Preparation of solar selective absorption coatings is a key technique in solar thermal utilization and it is crucial for enhancing the efficiency of solar collectors. Selective absorption coatings can be prepared through many methods, including spraying method, electrochemical method, vacuum evaporation, magnetron sputtering method, etc. However, these methods all have some disadvantages, such as pollution to the environment, harsh preparation conditions, high cost, poor weatherability, high emissivity, etc. Transition-metal complex oxides with spinel structure are a kind of important solar selective coatings. They possess suitable energy gap, can absorb radiation of visible light and that in the near infrared region of the spectrum.

Researchers at R&D Center for Eco-material and Eco-chemistry, Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Sciences (CAS), have prepared solar selective absorption coatings with excellent selective absorbing ability using coating method and black spinel pigments as the light absorbing agent. They first prepared transition-metal complex oxides with spinel structure via sol-gel and self-propagating high-temperature synthesis (SHS). The powder was then calcined to obtain black spinel pigments which were used to prepare solar selective absorption coatings. The technique received a Chinese invention patent on October 8(Method for preparing solar selective absorption coatings using spinel pigments as light absorption agent; patent number: ZL201110260189.4).

The coatings were prepared through the following steps. First, spinel pigments, resin, solvent and promoter were evenly dispersed using ball milling method and then curing agent was added to obtain solar selective absorption paint. The paint was coated evenly to metal substrate to form solar selective absorption coatings after solidified at room temperature.

The experimental tests show that the coatings possess high solar absorbing rate, low emissivity and can enhance the heat-light conversion efficiency of solar collectors. In addition, the coatings have excellent mechanical property and good weatherability. It has a service life of more than 45 years.