Effects of Addition of Polyacrylic Ammonium on Colloidal Processing of α-Alumina

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Effects of addition of polyacrylic ammonium (shorten as PAN) on colloidal processing of ceramics were investigated with aqueous suspensions of α-alumina powder with an equivalent spherical diameter of 0.35μm. The added PAN was completely adsorbed onto the alumina surface below a critical amount of -3.5×10-6mol carboxyl group per m2 of alumina surface at pH9.0. The zeta potential of alumina powder (-24mV at pH9.0) reached -40mV with addition of the critical amount of negatively charged polymer, and free polymer above the critical amount didn't affect the zeta potential.The viscosities of PAN-added alumina suspensions showed a first minimum at the critical amount of PAN, and increased with increasing free polymer content. Further addition of PAN caused the secondary minimum of viscosity as a function of PAN content. It was possible to prepare a highly concentrated fluid alumina suspension of ≈63vol% solids by addition of the critical amount of PAN. However, it was difficult to increase the solids content above 53vol% at pH2.5 in electrostatic stabilization with HCl solution.The green densities of alumina compacts consolidated by filtration of electrostatically stabilized suspensions were independent of solids content in suspension stage, while the green densities for electrosteric stabilization with PAN showed a minimum at around solids content of 40vol%. The pore size distributions depended on both green densities and consolidation processes of powder (filtration or doctor blade method). A narrow pore size distribution was measured in a high-density green compact consolidated by filtration. The median size of pore in a green compact was small for electrostatic stabilization than electrosteric stabilization at a similar density, but PAN-addition was effective to produce a more continuous pore size distribution and a more uniform green structure.Although the densification behavior was mainly dominated by green density, PAN-addition in suspension stage was effective in achieving a higher density and a more fine texture than electrostatic stabilization at the final stage of sintering. The densification of a high-density green compact proceeded with disappearance of closed pores which were formed at around 92-95% T. D. The low-density green compacts were densified without the significant formation of closed pores. The texture of sintered alumina was greatly affected by the pore size distribution of a green compact. A wide pore size distribution formed a small number of large pores at the grain boundaries of large grains, while a narrow pore size distribution gave a microstructure with many small closed pores located at the grain boundaries of small grains.