How to modify the surface of silicon nitride powder?
Surface modification of silicon nitride powder primarily involves treating the surface of the powder through various physical and chemical methods to improve the physical and chemical properties of the particles.
Surface modification can reduce the mutual attraction between powder particles, allowing for better dispersion of the powder in the medium and improving the dispersibility of the powder slurry. It can also enhance the surface activity of the silicon nitride powder, increasing its compatibility with other substances and thus developing new properties.
The main principle of powder surface modification is that the interaction between the powder and the surface modifier enhances the wettability of the powder surface and improves its dispersion in aqueous or organic media.
1. Surface Coating Modification
Surface coating modification technology utilizes physical or chemical adsorption to uniformly attach the coating material to the surface of the coated object, forming a uniform and complete coating layer. The coating layer formed during the coating process is typically a monolayer.
Coating modification is generally categorized as inorganic and organic. Inorganic coating primarily involves depositing appropriate oxides or hydroxides on the surface of ceramic particles to modify the powder, but this modification only affects physical properties. Organic coating, on the other hand, involves selecting organic substances as coating materials. These organic substances bond with groups on the surface of the powder particles and selectively adsorb onto the surface, imparting the properties of the coating layer to the powder.
This modification technology offers low cost, simple steps, and easy control, but the resulting results are often limited.
2. Surface Acid and Alkali Treatment
Ceramic molding processes generally require ceramic slurries with high solids content and low viscosity. The charge density on the powder surface significantly influences the rheological and dispersibility of the slurry. Washing the ceramic powder surface (acid and alkaline treatments) can alter the surface charge properties of the powder. As the name suggests, this modification method involves thoroughly mixing and washing the silicon nitride powder with acid or alkaline solutions of varying concentrations.
At the same time, alkaline treatment at a certain concentration can also react with the surface of ceramic powders. Research by Wang Yongming et al. has shown that alkaline washing can reduce the silanol content on the surface of silicon carbide powder, lowering its degree of oxidation, altering the electrostatic repulsion between particles, and improving the rheological properties of the slurry.
3. Dispersant Modification
Based on the differences between different types of ceramic powders, selecting an appropriate dispersant or designing a new one plays a key role in increasing the solid content of the ceramic slurry. The type and amount of dispersant added can significantly alter the effect on ceramic properties.
Dispersants generally have both hydrophilic and hydrophobic structures, and it is through the interaction between these hydrophilic and hydrophobic groups that they adjust the dispersion properties of the ceramic slurry. Dispersants include surfactants or polymer electrolytes, with surfactants including cationic and anionic surfactants.
Polymer electrolytes include polyvinyl sulfonic acid, polyacrylic acid, polyvinyl pyridine, and polyethyleneimine. Dispersants can undergo adsorption reactions with the powder surface, including chemical and physical adsorption, leveraging interparticle forces (van der Waals forces and electrostatic repulsion) and the potential for steric effects.
4. Surface Hydrophobicity Modification
Surface hydrophobicity modification involves converting the hydroxyl groups in ceramic powder into hydrophobic groups, such as hydrocarbon groups, long-chain alkyl groups, and cycloalkyl groups. These organic groups bind to the ceramic powder surface, exerting a strong hydrophobic effect, enabling better dispersion in the dispersion medium and preventing agglomeration.
When polymers are grafted onto the surface of silicon nitride powder, the long polymer chains attach to the powder surface, while the hydrophilic chains at the other ends extend into the aqueous medium. Throughout the dispersion process, the powder particles experience both interparticle repulsion and steric hindrance created by the long polymer chains, resulting in better slurry dispersion.