Study On Preparation And Luminescent Properties Of Nano-scale Rare Earth Luminescent Materials

Rare earth luminescent materials have become the main materials in information display, light engineering and optical communication et al because the rare earth ion has the special structure of its 4f and 4f5d energy level and charge transition state. Rare earth luminescent materials used for commerce can be prepared by high temperature solid reaction or precipitation in solution. It is difficult to obtain rare earth luminescent materials with high quality and finely granularity through traditional methods so that the application of rare earth luminescent materials on the high-technology products has been restricted. Nano-technology on preparing functional materials has been studied widely by scholars. There are a lot of troubles and limitations on preparing rare earth luminescent materials, so new methods of preparing are required. According to the problems mentioned, we decided to use new methods to synthesize nano-sized or nano-level rare earth luminescent materials with fine particle and good luminescent properties.By means of two steps method of precursors and thermal decomposition, the nanometer rare earth red and green phosphors were obtained, which band mechanical alloying and salt-salt solid state reaction together. Nano-sized Y2O3: Eu3+ red phosphors and nano-level (Ceo.67Tbo.33) MgAl11O19 green phosphors were prepared by thermal decomposition of rare earth complex precursors and combustion. The characteristics of phosphors were investigated by means of TG-DSC, XRD, SEM, TEM. The reason for which surfactant effect on the sizeand morphology of powder were studied. Some creative results were acquired.Nano-sized Y2O3: Eu3+ phosphors have been prepared by mechanical solid state chemical reaction. The mixed precursors of Y2(C2O4)3 · 10H2O and Eu2(C2O4)3·10H2O have been prepared by raw materials of Y(NO3)3·6H2O and Eu(NO3)3·6H2O. Nano-sized Y2O3: Eu3+ powders can be obtained by mixed precursors decomposing at 600℃. The temperature of thermal decomposition for the precursors is lower and the time is shorter than high temperature solid state reaction method. The "ball milling method" is better than "hand milling method" . Y2O3: Eu3+ powder is cubic system structure, while its morphology is spherical and the average size of the particles is within 30~50nm with uniform grain size. Studying on the luminescent properties of Y2O3: Eu3+ phosphors, the results indicate that nano-scale Y2O3: Eu3+ phosphors have a high intensity and broad emission band corresponding to Eu3+ and O2- charge-transfer state transition, and the maximum excitation spectra peak is at λex=249nm, red-shift 10nm, while the maximum emission spectra peak is at λem=611nm corresponding to 5D0→7F2 transition, blue-shift happening. The luminescence intensity increases with the growth of Eu3+ concentration, and the Eu3+ optimal concentration is 10% improved obviously comparing with micron crystal (6%) . It is also found that the particle size of Y2O3: Eu3+ powders increases with the increasing temperature. The luminescence intensity of Y2O3: Eu3+ was increased slowly with annealing temperature up to 1100℃, and the obtained powder exhibits a normal size distribution, whose size is about 1~2μm.Mechanical solid state chemical reaction is a kind of new effective method with many advantages such as simplicity, directness, easy operation and lower thermal decomposition temperature for preparing nano-scale rare earth luminescent materials.Nano-sized LaPO4: Ce3+, Tb3+ green phosphors have been prepared by mechanical solid state chemical reaction. The results show that the increase of heat treatment temperature is beneficial to the crystallization of sample, and the particles with spherical shape, non-aggregation and 300~700nm size, are obtainedat 800 ℃. The photoluminescence spectra indicates that LaPO4:Ce3+, Tb3+ has a broad emission band at 250~290nm corresponding to the transition of 4f-?5d with the maximum excitation spectra peak at λ ex=272nm accompanied by green-shift, while the maximum emission spectra peak is at λ em=548nm with 272nm excitation, corresponding to 5D4→7FJ(J=6, 5,4, 3) transition. The concentration of most excitation energy on the green area of 540~550nm indicates that Ce3+ sensitizes the Tb3+ to luminescence effectually.The means of preparing nano-sized rare earth phosphors by the mechanical solid state chemical reaction has broadened the application areas and developed the method or measure of the research on the nano-sized rare earth phosphors.The study of the affection of surfactants on precursors and final products has been carried out in the thesis. The result indicates that, with surfactant, the precursors particles have spherical shape of 80~120nm, non-aggregation characteristics, and the final products are similar to them with spherical shape and uniform grain size of 30~50nm, without surfactant, the precursors and final products aggregate seriously and disperse badly, and the particle size is 50~250nm and 20~90nm respectively. The function of surfactant in the process of preparing LaPO4: Ce3+, Tb3+ is the same as the former.Above all, the surfactant is helpful to improve the dispersion and decrease the size of the products.The study on preparing nanometer Y2O3:Eu3+ by thermal decomposition of the rare earth complexes precursors has been carried out in the thesis. The experimental results show that the Y2O3: Eu3+is cubic system structure with net structure made up of spherical particles of 10~20nm size when the temperature is lower than 800 ℃. The maximum excitation spectra peak of Y2O3: Eu3+ is 248nm corresponding to Eu3+ and O2- charge-transfer state transition accompanying with red-shift, while the maximum emission spectra peak is 615nm corresponding to 5D0→7F2 transition accompanying with blue-shift. The luminescence intensity and particle size increase with the growth of temperature, and the Eu3+ optimal concentration is about 10%. The luminescence intensity and size of Y2O3: Eu3+increase slowly with annealing temperature up to 1100 ℃, and the particle size is about 1-2 μ m.The strongest emission and excitation spectra peak of the sample by thermal decomposition of the rare earth complexes precursors differ from the one by the mechanochemical method. It indicates that different preparations of nanometer particles induce different surface states with different luminescent properties.Nano-sized (Ce0.67Tb0.33) MgAl11O19 powders are prepared by combustion with the irregular shape and the size of 100~500nm. If the quantities of the reducer are added, grains will become augmented with the size of 250~280nm and the irregular shape. The maximum excitation spectra peak of the sample is 278nm corresponding to 4f→5d transition, and the maximum emission spectra peak is 543nm with 272nm excitation corresponding to 5D4→7FJ(J=6, 5,4, 3) transition.