Preparation, Surface Modification And Spectrum Characteristics Of Nano-sized BaMgAl₁₀O₁₇:Eu～(2+) Blue Phosphor For PDP Application

Cathode-ray tube (CRT) screen and liquid-crystal display (LCD) are the most common display devices in this information age. Whereas, as a large screen application, a conventional CRT is too bulky and heavy, a LCD is restricted by a small viewing angle and by a slow response. A plasma display panel (PDP) provides a rapid response and large viewing angle. Hence, a PDP becomes a promising candidate for high-definition wall-attached TVs. The phosphor plays an important role in the working of a PDP. Currently, BaMgAl₁₀O₁₇:Eu²⁺ (BAM) has been widely used in PDP as a blue component due to its merits such as ideal chromaticity, high quantum efficiency and luminescent properties under the excitation of vacuum ultraviolet (VUV) light.Thus far, commercialized BAM phosphors have been prepared by conventional solid-state synthesis route. The solid-state route requires high reaction temperatures typically more than 1600℃, long heating times, and a milling process to obtain the pure phase and small particles. Therefore, particles produced by this route are agglomerated particles of irregular shape. In addition, the milling process reduces the brightness due to the destruction of surface structure of the phosphor materials.As a result, many attempts have been carried out to find alternative methods for the preparation of BAM phosphor so as to lower the production cost, simultaneously improve the brightness, the lifetime, and the resolution.Recently, a solution combustion synthesis (abbr. SCS) has attracted considerable attentions as a novel soft chemical method for preparing superfine or nano-sized oxide powders because it offers better homogeneity and lowers sintering temperature. In this paper, the high quality nano-sized BAM phosphor was first prepared by this novel SCS method. Simultaneously, the surface modification of nano-BAM particles was investigation, the mechanism of morphology control of BAM nanocrystals was researched, the influence of key processing parameters was studied on the crystalline, particle size, and luminescent performance of BAM phosphor. Also, the spectrum characteristics of BAM on UV and VUV regions were analyzed in detail. Furthermore, the comparison analysis of BAM phosphors prepared by SCS and Sol-Gel method respectively was carried out systematically. The main research results are as followings.Nanocrystalline BAM powders have been rapidly prepared by SCS method. The results of XRD and TEM analysis show the sample is monophasic and well-crystallized, in which displace as well as twin crystal do not exist. Moreover, BAM nanocrystals exhibit an ideal spherical morphology with grain size of about 20nm. The emission spectrum of the BAM consists of a wide band with the peak at 450nm, which corresponds to the transition from the 4f65d excited state to the 4f7 ground state of Eu²⁺ ion. The blue emission is excellently consistent with CIE chromaticity and quite suitable for Plasma display panels.The processing parameters have an obvious influence on the luminescence properties of BAM phosphors. When ignition temperature is low, the mixture reacts very slowly and the combustion is not sufficient leading to the final products including organic impurities and decreasing the luminescent intensity of BAM. But, if the ignition temperature is too high,the morphology of BAM will be destroyed. The optimal ignition temperature is 600℃, at which the obtained BAM has a regular spherical shape, narrow size distribution and high luminous performance. The fuel/oxidizer ratio strongly affects the spectrum character of BAM. With the fuel/oxidizer ratio reducing, the main peak in the emission spectrum shifts to the long wavelength and the luminescent intensity increases. As the fuel/oxidizer ratio is 1/4, the maximum luminescent intensity is reached.In appropriate Eu²⁺ doping concentration range, the intensity of the fluorescence of BAM is increased obviously with increasing the Eu²⁺ doping concentration, owing to adding the number of luminescent centers and enhancing the energy transfer between Eu²⁺ ions. The maximum emission intensity is reached when Eu²⁺ molar concentration is 0.2. However, as the Eu²⁺ concentration is higher than 0.2, the intensity of the fluorescence is reduced reversely, due to the concentration quenching occurring. Furthermore, the quenching concentration of Eu²⁺ in BAM nanocrystal is improved owing to the decrease of non-radiative transition probability.Moreover, the luminescent property of BAM phosphor is considerably influenced by Ba/Mg ratio. The relative luminous intensity of BAM phosphors is increased obviously with increasing the Ba/Mg ratio, and the maximum emission intensity is reached at Ba/Mg = 0.9 . Furthermore, with the decrease of the Ba/Mg ratio, the emission of the phosphors shifts to the longer wavelength due to the crystal field enhancing and electron cloud expansion effect. The use of H₃BO₃ as flux can help to crystallize the phosphor, and enhances the intensities of excitation and emission spectra of BAM. The optimum content of H₃BO₃ is about 1.0wt.%, which increased the relative emission intensity of BAM over 30%. The flux mechanism to improve photoluminescence characteristics of BAM is that the H₃BO₃ melts at a low temperature to form liquid phase leading to accelerate the velocity of chemical reaction, prompt the Eu²⁺ enter the BAM lattice through the grain boundary diffusion and improve particles morphology with round edge, smooth surface and nearly spherical shape.Morphology control of BAM phosphor particles has been attempted by using polyethylene glycol (PEG) as additive. The obtained BAM phosphors resulted from PEG additive have spherical morphology, narrow size distribution and strong blue emission under VUV light excitation compared to that without PEG additive, and the maximum emission intensity of BAM is achieved when the concentration of PEG(MW=10000)is 5.0 wt.% in the precursor solution.VUV luminescent properties of BAM have been systematically studied. The excitation spectrum shows a wide band with the peak at 173nm, which is well coincidence with the Xe2 molecular emission band of admixture of inert gases utilized in PDP's spark plasma source. Furthermore, the BAM shows a sufficient absorbance in 150-190nm VUV range, which indicates there is a efficient energy transfer from host lattice to Eu²⁺. Therefore, the BAM resulted from SCS method is promising for use in PDP.The surface-modified nano-sized BAM phosphors have been successfully achieved by an emulsion method. The luminescent performances of BAM with perfect surface modification is greatly improved due to the hanging-bonds and defects on the surface of nano-scale BAM particles being removed as well as non-radiative transition probability decreasing. Thermal degradation experiment results indicate that the property of resistance to thermal degradation of surface-modified BAM is enhanced obviously.In order to compare luminescent properties of BAM derived from different wet-chemical methods, the BAM has also been prepared by a modified Sol-Gel method. The results indicate that, for the sample prepared in air, the excitation lines is observed at 363,381,393,413nm, correspond to ⁷F₀→⁵D₄,⁷F₀→⁵G₂,⁷F₀→⁵L₆ and ⁷F₀→⁵D₃ transitions of Eu³⁺ ; The emission prominent peak locates at 617nm, being attribute to ⁵D₀ →⁷F₂ electric-dipole transitions of Eu³⁺ ions lie in non-centrosysmetrical sites. After the sample is reduced in the reducing atmosphere, a strong blue emission with a peak at 444.9nm is observed, which shows the Eu³⁺ ions has been completely reduced to Eu²⁺. Therefore, the Sol-Gel method is also potential method to prepare BAM blue-emitting phosphors suitable for PDP application.Comparison analysis of the BAM phosphors fabricated by Sol-Gel and SCS methods respectively indicates that the BAM phosphors originated SCS have more regular spherical shape and more narrow size distribution leading to the better luminescent performance. Further, the SCS method itself generated the reducing gases and inert gases (for instance NO_x, H₂O, CO₂, and NH₃) during the combustion produces to reduce the Eu³⁺ ion to Eu²⁺ ion effectively, avoiding the use of reducing atmosphere during the post-treatment to reduce Eu³⁺ to Eu²⁺, which shows the SCS is more energy-saving. Therefore, the SCS method is an obviously better approach to prepare BAM blue phosphor.