Elsevier

Sensors and Actuators A: Physical

Volume 234, 1 October 2015, Pages 339-345
Sensors and Actuators A: Physical

A detailed characterization of BaMgAl10O17:Eu phosphor as a thermal history sensor for harsh environments

https://doi.org/10.1016/j.sna.2015.09.020Get rights and content

Highlights

  • A thermographic phosphor is investigated regarding thermal history sensing.

  • Sensing capabilities of BAM:Eu are studied in the temperature range 700–1200 °C.

  • Intensity ratio and lifetime decay methods are successfully reported.

  • Influence of several factors on measurement accuracy and sensitivity is characterised.

Abstract

Knowledge of component temperatures in gas turbines is essential for the design of thermal management systems and to maintain the lifetime of highly loaded parts as the firing temperature increases in pursuit of improved thermal efficiency. When on-line methods such as pyrometers and thermocouples are not suitable, a thermal history sensor can be used to record the maximum temperatures and read them out after operation. Currently, temperature sensitive paints are applied to obtain temperature profiles in gas turbine components but they present some limitations. A new method based on irreversible changes in the optical properties of thermographic phosphors can potentially overcome some of these difficulties. In particular, a sensor based on the oxidation of europium based phosphors has shown great potential. In this work the temperature sensing capabilities of the phosphor BaMgAl10O17:Eu are investigated in the temperature range from 700 °C to 1200 °C, and suitable measurands defined. The influence of practical factors comprising excitation fluence, exposure time, dopant concentration, cooling down time and atmosphere composition, on measurement accuracy and sensitivity are also reported.

Introduction

Increased thermal efficiency has been a main driver of gas turbine development since their inception in both their aerospace and land based forms and is set to remain so due to fuel costs and concerns about emissions. The firing temperature (analogous to turbine inlet temperature for jet engines) is the determining factor for thermal efficiency but is restricted by the material limits of the metallic components in the high-pressure turbine. This problem has been addressed by the development of new alloys, the implementation of complex air and steam cooling systems and the development of coatings that thermally insulate and protect against corrosion. Nevertheless, the lifetime of components remains dependent on their temperature and therefore it is essential to determine this as part of the engine development. It could also be of great value in service as part of the engine control system [1].

Temperature measurements can be performed in real time, using thermocouples or pyrometers for example, but all such conventional techniques are limited in the gas turbine environment. Thermocouples are intrusive, potentially affecting the conditions they are intended to measure, and giving erroneous temperature data. Furthermore, their installation is complicated (they cannot easily be installed on rotating components) and they provide only point measurements so that multiple sensors need to be set-up in order to acquire temperature maps. Pyrometers are passive optical devices and therefore are non-intrusive. However, their accuracy is compromised under conditions such as those present in the hot sections of gas turbines. The two main sources of error are the presence of secondary radiation reflected from other components of the turbine and the difficulty in correctly evaluating the emissivity of the surface whose temperature is being measured [2].

Off-line temperature measurements, performed after the operation of the system, are an alternative when on-line measurements are not possible. In this technique, sensors that remember the conditions to which they have been exposed are used and interrogated post exposure to determine them. Such sensors typically record a measure of the thermal exposure that is a function of temperature and exposure time so that steady state exposures of known duration are required if the temperature is to be accurately determined. Despite this apparent limitation such sensors, sometimes referred to as thermal history sensors, are widely used with temperature sensitive paints (or thermal paints) the most common example. These incorporate a metallic pigment that permanently changes colour upon heating. This pigment is mixed with a binder and solvent to make a paint that can be applied on complex surfaces such as blades, nozzle guide vanes and combustor walls [3], [4]. Temperatures from 120 °C up to 1300 °C can be measured using different paints that are normally calibrated for exposure times from 3 to 60 min [5]. Despite their undoubted utility, these paints present some practical difficulties. Their application, normally via spray, needs to be carefully performed in order to obtain good surface finish that permits correct visual identification of the colour changes. The thickness of the deposited layer should be kept below a few microns to avoid interference with the heat flux of the component. However, this can result in spallation of the layer and loss of temperature information for long exposure times. Colour changes are sometimes subtle, limited to a fixed number (poor resolution) and require interpretation by an experienced operator typically under controlled conditions after dismantling of the engine, although the automation of the process is currently being investigated [6]. Finally, some of the substances present in thermal paints are now restricted by the EU REACH regulation [7].

An alternative off-line method based on the use of thermographic phosphors was has been devised by Feist et al. [8]. Thermographic phosphor materials usually consist of a ceramic host and a rare-earth dopant that emit luminescence upon excitation by a UV or visible light source. They have been studied for many years as on-line temperature sensors [9], [10], [11], [12], [13] with applications on surfaces [14], [15], [16], [17] and for gas flow temperature measurements [18], [19], [20], [21]. Off-line measurements using thermographic phosphors are based on irreversible changes in their optical properties caused by the exposure to high temperatures and oxidising environments. These optical properties can be measured using the same methods as on-line thermographic phosphors, such as the lifetime decay and intensity ratio methods widely described in the literature [9], [13]. Thermographic phosphors present some advantages when compared to thermal paints: the measurement is objective and unaffected by errors due to visual interpretation, temperature data is continuous and therefore resolution is improved, in-situ measurements can be made more easily which can remove the need to dismantle the engine and the chemicals used are not restricted by REACH regulations.

Two processes that cause permanent changes in phosphors have been the main focus of studies conducted to date: the amorphous to crystalline transformation [22] and oxidation [23]. The former requires an initially amorphous phosphor but normally manufacturers sell them fully crystallised so that custom-made materials are usually required. During crystallisation, the local environment of the dopant ions becomes more regular and luminescence becomes more intense and long lived. This concept has been investigated in several phosphors, for example Y2SiO5:Tb powder showed a dynamic range extending up to 1000 °C [22], a Eu based phosphor applied to a surface in paint form went up to 800 °C [24] and a YAG:Ln air plasma sprayed coating up to 900 °C [24].

The second mechanism makes use of the change in optical properties that results from oxidation of divalent rare-earth ions, such as europium, to a trivalent state. This has the advantage that well characterised commercially available, fully crystalline phosphors can be employed [23]. The concept has been investigated for a number of phosphors [23] sensitive to 1400 °C. BaMgAl10O17:Eu (BAM:Eu) phosphor shows great potential and an initial investigation of its sensing capabilities can be found in [23], [25]. In the literature thermally driven changes in BAM:Eu are usually regarded as a form of degradation due to the reduction in blue emission that the phosphor is commercially employed to provide in plasma display panels. In the context of thermal history sensing the process is desirable so that degradation seems an inappropriate term and is replaced herein by reference to oxidation notwithstanding the fact that this is not the only effect of heat treatment of the phosphor.

In this work, a detailed study of the temperature sensing capabilities of BAM:Eu from 700 °C to 1200 °C is presented. The influence of several factors that might affect the accuracy, sensitivity or practical utility of the phosphor is also discussed. These include excitation fluence, excitation wavelength, exposure time, dopant concentration, cooling down time and bath gas composition. The intensity ratio method was the focus of the investigation as it is better suited to 2-D surface temperature measurements. However, the lifetime decay method is also reported for the first time for this material.

Section snippets

Experiments

Samples of commercially available BAM:Eu phosphor powder (KEMK63 UF-P1, Phosphor Technology) were studied in this work. BAM:Eu samples were also manufactured by the sol–gel process reported previously [25] in order to study the effect of dopant concentration.

The luminescence properties of the samples were examined at room temperature and a diagram of the set-up is shown in Fig. 1. The samples were excited by using the third and fourth harmonics (355 nm and 266 nm respectively) of a pulsed Nd:YAG

Excitation and emission: definition of a suitable measurand

The emission spectrum of BAM:Eu after heat treatment in air, as shown in Fig. 2 for a sample heat treated at 1000 °C for 20 min, consists of a broad band emission centred at 445 nm (due to 4f65d  4f7 transitions of Eu2+) and a series of peaks between 550 nm and 750 nm (due to 5D0 to 7F0,1,2,3,4 transitions of Eu3+). The phosphor can be excited using both the third and fourth harmonics of the YAG laser and the shape of the emission spectra obtained for both is the same. Eu3+-based phosphors, such as Y2

Conclusion

The performance of the phosphor BAM:Eu as a temperature history sensor has been evaluated in a range of temperatures from 700 °C to 1200 °C. Oxidation of Eu2+ to Eu3+ generates changes in the optical spectrum and lifetime decay of the phosphor that can both be used to measure the temperature to which the material has been exposed. Measurands composed of the intensity ratio between spectral lines representing the Eu2+ and Eu3+ ion emissions and the time constant of the lifetime decay of emission

Acknowledgement

The authors would like to thank the financial support from The Energy Futures Lab.

Álvaro Yáñez González is a P.h.D. candidate in Mechanical Engineering at Imperial College London. He has a Licentiate degree (BSc + MSc) in Mechanical Engineering (2011) from the Polytechnic University of Madrid, Spain. The study of thermal history sensors is the main subject of his thesis and his research interests lie in materials sciences and thermometry.

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      Luminescence thermal history sensors record the maximum surface temperatures reached during operation by following temperature-induced changes in luminescent properties of probe material which can be measured after the system has cooled down, thus enabling non-contact, off-line monitoring of components’ temperature. Four main temperature-driven processes have been defined to cause permanent luminescent changes because of exposure to high temperatures that can be used to govern thermal history: i) the phase transformation of phosphor [11–14], diffusion of ions into the phosphor host material, and quantum confinement effects [15,16], oxidation of dopant activator ions [17–19] and crystallization of the phosphor i.e., amorphous-to-crystalline transition [20–25], please see Table 1. These thermally activated changes within the thermographic phosphor have an impact on the energy level distribution of dopant ions and the probability of nonradiative electronic transitions, potentially influencing temporal (rise and decay time) and/or spectral (intensity, band shape, spectral peak position, bandwidth, and polarization) emission properties.

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    Álvaro Yáñez González is a P.h.D. candidate in Mechanical Engineering at Imperial College London. He has a Licentiate degree (BSc + MSc) in Mechanical Engineering (2011) from the Polytechnic University of Madrid, Spain. The study of thermal history sensors is the main subject of his thesis and his research interests lie in materials sciences and thermometry.

    Enrique Ruiz-Trejo obtained his P.h.D. in Materials from Imperial College and immediately after was appointed lecturer at Universidad Nacional Autónoma de México. He was then awarded a Humboldt scholarship at the Max Planck Institute for Solid State Research. In 2009 he moved to Denmark as Senior Scientist at Risoe National Laboratories for Sustainable Energy followed by a position as Research Fellow at the University of St Andrews. Since, 2012 he is Research Associate in Fuel Cells and Materials Processing at Imperial College. His areas of interest include materials for energy applications and gas separation membranes, the development of electrodes for fuel cells and the manufacture of metal-ceramic composites.

    Berend van Wachem obtained his M.Sc. and P.h.D. degree at Delft University of Technology on the modelling of dense gas–solid flows. After spending a number of years as a lecturer in Sweden, he joined the Department of Mechanical Engineering of Imperial College London in 2008. Berend van Wachem works on research projects involving multiphase flow modelling, ranging from understanding the behaviour of turbulence on individual particles, to the large-scale modelling of gas–solid and gas–liquid flows.

    Stephen Skinner joined Imperial College in 1998 and was promoted to Professor in 2014. His research interests are in materials for new energy technologies and he is primarily concerned with the chemical and physical properties of solid oxide fuel cell electrolytes and electrodes and encompasses the electrical and structural characteristics of materials.

    Frank Beyrau After his degree in Physics from Oldenburg University in Germany, Dr Beyrau did his P.h.D. studies in Engineering Thermodynamics at the University of Erlangen-Nuernberg with the focus on laser spectroscopy for combustion analysis. After leading the group “combustion technology” at the same institute for 3 years he moved to Imperial College in 2008. In 2014 he joined as a Professor in the Otto von Guericke University in Magdeburg.

    Andrew Heyes Professor Heyes studied Mechanical Engineering at the University of Manchester. He obtained a B.Eng. degree in 1989 graduating with first class honours. He then obtained an M.Sc. in 1991 and P.h.D. in 1994, also from Manchester. In 1995 Professor Heyes moved to Imperial College London joining the Department of Mechanical Engineering as a lecturer in Thermofluids. In 2013 he joined the University of Leeds as a Professor of Energy Technology and Environment and in 2015 went on to become head of the Department of Mechanical and Aerospace Engineering at the University of Strathclyde.

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