M₂SiO₄（M=Sr,Ba） Long Afterglow Phosphorescent Materials Synthesis And Luminescent Properties

Rare-earth-doped silicates are new energy-efficient green materials which were popular by human due to their function of light-absorbing and luminescent. As the high performance long after glow phosphors, Eu actived silicate phosphor, made by cheap and abundance raw material SiO2, it has more chemical and thermal stability,lower sintering temperature and better consistent to ceramics than aluminate phosphor. It is doomed to have wide application in the future.So, rare-earth-doped silicates are still researched by many researchers.The Eu2+was widely used in rare-earth-doped materials. In general, Eu2+ion was used as luminescent center. The electron configuration of Eu2+ion is4f75d0and the luminescence of Eu2+is electron transition by4f65d1-4f75d0. The energy level of5d is very sensitive to the change of crystal field due to the naked electronic shell. So, the split of5d energy level is quite different in different matrix crystalline field, and the luminescence wavelength of Eu2+is quite different. In this work, Sr2SiO4:Eu2+(Dy3+) is synthesized as matrix by a high temperature solid state method. In order to study the luminescence of Eu2+in different crystal structure, we doped Ba2+into raw materials, and changed the amount of Ba2+to replace the Sr2+in matrix. The role of Sr/Ba ratio in adjusting the phase structure, the emission colors and decay times of phosphor samples has been investigated. And the mechanism of long persistence also has been discussed, according to the result of thermoluminescence spectra.Sr1.97SiO4:Eu2+0.03phosphors were synthesized by the solid-state reaction technique. The X-ray diffraction shows that the phase of the phosphors is orthorhombic α-Sr2SiO4. The produced phosphors show one intense emission band located at490nm. The phosphor shows a long afterglow properties excited by the sunlight. The decay characteristic shows that the phosphor consist of a quick decay process and a slow decay process. The experimental results demonstrate that the thermoluminescence (TL) curves of the samples containing four peaks, located at73℃,147℃,183℃and279℃, respectively. Meanwhile, the different peaks show the different decay characteristics and the electron transfer between the trap levels was measured.Sr1.99SiO4:Eu2+0.01,and Sr1.98SiO4:Eu2+0.01, Dy3+0.01long afterglow luminescent materials were prepared by a high temperature solid-state reaction method. XRD shows that the samples are α’-Sr2SiO4orthorhombic structure. The photoluminescence(PL) shows that Eu2+ion is the sole luminescence center of the samples. But Sr1.98SiO4:Eu2+0.01Dy3+0.01displays higher luminescent intensity than Sr1.99SiO4:Eu2+0.01-As t mechanism of the long afterglow of this phosphor, Dy3+acted as the trap level and captured the free holes in the system. The decay characteristics show that the phosphors consist of a rapid decay process and a slow decay process. And the initial intensity of afterglow of the sample with co-doping is higher than the single-doping. The measurement of thermoluminescence reveals that the concentrations and depth of traps are increased by the amount of Dy3+ion.The rare-earth doped strontium silicate phosphors Sr2-xBax SiO4:Eu2+, Dy3+(x=0,0.5,1.5,2) were synthesized by a high temperature solid-state reaction method, too. The X-ray diffraction spectra show the samples are α’-Sr2SiO4orthorhombic structure At the meantime, some shift was found in the emission spectra due to the change of the Ba2+surroundings. The decay characteristics show that that the phosphor samples with different Ba2+content have different afterglow time, and the afterglow times also change with the value of x. The measurement of thermoluminescence reveals that the trap depth of the phosphor samples with different Ba2+content is different. The Samples with deeper traps have longer afterglow time. The results revealed that the Ba2SiO4:Eu2+, Dy3+can be used as a green phosphor in white LED.