Monodispersed Spherical Particles Of Rare Earth Borates:Controlled Processing,Phase Structure And Luminescent Properties

In this dissertation,binary system precursors of(Y,Eu)(B(OH4)CO3,ternary system precursors of(Y,Gd,Eu)(B(OH)4)CO3 and amorphous precursors of Lnm(CO3)n(OH)p(BO3)l·wH2O(Ln=Eu-Yb,Y)have been successfully synthesized via urea-based homogeneous precipitation(UBHP)from mixed solutions of the component nitrates in the presence of ethylene glycol(EG).Monodispersed spheres of(Y,Eu)BO3 and(Y,Gd,Eu)BO3(hexagonal crystal structure)with a diameter of 0.8-1.5μm,and Ln3BO6(Ln=Eu-Yb,Y)(monoclinic crystal structure,～50-800 nm in diameter)have been obtained by calcining the precursors,respectively.The effects of particle morphology,calcination temperature,doping content and coordination symmetry on various aspects of luminescence of the resultant phosphors have been discussed in detail.Detailed characterizations of the samples were achieved by combined techniques of XRD,FT-IR,Raman,FE-SEM,HR-TEM,TG-DTA,PLE/PL,and fluorescence decay analysis,and the main achievements are listed as follows.Ln(B(OH)4)CO3(Ln=Y,Gd,Eu)cubic particles with lateral side of 0.8-1.5 μm and hexagonal crystal structure were synthesized via UBHP.Proper EG addition leads to the formation of spherical particles,but nano-sized amorphous particles were resulted from the excessive EG content.Longing reaction time or elevating the reaction temperature induced the appearance of cubic ones.Calcining precursors at 700℃ and 800-1000℃ yielded hexagonal structured LnBO3 phase with the space group of P63/m and P63/mmc,respectively.The spherical shape and excellent dispersion of the original particles can be well retained up to 800℃.Upon UV excitation at 214 nm,the difference of space-group vibrations between in 700℃-phase and 800-1000℃ phase resulted in the blue shift of CTB centre,but they did not significantly affect the lifetimes.A higher Eu3+content(x=0.01-0.10)in(Y1-xEux)BO3 leaded more Eu3+ions occupying the non-centrosymmetric site insdead of S6 one,thus inducing enhanced emission of 5D0→7F2 transition,larger asymmetry factor and red shift of the CIE chromaticity coordinates.Upon UV excitation at 207.nm,doping Gd3+ in(Y0.95-xGdxEu0.05)BO3 resulted in the red shift of CTB centre.Due to the energy transfer from Gd3+ to Eu3+,enhanced fluorescence intensity was found at a higher Gd3+ content,and the strongest emission was observed at the x value of 0.15.The compounds of Lnm(CO3)n(OH)p(BO3)l·wH2O(Ln=Eu-Yb,Y)in a wide range of lanthanides were obtained via optimizing the synthesis conditions and the resultant spherical particles with the diamater of～50-800 nm were amorphous spherical particles.The spherical shape and excellent dispersion of the Ln17.33(BO3)4(B2O5)2O16(Ln3BO6)can be well retained up to 900℃.The spherical particles of lanthanide(Eu-Yb)rare earth borates were synthesized for the first time.Upon UV excitation at 209 nm,the Y17.33(BO3)4(B2O5)2O16:Eu 3+spheres exhibited a typical red emission at 613 nm with the 5D0-7F2 electric dipole transition of Eu3+taking the dominant role.