Sol–gel deposition of multiply doped thermographic phosphor coatings Al2O3:(Cr3+, M3+) (M = Dy, Tm) for wide range surface temperature measurement application

A promising method of measuring surface temperatures in harsh environments is the use of thermographic phosphor coatings. There, the surface temperature is evaluated from the phosphorescence decay lifetime following a pulsed laser or flash lamp light excitation. Depending on the used dopant, single doped M3+:α-Al2O3 (M = Cr, Dy, Tm) emit at 694 nm (Cr3+), 488 nm (Dy3+), 584 nm (Dy3+), and 459 nm (Tm3+), respectively. However, the accessible temperature range with a single dopant is limited: for the Cr3+-transition from 293 K up to 900 K, and for the Dy3+ and Tm3+-transitions both from 1073 K up to 1473 K. In the present study a new approach is followed to extend these limitations by co-doping two dopants using the sol–gel method and dip coating of α-Al2O3 thin films. For that application (Dy3+ + Cr3+) co-doped thin α-Al2O3 films and (Tm3+ + Cr3+) co-doped α-Al2O3 films with thicknesses of 4–6 μm were prepared, and the temperature-dependent luminescence properties (emission spectra and lifetimes) were analysed after pulsed laser excitation in the UV (355 nm). The phosphorescence lifetime as a function of temperature were measured between 293 K and 1473 K. A considerably extended range for surface temperature evaluation was established following this new approach by combining different dopants on the molecular level.