Near-infrared reflective Ni, La-doped hibonite pigments for cool blue coatings

Blue pigments based on Ni2+ and La3+- doped hibonite, Ca1-xLaxAl12-xNixO19, show high total solar reflectance and act as cool pigments to prevent surface overheating. During exposure to NIR radiation, the surface tem-perature of Al sheet coated with Ca0.7La0.3Al11.7Ni0.3O19-based acrylic coating was 15.1 degrees C lower than the temperature of a reference sample coated with a similar color blue pigment. The preparation technique proposed for the synthesis of these pigments, namely the combustion method, is very fast and does not require additional annealing. The elevated combustion temperatures recorded during the obtaining reaction of the pigments (1463-1565 degrees C) were correlated to the thermodynamic calculations (standard enthalpy of reaction and adiabatic temperature) and explain the direct formation of the crystalline structure of Ca0.7La0.3Al11.7Ni0.3O19 without annealing. Increasing the chromophore proportion, x, from 0 to 0.5 leads to a decrease of the crystallite size (from 55 to 34 nm), color enhancement (CIEL*/a*/b*: from 97.7/0.8/1.2 to 75.6/-11.6/-28.3) and decrease of the total solar reflectance (from 89.7% to 67.1%). Depending on the substitution degree, the pigments show specific surface areas between 3.5 and 4.4 m2/g.