Sol-gel deposited thermographic phosphors as possible thermal history coatings

In many engineering applications surface temperatures have to be measured accurately. Sometimes the maximum temperature reached by a surface within its lifetime is even more important than its actual temperature. In order to obtain information about such maximum temperatures, so called temperature history sensors can be applied. One idea to obtain such information, which is followed in the actual work, is to deposit doped oxides with phosphorescence properties which are depending on the crystal structure of the host oxide. Thus, if the crystal structure is changing at a certain temperature, this change could also be detected by a simple phosphorescence lifetime measurement afterwards. In the present work, two materials were investigated. Yttria doped with terbium and europium and alumina doped with europium. Both materials were deposited as thin films with the sol gel technique using a newly developed automated spray coating set-up. The samples were heated twice: once for calcination at a given temperature and then a second time by annealing them at different temperatures. After each of the two treatments the phosphorescence lifetimes were investigated. The first procedure is intrinsic for the production of phosphorescing material. The second annealing simulates the high temperature treatment within an application. It is found that the phosphorescence lifetime is changed in most cases, when the second annealing temperature is higher than the first calcination temperature. Thus, a certain design or alignment of the detected maximum temperature is possible. For the doped yttria, the lifetimes increase by ca. 20% after the second treatment at higher temperatures, while for doped alumina a reduction of the lifetimes by more than 50% is observed. In total this may be a promising way for obtaining adjustable temperature his