Synthesis And Spectroscopic Properties Of Rare Earth Doped Lanthanum Phosphate Luminescent Materials

Light emitting diode(LED) is regarded as a new generation of the green lighting source in the 21 st century due to its advantages of energy-saving, environmental protection and so on. Rare earth phosphate has the properties of high luminous efficiency and good stability, which has the important application value in the fields of white LED and display. Lanthanum phosphate has monoclinic monazite crystal structure, which has the excellent chemical stability, low phonon energy and abundant cells for the cations. Therefore it is usually used as the matrix of the luminescent materials. In this research, we studied the synthesis method, luminous performance and the application in the white LED of the rare earth cations doping or co-doping lanthanum phosphate luminescent materials. The study includes four aspects:(1) La0.85-xPO4: xCe3+,0.15Tb3+powers are synthesized by high temperature solid method. The effects of doping concentration, calcining temperature, calcining time and the fluxing agents on the crystal structure, particle morphology and luminescent properties are studied in this research. The results show that the powders are the spherical particles with smooth surface and uniformity size(0.8~1.0 μm) after annealed at temperature of 1250 ℃ for 2 h. There exists the energy transfer of Ce3+→ Tb3+in LaPO4:Ce3+,Tb3+phosphors. What is more, the luminous intensity is strongest when the concentration ratio of Ce3+ and Tb3+ is 2:1. In addition, adding K2CO3, Na2CO3 or Li2CO3 as fluxing agents(Li2CO3 achieves the best effect) can reduce the reaction temperature meanwhile increase the powder crystallinity and luminous intensity.(2) La1-3xPO4:2xCe3+,xTb phosphors are synthesized by coprecipitation method with precipitant(NH4)2HPO4.We studied the effects of solution pH on crystal structure,particle morphology and luminescent properties of precursor. Furthermore, we discussed the growth process of the precursor in different pH condition. The results show that the precursor with the morphology of nanowires, nanorods and nanospheres can be prepared by adjusting the solution pH value. Besides, LaPO4:Ce3+,Tb3+nanopowder shows high crystallinity and good luminescent properties after calcinations at 1000 ℃ for 2 h. In addition, adding polyethylene glycol(PEG) as dispersant is helpful to obtain the dispersive powder with less surface defects.(3) LaPO4:Ce3+,Tb3+nanophosphors are synthesized by microwave-assistedcoprecipitation method.The effects of microwave calcining temperature on the crystal structure, particle morphology and luminescent properties are studied. The results show that the crystal structure of precursor is changed when sintering temperature is700 ℃. And the dispersed smooth spherical particles are obtained when sintering temperature is 900 ℃. Therefore, the calcining temperature is greatly decreased by microwave-assisted coprecipitation method. La PO4:Ce3+,Tb3+ luminescent ceramics prepared by the nanopowders show the good chemical stability and luminescence properties, which has wide application prospects of ceramic lighting lamps and lanterns and new conversion materials for LED light.(4) La1-x-yPO4:xTb3+,yEu3+ nanopowders are synthesized by microwave-assisted coprecipitation method.The effects of Tb3+and Eu3+doping concentration on the luminescent properties has been studied systematically. The results show that the phosphors doping Tb3+or Eu3+can emitting the green or red light emission under UV excitation respectively. There exists the energy transfer of Tb3+→Eu3+ in LaPO4:Tb3+,Eu3+phosphors. The intensity proportional of emission peaks can be adjusted via changing the Tb3+/Eu3+ concentration ratio. In addition, the La1-x-yPO4:xTb3+,yEu3+phosphors can emit multiple color light and can be combined into white light. There are several excitation peaks in the range of 340~400 nm ofLa1-x-yPO4:xTb3+,yEu3+phosphors and these peaks can match with the emission peak of near ultraviolet area chips. The research provide a new type white LED which is made by La0.86PO4:0.1Tb3+,0.04Eu3+and In GaN chips. Its color coordinates is(0.3807,0.3578), color rendering index is 81.5 and color temperature is 3844 K.