Phosphor

What’s a Phosphor?

 

A Phosphor is a substance that exhibits the phenomenon of luminescence; it emits light when exposed to some type of radiant energy. The term is used both for fluorescent or phosphorescent substances which glow on exposure to ultraviolet or visible light, and cathodoluminescent substances which glow when struck by an electron beam (cathode rays) in a cathode ray tube.

When a phosphor is exposed to radiation, the orbital electrons in its molecules are excited to a higher energy level; when they return to their former level they emit the energy as light of a certain color. Phosphors can be classified into two categories: fluorescent substances which emit the energy immediately and stop glowing when the exciting radiation is turned off, and phosphorescent substances which emit the energy after a delay, so they keep glowing after the radiation is turned off, decaying in brightness over a period of milliseconds to days.

Fluorescent materials are used in applications in which the phosphor is excited continuously: cathode ray tubes (CRT) and plasma video display screens, fluoroscope screens, fluorescent lights, scintillation sensors, and white LEDs, and luminous paints for black light art. Phosphorescent materials are used where a persistent light is needed, such as glow-in-the-dark watch faces and aircraft instruments, and in radar screens to allow the target ‘blips’ to remain visible as the radar beam rotates. CRT phosphors were standardized beginning around World War II and designated by the letter “P” followed by a number.

 

Phosphorus, the light-emitting chemical element for which phosphors are named, emits light due to chemiluminescence, not phosphorescence.

 

Phosphor Light Emission Process

 

The scintillation process in inorganic materials is due to the electronic band structure found in the crystals. An incoming particle can excite an electron from the valence band to either the conduction band or the exciton band (located just below the conduction band and separated from the valence band by an energy gap). This leaves an associated hole behind, in the valence band. Impurities create electronic levels in the forbidden gap. The excitons are loosely bound electron–hole pairs that wander through the crystal lattice until they are captured as a whole by impurity centers. The latter then rapidly de-excite by emitting scintillation light (fast component). In the case of inorganic scintillators, the activator impurities are typically chosen so that the emitted light is in the visible range or near-UV, where photomultipliers are effective. The holes associated with electrons in the conduction band are independent from the latter. Those holes and electrons are captured successively by impurity centers exciting certain metastable states not accessible to the excitons. The delayed de-excitation of those metastable impurity states, slowed by reliance on the low-probability forbidden mechanism, again results in light emission (slow component).

Phosphors are often transition-metal compounds or rare-earth compounds of various types. In inorganic phosphors, these inhomogeneities in the crystal structure are created usually by addition of a trace amount of dopants, impurities called activators. (In rare cases dislocations or other crystal defects can play the role of the impurity.) The wavelength emitted by the emission center is dependent on the atom itself and on the surrounding crystal structure.

 

Materials

 

Phosphors are usually made from a suitable host material with an added activator. The best-known type is a copper-activated zinc sulfide (ZnS) and the silver-activated zinc sulfide (zinc sulfide silver).

The host materials are typically oxides, nitrides and oxynitrides, sulfides, selenides, halides or silicates of zinc, cadmium, manganese, aluminium, silicon, or various rare-earth metals. The activators prolong the emission time (afterglow). In turn, other materials (such as nickel) can be used to quench the afterglow and shorten the decay part of the phosphor emission characteristics.

Many phosphor powders are produced in low-temperature processes, such as sol-gel, and usually require post-annealing at temperatures of ~1000 °C, which is undesirable for many applications. However, proper optimization of the growth process allows manufacturers to avoid the annealing.

Phosphors used for fluorescent lamps require a multi-step production process, with details that vary depending on the particular phosphor. Bulk material must be milled to obtain a desired particle size range, since large particles produce a poor-quality lamp coating, and small particles produce less light and degrade more quickly. During the firing of the phosphor, process conditions must be controlled to prevent oxidation of the phosphor activators or contamination from the process vessels. After milling, the phosphor may be washed to remove minor excess of activator elements. Volatile elements must not be allowed to escape during processing. Lamp manufacturers have changed compositions of phosphors to eliminate some toxic elements, such as beryllium, cadmium, or thallium, formerly used.

The commonly quoted parameters for phosphors are the wavelength of emission maximum (in nanometers, or alternatively color temperature in kelvins for white blends), the peak width (in nanometers at 50% of intensity), and decay time (in seconds).

Examples are as following:

• Calcium sulfide with strontium sulfide with bismuth as activator, (Ca,Sr)S:Bi, yields blue light with glow times up to 12 hours, red and orange are modifications of the zinc sulfide formula. Red color can be obtained from strontium sulfide.
• Zinc sulfide with about 5 ppm of a copper activator is the most common phosphor for the glow-in-the-dark toys and items. It is also called GS phosphor.
• Mix of zinc sulfide and cadmium sulfide emit color depending on their ratio; increasing of the CdS content shifts the output color towards longer wavelengths; its persistence ranges between 1–10 hours.
• Strontium aluminate activated by europium, SrAl2O4:Eu(II):Dy(III), is a material developed in 1993 by Nemoto & Co. engineer Yasumitsu Aoki with higher brightness and significantly longer glow persistence; it produces green and aqua hues, where green gives the highest brightness and aqua the longest glow time. SrAl2O4:Eu:Dy is about 10 times brighter, 10 times longer glowing, and 10 times more expensive than ZnS:Cu. The excitation wavelengths for strontium aluminate range from 200 to 450 nm. The wavelength for its green formulation is 520 nm, its blue-green version emits at 505 nm, and the blue one emits at 490 nm. Colors with longer wavelengths can be obtained from the strontium aluminate as well, though for the price of some loss of brightness.

 

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