Protectively Coated Phosphors for Flat Panel FED Devices

J.S. Sanghera, G. Villalobos, S.S. Bayya, and I.D. Aggarwal
Optical Sciences Division

The Problem: Cathode ray tube (CRT)-based displays operate on the principle that accelerated electrons excite phosphor particles on a screen, which subsequently emit visible light. However, there is a worldwide effort to reduce the bulkiness of the CRT displays. One approach is based on field emission display (FED) technology in which the electron gun in the CRT is replaced with a miniature field emitter (Fig. 1). Unfortunately, the zinc sulfide-based phosphors developed for use in CRTs are not optimized for use in the FED environment. FED devices work at much lower accelerating voltages, and therefore, driving currents have to be greatly increased to maintain adequate brightness. Surface degradation of the phosphor occurs due to reaction with residual gases in the vacuum, and this is exacerbated by the higher current density. This degradation leads to decreased brightness (aging). In addition, the volatile by-products from the phosphor surface cause poisoning of the field emitter tips. Consequently, the FED device ages at an accelerated rate.

FIGURE 1
FED device.

The NRL Solution: Worldwide attempts to produce new, durable, and high-efficiency phosphors have not been successful. The alternative approach used by NRL requires that the phosphor particles be hermetically coated with a protective film. The film protects the surface of the phosphor particle and prevents attack by residual gases in the FED environment. Our approach allows existing sulfide-based phosphors to be used.

Technical Approach: We selected silica as the coating material because it is passive and does not react with zinc sulfide. Traditional "bucket chemistry" approaches produce particulates of silica distributed sporadically on the surface of the phosphor particles as well as dispersed throughout as a secondary phase (Fig. 2(a)). However, we are able to obtain uniform and smooth coatings on the phosphor particles by spraying a slurry containing the phosphor particles and the dissolved coating precursor (Fig. 2 (b)). The spray system consists of an ultrasonic atomizer, a 3-m-long drying column, and a cyclone separator. The key is to prevent gelation or precipitation of the silica before spraying so that the phosphor particles can be individually coated with silica during flight. After spraying, the coated phosphor is heat-treated to remove residual organics and to further densify the coating. The thickness of the film is controlled by varying the coating precursor concentration and ratio of phosphor particles, but typically we apply a 10-nm-thick coating of silica. The coating integrity is confirmed using a simple HCl acid test.

FIGURE 2
(a) Traditional "bucket chemistry" approach highlighting secondary phase particles of SiO2 on ZnS:Ag phosphor particles, (b) a uniform and smooth coating of SiO2 (10-nm-thck) on ZnS:Ag phosphor particles using the NRL-developed spray coating technique.

Results: The efficiency, chromaticity, and aging characteristics are important properties that need to be measured if the coated phosphor is to be commercially used in an FED device. The efficiency and chromaticity of the coated phosphors are comparable to the uncoated phosphors. More importantly, the coating needs to provide protection against aging. Figure 3 shows the results for the accelerated aging experiments. The goal was to maintain 50% of the original brightness after charge loading of 1 C/cm². The coated phosphor brightness decreases to only 60% of its original value, which is acceptable, compared with 18% for the uncoated phosphor. These results generated significant industry interest, and the spray coating process was successfully scaled up to provide larger quantities of coated phosphor to commercial vendors for testing and evaluation in their proprietary FED environments.

FIGURE 3
Normalized aging curves for coated and uncoated ZnS:Ag phosphor.

Future: The results so far appear to be very encouraging and should result in licensing our patents and technology to industry. Although the coatings are uniform and smooth, we have identified nanoscale porosity in the coating, which probably leads to the small amount of aging observed in the coated phosphors. Even though our coated phosphor is acceptable for FED applications, we are now developing a double spraying process, whereby the coated phosphor is resprayed to fill in the initial nanoporosity. This could potentially result in no aging. These doublecoated phosphors will be available for industrial evaluation in the near future.

The spray coating process is very versatile. It has also been used to apply MgO, indium tin oxide, sodium phosphate, and alumina coatings and can be used for organic coatings as well. These and other coatings will be exploited in the future in new programs.

Acknowledgments: We acknowledge the efforts of Fritz Miklos (SF Associates) for assisting in the spray coating scale-up process; Lauren Shea (Sandia National Laboratories) for the aging measurements; industrial collaborations with Candescent Corp., Lumileds (Hewlett Packard-Philips), Gemfire, and Motorola; and Bruce Gnade (DARPA) for financial support and helpful discussions.

[Sponsored by NRL and DARPA]