Sol-Gel-Assisted Molten-Salt Synthesis of Co₂SiO₄ Pigments for Ceramic Tiles Application

Abstract

Cobalt orthosilicate has received considerable attention over the past decade due to its unique properties. However, the synthesis of the pure-phase Co₂SiO₄ at relatively low-temperature remains a challenge. In this study, cobalt orthosilicate pigment has been successfully prepared using a new alcohol-free sol-gel process combined with a molten salt method at a relatively low temperature. The precursors were synthesized via a simple non-alkoxide sol-gel method. The effects of pH and the thermal temperature conditions on the Co₂SiO₄ formation were investigated. The dried gel was transformed into the olivine oxide Co₂SiO₄ by reaction in molten NaCl at 1000 °C for 8 h. The heat-treatment of the gel without NaCl at the same temperature leads to a mixture of Co₂SiO₄, SiO₂, and Co₃O₄. The synthesis temperature (1000 °C) was much lower than that required by the conventional solid-state route or the classical sol-gel process. SEM-EDS analysis shows the formation of homogeneous powders with a molar ratio Co/Si close to 2. Optical spectrum of Co₂SiO₄ is dominated by the strong bands of Co²⁺ in octahedral sites. The colorimetric parameters show the formation of violet pigment (L* = 41.51, a* = 15.23, b* = −6.28). A content of 0.5 wt% of the prepared pigments is sufficient to achieve a deep blue-violet hue to ceramic glazes.