Particle Growth in Zinc Sulfide Phosphor Containing Halide Fluxes

Abstract

The mechanisms by which zinc sulfide powders grow under the action of heat and in the presence of a fluxing agent have been the subject of considerable discussion and speculation (1). Common fluxes employed in ZnS type phosphors include the halogen salts of ammonium and the alkali or alkaline earth metals, although compounds such as barium thiozincate (2) have been recommended also. According to the classical concept of flux action (3), the function of the additive is to provide a liquid phase in which the zinc sulfide is soluble, so that transport of material can occur readily. Apparently, no experimental evidence has been offered in support of this hypothesis. Indeed, the data of Gashurov and Levine (4) show that at 880~ ZnS has a very limited solubility of 0.0152 mole/1000g solvent in the common fluxing agent NaC1. More recent work (5-7) has emphasized the importance of diffusion phenomena in thermal recrystallization and sintering experiments on zinc sulfide powders.Thus, McQueen and Kuczynski (5) carried out work on the sintering of polycrystalline ZnS as a function of temperature and atmosphere of firing, and interpreted the results obtained in terms of material transport by diffusion mechanisms. Studies on the kinetics of hexagonal-cubic phase transformation in ZnS at 800~176 in zinc vapor (6) suggested a diffusion-controlled transition having an activation energy of 67 kcal. Brown (7) investigated the mechanism of flux action in ZnS:Ag phosphors containing either 2 or 10% NaC1. In these systems, and also for zinc sulfide without additives, the rate of increase in particle diameter, D, at a firing time, t, followed the expression D ~ = kt, where k is a constant. Two distinct regions of growth were noted. Below about 1000~ an activation energy of 29 kcal was obtained, corresponding to movement of zinc through interstitial channels. At higher temperatures the measured activation energy was 59 kcal, suggesting bulk diffusion of zinc. This change in mechanism corresponded to the onset of the cubic-hexagonal phase transformation.The objective of the present work was to extend the particle growth experiments reported previously (7) to include other fluxes, with the aim of obtaining a better understanding and control of the particle size characteristics of zinc sulfide phosphors. With this object in mind, measurements were carried out on the rate of increase in particle diameters, as a function of duration and temperature of heating, for the following systems: ZnS:0.015% Ag with either 2% ZnC12, 2% Ntt4C1, or 2% CaC12. Note that these values correspond to weight per cent (w/o) compositions. Particle dimensions were obtained by Coulter Counter techniques; the numbers quoted represent median point by weight of particles. Experimental and ResultsDetails of the precipitation of zinc sulfide (8) and the preparation and measurement of phosphors (7) have been given previously. A brief account will suffice here.Batches of about 200g of zinc sulfide were obtained from 2 liters of 1M zinc sulfate solution at 25~ using a h...