Sol-Gel-Assisted Molten-Salt Synthesis of Co 2 SiO 4 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 Co 2 SiO 4 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 Co 2 SiO 4 formation were investigated. The dried gel was transformed into the olivine oxide Co 2 SiO 4 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 Co 2 SiO 4 , SiO 2 , and Co 3 O 4 . 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 Co 2 SiO 4 is dominated by the strong bands of Co 2+ 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.