Preparation And Luminescence Property Regulation Of Polychromatic Emission Rare Earth Phosphates

With the increasing shortage of global resources,it is particularly important to find new green energy and develop new green technologies.In the field of solid luminescence,rare earth luminescent materials are regarded as a new generation of luminescent materials,which are widely used in LEDs,PDPs,scintillators,bioluminescence markers and other fields.Among them,rare earth phosphate phosphors have attracted much attention due to their excellent thermal stability,stable crystal structure and good absorption in the ultraviolet region.The application fields of that are mainly white LED lighting,liquid crystal display and plant lighting.In this paper,phosphate Na2BaCa(PO4)2 was used as the matrix material to synthesize Ce3+,Tb3+single-doped and double-doped Na2BaCa(PO4)2 phosphors and Na2BaCaixSrx(PO4)2:Eu2+phosphors with cation Sr2+ replacing Ca2+by high-temperature solid-state method.The crystal structure,surface morphology,spectral characteristics,energy transfer,luminescence mechanism and practicability evaluation of phosphors were discussed and analyzed by a series of characterization methods.The research contents and results are as follows:The phase of the Na2BaCa(PO4)2:xCe3+phosphors were obtained by X-ray powder diffraction(XRD),both in a single phase,and the spatial group of the sample was determined as P3ml according to the standard crystal structure information card of Na2BaCa(PO4)2.When considering the valence,ionic radius and PL spectra,it was speculated that Ce3+and Tb3+occupied Ba2+and Ca2+sites,respectively.Under n-UV excitation,Na2BaCa(PO4)2:Ce3+and Na2BaCa(PO4)2:Tb3+show a blue-violet and green emission dominated by the characteristic transition of Ce3+and Tb3+,respectively.According to the data of spectral characteristics,the concentration quenching point of Na2BaCa(PO4)2:xCe3+is 0.05,and the concentration quenching mechanism of single doped Ce3+ was found to be the dipole-dipole electrical multi-level resonance interaction.Based on the XRD analysis,Na2BaCa(PO4)2:0.03Ce3+,yTb3+ phosphors are single pure phase.Under the excitation of near ultraviolet light,the PL spectra show that Ce3+→Tb3+effective energy transfer exists in Ce3+/Tb3+co-doped phosphors,and the value of energy transfer efficiency is 91.5%,the energy transfer mechanism is dipole-dipole electrical multilevel interaction.By calculating the data of PL spectra,the multi-color emission of phosphor from blue to green was realized.Monitoring the thermal stability of the optimized composition Na2BaCa(PO4)2:0.03Ce3+,0.03Tb3+phosphor,it can be concluded that the emission intensity remains 94.5%of that room temperature at 150℃,at the same time,the absolute quantum efficiency of the sample is 41.3%.Based on the cation substitution method,a series of Na2BaCa1-xSrx(PO4)2:0.03Eu2+phosphors were synthesized.The Sr2+ concentrations of x=0,x=0.8 and x=1.0 were refined to confirm the crystal structure,which belong to the space group P3m1.Particularly,Eu2+mainly occupies the Ba2+site and a small amount occupies the Ca2+ site.Under near-UV excitation,the Na2BaCa1-xSrx(PO4)2:0.03Eu2+ emission spectra is blueshifted with a highest luminescence intensity at x=0.8,which is further explained from the perspective of crystal field splitting and centroid displacement.The external quantum efficiencies of the optimized Na2BaCa0.2Sr0.8(PO4)2:0.03Eu2+ phosphor is 40.78%,which was significantly improved compared with the external quantum efficiency of 19.92%without Sr2+ substituting.Moreover,by analyzing the thermal stability of the Na2BaCa0.2Sr0.8(PO4)2:0.03Eu2+phosphor,it was found that this sample achieved a zerothermal quenching characteristic with an efficiency of 100.3%at 150℃.And the characteristic of zero-thermal quenching was due to the creation of deep trap by the substitution of Sr2+ into Ca2+.Finally,the Na2BaCa0.2Sr0.8(PO4)2:0.03Eu2+ phosphor was applied to the white LED.Under the excitation of 365 nm near-UV chip,the phosphor was packaged with commercial green SrSi2O2N2:Eu2+phosphor and red Sr2Si5Ns:Eu2+phosphor to achieve a white LED device with excellent performance index.And the white CIE coordinate is(0.3532,0.3561),The color temperature is 4721 K,the rendering index is 91.9.It is concluded that the cationic substitution method provides a feasible reference to improve the thermal stability and quantum efficiency of phosphors used in WLEDs.