Design,Luminescence Mechanism And Application Of Novel Rare Earth Doped Aluminate Phosphors

Luminescence materials based on rare earth ions doped oxides have attracted great attention for their applications of field emission displays?FEDs?and solar cells.Because FEDs exhibit many advantages,such as wide view angle,high color purity,low cosumption,high brightness and fast response,this teachnique has been considered as one of the most promising flat plate display technologies.At present,sulfide-based luminescence materials are employed as the conductive phosphors for FEDs.However,the electron beam bormbardment will lead to the decomposition of the sulfide-based materials due to their poor chemical stability and the decreased brightness because of low vacuum and the polluted cathodic material.In addition,there exists the accumulated charges under the electron beam bombardment because the oxide-based phosphors are insulating and it will lead to the reduced luminescent intensity.Therefore,it is a hotspot for developing novel oxide-based conductive phosphors with high brightnss and color purity and the resistance to the electron beam bormbardment.In this dissertation,Sm³⁺ doped 12CaO·7Al₂O₃?C12A7?phosphors were prepared by a self-propagating combustion in combination with a thermal annealing.For these phosphors,four emission peaks at 564,601,648 and 749 nm appear upon 404 nm excitation and they originates from the 4G5/2-6HJ?J = 5/2,7/2,9/2 and 11/2?transitions of Sm³⁺ ions.The strongest luminescent intensity of Sm³⁺ doped C12A7 losphors was obtained via modulating Sm³⁺ doping concentrations and the optimized Sm³⁺ doping concentration is 1.0%.Our results suggest that the concentration quenching appears when the distance between Sm³⁺ ions is shorter than 27.37 A and the energy transfer through a dipole-dipole interaction has a main contribution to the concentration quenching of the luminescent intensity.On the basis of the unique feature of C12A7 with nanocage structure,red emitting C12A7:1.0%Sm³⁺ conductive phosphor was achieved by a thermal annealing at 1300 ? in H2 ambient for 6 h and followed with a subsequent UV-irradiation.The change in surface color with UV irradiation time,diffuse reflection spectra and electron paramagnetic resonance measurements of the H2-annealed C12A7:1.0%Sm³⁺ phosphor indicate that encaged electrons are formed in C12A7 and the calculated encaged electron concentration is about 1020 cm-3 and the obtained conductivity of the C12A7:1.0%Sm³⁺ pellet is about 1000 S.cm-1.The intense red emissions of the C12A7 conductive phosphor were obtained under the electron beam excitation?acceleration voltage:5 kV?and its chromaticity coordinate is?0.64,0.31?.Our results suggest that the red emitting C12A7:Sm³⁺ conductive phosphor with nanocage structure has potential application for FEDs.On the other hand,man made satellites and space vehicles have been used in many fields,such as navigation,radio and television,communication,weather and space science.In particular,solar cells have become the main power source of man made satellites and space vehicles due to their high reliability,fine flexibility and good sustainability.Since the working space solar cells are irradiated by deep UV light ranging from 200 to 300 nm,it will lead to the increased lattice thermalization,the higher temperature and the decreased lifetime of space silicon solar cells.Therefore,there is a remaining challenge for developing novel protecting layer of silicon solar cells for avoiding them from deep UV irradiation and the temperature rise.In this dissertation,Pr doped CaGdAlO4?CGA?phosphors were prepared by a self-propagating combustion along with a thermal annealing.On the basis of emission and excitation spectra data,it can be found that,in the case that the 5d states of Pr³⁺ ions locates below the 1S0 state,the abnormal blue and red emissions of Pr³⁺ appear via the energy transfer between Gd³⁺ and Pr³⁺ ions under 261 nm(the 4f-5d transition of Pr³⁺)excitation.These sharp emission peaks at 498,554,630 and 662 nm can be attributed to the 3P0-3HJ?J=4,5,6?and 3P0-3F2 transitions of Pr³⁺ ions,respectively.The strongest emission intensity can be obtained when the Pr³⁺ doping concentration is 0.5%.Furthermore,Pr³⁺/Yb³⁺ codoped CGA phosphors with different doping concentrations were prepared by a self-propagating combustion along with a thermal annealing.Based on the excitation and emission spectra of Pr³⁺/Yb³⁺ codoped CGA phosphors,it is found that the deep UV?DUV?to near infrared?NIR?quantum cutting under 261 nm excitation is realized through the energy transfers between Pr³⁺-Gd³⁺-Pr³⁺-Yb34+ ions.When the doping concentration of Yb³⁺ ions is 6%,the most efficient DUV to NIR quantum cutting is obtained and the quantum cutting efficiency is about 166%.The two-step energy transfer from Pr³⁺ to Yb³⁺ has a main contribution to the DUV to NIR quantum cutting.The temperature dependent emission intensity suggests that the CGA:0.5%Pr³⁺,6%Yb³⁺ phosphor with the higher thermal activation energy?320 meV?has good thermal stability.Especially,under 254 nm irradiation,the change in the surface temperature with irradiation time suggests that this material is resistant to the DUV irradiation.To further develop novel DUV to NIR quantum cutting materials,Tb³⁺/Yb³⁺ co-doped CGA phosphors were prepared by a self-propagating combustion along with a thermal annealing.From the emission and excitation spectra of Tb³⁺/Yb³⁺ co-doped CGA phosphors,it is found that the intense absorption peaks in the DUV range of 230-280 nm originate from the 4f-5d and 4f-4f transitions of Tb³⁺ ions,respectively,while the narrow absorption peak at 275 nm can be attributed to the 8S7/2-6IJ transitions of Gd³⁺ions,indicating that there exists the energy transfers between Gd³⁺ and Tb³⁺ ions.Upon 253 nm excitation,several sharp emission peaks are observed in the range of 300-1000 nm.The emission peaks in the visible range can be attributed to the 5D3-7FJ and 5D4-7FJ of Tb³⁺ ions and the NIR emission peak originates from the 2F5/2-2F7/2 transition of Yb³⁺ions.When the Yb³⁺ doping concentration is 4%,the most efficient DUV to NIR quantum cutting can be realized upon 253 nm excitation and it is attributed to the cooperative energy transfer from Pr³⁺ to Yb³⁺.Our results suggest that CGA:Re³⁺/Yb³⁺?Re=Pr or Tb?DUV to NIR quantum cutting phosphors are helpful for improving photoelectric conversion efficiency of space silicon solar cells and reducing their lattice thermalization.