Phosphorescent thermal history sensors for extreme environments

Abstract

The measurement of the surface temperature of many components in gas turbines has become increasingly important as the firing temperature raises to improve thermal efficiency and reduce CO2 emissions. Traditional methods to measure temperatures in real time, such as thermocouples or pyrometers, are sometimes not suitable and an alternative must be sought. Thermal history sensors record the maximum temperatures reached during operation, which can then be measured after the engine has cooled down. Currently, temperature sensitive paints are mainly used to obtain temperature profiles on gas turbine components but they present some limitations such as subjectivity of the measurement, poor resolution and toxicity. Permanent changes in the optical properties of thermographic phosphors have been proposed as an alternative to record temperatures and can potentially overcome some of the difficulties associated with traditional paints. The changes in the optical properties of some europium doped phosphors after oxidation can be used to sense temperatures up to 1400 °C. The oxidation mechanism of BaMgAl10O17:Eu are investigated in detail by means of standard material characterisation techniques and laser induced phosphorescence. Variations in the luminescence properties of the phosphor (intensity ratio and lifetime decay) are related to microstructural and chemical changes and permitted to measure temperatures in the range 700 - 1200 °C. The influence of practical factors that can affect the measurement accuracy and sensitivity are thoroughly characterised. These include the energy fluence used for excitation, duration of the exposure at high temperatures, dopant concentration, time spent during cooling down, composition of the atmosphere during the heat treatment and particle size. The reversibility of the changes in the optical properties of the phosphor is studied by applying a heat treatment in a reducing atmosphere, and thus reusability of the sensor demonstrated. The development of a coating made of this phosphor is explored for the first time with regards to its application as a sensor. The difficulties to manufacture such a coating are mainly related to the complex stoichiometry and high processing temperature of the phosphor material. BaMgAl10O17:Eu coatings onto metallic substrates are manufactured by the screen printing method. In these coatings, diffusion of elements from the substrate undesirably affects the optical properties of the sensor after exposure to high temperatures. The use of a diffusion barrier permits to perform temperature measurements at temperatures up to 1100 °C comparable to the powder material. Thermal gradients across the ceramic coating can drastically affect the accuracy of the temperature measurements performed by using luminescence. Investigations in thermal barrier coating sensors in controlled gradient conditions are performed that permit evaluation of the temperature error introduced by these gradients. Comparison of experimental data and a theoretical model indicates that significant temperature measurement errors can be expected in BAM:Eu coatings when a thermal gradient is present.