Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2001-08-13
2004-10-12
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S263000, C423S607000, C423S608000, C423S610000, C423S625000, C423S594170, C423S593100, C423S600000, C423S595000, C423S596000, C423S594120, C423S598000, C423S594800
Reexamination Certificate
active
06803027
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a process for forming ceramic powders with fine nanosize particles.
BACKGROUND OF THE INVENTION
Nanosize powders are generally considered to be powders having very fine particles in the nanometer range, i.e., less than a few nanometers, e.g., 100 nanometers or less, usually 10 nanometers or less.
Nanosize powders have numerous applications such as catalysts, electrocatalysts, catalyst supports, electrodes, active powders for the fabrication of dense bodies, semiconductors for energy storage, photovoltaics, ultrafine magnetic materials for information storage, environmental clean-up as destructive adsorbents, water purification, information storage, and optical computers, to name a few. Some of the numerous examples include the following: nanosize.(3 to 4 nm) platinum for oxygen reduction in acid electrolytes, many metallic powders made by precipitation in aqueous and non-aqueous media for alloy fabrication and for catalysis, nanosize iron oxide catalyst for coal liquefaction, nanosize iron oxide particles for magnetic applications, tetragonal zirconia powder by a hydrothermal treatment at high pressures for structural applications, carbides and nitrides using non-aqueous media, nanosize BaTiO
3
by a gas-condensation process, etc. Many oxides have potential applications as nanosize powders. These include: CeO
(2-x)
for catalytic reduction of SO
2
,&ggr;-alumina as a catalyst support and for enhancing ionic conductivity of lithium iodide, V
2
O
5
as a catalyst for NO
x
reduction, and etc. Several processes currently used for the synthesis of nanosize powders include:(1) Gas-phase condensation, (2) Mechanical milling, (3) Thermal crystallization, (4) Chemical precipitation, (5) Sol-gel processing, (6) Aerosol spray pyrolysis, and etc.
In gas-phase condensation, evaporation of precursors and their interaction with an inert gas leads to loss of kinetic energy, and homogeneous nucleation of nanosize powders occurs in a supersaturated vapor. Nanocrystalline powders of TiO
2
, Li
2
O-doped MgO, CeO
2
, Y-doped ZrO
2
, etc. have been produced by gas-phase condensation. Aerosol spray pyrolysis has been used to synthesize BaFe
12
O
19
, Fe
2
O
3
among some other materials. High-energy mechanical milling is used extensively to produce nanostructured materials, especially when large quantities of materials are required. Very fine particles of nickel-aluminum alloy, Fe—Co—Ni—Si alloys, Ni—Mo alloys, for example, have been produced by mechanical milling. Contamination by the milling process, however, is a shortcoming of this process. Also, although very fine (nm size) particles can be made, agglomeration is a problem leading to cluster sizes in the micron range.
Chemical coprecipitation has received considerable attention for the synthesis of nanosize powders. Metallic as well as ceramic powders can be made by a careful control of chemistry. Alkali metal borohydride, MBH
4
where M is an alkali metal, for example, has been used as a reducing agent in aqueous media for the synthesis of metallic powders. Similarly, hydroorganoborates of the general formula MH
v
(BR
3
) or MH
v
[BR
n
(OR′)
3-n
]
v
where M is an alkali or alkaline earth metal, v=1, 2, and R, R′ are alkyl or aryl groups have been used as reducing and precipitating agents. It is important to control pH and ionic strength in aqueous media to prevent Ostwald ripening. In the synthesis of nanosize iron oxide, for example, it has been shown that the higher the pH and the higher the ionic strength, the smaller is the size of nanosize Fe
3
O
4
particles.
In most methods for the synthesis of nanosize powders, two issues are particularly important; (1) the formation of fine, uniform size particles, and (2) the prevention of agglomeration. Nanoparticles of a uniform size can in principle be formed by carefully controlling nucleation and growth. Often, a variety of encapsulating methods are necessary to control the size of nanoparticles.
Agglomeration is often the result of Van der Waal's forces. The adverse effect of agglomeration on the sintering behavior of ceramic powders is well documented. Even in catalysis, the need for dispersed powders is well known. Often, supercritical drying can be used to obtain nonagglomerated powders. In liquid media, agglomeration can be suppressed through steric hindrance or through the manipulation of electrostatic interactions. The latter in polar liquids can be achieved by changing the pH and the ionic strength of the solution. Many techniques involve the use of surfactants. Often the powders which are nonagglomerated and well dispersed in a liquid, tend to agglomerate during the drying stage. Fortunately, methods such as slip-casting, gel-casting, pressure slip casting can be used to achieve powder compaction in a wet state. Such has been demonstrated using submicron ceramic powders.
With the exception of milling, all the above methods are based on molecular synthesis of nanoparticles wherein the particles are built-up by atom-by-atom, or molecule-by-molecule, addition. Even in processes based on the decomposition of metal carbonyls, the growth of particles occurs by a layer-by-layer addition of atoms. As a result, a control of nucleation and growth is necessary to ensure the formation of nanosize particles. This often requires a very precise and difficult control of the reaction system, which renders the manufacture of the nanosize powder in large quantities impractical or impossible. In addition, the molecular synthesis processes are costly because of the relatively large capital expenditures required for the equipment to control the formation of only a small quantity of nanosize product.
OBJECTS OF THE INVENTION
It is, therefore, an object of the invention to provide method for the formation of nanosize powders that is easy to implement on an industrial scale and in relatively inexpensive when compared to molecular synthesis methods.
Another object of the invention is to provide a method in which nanosize powders are formed by a process other than precipitation or deposition from solutions, thus eliminating the possibility of unwanted deposition and growth of the nanosize powders.
Another object of the invention is to provide a method which forms nanosize powders that have a reduced tendency to agglomerate.
Another object of the invention is to provide a method for the formation of nanosize powders that can be applied to forming a variety of powder compositions.
Further objects of the invention will become evident in the description below.
BRIEF SUMMARY OF THE INVENTION
In order to overcome the problems associated with molecular synthesis and milling to form nanosize powders, the present invention presents an alternative approach for the synthesis of nanosize powders. In the present invention, a precursor inorganic compound is formed from which the unwanted component is leached away so that a fine, nanosize powder is left as a residue. Thus, the present invention is based on molecular decomposition, rather than molecular synthesis, or deposition.
As discussed above, one of the problems with many methods of synthesis of nanosize powders is that often it is difficult to synthesize large quantities of materials. By contrast, the present invention is suitable for making large quantities of nanosize powders of a number of materials.
In summary, the present invention is a process for forming nanosize powders . The process comprises:
forming a precursor ceramic material comprising a fugitive constituent and a non-soluble constituent in a single phase;
contacting the precursor material a selective solvent to form a solution of the fugitive constituent and a residue of the non-soluble constituent, the precursor sufficiently reactive with the solvent to form the solution of the fugitive constituent in the solvent and the residue of the non-soluble constituent the precursor material and the non-soluble residue sufficiently insoluble in the solvent such that there is insufficient precursor material and non-soluble residue
Bhide Sanjeevani Vidyadhar
Virkar Anil Vasudeo
Bos Steven
Thorpe North & Western LLP
University of Utah Research Foundation
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