Study On Modification And Stability Of ?-Ce₂S₃ Red Pigment Via Ion Doping And Coating

Red is one of the most abundant cultural connotation colors of the Chinese nation for thousands of years,and plays an important role in various decoration fields in daily life.Recently,the widely used red pigments often contain toxic heavy metal elements such as Pb,Cr and Cd.The extensive use of such pigments is harmful to health and does not accord with China's sustainable development strategy.Rare earth sesquivalent cerium sulfide(?-Ce₂S₃)is an environment-friendly red pigment with stable color,strong coloring,UV radiation resistance and no heavy metal elements such as Pb and Cd elements.Therefore,it is considered to be the preferred material to replace traditional toxic red pigments such as cadmium sulfide selenide.However,?-Ce₂S₃ is a high-temperature phase of cerium sulfide.The synthesis temperature of?-Ce₂S₃ prepared by direct reduction method is as high as 1300°C,which leads to the problem of poor redness for the pigment.In addition,some practical issues for?-Ce₂S₃ pigment still remain:poor high-temperature stability(oxidation at about 350°C)and easy decomposition in presence of acid.To overcome these problems,substantial effort has been devoted to the preparation,doping and coating modification of?-Ce₂S₃ red pigment.The main results and contents are as follows:Firstly,the reaction process of pure CeO₂ and CeO₂ doped with Na⁺or Sr²⁺in vulcanization atmosphere was studied by gas-solid reaction.The mechanism of Na⁺and Sr²⁺ion doping to reduce the synthesis temperature of?-Ce₂S₃ was analyzed by comparing the vulcanization behavior of pure CeO₂ and CeO₂ doped with Na⁺or Sr²⁺at different temperatures.The results showed that the synthesis process of?-Ce₂S₃ included three main steps without ion additives:(1)The deoxidation process in which oxygen in CeO₂ is replaced by sulfur to form Ce S₂;(2)The reduction process of Ce S₂ to?-Ce₂S₃;(3)The phase transition process of?-Ce₂S₃??-Ce₂S₃??-Ce₂S₃.This makes the synthesis temperature of?-Ce₂S₃ as high as 1300°C.After the introduction of Na⁺or Sr²⁺,the reaction path between CeO₂ and CS₂ was changed so that it did not need to go through the high temperature phase transition process of?????.The temperature of synthesizing pure?-Ce₂S₃ phase can be reduced to 900°C when Na⁺is introduced,while the mixture phase of?-Ce₂S₃ and Ce₁₀S₁₄O was obtained at 900°C in the presence of Sr²⁺.This result showed the decreasing synthesis temperature effect on?-Ce₂S₃ of Sr²⁺doping was not as good as that of Na⁺.Besides,the resulting?-[Na]-Ce₂S₃solid solution along with the change of band gap markedly increased the color performance and stability of?-[Na]-Ce₂S₃compared to undoped?-Ce₂S₃.The initial exothermic peak temperature in TG-DTA of?-[Na]-Ce₂S₃ is 480°C and the chromaticity values are:L^*=26.07,a^*=29.29,b^*=12.80.However,the initial exothermic peak temperature of?-Ce₂S₃ is 458.6°C and the chromaticity values are:L^*=24.95,a^*=26.54,b^*=11.20.Subsequently,the?-[Li]-Ce₂S₃,?-[Na]-Ce₂S₃ and?-[Ag]-Ce₂S₃red pigments were prepared by using Li⁺,Na⁺and Ag⁺as ion doping sources respectively and then Li&Y Co-doped pigments?-[Li]-Ce_2-xY_xS₃(x=0.0,0.1,0.2,0.3,0.4)were obtained on the basis of Li⁺doping.The Rietveled structure refinement for different ion doped pigments was performed with TOPAS program(Bruker-axs Company,Germany).The effects of doping amount of n _M/Ce(the molar ration of M/Ce)on the structure,chromatic properties and thermal stability of?-[M]-Ce₂S₃(M=Li,Na,Ag)pigment were systematically studied.The results showed that the solid solution limits of?-Ce₂S₃ for Li⁺,Na⁺,Ag⁺ions were all n _M/Ce=0.20(M=Li,Na,Ag)and there was no change in the structure parameters of?-Ce₂S₃ over n _M/Ce=0.20.In addition,it was found that the Ce-S bond length,cell parameters and band gap of?-[M]-Ce₂S₃ showed different change rules for different doping ions.For?-[Li]-Ce₂S₃,the Ce-S bond length and cell parameters decreased with the increase of Li⁺content.While these parameters increased with increasing Na⁺content for?-[Na]-Ce₂S₃.Due to the similar difference of ion radius and electronegativity,the band gaps of?-[Li]-Ce₂S₃and?-[Na]-Ce₂S₃increased with the increase of doping amount.However,the larger ion Ag⁺(1.28?,CN=8)replaced Ce³⁺(1.14?,CN=8)to form?-[Ag]-Ce₂S₃ solid solution,resulting in the abnormal phenomenon inculding lattice contraction and the change rule of Ce-S bond length and cell parameters accord with doping content.This is mainly due to the strong polarization between Ag⁺and S^2-,causing the bond type of?-Ce₂S₃ transition from ionic bond to covalent bond,and the color from red to black with the increase of Ag content.In a word,the comprehensive performance of the?-[Li]-Ce₂S₃ pigment was better than Na⁺or Ag⁺doped?-Ce₂S₃.The stable temperature of?-[Li]-Ce₂S₃(n _Li/Ce=0.20)can reach 450°C and displayed excellent chromaticity properties:L^*=27.25,a^*=32.83,b^*=13.23.When Y³⁺was introduced for Li&Y co-doping,?-[Li]-Ce_2-xY_xS₃ pigments exihibited the enhanced color rendering as a result of the smaller grain size,but its stability declined to a great extent.When the doping amount x=0.3,the initial exothermic peak temperature of the pigment on the TG-DSC curve decreased from 503.8°C to 442.5°C.Based on the preparation of?-[Li]-Ce₂S₃ by Li⁺doping,we developed an in-situ coating method for the synthesis of pomegranate-like?-[Li]-Ce₂S₃@c-SiO₂ by microemulsion method.Through orthogonal experiment,the optimum parameters for preparing precursor of?-[Li]-Ce₂S₃@c-SiO₂ in microemulsion system were the Ce/Si mole ratio of 1:9,45°C of reaction temperature,and p H=11.7.The phase composition,properties and coating structure of the as-obtained?-[Li]-Ce₂S₃@c-SiO₂ coating pigment were systematically analyzed.The results showed that the pomegranate-like?-[Li]-Ce₂S₃@c-SiO₂ exhibited excellent acid corrosion resistance and high-temperature stability.After acid treatment for 24 h followed by heating treatment at 900°C in air,the pomegranate-like?-[Li]-Ce₂S₃@c-SiO₂ still showed a bright red color(L*=29.02,a*=30.62,b*=15.45).These excellent acid corrosion and oxidation resistance were attributed to pomegranate-like embedded coating structure and the formation of Si-O-Ce bond between?-[Li]-Ce₂S₃ and c-SiO₂ coating layer.Meanwhile,combining with the growth characteristics of particles in microemulsion system,the formation prcocess of embedded coating structure of?-[Li]-Ce₂S₃@c-SiO₂encapsulated pigment was discussed.