On the design of phosphors for high-temperature thermometry

The temperature-dependent luminescence of certain thermographic phosphors has been used in surface thermometry for a number of years. Of the two practical methods by which temperature can be deduced from the emission [Heyes, A.L., Thermographic Phosphor Thermometry- Physical Principles and Measurement Capability, In: C.H. Sieverding, J.-F. Brouckaert (Eds.), VKI Lecture Series on Advanced Measurement Techniques for Aero and Stationary Gas Turbines. 2004, von Karman Institute for Fluid Dynamics, Brussels, (Vol. Lecture Series 2004-04)], the one considered herein relies on thermally activated non-radiative transitions from the excited to ground state making the decay rate in luminescence temperature dependent. The time constant of the exponential decay in luminescence can then be used to derive the temperature. The authors and co-workers have been working for some years now on the application of this method for condition monitoring and non-destruction evaluation in the hot sections of gas turbines. Turbine entry temperature is linked to efficiency and thereby to CO₂ emissions so that there is a clear imperative to increase it. Current entry temperatures exceed the melting points of the metals from which local components such as combustion liners, nozzle guide vanes and turbine blades are made. Thermal management of these components is therefore critical to the operation of the engine and by implication sensors and instrumentation are required to aid the design of insulation and cooling schemes and to monitor their performance in service. Thermographic phosphors typically consisting of a ceramic host (e.g. YAG) with a rare-earth dopant are excellent candidates. These materials are stable at the required temperatures and the temperature dependence of their luminescence can provide a dynamic range that matches the expected environment. In this paper, the physics of the multi-phonon non-radiative decay process are reviewed and used to construct a simple design model for thermographic phosphors. Comparison with experimental data shows general agreement but there are several anomalous cases. These are reviewed and alternative thermal quenching mechanisms considered.