Without enough trees, urban areas are much warmer than the surrounding countryside. This “urban heat island” effect is mostly caused by sunlight’s near-infrared (NIR) radiation absorption. Therefore, NIR-reflective pigments that can reduce these heating effects are widely desired.
Functional inorganic pigments in particular are a desirable choice on this front. In reality, layered perovskite ceramic compounds of type A2BO4 have been shown by Dr. Ryohei Oka and a colleague from Japan’s Nagoya Institute of Technology to be excellent at reflecting NIR.
In his earlier research, it was shown that new perovskites, such as ceramics with titanium-added calcium manganese oxide (Ca2(Mn,Ti)O4), reflect NIR photons significantly better than currently available black pigments. However, it is still unclear how Ca2(Mn,Ti)O4 manages to accomplish this amazing feat.
To research the variables influencing Ca2(Mn,higher Ti)O4’s NIR reflectivity, Dr. Oka and his colleague Dr. Tomokatsu Hayakawa recently published a study in the journal Inorganic Chemistry that examined the structure and composition of the compound.
This paper was made available online on April 19, 2022, and published in Volume 61 Issue 17 of the journal on May 2, 2022.
The pair effectively extracted missing information regarding the crystal structure and electronic states of Ca2(Mn,Ti)O4 by combining X-Ray diffraction (XRD), Raman spectroscopy, and the computational technique known as “density functional theory” (DFT).
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.
Dr. Ryohei Oka
“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,” says Dr. Oka, explaining the motivation behind their approach.
The researchers examined the calcium manganese oxide (Ca2MnO4) crystal structure and monitored the structural alterations brought on by the presence of Ti impurities.
They also discovered how the addition of Ti impurities alters the chemical linkages inside the perovskite. They discovered that Ca2(Mn,Ti)O4 displayed an extra Raman peak in comparison to Ca2MnO4, which was probably brought on by the Ti impurities activating a “silent mode.”
However, Ca2MnO4 and Ca2(Mn,Ti)O4 have the same XRD patterns. The researchers explained this by a Ti-Ti connection at a specific distance.
Their study’s startling consistency between computational DFT results and experimental data was another highlight. The energy gaps found using the three Ca2(Mn,Ti)O4 models that the researchers employed in their calculations were in agreement with each other and the experimental result.
Furthermore, the outcome was unaffected by Ti-substitution or the crystal’s orientation. The calculations also showed that a reduction in “density of states” (the number of electronic states per unit volume per unit energy) around the Fermi level (the highest energy level an electron can occupy at absolute zero temperature) was the cause of the increased NIR reflectivity when Ti ions were added.
These results move us one step closer to understanding perovskite ceramics’ thermal shielding ability. The ideal fusion of experimental and theoretical methods created in this study offers a generic formula for comprehending the structure and characteristics of a variety of complicated perovskite ceramics, not just those of the A2BO4 type.
As Dr. Oka puts it, “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.”
In fact, if inorganic pigments become more widely used as improved thermal coverings for urban buildings, a thorough knowledge of the enhanced NIR reflection process will be quite helpful.
