A green inorganic binder for material extrusion of ultra-low shrinkage and relatively high strength metakaolin ceramics at low sintering temperature

Ultra-low shrinkage and relatively high strength metakaolin ceramics printed by material extrusion were innovatively fabricated using inorganic aluminum dihydrogen phosphate (Al(H₂PO₄)₃) as a binder and low sintering temperature. The optical microscopy, scanning electron microscopy (SEM), electronic vernier caliper, three-point bending test, Archimedes method and X-ray diffraction (XRD) were used to measure and evaluate the surface morphology, microstructure, dimensional shrinkage, flexural strength, porosity and phase composition of the printed ceramics sintered at different temperatures. The results showed that when the mass ratio of metakaolin, Al(H₂PO₄)₃ and deionized water was 17:6:2, the rheological characteristic of the purely green inorganic ceramic slurry was very suitable for material extrusion additive manufacturing, and the corresponding printed ceramic green bodies possessed high-quality formability. The ceramic samples sintered at 750 °C possessed the best whiteness, the lowest shrinkage (<2 %), relatively high flexural strength (9.02 MPa). As the sintering temperature increased, Al(H₂PO₄)₃ transformed to aluminum metaphosphate Al(PO₃)₃, and then decomposed into aluminum phosphate (AlPO₄) and phosphorus pentoxide gas (P₂O₅), which caused pores and reduced strength, and alumina oxide (Al₂O₃) and silicon oxide (SiO₂) in metakaolin gradually transformed to mullite and cristobalite with more stable structure and higher strength.