Preparation And Luminescence Properties Of (Ca 1-x M X ) 2 SiO 4 :Ce 3+ , Al 3+ (M=Mg, Sr, Ba) Phosphors

The white light-emitting diode?LED?as a new generation of green lighting source has many realization ways.The commercial white LED was realized by the encapsulation of blue LED chip and yellow phosphor-Y3Al5O12:Ce³⁺.However,this type of white LED is still suffering the drawbacks of lower color rendering index?CRI?and higher color temperature?CT?due to the scarcity of red light component.At present,the researches of yellow phosphors and performance improvement have not been stopped.Owing to its excellent performance,simple synthesis process,wide excitation wavelength?250⁵00 nm?and narrow emission peak width and so on,the silicates phosphors has attracted many intention and has been a new research hotspot.In this paper,in order to find a new yellow fluorescent material with high performance and stable matrix,a series of silicates phosphors-?Ca1-xMx?2SiO₄:Ce³⁺,Al³⁺?M=Mg,Sr,Ba?were systematic investigated.These yellow phosphors is hoped to employed in white LED field.The main work and results are as follows:?1?A series of ?-Ca₂SiO₄:xCe³⁺,yAl³⁺ yellow phosphors were prepared by high temperature solid-state reactions method.The structures of Ca₂ Si O₄ almost unchanged with Al³⁺/Ce³⁺ co-doped and N doped.These phosphors exhibits many following characteristic,such as good crystallization,regular morphology and no obvious aggregate performance,as well as a narrow normal distribution and average size of approximately 10¹5?m.The optimum doping concentration of Ce³⁺ and Al³⁺ are x=0.5mol% and y=4mol%,respectively.The Al³⁺ doped can have effect of change balance and stabilize ?-Ca₂SiO₄ phase.In the250⁵00 nm range of excitation wavelengths,the excitation peaks are located at 350 nm and450nm,respectively.And the main emission peak of sample is located at 565 nm,which can emit bright yellow light.The color coordinates are located in yellow region and there is no influence in luminescence properties which nitrogen doped into lattice.The fluorescence lifetime at 365 nm and 450 nm of ?-Ca₂ Si O₄:0.5mol%Ce³⁺,4mol%Al³⁺ sample are determined to be ?1=103.9 ns and ?2 = 104.2 ns,respectively,which is basically consistent with the transition types as well as ionic valence of Ce³⁺.Under the 450 nm excitation,the luminescence intensity of sample is about 63% at 475 K than that at room temperature.The results reveals that ?-Ca₂SiO₄:Ce³⁺,Al³⁺ yellow phosphor is suitable for blue LED chip and it has great potential application.?2?Based on ?-Ca₂SiO₄:0.5mol%Ce³⁺,4mol%Al³⁺ yellow phosphor,a series ofMxCa_2-xSiO₄:0.5mol%Ce³⁺,4 mol%Al³⁺?M=Mg,Sr,Ba;x=0.05,0.1,0.15,0.2?phosphors was prepared by high temperature solid-state reaction.The structure,morphology and luminescence properties of the phosphors were measured by X-ray diffraction,scanning electron microscope and fluorescence spectrometer.The results show that the Mg²⁺ doped with x=0.05 into samples maintain the primary structure of ?-Ca₂SiO₄ phase.Under the excitation of 450 nm light,a broad band emission centered at 565 nm is observed,which mainly is ascribed to 5d?2F7/2 and 5d?2F5/2 transitions of Ce³⁺.When Mg²⁺ doping concentration grows up to x=0.1-0.2,the phase is changed to ?-Ca₂ Si O₄ phase and come to occur impurity phase with the increase of the Mg²⁺ contents,the emission peak of MgxCa_2-xSiO₄:Ce³⁺,Al³⁺ shifts from 503 nm to 423 nm under the excitation of 365 nm.When Sr²⁺doped with x=0.05-0.15 for SrxCa_2-x SiO₄:Ce³⁺,Al³⁺phosphor,the sample maintain the?-Ca₂ Si O₄ phase,the excitation and emission spectra are almost same as those of the Sr-free phosphor.When Sr²⁺ doped with x=0.2,the phase is changed to ?'H-Ca₂SiO₄ phase.Under the excitation of 365 nm,the emission peak is locates at 465 nm.When Ba²⁺doped with x=0.05-0.1 for Ba_xCa_2-xSiO₄:Ce³⁺,Al³⁺ phosphor,the structure are almost unchanged,but increasing Ba²⁺ doping contents up to x=0.15-0.2,the phase transforms to ?-Ca₂SiO₄ phase.The emission peak is located at 460 nm under the excitation of 365 nm.The actual reason for phase transformation and the change of luminescence properties of these phosphors may be ascribed to the fact that the difference of melting points of alkali-earth metal oxides.The tendency of phase transformation from ?-Ca₂SiO₄ to other phases is enhanced with the doping from Mg²⁺ to Sr²⁺ and Ba²⁺.Through the calculation of crystal cell lattice parameters,the results show that the phase transformation is mainly affected by the amount of alkaline earth metal doped.When the alkaline earth metal doped into main matrix,the change of crystal cell volume can leads to the change of the crystal field environment and the ionic bond length,which causes the differences in structure and change of crystal field changes.The 5d configuration energy of Ce³⁺ions in ?-Ca₂SiO₄? ?'H-Ca₂SiO₄??-Ca₂SiO₄? ?-Ca₂SiO₄ is gradually increasing,which leads to the emission peak of phosphors is shifted to blue region.