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Abstract
In the manufacturing of screens for advanced displays, layers of small luminescent particles may be deposited by electrophoretic deposition (EPD). Electrophoretic deposition of phosphor particles is currently used, particularly for fine particles in the range of 1 to 10 $\mu$m in diameter, in the manufacturing of high resolution screens. The EPD bath of interest is a suspension of phosphor particles in isopropyl alcohol (IPA) which contains dissolved Mg(NO$\sb3)\sb2$. The Mg(NO$\sb3)\sb2$ dissociates slightly, providing the ions to charge the particles positively, as well as form the binder material.
A method for testing and quantifying the adhesion strength of EPD phosphor screens was developed. A gas impingement test was developed which can measure adhesion forces in the range of 0.08 to 0.4 N. A high pressure jet of N$\sb2$ gas is directed perpendicularly to a particulate deposit. The particles are removed from the substrate in a ring pattern which can be correlated to the skin friction on the substrate.
The mechanism of adhesion in EPD phosphor screens was further investigated, particularly the effects of water in the IPA solution. The chemistry of the formation of the binder material has been shown to change with the amount of water in the deposition bath. For very low amounts of water, the binder material is an alkoxide, Mg(C$\rm\sb3H\sb7O)\sb2$. For high amounts of water in the bath ($>$5 vol. %), the binder is predominantly a hydroxide, Mg(OH)$\sb2$, which is in agreement with previous studies. At intermediate water concentrations, the binder is a mixture of the two materials.
A set of processing variables that enhance the adhesion strength of EPD phosphor screens have been determined. Post-deposition baking at 425$\sp\circ$C for 1 hour, added water, up to 5 vol. %, and added glycerin, up to 2 vol. %, to the deposition bath, using Y(NO$\sb3)\sb3$ instead of Mg(NO$\sb3)\sb2$, and particle size distribution, 20% 6 $\mu$m and 80% 3 $\mu$m mean diameter, all enhance the adhesion strength of EPD phosphor screens.
The color of the screens when excited by electron bombardment was unchanged by the processing conditions. The brightness was slightly reduced, but all screens were still relatively very bright.
