Combustion Physics

Lund University

Laser-induced phosphorescence (LIP) for temperature measurements

Johannes Lindén, Christoph Knappe, Mattias Richter and Marcus Aldén

Contact person: Christoph Knappe

Temperature is an important parameter in many technical applications. When making technical models of combustion and fire to be able to perform computer simulations, temperature is a crucial parameter. In engines one of the biggest loss terms is heat. Surface temperature is of course important in that context both for the proper design of engine parts and as input for computer simulations.

Phosphorescence is the long afterglow from phosphor materials after excitation of electrons in the material to higher electronic states. Phosphorescence has a relatively longer duration originating from partly forbidden transitions from triplet-to-singlet states, as opposed to the faster fluorescence that originates from singlet-to-singlet transitions. Typical emission lifetimes for so called  thermographic phosphors are in the range from a few milliseconds down to a few nanosecond. Other phosphor materials can however have emission with lifetimes up to hours.

Thermographic phosphors are inorganic crystalline materials with low absorption coefficients doped with some impurity. The impurity is usually a rare earth metal or a transition metal. The impurity concentration is typically in the order of a few mole percent, a small enough part to ensure that the added metal ions are isolated from each other. When these ions are isolated they start to act as color-centers where the absorption and emission process is essentially free of quenching terms from neighboring atoms. Since the host material has no absorption, the isolated ions will be the only active atoms in absorbing and emitting light. The emission process can then be described using a single atom model where electrons are excited to higher electronic states from where they can relax by emitting a photon, much like the theory of fluorescence. Emission from phosphors can however be much more efficient due to very small quenching terms.

For some phosphors with doping materials as e.g. IIb-VIb compounds, a solid state model with energy bands is more appropriate. Electrons are then excited from the valence band to the conduction band in the material creating a mobile electron / electron hole. The electron and electron hole are sometimes bound to each other in a manner similar to a covalent binding and will then move as a single particle. This particle is called an exciton. Impurities can in some cases cause so called electron traps, from where extremely long emission lifetimes are can be noted. Emission from the recombination process of an exciton is called near band edge emission and has a very small shift to the absorption wavelength. Near band edge emission that is partially allowed can have very short emission lifetimes, in the order of picoseconds.

Emission from thermographic phosphors is temperature dependant in different ways for different phosphors. Two main emission property changes are used to extract temperature information. The emission lifetime can decrease with temperature and the emission spectrum can change with temperature inducing intensity ratio changes for chosen wavelengths. These two methods for thermometry using thermographic phosphors are called:

The lifetime method

The intensity ratio method

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