Research On Synthesis And Luminescence Properties Of Rare Earth Or Transition Metal Lons Doped Yttrium Aluminum Garnets

Garnets are a group of minerals which have been widely used as gems as early as Bronze Age. Garnets which can be synthesized artificially to be used as luminescent materials include yttrium aluminium garnet (Y3Al5O12, YAG) and some modifications of its. In 1960s, the single crystal of yttrium aluminium garnet was synthesized for the first time. In 1970s, people found that Ce3+ doped YAG can emit intense yellow light. When it comes to 1990s, phosphor YAG:Ce3+ began to be combined with blue light-emitting diodes (LEDs). Obtained white light can be used as a light source for interior illumination. In the subsequent nearly twenty years, people spent plenty of time and efforts in the investigation of this phosphor and obtained remarkable achievements. These investigations mainly focus on the preparation techniques, the improvement or modification of luminescence properties by doping, chromaticity, the enhancement of brightness, the controlment of particle size, and so on, to fulfill the requirements in practical production. However, the other problems were ignored such as the photoluminescence and thermoluminescence (TL) properties of the other rare earth (RE) ions and transition metal (TM) ions in garnets, the effect of host substitution on the luminescence properties of RE or TM ions in garnets, luminescence energy transfer of these ions in garnets, and so on. Thus, a series of RE or TM ions doped YAG or its modifications were synthesized with solid state reactions. The effect of the doping of RE or TM ions on the structure and luminescence properties were investigated. The main conclusions obtained in this dissertation were as follows.1. Afterglow and TL properties of Ce3+ in YAGPure YAG can be obtained by sintering the raw material for 4h at 1550℃. The doping of Ce3+ with a small amount does not change the structure of host YAG.The measurement of fluorescence spectra show that the sample prepared in reducing atmosphere can emit a broad band peaking at 531nm under the excitation of 341 and 455nm. Ce4+in the sample prepared in air can be partly reduced to Ce3+ spontaneously. Thus it can also emit yellow light peaking at 531nm. The luminescent intensity of the sample prepared in air is about one third of the intensity of the sample prepared in a reducing atmosphere. This indicates that the spontaneous reducing in air is incomplete. Under the ultraviolet excitation, yellow long afterglow can be found in the sample prepared in reducing atmosphere. The afterglow time is as long as 35 minutes. While there is no long afterglow is observed in Ce3+ doped YAG prepared in air and undoped YAG:Ce3+ doped samples show obvious thermoluminescence after they are excited by ultraviolet (UV) light.There are two TL peaks at 112 and 256℃respectively in the TL spectrum of reduced sample. However there is only one at 128℃in the sample prepared in air. The TL of Ce3+ doped samples is obviously stronger than the undoped one. This indicates that TL is originated from doped Ce3+. There is also TL in pure YAG after UV excitation which is originated from the extrinsic defects introduced by the impurity formed in the synthesis process and intrinsic defects mainly composed or anti-site defects. The doping of Ce3+ increases the concentration of defects remarkably and decreases the depth of defect energy level. This results in the long afterglow of samples under UV excitation.2. Effect of host substitution on the structure and luminescence properties of Ce3+ in YAGThe cations in YAG were gradually substituted with two methods. Then the effect of host substitution on the structure and luminescence properties was investigated in detail. One method is to substitute Y3+ with Dy3+. The other one is to substitute Y3+ and Al3+ with Ca2+ and Si4+ respectively.For the first substitution method Dy3+ gradually replaces Y3+ in Y2.95Al5O12:Ce3+ 0.05 with the increment of Dy3+ content. This indicates that YAG and DyAG can form complete solid solution. The crystal lattice expand with the increment of Dy3+ content but keep original garnet structure unchanged. The expansion extent of crystal lattice is proportional with Dy3+ content. The emission of all the samples is in a broad band. However, with the increment of Dy3+ content, a red-shift of emission peak is observed. The red-shift extent is also proportional with Dy3+ content. Long afterglow can be observed for all the samples. Afterglow time decreases with the increment of Dy3+ content with the longest afterglow time of 35 minutes. Via the measurements and calculation on TL spectra, two TL peaks can be found in the TL spectra. Position of the peak with lower temperature moves to lower temperature with the increasing Dy3+ content. This indicates that the depth corresponding to the peak with lower temperature decreases with the increasing Dy3+ content which is advantageous to the release of trapped electrons. The satisfying TL efficiency indicates our phosphor have potential applications in UV radiation dosemeters.For the second method, with the increment of co-substitution content, Ca2+ and Si4+ replaced Y3+ and Al3+ respectively, which results in the shrink of crystal lattice of samples whereas it presents single garnet structure all along. The decrement of cell parameter is in linear with the increment of co-substitution content. The emission spectra of all samples are in a broad band. The peak wavelength of the emitted broad band shows a blue-shift and the amount of the blue-shift is also in linear with the co-substitution content.3. Luminescence properties of the other RE and TM ions in YAGSi4+is co-doped into YAG with Mn2+ as a charge and volume compensator. This avoids charge unbalance and increases the doping content of Mn2+ in YAG. On the basis of this, the luminescence properties of Mn2+ in YAG are investigated. According to the radii of introduced ions, we design two methods for the substitution of Mn2+ and Si4+. One is to replace Y3+ and Al3+ with Mn2+ and Si4+ respectively. The other is to replace Al3+ with both Mn2+ and Si4+. The results of X-ray diffraction (XRD) analysis indicate that both methods are practical. The maximal doping concentration of Mn2+ in two methods can be enhanced to 30 and 40mol% respectively.For the first method, the substitution of Mn2+ and Si4+ to Y3+ and Al3+ make the interplanar distance decrease but does not change the single garnet crystal phase of the samples because the radii of both doped ions are smaller than replace ones.The emission spectra show that samples can emit yellow-orange light in a broad band peaking from 579 to 592 nm with long afterglow under the excitation of UV and blue-violet light. With the increment of substitution content, the emission intensity of samples increases firstly, decreases subsequently. The highest emission intensity occurs when x=0.075. The emission peaks move to longer wavelength. Afterglow spectra and decay curves show that all the Mn2+ and Si4+ co-doped samples emit yellow-orange light with long afterglow after the irradiation of UV light. The longest afterglow time is 18 minutes for Sam3.Two kinds of traps with different energy level depth in co-doped samples were observed by means of TL detection, and their depth decreases with the increment of substitution content. Higher TL efficiency shows that the phosphors present a good potential for UV irradiation dosimeter applications.For the second method, the co-doping of Mn2+ and Si4+ makes the interplanar distance decrease but does not change the single garnet crystal phase of the samples in a certain extent. The excitation and emission spectra of samples with pure crystal phase show that samples can broadband emit orange light peaking at 586nm to 593nm under the excitation of UV light and blue-violet light. With the increment of co-doping content, the emission intensity of the phosphors increases when Mn2+ content is lower than 10mol% while decreases when it is higher than 10mol%. That is to say, the emission intensity is biggest when Mn2+ content is 10mol%. The emission peak moves to longer wavelength with the increment of co-doping content.4. Luminescence energy transfer of RE and TM ions in YAGBy employing Si4+ as a charge compensator, the doping content of Mn2+ in YAG was increased to a certain extent. On this basis, the structure and luminescence properties of Mn2+ singly doped phosphor were investigated. Mn2+ in YAG emits orange light in a broad band. With the increment of the doping content, the emission peak shifts to longer wavelength and the emission intensity of phosphors increases firstly, decreases subsequently. The emission intensity reaches its maximum when Mn2+ content is 10mol%. Two groups of emission peaks, corresponding to the optical transitions from excited states 5D3 and 5D4, to ground state 7F1(J=6,5,4,3), respectively, were observed in the emission spectra of Tb3+ singly doped phosphors. When Tb3+ and Mn2+ were co-doped into YAG, the obvious sensitization effect of Tb+ on Mn+ was observed which indicates that the energy transfer from Tb3+ to Mn2+ occurs. The reason for energy transfer from Tb3+ to Mn2+ is verified that there is a perfect overlapping between the emission spectrum of Tb3+ and excitation spectrum of Mn2+. The experiments and analysis confirm that charge compensation and the sensitization of the other luminescent ions are two effective methods which can be utilized to enhance the luminescence of Mn2+ in YAG.A series of Bi3+ and Dy3+ doped YAG were synthesized with solid state reactions. The luminescence properties of Bi3+ and Dy3+, and the energy transfer from Bi3+ to Dy3+ were investigated. Bi3+ in YAG emits one broad band peaking at 304nm under the excitation of UV light (276nm) which is attributed to the transition from excited states 3P0.1 to ground state1So.Dy3+ in YAG emits two groups of emission lines near 484 and 583nm respectively which are attributed to the transitions from excited state 4F9/2 to ground states 6H15/2 and 6H13/2. The co-doping of Bi3+ into Dy3+ doped YAG enhances the luminescent intensity of Dy3+ by-7 times. The remarkable enhancement of Dy3+ emission obtained by Bi3+ co-doping makes it possible that Dy3+ doped phosphors are used in w-LEDs. The enhancement also proves the occurrence of resonance energy transfer from Bi3+ to Dy3+ which results from the overlapping between the emission spectrum of Bi3+ and the excitation spectrum of Dy3+. The resonance energy transfer from Bi3+ to Dy3+ is performed through dipole-quadrupole interactions.5. Charge compensation for the luminescence of Eu3+ in Sr2 CeO4 and ZnB2O4For the first system (Eu3+ doped Sr2CeO4), the phosphors can strongly absorb UV (broad band peaking at 276nm and overlapping 254nm) which is coupled well with the emission of low pressure mercury vapor (LPMV) lamps. In our experiments, the optimal doping concentration of Eu3+ in Sr2CeO4 is ascertained(x=0.13). All four charge compensators can enhance the red emission of Eu3+, but they do not significantly change the shape and positions of excitation and emission spectra in the same host lattices. The introduction of Li+, Na+ and K+ at Sr2+ site, Al3+ at Ce4+ site can enhance the luminescent of red emission by 1.3,1.6,2.1 and 1.4 times respectively. The investigation in this paper indicates that Eu3+ heavy doped Sr2CeO4 phosphors with charge compensation are potential candidates of red emitting phosphors for LPMV lamps or ascending AlxGa1-xN-based UV LEDs.For the second system (Eu3+ doped ZnB2O4) the results of XRD analysis show that all the prepared samples can be indexed to pure phase ZnB2O4. The excitation and emission spectra of samples show that Eu3+ doped ZnB2O4 can strongly absorb 393nm UV light which is coupled well with the emission of currently used InGaN-based near UV LEDs and emit red light with good color purity. Employed four charge compensation approaches can not only improve the crystallinity but also enhance the red emission of phosphors. Different charge compensation approaches do not affect the shape and position of excitation and emission spectra. The compensation effect of Li+ is the optimal followed by Na+, K+ and self-compensation. They can enhance the red emission intensity of Eu3+ by 4.4,3.7,3.4 and 2.2 times, respectively. Morphology analysis of samples with a scanning electronic microscope indicates that prepared samples are inhomogeneous and the introduction of charge compensators improves the crystallinity and increases the mean particle size of phosphors.When an ion as a luminescent center replaces another one which has the different valency(such as the substitution of Eu3+ for Sr2+, Ca2+, Ba2+ or Zn2+), the substitution content will be restricted. On this condition, charge compensation is an effective method for the enhancement of luminescence intensity. If just charge balance is taken into consideration, selected compensating ion should own less positive charges than replace ion when luminescent center ion has more positive charges than replaced ion, vice versa. Luminescent center ion and compensating ion may replace the same ion in the host. They can also replace the different ions. However, it is deficient to take just charge balance into consideration. When a charge compensator is selected, two other aspects should also be thought. One is the equilibrium of mole number which requires that one ion can only replace one. The other is volume balance. If luminescent center ion is bigger than replaced ion, the compensating ion should own a smaller radius than replaced ion by it, vice versa.