Investigations On Zircon Pigments From Monodisperse CdS_xSe_1-x Colorants By Low-temperature Solvothermal Method

The CdS_xSe_1-x@ZrSiO₄pigments, as one of excellent heterogeneous crystalline stainsapplied in ceramic and glass industries, are always demonstrating their irreplaceable roles forcharacteristics of pure bright red color, abundant hues and strong tinting capacity andtherefore regarded as a paragon to all red pigments. Their unparallel brilliant red hues fordecoration function have imposed unlimited infatuation upon most beauty-pursuers. Ourresearching subjects are currently focusing on these kinds of pigments, which are of particularimportance and have received large-scaled industrious application but still by far exhibit theirinherent disadvantages such as high synthetic temperatures, poor stability under hightemperatures, low encapsulation coefficient and complicated production flow, etc. Thewhite-hot requirement for advanced red pigments in the ink-jet printing industry gives usanother motivation to pursue these subjects.In view of the problems and requirement for red pigments metioned above, the author’shaved achieved the current investigations which relates to: syntheses of monodispersecolorants CdS and CdS_xSe_1-xby solvothermal and hydrothermal methods, the optimizationselection of raw reagents, control factors such as reaction temperature, duration, pH valuesand alternative materials for synthesizing of ZrSiO₄（zirconium silicate） under hydrothermalconditions, the improvement for the hydrothermal method used for the encapsulatingpigments into the ZrSiO₄crystalline matrix, morphological revolution mechanism for roundor square layered zircons, the preparation of core-shell shaped CdS@SiO₂pigments,syntheses of red pigments CdS_xSe_1-x@ZrSiO₄under hydrothermal conditions as the ultimateobjective, and the calculation of encapsulation coefficient by the model establishment fromthe CdS_xSe_1-x@ZrSiO₄system and the extension of calculation modeling to all similarheterogeneous-crystalline encapsulation system.It is a premise for advanced monodisperse inclusion pigments to originate from themonodisperse colorant cores. The author have firstly prepared monodisperse orquasi-monodisperse CdS spherical particles in the size range of200-900nm at120-180℃bysolvothermal reaction. These colorants are of excellent brilliant yellow hues and have anincreasing size deviation with the increase of their particle scale. PVP and CTAB wereselected as surfactants for encapsulating SiO₂from TEOS hydrolysis upon to the obtainedCdS colorants by the St ber method. The ultra-thick pigments of CdS@SiO₂are preferablyobtained by CTAB as surfactant and they have an improved thermal stability when compared with those unencapsulated CdS pigments. The micro-meso pores in the outer SiO₂layers havehindered the further promotion of thermal stability.The brilliant red pigments were obtained by in-situ reaction from the obtained CdScolorants, which is based on the distinct disparity of solubility product between CdS andCdSe. When PVP was used as surfactant and synthesis conditions were concisely controlled,the resultant CdS_xSe_1-xcan still maintain its monodisperity and qusai-monodisperity. Thesesub-micron CdS_xSe_1-xpigments used for ceramics have not covered by previous researchers,which are most focused on the nano-sized CdS_xSe_1-xquantum dots for their photo-electricfuntions in this erea. The special raw materials and complicated processing ways applied bythem can not function well for the ceramic pigment industry. The22°-52°XRD diffractionpeaks of CdS_xSe_1-xexhibit a superb linear variation with the contents of CdS and CdSe. Theauthor has applied the relation for the proportion determination of sulfur and seleniumelements in CdS_xSe_1-xand the resulting can be guaranteed as accurate as the EDS testing.It is of great significance in low temperatures to synthesizing ZrSiO₄as theencapsulation body for inclusion pigments. In this study, comprehensive investigations offactors of temperature, duration, concentration, mineralizers, pH and alternative raw materialsand other factors have been conducted under the hydrothermal temperatures below300℃.The controllable syntheses of zircons of round single-layer to multi-layer and still toultra-thick doughnut-shape can be accomplished by varying different factors. When themineralizers of CaF₂and MgF₂are applied, square lamellar ZrSiO₄are first prepared andanother phase ZrO₂commonly emerges as an intermediate only under this kind ofmineralizers. Therefore the different catalyzing role of alkali metal and alkaline mineralizersare also discussed. The hydrothermal ZrSiO₄products have pore contents of about0.10cm3/gand specific surface areas of more than100m2/g and so they can be used as candidatematerials as catalyst carriers. Based on these researches, the author presents a four-stagemechanism for the morphological evolution of the hydrothermal zircons and interprets thepreferable crystallization orientation. The modification processing of Pechini method,pH-elevating by acetic acid and in-situ matrix ball preparation, are also explored for thepurpose of subsequent pigment encapsulation.The as-prepared monodisperse colorants, CdS@SiO₂and CdS_xSe_1-x, were applied asstarting pigments for the syntheses of CdS_xSe_1-x-ZrSiO₄, where citric acid functioned aszirconium source complexing agent and NaF as a mineralizer. The brilliant redCdS_xSe_1-x-ZrSiO₄pigments were prepared through the hydrothermal reaction at180℃for 5-12h. Separation of colorant cores and ZrSiO₄has been observed, as is mainly due to thesilicon phase migration and zircon’s preferable crystallization orientation under the complexhydrothermal system. In response to this problem, combination tests of preheating andhydrothermal treatment were conducted and the improvement work for the layer-by-layerinclusion and subsequent generation of ZrSiO₄is still requesting further investigations.Established on the above bases and auxiliary syntheses of red pigments by solid-statecalcination with the prepared CdS_xSe_1-xcolorants, the author has established a model for thecalculation of encapsulation coefficient for heterogeneous-crystalline inclusion pigments andhas proposed the calculation formula with three independent parameters as a_max=[1-2（k+1）r/L]³×100%and also its simplified calculation method. Meanwhile, theconcept of "Marginal Colorant Cluster" has been introduced. The calculation modeling for theencapsulation coefficient is finally extended to the encapsulation of other kind of pigments ornon-pigments. For the first time, the author points out that the upper limits of encapsulationcoefficient of CdS_xSe_1-x@ZrSiO₄will not be higher than29.63%-42.19%for the currentproduction level, which denotes the unreachability for a higher coefficient in thepigmentindustry. It has first presented the author’s original proposals and suggestions against theproblems mentioned above in the red pigment industry.The routes for syntheses of brilliant red pigments discussed above significantly varyfrom those routes applied by previous researchers in the terms of preparation and theencapsulation of monodisperse colorants, hydrothermal syntheses, the modeling andcalculation of encapsulation coefficient. These efforts will be of good significance forsubsequent researchers, and the brilliant red pigments and their alternatives will have moreconspicuous prospects for researching and applications of their products.