The performance of advanced ceramic materials is to a certain extent determined by its microstructure, and the quality of the microstructure depends on the preparation process. The preparation process of advanced ceramics is mainly composed of powder preparation, molding and sintering. In the three links, powder preparation is the basis. If the quality of the basic powder is not high, even with great efforts in forming and sintering, it is difficult to obtain the ideal microstructure and high-quality advanced ceramic products.
The quality of the powder will directly affect the quality of molding and sintering. For example, the powder particles are coarse, severely agglomerated, and poor in fluidity. No matter what molding method is used, it is difficult to obtain a green body with uniform texture, few defects, and high density. Such a green body will inevitably lead to a narrow sintering temperature, making the sintering conditions difficult Controlled, it is impossible to produce a sintered body with uniform microstructure, high density, few internal defects and smooth external surface. Therefore, the quality of the powder is critical to the performance of advanced ceramic materials.
For example,the key to preparing high-performance alumina ceramics lies in the purity, particle morphology and particle size distribution of the raw material powder. The raw material for the preparation of transparent alumina ceramics is high-purity ultrafine powder with a purity of up to 99.99% and a particle size of less than 100 nm, and the growth of sapphire crystal requires the purity of the raw material to reach 99.999%.
Stable powder raw materials are still the key to the preparation of high-performance zirconia ceramics. PSZ ceramics often use 3% yttria-stabilized zirconia nanopowder as the raw material. The zirconia nanopowder is generally prepared by chemical co-precipitation with an average particle size of 20-40 nm. It is easy to reunite due to its high activity. Solving the problem of agglomeration of zirconia nanopowders is the key to preparing zirconia ceramics with uniform structure and stable performance.
In general, advanced ceramics have the following requirements on the characteristics of powder raw materials:
1. The chemical composition is precise. This is a basic requirement. For advanced ceramics, the chemical composition directly determines the crystal phase and performance of the product.
2. Good chemical composition uniformity. That is, the chemical composition distribution is uniform and uniform. If the chemical composition is not uniform, it will lead to the deviation of the local chemical composition, which will result in the segregation of the local phase and the difference and abnormality of the microstructure. At the same time, the performance of the final sintered body will decrease.
3. High purity. It is required that the content of impurities in the powder be low, especially the content of harmful impurities as low as possible, because the presence of impurities will affect the process performance of the powder and the physical and chemical properties of the sintered body.
4. Appropriately small particle size. Properly small particle size can reduce the sintering temperature and effectively reduce the particle size of the sintered body, and at the same time can meet the requirements of ceramic materials less than the critical size.
5. Spherical particles with uniform size. Spherical particles have good fluidity, high particle packing density, and uniform pore distribution, so that the growth of crystal grains and the elimination and distribution of pores can be effectively controlled during the forming and sintering densification process to obtain uniform microstructure and performance Excellent and consistent products.
6. Good dispersion and less reunion. Minimize soft and hard reunions.