Chemistry: physical processes – Physical processes – Crystallization
Patent
1997-10-29
1999-08-10
Straub, Gary P.
Chemistry: physical processes
Physical processes
Crystallization
23293R, 423593, 423263, 427220, 428403, B05D 700, B01D 900
Patent
active
059352752
DESCRIPTION:
BRIEF SUMMARY
TITLE OF THE INVENTION
The present invention relates to a process for producing weakly agglomerated, densified and/or crystallized nanosize particles.
DESCRIPTION OF THE BACKGROUND
Processes for producing powders having primary particle sizes below 100 nm (nanosize powders) have attracted increasing interest in recent years since these powders have the potential to enable completely new materials, for example ceramics or composites, based on them to be developed. As in the case of the submicron powders (particle diameters of from 0.1 to 1 .mu.m) already available, high demands in terms of quality are also made of nanosize powders but these demands are different depending on material and application. For ceramic powders, important criteria are, for example:
The last point in particular is of great importance for the use of nanosize powders in powder metallurgical processing and manufacturing processes. In general, the powder particles should have a density which is as high as possible and/or possess crystalline structures. The agglomerates which are inevitably present should have a nature such that they can be broken up again to their primary particle size during processing. The potential of nanosize powders can be optimally utilized only subject to these prerequisites. This means that soft agglomerates are required. Particles which allow the state of agglomeration between nanosize particles to be adjusted are therefore necessary. This can be carried out during the synthesis or in a downstream process.
Physical and chemical processes for producing nanosize (ceramic) powders are described in the literature. The physical processes are divided into three categories, namely vacuum, gas-phase and condensed-phase syntheses. However, their applicability is restricted by the low material conversion to the production of small amounts of powder.
Processes which include chemical reactions are becoming increasingly important in the powder synthesis, for example hydrothermal synthesis, precipitation reactions, flame hydrolysis, plasma synthesis, the sol-gel process or emulsion processes.
In the hydrothermal synthesis, inorganic salts are converted by means of precipitation reactions under increased pressure and elevated temperatures (above the critical data of the solvent) into the corresponding oxide, hydrated oxide or hydroxide. Setting the optimum reaction parameters (pH, type and concentration of the starting compounds, pressure, temperature) enables crystallite sizes of about 20 nm to be achieved. However, a disadvantage in this process is the formation of agglomerates which can no longer be broken up. These agglomerates are formed as a result of metal-OH groups present on the particle surface undergoing condensation reactions during drying and calcination of the powder. Since agglomerate formation is generally not reversible the potential of this technique can at present be utilized only to a restricted extent.
Flame hydrolysis is a standard method of producing aerosils. It gives high powder yields and can be applied to many materials. In this process, volatile compounds such as SiCl.sub.4, TiCl.sub.4 or ZrCl.sub.4 are reacted in a hydrogen/oxygen flame to give very fine oxide particles. Although oxide powders having particle sizes of from 5 to 50 nm can be produced by means of flame hydrolysis, a disadvantage of this process is the great degree of agglomerate formation, since the cohesion between the particles increases greatly with decreasing particle size. Redispersion of these powders to their primary particle size is usually possible to only a small extent, if at all.
Plasma synthesis enables not only oxidic powders but also nitrides and carbides to be produced. In this process, for example, metal powders or suitable metal compounds are vaporized in an inductively coupled plasma and reacted with ammonia to produce nitrides or with methane to produce carbides. This process enables highly pure, very fine spherical powders to be produced and, when the reaction parameters are optimally set, it gives particles
REFERENCES:
patent: 5593781 (1997-01-01), Nass et al.
Charty et al. The Role of Complexing Ligands in the Formation of Non Aggregated Nanoparticles of Zirconia Journal of Sol-Gel Science & Technology 1(1994) #3 Dordrecht NL.
Lerot et al, Chemical Control in Precipitation of Spherical Zirconia Particles, J. Material Science 26(1991) May 1, #9 London GB.
Burgard Detlef
Nass Rudiger
Schmidt Helmut
Institut fur Neue Materialien gemeinnutzige GmbH
Straub Gary P.
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