Dialuminum strontium oxygen(2-)
|3D model (Jmol)||Interactive image|
|Molar mass||205.58 g/mol|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Strontium aluminate (SRA, SrAl, SrAl2O4) is a solid odorless, nonflammable, pale yellow, monoclinic crystalline powder, heavier than water. It is chemically and biologically inert. When activated with a suitable dopant (e.g. europium, then it is labeled SrAl2O4:Eu), it acts as a photoluminescent phosphor with long persistence of phosphorescence.
There are also other strontium aluminates, e.g. SrAl4O7 (monoclinic), Sr3Al2O6 (cubic), SrAl12O19 (hexagonal), Sr4Al14O25 (orthorhombic).
For many phosphorescent-based purposes, strontium aluminate is a vastly superior phosphor to its predecessor, copper-activated zinc sulfide; it is about 10 times brighter and 10 times longer glowing, however it is about 10 times more expensive than ZnS:Cu and it cannot produce the unique red phosphorescence of the latter. It is frequently used in glow in the dark toys, where it displaces the cheaper but less efficient ZnS:Cu. However, the material has high hardness, causing abrasion to the machinery handling it; manufacturers frequently coat the particles with a suitable lubricant when adding them to a plastic.
Different aluminates can be used as the host matrix. This influences the wavelength of emission of the europium ion, by its covalent interaction with surrounding oxygens, and crystal field splitting of the 5d orbital energy levels.
Strontium aluminate phosphors produce green and aqua hues, where green gives the highest brightness and aqua the longest glow time. 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.
For europium-dysprosium doped aluminates, the peak emission wavelengths are 520 nm for SrAl2O4, 480 nm for SrAl4O7, and 400 nm for SrAl12O19.
The most described kind is the stoichiometric green-emitting (approx. 530 nm) SrAl2O4:Eu2+. SrAl2O4:Eu2+,Dy3+,B3+ shows significantly longer afterglow than the europium-only doped material. The Eu2+ dopant shows high afterglow, while Eu3+ has almost none. Polycrystalline SrAl12O19:Mn is used as a green phosphor for plasma displays, and when doped with praseodymium or neodymium it can act as a good active laser medium. Sr0.95Ce0.05Mg0.05Al11.95O19 is a phosphor emitting at 305 nm, with quantum efficiency of 70%. Several strontium aluminates can be prepared by the sol-gel process.
The wavelengths produced depend on the internal crystal structure of the material. Slight modifications in the manufacturing process (the type of reducing atmosphere, small variations of stoichiometry of the reagents, addition of carbon or rare-earth halides) can significantly influence the emission wavelengths.
Strontium aluminate phosphor is usually fired at about 1250 °C, though higher temperatures are possible. Subsequent exposure to temperatures above 1090 °C is likely to cause loss of its phosphorescent properties. At higher firing temperatures, the Sr3Al2O6 undergoes transformation to SrAl2O4.
The glow intensity depends on the particle size; generally, the bigger the particles, the better the glow.
Strontium aluminate doped with europium and dysprosium is called Lumibrite developed by Seiko and claimed to be brighter and longer glow time compare to ordinary strontium aluminate doped with only europium.
Europium-doped strontium aluminate nanoparticles are proposed as indicators of stress and cracks in materials, as they emit light when subjected to mechanical stress (mechanoluminescence). They are also useful for fabricating mechano-optical nanodevices. Non-agglomerated particles are needed for this purpose; they are difficult to prepare conventionally but can be made by ultrasonic spray pyrolysis of a mixture of strontium acetylacetonate, aluminium acetylacetonate and europium acetylacetonate in reducing atmosphere (argon with 5% of hydrogen).
Cerium and manganese doped strontium aluminate (SrAl12O19:Ce,Mn) shows intense narrowband (22 nm wide) phosphorescence at 515 nm when excited by ultraviolet radiation (253.7 nm mercury emission line, to lesser degree 365 nm). It can be used as a phosphor in fluorescent lamps for e.g. photocopiers. A small amount of silicon substituting the aluminium can increase emission intensity by about 5%; the preferred composition of the phosphor is SrAl11Si0.75O19:Ce0.15Mn0.15.
Strontium aluminate cement (SrAl2O4, or SrO.Al2O3) can be used as refractory structural material. It can be prepared by sintering of a blend of strontium oxide or strontium carbonate with alumina, in a roughly equimolar ratio, at about 1500 °C. It can be used as a cement for refractory concrete for temperatures up to 2000 °C. Barium aluminate cement has similar characteristics, and can be furthermore used as radiation shielding. The use of barium and especially strontium aluminate cements is limited by the availability of the raw materials.
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