Preparation And Spectroscopic Properties Of Y₂O₃:Eu³⁺ Nano-powders And Transparent Ceramics

This thesis focuses on the preparation and spectral properties of Y₂O₃:Eu³⁺ nano-powders and ceramics,which is composed of four chapters.In the preface,the background of my work is introduced.In chapter 2 to 4,the synthesis of Y₂O₃:Eu³⁺ nano-powders and ceramics as well as the morphological,structural and spectral properties of the resultant samples are repoted.In chapter 1,an introduction of some important rare earth luminescent materials is given at first.Then the basic principals ofphotoluminescence,spectral properties of trivalent rare-earth ions as well as knowledge of transparent ceramics are stated.At last,combustion and homogeneous precipitation methods for synthesis of nano-powders are briefly introduced.In chapter 2,Y₂O₃:Eu³⁺ nano-powders were synthesized by combustion method and the influence of pH value on properties of combustion-derived powders was studied.The structure and morphology analyses display that with the increase of pH value,the crystallites become bigger,and the powder becomes more and more compact.TG-DAT curves indicate that longer combustion time is needed under higher pH value,which may account for the severer agglomeration in corresponding powders. The emission spectra of powders contain the typical Eu³⁺ luminescent components in cubic Y₂O₃ with the strongest emission at 612 nm assigned to ⁵D₀-⁷F₂ forced electric dipole transition.The emission spectra show that with the increase of pH value,the luminescent intensity of Y₂O₃:Eu³⁺ nano-powder is enhanced.The excitation spectra consist of three parts:host absorption,charge transfer band and transition within 4f orbital and the relative intensity of host absorption weakens with the increase of pH value.The decay curves reveal the observed lifetime of Y₂O₃:Eu³⁺ nano-powder shortens with the increase of crystalline size.Agglomeration,which is deleterious to the transparency of ceramic,is easily present in the powders prepared by combustion method,so PEG2000 was used as dispersant to improve the dispersivity of the resultant powders.Small amount of dispersant increases the crystalline size slightly,while with the continuous increase of dispersant,the crystallites become smaller and smaller.What is the most important is that appropriate amount of dispersant(10wt%) reduces the size of agglomerate obviously.Emission spectra confirm that the reduction of agglomerate size can enhance the luminescent intensity apparently when the powders possess almost the same crystalline size.In the excitation spectra,red-shift of charge transfer band with the decrease of crystalline size is observed,resulting from the increase of surface area. The decay curves reveal that powders with different crystalline size have almost the same lifetime.In chapter 3,Y₂O₃:Eu³⁺nano-powders were prepared by gel combustion,and the products were compared with those produced by direct combustion.The resultant powders were sintered into transparent ceramics and the influence of properties of powders on microstructures of ceramics was studied.XRD analysis indicates the gel process before combustion has no influence on the crystalline size,which is mainly determined by the molar ratio of glycine to metal ions.However,TEM and SEM images reveal that the gel process can reduce the size of agglomerate significantly. The surface morphologies of ceramics show that smaller agglomerates lead to even surface and homogeneous structure while cracks and pores are present in the ceramics derived from powders prepared by direct combustion.Powders and the corresponding ceramics have just the same emission spectra in terms of peak position.But there are some distinct differences in the excitation spectra. In chapter 4,homogeneous precipitation was used to prepare Y₂O₃:Eu³⁺ nano-powders,which were subsequently sintered into ceramics.In the experiment condition range of this thesis,it is found that the concentration of reactant,the water-bath temperature and calcination temperature have no significant influence on the morphologies and structures of the resultant powders,all of which are composed of crystallite-agglomerated spherical particles.Nevertheless,the pH value of solution makes striking difference.When the pH value of solution was adjusted to 6.5,the synthesized powder has good dispersivity and the corresponding ceramic shows better transparency.SEM images of ceramics confirm that agglomeration in powder results in wide crystalline size distribution and heterogeneous morphology while good dispersivity ensures homogeneous microstructure.The decay curves of ceramic and powder reveal that former has much shorter lifetime.