Stabilization Mechanisms And In-situ Coagulation Casting Of Alumina Slurry Dispersed In Silica Sol Without De-airing Process To Prepare Mullite Ceramics

Colloidal in-situ consolidation forming technique has been one of the research hotspots on thea high performance ceramics in the recent decades, due to their significant advantages, such as near-net shape capability, complex shape capability, simplicity, versatility, and well-distribution, minor defect, as well as reducing the cost of manufacturing, which possesses broad prospects in the future. At present, such new forming methods have been widely studied and used to prepare Al₂O₃, ZrO₂, SiC, Si₃ N₄, PZT, etc. Mullite (3Al₂O₃·2SiO₂) ceramics possess low density, low thermal conductivity, low thermal expansion, high bend strengths and low creep rate at high temperatures, which make mullite a promising and important matrix candidate for ceramic matrix composites for use in advanced structure materials. However, few studies have been focused on the colloidal in-situ consolidation forming methods, especially the without-de-airing-process-forming technologies, of mullite ceramics.In this paper, present situations of colloidal in-situ consolidation forming processes together with composing, shaping and sintering of mullite material have been systematically reviewed. Dispersion behavior, stabilization and de-stabilization mechanisms of the alumina slurry dispersed in the silica sol have been systematically studied. Then a without-de-airing-process gelcasting technology and a without-de-airing-process hydrolysis-assisted in-situ consolidation forming method have been developed to prepare mullite green body, based on the stabilization mechanism of the colloidal gas aphrons (CGA) in the slurry. Simultaneously, nano-SiO₂-coated-Al₂O₃ (NSCA) micro-composite systems have been derived to realize the transient viscous sintering of mullite ceramics, utilizing the sol-gel transition of the silica sol during forming process of the green body. Furthermore, mullite seeds have been in-situ composed at about 1100¹250℃ to accelerate mullitization and densification of the sintering body, making use of the formation of minor alumina-silica gel in the green body through hydrolysis-assisted coagulation casting technology. The main achievements obtained are as follows:(1) Dispersion behavior, stabilization and de-stabilization mechanisms of the alumina slurry dispersed in the silica sol have been systematically studied at first. The results showed that the silica sol could enhance the dispersion and the stability of alumina suspension at about pH 9.5. It is proposed that the dispersion of alumina in silica sol at basic condition is dominated by a depletion stabilization mechanism, because there is no evidence of colloidal silica nanoparticle adsorption on the aluminaparticle surface exposed to the silica sol. The slurries showed less stable with decreasing the pH values and showed the highest viscosity in the range of pH 4.0-5.5, with the variations of the concentration of the silica sol and the alumina volume loading in the slurry. It is found that the de-stabilization mechanisms of the slurry belong to the sol-gel effect, due to the formation of the Al + which results in the sol-gel transition of the silica sol while adjusting the pH value of the slurry by adding HCl solution. With further decreasing the pH values, the slurry showed more and more stable gradually and showed the most stable at about pH 2.0. The re-stabilization of the slurry is found to be caused by the short-range hydration force between particles in the slurry, due to the high electrolyte concentrations in the slurry.(2) Based on the stabilization mechanism of the colloidal gas aphrons (CGA) in the slurry, a type of gelcasting technology without de-airing process has been developed to prepare mullite ceramics. Concentrated alumina slurry dispersed in silica sol (with chemical composition of stoichiometric pure mullite) with solid loading of 50 vol.% and with viscosity as low as 956 mPa-s was successfully prepared at about pH9.5, utilizing the depletion stabilization effect of the silica colloidal particles on the dispersion of the alumina powders. In-situ consolidation of the slurry was realized through adjustment of NH4C1 concentration to control the gelation time of the slurry. The defect-free dried green body with bending strength of 4.83 MPa could be obtained by gelcasting technology without de-airing process, because there existed no surfactants in the slurry, thus only a small amount of unstable bubble were entrapped, which can be automatically eliminated without de-airing process. The linear shrinkage and the relative density of the green body are about 4.2% and 57.6%, respectively. It indicated that a nano-SiC^-coated-AbC^ (NSCA) micro-composite system has been derived due to the sol-gel transition of the silica sol, which is beneficial to the transient viscous sintering of the mullite ceramics. The initial and final mullitization temperatures were about 1400°C and 1600°C respectively. The densification behavior mainly happened before the final mullitization through transient viscous sintering of the amorphous silica, and the relative density was about 95.0% after sintering at 1650°C for 2 h.(3) Utilizing the sol-gel de-stabilization effect caused by the formation of the Al34 in the slurry, a type of hydrolysis-assisted coagulation casting technology without de-airing process has been developed to prepare mullite ceramics. At about pH 2.0, concentrated alumina slurry dispersed in silica sol (with chemical composition of stoichiometric pure mullite) with solid loading of 55 vol.% and withviscosity as low as 976 mPa-s was successfully prepared. In-situ consolidation of the slurry was realized through adjustment of pH values from 2.0 to 5.0 utilizing the hydrolysis of the urea at 80°C. The defect-free dried green body with bending strength of 5.14 MPa could be obtained by hydrolysis-assisted coagulation technology without de-airing process, because urea has no lyophobic tail end and don't show surface-active, thus only a small amount of unstable bubble were entrapped in the slurry, which can also be automatically eliminated without de-airing process. The linear shrinkage and the relative density of the green body are about 3.4% and 61.3%, respectively. It is found that a nano-SiO2-coated-Al2O3 micro-composite system has also been derived after the gelation of the slurry. The results indicated that the initial and final mullitization temperatures have declined to 1300°C and 1500°C, respectively, due to the in-situ formation of the even-distributed mullite seeds at about 1100— 1250°C, which could enhance the mullitization of the specimen. And densification occurs in two stages: in the presence of a silica-alumina mixture and after conversion to mullite. The first stage of densification occurs through transient viscous sintering and a further sintering stage was observed following mullitization, because of the promotion of the solid-state sintering by the in-situ formed mullite seeds. And the relative density was about 97.5% after sintering at 1650°C for 2 h.