Jiang Jiawei 1, 2, Zhang Lijuan 1, 2, 3, Wen Guangwu 1, 2, Hou Yongzhao 1, 2,
Zhang Zhongya 1, 2, Zhong Cheng 1, 2
(1. School of Materials Science and Engineering, Shandong University of Science and Technology, Zibo 255000, Shandong, China;
2. Engineering Ceramics Research Institute, Shandong University of Science and Technology, Zibo 255000, Shandong, China;
3. Additive Manufacturing Research Institute, Shandong University of Science and Technology, Zibo 255000, Shandong, China)
Extended abstract:
[Background and purposes] Si3N4 ceramics are widely used in aerospace, new energy vehicles and semiconductors, due to their high hardness, high temperature resistance and thermal shock resistance, but forming complex components still faces challenges, such as difficulty in traditional processes to adapt to irregular-shaped structures, poor dimensional accuracy and low processing efficiency. Binder jetting technology, with its high efficiency, high precision and good material adaptability, is used to construct green bodies by spreading powder and jetting binder layer-by-layer, and then obtain parts through debinding and sintering, demonstrating significant advantages. Among them, powder flowability is the key to high-quality powder beds. Currently, irregular powders are difficult to meet the requirements, due to their poor flowability. By granulating submicron powders into micron-sized spherical powders, flowability can be effectively increased, facilitating uniform spreading, increasing packing density and optimizing particle size distribution. Spray drying is the main method for preparing spherical ceramic powders, due to its low cost and high efficiency. Its principle is to atomize the slurry and then rapidly dry it with hot air, causing the droplets to shrink into spheres under the action of surface tension. High solid content slurries help to increase powder density, where dispersants need to be added to reduce viscosity and binders are added to maintain spherical shape.
[Methods] Si3N4 slurries for spray drying were prepared by dissolving polyvinyl alcohol in deionized water at 80 ℃, into which Si3N4 and dispersant were added, after cooling down, followed by ball milling. An airflow-type spray dryer was used for spray drying, at atomization pressure of 0.1 MPa, temperature of 190 ℃, peristaltic pump speed of 20 r·min−1, nozzle diameter of 1 mm and cyclone separator frequency of 50 Hz. The resulting spherical powder was classified by airflow to remove fine particles, then heat-treated in a tubular furnace under in N2. It was first heated at 3 ℃·min−1 to 200 ℃ and held for 30 min, then heated at 1 ℃·min−1 to 600 ℃ and held for 2 h, and finally cooled at 3 ℃·min−1 to room temperature.
[Results] This study was aimed to systematically evaluate the effects of dispersants and binders on the properties of Si3N4 slurries and spray-dried spherical powders. The specially formulated ammonium salt 4705, at an addition level of 0.4 wt.%, can be used to reduce the viscosity of a 50 wt.% high-solid-content slurry to 90.7 mPa·s, significantly outperforming sodium dodecyl sulfate (SDS) and ammonium polyacrylate (PAA-NH4), which does not cause foaming, thus meeting the requirements of the spray-drying process. The spherical powder prepared with PVA2488 (0.8 wt.%) exhibited the highest overall performance, with a D50 of 24.5 µm, a repose angle of 43.6° and a sphericity of 84%. After air classification, the powder's D50 increased to 37 µm, the repose angle decreased to 35.3° and its flowability was significantly increased. TG analysis and infrared spectroscopy showed that heat treatment at 600 ℃ in N2 can effectively remove organics, and the powder retained complete spherical morphology, with a repose angle of about 34.9° and sphericity of 89%. Using the optimized spherical powder for binder jetting printing, well-formed green bodies were obtained, without obvious defects, demonstrating its feasibility in the preparation of small Si3N4 ceramic prototypes.
[Conclusions] The problem of high viscosity and poor flowability of high-solid-content slurries was addressed with spray drying. 0.4 wt.% specially formulated ammonium salt 4705 was used as a dispersant to reduce the viscosity of Si3N4 slurry with a solid content as high as 50 wt.% to only 90 mPa·s, making it suitable for spray drying. The binder affects the morphology and flowability of the powder, with PVA2488 as the binder producing spherical powder with few anisotropic structures and a narrow particle size distribution. By combining with airflow classification technology, performance of the spherical powder was further optimized, achieving a D50 of 37.4 µm, a repose angle of 34.9° and a sphericity of up to 89%. The resulting Si3N4 spherical powders exhibited excellent flowability, meeting the process requirements for binder jetting. The powder has significant processing advantages in preparing small-sized geometrically simple Si3N4 ceramic prototypes.
Key words: Si3N4; spray drying; dispersant; binder; spherical powder