Fluorescent Cooling of Objects Exposted to Sunlight - The Ruby Example

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Journal Article

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<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>


Solar Energy Materials and Solar Cells



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