Preparation Of γ-Ce₂S₃ Inclusion Pigments And Study Of Colour Performance

In recent years, the scientific research workers urgently seek a kind of green environmental red pigment, in order to replace commonly used traditional inorganic pigment. y-Ce₂S₃ is a new type of rare earth red ceramic pigment, which has low toxicity, bright color, good dispersion and other excellent performance. At present, the main technique for the synthesis of y-Ce₂S₃ is solid phase method. However, in the process of preparation, the use of H₂S will be harmful to the environment and person. γ-Ce₂S₃ red pigment is easily oxidized to a colorlessness material above 350℃, thus reducing the scope of its use in the field of ceramic pigment. In order to make y-Ce₂S₃ present red hue stably at high temperature, the most common approach is to wrap y-Ce₂S₃ with the colorless transparent encapsulating layer.（1） γ-Ce₂S₃ pigments with quadrate morphology were prepared via traditional hydrothermal process using CeCl₃·7H₂O and CH₃CSNH₂ as raw materials. The influence of raw materials （cerium source、sulfur source）、mole ratio of S to Ce、pH values of the solution and hydrothermal time on the crystallites structure and morphology of prepared pigments were investigated. The results show, for many raw materials, CeCl₃·7H₂O and CH₃CSNH₂ are suitable for the preparation of γ-Ce₂S₃. For different mole ratio of S to Ce, the purity of γ-Ce₂S₃ increase gradually with mole ratio of S to Ce of 2. Different pH values of the solution also have great effect on the samples, the product contains small amount of γ-Ce₂S₃ crystal phase with pH values of the solution of 9. The diffraction peaks intensity of γ-Ce₂S₃、S、and Ce₂（SO4）3 become gradually stronger with the increase of hydrothermal time. However, the diffraction peaks of γ-Ce₂S₃ are less when hydrothermal time reaches 30 h.（2） γ-Ce₂S₃@SiO₂ red inclusion pigments were prepared via sol-gel method using γ-Ce₂S₃ and TEOS as raw materials. The influence of mole ratio of Si to Ce、mole ratio of H₂O to Si and reaction temperature on the chromatic performance and coated situation of inclusion pigments were investigated. Furthermore, y-Ce₂S₃@SiO₂ red inclusion pigments were calcined at 500 ℃ in order to investigate thermal stability of the inclusion pigments. The results show, for different mole ratio of Si to Ce, γ-Ce₂S₃@SiO₂ inclusion pigments have excellent performance with mole ratio of Si to Ce of 2, a dense and uniform encapsulating layer is formed on the surface of y-Ce₂S₃. The inclusion pigments have good thermal stability with mole ratio of H₂O to Si of 16. Hydrolysis rate of TEOS is low when the mole ratio of H₂O to Si is less than 16:1. As a result, the formed SiO₂ encapsulating layer is not dense enough; with the increase of water content, the size of generated silica also increases. However, it is not in favor of γ-Ce₂S₃@SiO₂ when the particle size of SiO₂ is too large. A dense and uniform SiO₂ encapsulating layer is formed on the surface of γ-Ce₂S₃ pigments when the reaction temperature reaches 40 ℃. Hydrolysis rate of TEOS is low when the reaction temperature is less than 40 ℃. However, the particle size of SiO₂ is too small and easily agglomerate with high activity, when the reaction temperature is too high. γ-Ce₂S₃@SiO₂ pigments have good thermal stability and hues under the following synthesis conditions:a mole ratio of Si to Ce is 2, a mole ratio of H₂O to Si is 16:1 and reaction temperature at 40 ℃. The subsequent thermal stability analysis indicates:the XRD curves of all the samples detect the diffraction peak of CeS2 and CeO₂, which displays the formed SiO₂ encapsulating layer indeed play an important role in protection of y-Ce₂S₃ red pigments.（3） In addition, γ-Ce₂S₃ red pigments were encapsulated by SiO₂ twice in the experiment. The influence of encapsulating time on thermal stability of y-Ce₂S₃ red pigments is investigated. SiO₂ encapsulating layer is thinner and easy to peel with encapsulating time of 1, which will influence encapsulating effect to some extent. Therefore, y-Ce₂S₃ red pigments were encapsulated by SiO₂ twice. The result shows that:the encapsulating layer on the surface of y-Ce₂S₃ pigments becomes thicker. The subsequent thermal stability analysis indicates:Compared with γ-Ce₂S₃@SiO₂ pigments with encapsulating time of 1, the thermal stability of the samples with encapsulating time of 2 has improved, thus expanding the scope of its use in the field of ceramic pigment.