A reusable thermal history sensor based on the oxidation of a divalent rare earth ion based phosphor synthesized by the sol-gel process

Reversible changes in thermographic phosphors have been used for the last 20 years to perform on-line surface temperature measurements in harsh environments such as in gas turbines. However, when on-line measurements cannot be performed, thermal history sensors are used and the temperatures are recorded during operation and read later offline. Thermal paints have been used for many years as thermal history sensors, but they present some disadvantages. Recently, a new method based in irreversible changes in phosphor materials has been proposed and, in particular, oxidation of Eu²⁺ to Eu³⁺ in BaMgAl₁₀O₁₇:Eu (BAM:Eu) proved to be temperature sensitive up to 1400 °C. In this work, BAM:Eu powder was manufactured using the sol-gel process. This process can be adapted to create a coating without using any binder via the sol-gel dip coating process. Optical properties of the powder after treatment in air and argon atmospheres are reported, which proved that after treatment in argon the oxidation process Eu²⁺ to Eu³⁺ can be reversed and therefore a possible reusability of the thermal history phosphor. The change of optical properties with temperature and time has also been investigated for BAM, and a measurand based on the intensity ratio of the emission lines from Eu²⁺ and Eu³⁺ has been defined, with a temperature range from 700 °C to 1100 °C.