Rheological engineering and sintering control for colloidal shaping of high-performance fused silica ceramics

Using the fused silica powders with unimodal distribution as raw materials, the 50 vol% slurries possessing outstanding flowability were prepared by optimizing slurries' pH value, fused silica's zeta potential, dispersant (acrylic acid-acrylamido-2-methypropane sulfonic acid copolymer) content, and ball milling time. Despite of high absolute value of ζ of powders under alkaline solution, the acidic and neutral solutions were more effective than alkaline solution in dispersing fused silica powders. When the AA-AMPS content and ball milling time were 1.2 wt% and 6 h, not only the powder agglomerates can be effectively opened, but also the AA-AMPS molecules can form a homogeneous monolayer adsorption state on the particle surface and generated strong electrostatic repulsion and steric hindrance effects. The enhancement of solid loading consumed more free water in slurries, simultaneously shortened particle distance and heightened particle collision or friction, thus impairing fluidity. The slurry exhibited a low viscosity of just 0.45Pa·s at 50 vol%, which can meet requirement of gelcasting. At a standing time of 120 h, the fused silica slurries still maintained good dispersion stability, with a volume sedimentation rate of only 2.89 %. For gelcasting-pressureless sintered fused silica ceramics, prolonged holding time at 1250 °C from 1 h to 6 h enhanced densification, but simultaneously promoted more detrimental cristobalite precipitation. The material achieved optimal performance at 3 h, exhibiting a bulk density of 1.89 g/cm 3 and flexural strength of 69.5 MPa. This work provides certain insights into preparing high-solid-loading, well-flowing aqueous fused silica slurries and high-performance fused silica ceramics.