Effect of Thermal History on Color of Zinc Beryllium Silicate (Mn) Phosphors

Abstract

The fluorescent colors at room temperature of various completely‐reacted manganese‐activated zinc beryllium silicate phosphors can be changed by reheating them to temperatures lower than the original firing temperature with subsequent cooling at a rate in effect similar to the original cooling rate. A radical change in the cooling rate, as by quenching in water, also produces color shifts apart from the change due to reheating alone.When a zinc beryllium silicate (Mn) phosphor is reheated at various temperatures from 700°C to 1200°C with subsequent normal cooling, shifts to the green occur in all cases with a maximum shift at 950°C. With increased time of retiring, especially at the higher temperatures, the colors shift back toward the original phosphor color.With reheating and subsequent water quenching, the color shift to the green increases almost linearly with temperature over the range 800°C to 1200°C and extrapolates to zero color shift at 700°C. The magnitude of the color shift is greater with quenching than with normal cooling.The data require two equilibrium approach rates for interpretation. The first or rapid change involves the equilibrium of manganese between its red‐emitting and green‐emitting states, with the green‐emitting structure predominating at high temperatures. The second or slow change involves the migration of beryllium between substitutional positions either for silicon or zinc in the crystal. The equilibrium favors the beryllium‐for‐zinc substitution more for higher temperatures.The beryllium position in turn modifies the manganese‐state distribution, and this third equilibrium between the primary equilibria accounts for the time dependence of the color shifts.