Combustion synthesis of a blue Co-doped zinc aluminate near-infrared reflective pigment

- A low cobalt content blue spinel Zn0.9Co0.1Al2O4 pigment was obtained.
- The time effective and energy efficient combustion synthesis was used.
- The pigment shows an intense blue color and high total solar reflectance.
- Pigment features (TSR = 63%) recommend it as a NIR-reflective pigment.

A near-infrared reflective blue pigment with a low cobalt content, Zn0.9Co0.1Al2O4, was prepared by solution combustion synthesis, which proved to be time-effective and energy-efficient. No additional annealing was required as the elevated temperature developed during the exothermic self-propagating combustion reaction (1428 °C) enabled the direct formation of a Zn0.9Co0.1Al2O4 solid solution having an appealing blue color: L* = 67.9, a* = −3.7, b* = −39.0. The resulting blue pigment had an average particle size of 6.1 μm and a BET surface area of 6.5 m2/g. The total solar reflectance of the pigment (63%) recommends the use of Zn0.9Co0.1Al2O4 as a near-infrared reflective blue pigment for cool coatings. The ability of the pigment to reflect the near-infrared radiation was confirmed by measuring the surface temperature of an aluminum sheet painted with an acrylic coating containing 30 % wt. pigment, that had a total solar reflectance of 59%. The excellent properties of Zn0.9Co0.1Al2O4, color and solar reflectance, together with the low cobalt content and the advantages of self-propagating combustion reaction recommend the use of combustion synthesis for the preparation of blue Zn0.9Co0.1Al2O4 near-infrared reflective pigments.