Long-Afterglow Phosphor

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). 

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. 

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. 

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... 

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. 

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. 

As discussed previously, Eu -doped Ms(P04)3X (M = Ca, Sr, Ba X = F, Cl, Br) phosphors generally show efircient blue emission, which can he used in tricolor fluorescent lamps and LEDs [53]. Except for this, it was interestingly found that Ba5(P04)3Cl Eu and Ba5(P04)3Cl Eu % Ce " can be used as long-persistent phosphors, but their afterglow intensity is very weak. The persistent luminescence of Ba5(P04)3Cl Eu, was enhanced with co-dopants La ", Ce ", Gd, Tb -", and Lu ... 

Y202S Eu " oxysulfide is a new kind of long-persistent phosphor. Figures 16.54 and 16.55 display the emission spectram of Y202S Eu and the afterglow decay... 

Willemite is used as a cathode-ray phosphor in terminal displays and oscilloscope tubes. The decay time is very long, viz. 25 ms [I]. This is mainly due to the spin- and parity- forbidden nature of the Ti -> Ai emission transition in the configuration of the Mn ion (Sect. 3.3.4.C), but there is also a contribution of afterglow. An even longer persislance is observed for samples to which As has been added. As a re.sult of the As addition, electron traps are formed ich trap the electrons for a certain time, so that the emission is delayed (Sect. 3.4). 

Authors of paper [51] propose polycrystalline powder AIN Mn as a source of red long-lasting emission - a red long-lasting phosphorescence phosphor, as they call it. The observed afterglow emission with a band centered at 600 nm lasts for about an hour and is assigned to emission of manganese ions. 

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