Precipitation process

Details Precipitation process Precipitation process

2001-11-15

2004-11-02

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Rare earth metal

C423S142000, C502S300000, C502S304000, C502S338000

Reexamination Certificate

active

06811758

ABSTRACT:

The present invention is concerned with a process for inducing homogeneous precipitation of metal oxides and with the application of such a process to the preparation of weakly agglomerated nanacrystalline powders of said metal oxides.
In the present specification the term metal oxides is intended to include hydroxides, hydrated oxides, oxohydroxides, or oxoperoxohydroxides of metals.
The increased mechanical performance demanded of advanced ceramic materials imposes increasingly stricter requirements on the ceramic powders from which they are made. The use of monodisperse nanocrystalline powders as starting materials has demonstrated considerable potential for improving the functional properties of existing ceramic compositions. For example, the use of monodisperse nanocrystalline powders as starting materials provides ceramic compositions with finer porosity (e.g. for use in ceramic filters), and greater surface area (e.g. for use in catalysts). Such materials are also capable of forming better thin ceramic coatings.
In the present specification the term “nanocrystalline powder” is intended to mean a powder wherein substantially all of the constituent particles have a crystal size of less than 100 nm.
The term “weakly agglomerated powder” is intended to mean a powder containing agglomerates that break up during normal processing or forming operations.
The term “monodisperse powder” is intended to mean a powder whose particle size distribution has a geometric standard deviation, &sgr;
g
, less than or equal to 1.1. For many conventional powders &sgr;
g
would be in the range of from 1.8 to 2.2.
Cerium (IV) oxide, CeO
2
, is an example of a material where the number of applications has increased rapidly, for example in glasses, phosphors, catalysis, and chemical applications, and for which the use of nanocrystalline powders is an important factor. Unfortunately, the high specific surface areas of nanocrystalline powders, in which the primary particle size is often smaller than 5 nm, also results in a stronger tendency of the powder to agglomerate which can make processing difficult. In the present specification the term “primary particle” distinguishes the small individual particles (typically less than 5 nm in diameter) that are formed in the first stage of the homogeneous precipitation process from the larger agglomerates of such particles (typically 50-100 nm in diameter) that may form later. These large spherical agglomerates are referred to as “secondary particles” and may contain hundreds of primary particles. Weakly-agglomerated powder is needed both for dry processing methods, for example powder compaction, and for the preparation of stable suspensions in liquids, for example for thin or thick film production. Unless weakly-agglomerated nanoscale powders can be produced, the benefits expected from highly-uniform nanocrystalline powders are easily lost during the manufacture of components. The strength of agglomerates depends on the surface properties of the nanocrystalline particles in the powder and these properties are sensitively dependent on the powder synthesis procedures.
Precipitation from aqueous metal salt solution is widely used in industry for producing ceramic oxide powders, but for nanocrystalline materials, such powders tend to form excessively hard agglomerates. The precipitated species is usually a precursor, for example a hydroxide, rather than the required oxide and a thermal decomposition treatment is needed to obtain the final product. In densely agglomerated nanostructured powders there are many points of contact between primary particles and even a low-temperature thermal decomposition treatment allows sufficient diffusion to occur to produce agglomerates too hard to be easily redispersed. Dense agglomerates must therefore be avoided during the precipitation process if easily processable powders are to be obtained. Control of agglomerate morphology requires control of the chemistry of the precipitation reaction.
Precipitation occurs by adding a precipitating ligand (anion) to a solution containing cations of the appropriate metal. If the precipitating ligand is added,directly by simply pouring one solution into another then there is little control of the chemistry during precipitation because of the large and inhomogeneous gradients in solution concentration. A better control of chemical and morphological characteristics can be achieved if the precipitating ligands are generated “in situ”, simultaneously and uniformly throughout the solution, this results in what is known as a “homogeneous” precipitation process.
A homogeneous precipitation process based on forced hydrolysis is quite widely applicable and has been used to produce various monodisperse metal oxide precursor particles of various shapes and sizes [see MATIJEVIC, in High Tech Ceramics, edited by P. Vincenzini, (Elsevier, Amsterdam, 1987) p. 441-4583]. Forced hydrolysis is usually accomplished either by increasing the pH of the solution, or by heating the solution, in some cases at temperatures up to boiling point at atmospheric pressure, but more usually to higher temperatures under pressure, i.e. hydrothermal treatment.
In the present specification, the term “hydrothermal treatment” of a substrate means heating said substrate in the presence of water at a temperature above the normal boiling point of the water under applied or autogenous pressure sufficient to prevent boiling of the water.
Homogeneous precipitation by an increase in the pH of the solution can be achieved by the thermal decomposition of urea or hexamethylenetetramine to form ammonia thereby generating OH as the precipitating ligand [see MATIJEVIC, in High Tech Ceramics, edited by P. Vincenzini, (Elsevier, Amsterdam, 1987) p. 441-458]. Monodispersity of the precipitated particle results from the occurrence of nucleation in a single burst followed by a uniform growth process, for example according to the LaMer theory. The primary particles resulting from such a nucleation process are usually monodisperse and several nanometers in size. Amorphous precipitates such as aluminium hydroxide usually consist of spherical particles whereas crystalline precipitates often consist of faceted particles. Ageing of such a solution usually leads to agglomeration of the primary particles to form densely packed agglomerates. The agglomerates often have a fairly narrow size distribution and tend to be spherical in the case of amorphous precipitates where there is no ordering force such as a dipole moment or a difference in the surface energy between the crystal facets of the primary particles. Agglomerates as large as 1 micron in diameter can be obtained
There are several reports of methods for the preparation of cerium oxide that depend on an increase in pH to cause precipitation. Matjevic and Hsu [see MATIJEVIC and W. P. HSU,
J. Colloidal Interface Sci
., 118 (1987) 506-523] obtained non-spherical crystalline particles of CeO(CO
3
)
2
H
2
O by precipitation with urea. Aiken et al [see AIKEN, W. P. HSU and E. MATIJEVIC,
J. Am. Ceram. Soc
., 71 (1988) 845-85] used the same method to obtain spherical particles of a mixed Y(III)/Ce(III) compound. Akinc and Sordelet (see AKINC and D. SORDELET,
Advanced Ceramic Materials
, 2 (1987) 232-238) prepared non-spherical well-crystallised CeOHCO
3
particles. Chen and Chen (see CHEN and I. W. CHEN,
J. Am. Ceram. Soc
., 76 (1993) 1577-1583] used hexamethylenetetramine decomposition to prepare cerium oxide powders and compared them with those precipitated with ammonium hydroxide.
Heating the solution to force hydrolysis has been reported by several authors. Briois et al [see BRIOIS, C. E. WILLIAMS, H. DEXPERT, F. VILLAIN, B. CABANE, F. DENEUVE and C. MAGNIER,
J. Mat. Sci
., 28 (1993) 5019-5031] reported the preparation of 3 nm particles of CeOSO
4
H
2
O from Ce(IV) sulphate at 90° C., but this is not an attractive precursor for cerium oxide due to the presence of the sulphate group.
Hydrothermal conditions appear more suitable for the direct

Affiliated with

Also associated with

Rating

Precipitation process does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Precipitation process, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Precipitation process will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3340743