Study On The Preparation Of Red Phosphor Y₂O₂S:Eu³⁺ And The Surface Modification By Coating Process

Along with the quick development of all kinds of new flat-panel displays, the history of dominating the world by the cathode ray tube (CRT) displays has ended. There are various flat-panel displays(FPD) such as liquid crystal display (LCD), plasma display panel (PDP), field emission display (FED), electro luminescent display (ELD), vacuum fluorescence display (VFD) and light emitting diodes (LED), et al. Among these FPDs, CRT, VFD and FED have a common fundamental in luminescent principle, that is, they all belong to cathode-ray luminescent devices. The phosphors used on them are cathode-ray luminescent materials, and some phosphors are generally utilized. The difference among them is that CRT usually works under about 20KV, attributed to high-voltage luminescence, but FED and VFD work under below 2000V and 100V respectively, attributed to low-voltage luminescence. In order to enable Y2O₂S:Eu³⁺, which is a kind of high-voltage red phosphor widely used in CRT, can be applied to FED and VFD devices surface modification of the phosphor must be executed. Usually one-layer or multi-layer functional film coated on the surface of phosphor particles can make the particles isolated from outside ambence, improve the dispersion and stability of the phosphor and retard the decline of phosphor's capability caused by the electrical property and surface chemical activity. At the same time, functional materials coated on phosphors can also improve the luminescent properties.This dissertation carried out the following research on the preparation of red phosphor Y2O₂S:Eu³⁺ and the surface modification by coating transparent and conductive films.1. A flux fusion method has been used to prepares a eries of Y2O₂S:Eux3 + phosphors which have a hexagonal structure. Eu³⁺ is expected to substitute the Y³⁺. Since the ionic radius of Eu³⁺ is slightly larger than that of Y³⁺, the type of crystal structure do not change and the lattice contants a and c increase with the Eu³⁺ substitution doping. The average size of samples is about 10μand some aggregation phenomenon occurs between particles. The photoluminescence(PL) emission intensity ratio I626/I539 gradually increases and I626/I596 keept unchanged with the increacing Eu³⁺ doping, when the excitation wavelength is 340nm. The main reason might be that the concentration quenching occurs between Eu³⁺ in higher excited states. The effects of annealing temperature and time on the growth of Y2O₂S:Eu³⁺ particles and the fluorescent properties have been optimized and 1200℃for 2h are suitable annealing conditions2. A dip-coating method has been used to prepare the SnO₂:Sb films which have rutile structure and about 300nm particles size. The thickness of SnO₂:Sb films is about 4μm under the experiment conditions. The effects of different coating layers, doping concentration nSb:nSn and annealing temperature on the optical transmission and electrical conductivity of the films have been investigated respectively. When the films are 8 layers, nSb:nSn=10% and annealing at 500℃for 1h in oxygen flow, an over 80 % transmittance in the visible light can be obtained. The electrical resistivity of the films is about 5×10-2??cm. Y2O₂S:Eu³⁺ phosphors have been coated under the optimized processing conditions. It can be seen that the phosphor particles are covered by a spiccato layer of SnO₂:Sb film with the thickness of about 1μm. The measurements of PL indicate that the coating process do not influence the location of emission peak. The relative fluorescent intensity decline a little due to the role of reflection, scattering and absorption of SnO₂:Sb layer. The effect of nSnO₂/nY2O₂S on resistivity and relative fluorescent intensity of the phosphors was investigated under the fixed treatment conditisions. If nSb/nSn=10% , nSnO₂/nY2O₂S = 6%, and annealing at 500℃for 1h in oxygen flow, the modified phosphors have good electrical conductivity with relative fluorescent intensity of 83% .3. The ZnO:Al films obtained by dip-coating method under the experiment conditions are highly c-axis oriented with about 200nm particles size. The effects of different coating layers, doped concentration nAl:nZn and annealing temperature on the optical transmission and electrical conductivity of the films have been studied respectively. If films consists 12 layers, nAl:nZn=1% and annealing at 550℃for 1h in air, a more than 85 % optical transmittance in the visible light and about 0.25??cm electrical resistivity have been observed. The optimized processing conditions have been used to modify Y2O₂S:Eu³⁺ phosphors. As a result, the phosphor particles are covered by a spiccato layer of ZnO:Al film with the thickness of about 1μm. The coating process do not influence the location of emission peak from PL measurements but the relative fluorescent intensity declines due to the role of reflection, scattering and absorption of ZnO:Al layer. The effect of nZnO/nY2O₂S on resistivity and relative fluorescent intensity of the phosphors was inspected under the fixed treatment conditisions: nAl/nZn=1% and annealing at 550℃for 1h in air. The coated phosphors have good electrical conductivity with relative fluorescent intensity of 80% when the ratio of nZnO/nY2O₂S is 9%.