The evaluation of thermal performance of cool coatings colored with high near-infrared reflective nano-brown inorganic pigments: Magnesium doped ZnFe2O4 compounds

• A novel inorganic pigments Zn1−xMgxFe2O4 have been synthesized with a Sol–Gel method.
• Replacing Mg2+ for Zn2+ in ZnFe2O4 changed the color from brown to dark brown.
• The pigments possess high NIR solar reflectance (>51%).
• The surface temperature difference of the two pigmented coatings is above 7 °C.
• Indoor air temperature difference of boxes in the simulation test reaches more than 6 °C.

A new class of environmentally benign inorganic pigments with general formula Zn1−xMgxFe2O4 (x = 0, 0.1, 0.2, 0.3, 0.4) were successfully prepared by the Sol–Gel method. The developed nano-crystalline powders were characterized by TG-DSC, XRD, FESEM, UV–vis–NIR diffuse reflectance spectra, FLIR, and CIE-L∗a∗b∗ 1976 color scales. The results demonstrate that the single-phase Zn1−xMgxFe2O4 is synthesized at an optimum temperature of 800 °C. The resulting calcined powders are nano-sized particles with a well-developed cubic spinel structure. The doping of magnesium in zinc ferrite changes the color from brick red to dark brown and the band gap shifts from 2.23 to 2.18 eV. Mg doped pigments possess high near-infrared solar reflectance (>51%). In the simulated experiment, the analyses of thermal performance reveal that the surface temperature and the interior temperature of the devices covered with the coating we prepared are lower than those with the conventional coating, and the coating colored with the pigment we prepared absorbs less NIR portion and decreases the heat flux transferred into indoor space, which can decrease the need of cooling and increase thermal comfort in the summer. Therefore, these Zn1−xMgxFe2O4 powders have great potential in serving as cool pigments for building coatings.

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