High-solids SiO2 dispersions, process for producing them,...

Details High-solids SiO2 dispersions, process for producing them,... High-solids SiO2 dispersions, process for producing them,...

2002-03-01

2004-03-02

Sample, David (Department: 1755)

Compositions: ceramic

Ceramic compositions

Glass compositions, compositions containing glass other than...

C501S054000, C501S055000, C501S021000, C423S335000, C423S339000, C065S395000, C065S398000

Reexamination Certificate

active

06699808

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to high-solids SiO
2
dispersions, a process for producing it and also a process for producing porous amorphous shaped SiO
2
bodies having an extremely high solids content, these shaped bodies and their production.
2. Description of the Related Art
Porous, amorphous shaped SiO
2
bodies are used in many industrial fields. Examples which may be mentioned are filter materials, thermal insulation materials and heat shields.
In addition, fused silica goods of all types can be produced from amorphous, porous shaped SiO
2
bodies by sintering and/or melting. Highly pure porous shaped SiO
2
bodies can serve, for example, as “preform” for glass fibers or optical fibers. Furthermore, crucibles for drawing single silicon crystals can also be produced in this way.
Regardless of the use of the porous shaped bodies, efforts are always made to produce a shaped body which is as close as possible to the final dimensions. This means that little if any shrinkage may occur during production of the shaped body.
Porous shaped SiO
2
bodies can in principle be produced by pressing of appropriate SiO
2
powders or by way of a wet chemical process.
In the processes known from the ceramics field for pressing powders, e.g. cold or hot isostatic pressing processes, it is generally necessary to add organic binders in order to obtain a strong green body. These binders have to be leached out again or burnt in a later step. This is technically complicated, expensive and leads to undesired impurities which, particularly in the production of crucibles for drawing single silicon crystals, have to be avoided at all costs.
The preferred way of producing porous shaped SiO
2
bodies is therefore the wet chemical route. A process known from the literature is the sol-gel process. This generally starts from silicon-containing monomers dissolved in solvents (sol) which are converted by means of hydrolysis and/or polycondensation into a nanoporous three-dimensional SiO
2
network (gel). The porous shaped body is then obtained by subcritical or supercritical drying. Apart from the usually expensive starting materials, this process can only produce gels having a solids content of about 10-20% by weight. Subcritical drying therefore results in extremely high shrinkage, as a result of which shaped bodies close to the final dimensions cannot be produced reproducibly. If supercritical drying is carried out, the shaped body does not shrink but it then has a solids content of only 10-20% by weight.
One way of obtaining shaped SiO
2
bodies having a higher solids content is described in EP 705797. Here, finely divided silica particles (fumed silica) are added to the sol. This enables a solids content of about 40% by weight to be obtained. However, the preparation of the sol is still expensive and drying is complicated.
A further method is described in EP 318100. Here, a dispersion of finely divided silica (fumed silica) having particle sizes in the range 10-500 nm in water is prepared. After shaping and solidification of the dispersion, the corresponding shaped body is obtained by means of drying. This gives solids contents of up to 60% by weight.
EP 653381 and DE-A 2218766 disclose a slip casting process in which a dispersion of fused silica articles having a particle size of from 0.45 to 70 &mgr;m, preferably 1-10 &mgr;m, in water is prepared. The achievable solids content of the dispersion is from 78 to 79% by weight. The dispersion is subsequently solidified in a porous mold by slow withdrawal of water and is dried after removal from the mold. Although shaped bodies having quite high solids contents can be produced by this process, the slip casting process is very time-consuming due to the diffusion-dependent withdrawal of water and can only be employed for thin-walled shaped bodies. Furthermore, the solidification by withdrawal of water by means of porous molds leads to an undesirable density gradient within the shaped body, which during later sintering causes differing sintering temperatures, sintering times and also density differences.
If a complicated slip casting process is to be avoided and shaped bodies as close as possible to the final dimensions are nevertheless to be produced, dispersions having an extremely high solids content have to be realized. In practice, this leads to great problems, since dispersed SiO
2
particles have a strong thixotropic effect. During the dispersion procedure, a dilatant phase occurs. This is shown by a rise in the viscosity of the suspension with increasing shear. In order to obtain a high-solids dispersion which is still pourable, it is necessary to employ a complicated process with a change from low shear during stirring-in and high shear during homogenization. Due to the very rapid solidification of such a high-solids suspension, problems are also encountered in achieving homogeneous shaping of the dispersion.
GB-B-2329893 discloses a composition for producing vitreous silica, which composition contains pyrogenic silica having a mean particle diameter of from 5×10
−3
to 1×10
−1
&mgr;m and a specific surface area of from 50 to 400 m
2
/g and also heat-treated silica, as agglomerate of a pyrogenic silica, having a mean diameter of from 2 to 15 &mgr;m and a lower specific surface area than the pyrogenic silica together with plasticizer (e.g. tetramethylammonium hydroxide), dispersant (e.g. polyethyloxazoline, glycerol) and binder (e.g. methyl formate). Due to its content of different organic and/or inorganic additives, such a composition is quite unsuitable for producing highly pure sintered bodies. In addition, solids contents of only up to 51% by weight of solids are achieved in the compositions disclosed in this document and the green bodies resulting therefrom.
JP 5294610 discloses a process for producing amorphous shaped SiO
2
bodies which have a solids content of up to 80% by weight. Here, SiO
2
particles having a mean particle diameter of from 0.1 to 100 &mgr;m are used. To be able to achieve such high solids contents at all, the particles have to be dispersed in basic water (pH>10, e.g. by means of TMAH) by means of application of high shear for a long time (e.g. ball mill). Owing to the unavoidable contamination due to the use of bases and abrasion (abrasive nature of the SiO
2
particles) during the long and intensive shear process, such a composition is completely unsuitable for producing high-purity shaped SiO
2
bodies.
U.S. Pat. No. 4,929,579 discloses a process by means of which refractory articles are produced from silica using a dispersion containing amorphous SiO
2
particles having a trimodal particle size distribution. Such a dispersion is very complicated to prepare. In addition, owing to the proportion of very large particles (mean particle diameter>300 &mgr;m), inhomogeneities in the dispersion and sedimentation phenomena occur. This leads to density fluctuations within the dispersion or the shaped bodies, which results in considerable difficulties in respect of shape accuracy and isotropic shrinkage during subsequent shaping processes and sintering.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a homogeneous and very readily pourable dispersion which has a high solids content of SiO
2
particles and does not have the disadvantages known from the prior art.
This object is achieved by a homogeneous dispersion of amorphous SiO
2
particles in a dispersion medium, wherein the dispersion has a solids content of at least 80% by weight of amorphous SiO
2
particles and the amorphous SiO
2
particles have a bimodal particle size distribution.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The dispersion preferably has a solids content of at least 83% by weight of amorphous SiO
2
particles.
The dispersion particularly preferably has a solids content of at least 86% by weight of amorphous SiO
2
particles.
The upper limit of the solids content of amorphous SiO
2
particles is preferably about 95% by weight when water is used

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