In the era of new materials, advanced ceramics are one of the most important pillar industries in the field of ceramics and inorganic materials. Compared with metal and polymer materials, advanced ceramic materials have incomparable high hardness, high modulus, high temperature resistance, corrosion resistance and other structural characteristics, as well as excellent electrical insulation, light transmission, wave transmission and other functional characteristics, so they are more and more used in aerospace, information technology, national defense, biomedical and new energy fields. Basically maintain 7%~10% annual growth rate.

The reason why advanced ceramics have such excellent characteristics is that they use higher purity, smaller particle size raw materials and more complex preparation processes than traditional ceramics. With the continuous expansion of application fields, the requirements for the performance of advanced ceramics are also increasing. Therefore, based on the traditional preparation process, researchers have continuously developed new technologies specifically for improving the properties of special ceramics to promote their further development. Among these new technologies, forming and sintering technology has achieved a significant improvement and leap because of the introduction of vacuum technology.

Application of vacuum technology in ceramic molding

Good molding technology is the key to obtain high quality ceramic products. The green body forming methods are mainly divided into three categories: dry forming, plastic forming and wet forming. Among them, wet molding is a commonly used molding method, which is suitable for the preparation of ceramics with complex shapes and uniform composition. The preparation process is to prepare the powder into a slurry with a certain fluidity, and then fill the model, and then form after drying, including grouting molding, flow casting molding, centrifugal molding and gel injection molding.

The quality of the slurry is the key to determine the molding process, the slurry should have good fluidity and stability, but also need to reduce the bubble content of the slurry. Because water is often used as the solvent in slurry preparation, and the water solvent has a large surface tension and poor wetness of the powder, so it is easy to produce a large number of bubbles in the slurry configuration process, which seriously affects the quality of the green body. To this end, the researchers introduced vacuum technology into the preparation.

There are two main ways to introduce vacuum technology into the grouting molding method: one is to vacuum outside the plaster mold to increase the pressure difference between the inside and outside the mold; The other is grouting in a vacuum chamber under negative pressure. Both methods can effectively reduce air bubbles in the slurry.

Flow casting is often used to prepare flake ceramic materials, which has the advantages of continuous molding, simple operation, stable process, and less substrate defects. The preparation process is to first dissolve the powder raw material and the binder, plasticizer and dispersant in water-based or organic solvent, and get a uniform and stable suspension slurry after mixing. Then the slurry is put into the hopper of the flow casting machine to make it flow down to the conveyor belt, and on the conveyor belt is scraped and coated with a scraper. After drying and curing, the film green is obtained. In order to reduce the influence of bubbles, the slurry is treated with stirring and vacuum degassing before pouring into the casting machine, which can effectively improve the density and quality of the green body.

Gel injection molding is a new ceramic molding process developed by Oak Ridge National Laboratory in the early 1990s. The core is the use of organic monomer solutions that can polymerize into high-strength, laterally connected polymer-solvent gels. The green body obtained by this method has good uniformity, high strength, and small shrinkage during sintering, which is suitable for the molding of precision size ceramics. However, this method requires a long time of mechanical stirring in the process of preparation of slurry. Due to the presence of a large number of organic additives, high-speed stirring will make the slurry mixed with a large number of bubbles, and when the slurry solid content is high, these bubbles are usually difficult to rely on their buoyage to emerge from the liquid surface. The existence of bubbles is bound to form defects in the cross-linking process and affect the quality of the green body.