B (en) * | 2014-12-30 | 2018-12-18 | 广西梧州制药(集团)股份有限公司 | Purposes of the monodisperse polymer silica gel in Rg1, Re and Rb1 purification |
WO2017155848A1 (en) | 2016-03-06 | 2017-09-14 | Waters Technologies Corporation | Porous materials with controlled porosity; process for the preparation thereof; and use thereof for chromatographic separations |
CN108190896B (en) * | 2018-01-20 | 2021-08-10 | 陕西科技大学 | Preparation method of ordered mesoporous nano-silica microspheres |
CN110330583B (en) * | 2019-06-05 | 2022-03-08 | 南京亘闪生物科技有限公司 | Preparation process of uniform-particle functional high-molecular polymer microspheres |
CN112156730B (en) * | 2020-08-25 | 2022-05-03 | 安徽壹石通材料科技股份有限公司 | Preparation method of high-purity monodisperse porous silicon oxide spheres |
CN114804120B (en) * | 2021-01-29 | 2023-11-28 | 中国科学院化学研究所 | Silicon dioxide micro-nanospheres, preparation method and application thereof |
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EP0216278A2 (en) * | 1985-09-25 | 1987-04-01 | MERCK PATENT GmbH | Spherically shaped silica particles |
EP0275688A1 (en) * | 1986-12-27 | 1988-07-27 | Kabushiki Kaisya Advance | Process for manufacture of metal oxide |
US4983369A (en) * | 1989-11-22 | 1991-01-08 | Allied-Signal Inc. | Process for forming highly uniform silica spheres |
JPH03208813A (en) * | 1990-01-11 | 1991-09-12 | Mitsubishi Kasei Corp | Production of globular silica |
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US3634558A (en) * | 1968-10-29 | 1972-01-11 | Atomic Energy Commission | Method of producing monodisperse silica spheres having a dispersed radioactive tracer |
JPS58110414A (en) * | 1981-12-23 | 1983-07-01 | Tokuyama Soda Co Ltd | Inorganic oxide and its manufacture |
JPS5930730A (en) * | 1982-08-16 | 1984-02-18 | Seiko Epson Corp | Manufacture of quartz glass |
FR2589871B1 (en) * | 1985-09-13 | 1987-12-11 | Rhone Poulenc Chim Base | REINFORCING FILLER FOR SILICA-BASED ELASTOMER |
JPS6374911A (en) * | 1986-09-19 | 1988-04-05 | Shin Etsu Chem Co Ltd | Production of fine spherical silica |
US4940571A (en) * | 1988-04-01 | 1990-07-10 | Gte Laboratories Incorporated | Method of making large particle size, high purity dense silica |
JP2722627B2 (en) * | 1989-03-16 | 1998-03-04 | 日産化学工業株式会社 | Method for growing silica core particles |
-
1993
- 1993-09-17 US US08/123,283 patent/US5425930A/en not_active Expired - Lifetime
-
1994
- 1994-09-13 EP EP94929162A patent/EP0719237A1/en not_active Ceased
- 1994-09-13 WO PCT/US1994/010155 patent/WO1995007858A1/en not_active Application Discontinuation
- 1994-09-13 JP JP7509263A patent/JPH09502693A/en active Pending
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EP0216278A2 (en) * | 1985-09-25 | 1987-04-01 | MERCK PATENT GmbH | Spherically shaped silica particles |
US4775520A (en) * | 1985-09-25 | 1988-10-04 | Merck Patent Gesellschaft Mit Beschrankter Haftung | Spherical SiO2 particles |
EP0275688A1 (en) * | 1986-12-27 | 1988-07-27 | Kabushiki Kaisya Advance | Process for manufacture of metal oxide |
US4983369A (en) * | 1989-11-22 | 1991-01-08 | Allied-Signal Inc. | Process for forming highly uniform silica spheres |
JPH03208813A (en) * | 1990-01-11 | 1991-09-12 | Mitsubishi Kasei Corp | Production of globular silica |
Non-Patent Citations (2)
Title |
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CHEMICAL ABSTRACTS, vol. 115, no. 24, 16 December 1991, Columbus, Ohio, US; abstract no. 259311k, page 190; * |
See also references of EP0719237A1 * |
Also Published As
Publication number | Publication date |
---|---|
EP0719237A1 (en) | 1996-07-03 |
US5425930A (en) | 1995-06-20 |
JPH09502693A (en) | 1997-03-18 |
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