Effects of rare earth elements on phosphorescent properties in magnesium silicate based phosphorescence materials

Nowadays, the persistent luminescence materials and mechanism are the most significiant issues in the storage phosphor field, for they can store the energy like sunlight, UV light or fluorescent lamp and let off the stored energy for a length of time. G. Blasse describe a phosphor as " A solid which converts certain types of energy into electromagnetic radition over and above thermal radition." Phosphor is a solid material which can emit light when exposed to radition for instance ultraviolet light or an electron beam. They are broadly used in dark enviroment. Phosphor, which can continue to emit light upon induction, is widely used in clock faces, luminescent dyes, traffic signs, textile and accessory materials as well as LED and plasma display panels. Zinc sulfide doped with copper is the first, used in long persistent phosphor on the market. It has a green emission of around 530 nm. However, luminous intensity and scintillation duration of ZnS, which is traditional phosphor, is not sufficient for such applications. Eu2+ and Dy3+ co-doped aluminates and silicates are thought up at the end of 19th - century. Their luminescent brightness, long lasting phosphorescence properties and stability were clearly better than sulfide series products. Studies on aluminates that have been performed by using rare earth elements mainly focus on Sr2AlO4:Eu2+, Dy3+ and it is observed that their luminescent characteristics are enhanced. It is the best afterglow phosphor and can emit light for more than 20 hours however, exposition to water may damage the luminescence features and this causes to restrict their usage in the paint as a pigment. The facts that silicates are not damaged when they are subjected to humidity and their luminescent characteristics are comparable to aluminates make Sr2MgSi2O7:Eu2+, Dy3+ featured. The best known persistent luminescent silicate is Sr2MgSi2O7:Eu2+, Dy3+. The family of materials M2MgSi2O7 (M: Ca, Sr, Ba) are called alkaline earth akermanites. Additionally, they are the most widely known persistent luminescent silicates. Solid-state raction at 1200-1400 °C, sol-gel, co-precipitation and combustion methods are generally use for preparing M2MgSi2O7. The Eu2+ and Dy3+ activated silicate host is broadly used owing to chemical and physical stability, easy preparation, low cost, its high luminance and long afterglow. We can observe various color from blue to red because the emission of 4f7→4f65d1 from Eu2+ is quite effective and its emission wavelenght is strongly dependent on host lattice. The phosphorescent characteristic of Sr2MgSi2O7: Eu2+, Dy3+ is related to its radiation center and trapping mechanisms. It emits blue and blue-green light and have afterglow time longer than 20 hours visible to human eye. While Eu2+ ions act as emission center, Dy3+ ions take role as space trapping center which can extend the afterglow in the mechanism. If the traps in the long afterglow phosphor are plentiful, it will cause the long afterglow. Eu2+ and Dy3+ ions settle in the spaces of Sr2+ ion that is in the tetragonal structure of Sr2MgSi2O7. The first aim of this study is to analyze the effects of rare earth elements in the range of La-Lu in terms of phosphorescence characteristics and radiation duration on Sr2MgSi2O7: Eu2+, Re3+ which is produced by solid state reaction. The second aim is to observe the effects of boric acid on the samples, which have better radiation performance. In the first part of the experimental work, samples which consist of La, Ce, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu lanthanides were prepared according to Sr1,92MgSi2O7: Eu0,022+, Re0,063+ (8% H3BO3) formula. Samples were produced using solid state reaction technique. Carbonates such as SrCO3, oxides like SiO2, MgO, Eu2O3, Dy2O3, Er2O3, La2O3, Nd2O3, Sm2O3, CeO2, Pr2O3, Gd2O3, Tb4O7, Ho2O3, Tm2O3, Yb2O3, Lu2O3 and H3BO3 with high purities were mixed and grinded homogeneously via mill-type grinder in ethanol environment for 2 hours and then ethanol was removed by incubation at 115 oC for 12 hours. Powder and pressed samples were heated until 1300 oC with a rate of 10 °C/min and they are heat treated in reductive atmosphere (5% H2, 95% N2) followed by cooling in nitrogen atmosphere. In the second step, compositions, which comprise 0% and 16% boric acid for samples containing europium and dysprosium were prepared. Samples were produced by using the same solid state reaction. Samples which are obtained by these processes were analyzed for their phase and optical characteristics by using XRD and spectrophotometric techniques. XRD powder patterns were obtained using a difractometer PhilipsTM 3710 X-Ray equipped with Cu-Kα radition source. Excitation and emission spectra were measured using VARIANTM fluorescence spectrophotometer. Totally 22 samples were synthesized and characterized to investigate the effects of rare earth elements and boric acid on structural and optical performance of phosphors. Sr2MgSi2O7 photoluminescent pigment codoped with rare earth elements was prepared which showed long afterglow properties. The main excitation peaks at 365-370 nm and the main emission peaks at 465-470 nm are all attributed to 4f7→4f65d1 transtion of Eu2+. Spectrophotometric analysis showed that all samples have different emission and excitation values and intensities. All the samples can be indexed to tetragonal Sr2MgSi2O7 structure in the space group P421m. Results of this study indicate that additives which were introduced to the system at 2% and 6% ratios did not damage the crystal structure, single-phased Sr2MgSi2O7 was obtained so that targeted structure was achieved. The doping with small amount of the different ion has no important influence on the crystal structure. The obtained Sr2MgSi2O7:Eu phosphors showed a blue emission approximately 470 nm. The effects of co-activated trivalent rare earth ions on the luminescence properties of the phosphor were not only trap centers but also a luminescence centers. All of the samples had different excitation and emission wavelenghts, intensities and decay times as it can be seen clearly from the graphics. These results are associated with the trap densities and trap depthes. Therefore, rare earth elements affect characters of phosphors seriously. Similar characterization tests were made on target samples which contain different amounts for boric acid. Different phases resulting from low purity of Sr2MgSi2O7 when the ratio for boric acid deviates from 8% in the composition and the formation of borate are observable in X ray diffraction results. In addition to this, it had been clarified that boric acid is not only an inert high temperature solvent to accelarate grain growth but also substitutional effect. If the concentration of boric acid increases, it can react with SiO2, MgO, SrO or Sr2MgSi2O7 and it brings about the formation of the borate phases. High boric acid content damages the tetragonal crystal structure. B2O3 plays a significiant role in stabilizing phases involving chemical reactions.