Fluorescent Cooling of Objects Exposted to Sunlight - The Ruby Example

TitleFluorescent Cooling of Objects Exposted to Sunlight - The Ruby Example
Publication TypeJournal Article
Year of Publication2016
AuthorsBerdahl, Paul, Sharon S. Chen, Hugo Destaillats, Thomas W. Kirchstetter, Ronnen M. Levinson, and Michael A. Zalich
JournalSolar Energy Materials and Solar Cells
KeywordsAl2O3, Cr, Fluorescent cooling, Quantum efficiency, Ruby

<p>Particularly in hot climates, various pigments are used to formulate desired non-white colors that stay cooler in the sun than alternatives. These cool pigments provide a high near-infrared (NIR) reflectance in the solar infrared range of 700–2500&nbsp;nm, and also a color specified by a reflectance spectrum in the 400–700&nbsp;nm visible range. Still cooler materials can be formulated by also utilizing the phenomenon of fluorescence (photoluminescence). Ruby, Al<sub>2</sub>O<sub>3</sub>:Cr, is a prime example, with efficient emission in the deep red (~694&nbsp;nm) and near infrared (700–800&nbsp;nm). A layer of synthetic ruby crystals on a white surface having an attractive red color can remain cooler in the sun than conventional red materials. Ruby particles can also be used as a red/pink pigment. Increasing the Cr:Al ratio produces a stronger (darker) pigment but doping above ~3&nbsp;wt% Cr<sub>2</sub>O<sub>3</sub> causes concentration quenching of the fluorescence. The system quantum efficiency for lightly doped ruby-pigmented coatings over white is high, 0.83±0.10.</p>

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