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Phosphorescent Thermal History Sensors for Extreme Environments Imperial College London Department of Mechanical Engineering Phosphorescent thermal history sensors for extreme environments Álvaro Yáñez González Submitted in part fulfilment of the requirements for the degree of Doctor of Philosophy in Mechanical Engineering of Imperial College London and the Diploma of Imperial College London September 2015 2 DECLARATION OF ORIGINALITY I, Álvaro Yáñez González, certify that the thesis entitled “Phosphorescent thermal history sensors for extreme environments” and the work presented herein are my own and any information derived from published or unpublished work of others has been properly acknowledged and a list of references is provided at the end of the thesis. COPYRIGHT DECLARATION The copyright of this thesis rests with the author and is made available under a Creative Commons Attribution Non-Commercial No Derivatives licence. Researchers are free to copy, distribute or transmit the thesis on the condition that they attribute it, that they do not use it for commercial purposes and that they do not alter, transform or build upon it. For any reuse or redistribution, researchers must make clear to others the licence terms of this work 3 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 4 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. 5 PUBLICATIONS The work conducted during the PhD and presented in this thesis is associated with the following publications. Journal Publications 1. Yáñez González, Á., Ruiz Trejo, E., van Wachem, B., Skinner, S., Beyrau, F., Heyes, A.L., A detailed characterization of BaMgAl10O17:Eu phosphor as a thermal history sensor for harsh environments, Sensors and Actuators A: Physical, 234 339-345 (2015) [1]. 2. Yáñez González, Á., Pilgrim, C.C., Feist, J.P., Sollazzo, P.Y., Beyrau, F., Heyes, A.L., On-line temperature measurement inside a thermal barrier sensor coating during engine operation, Journal of Turbomachinery, 137(10) 101004-101004, (2015) [2]. 3. Yáñez González, Á., Skinner, S., Beyrau, F., Heyes, A.L., Reusable thermal history sensing via oxidation of a divalent rare earth ion based phosphor synthesized by the sol-gel process, 36(14-15) 1275-1281, (2015) [3]. Conference proceedings 4. Yáñez González, Á., Ruiz Trejo, E., van Wachem, B., Skinner, S., Beyrau, F., Heyes, A.L., Development of an optical thermal history sensor based on the oxidation of divalent rare earth ion phosphor. Proceedings of the 14th UK Heat Transfer Conference, Edinburgh, UK, 2015 [4]. 5. Araguas Rodriguez, S., Feist, J.P., Pilgrim, C.C., Berthier, S., Biswas, K., Skinner, S., Yáñez González, Á., Heyes, A.L., Luminescent Thermal History Sensors: Fundamentals and Applications for Thermal Profiling, NATO AVT Symposium, Rzeszów, Poland, 2015 [5]. 6. Yáñez González, Á., Pilgrim, C.C., Feist, J.P., Sollazzo, P.Y., Beyrau, F., Heyes, A.L., On-line temperature measurement inside a thermal barrier sensor coating during engine operation, ASME Turbo Expo 2014, Düsseldorf, Germany, 2014 [6]. 7. Yáñez González, Á., Skinner, S., Beyrau, F., Heyes, A.L., A reusable thermal history sensor based on the oxidation of a divalent rare earth ion based phosphor synthesized 6 by the sol-gel process, IET & ISA 60th International Instrumentation Symposium, London, UK, 2014 [7]. 8. Yáñez González, Á., Pilgrim, C.C., Sollazzo, P.Y., Heyes, A.L., Feist, J.P., Nicholls, J.R., Beyrau, F., Temperature Sensing inside Thermal Barrier Coatings using Phosphor Thermometry, IET & ISA 60th International Instrumentation Symposium, London, UK, 2014 [8]. 9. Yáñez González, Á., Skinner, S., Beyrau, F., Heyes, A.L., Optical Thermal History Sensing via Oxidation of Divalent Rare Earth Ion Based Phosphors Synthesized by the Sol-Gel Process, Proceedings of the 13th UK Heat Transfer Conference, London, UK, 2013 [9]. 7 NOMENCLATURE APS – Atmospheric Plasma Spray BAL – Ba0.75Al11O17.25 BAM – BaMgAl10O17 BR – Beever-Ross CCD – Charge-Coupled Device CTE – Coefficient of Thermal Expansion CVD – Chemical Vapour Deposition DTA – Differential Thermal Analysis EBPVD – Electron Beam Physical Vapour Deposition ECCP – European Climate Change Programme EDS – Energy Dispersive x-ray Spectroscopy emf – electromotive force EPD – Electrophoretic deposition ESM – Exponential Series Method ESR – Electron Spin Resonance ICDD – International Centre for Diffraction Data KJMA – Kolmogorov-Johnson-Mehl-Avrami MEM – Maximum Entropy Method MOS – Metal Oxide Semiconductor PDP – Plasma Display Panel PIV – Particle Image Velocimetry PLD – Pulsed Laser Deposition PMT – Photomultiplier tube 8 PVD – Physical Vapour Deposition REACH – Regulation on Registration, Evaluation, Authorisation and Restriction of Chemicals SAE – SrAl14O25 SCS – Sensor Coating Systems Ltd. SEM – Scanning Electron Microscope SOFC – Solid Oxide Fuel Cells SSZ – Scandia-stabilised Zirconia SVHC – Substances of Very High Concern TBC – Thermal Barrier Coating TGA – Thermogravimetric Analysis VUV – Vacuum Ultraviolet XANES – X-Ray Absorption Near Edge Structure XPS – X-ray Photoelectron Spectroscopy XRD – X-Ray Diffraction YAG – Y3Al5O12 YSZ – Y2O3-stabilised ZrO2 ρ – Intensity ratio 9 ACKNOWLEDEMENTS I would like to thank Professor Berend van Wachem, Professor Frank Beyrau and Professor Andrew Heyes for their supervision and support during my PhD. They all have made this work possible and have provided personal advice, useful discussion and proof-reading which I am grateful for. I thank the Thermofluids Section, headed initially by Professor R. P. Lindstedt and now by Professor Ricardo Martinez-Botas for permitting me to undertake my work in this section. I am also grateful to all the people in the Department of Mechanical Engineering who made this research possible, especially to Serena Dalrymple, our secretary, Ian Wright, head technician and Kate Lewis, postgraduate administrator, for making everything run smoothly during the PhD. I would also like to thank the technicians Judith Thei, Asanka Munasinghe, Stuart Marchant and Guljar Singh for their invaluable help during this work. I would like to thank Dr. Jörg Feist for his support, advice and encouragement. I am also grateful for the technical and material support provided through the company Sensor Coating Systems. Especial thanks to Christopher Pilgrim who provided essential knowledge on numerical models and material synthesis and characterisation. Many thanks to Professor Stephen Skinner, from the Materials Department, who provided technical support and useful discussions during the whole PhD programme,
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