Study On Gelcasting Forming Process Of Composite Ceramic Materials Based On Silicon Nitride

Gelcasting technology for silicon nitride based ceramics forming was researched in this dissertation. Gel polymerization process, slurry preparation and green body drying were studied in view of technical design. To seek a practical gelcasting technical route applied for non-oxides ceramic forming, particularly for silicon nitride-based ceramics, mechanism of the gelcasting process was discussed. The chief investigation and innovation in this dissertation were as follows: (1) The polymerization reaction kinetics and the effect mechanism of initiator and catalyst in acrylamide gel system were studied. Polymerization reaction rate equation was fitted by trial-and-error method. The factors affecting the rate of polymerization were analyzed, and the effect of monomer and initiator concentration on the polymerization reaction rate and extent was studied. The results show that the rate of polymerization is proportion to the square root of initiator concentration and the monomer concentration in premixed solution. (2) High property silicon nitride slurry with solid loading of 60vol% and viscosity of 1Pa·s was prepared using poly(acrylic acid) ammonium (PAA-NH4) as dispersant and polyethylene glycol 400 (PEG400) as wetting reagent. The effect of PAA-NH4 on stability of slurry was researched. The wetting ability of PEG400 on particles was studied and adsorption characteristic of PEG400 on particle surface were discussed. The mechanism of this impact was explained through double-sphere model based on particle tangent compacts. (3) In this dissertation, surface modification of Si particle was carried out to solve the problem of original powder foaming in water and badly wetting ability. Slurry with high solid loading and thixotropy was prepared by high speed dispersing process. The adsorption state of dispersant on the particle surface was studied by thixotropic network model and the mechanism of PAA-NH4 on thixotropic slurry process was discussed. (4) The various factors impacting rheology and stability of silicon nitride-silica-sintering additive composite slurry were studied in this work. PAA-NH4 and polysiloxane S-17 were used as composite dispersant to meet the stability requirements of the various powders. S-17 was used as slurry defoamer in order to eliminate bubbles. Since the introduction of surfactant S-17 into the slurry, lowering internal liquid surface tension, thus reducing capillary force between particles, the problem of cracking could be solved. (5) Based on the theory of particle accumulation, the effect of dual and ternary grain-composition on the rheology and stability of slurry were researched using quartz sand as coarse powder, silicon nitride powder for fine particles, the nano-silicon nitride as a superfine powder. The results of experiments show that the stability of slurry was improved due to the introduction of a small amount of nano-silicon nitride. And the problem of increasing the solid loading of slurry containing nano-powder was solved. (6) Performance of green body was characterized. High intensity (20 MPa) was presented before burning out the plasticity. Body shrinkage rate nearly became zero when the solid loading came up to 60vol%. Uniformity and compaction of the internal structure were remained after burning out the plasticity.