Research Of Surface Coating On BaMgAl₁₀O₁₇: Eu～(2+) Blue Phosphor For PDP

Being a critical material in PDP apparatus, phosphor is of extraordinary importance to the development of PDP technique and its application properties. Aluminate base BaMgAl10O17:Eu2+ blue phosphor is of very importance due to its relatively high quantum efficiency, pure chromaticity and chemical stability. However, when after baked at 500⁶00℃, severe thermal degradation would happen to BAM, presents luminescence decreasing and chromaticity shift. this kind degradation will be a detrimental factor to both phosphor's application property and working life of PDP apparatus. Up to now, some measures posed still could not solve this problem effectively.The relationship between phosphor luminescence properties and baking temperature and baking time were studied with thermal degradation experiment. Results indicated that both luminance and chromaticity decreased gradually with baking temperature rising to 500℃and above. As temperature reached 550℃. the degradation extent aggravated further. When at 600℃, luminance only maintained at 69%, and chromaticity coordinate y decreased from initial 0.0650 to 0.0563. For the purpose to study the thermal degradation mechanism, electron binding energy of BAM luminescence center Eu and crystal structure were investigated by X-ray Photoelectron Spectrum (XPS) and X-ray diffraction (XRD), respectively. Results illustrated that both oxidation of Eu2+ into Eu3+ and changing of crystal structure resulted in the thermal degradation.Firstly, Al(OH)3 precipitation method and NH4AlO(OH)HCO3 precipitation method were developed respectively based on heterogeneous precipitation, and all obtainedγ- Al2O3 coating layer. Results indicated that as to the former method, when pH =4.5 or 4.8, addition proportion of precursor was 1.0% or 1.5%, the coating layer had relative high coating ratio, and distributed uniformly on the surface of phosphor in the form of tiny particle. While pH = 5.2 or 6.0, coating layer grew into spicule with high rate of length to radius. When addition proportion is 0.5².0%, the thickness of coating layer increased gradually, and the morphology transformed from former uniform state into accidented state. As to letter method, when pH =8.5, addition proportion was 0.5%, the layer with relative high coating ratio and uniformity can be obtained, and its uniformity would decrease as addition proportion increased further. Thermal degradation experiment results illustrated that both these coating phosphor did not present favorable protective effect.Based upon explored results of Al2O3 coating work, a simple and stable SiO2 coating technique was explored, and the critical problem how to effectively control morphologies of coating film through pH value, addition proportion of precursor and addition velocity were explored. Experimental results indicated that when pH = 7¹1, addition proportion =5¹5%, aging time = 2⁴h and addition velocity =5ml/min, the coating film with high coating ratio, uniformity and compact degree was prepared. This kind coated BAM presented excellent anti-thermal degradation effect. On the premise of initial luminance (105%), SiO₂ coated BAM luminance kept at 92% at 600℃, which is higher than that of uncoated BAM by 23%, and chromaticity was 0.058. Furthermore, the contact mechanism between coating layer and phosphor, coating mechanism and anti-thermal degradation mechanism were discussed. Results showed that SiO₂ coating film contacted with phosphor in the form of chemical bonding, and served as a block layer to obstruct oxygen atom. These results also confirmed that the main reason of thermal degradation mechanism of BAM was oxidation of Eu²⁺.Finally, Kublka-Munk optical model was applied to calculate the optical behavior of coating phosphor layer. Results illustrated that the lower refractive index, the lower descendent extent of luminescence efficiency caused by absorption and dispersion of coating layer to incident light and emergent light, which confirmed that the reasons why SiO₂ coating film had advantage over Al₂O₃ coating film from aspect of optics and provide effective theoretical reference on how to choose proper coating material.