Preparation of infrared reflective blue and purple pigments in BaO–CuO–SiO<sub>2</sub> system and application to paints

Five types of three-component crystals have been reported in the BaO–CuO–SiO2 system. Among them, BaCuSi2O7 (Colinowensite, Han Purple), BaCuSi4O10 (Effenbergerite, Han Blue), and BaCu2Si2O7 (Scottyite, Dark Blue) have been known as blue/purple pigments since ancient times. Although there have been detailed reports on the structure and optical properties of these crystals in the visible region, there are no reports on their optical properties in the near-infrared region. In this study, these crystals were synthesized by heat treatment of BaCO3, CuO, and SiO2 mixtures at 900–1050 °C, and their visible and near-infrared reflectance was examined. It was found that three crystals exhibit an inherent blue or violet color and brightness, and all three crystals have extremely high near-infrared reflectance. Painted samples show brightness L* of 34–42, near-infrared reflectance of 62–67 %, and solar reflectance of 36–43 %, indicating its potential application as a blue/purple cool-roof paint. These pigments are extremely promising as infrared selective reflective blue/purple pigments needed for heat island measures.