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

Posted on:2022-11-08Degree:DoctorType:Dissertation
Country:ChinaCandidate:F S SongFull Text:PDF
GTID:1481306605959139Subject:Materials Science and Engineering
Abstract/Summary:PDF Full Text Request
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]-Ce2-xYxS3(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 S2-,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]-Ce2-xYxS3 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.
Keywords/Search Tags:?-Ce2S3 red pigment, Cristobalite, Ion doping, In situ coating, Acid resistance
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