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

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 degrees 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 degrees 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.