Development of efficient, small particle size luminescent oxides using combustion synthesis.

摘要

Luminescent materials (phosphors) find application in cathode-ray tubes (CRTs), medical and industrial equipment monitors, fluorescent lamps, xerography, and many types of flat panel displays. Many commercially available phosphors were optimized in the 1960s for high voltage ({dollar}>{dollar}10 kV) CRT applications. Recently, a great deal of emphasis has been placed on the development and improvement of phosphors for flat panel displays that operate at low voltages ({dollar}<{dollar}5 kV). In addition to high efficiency at low voltages, these displays demand high resolution phosphor screens which can only be realized using phosphors with smaller particle size ({dollar}<{dollar}3 {dollar}mu{dollar}m). Conventional methods of preparing phosphors (e.g., high temperature solid-state reaction) cannot easily produce a homogeneous product with uniform, small particle size. In this work, a novel ceramic synthesis technique, combustion synthesis, was used for the first time to produce submicron-sized oxide phosphors more efficiently for use in flat panel displays. This technique exploits the exothermic redox reaction of metal nitrates (oxidizers) with an organic fuel (reducing agent). Typical fuels include urea {dollar}rm(CHsb4Nsb2O),{dollar} carbohydrazide {dollar}rm(CHsb6Nsb4O),{dollar} or glycine {dollar}rm(Csb2Hsb5NOsb2).{dollar} Resulting powders were well-crystallized, with a large surface area and small particle size. Phosphor powders were exposed to photoluminescence excitation by high energy (254 nm, E = 4.88 eV) and low energy photons (365 nm, E = 3.4 eV and 435 nm, E = 2.85 eV) and cathodoluminescence excitation by a low-voltage (100-1000 V) electron beam. Photoluminescence (PL) techniques resulted in the measurement of spectral energy distribution and relative intensities. Phosphor efficiencies in lumens per watt (lm/W) were obtained by low-voltage cathodoluminescence measurements. The effects of processing parameters such as type of fuel, fuel to oxidizer ratio, and heating rate were studied. The combustion process was optimized based on these processing parameters in order to maximize the luminescence of the phosphor powders in the as-synthesized condition. An increase in PL intensity with increasing flame temperature of reaction was observed. Post-reaction annealing (1000-1600{dollar}spcirc{dollar}C) increased the PL intensity and CL efficiency of the as-synthesized powders. Diffusion of the activator ions, particle growth, reduction of residual carbon impurities, disorder surrounding the activator ions, and crystallite size increase were investigated as possible explanations for the increase in luminescence intensity with increasing annealing temperature. A model of cathodoluminescence which includes the effects of the crystallite size, the probability of radiative recombination, and surface bound electrons, was developed to predict phosphor efficiency. The efficiencies predicted by the model are in very good agreement with experimental results.