Researchers Analyze NIR-Reflective Pigments
To combat urban heat island effects and the absorption of near-infrared (NIR) radiation in sunlight, researchers from Japan have recently analyzed NIR-reflective pigments to mitigate the issue.
In a recent study, since published in Inorganic Chemistry, Ryohei Oka and his colleague Tomokatsu Hayakawa from Nagoya Institute of Technology demonstrated that layered perovskite ceramic compounds of the type A2BO4 are ideal for reflecting NIR.
Studying NIR Reflectivity
Prior to looking at layered perovskite ceramic compounds of the type A2BO4, Oka found in a previous study that novel perovskites such as titanium-added calcium manganese oxide (Ca2(Mn,Ti)O4) ceramics are much better at reflecting NIR radiation than commercially available black pigments.
However, it is unknown what mechanism in Ca2(Mn,Ti)O4 achieves this feature.
To look more closely at the novel perovskite, the duo analyzed the structure and composition through standard theoretical and experimental techniques to investigate the factors contributing to its enhanced NIR reflectivity.
In their work, Oka and Hayakawa were reported to employ X-Ray diffraction (XRD) and Raman spectroscopy. The team also utilized a computational method called “density functional theory” to extract missing details about the crystal structure and various electronic states of Ca2(Mn,Ti)O4.
“Few studies so far have conducted Raman spectroscopy of Ca2(Mn,Ti)O4. Furthermore, they have not provided any detail of its vibrational modes. However, information about its electronic states and vibrational modes is crucial to understand how these perovskites turn out to be such great NIR reflectors,” said Oka.
By analyzing the crystal structure of calcium manganese oxide (Ca2MnO4) and occurring structural changes with Ti impurities were added, the duo was better able to identify how the chemical bonds within the perovskite were modified.
Through the study, it was discovered that as compared to Ca2MnO4, Ca2(Mn,Ti)O4 exhibited an additional Raman peak—likely due to the activation of a “silent mode” caused by the Ti impurities. However, the XRD patterns of both compounds were identical. The researchers believe that this was because of the Ti-Ti correlations at certain distances.
As a result of the research, the duo also highlighted that they observed a striking agreement between the computational results from DFT and experimental data. The researchers further explained that the obtained energy gaps from the three models for Ca2(Mn,Ti)O4 had agreed with one another, in addition to agreeing with the experimental value.
In addition, the result was noted to be independent of the Ti-substitution or its position in the crystal.
The study results also revealed that, “the enhanced NIR reflectivity upon adding Ti ions resulted from a lowering of "density of states" (the number of electronic states per unit volume per unit energy) near the Fermi level (the highest energy level an electron can occupy at absolute zero temperature).”
According to Oka and Hayakawa, the research findings are one step closer to understanding the thermal shielding properties of perovskite ceramics and provides a “general recipe” for understanding the structure and properties of not only A2BO4 type ceramics but a range of complex perovskite ceramics.
“This combinational approach is applicable to a wide range of functionalized crystalline ceramics to understand their optical, electronic, and magnetic properties in a much better way with more reliable structural models obtained computationally,” said Oka.
In continuing to unveil the works of enhanced NIR reflection mechanism, the researchers conclude that industry would benefit as inorganic pigments could be properly attributed to offer superior thermal coatings on buildings in urban areas.