Some electroluminescent ZnS; Cu, Pb phosphors of powder type have been prepared with different concentrations of impurities at two firing temperatures, 1000°C and 1100°C. They are excited by ac field of 60 c. p. s. between 2 kV/cm and 15 kV/cm and the variation of such characters as reflection color, after glow, electroluminescency, homogeneity and hardness of the burn-ups, with the impurity concentration and firing temperature are tabulated.
From this experiment, it seems that the brightness of electroluminescence is proportional to
V5, where
V is the applied voltage within the range of brightness ratio 1: 10
4. The build up of light intensity after application of ac field is expressed experimentally as
Lo{1-exp (-α
t)}, and the succeeding decay for a few hours, as
Lo (1+
kt). The values of α and
k are tabulated for typical voltages and binders. Although this decay is of the same type as that reported by THORNTON, the sample does not deteriorate; it revives after a few hours by the removal of field and the next application of field gives rise to an intensity higher than before.
Using Maxwellian distribution for freed electrons, estimation is made of the portion of electrons that are acceralated by field and have energy greater than 3.7 eV, the band gap of host crystal ZnS. The portion increases with a high power of V within an adequate range of field strength. It is then inferred that the light intensity is proportional to the number of such electrons and that the acceralating field would be about 10 times greater than the average applied field if the collision relaxation time is assumed to be 10
-13 sec.
Assuming a simple time-dependent equation for electrons freed by field, formulas similar to those obtained experimentally are derived for the time-dependence of light intensity. The coeffi-cients are compared and the light emission efficiency is estimated to be of the order of 10
4 photons per electron.
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