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Synthesis And Investigation On Lanthanides Incorporated Sr2MgSi2O7,CaWO4 Long Afterglow Materials

Posted on:2012-04-10Degree:DoctorType:Dissertation
Country:ChinaCandidate:H Y WuFull Text:PDF
GTID:1111330368983093Subject:Applied Chemistry
Abstract/Summary:PDF Full Text Request
A series long afterglow phosphors samples of M2MgSi2O7:Eu2+, Re3+(M:Ba, Sr, Ca; Re3+:Dy3+, Nd3+, Ho3+, Er3+,Yb3+)and CaWO4:Re3+(Eu3+, Tb3+, Pr3+) were synthesized via high temperature solid state reaction. The XRD and photoluminescence, decay and thermoluminescence (TL) are employed to investigate the structure and the luminescence of the phosphors and the results are as follows:1) It is found that different Sr/Ca or different Sr/Ba ratios of M2MgSi207:Eu2+, Dy3+ matrix change the crystal plane spacing of the crystal with the same structure. The smaller crystal plane spacing resulting from the smaller atomic radius leads to the further splitting of the 5d levels of Eu2+ ions and then the red shift of the emission occurs. No matter Ca2+ ions or Ba2+ ions doping in the matrix, the afterglow properties will be declined. The lifetime of the afterglow is shortened. The analysis of TL reveals that the doping of Ca2+ ions or Ba2+ ions decreases the trap concentration, and then the trapping and liberation of carriers are decreased so as to the weak properties.2) With the investigation of the influence of environmental temperature on afterglow properties, the long afterglow Sr2MgSi2O7:Eu2+, Dy3+ is observed at room temperature, while Ba2MgSi2O7:Eu2+, Dy3+ does not show an obvious afterglow. However, in the case of higher temperature environment, the latter exhibits obvious long afterglow while the afterglow of the former can not be observed. Therefore, Ba2MgSi2O7:Eu2+, Dy3+ is a long afterglow phosphor available at higher environmental temperature.3) In addition, H3BO3 and Li2CO3 which act as fluxes during the synthesis process of Sr2MgSi2O7:Eu2+, Dy3+, substantially reduce the sintering temperature of the samples. Both of them do not have a significant influence on the photoluminescence of the samples. Through the analysis of TL, it is found that the employment of H3BO3 deepens the trap depth of phosphors so that the liberation of trapped carriers is delayed, in which way the duration of afterglow is prolonged. Contrarily, the involvement of Li2CO3 reduces the trap depth significantly. So the attraction of traps to carriers is declined so as to shorten the afterglow duration.4) On the other hand, influences of Dy3+, Nd3+, Ho3+, Er3+ and Yb3+ ions on the afterglow properties of Sr2MgSi2O7:Eu2+ are also studies. To some extent, Dy3+, Nd3+, Ho3+ and Er3+ enhance the afterglow properties because the divalence of these ions are close to the bottom of conduction band and the intrinsic traps induced by lattices defects can be reinforced by these ions. However, the traps induced by Yb3+ are too deep to liberate carriers, thus the afterglow is suppressed.5) The mechanism for the generation of afterglow is probably associated with the oxygen vacancies and cation vacancies which act as electron traps and hole traps, respectively. The aggregation of oxygen vacancies, Dy3+ ions and Sr2+ vacancies forms clusters. Some electrons in oxygen vacancies can be released to Dy3+ ions and then the afterglow duration is prolonged. On the other hand, some holes in ground state can be trapped by Sr2+ vacancies. The inverse of these process results in the afterglow. Not all Eu3+ ions can be reduced to Eu2+ ions. Holes in ground states of the reserved Eu3+ can be trapped by hole traps. The kinetics of the afterglow and TL of the phosphors confirms to second order. It means most of the liberated carriers can be retrapped so that the afterglow can be prolonged.6) Finally, the afterglow properties of Eu3+, Tb3+ and Pr3+ at CaW04 matrix are investigated. The incorporation of Ti4+ and Mg2+ ions enhance the efficiency of the energy transfer and the luminescence efficiency of the host, respectively, thus the afterglow properties of Eu3+ can be enhanced. Both Tb3+ and Pr3+ shows afterglow phenomenon at CaWO4 matrix. It is probably due to the charge transfer (CT) from the 2p states of O2- to the 5d states of W6+ accompanied by the energy transfer to luminescent centers. Holes in valence band can be trapped by hole traps and their liberation leads to the CT and then the afterglow of Tb3+ and Pr3+ is generated by energy transfer from CT to the ions.
Keywords/Search Tags:Sr2MgSi2O7:Eu2+,Dy3+, Long afterglow, Photoluminescence, Thermoluminescence
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