Stephen W. Allison

Remote thermometry with thermographic phosphors: Instrumentation and applications

The temperature-dependent characteristics of fluorescence of several rare-earth-doped ceramic phosphors has made these materials the focus of a major effort in the field of noncontact thermometry over the past few decades. These “thermographic phosphors,” e.g., Y2O3:Eu, have been used for remote measurements of the temperatures of both static and moving surfaces, and have performed many other tasks that standard sensors (thermocouples, thermistors, etc.) cannot. The range of usefulness of this class of materials extends from cryogenic temperatures to those approaching 2000 °C. The instrumentation needed for this type of thermometry has followed many different lines of development, and this evolution has produced a wide variety of both field- and laboratory-grade systems that are now described in the literature. In general, the technique offers high sensitivity (≈0.05 °C), robustness (e.g., stability of the sensor sample in harsh environments), and NIST traceability. In addition, such systems have been suc...

Nanoscale thermometry via the fluorescence of YAG:Ce phosphor particles: measurements from 7 to 77?C

The laser-induced fluorescence lifetime of 30 nm particles of YAG:Ce was measured as a function of temperature from 7 to 77°C. The fluorescence decay lifetimes for the nanoparticles of this phosphor varied from ≈18 to 27 ns, i.e. ≈33% relative to the longest lifetime measured. This large variation in lifetime, coupled with the high signal strength that was observed, suggest that YAG:Ce nanoparticles will be useful thermographic phosphors. We describe the material and the apparatus used to characterize its fluorescence, present the results of measurements made over the range of temperatures tested and comment on some possible applications for this novel material.

Preparation of Cr-doped Y3Al5O12 phosphors by heterogeneous precipitation methods and their luminescent properties

Abstract Chromium-doped Y 3 Al 5 O 12 (YAG-Cr) phosphor powders were prepared by a heterogeneous precipitation method. Hydroxide precursor powders were precipitated from their respective metal sulfate solutions with urea. The formation of the YAG phosphors was investigated by means of XRD. Phase pure YAG-Cr was formed by heating the precursors at 1300°C, which is more than 300°C lower than that required for the conventional solid-state reaction method. The emission intensity of YAG-Cr phosphors increased with increasing firing temperature, and the intensities observed for phosphor powders were brighter than that of the commercial YAG-Cr phosphors.

Monitoring permanent-magnet motor heating with phosphor thermometry

The fiber-optic-conveyed, laser-induced fluorescence of a thermographic phosphor was used to measure the surface temperature of the rotor in an operating permanent-magnet motor. The technique uses a pulsed laser to interrogate a surface-bonded film of europium-doped lanthanum oxysulfide for the purpose of determining its temperature-dependent, exponential-decay lifetime. By using digital-delay circuitry to control the measurement systems timing, the authors achieved spatial resolutions of better than 2 mm at speeds of up to 700 rotations/s. The precision of the temperature measurements was 0.3 degrees C. The details of the measurement system are presented and discussed. >

Characterization of high-temperature thermographic phosphors: spectral properties of LuPO 4 :Dy(1%),Eu(2%)

The fluorescence properties of a thermographic phosphor in the form of single crystals of LuPO(4):Dy(1%),Eu(2%) have been studied as a function of temperature by the use of a high-temperature oven integrated into a spectrophotometer. The results reveal differing rates of decrease in the fluorescence intensity with increasing temperature for various emission lines of dysprosium and europium, thus suggesting a new differential method of remote thermometry for high-temperature applications and for temperature measurements on moving components.

Depth-penetrating temperature measurements of thermal barrier coatings incorporating thermographic phosphors

Thermographic phosphors have been previously demonstrated to provide effective non-contact, emissivity-independent surface temperature measurements. Due to the translucent nature of thermal barrier coatings (TBCs), thermographic-phosphor-based temperature measurements can be extended beyond the surface to provide depth-selective temperature measurements by incorporating the thermographic phosphor layer at the depth where the temperature measurement is desired. In this paper, thermographic phosphor (Y2O3:Eu) fluorescence decay time measurements are demonstrated to provide through-the-coating-thickness temperature readings up to 1100 °C with the phosphor layer residing beneath a 100-µm-thick TBC (plasmasprayed 8 wt.% yttria-stabilized zirconia). With an appropriately chosen excitation wavelength and detection configuration, it is shown that sufficient phosphor emission is generated to provide effective temperature measurements, despite the attenuation of both the excitation and emission intensities by the overlying TBC. This depth-penetrating temperature measurement capability should prove particularly useful for TBC diagnostics where a large thermal gradient is typically present across the TBC thickness. The fluorescence decay from the Y2O3:Eu layer exhibited both an initial short-term exponential rise and a longer-term exponential decay. The rise time constant was demonstrated to provide better temperature indication below 500 °C while the decay time constant was a better indicator at higher temperatures.

Temperature-dependent luminescence of Ce3+ in gallium-substituted garnets

The luminescent lifetime of cerium-doped yttrium aluminum garnet has been determined as a function of temperature and as a function of gallium content. We have shown that increasing gallium content decreases the decay lifetime and results in luminescence quenching at lower temperatures. The results are quantitatively explained using a configurational coordinate diagram.

Temperature-dependent fluorescence decay lifetimes of the phosphor Y3(Al0.5Ga0.5)5O12:Ce 1%

The decay time of the phosphor yttrium aluminum garnet doped with cerium (YAG:Ce) is temperature dependent. Selective incorporation of gallium into the YAG:Ce matrix permits tuning the temperature at which quenching begins. Also, the size of the phosphor particle and processing method affect this characteristic. We describe one such situation in which the quenching of the combustion synthesized nanophosphor Y3(Al0.5Ga0.5)5O12:Ce 1% was observed from ambient to 125 °C. By signal averaging of laser excited fluorescence, temperature uncertainties ranged from 0.05 to 0.15 °C. The single shot temperature uncertainty at 115 °C was ±3 °C, indicating the feasibility for transient thermometry with response rate exceeding 1 MHz.

Phosphor thermometry at cryogenic temperatures

The temperature dependence of the lifetimes of two phosphors, La2O2S:Eu and Mg4FGeO6:Mn, is presented from 300 to 11 and 140 to 8 K, respectively. Emission from the 5D3 state of La2O2S:Eu reveals a strong temperature dependence from 11 to about 130 K, where the signal becomes too weak to measure accurately. Emission from the 4F2 state of Mg4FGeO6:Mn changes throughout the temperature range measured. Both of these materials are used for higher temperature thermometry applications. This work illustrates their utility down to liquid helium temperatures.

Project SEE (Satellite Energy Exchange): proposal for space-based gravitational measurements

Project SEE (Satellite Energy Exchange) is an international effort to organize a new space mission for fundamental measurements in gravitation, including tests of the equivalence principle (EP) by composition dependence (CD) and inverse-square-law (ISL) violations, determination of G, and a test for non-zero G-dot. The CD tests will be both at intermediate distances (a few metres) and at long distances (radius of the Earth, RE). Thus, a SEE mission would obtain accurate information self-consistently on a number of distinct gravitational effects. The EP test by CD at distances of a few metres would provide confirmation of earlier, more precise experiments. All other tests would significantly improve our knowledge of gravity. In particular, the error in G is projected to be less than 1 ppm. Project SEE entails launching a dedicated satellite and making detailed observations of free-floating test bodies within its experimental chamber.