Synthesis of iron zircon coral by coprecipitation routes

An iron-doped (15 mol-% of Fe2O3) zircon ceramic pigment has been prepared using binary (ZrO2-Fe2O3 and SiO2-Fe2O3) and ternary (ZrO2-Fe2O3-SiO2) colloidal gels or coprecipitates as precursors. The obtained raw powders, precalcines, and fired pigments have been characterized by XRD, thermal analysis (TGA/DTA) and SEM/EDX to analyze the effect of binary interactions (occlusion and adsorption phenomena) on the synthesis of the iron-zircon coral as well as on the coloring yield. The use of a binary raw coprecipitate as precursor prepared with colloidal silica and ferrous sulphate leads to a higher efficiency in the hematite occlusion, since a more intense coral hue (L* = 59.6, a* = 29.3 and b* = 25.8 at 950°C) is obtained, similar to an optimal ceramic reference. The protection or occlusion of α-Fe2O3 in amorphous silica agglomerates of high specific surface appears to be a more effective reaction intermediate than the observed adsorption of micronic α-Fe2O3 particles on tetragonal zirconia monoliths. The necessary transformation of tetragonal zirconia into its monoclinic form prior to zircon formation, and the higher α-Fe2O3 segregation from the coprecipitate obtained with Zr and Fe precursors, seem to lower the efficiency of the hematite coarsening-occlusion process involved in the coral pigment formation.