Preparation of Alumina Ceramics
The key technology of alumina ceramic forming is the preparation of slurry. Alumina ceramics can be prepared in a variety of ways, including dry pressing, isostatic pressing, grouting, extrusion, etc. Among them, slip casting is a molding process directly related to slurry. Generally speaking, granulation, such as spray granulation, is necessary before dry pressing and isostatic pressing. This is why it is essential to granulate the ceramic slurry and then dry it out. The final performance of alumina ceramics is also directly impacted by the quality of granulation, which is closely related to the quality of the ceramic slurry.
Basic Requirements for Ceramic Slurry
A flowable slurry and a mold that absorbs water are the foundation of grouting molding. There are several fundamental requirements for the slurry in the grouting molding in order to achieve a high-quality green body, as follows:
(1) Strong liquidity;
(2) Improved stability;
(3) Appropriate thixotropy;
(4) Reduce the water content as much as possible;
(5) Better filtering;
(6) The formed body should be strong enough;
(7) The green body is simple to demold after forming;
(8) The slurry should contain as few air bubbles as possible.
Effects of High Concentration Slurry
High-concentration slurry can increase the yield of the product, ensure the performance of the product, and guarantee the strength of the ceramic body in the grouting molding. Additionally, it can reduce energy consumption and boost production capacity in the manufacture of granulated powder.
Additives for Alumina Ceramic Slurry
Alumina ceramic slurry should have good thixotropy and fluidity in addition to the performance of general ceramic slurry, as well as a high homogeneous content to increase the slurry's filling capacity. Consequently, some additives are necessary.
Defoamer for Alumina Ceramic Slurry
It is necessary to add a defoamer, typically a low molecular weight alcohol or polysiloxane, to prevent the slurry from foaming during the process of extracting stains and extruding excess slurry, which would negatively impact the product's performance.
Dispersant for Alumina Ceramic Slurry
Dispersants can enhance the slurry's stability by preventing the re-agglomeration of particles. Both water-based and non-water-based systems need to add dispersants to increase the slurry's solid content. In general, the effects of dispersants vary for various powder systems. The slurry's solid phase content can be raised by selecting a suitable dispersant.
Surfactant for Alumina Ceramic Slurry
When the slurry is impregnated, there will be thicker slurry attached to the cross part of the foam structure, while the very thin slurry will attach to the bridge and ridge parts if the ceramic slurry is water-based and there is poor wettability between the organic foam and the slurry. If the problem is severe enough, the green body will crack during the sintering process and the porous ceramic's strength will be significantly diminished. To resolve this issue, the addition of a surfactant is typically used to improve the adhesion between the ceramic slurry and the organic foam. Surfactants like polyethylenediamine are most commonly used.
Rheological Agent for Alumina Ceramic Slurry
In order for a slurry to form, it must not only have a certain degree of fluidity but also good thixotropy, or the ability to solidify when still and regain its fluidity in response to an external force.
The slurry's fluidity makes sure that it permeates the organic foam during the impregnation process and coats the foam network's pore walls uniformly. The thixotropy of the slurry makes sure that the viscosity of the slurry can be reduced under the action of shear, the fluidity of the slurry can be improved, and it can be helpful for forming; however, at the end of the forming, the viscosity of the slurry increases and the fluidity decreases . Due to the serious blockage of the green body caused by the flow of the slurry, this makes the slurry attached to the hole wall easy to solidify and shape, preserving the uniformity of the final product.
Natural clays like bentonite and kaolin, as well as carboxymethyl cellulose, hydroxyethyl cellulose, etc. are frequently used as additional rheological agents.
Conclusion
The preparation of ceramic slurry is a critical step in the molding process of alumina ceramics, and it also has an impact on the final performance of alumina ceramics. Consequently, it is necessary to prepare a high-concentration alumina slurry with excellent fluidity, thixotropy, and stability.