Phosphorescent thermal history sensing for advanced condition monitoring in high temperature environment
Creators: name: Rabhiou, Abderahman
Publisher: name: Imperial College London
Year: 2012

description: Knowledge of the temperatures surfaces are exposed to is crucial in many processes, particularly above 300°C. It is often not possible to measure temperature of all surfaces of interest in real-time. Off-line temperature measurement techniques can be used to record exposure temperatures in such a way that these can be retrieved later off-line, at room temperature. Thermal paint changes colour or surface properties permanently, depending on the maximum temperature they have been exposed to. Thermal paints have been a valuable tool of engine developers for many years, but the use of the existing paints presents a number of challenges if reliable results are to be obtained. Feist et al. [1] proposed a thermal history sensor based on phosphors that undergo permanent changes in their luminescence properties when exposed to high temperatures. These luminescence properties can be interrogated with a light source and measured with standard spectroscopic instrumentation. Phosphorescent thermal history sensors might be applied as paint, coating or as point-sensors. The proposed concept has several advantages over the existing sensors. As the main embodiment is a paint or coating, the proposed sensor would be a direct competing technology to thermal paints. The present study reports on the three main concepts responsible for the permanent changes in some phosphors that have been identified. The mechanisms behind these concepts are outlined and examples are given of phosphors which could be used as sensors. The amorphous-to-crystalline and the thermal damaging concepts were demonstrated in laboratory tests in this work. An extensive characterisation of the luminescence properties dependency on thermal history was conducted on several phosphors: BAM : Eu, BAM : Eu;Mn, Y2O2S : Eu, SrAl2O4 : Eu, Y2SiO5 : Tb, Y AG : Dy and Y SZ=Y AG : Dy. Calibration curves of powder, paint and/or coating embodiments of these phosphors are presented. The amorphous-to-crystalline concept revealed to be the most promising for the application as thermal history sensor, covering a temperature range from 300°C to 1300°C. The amorphous-to-crystalline phosphor Y2SiO5 : Tb was synthesised by sol-gel technique and applied as a paint on a stainless steel disk, which was subsequently exposed to a jet impinging flame. A luminescence readout of the paint after the exposure to the flame revealed a map of temperatures between 300°C and 800°C. The paint was able to highlight the asymmetrical set-up of the impinging flame. The phosphor Y SZ=Y AG : Dy was APS-coated on a rotating turbine blade and inner flame tube of a Rolls-Royce Viper 201 jet engine, which was operated for several hours. Temperature profile maps were obtained after interrogation of the luminescence changes of the coating. These revealed that the probed surface of inner flame tube was exposed to temperatures generally not exceeding than 500°C, apart from local hotspots of 700°C located above the dilution holes. The interrogation of the turbine blade indicated the areas cooled by air streams and maximum temperatures in excess of 800°C.