Synthesis Of Novel Phosphate Phosphors By Structural Modification

White LEDs are known as the fourth generation of lighting source because of its advantages of energy saving and environmental protection,long service life,high brightness.White LEDs also called as green light source in the 21st century.In recent years,white LEDs have been widely used in various fields.At present,the main way to prepare white LEDs is to excite phosphors with chip.The research progress of phosphor plays a crucial role in the development of white LEDs.Phosphate-based phosphors have received increasing attention in this field because of their stable physicochemical properties,lower synthesis temperature,and excellent luminescence properties.In this thesis,the cationic substitution and chemical unit substitution was used as the synthesis strategy to prepare the self-activated Ca₂Ba₃?PO₄?₂SiO₄ phosphor.The high-temperature solid-state method was used as the synthesis method.The self-activated luminescence mechanism and thermal stability were discussed.And the spectrum regulation is realized by cation replacement.Firstly,the self-activated phosphosilicate phosphor was prepared by high-temperature solid-state method,and its luminescence properties and self-activated luminescence mechanism were studied.The results show that pure crystalline phase product of Ca₂Ba₃?PO₄?₂SiO₄ can be obtained under reducing atmosphere at 1000�C.The self-activated Ca₂Ba₃?PO₄?₂SiO₄ phosphor exhibits a broad emission spectrum from 400-600 nm with the strongest peak at 460 nm under254 nm excitation.The luminescence mechanism of the self-activated Ca₂Ba₃?PO₄?₂SiO₄ phosphor may be the recombination luminescence of conduction band electrons and holes caused by oxygen vacancy defects.Since the phosphor synthesis condition is a CO reducing atmosphere,there may also be a carbon defect luminescence.Secondly,in order to further study the effect of rare earth ion doping on the luminescent properties of the self-activated phosphor Ca₂Ba₃?PO₄?₂SiO₄,the self-activated phosphor Ca_2-xBa₃?PO₄?₂SiO₄:xEu²⁺was successfully prepared using the same synthesis method and synthesis conditions.And the effect of Eu²⁺doping concentration on phosphor emission was also studied.The excitation and emission spectra of the phosphor Ca₂Ba₃?PO₄?₂SiO₄ did not change with the increasing concentration of doped Eu²⁺ions,but the emission intensity gradually decreased.This may be due to the fact that Eu²⁺ion radius is similar to Ca²⁺ion radius and Eu²⁺ions can easily replace Ca²⁺ions.The interactions between Eu²⁺ions and the small clusters on the phosphor surface or grain boundaries result in recombination of electrons and holes on the surface of the phosphor,which produces a non-radiative transition.Thirdly,in order to further study the effect of cation substitution on the luminescent properties of the self-activated phosphor Ca₂Ba₃?PO₄?₂SiO₄,Ca₂Sr_xBa_3-x?PO₄?₂SiO₄ and Ca_2-xMg_xBa₃?PO₄?₂SiO₄ were prepared using the same synthesis method and conditions.And the effects of different concentrations of cation substitution on their luminescence properties were investigated.The study found that the substitution by Sr²⁺and Mg²⁺have little effect on the spectra profile of the Ca₂Ba₃?PO₄?₂SiO₄ phosphor.However,the emission peaks showed redshift in different degrees with the increase of the cation substitution concentration.When Sr²⁺ions gradually substitute for Ba²⁺,the emission peak red shifts about 18 nm from 460 to 478 nm.When Mg²⁺ions gradually substitute for Ca²⁺,the emission peak shows a red shift about 34 nm from 460 to 494 nm.This may be due to the radii difference between Sr²⁺and Ba²⁺,Mg²⁺and Ca²⁺,which results in bandgap decrease of the phosphors.So the emission peaks of the phosphors shift to larger wavelengths.By comparison,it can be seen that the Mg²⁺ion substitutions produce a larger red shift than that of Sr²⁺,which may be due to the greater difference of radii between the Mg²⁺and Ca²⁺.The greater difference results in a smaller bandgap,resulting in a larger redshift.Fourth,a series of self-activated phosphor Ca_2-x-x R_xBa₃?PO₄?₂SiO₄?R=Zn,Cd?was successfully synthesized using the traditional traditional high-temperature solid-phase method in the CO reduction atmosphere.The XRD spectra show that the introduction of Zn²⁺and Cd²⁺may destroy the original crystal structure of self activated phosphor,resulting in impurity peaks.The study of luminescence properties of Ca_2-xZnxBa₃?PO₄?₂SiO₄ phosphor and Ca_2-xCd_xBa₃?PO₄?₂SiO₄phosphor shows that the emission peaks of phosphors appear 19 and 24 nm redshift with the increase of the concentration of Zn²⁺and Cd²⁺,respectively.However,the emission intensity of phosphor greatly decreases as the concentration of Zn²⁺and Cd²⁺increases.The reason may that the luminescence mechanism of the electron and hole recombination mode in the self activated phosphor Ca₂Ba₃?PO₄?₂SiO₄ is completely broken when Zn²⁺and Cd²⁺completely replace Ca²⁺.So this luminescence process can not continue and the luminescent properties of the phosphors are nearly lost.