| Silicon nitride ceramics have advantages with high temperature resistance, corrosion resistance and high mechanical strength, it is widely used in areas such as filtration and separation, and the introduction of the pore structure in the ceramic and the growth of whiskers in the pores can greatly improve filtration performance of silicon nitride porous ceramics. In this paper, we firstly utilized silicon(Si) and silicon nitride(Si3N4) powers serving as skeletal materials and sodium alginate serving as gels to prepare sodium alginate-silicon(NaAlg-Si) slurry and sodium alginate-silicon nitride(NaAlg-Si3N4) slurry, and used ionotropic gelation of sodium alginate to make the slurry immobilization in situ to form Si and Si3N4 green bodies and then pore structure was introduced into green bodies. Thus, we can obtain green body with honeycomb-like pore structure. Subsequently, as-prepared green bodies were sintered through nitridation of silicon and hot pressing sintering method to fabricate silicon nitride honeycomb ceramics(sintered bodies). Then, the stability of slurry was studied by Zeta potential instrument and the pore size of green bodies were measured by mercury injection apparatus, morphology and phase composition of green bodies and sintered bodies were characterized by scanning electron microscope(SEM) and X-ray diffraction(XRD). We also studied the influence of sodium alginate concentration and solid content on the formation of channels, and explored the effects of different sintering temperature and holding time on the formation of silicon nitride ceramics and crystal phase composition. The results we obtained are as follows.(1) By the slurry sedimentation experiments, we found that, for either NaAlg-Si slurry or NaAlg-Si3N4 slurry, in the whole standing time(72 h), settlement amount of the slurry with dispersant was smaller than the slurry without dispersant, therefore the stability of the slurry with dispersant was better. After spraying CaCl2 solution onto the surface of NaAlg-Si slurry and NaAlg-Si3N4 slurry with dispersant, the slurry height kept constant in the whole standing time(72 h), and it was demonstrated that the slurry was still stable during ionotropic gelation process and the solidified body had relatively uniformity.(2) In the experiment, we used scanning electron microscope(SEM) to observe the pore structure of green bodies. We observed that the channels in the silicon and silicon nitride green bodies were parallel to the direction of Ca2+ diffusion uniformly and were honeycomb-like. Moreover, the pore size of green bodies decreased while the concentration of sodium alginate and solid content increased. The data of the pore size measured by the mercury better explained the impacts of concentration of sodium alginate and solid content on the pore size.(3)We use nitridation of silicon to sinter Si green bodies and Si was all transferred to Si3N4 by reaction of Si and N2. At lower temperatures, there was mainly α-Si3N4 in the sintered bodies by nitridation of silicon. Its content is far more than β-Si3N4. And α-Si3N4 and β-Si3N4 content decreased with decrease of the sintering temperature and the holding time. At higher temperatures, nitrided Si bodies were all converted to β-Si3N4. Si3N4 green bodies were sintered directly through high temperature sintering in nitrogen atmosphere to form silicon nitride honeycomb ceramic only containing β-Si3N4.(4)After morphology characterization of sintered bodies by scanning electron microscopy, pores and whiskers structure can be observed clearly in the bodies. At low temperature, pore wall of sintered bodies gained by nitridation of silicon were occupied with linear α-Si3N4 whiskers, and at high temperature pore wall were occupied with rod-like β-Si3N4 whiskers. In addition, by directly sintering silicon nitride green bodies at high temperature, the rod-like β-Si3N4 whiskers covered the entire channels in Si3N4 sintered bodies sintered directly at high temperature. |
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