Synthesis And Properties Of Rare-earth(Holmium,Neodymium) Molybdate Functional Pigments With Allochroic Effect

Modern inkjet printing is widely used in architectural ceramic printing,making ceramic decoration simple,low-carbon and intelligent.However,the problems of homogeneity and lack of functionality of jetinks in the printing restrict its applications.Therefore,ceramic tiles with functional decoration based on sinking,antibacterial,moisturizing,metal,flashing and other effects are developed.Pigment with allochroic effect is a novel functional inorganic material,having a reversible color transformation under various illuminants with different spectral power distributions.In this dissertation,rare-earth（i.e.,holmium,neodymium）molybdate pigments with allochroic effect were synthesized by different methods（i.e.,mechanical activation-assisted solid-state reaction,chemical co-precipitation method and solution combustion method）.In addition,the application of the synthesized pigments with allochroic effect in the ceramic decoration and anti-counterfeit label were also explored.The main research aspects and the obtained results are given as follows:（1）Ho₂Mo O₆ pigment with allochroic effect was synthesized via mechanical activation-assisted solid-state reaction.The effects of mechanical activation mode and time as well as sintering temperature on the properties of synthesized pigment were investigated.The results show that mechanical activation pretreatment of precursor mixtures decreases the average particle/crystallite sizes and the crystallinities,increases the surface activity of precursors particle,thus leading to the reduction of subsequent calcination temperature for synthesis of Ho₂Mo O₆.Ho₂Mo O₆ pigment exhibits different colors under different standard illuminants（i.e.,D65,A,F2 and F11）,implying that Ho₂Mo O₆ is a functional pigment with allochroic effect.Also,according to the spectroscopic analysis,the discoloration mechanism of Ho₂Mo O₆ is attributed to the rich energy level structure of Ho³⁺ions in the visible range and the relative spectral power difference of different standard illuminants.According to colorimetry analysis,the influence of the characteristic absorption peaks at wavelengths of 537 nm,455 nm and 641nm of Ho₂Mo O₆pigment on the color performance and allochroic effect are degressive in a decreasing order,and the allochroic effect is most significant between illuminants D65 and F11.（2）Ho₂Mo O₆ pigment with allochroic effect was also synthesized via chemical co-precipitate method and subsequent heat-treatment in the presence of different fluxes（i.e.,lithium chloride,sodium fluoride and boric acid）.The effects of reaction solution p H value,type/content of flux and heat-treatment temperature on the formation,color performance and allochroic effect of Ho₂Mo O₆ were investigated.The results show that the solution p H value affects the ratio of Mo and Ho elements in the co-precipitation precursor and the purity of the pigment after heat-treatment.Sole Ho₂Mo O₆phase can be formed via the co-precipitate reaction at p H value 7.0 and heat-treatment at 1000℃.The flux type has a dominant effect on the purity of the product.The pigments obtioned from co-precipitated product with 5 wt.%boric acid or sodium fluoride as a flux are accompanied with some impurity phases.The sintering temperature of forming sole Ho₂Mo O₆ phase is decreased by 200℃in the presence of 5 wt.%lithium chloride in co-precipitated product.The crystallization activation energies calculated by the Kissinger formula and the Ozawa formula indicate that the activation energy of Ho₂Mo O₆ phase crystallization decreases from 154.1k J/mol and 166.6 k J/mol in the absence of Li Cl as a flux to 68.4 k J/mol and 77.7 k J/mol in the presence of Li Cl,respectively,thus reducing the required crystallization temperature.And lithium chloride is conducive to the crystallization and grain growth of Ho₂Mo O₆,resulting in the excellent color performance and allochroic effect.（3）Ho₂Mo O₆ pigment with allochroic effect was synthesized by a solution combustion method using different fuels（i.e.,D-alanine,urea,citric acid,glycine and glucose）.The effects of ignition temperature,amount and type of fuel on the properties of the pigment and the synthesis mechanism were investigated.The results show that the optimum ignition temperature of Ho₂Mo O₆ synthesized in solution combustion is 1000℃.When the molar ratio of the fuel to the oxidizer（φ）=1,the oxidizer can effectively provide the oxygen required by the fuel,and the reaction system can reach a higher temperature,which is conducive to the formation of products and grain growth.In addition,there exist differences in particle size and morphology,particle uniformity,degree of dispersion/agglomeration,and chemical groups on the particle surface of the products obtained with different fuels.The products obtained with glucose as a fuel are fluffy and the connections between particles are loose.The particle morphology is spherical,its size is lower than that obtained with other fuels,and the more hydroxyl groups occur on the partical surface.Based on the results of thermodynamic analysis,the reaction enthalpy change and adiabatic flame temperature(T_ad)are the maximum and the minimum,respectively,when glucose and urea are used as fuels,indicating that glucose is more conducive to the progression of combustion reactions.（4）Besides,Nd³⁺ion doped Ho₂Mo O₆ pigment with allochroic effect was prepared by a solid-state reaction method.The effects of Nd³⁺ion doping amount,heat-treatment temperature and time on the crystal structure,particle morphology,color performance and allochroic effect of the synthesized pigment were investigated.The results show that Nd³⁺ions doped Ho₂Mo O₆formed a solid solution.The lattice expansion and parameters show a regular growth of the crystalline when the doping amount of Nd³⁺ions increases,which is attributed to the massive Nd³⁺ions dissolve in the Ho₂Mo O₆ lattice.Ho₂Mo O₆ maintains a stable monoclinic structure,while some Nd³⁺ions doped Ho₂Mo O₆ have a phase transition from a monoclinic structure to a tetragonal structure as the heat-treatment temperature increases,and the degree of phase transition increases with the increase of doping amount.For the pigment with Nd³⁺ions doping amount of 0.2,theΔE*value between D65 and F11 is 14.58,and theΔE*value between any two standard illuminants is greater than 5.6.Ho₂Mo O₆ and Nd₂Mo O₆ both have a good compatibility according to the Vegard law analysis.According to the results of first-principle calculation based on density function theory,Ho³⁺ions are readily replaced by Nd³⁺ions to form stable doping structures,increasing the hybridization between tracks,narrowing the band gap and making the band gaps of(Ho_1-xNd_x)₂Mo O₆tunable.（5）The potential applications of the synthesized pigment in ceramic decoration and anti-counterfeit label were investigated.The results indicate that a waterborne ceramic ink obtioned with Ho₂Mo O₆ pigment with allochroic effect synthesized via solid-state reactions has a good dispersion stability and adaptability in inkjet printers,and the ceramic tiles printed with the waterborne ink exhibit an allochroic effect after high-temperature sintering.For the pigment prepared by a solution combustion method with glucose as a fuel just needs a simple dispersion instead of a grinding process to meet the application requirements of ceramic ink.In addition,(Ho_1-xNd_x)₂Mo O₆（x=0-0.8）pigments and a mixed Ho₂Mo O₆pigment and Sr Al₂O₄:Eu²⁺,Dy³⁺,Zr⁴⁺phosphor in a mass proportion of 1:1 were also used to prepare anti-counterfeit labels,having superior allochroic effect and photo-/mechano-luminescence.Finally,the major conclusions of this dissertation were summerized,and some prospects for the future research,improvement and application of rare-earth molybdate pigments with allochroic effect were also proposed.