Strontium aluminate

Strontium aluminate (SRA, SrAl) is an aluminate compound with the chemical formula SrAl2O4 (sometimes written as SrO.Al2O3). It is a pale yellow, monoclinic crystalline powder that is odorless and non-flammable. When activated with a suitable dopant (e.g. europium, written as Eu:SrAl2O4), it acts as a photoluminescent phosphor with long persistence of phosphorescence.

Strontium aluminate
Names
IUPAC name
Dialuminum strontium oxygen(2-)
Identifiers
3D model (JSmol)
ECHA InfoCard 100.031.310
EC Number
  • 234-455-3
Properties
SrAl2O4
Molar mass 205.58 g/mol
Appearance Pale yellow powder
Density 3.559 g/cm3
Structure
Monoclinic
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Strontium aluminates exist in other compositions including SrAl4O7 (monoclinic), Sr3Al2O6 (cubic), SrAl12O19 (hexagonal), and Sr4Al14O25 (orthorhombic).

Phosphor

For many phosphorescent-based purposes, strontium aluminate is a vastly superior phosphor to its predecessor, copper-activated zinc sulfide, being about 10 times brighter and 10 times longer glowing. It is frequently used in glow in the dark toys, where it displaces the cheaper but less efficient Cu:ZnS. However, the material has high hardness, causing abrasion to the machinery used in processing 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.[1]

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 aqua, or blue-green, version emits at 505 nm, and its blue emits at 490 nm. Strontium aluminate can be formulated to phosphoresce at longer (yellow to red) wavelengths as well, though such emission is often dimmer than that of more common phosphorescence at shorter wavelengths.

For europium-dysprosium doped aluminates, the peak emission wavelengths are 520 nm for SrAl2O4, 480 nm for SrAl4O7, and 400 nm for SrAl12O19.[2]

Eu2+,Dy3+:SrAl2O4 is important as a persistently luminescent phosphor for industrial applications. It can be produced by molten salt assisted process at 900 °C.[3]

The most described type is the stoichiometric green-emitting (approx. 530 nm) Eu2+:SrAl2O4. Eu2+,Dy3+,B:SrAl2O4 shows significantly longer afterglow than the europium-only doped material. The Eu2+ dopant shows high afterglow, while Eu3+ has almost none. Polycrystalline Mn:SrAl12O19 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.[4]

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.[5]

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

Strontium aluminate based afterglow pigments are marketed under numerous brand names such as Super-LumiNova [6] and Lumibrite, developed by Seiko.

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).[7]

Cerium and manganese doped strontium aluminate (Ce,Mn:SrAl12O19) 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 in photocopiers and other devices. A small amount of silicon substituting the aluminium can increase emission intensity by about 5%; the preferred composition of the phosphor is Ce0.15Mn0.15:SrAl11Si0.75O19.[8]

Structural material

Strontium aluminate cement 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 as well as for radiation shielding. The use of strontium aluminate cements is limited by the availability of the raw materials.[9]

Strontium aluminates have been examined as proposed materials for immobilization of fission products of radioactive waste, namely strontium-90.[10]

References
  1. Dutczak, D.; Jüstel, T.; Ronda, C.; Meijerink, A. (2015). "Eu2+ luminescence in strontium aluminates". Phys. Chem. Chem. Phys. 17 (23): 15236–15249. Bibcode:2015PCCP...1715236D. doi:10.1039/C5CP01095K. hdl:1874/320864. PMID 25993133.
  2. Katsumata, Tooru; Sasajima, Kazuhito; Nabae, Takehiko; Komuro, Shuji; Morikawa, Takitaro (20 January 2005). "Characteristics of Strontium Aluminate Crystals Used for Long-Duration Phosphors". Journal of the American Ceramic Society. 81 (2): 413–416. doi:10.1111/j.1151-2916.1998.tb02349.x.
  3. Rojas-Hernandez, Rocío Estefanía; Rubio-Marcos, Fernando; Gonçalves, Ricardo Henrique; Rodriguez, Miguel Ángel; Véron, Emmanuel; Allix, Mathieu; Bessada, Catherine; Fernandez, José Francisco (19 October 2015). "Original Synthetic Route To Obtain a SrAlO Phosphor by the Molten Salt Method: Insights into the Reaction Mechanism and Enhancement of the Persistent Luminescence". Inorganic Chemistry. 54 (20): 9896–9907. doi:10.1021/acs.inorgchem.5b01656. PMID 26447865.
  4. Misevičius, Martynas; Jørgensen, Jens Erik; Kareiva, Aivaras (2013). "Sol-Gel Synthesis, Structural and Optical Properties of Cerium-Doped Strontium Aluminates, Sr3Al2O6 and SrAl12O19". Materials Science. 19 (4). doi:10.5755/j01.ms.19.4.2670.
  5. Liu, Yun; Xu, Chao-Nan (May 2003). "Influence of Calcining Temperature on Photoluminescence and Triboluminescence of Europium-Doped Strontium Aluminate Particles Prepared by Sol−Gel Process". The Journal of Physical Chemistry B. 107 (17): 3991–3995. doi:10.1021/jp022062c.
  6. "RC TRITEC Ltd. : Swiss Super-LumiNova®". Retrieved 3 March 2016.
  7. Acers (American Ceramics Society, The) (2010-01-14). Progress in Nanotechnology. ISBN 9780470588239. Retrieved 3 March 2016.
  8. http://patentimages.storage.googleapis.com/pdfs/US3836477.pdf
  9. Odler, Ivan (2003-09-02). Special Inorganic Cements. ISBN 9780203302118. Retrieved 3 March 2016.
  10. https://inldigitallibrary.inl.gov/sti/4655305.pdf
  • R C Ropp Elsevier (2013-03-06). Encyclopedia of the alkaline earth compounds. Elsevier. p. 555. ISBN 9780444595508.
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