Na⁺-Doped γ-Ce₂S₃ Bright Red Pigment Coated with Mullite

γ-Ce₂S₃ is a kind of non-toxic environmental protection red inorganic pigment due to its bright color, intense hiding power and superior ultraviolet resistance. However, its antioxidant temperature is 350 ℃. To improve the oxidation resistance temperature of γ-Ce₂S₃, a mullite coated Na⁺ ion-doped γ-Ce₂S₃ (i.e., γ-[Na]-Ce₂S₃@mullite) was synthesized by a sol–gel method and a subsequent sulfuration-heating process. The synthetic conditions of mullite as well as the effect of pre-firing temperature on the morphology of precursor and chromaticity value of the coated pigments were investigated, and the thermal stability of the coated pigments was also analyzed. The results show that short rod-like and spheroidal mullite crystals can be synthesized at 1300 ℃ for 3 h as n_Al:n_Si=3.2:2.0, and the optimum chromaticity value (i.e., L*=36.24, a*=41.86, b*=36.26) of the coated pigment can be obtained when the pre-firing temperature is 400 ℃. The pigment has a good chromaticity value (i.e., L*=29.84, a*=20.83, b*=18.17) when being in air at 800 ℃ for 10 min. This study indicates that γ-[Na]-Ce₂S₃@mullite can increase the antioxidant temperature of γ-Ce₂S₃ from 350 ℃ to 800 ℃.