Afterglow Phosphors

In long-afterglow phosphors, optical excitation energy is stored in the lattice by trapping photoexcited charge carriers. The most prominent example is SrAl204 Eu,Dy after optical excitation of Eu, Eu is oxidized to Eu and Dy is reduced to Dy. Thermal excitation of Dy to Dy, followed by capture of the electron by Eu and subsequent Eu emission results in time-delayed Eu emission. The thermal excitation process of Dy determines the time delay. This particular material still generates visible emission after several hours in the dark. 

Long-afterglow phosphors can be used in watch fingers, and also in safety applications, e.g., in exit signs, which still operate in the case of a current blackout. Other long-afterglow materials are e.g. ZnS Cu or SrS Bi. 

---

In the 1990s a breakthrough was achieved in the development of long-lived afterglow phosphors. It was discovered that by co-doping rare earth aluminates, especially strontium, with europium and dysprosium gave phosphors with around ten times the afterglow of copper activated zinc sulfide and also with ten times the... 

Since the 1960 s, stamp paper has been treated with fluorescent or phosphorescent phosphors. Since the stamp is stuck to the top right-hand corner of the letter, its recognition by the machine serves to establish the position of the letter and enables machine control of the correct positioning for postmarking and reading. Doped zinc sulfides with average particle sizes of 5-6 pm are used as afterglow phosphors. 

Table 59. Properties of industrially important long-afterglow phosphors for accident prevention... 

For long-afterglow phosphors, the brightness as a function of time after standardized excitation is measured (DIN 67 510). 

Keywords long-afterglow phosphor, red emitting, visible light, strontium sulfide. Lanthanide ions... 

Copper-activated zinc and cadmium sulphides exhibit a rather long afterglow when their irradiation has ceased, which is favourable for application in radar screens and self-luminous phosphors. 

Industrial silver-activated zinc sulfide phosphors use the intense blue emission exclusively. The ZnS Ag phosphor for cathode ray tubes is obtained by firing zinc sulfide and silver nitrate at ca. 1000 °C in the presence of sodium chloride (coactivator Cl-) [5.318]. The afterglow can be further reduced by addition of 10-3-10-4% of nickel ions. 

Among the long afterglow alkaline-earth sulfides, only (Ca, Sr)S Bi3+, CaS Bi3+, and CaS Eu2+, Tm2+ still have any real importance because their respective blue, violet, and red luminescence cannot yet be achieved with the less hydrolysis-sensitive zinc sulfide phosphors. The last-mentioned phosphor gives an intensive red afterglow and can substitute the red zinc-cadmium phosphor. 

When fired in a reducing atmosphere, Y3A15Oi2 exhibits a pronounced afterglow due to traps formed by oxygen vacancies [5.356]. Subsequent annealing in air diminishes this effect and leads to decay times of 200 300 ns therefore, Y3Al5Ol2 Ce3+ is used in flying-spot scanner tubes. The emission maximum is at 550 nm. This phosphor is classified under P46 (TEPAC) and KG (WTDS) (see Section 5.5.4.3). 

These orange-emitting phosphors have a long afterglow time and are therefore still used in special radar tubes and oscilloscopes [5.296], [5.307], [5.418], despite their low stability towards burn-in compared with other cathode-ray phosphors. 

UV light in the presence of water, due to photochemical reduction of Zn2+ to Zn°. Outdoor application of these phosphors is limited. The phosphors are used in afterglow paints, plastics, ceramics, and enamels. 

Infrared radiation can be detected by phosphors by various mechanisms. Upon excitation with UV radiation, ZnS Cu +, Co2+ emits an afterglow that is quenched by IR radiation, whereas ZnS Cu+, Pb2+ stores excitation energy and emits it spontaneously under the action of IR radiation. CaS Sm3 +, Ce3+ and SrS Sm3 +, Eu2+ only exhibit low fluorescence on excitation by UV radiation and daylight. Through the subsequent action of IR radiation these phosphors can be stimulated to emit green or orange-red light. 

Fluorescent and afterglow products are used to attain special effects in cinema films, on the stage, and in works of art. These effect pigments are physiologically harmless phosphors, mainly based on zinc sulfides. 

---