Synthesis And Photoluminescence Properties Of Barium Titanate Based Phosphors

All-solid-state white light-emitting diodes（w-LEDs）are regarded as the fourth generation lighting source due to their advantages,including high efficiency,small volume,long lifetime,low power consumption,wide range spectra,environmental safety and quick response.The design concept of w-LEDs is trichromatic theory of light,and white light which has high color rendering index,luminous efficiency and controllable color temperature is obtained by allocating red,green and blue light together.The realization method of white lighting is the use of light conversion approach,which combines solid state phosphors activated by rare earth ions and blue/ultraviolet LED chips.Currently,the most widely acquisition of white light is the combination of yellow emitting phosphor YAG:Ce³⁺and the blue InGaN LED chip,owing to its high brightness,wide emission peak,low cost,but poor color rendering index and bad thermal stability when hybrid packaging with epoxy resin,largely affecting the lighting efficiency.It is necessary that phosphors have excellent luminous performance,as well as outstanding thermal stability in LED devices.Therefore,the development of phosphors with good thermal stability is one of the research focuses in LED filed.At present,combinatorial method is an excellent technology,which enables rapid preparation of sample libraries.With the structure analysis and spectral characterization of sample libraries,the optimal results can be obtained quickly.However,it is necessary to avoid the phenomenon of swelling and burning that may affect the performance of the samples during reactions in the confined space of microreactor,regarding the preparation of array powder sample librares by combination solution injection method.Therefore,a suitable preparation method of phosphors is very important for the successful combinatorial fabrication and screening.Based on the above background,the nitrate pyrolysis method is selected to successfully synthesis phosphors at 750℃,and it is an effective path for combinatorial preparation of phosphors.And the core contents of this dissertation are as follows:Firstly,to design and to prepare BaTi O₃ phosphors singly doped with Eu³⁺and Dy³⁺.Secondly,small radius ions Sr²⁺is substituted for partial Ba²⁺to prepare（Ba,Sr）TiO₃ phosphors doped with rare earth ions;by changing the crystal field of materials,the luminescence spectrum of the phosphors can be controlled to achieve a variety of LED lighting requirements.Thirdly,to explore the performance differences between two different matrix phosphors from phase structure,morphology,luminescent properties and thermal stability,and other aspects.Eventually,it is found that the phosphors have good thermal quenching resistance.In the first part of this research,Eu³⁺-doped titanate phosphors were synthesized.The optimal calcination temperature,optimal doping concentration of Eu³⁺,and optimum substitution of Sr²⁺were studied.The phase structures,morphologies,luminescence properties and thermal stability of the two different matrix phosphors were studied.It was found that the doping concentration of Eu³⁺and calcining temperature have a great influence on the luminescence intensity of the red emitting phosphor,and the optimal doping concentration of Eu³⁺is 8 mol%,and the calcination temperature is 750℃.The substitution of 30%Sr²⁺further enhances the photoluminescence intensity of the（Ba,Sr）Ti O₃:Eu³⁺phosphor.Interestingly,the thermal stability of BaTi O₃:Eu³⁺（8 mol%）is better than that of Ba_0.7Sr_0.3TiO₃:Eu³⁺（8mol%）,and the emission intensity remains about 70%and 60%of those at room temperature respectively,measured at higher 100℃.While cooling down to the room temperature again,the emission intensities of these two phosphors can be restored approximately 85%of the value before heating up.When the phosphors are subjected to a temperature increase and then to room temperature,the luminescence intensity of both phosphors can be restored to about 85%of the room temperature intensity.The fluorescence decay curve illustrates that Eu³⁺occupies two different luminescent centers in the serial（Ba,Sr）TiO₃:Eu³⁺phosphors.Besides,the unfilled 4f electron layer of Eu³⁺ions not only allows the phosphor to emit red light under excitation,but also makes the sample exhibit paramagnetic characteristics under excitation of magnetic field.In the second part of the research,the Dy³⁺-doped titanate phosphors were prepared,and the calcination temperature,doping concentration of Dy³⁺,and substitution of Sr²⁺were also optimized.The properties of two different phosphors such as phase transformation,morphology,photoluminescence spectra（emission spectra,excitation spectra,and thermal stability）were studied.When the doping concentration of Dy³⁺and calcination temperature are 4 mol%and 750℃respectively,the prepared yellow-emitting BaTiO₃:Dy³⁺（4 mol%）phosphors have high purity and good luminescent properties.The substitution of 20%Sr²⁺further enhances the photoluminescence intensity,but a reverse effect on the thermal stability of the（Ba,Sr）TiO₃:Dy³⁺（4 mol%）phosphors.In contrast,the thermal stability of BaTiO₃:Dy³⁺（4 mol%）is excellent,and the emission intensity remains about 90%at higher 100℃of that at room temperature.While cooling down from 210℃to room temperature,BaTiO₃:Dy³⁺（4 mol%）phosphor shows a stronger recovery of PL emission than others,which can be restored to about 95%of its original emission intensity.The fluorescence decay curve illustrates that Dy³⁺occupies two different luminescent centers in the serial（Ba,Sr）TiO₃:Dy³⁺phosphors,and the magnetic properties of phosphors would change from diamagnetic to paramagnetic in the presence of Dy³⁺or Sr²⁺under excitation of magnetic field.