Strontium aluminate

Strontium aluminate (SRA, SrAl, SrAl₂O₄) is a solid odorless nonflammable pale yellow powder, heavier than water. It is chemically and biologically inert. When activated with a suitable dopant (eg. europium, then it is labeled SrAl₂O₄:Eu), it acts as a photoluminescent phosphor with long persistence of phosphorescence. Its CAS number is [12004-37-4].

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Strontium aluminate is vastly superior phosphor than its predecessor, copper-activated zinc sulfide; it is about 10 times brighter, 10 times longer glowing, and 10 times more expensive than ZnS:Cu. 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; coating the particles with a suitable lubricant is usually used when strontium aluminate is added to plastics.

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 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.

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 fired at about 1250 °C. Subjecting it to temperatures above 1090 °C is likely to cause loss of its phosphorescent properties.

The glow intensity depends on the particle size; generally, the bigger the particles, the better the glow.