Synergistic Temperature and Electron Irradiation Effects on the Degradation of Cathodoluminescent ZnS:Ag,Cl Powder Phosphors

Abstract

Synergistic temperature and electron irradiation effects on cathodoluminescent (CL) degradation of ZnS:Ag,Cl powder phosphor have been investigated. After continuous 2 keV electron beam exposure at 250°C for a 24 C/cm2 dose, the CL intensity upon cooling to room temperature decreased to 40% of the original intensity which is less than room temperature degradation for the same experimental conditions. Reduced degradation at high temperatures is consistent with the electron stimulated surface chemical reaction model of degradation which predicts that elevated temperatures reduce the mean stay time of physisorbed gases, decreasing the rate of the surface reactions leading to CL degradation. In a water-dominated ambient, the electron beam stimulates the formation of nonluminescent ZnO dead layers, while dissociation and reaction of leads to volatilization of S and Zn. Vaporization of Zn is accelerated by electron beam heating of the phosphor. The amount of heating was calculated using a heat transfer model to be ∼200°C for a primary beam of 2 keV, 25 C/cm2. This is consistent with morphological erosion on the surface of ZnS particles degraded at elevated temperatures or high power densities in -rich ambients. It is speculated that at a temperatures of about 300°C, surface chemical reactions with in combination with heating leads to removal of S as and evaporation of Zn. The consequences of beam heating on current saturation measurements is discussed. © 2003 The Electrochemical Society. All rights reserved.