Chemistry: physical processes – Physical processes
Patent
1985-06-26
1987-03-31
Doll, John
Chemistry: physical processes
Physical processes
501 88, 501 94, 501103, 501127, B01J 208, C04B 3510, C04B 3549, C04B 3552
Patent
active
046540482
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a process for the preparation of a powder in the form of grains, and which is particularly suitable for fritting, especially for producing large members having improved mechanical properties.
It is known to use alumina or alumina-based refractory materials e.g. in place of metal parts in mechanical assemblies operating at high temperatures, i.e. above 1000.degree. C., which is the case in heat motors. By comparison with other refractory ceramics, alumina or alumina-based composite materials have the advantage of a good thermal shock resistance. Moreover, their bending resistance is improved on carrying out a zirconia microdispersion in a finely divided alumina. This resistance rises from 200 to 350 MPa, when the crystallites have a size not exceeding a few microns.
The alumina or alumina-based composite materials are shaped in a conventional manner in accordance with the following industrial procedure:
1. preliminary treatment of the powder;
2. compression of a rough blank;
3. thermal treatment and natural fritting;
4. obtaining the required dimensions.
It is vital to obtain a rough compressed blank as close as possible to the final dimensions, if it is wished to industrially produce dense parts of large size and complicated geometry. It is therefore the capacity to produce such rough parts, which controls the practicability of the complete process. However, at present, rough blanks do not have an adequate strength to enable anything other than small simple objects to be produced on an industrial scale. Such a situation is very prejudicial to substituting metal parts operating at high temperatures by alumina-based ceramics, particularly in the case of large, complicated parts, despite the excellent performance levels which can be expected from alumina.
These difficulties are mainly due to the fact that the microcrystallization necessary for bringing about the mechanical reinforcement of the part can only be obtained by fritting the compressed object at a temperature equal to or below 1500.degree. C. This can only be obtained with agglomerated, extremely finely ground powders which, as a result of the grinding lack cohesion and are consequently unsuitable for agglomeration. The granulation of the powder by compression and crushing is not possible, as a result of the lack of intrinsic cohesion of granules.
SUMMARY OF THE INVENTION
The object of the present invention is to eliminate these disadvantages by a process making it possible to obtain a powder with a good cohesion so that it can be shaped without prejudicing its densification capacity.
More specifically, the present invention relates to a process for the preparation of a powder suitable for compression and fritting and which comprises the stages of forming an emulsion, breaking up the emulsion into the form of microdroplets which are solidified, dried and heat treated, wherein the emulsion is formed from at least one powder suspended in a solution of at least one precursor salt.
The term precursor salt is understood to mean any salt, whereof the cation or cations will form constituents of the final powder.
The process is applicable to powders of metal oxides, particularly alumina, zirconia, titanium dioxide and hafnium oxide, either considered singly or in mixed form.
The invention is particularly suitable for treating alumina powder with a specific surface equal to or greater than 5 m.sup.2 /g. According to the invention, the powder, particularly a metal oxide is suspended in a solution of a mineral salt, whose cation is a constituent element of said powder (precursor), e.g. an aluminium salt in the case of alumina. The mineral salt or salts forming the precursor may or may not be one of the constituents of the powder to be treated, e.g. alumina--aluminium salt, alumina--zirconium salt. This suspension is emulsified in an organic liquid and is then broken up or fractionated. The fractionation of the emulsion in the form of microdroplets can take place by any known process, such as pneu
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Braun Camille
de Neve Michel
Commissariat a l''Energie Atomique
Doll John
Kunemund Robert
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