In the era of new materials, in the field of ceramics and inorganic materials, advanced ceramics are one of the most important pillar industries at present. Compared with metals and polymer materials, advanced ceramic materials possess unparalleled structural properties such as high hardness, high modulus, high temperature resistance, and corrosion resistance, as well as excellent functional properties like electrical insulation, light transmission, and wave transmission. Therefore, they are increasingly being applied in fields such as aerospace, information technology, national defense and military industry, biomedicine, and new energy. Basically maintain an annual growth rate of 7% to 10%.

Advanced ceramics possess such outstanding properties because, compared with traditional ceramics, they use raw materials of higher purity and finer particle size as well as more complex preparation processes. With the continuous expansion of application fields, the requirements for the performance of advanced ceramics are also constantly increasing. For this reason, researchers have been constantly developing new technologies based on traditional preparation processes, specifically designed to enhance the performance of special ceramics, in order to promote their further development. Among these new technologies, the forming and sintering technologies have achieved remarkable improvements and leaps due to the introduction of vacuum technology.

01 Application of Vacuum Technology in Ceramic Forming

Good forming technology is the key to obtaining high-quality ceramic products. The main methods of green body forming are divided into three categories: dry forming, plastic forming and wet forming. Among them, wet forming is currently a commonly used forming method, suitable for preparing ceramic products with complex shapes and uniform compositions. The preparation process involves converting powder into a slurry with certain fluidity, then filling it into a mold, and after drying, forming it. It includes grouting forming, casting forming, centrifugal forming, and gel injection molding, etc.

The quality of the slurry is the key to determining the molding process. It is required that the slurry should have good fluidity and stability, and the bubble content of the slurry also needs to be reduced. Since water is often used as the solvent in slurry preparation, and the surface tension of water solvents is large, their wettability to powder materials is poor. Therefore, a large number of bubbles are prone to occur during the slurry configuration process, which seriously affects the quality of the formed green body. For this reason, researchers introduced vacuum technology in the preparation.

02 Application of Vacuum Technology in Ceramic Sintering

The influence of atmosphere during the sintering process is complex and significant. Sintering atmospheres are generally classified into three types: oxidizing, reducing and neutral. Generally speaking, a reducing atmosphere promotes the detachment of anions from the crystal surface, increases the diffusion coefficient, and is conducive to sintering. An oxidizing atmosphere is conducive to the diffusion of cations. Since oxide sintering is mostly controlled by O2-diffusion, a reducing atmosphere (especially vacuum) is more suitable. Vacuum also promotes the expulsion of internal gases from the green body, accelerating densification. Thus, heating in a vacuum atmosphere can yield dense ceramics.

1. Vacuum Sintering

The influence of atmosphere during the sintering process is complex and significant. Sintering atmospheres are generally classified into three types: oxidizing, reducing and neutral. Generally speaking, a reducing atmosphere promotes the detachment of anions from the crystal surface, increases the diffusion coefficient, and is conducive to sintering. An oxidizing atmosphere is conducive to the diffusion of cations. Since oxide sintering is mostly controlled by O2-diffusion, a reducing atmosphere (especially vacuum) is more suitable. Vacuum also promotes the expulsion of internal gases from the green body, accelerating densification. Thus, heating in a vacuum atmosphere can yield dense ceramics.

The emergence of vacuum sintering technology has laid a technical foundation for the development of various optical functional ceramics. In 1995, Ikesue et al. mixed refined Y2O3 with high-purity Al2O3 and Nd2O3 powders, added ball milling AIDS, dispersants, etc., and evenly mixed them by ball milling. After isostatic pressing and vacuum sintering at 170-1800 ℃, high-transparency YAG:Nd ceramics with a relative density of 99.98% were obtained. Its average grain size is 50µm, and it has achieved laser output for the first time.

2. Vacuum hot-pressing sintering

Hot-pressing sintering is a process in which pressure is applied to powders that are difficult to sinter in a mold while the temperature is raised for sintering. The mold is required to be conductive and have strong pressure resistance. Currently, graphite is the most widely selected material for molds. Graphite is prone to oxidation at high temperatures, but it is precisely under a sealed vacuum condition that the service life of graphite can be effectively prolonged. Meanwhile, vacuum conditions can also promote the diffusion of anions. Therefore, vacuum hot-pressing sintering has now become an important sintering method.

Optical functional ceramics can be prepared by vacuum hot pressing. The assistance of external pressure can effectively promote densification, shorten sintering time and reduce energy consumption. In addition, for other ceramics that are difficult to sint densely under normal pressure, vacuum hot pressing technology has also achieved good performance, such as the widely used ZnO and MgO ceramics.

3. Spark Plasma Sintering (SPS）

Discharge plasma sintering technology is an improvement on hot-press sintering technology, introducing the activation effect of plasma and enhancing the heating efficiency. The SPS technology still uses graphite molds similar to those used in hot-press sintering, so the vacuum atmosphere remains an important condition affecting the sintering process. Sintering under vacuum conditions can, on the one hand, protect the material itself from oxidation, and on the other hand, prevent the resistance change caused by the oxidation of graphite, thus

avoiding the impact on the transmission of pulse current and the densification process.

SPS can efficiently prepare difficult-to-sinter ceramic materials. Besides common ceramics such as Al2O3 and ZrO2, SPS technology can also be applied to the preparation of many difficult-to-sinter materials, including ultra-high-temperature ceramics like ZrB2, HfB2, ZrC, and TiN, as well as refractory metals such as W, Re, Ta, and Mo and their alloys.