Preparation And Optical Properties Control Of Silicate-based Electron-trapping Materials

Silicate electron trapping phosphors as the environment-friendly materials can last a long time after the removal of excitation.Silicate electron trapping phosphors are stable in chemical stability and thermal stability,good in water-resistant property,cheap in raw materials,and they are widely used for architectural ornament,security or emergency indication system,luminous paint and electrical switchgear.Therefore,silicate phosphors have received extensive attention in recent years.The M₂MgSi₂O₇:Eu²⁺,R³⁺?M=Ca,Sr,Ba?silicate electron trapping phosphors were successfully synthesized by high-temperature solid-state reaction in this paper.Systematical study on optimization of preparation technology,implementation of optical tuning,improvement of afterglow property and investigation of afterglow mechanism is mainly conducted in this paper.The main research results of this paper are as follows:1.Electron trapping phosphors M₂MgSi₂O₇:Eu²⁺,R³⁺?M=Ca,Sr,Ba?were successfully synthesized.The manufacture process of phosphors is optimized,and the reaction temperature is reduced,so the purity and lattice imperfection are improved and the photoluminescence and afterglow properties are enhanced.In this paper,effects of sintering temperature,flux H₃BO₃ and SiO₂ addition on purity,morphology and structure,photoluminescence and persistence luminescence properties were studied.The research results included the following:?1?The higher sintering temperature can improve crystallization capacity and facilitate activator and auxiliary activator entering into the lattice,the photoluminescence and afterglow properties are improved as sintering temperature rises.?2?The decomposition temperature of flux H₃BO₃ is 165? and B₂O₃ which is obtained by thermally decomposing melts at about 450?.A condition of semi-solid flow is formed by the molten B₂O₃ and the rate of solid state reaction is increased.The proper amount of flux H₃BO₃ not only can reduce the sintering temperature,but also can facilitate the growth of crystalline grain,so the photoluminescence and afterglow properties are improved with the addition of flux H₃BO₃.?3?The impurity phases which contain isolate [SiO₄]^4-silicate anion groups do not disappear as the sintering temperature or the amount of flux H₃BO₃ rises.However,excess SiO₂ not only can enhance the rate of solid-state reaction and reduce the reaction temperature,but also can increase the Si/O ratio in the samples and facilitate the transformation from impurity phases to M₂MgSi₂O₇.Therefore,the photoluminescence properties of samples are improved by adding SiO₂ within an appropriate range.2.The color-tunable electron trapping phosphors with different proportions of hosts or differernt luminescent centers were investigated,and the emitting color was tuned by excitation wavelength.The emitting color can vary from bluish purple to white to pale yellow green by adjusting proportions of hosts,doping concentrations of different activators or excitation wavelength.Concrete research content including following several aspects:?1?Sr_1.94-xCa_xMgSi₂O₇: Eu²⁺_0.01,Dy³⁺_0.05?x=0,0.388,0.776,1.164,1.552,1.94?were successfully synthesized by high-temperature solid-state reaction under the same sintering temperature?1300??with addition of excess SiO₂.The introduction of Ca²⁺ leads to the decrease of the distance between Eu²⁺ and ligands and the crystal field strength becomes stronger.Therefore,a red shift?from 468 to 529 nm?phenomenon is observed in the emission spectra.The afterglow emission peaks also change from 466 to 529 nm when the proportion of Ca²⁺ ions increases in the host.Both luminescence and afterglow colors can continuously vary from blue to green by adjusting the Ca/Sr ratio in the host,and emission wavelength is linear with the concentration of Ca²⁺?x?.?2?The CaMgSi₂O₆ phase becomes more and more obvious in the Ca_xMgSi₂O_x+5: Eu²⁺_0.01,Dy³⁺_0.05?x=2,1.6,1.2,1?samples and turns into the dominant phase when the Ca²⁺?x?content is less than 1.2.The emission band centering at about 449 nm becomes strong and the emission band centering at about 529 nm becomes weak with decreasing concentration of Ca²⁺?x?.The luminescence colors continuously vary from pale yellow green to bluish purple by adjusting the concentration of Ca²⁺?x?in the hosts.The luminescence color can change from pale yellow green to white or from white to light purple blue by adjusting excitation wavelengths of x=1.6 sample from 320 to 419 nm,and the sample emits white light with the excitation wavelength in the range of 330-356 nm.The luminescence color of x=1.2 sample can vary from white to light purple blue when excitation wavelength changes from 419 to 349 nm,and the sample emits white light with the excitation wavelength in the range of 397-419 nm.?3?Because the emission band of Ca₂MgSi₂O₇: Eu²⁺ is located at about 527 nm and emission band of Ca₂MgSi₂O₇: Ce³⁺ is located at about 400 nm,and the emission peak positions of different luminescent centers keep the same when Ca₂MgSi₂O₇ codoped with Eu²⁺ and Ce³⁺,but the luminescent intensity decreases due to the quenching.The luminescence colors vary from pale yellow green to light purple blue by adjusting the concentrations of Eu²⁺?x?and Ce³⁺?y?in the Ca_2-x-yMgSi₂O₇: Eu²⁺_x,Ce³⁺_y samples.Moreover,the luminescence color of Ca_2-x-yMgSi₂O₇: Eu²⁺_x,Ce³⁺_y?x=0.01,y=0.01?sample can change from pale yellow green to white,and then change from white to green when excitation wavelength changes from 310 to 370 nm,the sample emits white light with the excitation wavelength in the range of 329-361 nm.The luminescence color of Ca_2-x-yMgSi₂O₇: Eu²⁺_x,Ce³⁺_y?x=0.01,y=0.03?sample can vary from green to light purple blue when excitation wavelength changes from 300 to 370 nm,and the sample emits white light with the excitation wavelength in the range of 320-326 nm and 364-370 nm.3.The effects of assistant activators on long afterglow properties of M₂MgSi₂O₇:Eu²⁺,R³⁺?M=Ca,Sr,Ba?were researched to improve the afterglow performance.The results included the following:?1?Because of the shallow traps,the initial afterglow intensity of Ca_1.94MgSi₂O₇: Eu²⁺0.01,Tm³⁺0.05 sample is higher than other samples,but the duration of afterglow is shorter.By contrast,the traps of samples Ca_1.94MgSi₂O₇: Eu²⁺0.01,R³⁺0.05?R=Dy or Tb?become deeper,and afterglow decay rates become slower and the duration of afterglow become longer.The afterglow performance of Ca_1.94MgSi₂O₇: Eu²⁺0.01,Nd³⁺0.05 sample is poor due to the too deep traps.?2?The Sr_1.94MgSi₂O₇: Eu²⁺_0.01,Dy³⁺_0.05 sample has the best afterglow performance.The traps caused by Nd³⁺ or Ho³⁺ are shallower than Dy³⁺,and the duration of afterglow becomes shorter.The traps formed by Er³⁺ are too deep,the afterglow performance is poor.?3?The trap depths of Nd³⁺ and Tm³⁺ in Ba_1.94MgSi₂O₇: Eu²⁺0.01,R³⁺0.05?R=Nd,Tm?are suitable for afterglow emission,so the afterglow properties get better than the deep traps caused by Dy³⁺,and the traps formed by Er³⁺ are too shallow.4.The afterglow mechanism of M₂MgSi₂O₇: Eu²⁺,Dy³⁺?M=Ca,Sr?was studied by first-principle calculation.By comparing the calculational results of the two materials,the possible factors which influence the afterglow performance were obtained.The calculation results included the following:?1?The computed band-gap widths of Sr₂MgSi₂O₇ and Ca₂MgSi₂O₇ are 4.49 eV and 4.465 eV,respectively.?2?The 5d levels of Eu²⁺ ions in the Sr₂MgSi₂O₇: Eu²⁺,Dy³⁺sample are 0.13 e V below the conduction band,and the 5d levels of Eu²⁺ ions in the Ca₂MgSi₂O₇: Eu²⁺,Dy³⁺sample are 0.915 eV up the conduction band by using the scissor correction method.The electrons on 4f levels of Eu²⁺ ions can be excited into the 5d levels of Eu²⁺ ions under the radiation of ultraviolet lamp.Because the 5d levels of Eu²⁺ ions in the M₂MgSi₂O₇: Eu²⁺,Dy³⁺?M=Ca,Sr?samples are closer to conduction band,so the excited electrons on 5d levels can easily move to the conduction band.The electrons in conduction band could be trapped by the nearby electron traps and stay for a while.When the trapped electrons return to conduction band,they could back to the 5d levels of Eu²⁺ ions and move to the 4f levels accompanied by afterglow emission.?3?The electrons in conduction band could easily move to the 5d levels of Eu²⁺ ions in the Sr₂MgSi₂O₇: Eu²⁺,Dy³⁺ sample,because the empty 5d levels are 0.13 eV below the conduction band,and the afterglow property of Sr₂MgSi₂O₇: Eu²⁺,Dy³⁺ sample is better.?4?The 4f levels of Dy in Sr₂MgSi₂O₇: Eu²⁺,Dy³⁺ sample are distributed with the energy rage of 2.61 eV,and the empty Dy 4f levels are 0.92-2.114 eV below the conduction band by using the scissor correction method.The 4f levels of Dy in Ca₂MgSi₂O₇: Eu²⁺,Dy³⁺ sample are distributed with the energy rage of 2.5 eV,and the empty Dy 4f levels are 1.01-2.145 eV below the conduction band by using the scissor correction method.Therefore,Dy 4f levels in the Ca₂MgSi₂O₇: Eu²⁺,Dy³⁺ sample are deeper and the afterglow decay rate is slower.