Design And Investigation Of N-UV WLED-used Oxide-based Luminescent Materials With Long-wavelength Emission

The white light-emitting diode(WLED)with the superiority of efficiency and environmental friendliness is widely used in the field of lighting.However,the lighting with higher quality is required with the progress of human social production and the improvement in living standards.The approach to obtain white light by combing near-ultraviolet LED with multicolor phosphors(“n-UV chip+multi-color phosphors”)attracts extensive attention because of its unique advantages in color rendering index(CRI)and correlated color temperature(CCT).The phosphors with blue to green-range-emitting under n-UV are widely and well-investigated at present.However,luminescent materials with long-wavelength emission remain a lot of problems,such as the harsh or toxic synthesis condition in nitrides and fluorides,the mismatch with the n-UV chip,the reabsorption of blue(green)light,the knowledge of relationship between materials and their luminescence properties needs to be deepened and the related mechanisms of controlling luminescent properties are expected to be clarified,etc.Considering these facts,in this thesis,Eu³⁺/Eu²⁺/Mn²⁺activated oxides were focused.The oxide-based materials with long-wavelength emission under n-UV excitation were designed and synthesized,followed by the exploration of the relationship between the composition,crystal structure of the compounds,and the occupancy of activators in the hosts as well as the luminescent properties.The specific work of this thesis reads as follows:1)The trigonal system-crystalized Na₂Ba Ca(PO₄)₂(NBCP)with multi-cation-site peculiarity was selected as the matrix and Mn²⁺with d-d transition to be the activator;Eu²⁺with d-f transition to be the sensitizer.NBCP:Mn²⁺could perform yellowish orange emission band centered at 576 nm due to the occupancy of the octahedral field provided by the six-coordinated Ca²⁺site.The emission intensity would be greatly enhanced by the Eu²⁺-Mn²⁺energy transfer through the radiation and resonance approaches.The defect chemistry has been considered and analyzed when Mn²⁺and Eu²⁺ions are introduced into the crystal lattice.The occupancy of the doping ions could influence the properties of the two activators.Simultaneously,the compact structure of NBCP that formed by coplanar cationic polyhedra ensures the excellent thermal stability and the PL intensity of NBCP:Eu,Mn keeps 96%of the initial(at room temperature)at 150?.2)Generally,Eu²⁺could be used as an independent activator with much higher emission intensity compared to Mn²⁺.The(Ca_1-xSr_x)₉Bi(PO₄)₇:Eu(0?x?0.8)samples were designed based on the analysis of the structures of?-Ca₃(PO₄)₂ and Sr₉In(PO₄)₇ and synthesized by the solid-state reaction in reducing atmosphere.By introducing Sr into the Ca₉Bi(PO₄)₇(CBP)system,the structure of the host and occupancy of Eu²⁺were changed,altering the type and number of luminescence centers.Therefore,the emission redshift occurs with the band broadening and finally,a broad orange emission phosphor was obtained in the single phase of(Ca_0.2Sr_0.8)₉Bi(PO₄)₇(CSBP).After that,the influence of R³⁺in Ca₉R(PO₄)₇(CRP,R=La,Gd,Y,Lu,Sc)samples on the structure and luminescence properties was investigated.By the comparison of structure and luminescence kinetics with(Ca_0.2Sr_0.8)₉R(PO₄)₇:Eu,the influence of chemical environment around Eu²⁺/Eu³⁺on the spectral characteristics was formulated.3)The previous two chapters discussed the luminescent properties of NBCP:Eu,Mn and CSBP:Eu while the phenomenon that Eu³⁺could not be reduced to Eu²⁺efficiently were depicted and explained.In order to give supplemental instruction and investigation of the affecting factors for the unreducible Eu³⁺in phosphors,the apatites with well-researched structure and flexible frameworks were selected as objects.Firstly,the apatite-type germanate Na La₉(Ge O₄)₆O₂(NLGO)was focused,and Eu³⁺has been found unreducible to dyadic in it,after which further investigations were proceeded.The results show that the unreducible characteristic of Eu³⁺in NLGO is related to the synthesis condition,which make great loss of the raw material(Ge O₂),hindering the formation of NLGO.Besides,the possible affecting factors for unreduced Eu³⁺in apatites,and other common materials were summarized.