Phosphors


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Phosphors

 

solids and liquids characterized by luminescence under the action of various types of excitation. A distinction is made among photophosphors, roentgenophosphors, radiophosphors, cathodophosphors, and electrophosphors, depending on the type of excitation. Certain phosphors may be of composite types (for example, ZnS-Cu is classified as a photophosphor, cathodophosphor, or electrophosphor). A distinction is made between organic phosphors (“luminophors”) and inorganic phosphors. Phosphors of crystalline structure are called crystal phosphors.

The fluorescence of phosphors may be determined both by the properties of the base substance and by the presence of an additive (activator), which forms luminescence centers in the base. The names of activated phosphors combine the name of the base with that of the activators—for example, ZnS-Cu, Co denotes the phosphor ZnS, activated by Cu and Co. In the case of composite bases, the names of the bases are given first, followed by the activators (for example, ZnS, CdS-Cu, Co).

Phosphors are used to transform various types of energy into light energy. Specific requirements exist for the parameters of phosphors, such as the type of excitation, the excitation spectrum (for photophosphors), the radiation spectrum, the radiation efficiency (the ratio of radiated energy to absorbed energy), and the time characteristics (fluorescence excitation time and duration of afterglow), depending on the conditions of use. The widest variety of parameters may be produced in crystal phosphors by varying the activators (mainly heavy metals) and the composition of the base; in addition, the properties of phosphors change considerably, depending on the activator concentration. For example, optimum photoexcitation is achieved for ZnS-Cu with a copper concentration of 10-5 g/g, and optimum electron excitation is obtained at copper concentrations greater than 10-4 g/g.

The excitation spectrum for various photophosphors ranges from the shortwave ultraviolet to the nearinfrared. The emission spectrum may lie in the visible, infrared, or ultraviolet regions. The width of spectral emission bands for individual phosphors varies from thousands of angstroms (for organic phosphors) to single angstroms (for crystal phosphors activated by rare earths) and is strongly dependent on the concentration of the phosphor and the activator, as well as on temperature.

The energy efficiency of phosphor emission depends on the type of excitation and its spectrum (in the case of photoluminescence), as well as on the mechanism involved in the transformation of energy into light. The energy efficiency drops markedly with an increase in phosphor and activator concentrations (concentration quenching) and in temperature (temperature quenching). The brilliance of phosphor luminescence intensifies from the onset of excitation over an interval ranging from 10-9 sec to several minutes. The duration of afterglow for various phosphors ranges from 10-9 sec to several hours and is determined by the type of energy transformation and the lifetime of the excited state. Organic phosphors exhibit the shortest afterglow, and crystal phosphors have the longest period. Other properties of phosphors (for example, resistance to light, heat, and moisture) may also play a significant role, depending on the conditions of use.

The main types of phosphors are crystal phosphors, organic phosphors, and luminescent glass. Crystal phosphors are the most widespread. A considerable number of them are semiconductor compounds with a forbidden band 1-10 electron volts wide, whose luminescence is determined by the activator or by lattice defects; the activator concentration is 10-3-10-7 g/g. Certain impurities (for example, iron) in concentrations as small as 10-6 g/g can reduce luminosity; therefore, the purity of the initial materials, used in the preparation of phosphors must be carefully controlled. Such phosphors are produced by roasting a charge. Fusing agents—for example, salts such as KC1, LiF, and CaCl2—are added to the mixture to improve the crystallization process. Luminescent single crystals are grown from a melt or solution or from the gaseous phase.

Fluorescent lamps contain mixtures of crystal phosphors, such as mixtures of MgWO4 and (ZnBe)2SiO4·Mn, or single-component phosphors, such as calcium halophosphate activated by antimony and manganese. The phosphors are selected in such a way that their fluorescence has a spectral distribution similar to that of daylight. Cathodophosphors are used for screens in cathoderay tubes, oscilloscopes, and black-and-white and color kinescopes. Special phosphors have been developed for color kinescopes to produce fluorescence in the three primary colors: blue (ZnS-Ag), green (Zn, Se · Ag), and red [Zn3(PO4)2Mn]. For fluoroscopy, (Zn, Cd)S · Ag and CaWO4 are used; they generate fluorescence in the region of maximum optical sensitivity and make possible maximum use of the sensitivity of X-ray film and reduction of the radiation dosage. Electrophosphors on a ZnS · Cu base are used to create luminous indicators, displays, and panels.

Organic phosphors can fluoresce in solutions (fluorescein and rhodamine) and in the solid state (plastics and anthracene, stilbene, and other organic crystals). They may have great brightness and high speed of action; the color of luminescence may be selected for any region of the visible spectrum. Organic phosphors are used in fluorometric analysis, the preparation of luminous paints and indicators, and optical bleaching of fabrics. Many organic phosphors, such as dyes of the cyanine and polymethine series, are the active elements in liquid lasers. Crystalline organic phosphors are used as scintillators in recording gamma rays and high-speed particles. Organic phosphors are manufactured commercially in the USSR under the trade name liuminor.

Luminescent glass is prepared from glass dies of various compositions. Activators, usually salts of rare earths or actinides, are added to the mixture during the glassmaking process. The efficiency, spectrum, and fluorescence period of luminescent glasses are determined by the properties of the activator. The glass has good optical transparency, and several types may be used as laser materials. It may also be used in the visual representation of images produced by ultraviolet radiation.

E. A. SVIRIDENKOV

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