Syntheses Of Fe₂O₃/SiO₂ Red Ceramic Pigments Via A Microwave Hydrothermal Approach

Fe2O3/SiO2 red pigment is an excellent ceramic inclusion pigment mainly used as colorant for ceramic body coloration.In this research,Fe2O3/SiO2 red pigments were synthesized via a microwave hydrothermal approach in an attempt to improve the performance and broaden the application areas of the pigments.TEOS(ethyl orthosilicate)and FeCl3-6H2O were used as the starting raw materials and the influence of mineralizer,pH value,SiO2/Fe2O3(mass ratio)and water content on the coloration of the as-prepared samples was mainly investigated.The process parameters of Fe2O3/silica red pigments were optimized by changing the reaction temperature,duration time,addition of various surfactants and alcohol solution.The micromorphology,phase and elemental composition and coloration of the samples were characterized by using TEM,XRD,EDS and colorimetric analysis.The results showed that the highest inclusion rate with the redness a*value of 22.67 was obtained when pH value of 3 and NaF as mineralizer with a concentration of 3%.However,the color of the as-prepared samples turned to yellow and white hues with further increasing of pH value.The inclusion statue shifted from SiO2 loosely packed on the surface of Fe2O3 to needle covered,particle coating with cross-linked structure and ultimately thick sheet-like covered structure.The pH value with which samples attained the optimal inclusion status is of 8.Higher SiO2/Fe2O3 mass ratio would result a thicker layer of inclusion and consequently,a lighter coloration.On the contrary,lower SiO2/Fe2O3 mass ratio would lead to a thinner layer of inclusion and a more intense coloration.Appropriate inclusion morphology could be obtained with a SiO2/Fe2O3 mass ratio of 85/15.It is found through TEM analysis of samples that NaOH as precipitant is more suitable than NH3·H2O in the enhancement of inclusion efficiency and control over particle morphology.Samples with excellent inclusion morphologies were synthesized at 190? for more than 1.5 h.The optimal deionized water content is 40ml;low water content can be adverse to the dispersion of colorant,by contrast,excessive content of water could result an increased size of cross-linked structure thus a thicker inclusion layer which deteriorate the coloration of the resultant samples.Using PVP as surfactant,agglomeration of colorants could be prevented and development of spherulitic morphology of colorant and inclusion phase could be facilitated without affecting the inclusion rate.The addition of organic solvents plays a role of "breaking up" the cross-linked structure;and ethanol can be beneficial to the dispersion and size control of pigment particles.Cross-linked inclusion structure can be reached with a TEOS to ethanol volume ratio of 1:3;however,monodisperse spherical inclusion can be reached with a TEOS to ethanol volume ratio of 1:7.