Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Particulate matter
Patent
1994-11-03
1997-01-14
Beck, Shrive
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Particulate matter
427220, 4273722, 428593, 428600, 501 12, 501153, B32B 516, B05D 700
Patent
active
055937810
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of manufacturing surface-modified ceramic powders in the nanometer size range. Particularly, the present invention relates to a method of the mentioned type by means of which the state of agglomeration of particles in the nanometer size range in a suspension can be controlled and which can thus be employed for producing suspensions and compositions, respectively with high solids contents.
In the following "particles in the nanometer size range" and "powders in the nanometer size range", respectively is to denote particles and powders, respectively, wherein the average particle size is not higher than 100 nm, particularly not higher than 50 nm, and particularly preferred not higher than 30 nm.
In the processing of nanodisperse materials (particles, powders) substantially two problems are encountered, i.e.: materials, and contents.
Regarding problem (a) it is to be noted that when going from ceramic powders in the submicron size range to ceramic powders in the nanometer size range, an increase of agglomeration is generally observed. This is due to the fact that as the particle size decreases, weak forces of interaction such as van der Waals forces also gain significant importance and dominate, respectively. Further, the particle surface is always occupied by functional groups, i.e., groups capable of undergoing condensation. With conventional powders in the submicron size range, said groups are of importance only insofar as they can be used as centers of interaction for necessary organic processing aids (dispersion aids, binders, etc.). However, due to the high surface to volume ratio in nanodisperse materials, the surface groups are of importance also in another respect. On the one hand, they can also serve as reaction centers for organic processing aids. On the other hand, however, they can also result in the formation of hard agglomerates due to condensation reactions between individual particles. The particles are connected to each other quasi by "sinternecks". Thus, it is desirable to develop methods by means of which said agglomeration can be controlled to such an extent that agglomerated powders can be obtained in a controlled manner. Further it would be beneficial if by said method the reactive surface could be shielded outwardly and thereby a condensation between the particles could be prevented.
Regarding the above problem (b) it can be noted that the production of ceramic compositions having high solids contents and properties adapted for being suitable for a molding process is quite difficult. In oder to avoid agglomerates which can result in severe defects both in green and in sintered bodies, suspensions are generally employed. For stabilizing the suspensions, dispersion aids are added which are to prevent an agglomeration and to impart the required processing properties to said suspension. On principle, there are two different ways of stabilizing a suspension, i.e., the electrostatic and the steric stabilization. The electrostatic stabilization is disadvantageous in that due to the high hydrodynamic radius of the suspended particles, only low solids contents may be realized. In contrast thereto, the steric stabilization on principle allows the production of suspensions having high solids contents from materials in the nanometer size range since in this case the hydrodynamic particle radius is much smaller.
The advantages of the steric stabilization have already been demonstrated with nanodisperse SiO.sub.2 as example. As dispersion aids, non-ionic organic polymers (e.g. polymethyl methacrylate) which are adsorbed on the particle surface were generally employed in said case. The disadvantage of such a stabilization is that even thereby only maximum solids contents of 20 to 30% by volume may be realized and that the applicability of said stabilization to systems different from SiO.sub.2 is possible only with significant limitations. The reason therefor mainly is that the surface-chemical properties which are characteristic for a certain material (e.g. ac
REFERENCES:
patent: 3945945 (1976-03-01), Kiovsky
patent: 4744802 (1988-05-01), Schwabel
Nass Rudiger
Schmidt Helmut
Schmitt Peter
Beck Shrive
Institut Fue Neue Materialien Gemeinnutzige GMBH
Maiorana David M.
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