Thermally-induced structural phase transition in rare earth orthophosphate nanocrystals for highly sensitive thermal history paints

It was demonstrated that he temperature-driven irreversible structural phase transformations in the Eu3+ activated LaPO4, La0.5Y0.5PO4, and YPO4 nanocrystals can be used for the thermal history measurements over the 200oC-1000oC temperature range. Increase of the temperature caused significant changes in the Eu3+ local surroundings and, in consequence modified the intensity and the emission spectra. We showed that the intensity ratio of Eu3+ 5D0→ 7F2 to 5D0→7F1 emissions may serve as a highly-sensitive indicator of the highest temperature to which the nanocrystals were exposed to. The relative sensitivity of Y0.5La0.5PO4:2 %Eu3+ based thermal history paint was as high as 0.8 %/°C at 1000 °C. The thermal history surface map of the square steel plate covered with Y0.5La0.5PO4:2 %Eu3+ paint, heated locally at one of the corner is obtained as proof-of-concept experiment. The proposed ratiometric readout strategy facilitates measurement and enhances the accuracy of thermal history analysis.