Phosphor

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A phosphor is a substance that exhibits the phenomenon of phosphorescence (sustained glowing after exposure to light or energised particles such as electrons).

The chemical element phosphorus (Greek. phosphoros, meaning "light bearer") was discovered by German alchemist Hennig Brand in 1669. Working in Hamburg, Brand attempted to distill some kind of "life essence" from his urine, and in the process produced a white material that glowed in the dark. Since that time, the term phosphorescence has been used to describe substances that shine in the dark without burning.

Phosphorus itself is not a phosphor; it is highly reactive and gives-off a faint chemoluminescent glow upon uniting with oxygen. The glow observed by Brand was actually caused by the very slow burning of the phosphorus, but as he saw no flame nor felt any heat he did not recognize it as burning.

Phosphors are transition metal compounds or rare earth compounds of various types. The most common uses of phosphors are in CRT displays and fluorescent lights. CRT phosphors were standardized beginning around World War II and designated by the letter "P" followed by a number.

Contents

[edit] Materials

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

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

[edit] Glow-in-the-dark toys

Spectra of constituent blue, green and red phosphors in a common cathode ray tube.
Spectra of constituent blue, green and red phosphors in a common cathode ray tube.
  • Calcium sulfide with strontium sulfide with bismuth as activator, (Ca,Sr)S:Bi, yields blue light with glow times up to 12 hours, [1] 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:Dy, is a newer material 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 520nm, its blue-green version emits at 505nm, and the blue one emits at 490nm. Colors with longer wavelengths can be obtained from the strontium aluminate as well, though for the price of some loss of brightness.

In these applications, the phosphor is directly added to the plastic from which the toys are molded, or mixed with a binder for use as paints.

ZnS:Cu phosphor is used in glow-in-the-dark cosmetic creams frequently used for Halloween make-ups. [2]

Generally, the persistence of the phosphor increases as the wavelength decreases. Red and orange phosphors therefore do not have sufficiently long glow times to be commercially attractive for glow-in-the-dark applications.

See also lightstick for chemiluminescence-based glowing items.

[edit] Radioactive light sources

Mixtures of zinc sulfide with radioactive materials, where the phosphor was excited by the alpha- and beta-decaying isotopes, were used to paint dials of watches and instruments. The formula used on watch dials between 1913 and 1950 was a mix of radium-228 and radium-226 with a scintillator made of zinc sulfide and silver (ZnS:Ag). [3] However, zinc sulfide undergoes degradation of its crystal lattice structure, leading to gradual loss of brightness significantly faster than the depletion of radium.

The ZnS:Ag phosphor yields greenish glow. It is not suitable to be used in layers thicker than 25 mg/cm², as the self-absorption of the light then becomes a problem. ZnS:Ag coated screens were used by Ernest Rutherford in his experiments discovering atomic nucleus.

Copper-activated zinc sulfide (ZnS:Cu) is the most common phosphor used. It yields blue-green light.

Copper and magnesium activated zinc sulfide (ZnS:Cu,Mg) yields yellow-orange light.

Trasers are light producing devices composed of a sealed borosilicate glass tube with inner coat of a phosphor, filled with tritium. Betalights use tritium as energy source as well.

[edit] Electroluminescence

Main article: Electroluminescence

Electroluminescence can be exploited in light sources. Such sources typically emit from a large area, which makes them suitable for backlights of eg. LCD displays. The excitation of the phosphor is usually achieved by application of high-intensity electric field, usually with suitable frequency. The electroluminescent light sources so far tend to degrade with use, resulting in their relatively short operation lifetimes.

Phosphorwhite in powder
Phosphorwhite in powder
  • ZnS:Cu was the first formulation successfully displaying electroluminescence, tested at 1936 by Georges Destriau in Madame Marie Curie laboratories in Paris.

Indium tin oxide (ITO, also known under trade name IndiGlo) composite is used as a phosphor in some Timex watches, though as the electrode material, not as a phosphor itself. "Californeon" is another trade name of an electroluminescent material, used in electroluminescent light strips.

See also a history of electroluminescent displays.

[edit] White LEDs

White light-emitting diodes are usually blue InGaN LEDs with a coating of a suitable phosphor. Cerium(III)-doped YAG (YAG:Ce3+, or Y3Al5O12:Ce3+) is often used; it absorbs the light from the blue LED and emits in a broad range from greenish to reddish, with most of output in yellow. The pale yellow emission of the Ce3+:YAG can be tuned by substituting the cerium with other rare earth elements such as terbium and gadolinium and can even be further adjusted by substituting some or all of the aluminium in the YAG with gallium.

White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium based red and blue emitting phosphors plus green emitting copper and aluminium doped zinc sulfide (ZnS:Cu,Al). This is a method analogous to the way fluorescent lamps work.

[edit] Cathode ray tubes

The phosphors are usually poor electrical conductors. This may lead to deposition of residual charge on the screen, effectively decreasing the energy of the impacting electrons due to electrostatic repulsion (an effect known as "sticking"). To eliminate this, a thin layer of aluminium is deposited over the phosphors and connected to the conductive layer inside the tube. This layer also reflects the phosphor light to the desired direction, and protects the phosphor from ion bombardment resulting from an imperfect vacuum.

Combination of zinc sulfide with copper, the P31 phosphor or ZnS:Cu, provides green light peaking at 531 nm, with long glow.

Combination of zinc sulfide with few ppm of silver, the ZnS:Ag, when excited by electrons, provides strong blue glow with maximum at 450 nm, with short afterglow with 200 nanosecond duration. It is known as the P22B phosphor. [4] This material, zinc sulfide silver, is still one of the most efficient phosphors in cathode ray tubes. It is used as a blue phosphor in color CRTs.

When mixed with cadmium sulfide, the resulting zinc cadmium sulfide (Zn,Cd)S:Ag, provides strong yellow light.

The mix of zinc cadmium sulfide and zinc sulfide silver, the ZnS:Ag+(Zn,Cd)S:Ag is the white P4 phosphor used in black and white television CRTs.

Yttrium oxide-sulfide activated with europium is used as red phosphor in color CRTs. The development of color TVs took a long time due to the long search for a red phosphor.

  • ZnS:Ag+(Zn,Cd)S:Ag (P4), white phosphor for black and white TV screens and display tubes
  • ZnS:Cu,Al (P22G), green phosphor for TV screens
  • ZnS:Ag+Co-on-Al2O3 (P22B), blue phosphor for TV screens
  • ZnS:Ag,Cl or ZnS:Zn (P11, BE), blue (460 nm), 0.01-1 ms persistence, for display tubes and vacuum fluorescent displays
  • (Zn,Cd)S:Ag or (Zn,Cd)S:Cu (P20, KA), yellow-green, 1-100 ms persistence, for display tubes
  • (Zn,Cd)S:Cu,Cl (P28, KE), yellow, for display tubes
  • ZnS:Cu or ZnS:Cu,Ag (P31, GH), yellowish-green, 0.01-1 ms persistence, for oscilloscopes
  • ZnS:Ag+(Zn,Cd)S:Cu (P40, GA), white, for display tubes
  • ZnS:Ag,Al (P55, BM), blue (450 nm), for projection tubes
  • ZnS:Ag, blue (450 nm)
  • ZnS:Cu,Al or ZnS:Cu,Au,Al, green (530 nm)
  • (Zn,Cd)S:Cu,Cl+(Zn,Cd)S:Ag,Cl, white
  • ZnS:Ag+ZnS:Cu+Y2O2S:Eu, white, Cd-free replacement for P4, black and white CRT tubes, display tubes
  • Zn2SiO4:Mn (P1, GJ), yellowish-green (525 nm), 1-100 ms persistence, for display tubes
  • Zn2SiO4:Mn,As (P39, GR), green (525 nm), for display tubes
  • Y2SiO5:Ce (P47, BH), blue (400 nm), for beam index tubes
  • Y2SiO5:Tb, green (545 nm), for projection tubes
  • ZnO:Zn (P24, GE), green (505 nm), 1-10 µs persistence, for vacuum fluorescent displays
  • Gd2O2S:Tb (P43, GY), yellow-green (545 nm), for display tubes
  • Y2O2S:Eu+Fe2O3 (P22R), red phosphor for TV screens
  • Y2O2S:Tb (P45, WB), white (545 nm), for viewfinders
  • Y2O2S:Tb, green (545 nm), for display tubes
  • Y3Al5O12:Ce (P46, KG), green (530 nm), for beam index tubes
  • Y3(Al,Ga)5O12:Ce, green (520 nm), for beam index tubes
  • Y3Al5O12:Tb (P53, KJ), yellow-green (544 nm), for projection tubes
  • Y3(Al,Ga)5O12:Tb, yellow-green (544 nm), for projection tubes
  • MgF2:Mn (P33, LD), orange (590 nm), over 1 second persistence, for radar screens
  • (KF,MgF2):Mn (P19, LF), yellow (590 nm), for radar screens
  • (KF,MgF2):Mn (P26, LC), orange (595 nm), over 1 second persistence, for radar screens
  • (Zn,Mg)F2:Mn (P38, LK), orange (590 nm), for radar screens
  • InBO3:Tb, yellow-green (550 nm)
  • InBO3:Eu, yellow (588 nm)
  • InBO3:Tb+InBO3:Eu, amber
  • InBO3:Tb+InBO3:Eu+ZnS:Ag, white

[edit] Fluorescent lamps

  • (Ba,Eu)Mg2Al16O27, blue phosphor for trichromatic fluorescent lamps
  • (Ce,Tb)MgAl11O19, green phosphor for trichromatic fluorescent lamps
  • Ce0.67Tb0.33MgAl11O19:Ce,Tb, green (543 nm), for trichromatic lamps
  • BaMgAl10O17:Eu,Mn, blue-green (456/514 nm)
  • BaMgAl10O17:Eu,Mn, blue (450 nm), for trichromatic lamps
  • BaMg2Al16O27:Eu(II), blue (452 nm)
  • BaMg2Al16O27:Eu(II),Mn(II), blue (450+515 nm)
  • (Ce,Tb)MgAl11O19, green
  • Zn2SiO4:Mn, green (528 nm)
  • Zn2SiO4:Mn,Sb2O3, green (528 nm)
  • CaSiO3:Pb,Mn, orange-pink (615 nm)
  • MgWO4, pale blue (473 nm), wide bandwidth, deluxe blend component
  • CaWO4, blue (417 nm)
  • CaWO4:Pb (scheelite), blue (433 nm), wide bandwidth
  • (Sr,Eu,Ba,Ca)5(PO4)3Cl, blue phosphor for trichromatic fluorescent lamps
  • (La,Ce,Tb)PO4, green phosphor for trichromatic fluorescent lamps
  • (La,Ce,Tb)PO4:Ce,Tb, green (546 nm), for trichromatic lamps
  • (Ba,Ti)2P2O7:Ti, blue-green (494 nm), wide bandwidth, deluxe blend component
  • Sr2P2O7:Sn, blue (460 nm), wide bandwidth, deluxe blend component
  • Ca5F(PO4)3:Sb, blue (482 nm), wide bandwidth
  • Sr5F(PO4)3:Sb,Mn, blue-green (509 nm), wide bandwidth
  • LaPO4:Ce,Tb, green phosphor (544 nm), for trichromatic blends
  • (Sr,Ca,Ba)10(PO4)6Cl2:Eu, blue phosphor (453 nm) for trichromatic blends
  • (Ca,Zn,Mg)3(PO4)2:Sn, orange-pink (610 nm), wide bandwidth, blend component
  • (Sr,Mg)3(PO4)2:Sn, orange-pinkish white (626 nm), wide bandwidth, deluxe blend component
  • Ca5F(PO4)3:Sb,Mn, yellow, for Lite-white blend
  • Ca5(F,Cl)(PO4)3:Sb,Mn, warm white to cool white or blue or daylight
  • (Ca,Sr,Ba)3(PO4)2Cl2:Eu, blue (452 nm)
  • 3 Sr3(PO4)2.SrF2:Sb,Mn, blue (502 nm)
  • (Zn,Sr)3(PO4)2:Mn, orange-red (625 nm)
  • (Sr,Mg)3(PO4)2:Sn(II), orange-red (630 nm)
  • (Y,Eu)2O3, red phosphor for trichromatic fluorescent lamps
  • Y2O3:Eu, red phosphor (611 nm), for trichromatic blends
  • Y2O3:Eu(III), red (611 nm), for trichromatic lamps
  • Mg4(F)GeO6:Mn, red (658 nm)
  • Mg4(F)(Ge,Sn)O6:Mn, red (658 nm)
  • Sr5Cl(PO4)3:Eu(II), blue (447 nm)
  • Sr6P5BO20:Eu, blue-green (480 nm)
  • Y(P,V)O4:Eu, orange-red (619 nm)
  • Y2O2S:Eu, red (626 nm)
  • 3.5 MgO . 0.5 MgF2 . GeO2 :Mn, red (655 nm)
  • Mg5As2O11:Mn, red (660 nm)
  • Ca3(PO4)2.CaF2:Ce,Mn, yellow (568 nm)
  • SrAl2O7:Pb, ultraviolet (313 nm)
  • BaSi2O5:Pb, ultraviolet (355 nm)
  • SrFB2O3:Eu(II), ultraviolet (366 nm)
  • SrB4O7:Eu, ultraviolet (368 nm)
  • MgGa2O4:Mn(II), blue-green, used in black light displays

[edit] Detergents

Optical brighteners act as UV-sensitive phosphors with close-to-zero afterglow. Usually they are organic compounds.

[edit] Various

Some other phosphors commercially available, for use as X-ray screens, neutron detectors, alpha-particle scintillators, etc, are:

  • Gd2O2S:Tb (P43), green (peak at 545 nm), 1.5 ms decay to 10%, low afterglow, high X-ray absorption, for X-ray, neutrons and gamma
  • Gd2O2S:Eu, red (627 nm), 850 µs decay, afterglow, high X-ray absorption, for X-ray, neutrons and gamma
  • Gd2O2S:Pr, green (513 nm), 7 µs decay, no afterglow, high X-ray absorption, for X-ray, neutrons and gamma
  • Gd2O2S:Pr,Ce,F, green (513 nm), 4 µs decay, no afterglow, high X-ray absorption, for X-ray, neutrons and gamma
  • Y2O2S:Tb (P45), white (545 nm), 1.5 ms decay, low afterglow, for low-energy X-ray
  • Y2O2S:Tb (P22R), red (627 nm), 850 µs decay, afterglow, for low-energy X-ray
  • Y2O2S:Tb, white (513 nm), 7 µs decay, no afterglow, for low-energy X-ray
  • Zn(0.5)Cd(0.4)S:Ag (HS), green (560 nm), 80 µs decay, afterglow, efficient but low-res X-ray
  • Zn(0.4)Cd(0.6)S:Ag (HSr), red (630 nm), 80 µs decay, afterglow, efficient but low-res X-ray
  • CdWO4, blue (475 nm), 28 µs decay, no afterglow, intensifying phosphor for X-ray and gamma
  • CaWO4, blue (410 nm), 20 µs decay, no afterglow, intensifying phosphor for X-ray
  • MgWO4, white (500 nm), 80 µs decay, no afterglow, intensifying phosphor
  • Y2SiO5:Ce (P47), blue (400 nm), 120 ns decay, no afterglow, for electrons, suitable for photomultipliers
  • YAlO3:Ce (YAP), blue (370 nm), 25 ns decay, no afterglow, for electrons, suitable for photomultipliers
  • Y3Al5O12:Ce (YAG), green (550 nm), 70 ns decay, no afterglow, for electrons, suitable for photomultipliers
  • Y3(Al,Ga)5O12:Ce (YGG), green (530 nm), 250 ns decay, low afterglow, for electrons, suitable for photomultipliers
  • CdS:In, green (525 nm), <1 ns decay, no afterglow, ultrafast, for electrons
  • ZnO:Ga, blue (390 nm), <5 ns decay, no afterglow, ultrafast, for electrons
  • ZnO:Zn (P15), blue (495 nm), 8 µs decay, no afterglow, for low-energy electrons
  • (Zn,Cd)S:Cu,Al (P22G), green (565 nm), 35 µs decay, low afterglow, for electrons
  • ZnS:Cu,Al,Au (P22G), green (540 nm), 35 µs decay, low afterglow, for electrons
  • ZnCdS:Ag,Cu (P20), green (530 nm), 80 µs decay, low afterglow, for electrons
  • ZnS:Ag (P11), blue (455 nm), 80 µs decay, low afterglow, for alpha particles and electrons
  • anthracene, blue (447 nm), 32 ns decay, no afterglow, for alpha particles and electrons
  • plastic (EJ-212), blue (400 nm), 2.4 ns decay, no afterglow, for alpha particles and electrons
  • Zn2SiO4:Mn (P1), green (530 nm), 11 ms decay, low afterglow, for electrons
  • ZnS:Cu (GS), green (520 nm), decay in minutes, long afterglow, for X-rays
  • NaI:Tl, for X-ray, alpha, and electrons
  • CsI:Tl, green (545 nm), 5 µs decay, afterglow, for X-ray, alpha, and electrons

[edit] See also

[edit] External links

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