The surface modification of silicon nitride powder is mainly achieved through physical and chemical methods to improve the physical and chemical properties of silicon nitride particles.
Surface modification of the powder can reduce the mutual attraction between powder particles, improve the dispersion of the powder in the medium, and enhance the dispersibility of the powder slurry. At the same time, it is also possible to enhance the surface activity of silicon nitride powder, increase the compatibility of powder particles with other substances, and thus generate new properties of the powder.
The main principle of surface modification of powders is the interaction between powders and surface modifiers to improve the wettability of the powder surface and enhance its dispersion in aqueous or organic media.
1. Surface coating modification
Surface coating modification technology utilizes the principles of physical adsorption or chemical adsorption to uniformly attach coating materials to the coated body, forming a uniform and complete coating layer on its surface. The coating layer formed throughout the entire coating process is usually a monolayer.
Coating modification is generally divided into inorganic coating and organic coating. Inorganic coating mainly refers to the deposition of suitable oxides or hydroxides on the surface of ceramic particles to modify the powder, but only in terms of physical properties. And organic coating is the selection of some organic substances as coating materials, which bond with the functional groups on the surface of the powder particles and selectively adsorb on the surface of the ceramic particles, thus making the powder exhibit the properties of a coating layer.
SAT NANO adds zirconia and rare earth oxide yttrium oxide as sintering additives to coat the surface of silicon nitride, resulting in increased density and improved fracture toughness of the modified silicon nitride.
SAT NANO uses the alcohol thermal reduction method to coat the surface of silicon nitride powder with ruthenium, and characterizes the powder using infrared spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy.
This modification technology has low cost, simple method steps, and easy control, but the effect after modification is often average.
2. Surface acid-base treatment modification
Ceramic molding processes generally require ceramic slurries to achieve high solid content and low viscosity, and the charge density on the surface of the powder has a significant impact on the rheological and dispersibility of the powder slurry. The surface charge properties of ceramic powders can be altered through washing treatments (acid washing and alkali washing). As the name suggests, this modification method involves thoroughly mixing and washing silicon nitride powder with a series of concentrated acid or alkali solutions.
SAT NANO effectively removes impurities such as Mg2+and Ca2+from the surface of silicon nitride powder through acid washing treatment, thereby changing the ion conductivity of the powder slurry and altering the properties of the powder surface.
SAT NANO analyzed the XPS of the powder after acid washing treatment and found that the surface oxidation degree of the modified powder decreased, the isoelectric point of the powder slurry increased, and the maximum solid content of the slurry reached 55%. A ceramic slurry capable of casting process was prepared.
At the same time, a certain concentration of alkali treatment can also react with the surface of ceramic powder. According to the research of Wang Yongming et al., alkali washing treatment can reduce the content of silicon hydroxyl groups on the surface of silicon carbide powder, reduce its oxidation degree, change the electrostatic repulsion between particles, and improve the rheological properties of the slurry.
3. Dispersant modification
Choosing appropriate or designing new dispersants based on the differences in different types of ceramic powders plays a key role in improving the solid content of ceramic slurries. The types and amounts of dispersants have significant differences in their effects on changing the properties of ceramics.
Dispersants generally have both hydrophilic and hydrophobic structures, and it is precisely through the interaction between their own hydrophilic and hydrophobic water groups that the dispersibility of ceramic slurries is adjusted. The types of dispersants include surfactants or polymer electrolytes, among which surfactants include cationic surfactants and anionic surfactants.
Polymer electrolytes include polyethylene sulfonic acid, polyacrylic acid, polyethylene pyridine, polyethylene imine, etc. Dispersants can undergo adsorption reactions with the surface of powders, including chemical adsorption and physical adsorption, utilizing the interaction forces between particles (van der Waals forces, electrostatic repulsion) and the principle of potential steric hindrance effects.
In terms of adding dispersants, SAT NANO successfully prepared silicon nitride slurries with solid contents ranging from 51% to 55% using tetramethylammonium hydroxide (TMAH) and tetraethylammonium hydroxide (TEAH) as additives.
SAT NANO studied the modification effects of several different dispersants on the surface of silicon nitride powder. By measuring the Zeta potential of the modified powder slurry, it was found that adding ammonium polyacrylate is more conducive to preparing high solid content silicon nitride powder slurry.
4. Surface hydrophobicity modification
Surface hydrophobicity modification is the process of converting hydroxyl groups in ceramic powders into hydrophobic groups, such as alkyl, long-chain alkyl, and cyclic organic compounds. These organic compounds exhibit a strong hydrophobic effect when combined with the surface of ceramic powders, allowing for better dispersion in the dispersion medium and reducing the likelihood of agglomeration.
When surface grafting treatment is carried out on silicon nitride powder, the long chains of the polymer are in contact with the surface of the powder, and the hydrophilic chains at the other end extend in the aqueous medium. During the entire dispersion process, there are both repulsive forces between the powder particles and steric hindrance generated by the long chains of the polymer, resulting in better dispersion effect of the slurry.
SAT NANO is a best supplier of silicon nitride powder in China, we can offer nano particle and micron particle, if you have any enquiry, please feel free to contact us at sales03@satnano.com
