Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
1998-11-23
2001-09-04
Marcantoni, Paul (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C423S625000, C423S628000, C051S293000, C264S653000, C264S662000
Reexamination Certificate
active
06284694
ABSTRACT:
The present invention relates to a moulded microcrystalline spherical Al
2
O
3
sintered body, a process for manufacturing this, and the use thereof.
The processes used to manufacture ceramic balls can be divided into mechanical, chemical, conventional (fusion) metallurgy, and powder metallurgy methods.
The mechanical process are essentially restricted to achieving the spherical shape by mechanical processing using such methods as grinding, polishing, or smoothing. One prerequisite is that an appropriately prepared moulded body be used, this then being subjected to further processing to form a ball. The mechanical production of balls frequently requires that a conventional metallurgy, chemical, or powder metallurgy method be used beforehand in order to obtain the appropriate moulded body that is then subjected to further processing.
The chemical processes are particularly suitable for obtaining materials that are as pure as possible. One process that has recently been used more and more frequently is the so-called sol-gel process, In this sol-gel process, so-called colloidal solutions are formed with suitable solvents, starting from metallic salts. The solvent is usually water and contains the metal compound in the form of nano-scale oxides or hydroxides that are present, dissolved colloidally with the help of appropriate dispersants or stabilisers. Gelling can be brought about by modifying the pH value, temperature change, or aging/adding electrolytes. Spherical gel particles are obtained by dropping the sol into a medium that promotes formation of the gel, or exposing it to such a medium in gaseous form. The spherical gel particles are then dried, calcined, and sintered.
Sol-gel processes for manufacturing ceramic balls are described, for example, in GB-A 1 032 105, DE-A 3 035 845, DE-A 2 147 472, DE-A 2 733 384, and DE-B 2 753 503. In most cases, these processes relate to the production of combustion or fuel particles based on thorium or uranium. EP-A 0 224 3 75 describes the production of transparent spherical microballs based on zirconium oxide, using the sol-gel method.
The sol-gel processes are techically costly, and require relatively costly raw materials; in addition, they are not without problems from the ecological standpoint because inorganic acids, such as nitric acid and hydrochloric acid, are frequently used a stabilisers for the sol; these are then liberated once again as chlorine or nitrous gases during the calcining or sintering processes.
Ceramic balls can be manufactured using conventional metallurgical methods by dropping the liquid smelt into a cooling medium, by blowing the smelt with air, or by atomizing the liquid smelt with an air/water mixture. One elegant process is the production of spherical ceramic particles using rotating disks, the smelt being poured onto the rotating disks that then throw off the still-liquid smelt in the form of droplets. The droplets harden relatively quickly to form ceramic balls. However, it is difficult to obtain pure and compact spherical ceramic particles using these processes, which are particulary well-suited for extracting metals.
Powder metallurgy processes have recently become increasingly important for the production of spherical ceramic particles. One of the most important processes within this group is agglomeration. The underlying principle of agglomeration is based on the clustering of individual powder particles as the result of systematic movement of a powder bed. In most instances, a binder must be added to the powder, when either a liquid or solid binder is selected, depending on the type of powder that is being used. From the technical standpoint, liquid binders are the most important; in these, the water and alcohol systems dominate because they are easier to handle. In the case of processes that use solid binders, in most instances waxes or stearates are added as agents that enhance adhesion.
Air humidity plays an important role in the dry processes, which work without the addition of adhesion-enhancing additives.
In the normal course of events, containers or mixers that can be moved systematically in different ways are uses for powder agglomeration; several types of movement can also be combined with each other.
GB-A 1 344 870 and GB-A 1 344 869 describe the production processes for moulded spherical ceramic bodies, in which wax and stearates are used as binders. JP-A 05 137 997 describes the production of moulded spherical zirconium oxide, aluminum oxide, and mullite bodies, using water, aqueous solutions of carboxy-methyl-cellulose, polyvinyl alcohol, and/or polyethylene-glycol as binder. DE-B 1 229 055 describes the production of argillaceous-earth balls by rolling activated argillaceous earth in a cylindrical ball-moulding machine while simultaneously spraying it with water.
The demand for low-priced, very pure, wear-resistant ceramic balls that possess great mechanical strength, to be used, for example, as grinding bodies, ball bearings, etc., cannot be satisfied, or can be satisfied to only a limited degree, by using the processes referred to above.
DE-A 3 507 376 describes a process and an apparatus for manufacturing granulates with a very narrow grain-size distribution, in which the product that is to be granulated is sprayed into a fluid bed and there applied to appropriate nuclei. The grain size is adjusted by the strength of the flow of separating gas of a zig-zag separator. Similar processes or developments of the so-called fluid-bed spray granulation process are described in DE-A 3 808 277 and DE-A 4 304 405.
The fluid-bed spray granulation process is usually used for drying and agglomerating agrochemical substances (fungicides, insecticides, herbicides, growth regulators, and fertilizers), pest control agents, pharmacologically effective substances, nutrients, sweeteners, colouring agents, and inorganic and organic chemicals. In addition to the active components and thinners, there may also be inert fillers, dispersants, binders, and/or other additives, for example, preservatives and colouring agents, in the liquid product that is to be sprayed in.
The granulate particles that are obtained by fluid-bed spray granulation are distinguished by their uniform shape and great solidity; these characteristics make it simpler to handle, measure and process the original, finely powdered material, and in some instances even make these operations possible for the first time. Because of their microporous structure and the large surface areas associated therewith, the granulates can be redispersed spontaneously, which means that the la process is predestined for processing agrocherical substances, pest-control agents, and pharmacologically effective substances.
It is the task of the present invention to provide moulded spherical sintered bodies that do not have the disadvantages found in the prior art. Surprisingly, it was found during drying trials with ceramic powder suspensions based on Al
2
O
3
that by using fluid-bed spray granulation, it is possible to obtain extremely dense green bodies that can be sintered directly to form a dense ceramic body, without any additional manipulation, such as compacting. Because of the high basic density of the granulate and the fineness and sinter activity of the initial powder, for all practical purposes it is possible to suppress grain growth almost completely during the sintering process, so that a moulded microcrystalline spherical sintered ceramic body that is distinguished by particular toughness and wear resistance results.
The object of the present invention are moulded microcrystalline spherical sintered bodies that are based on &agr;-aluminum oxide, the average grain size d
50
of the primary crystals being preferably smaller than 3 &mgr;m, the diameter of the moulded sintered bodies being between 0.01 and 10 mm, and the moulded sintered bodies being of a hardness of >16 Gpa (HV
200
) and a density of >95% of the theoretical density TD. The moulded microcrystalline spherical sintered bodies according to the present invention, which have an averag
Luette Martin
Moeltgen Paul
Wilhelm Pirmin
Cohen Jerry
Korund Laufenburg GmbH
Marcantoni Paul
Perkins Smith & Cohen
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