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Chemical composition of solid luminescent materials

Although gases and liquids also have many eye-catching luminescence phenomena, only solids have practical value for lighting.

The first solid luminescent materials that people learned about were those that can emit light after being exposed to daylight or sunlight. This material belongs to the sulfide of alkaline earth metals. At that time, the artificial manufacturing method of this material was of course quite primitive. It was not until the appropriate ultraviolet energy and experimental methods were available that appropriate progress was made. Later, it was learned that many natural minerals also fluoresce and can be artificially synthesized to make relatively pure and efficient phosphors that can maintain their original crystal structure. Later, many phosphors with various crystal shapes can be artificially synthesized. Some of the synthesized structures often do not exist in known minerals, and even if they exist, they may be a non-luminescent mineral. The characteristics of the crystal structure are almost indispensable properties of solid inorganic fluorescent materials, and people have conducted extensive research on this aspect with X-rays.

A common feature of solid inorganic fluorescent materials is that they all contain a small amount of doping elements called activators. These activators are very important for the luminescence of fluorescent materials. The most widely used activator is the transition metal-manganese.

Willemite, or zinc orthosilicate containing trace amounts of manganese, was once an important fluorescent material because it converts short-wave ultraviolet light into green light. This was a material used by Crookes in his initial research on discharge tubes. Synthetic zinc silicate, whose manganese content is properly controlled, is certainly more reliable than the unevenly doped ore specimens, so synthetic zinc silicate is still an important fluorescent material to this day. In the second half of the 1930s, it was discovered that when preparing this fluorescent material, adding a small amount of beryllium compounds can produce a series of fluorescence from green to yellow, then to yellow-orange, and even to magenta. This manganese-activated zinc beryllium silicate was once the main material for the production of fluorescent lamps.

Generally speaking, fluorescent powders for lamps are almost without exception oxygen-containing materials, such as silicates, phosphates, borates, tungstates, vanadates, etc. Some of these materials can also be used in cathode ray tubes and beta lamps. Some dual-element materials, such as sulfides, related selenides and tellurides, and fluorides, also have important applications in this regard. For example, some manganese-activated phosphors based on magnesium fluoride have a long afterglow and are very valuable for use in radar. Some sulfide phosphors have other uses. For example, although many materials show the characteristics of field-induced luminescence, only zinc sulfide has a relatively large use value. Luminescent pigments are also mainly based on sulfides.

Medical x-rays were originally converted into visible light using barium platinocyanate phosphor screens, but now zinc cadmium sulfide phosphor screens are widely used. Blue calcium tungstate phosphor screens can be used in contact with film, which can improve the photographic effect of x-rays.

Luminescent properties are not limited to inorganic solid materials. "Daylight-emitting" advertising paint is a well-known organic luminescent material. Some luminescent materials containing organic dyes such as rhodamine are deposited in synthetic plastics with a certain particle size to make suitable pigments. It can show natural daylight colors by relying on the effects of purple and ultraviolet radiation. There are also some colorless organic dyes that can show blue fluorescence under the action of ultraviolet light. They can be used as fluorescent whitening agents in detergents. In the production of white textiles and paper, they show blue or lavender fluorescence, which can offset the light yellow color brought by the fibers, thus producing a "bright white". Similar materials are used to make "secret traces" for paper or cards. The latest stamps are printed with luminescent dyes, and high-speed machines can be used to automatically sort mail.

Few luminescent materials are equally effective under several different forms of excitation. And generally speaking, the composition and structure of each phosphor must be adapted to the specific excitation energy required.

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