Granulation of Silicon Nitride Powders by Spray Drying: A Review

Abstract

Spray drying is a widely used method of converting liquid material (aqueous or organic solutions, emulsions and suspensions) into a dry powder. Good flowability, narrow size distribution, and controllable morphology are inherent in powders produced by spray drying. This review considers the granulation factors that influence the final properties of the silicon nitride dried powders. The first group includes the types of atomizers, manifolds, and drying chamber configurations. The process parameters fall into the second group and include the following: inlet temperature, atomizing air flow, feed flow rate, drying gas flow rate, outlet temperature, and drying time. Finally, the last group, feedstock parameters, includes many factors such as feed surface tension, feed viscosity, solvent type, solid particle concentration, and additives. Given the large number of factors affecting morphology, particle size and moisture, optimizing the spray drying process is usually achieved by the "trial and error" approach. Nevertheless, some factors such as the effect of a solvent, dispersant, binder, and sintering additives considered in the literature that affect the Si₃N₄ granulation process were reviewed in the work. By summarizing the data available on silicon nitride powder production, the authors attempt to tackle the problem of its emerging demand in science and industry.

Keywords: binder; dispersant; powders; silicon nitride; solvent; spray drying.

Figure 1

Diagram of a common laboratory scale spray dryer. Droplets and granules are depicted oversized for better visibility. Reprinted from [49].

Figure 2

Products with high extensional resistance inhibits droplet pinch-off, tending to result in larger drop sizes. Reprinted from [58].

Figure 3

Silicon nitride granules obtained in [72].

Figure 4

Scanning electron microscopic diagram showing a spherical powder spray granulation process produced in accordance with the embodiments of the [72].

Figure 5

Comparison of granule structure. Dispersion state of spray-dry slurry was varied by controlling pH of slurry. (a) dispersed (pH = 10.8) and (b) flocculated (pH = 7.9). Reprinted from [66].

Figure 6

SEM observation for spray-dried granules. Sintering aid concentration 5 wt%. Concentrations of water-soluble maleic anhydride dispersant were (a) 0 wt% and (b) 2 wt%. Reprinted from [69].

Figure 7

Surface structures of granules spray-dried from Si₃N₄-Y₂O₃-Al₂O₃-AlN-TiO₂/toluene slurries stabilized with PEI-OA ((a) 1.00 mg/m³, (b) 1.30 mg/m³, (c) 1.55 mg/m³ and PEI-ISA, (d) 0.80 mg/m³). Reprinted from [61].

Figure 8

SEM image of spray-dried granules from PEI-OA (1.30 mg/m²) stabilized Si₃N₄-Y₂O₃-Al₂O₃-AlN-TiO₂/toluene slurries using (a) paraffin and (b) PMAO as binders. Reprinted from [61].