Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Patent
1991-06-13
1993-03-23
Bell, Mark L.
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
423600, 423608, 501103, 501127, C01F 702, C01G 2500
Patent
active
051961806
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the production of ceramic powders for pressing.
Recent interest in the use of high-purity ceramics such as zirconia for a variety of specialised end uses has led to a demand for high grade ceramic powders. Ceramic components are commonly made by the dry pressing of such powders, and use of a high grade powder suitable for such a process (hereinafter referred to as a "pressing powder") will result in components which are free of large pores and uniformly dense. In order to achieve these ends, the pressing powders must be fine, free-flowing and as free as is practicable from non-uniform shapes. Uniformly dense spheres are desirable because they will give the most free-flowing highest bulk density powders. On pressing, such a powder will more readily form a "green" body with a uniform pore size than will a powder which is not uniform in density. Failure to achieve these properties to an acceptable degree will result in a ceramic article which will exhibit inferior performance.
A common method of preparing ceramic pressing powders is spray drying. In this method, a dispersion of a ceramic material such as zirconia of a controlled particle size is dispersed in water in the presence of a suitable surfactant and then sprayed to give a fine pressing powder which consists essentially of loose spheroidal aggregates of particles. These can then be loaded to a die and pressed to give a "green" body prior to firing. Before spray drying, there may be added other materials such as binders and plasticisers, these additives helping control the strength and uniformity of the "green" body. The problem with this approach is the fact that there remains with the particles the surfactant residues. The presence of such residues reduces the overall properties of the final articles, yet these residues are both difficult and expensive to get rid of.
It has now been found that this can be avoided and that it is possible to make a high-grade ceramic pressing powder which has a reduced content of organic residues and may even be free of them. There is therefore provided, according to the present invention, a process of preparing a ceramic powder suitable for pressing into articles by the steps of an alkylamine having a boiling point below that of water, present in a quantity sufficient to disperse the ceramic; and
The particulate ceramics suitable for use in this invention may be any of those which are useful in the production of ceramic articles. The most common of these are the various types of stabilised or partially stabilised zirconium dioxide, but other ceramic oxides such as alumina can also be used. The mean particle size should be small, 1.0 um maximum.
The process consists of the steps of dispersing a particulate ceramic and then spray drying the dispersion to give a pressing powder. The dispersion is carried out in the presence of a quantity of ammonia or an alkylamine having a boiling point below that of water. There was described in co-pending PCT Application PCT/AU89/00242 a grinding process for ceramics where there was disclosed the surprising discovery that ammonia and volatile alkylamines could be used as dispersants for ceramic powders. The same types of alkylamines which were useful in that invention, i.e. compounds of the formula NR.sup.1 R.sup.2 R.sup.3, where R.sup.1, R.sup.2 and R.sup.3 are selected from hydrogen and C.sub.1 -C.sub.4 alkyl groups, at least one of R.sup.1, R.sup.2 and R.sup.3 being an alkyl group, are also useful in this invention. Again, ammonia is preferred because of its cheapness and relatively low toxicity.
The quantity of ammonia or alkylamine necessary for the dispersion of the particulate ceramic cannot be stated with any degree of precision as the concentrations of particulate ceramic and evaporation rate of ammonia or alkylamine vary considerably. Sufficient dispersant is added to suspend whatever particulate ceramic is used--the skilled person can readily ascertain this. Other required materials such as binders and plasticisers may be added at this part of the
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Bell Mark L.
ICI Australia Operations Proprietary Limited
Jones Deborah
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