Activated phosphors

Donor and acceptor levels are the active centers in most phosphors, as in zinc sulfide [1314-98-3] ZnS, containing an activator such as Cu and various co-activators. Phosphors are coated onto the inside of fluorescent lamps to convert the intense ultraviolet and blue from the mercury emissions into lower energy light to provide a color balance closer to daylight as in Figure 11. Phosphors can also be stimulated directly by electricity as in the Destriau effect in electroluminescent panels and by an electron beam as in the cathodoluminescence used in television and cathode ray display tubes and in (usually blue) vacuum-fluorescence alphanumeric displays. 

Soft, silvery metal whose isolation is difficult. Ytterbium fluoride is added to dental fillings (as X-ray contrast agent). Ytterbium is a constituent of rust-free special steels. Yb2Co13Fe3Mn holds the magnetic world record, but is too expensive for commercial purposes. The element is occasionally applied in nuclear medicine and radiography. It also activates phosphors that convert infrared rays into visible light. 

Synthesis of glycosyl phosphates by reaction of hemiacetals with activated phosphoric acid derivatives... 

We will examine the example of red-emitting phosphor in some detail, since Eu "-activated phosphors provide the best example of the practical realization of these principles. First consider the effect of the spectral width. Figure 4, taken from Levine and Palilla O), shows a series of hypothetical spectra, of different widths, which yield the same overall red color, namely, the same ratio X/Y. (Blue response Z can be ignored in this example because of the remoteness of the z curve see Figure 3.)... 

Most of the successful rare earth activated phosphors comprise host lattices in which the host cation is also a rare earth. A principal reason for this relates to the optical inertness of La, Gd, Y, and Lu this is essential to avoid interference with activator emission spectra. Close chemical compatibility including amenability to substitutional Incorporation of rare earth activators are also essential features. Rare earth hosts such as oxides, oxysulfides, phosphates, vanadates and silicates also tend to be rugged materials compatible with high temperature tube processing operations and salvage. 

In summary, lamplight illuminates human activities, so lamp phosphors should feed their light into the human visual system with high visual efficiency. Rare-earth-activated phosphors tend to produce narrow, strongly saturated, brilliantly colored lights. 

Bril and Wanmaker (149) examined the fluorescent properties of some europium-activated phosphors. In particular, they studied gadolinium oxide, gadolinium borate, gadolinium phosphate, lanthanum borate, and europium phosphate. The phosphors all show an emission spectrum characteristic of europium. From the excitation spectra one can deduce that, in the gadolinium containing host energy absorbed by the gadolinium, ions can be transferred to the europium ions. 

ZnO Zn is a typical example of a self-activated phosphor. In the case of zinc oxide, it is an excess of zinc which enables the phosphor to luminesce. The production is carried out by thermal oxidation of crystallized zinc sulfide in air at ca. 400 °C. The green luminescence, with a broad maximum at 505 nm, has a very short decay time of 10-6 s. As a phosphor for cathode-ray tubes, ZnO.Zn is classified in the TEPAC list as P 24 and in the WTDS system as GE. 

Other Halide Phosphors. Information on the most important manganese-activated phosphors is summarized in Table 57. 

The Srs(P04)3Cl crystals are hexagonal needles with lattice parameters ah = 9.953 A. and ch = 7.194 A. The needle axis corresponds to the crystallographic c axis. The europium(II) doped sample is a phosphor, readily excitable with electrons, x-rays, and both short and long ultraviolet light. It emits in the blue with a peak at 445 nm. Crystals of strontium chloride vanadate(V) are orthorhombic platelets with lattice constants a = 7.43 A., b = 11.36 A., and c = 6.54 A., with the b axis corresponding to the thin dimension of the flakes. Strontium chloride vanadate(V) is a self-activated phosphor giving broadband emission with a peak at 423 nm. when excited with 2537-A. radiation. All compounds are insulators, with resistivities >1012 ft-cm. 

Figure 10 shows the schematic illustration of the sintered layer-type sensor chip. The catalyst powder is ground using an auto-grinder and only fine particles are selected using a mesh filter. When the rare-earth ion is doped into the catalyst, an aqueous solution of rare-earth salt (e.g., (Dy(N03)3) is mixed in and then the catalyst is calcined. The rare-earth-activated phosphors used for thermoluminescence (TL) measurements, e.g., BaSO Eu, CaSO Eu, and SrSO Eu, also act as CTL catalysts. 

The nanostructure dependence of the excited state dynamics can be derived from the interaction of the electronic excitation with the surrounding environment and its phonon modes. A variety of nanophenomena, particularly, the lifetime of excited states of lanthanide ions in nanostructures may exhibit strong size-dependence (Prasad, 2004). Energy transfer rate and luminescence efficiency in lanthanide activated phosphors are also sensitive to particle size and surrounding environment. 

Table IV shows the Stokes shift of the Ce emission in several surroundings. In the 2.2.1 cryptand the Ce Stokes shift is smaller than in some commercial Ce " -activated phosphors (Y2Si05-Ce, Ca2AlSiOv-Ce). It becomes very small in ScBOa (see above) and in CaF2 and CaS04, where it carries an effectively positive charge that will make the Ca site smaller than it is on basis of the Ca " ionic radius.

Calcium and phosphoras are both used stmcturally to form bones and teeth. Lesser known but vital func-ticns of calcium include its uses as an enzyme activator and as a regulator cf nerve and muscle activity. Phosphorous is also a component cf nucleotide molecules that are... 

Nesvadba, P. Krohnke, C. A new class of highly active phosphorous free processing stabilizers for polymers. Proceedings of the Sixth International Conference Additives 97, ECM New Orleans, 1997. 

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