Silicon nitride nanopowders are widely used in structural ceramics due to their excellent mechanical strength, high-temperature resistance, and chemical stability. The uniform suspension state of the powder before sintering directly determines the density and microstructure uniformity of the green body, thus affecting the performance of the final ceramic product. This is a core and critical step in the preparation of silicon nitride ceramics.
The inherent characteristics of nanopowders present a natural challenge to achieving uniform suspension. Silicon nitride nanoparticles have a large specific surface area and high surface energy, easily forming soft and hard agglomerates through van der Waals forces. If dispersion is insufficient, defects such as porosity and cracks are prone to appear in the sintered product, significantly reducing mechanical properties. Simultaneously, silicon nitride is a strongly covalent compound with complex surface chemistry. It easily undergoes surface dissolution in different media, generating specific ions that affect interparticle interactions, further increasing the difficulty of controlling suspension stability.
The choice of medium is fundamental to achieving uniform suspension. While water-based systems are inexpensive, silicon nitride is easily oxidized and its surface reactions are complex. Precise control of the system's pH value is necessary to adjust the electric double layer charge on the particle surface and enhance the repulsive force between particles. Organic media, such as anhydrous ethanol, can reduce powder oxidation and surface dissolution, and are commonly used dispersion media. When combined with suitable dispersants, highly stable suspensions can be achieved; some studies have achieved stable suspension systems with a solids volume fraction exceeding 50%.
Combined dispersion processes are the core means to optimize suspension effects. In physical dispersion, ultrasonic dispersion can break up soft agglomerates through energy transfer. Its dispersion effect is closely related to the ultrasonic time, and excessive ultrasonication should be avoided to prevent secondary damage to particles. Chemical dispersion modulates interfacial interactions by adding dispersants. Cationic surfactants have better dispersion effects than nonionic surfactants, and some specialized dispersants can adsorb onto the particle surface through chemical bonds, forming a steric hindrance effect to inhibit agglomeration.
Synergistic process control is key to ensuring suspension stability. Combining ultrasonic dispersion with surfactant modification can reduce the particle size of silicon nitride nanoparticle agglomerates to below 200 nm. Simultaneously, parameters such as the powder-to-media ratio and stirring rate must be controlled to avoid secondary agglomeration during the process. After being shaped, the uniformly suspended powder can increase the relative density of the green body to over 65%, laying the foundation for subsequent sintering and densification, ultimately achieving optimized and upgraded performance of silicon nitride ceramics.
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