ELECTROPHORETIC DEPOSITION OF PHOSPHORS .1. CONDUCTIVITY AND ZETA-POTENTIAL MEASUREMENTS

As part of a fundamental study of the electrophoretic deposition of phosphor particles, a suspension composed of Zn2SiO4: Mn (P-1) or ZnS:Ag (P-11) phosphor particles in isopropyl alcohol (IPA) containing various nitrate salts was characterized by conductivity and zeta-potential measurements. Dissociation constants and limiting ionic mobilities of NaNO3, Mg(NO3)2, and Y(NO3)3 in IPA were determined from conductivity measurements. It was found that the dissociation constants are very low and decrease as the metal ion valency increases. The zeta potential of the phosphors was measured in IPA under a wide range of salt concentrations and pH values. In all cases, the zeta potential was approximately -40 mV at salt concentrations less than 10(-6)M. As the concentration of Mg(NO3)2, Y(NO3)3, or Al(NO3)3 increased, the zeta potential increased and became positive, reaching a maximum of approximately 40 mV at 10(-5) M. In IPA or IPA containing 10(-5)M Mg(NO3)2, the zeta potential was positive at pH < 6, but became negative for pH > 6. The zeta potential remained positive at all pH values for Mg(NO3)2 concentrations of 10(-4) and 10(-3)M. The dependence of zeta potential on salt concentration was modeled using the Gouy-Chapman-Stern-Grahame model of the double layer. Using low values of capacitance for the inner and outer Helmholtz planes and a large adsorption potential, the model results agreed well with the experimental data. (C) 1994 Academic Press, Inc.