Electrically assisted synthesis of particles and film with...

Details Electrically assisted synthesis of particles and film with... Electrically assisted synthesis of particles and film with...

1999-05-26

2001-07-03

Meeks, Timothy (Department: 1762)

Coating processes

Direct application of electrical, magnetic, wave, or...

Pretreatment of coating supply or source outside of primary...

C427S563000, C427S564000, C427S569000, C427S577000, C427S578000, C427S248100, C423S613000, C423S337000, C423S625000, C423S608000, C423S622000, C423S618000, C423S289000, C423S351000, C423S439000, C423S447300, C501S087000, C501S096100

Reexamination Certificate

active

06254940

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to methods of manufacturing titanium dioxide, silicon dioxide, aluminum oxide powders and other ceramic powders having well-controlled size, crystallinity and specific surface area characteristics. The materials produced are useful as catalysts, pigments, reinforcing agents, optical fibers and for producing metallic and alloy powders and nanostructured films. This invention relates, more particularly, to a development in the synthesis of ceramic, metallic and alloy nanometer-sized particles of characteristically high purity and precise particle size through the use of electric fields during particle formation.
BACKGROUND OF THE INVENTION
Fine powder materials synthesis is finding particular application in the fields of powder metallurgy, semiconductors, magnetics and ceramics. In each of these fields, the synthesis of high-purity, nanometer-sized particles or “nano-particles” is considered highly desirable. Primary nanoparticles in the 1-100 nm size range permit the creation of materials with carefully controlled properties. In view of the desirability of the particles, as described, several methods for synthesizing sub-micron particles have been developed.
The generation of fine, pure, uniform, spherical, particles is of intense interest because of their recently recognized properties as suitable starting materials for producing high performance, dense ceramic articles. Densified bodies produced from such powders are predicted to be very strong and to have significantly enhanced property reproducibility. Silicon carbide (SiC) and silicon nitride (Si
3
N
4
) are two ceramic materials currently considered highly suitable for use in advanced military and civilian engines.
The direct synthesis of such ceramic powders from gas phase reactants has been achieved using lasers, RF plasma heating systems and heated flow tubes. The first two methods have the advantage over other methods, such as solid phase synthesis and chemical vapor deposition, of avoiding contact of the reactants or products with hot walls (a source of contamination). The latter two methods suffer from non-uniformities in the size of the reaction zone resulting in the production of undesirable wide particle size distribution, agglomeration, etc. The first system is difficult to scale from the laboratory to a production facility.
Various physical, chemical and mechanical methods have been devised for the synthesis of nanostructured powders (n-powders). These have been described in detail in the scientific literature (see “NanoStructured Materials,” Vols. I, II and III, 1992-4). Of particular relevance to this invention is the prior art on the synthesis of n-powders by (1) thermal decomposition of metallo-organic precursors using a focused laser beam, combustion flame or plasma torch as heat source, and (2) evaporation and condensation of volatile species in a reduced-pressure environment.
Nanosized particles have distinctly different properties compared to bulk materials because the number of atoms on the particle surface is comparable to that inside the particle (Andres, R. P., R. S. Averback, W. L. Brown, L. E. Brus, W. A. Goddard III, A. Kaldor, S. G. Louie, M. Moscovits, P. S. Peercy, S. J. Riely, R. W. Siegel, F. Spaepen, and Y. Wang, “Research Opportunities on Cluster and Cluster-Assembled Materials—A Department of Energy, Council on Materials Science Panel Report,
J. Meter. Res.,
4, 704 (1989)). As a result, these particles are characterized by lower melting point, better light absorption and structural properties. Nanosized particles are also used to form catalysts with high specific surface area and large density of active sites. Though a number of processes have been developed for synthesis of nanoparticles, their production cost remains high, limiting, thus, the development of their applications. Flame reactors, on the other hand, are routinely used in industrial synthesis of submicron powders with relatively narrow size distribution and high purity (Ulrich, G. D., “Flame Synthesis of Fine Particles”,
C
&
EN,
62(8), 22 (1984)).
Charging particles during their formation can have a profound effect on the product particle characteristics: primary particle size, crystallinity, degree of aggregation and agglomerate size. Hardesty and Weinberg (“Electrical Control of Particulate Pollutants from Flames”,
Fourteenth Symposium
(
International
)
on Combustion,
The Combustion Institute, Pittsburgh, 1365 (1973)) showed that the silica primary particle size can be reduced by a factor of three when an electric field is applied across a counterflow CH
4
/air diffusion flame. They attributed it to the rapid deposition of particles on the electrodes, thus decreasing the particle residence time in the high temperature region of the flame. Likewise, Katz and Hung (“Initial Studies of Electric Field Effects on Ceramic Powder Formation in Flames”,
Twenty
-
Third Symposium
(
International
)
on Combustion,
The Combustion Institute, Pittsburgh 1733 (1990)) showed that the size of TiO
2
, SiO
2
and GeO
2
particles made in a similar reactor were greatly influenced by the presence of electric fields. Xiong et al. (1992) showed theoretically that charging titania particles unipolarly during their synthesis can reduce the particle size and narrow the particle size distribution.
Titania is used as a pigmentary material (Mezey, E. J., “Pigments and Reinforcing Agents” in VAPOR DEPOSITION, C. F. Powell, J. H. Oxley and J. M. Blocher, Jr., (Eds.), John Wiley & Sons, New York, 423 (1966)), photocatalyst (Ollis, D. F., Pelizzetti, E., and N. Serpone, “Photocatalytic Destruction of Water Contaminants”,
Environ. Sci. Tech.,
25, 1523 (1991)), and as a catalyst support (Bankmann et al., 1992). Fumed silica particles are widely used for optical fibers, catalyst supports and as a filler and dispersing agent (Bautista, J. R., and R. M. Atkins, “The Formation and Deposition of SiO
2
Aerosols in Optical Fiber Manufacturing Torches”,
J. Aerosol Sci.,
22, 667 (1991)). Nanosized tin oxide powders are used as a semiconductor and gas sensor (Kim, E. U-K., and I. Yasui, “Synthesis of Hydrous SnO
2
and SnO
2
-Coated TiO
2
Powders by the Homogeneous Precipitation Method and their Characterization”,
J. Mater. Sci.,
23, 637 (1988)). The objectives of the present invention are to provide methods using plate electrodes across the premixed flame for synthesis of nanophase materials with closely controlled characteristics.
Flame aerosol technology refers to the formation of fine particles from gases in flames. This technology has been practices since prehistoric times as depicted with paintings in cave walls and Chinese ink artwork. Today flame technology is employed routinely in large scale manufacture of carbon blacks and ceramic commodities such as fumed silica and pigmentary titania and, to a lesser extent, for specialty chemicals such as zinc oxide and alumina powders. These powders find most of their applications as pigments and reinforcing agents and, relatively recently, in manufacture of optical waveguides. Today the production volume of this industry is in the order of millions metric tons per year worldwide. Though this is an established industrial process bringing sizable profits to the corresponding corporations, its fundamentals are not yet well understood. This lack of understanding makes truly difficult the process development and scale-up for manufacture of titania, silica and other ceramic particles of closely controlled size including nanoparticles.
According to flame technology, vapor of the precursor compound reacts at high temperature with oxygen or any other desirable oxidant or gas resulting in the product ceramic powder in the form of a cluster of cemented primary particles. The size of primary particles ranges from a few to several hundred nanometers in diameter depending on material and process conditions. In most industrial processes, especially in the oxidation of SiCl
4
or TiCl
4
, these reactions are exothermic so little additional fuel is needed to initia

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