De-stabilization mechanism and in situ solidification of alumina slurry dispersed in silica sol

Effects of pH values, NH4Cl concentration and AlCl3 concentration on stability of the silica sol were studied to help elucidating the de-stabilization mechanism of the alumina slurry dispersed in silica sol when pH value changes. The de-stabilization mechanism was proposed to be a sol–gel effect of the silica sol resulted from formation of the aluminium ions in the slurry while adjusting the pH values of the slurry in one-way direction by adding HCl solution, which will affect the stability behavior of the silica sol at various pH values. In situ solidification of the slurry was successfully realized utilizing the self-decomposition of formamide to shift the pH value from 2.0 to above 3.0 according to the de-stabilization mechanism. High compaction and uniform green body without air entrapped was obtained by the novel near-net-shape forming method without de-airing process. And dense near-net-shape mullite-based ceramic part with elongated crystals has been successfully fabricated.