Angle Dependence of Diffuse Reflectance for a Microencapsulated Thermochromic Coating
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Angle Dependence of Diffuse Reflectance for a Microencapsulated Thermochromic Coating John E. Sinko1,2 , Alexis J. Corbett1, Travis L. Hislop1, and Meredith E. Rupp1 1 Department of Physics & Astronomy, St. Cloud State University, St. Cloud, MN 56301 2 Institute for Materials, Energetics and Complexity, St. Cloud, MN 56301 ABSTRACT Traditional solar power applications largely avoid using the infrared spectrum. Nevertheless, this region makes up about 45% of the solar power spectrum and therefore represents an untapped resource. Temperature control of buildings represents a significant cost for both businesses and private consumers. We are interested in developing thermochromic materials for building coatings to help moderate solar infrared absorption and thereby offset temperature control costs for buildings. Our initial effort in this study has been to characterize materials which might represent starting points for our research. We previously designed and 3D-printed an optical test platform to perform reflectance measurements with an ultraviolet-visible-near infrared spectrometer over a spectral range from 200-1000nm. The test platform temperature can be adjusted in real time using Peltier modules. In this study, a sample of microencapsulated 7anilino-3-diethylamino-6-methyl fluoran was studied by diffuse reflectance spectroscopy from 15-40 degrees Celsius. Scanning electron microscopy was used to characterize the dye particles. Temperature and spectral data were monitored while the sample temperature was adjusted. The visible diffuse reflectance from the sample increased from around 15% below the transition to more than 40% above the transition. A modification of this fluoran which extends the switching behavior into the infrared may be viable for passive thermo-optical switching in building coatings. INTRODUCTION The looming energy crisis necessitates that humans continue reducing usage of energy resources while simultaneously discovering new accessible energy sources. One largely passedover option is infrared energy in sunlight, which accounts for 40-50% of total power in the solar spectrum1. It has been proposed that color-switching materials could help reduce heating and cooling usage and costs2,3. Thermochromic materials may present a reasonable option for mitigating urban heat island effect4,5. However, the optimal configuration and materials are still being explored. Reversible thermochromic leuco dye materials are typically comprised of three components: the leuco dye itself, an electron acceptor, and a solvent 6. In commercial applications, the entirety is often enveloped within protective microspheres of a polymer material like melamine. Given a film composed of particles, we can apply a Kubelka-Munk-type model to determine diffuse reflectance from concentration7,8: 𝑓 (𝑟 ) =
(1−𝑟)2 2𝑟
=
𝐾 𝑆
(1)
where K is the molar absorption coefficient in [mol-cm/L], S is the scattering coefficient, and r is a comparative reflectance calibrated to a laboratory high reflectance standard. Unfortunately, the
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