One-step synthesis of near-infrared reflective brown pigments based on iron-doped lanthanum aluminate, LaAl1-xFexO3

The paper explores the perspective of using an urea and glycine fuel mixture in order to obtain brown, heat reflective iron-doped lanthanum aluminate LaAl1-xFexO3 (x = 0, 0.25, 0.5, 0.75) pigments, directly from the combustion reaction. The exothermic redox reactions were characterized by high combustion temperatures, ranging between 1712 °C (x = 0) and 1017 °C (x = 0.75), which facilitated the formation of perovskite solid solutions within a very short period of time, no further annealing needed. The crystallite size, specific surface area, colour and total solar reflectance of the pigments depend on the substitution degree of Al3+ by Fe3+. The increase of Fe3+ content resulted in lower combustion temperature, shorter reaction time and increased gases release rate, which are reflected in the linear decrease of crystallite size from 57 nm (x = 0) to 22 nm (x = 0.75), as well as the linear increase of specific surface area from 3.0 (x = 0) to 15.6 m2/g (x = 0.75). The brown hue was enhanced by the increase of the chromophore content. The total solar reflectance of the pigments decreased from 87.5% to 42.7% as the Fe3+ content, x, increased from 0 to 0.75. At the same time, the total solar reflectance values of the coatings prepared using the obtained pigments decreased from 85.3% to 35.8% as the Fe3+ content, x, increased from 0 to 0.75.