In addition to being used in ancient Egyptian art, the Egyptian-blue family of IR phosphors with near-infrared (NIR) fluorescence and high quantum yields also have modern-day applications. For example, they can be found in cool roofing where the fluorescence can provide additional cooling, and in luminescent solar concentrators where the fluorescence can be captured by photovoltaic cells to produce electricity.
The phosphors’ efficient NIR fluorescence is related to their relatively high-quantum yields, which are defined as the ratio of fluoresced to absorbed photons. Compounds with high-quantum yields offer strong and efficient fluorescence and are desirable for various applications. In the Journal of Applied Physics, researchers reported a novel method for measuring Egyptian-blue IR phosphors’ quantum yield by taking temperature measurements of the compounds in full sunlight. Motivated by cool roof applications, they measured temperature changes in the materials under the sun to estimate the cooling effect of fluorescence, in addition to long wave emission and air convection.
In total, four Egyptian-blue IR phosphors were tested. The phosphors were applied to a thermally insulated support and flanked by calibrated nonfluorescent samples for comparison. The temperature measurement in full sunlight permits the assignment of an effective solar reflectance (ESR), which is larger than the ordinary solar reflectance. This fluorescence contribution to the ESR is then used to calculate the quantum yield for each phosphor.
If the phosphors are used in low concentrations the quantum efficiency even approaches unity, but the fluorescence is weak. At larger concentrations, the fluorescence reaches a maximum at which the quantum yield is still high, 0.7, about a factor of 7 higher than previously reported. Future infrared fluorescence applications can benefit from the high quantum yields of these compounds.
Source: “High quantum yield of the Egyptian blue family of infrared phosphors (MCuSi4O10, M = Ca, Sr, Ba),” by Paul Berdahl, Simon K. Boocock, George C.-Y. Chan, Sharon S. Chen, Ronnen M. Levinson, and Michael A. Zalich, Journal of Applied Physics (2018). The article can be accessed at https://doi.org/10.1063/1.5019808.