The Inhomogeneous Rare Earth Ions Distribution In YAG Transparent Ceramics And Study Of Spectroscopic Properties In Yb³⁺ Doped YAG Nano-crystals And Nano-ceramics

The thesis mainly consists of five chapters. The first chapter is the preface which widely gives introductions about the fundamental knowledge in this research field. Then the works about rare earth doped transparent ceramics are shown in the next four chapters. The contents will include:the inhomogeneous rare earth distribution in YAG transparent ceramics; the study of spectroscopic properties of in Yb3+doped YAG nano-crystals and nano-ceramics.The preface will give brief introduction about the background knowledge in this field. We will firstly talk about the electronic configuration of rare earth elements. Then the rare earth doped optical materials will be discussed. In addition the principle of Laser Scanning Confocal microscope which has been used in this research work will be shown. At last the definition of equilibrium segregation coefficient, effective segregation coefficient and the BPS model will be given and discussed.In the main body of the thesis, the research background of transparent ceramics will be shown firstly. We will talk about the definition and history of transparent ceramics. Then we will focus on the fabrication process of transparent ceramics and the recent progress of transparent laser ceramics and ceramic scintillators. At last we will introduce the YAG lattice that has a garnet structure which leads to its great performance as an important optical material in laser and scintillating applications.The main works of the thesis will include:1. In the first chapter, the characterization and analysis of the rare earth distribution have been done by using Laser scanning confocal microscope and transmission electron microscopy in Ce3+doped Y3Al5O12and (Y,Gd)3Al5O12 transparent ceramics. The results clearly show that the doping ions concentration in these transparent ceramics is not homogeneous. As a result, the evidence of Ce3+segregation in YAG transparent ceramics is given successfully.On the assumption that the polycrystalline ceramic growth is based on numerous independent growth of individual crystals, the segregation coefficient of rare earth ions and BPS model in fabrication process of single crystal is introduced to explain the inhomogeneous rare earth distribution in polycrystalline transparent ceramics. The high dopant concentration area near the interface of crystals and melts occurs when the effective segregation coefficient K is less than unit. This phenomenon will lead to the dopant segregation in polycrystalline transparent ceramics.According to the different behavior of Nd3+,Ce3+and Yb3+in dopant segregation phenomenon,we have built a reasonable curve that clearly depicts the segregation coefficients dependence on different radius of rare earth ions on the basis of calculations of segregation coefficients in other garnet lattice from other reports.2. The nano-crystal powders which have the average size of 20-40nm have been fabricated by Pechini method. The topographic image and particle size have been analysed by SEM and XRD.The emission spectra and life time of different dopant concentration have been measured under room temperature 298K and 12K.The Stark energy levels of Yb3+in YAG nano-crystals have obtained by comparison of emission spectra,absorption spectra and Raman spectra.The increasing process at low dopant concentration in dependence curve of life time on different Yb3+concentration is not observed compared to the Yb:YAG single crystal. It is probably due to the increasing self-quenching process that results from the nano scale crystal size.The upconversion spectra of different Yb3+concentration samples has also been measured. The cooperative emission spectra of Yb3+pairs is observed at about 505nm.The upconversion spectra of the impurities such as Tm3+and Er3+is also observed. The upconversion energy transfer process and scheme are analysed and shown.3. The low temperature high sintering pressure fabrication method has been used to produce the nano-ceramics from the nano-crystals powders discussed in the fourth chapter. The topographic image and particle size have been analysed by SEM,TEM and XRD. The transparency seems acceptable under high sintering pressure up to 8GPa. The preliminary spectroscopic analysis has been done for different doping concentration and different sintering pressure samples. The concentration quenching is observed in the 20% concentration sample which is different from the nano-crystals and single crystals. A little red shift occur in the emission spectra of high sintering pressure samples. The decay time of the nano-ceramics decrease fast with increasing the sintering pressure and is obviously shorter than that of nano-crystals. The equation of Meltzer has been used to explain these results.The upconversion spectra has also been measured with the observation Yb3+pairs emission and impurities emission.Some widen emission peaks are also observed.,The grain boundary model has been used to explain the experimental results. The clear concentration quenching process, the shorter decay time compared with nano-crystals and the increasing upconversion spectra all can be ascribed to the grain boundary model.