J. Brübach

Gas compositional and pressure effects on thermographic phosphor thermometry

In the present study, the influence of gas compositional and pressure conditions on thermographic phosphor thermometry was investigated. A heatable pressurized and optical accessible calibration chamber was built to measure the phosphorescence decay time at different temperatures as well as at different partial and absolute pressures. At room temperature, the absolute pressure could be increased to 30 bar. To vary the gas composition, nitrogen, oxygen, carbon dioxide, methane, helium as well as water vapour were used. Three different phosphors were investigated: Mg4FGeO6:Mn, La2O2S:Eu and Y2O3:Eu. Phosphorescence was excited by the third and the fourth harmonics of a pulsed Nd:YAG-laser (355 nm and 266 nm, respectively) and recorded temporally resolved by a photomultiplier. Mg4FGeO6:Mn as well as La2O2S:Eu were not influenced significantly by varying partial and absolute pressures. In contrast, Y2O3:Eu showed a strong sensitivity on the oxygen concentration of the surrounding gas phase as well as irreversible changes in the phosphorescence decay time after increasing the absolute pressure.

Characterization of manganese-activated magnesium fluorogermanate with regards to thermographic phosphor thermometry

Apart from the temperature, four parameters were investigated for their impact on the phosphorescence characteristics of Mg4FGeO6:Mn with regards to phosphor thermometry: the dopant concentration, the laser pulse energy, gas compositional and pressure effects as well as irreversible changes due to heat treatments. Five specially produced phosphors with different dopant concentrations as well as commercially available Mg4FGeO6:Mn were investigated in the form of coatings and pure powder. The phosphorescence was excited by the third harmonic of a pulsed Nd:YAG laser (355 nm). The lifetime decay as well as the emission spectra of the subsequently emitted phosphorescence were determined. Generally, the decay time decreased with increasing dopant concentration, with increasing laser pulse energy and for coatings also with increasing maximum temperature and duration of heat treatments, whereas the impact of the laser power was minimized by a modified evaluation routine of the decay time. Gas compositional and pressure effects did not have a significant influence on the decay time of Mg4FGeO6:Mn. Neither the variation of the dopant concentration nor the exposure to heat treatments influenced the shape of the emission spectra in any way.

High-speed phosphor thermometry.

Phosphor thermometry is a semi-invasive surface temperature measurement technique utilising the luminescence properties of doped ceramic materials. Typically, these phosphor materials are coated onto the object of interest and are excited by a short UV laser pulse. Up to now, primarily Q-switched laser systems with repetition rates of 10 Hz were employed for excitation. Accordingly, this diagnostic tool was not applicable to resolve correlated temperature transients at time scales shorter than 100 ms. This contribution reports on the first realisation of a high-speed phosphor thermometry system employing a highly repetitive laser in the kHz regime and a fast decaying phosphor. A suitable material was characterised regarding its temperature lifetime characteristic and its measurement precision. Additionally, the influence of laser power on the phosphor coating was investigated in terms of heating effects. A demonstration of this high-speed technique has been conducted inside the thermally highly transient system of an optically accessible internal combustion engine. Temperatures have been measured with a repetition rate of 6 kHz corresponding to one sample per crank angle degree at 1000 rpm.

Two-dimensional surface temperature diagnostics in a full-metal engine using thermographic phosphors

An optical system consisting of a rigid borescope was developed to measure surface temperatures inside full-metal internal combustion engines. The measurement principle is predicated on lifetime-based phosphor thermometry of the material Gd3Ga5O12: Cr. The system is designed to resolve the luminescence decay of thermographic phosphors temporally and two-dimensionally by the use of a CMOS high-speed camera. The device allows the visualization of the temperature distribution in an area of 9 mm in diameter. An application of this optical system inside an internal combustion engine is demonstrated, yielding temperature maps under fired and motored conditions in a full-metal engine for the first time.

Gd3Ga5O12:Cr—a phosphor for two-dimensional thermometry in internal combustion engines

The lifetime of thermographic phosphors needs to be short enough to resolve the temperature time scales and long enough to be properly resolved by the respective detector. Up until now, there has not been a thermographic phosphor that exhibits an adequate temperature lifetime characteristic for two-dimensional thermometry inside internal combustion engines using high speed cameras. Hence, this study suggests the material Gd3Ga5O12:Cr for this purpose. The emission spectra and the temperature lifetime characteristics were determined and the lifetime is shown to be independent of the composition and the absolute pressure of the surrounding gas phase.

The spectrally resolved luminescence decay of thermographic phosphors

A novel instrumentation for a simultaneous determination of the spectrally and temporally resolved luminescence characteristics of thermographic phosphors is introduced. For this purpose, a spectrometer was combined with a CMOS high-speed camera in order to analyse the materials La2O2S:Eu and LaAlO3:Eu. Lifetime and luminescence intensity plotted against the wavelength as well as luminescence decay curves are presented. Such information in future will guide the best choice for the monitored spectral windows in thermometry applications. The system is shown to be a powerful and efficient tool for a detailed analysis of thermographic phosphors.

Phosphor thermometry at an optically accessible internal combustion engine

Two-dimensional phosphor thermometry based on the temperature-dependent luminescence lifetime using a CMOS high-speed camera was characterized and applied to an optically accessible diesel engine. A temperature transient was monitored in trailed and in fired operation.

Phosphor thermometry at high repetition rates

Phosphor thermometry is a semi-invasive surface temperature measurement technique utilizing the luminescence properties of thermographic phosphors. Typically these ceramic materials are coated onto the object of interest and are excited by a short UV laser pulse. Photomultipliers and high-speed camera systems are used to transiently detect the subsequently emitted luminescence decay point wise or two-dimensionally resolved. Based on appropriate calibration measurements, the luminescence lifetime is converted to temperature. Up to now, primarily Q-switched laser systems with repetition rates of 10 Hz were employed for excitation. Accordingly, this diagnostic tool was not applicable to resolve correlated temperature transients at time scales shorter than 100 ms. For the first time, the authors realized a high-speed phosphor thermometry system combining a highly repetitive laser in the kHz regime and a fast decaying phosphor. A suitable material was characterized regarding its temperature lifetime characteri...

Thermometry of Surfaces: Application of a High Speed Camera as a Detector for Laser-Induced Phosphorescence

This study demonstrates the use of CMOS high speed camera systems for two-dimensional surface phosphor thermometry. Using Mg4FGeO6: Mn, a temperature map of a generic system was determined.

Spectral decomposition of phosphorescence decays

In phosphor thermometry, the fitting of decay curves is a key task in the robust and precise determination of temperatures. These decays are generally assumed to be mono-exponential in certain temporal boundaries, where fitting is performed. The present study suggests a multi-exponential method to determine the spectral distribution in terms of decay times in order to analyze phosphorescence decays and thereby complement the mono-exponential analysis. Therefore, two methods of choice are compared and verified using simulated data in the presence of noise. Addtionally, this spectral decomposition is applied to the thermographic phosphor Mg4FGeO6 : Mn and reveals changes in the exponential distributions of decay times upon a change of the excitation laser energy.