The importance of silicon carbide powder modification process
Silicon carbide (SiC) is an inorganic non-metallic material with a wide range of uses and good development prospects. After being made into ceramics, it is an excellent structural material. It has high elastic modulus and specific stiffness, is not easy to deform, and has High thermal conductivity and low thermal expansion coefficient have now become one of the main considerations for high-temperature heat engine materials, and can be used in high-temperature nozzles, turbine blades, turbocharger rotors, etc.
Therefore, the industry has put forward higher requirements for SiC ceramics in terms of geometric accuracy, strength, toughness and reliability, and the molding process is a crucial part. Different molding processes have a greater impact on the performance of ceramic products, such as difficulty in demoulding, difficulty in preparing products with complex shapes, insufficient density of ceramics, etc. The existence of these defects will restrict their application in high-end fields. Therefore, it is necessary to To prepare ceramic products with excellent performance and high reliability, it is necessary to explore the factors that affect the effectiveness of the molding process.
The silicon dioxide layer on the surface of silicon carbide will affect the dispersion of the powder in the aqueous phase. The silicon dioxide will form silicon hydroxyl groups “Si-OH” in the water phase. The silicon hydroxyl groups are acidic in the water phase, so the dispersion of silicon carbide is The isoelectric point is acidic. The more silicon dioxide there is, the closer the isoelectric point of silicon carbide is to the acidic end. When the pH value is lower than the isoelectric point of the powder, silanol will attract hydrogen ions, making the particle surface positively charged and thus the Zeta potential. becomes a positive value. Under alkaline conditions, silanol will react with the high concentration of OH- in the solution to form [Si-O]- on the surface of the powder, making the surface of the particles negatively charged, so the Zeta potential is also negative.
The dispersion of powder in the water phase is closely related to the absolute value of Zeta potential, so the silica layer formed on the surface of the powder plays a great role in the dispersion of the powder.
Chemical modification method refers to the chemical reaction that occurs during the surface coating process. This is the most common method in powder modification. Surface coating is divided into two types: inorganic coating and organic coating. It mainly deposits a layer of oxide, hydroxide or organic matter on the surface of inorganic powder. When the coating is an oxide or hydroxide, it is called inorganic coating. When the coating is organic, it is called organic coating.
Inorganic coating methods mainly include alkoxide hydrolysis method, uniform precipitation method, non-uniform nucleation method, and sol. Gel method, etc. Among them, the best method is the non-uniform nucleation method. Organic coating modification improves the electrostatic and steric hindrance of inorganic powder, thereby improving its dispersion. Organic coating methods mainly include organic surface grafting, surface adsorption coating and encapsulation modification. It is mainly used in the dispersion of inorganic composite materials or fillers to improve the wettability and compatibility of inorganic powders and organic matrices. It is also used to improve the dispersion of inorganic powder in water.
Highly dispersible micron-sized SiC powder is a necessary condition for obtaining ceramic products with high accuracy, strength, toughness and reliability. Therefore, it is of great significance to explore related technologies to prepare silicon carbide ceramics that can be used in high-end fields.