Field emission displays (FED) operate at lower voltages and significantly higher current densities than cathode ray tubes (CRTs). This makes it advantageous to consider how to adapt conventional CRT sulfur based phosphors, which have the highest luminescent efficiency, but which unstable under high electron bombardment, to FED applications. Thus to protect these phosphors from degradation it is necessary to develop thin layer coatings of stable materials, such as MgO, In 2 O 3 or polyphosphate. The goal of this study is to investigate the influences of these coatings on the cathodoluminescent properties of the blue phosphor, ZnS:Ag,Cl; and the green phosphor, [2 -5]. Due to the short distance between the anode and cathode, the acceleration voltages used in a FED does not have to be as high as in a CRT (20-30 kV), and typically are in the range from 200 V to 8 kV. However, to achieve CRT levels of brightness in a FED, the current densities need to be increased at least 20 times. For these excitation conditions, most conventional sulfur-containing CRT phosphors are not appropriate for field emitter arrays. Also it has been shown that irradiation with an electron beam causes decomposition and generates harmful gases, such as SO 2 , subsequently causing the cathode to deteriorate and the luminous efficiency of phosphors to decrease. In order to avoid these problems new coating techniques are being developed for sulfide phosphors to protect the phosphors from degradation. The published results demonstrated good aging behavior for ZnS phosphors with oxide or polyphosphate coating [6][7][8][9][10].In this paper, we present the low voltage cathodoluminescent properties of the ZnS:Ag,Cl blue phosphor coated with MgO and polyphosphate, and the SrGa 2 S 4 :Eu 2 + green phosphor coated with MgO and In 2 O 3 . These two phosphors have excellent chromaticity and high luminous efficiency at low voltages [11]. The green emitting thiogallate used in this study was prepared at the Phosphor Technology Center of Excellence (PTCOE) and the blue emitting ZnS:Ag,Cl phosphor was purchased from Kyokko (P22-B1). All the coatings were prepared at the PTCOE of the Georgia Institute of Technology using a wet chemistry method.The MgO and In 2 O 3 coatings were prepared by a sol-gel technique using magnesium ethoxide (Mg(OC 2 H 5 ) 2 ) and indium isopropoxide (In (OC 3 H 7 ) 3 ) as the precursors. The ratio of MgO to the strontium thiogallate was 0.5 wt.%, and to the zinc sulfide, 1.0 wt.%. The ratio of In 2 O 3 to strontium thiogallate was 0.5 wt.%. The precursors were first dissolved in ethanol, and then while continuous stirring, the phosphor materials were added to the ethanol solutions and mixed for :10 h. The final mixtures were evaporated and dried and baked at 450°C under an argon atmosphere. The polyphosphate coatings on zinc sulfide were prepared * Corresponding author.0921-5107/00/$ -see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 -5 1 0 7 ( 0 0 ) 0 0 4 4 5 -1

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