Michael R. Cates

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. >

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.

Excitation of thermographic phosphors using a blue light emitting diode: Spectral characteristics and instrumentation applications

A blue light emitting diode (LED) has been used as the excitation source in a series of sensor-related characterization studies carried out on thermographic phosphors. The motivation for the described effort is the potential utility of LEDs for fluorescence based thermometry applications. The phosphors that were evaluated included La2O2S:Tb, Gd2O2S:Tb, Y2O2S:Pr, and various other rare-earth activated ceramics. Periodic and pulsed excitation of the phosphors was demonstrated, with LED gate-on times of 10–12 μs at operating levels of ≈25 V. The spectral response of the phosphors under these conditions is described, and the implications of such devices for the design of electro-optic instrumentation systems (including remote thermometry applications) are discussed. A beneficial finding of this work is that a good match between the LED emission spectrum and the phosphor excitation spectrum is not required in order to produce useful, detectable fluorescence.

Laser-induced fluorescence of phosphors for remote cryogenic thermometry

Remote cryogenic temperature measurements can be made by inducing fluorescence in phosphors with temperature-dependent emissions and measuring the emission lifetimes. The thermographic phosphor technique can be used for making precision, noncontact, cryogenic-temperature measurements in electrically hostile environments, such as high dc electric or magnetic fields. The National Aeronautics and Space Administration is interested in using these thermographic phosphors for mapping hot spots on cryogenic tank walls. Europium-doped lanthanum oxysulfide (La2O2S:Eu) and magnesium fluorogermanate doped with manganese (Mg4FGeO6:Mn) are suitable for low-temperature surface thermometry. Several emission lines, excited by a 337-nm ultraviolet laser, provide fluorescence lifetimes having logarithmic dependence with temperature from 4 to above 125 K. A calibration curve for both La2O2S:Eu and Mg4FGeO6:Mn is presented, as well as emission spectra taken at room temperature and 11 K.

LED-induced fluorescence diagnostics for turbine and combustion engine thermometry

Fluorescence from phosphor coatings is the basis of an established technique for measuring temperature in a wide variety of turbine and combustion engine applications. Example surfaces include blades, vanes, combustors, intake valves, pistons, and rotors. Many situations that are remote and noncontact require the high intensity of a laser to illuminate the phosphor, especially if the surface is moving. Thermometric resolutions of 0.1 C are obtainable, and some laboratory versions of these systems have been calibrated against NIST standards to even higher precision. To improve the measurement signal-to-noise ratio, synchronous detection timing has been used to repeatedly interrogate the same blade in a high speed rotating turbine. High spatial resolution can be obtained by tightly focusing the interrogation beam in measurements of static surfaces, and by precise differential timing of the laser pulses on rotating surfaces. We report here the use of blue light emitting diodes (LEDs) as an illumination source for producing useable fluorescence from phosphors for temperature measurements. An LED can excite most of the same phosphors used to cover the temperature range from 8 to 1400 C. The advantages of using LEDs are obvious in terms of size, power requirements, space requirements and cost. There can also be advantages associated with very long operating lifetimes, wide range of available colors, and their broader emission bandwidths as compared to laser diodes. Temperature may be inferred either from phase or time-decay determinations.

Precision limits of waveform recovery and analysis in a signal processing oscilloscope

The precision limits of a waveform processing oscilloscope have been investigated for the situation where the instrument is used to recover a repetitive exponential signal from high levels of noise. In these studies, the oscilloscope was programmed to sample the signal, average it, and then calculate its exponential decay time constant. The measurement errors in the resulting time constants were found to decrease with continued averaging, but not without limit. In fact, we found that there was an optimum number of samples needed to minimize the error for any particular value of the signal‐to‐noise ratio at the input to the oscilloscope. We present here our empirically determined values of those optima, an intercomparison of them with the results of a theoretical model, and a description of the measurement system developed for this work.

A research bibliography on the temperature dependencies of thermographic phosphors

Abstract We have prepared a comprehensive research bibliography on the fundamental physics and engineering applications of thermographic phosphors. It is available upon request.

Advances in High Temperature Phosphor Thermometry for Aerospace Applications

Phosphor thermometry has been used for many years for non-contact temperature measurements in hostile environments. Aerospace systems are particularly prone to adverse high temperature environments, including large blackbody background, vibration, rotation, fire/flame, pressure, or noise. These environments often restrict the use of more common thermocouples or infrared thermometric techniques. Temperature measurements inside jet turbines, rocket engines, or similar devices are especially amenable to fluorescence techniques. Often the phosphor powders are suspended in binders and applied like paint or applied as high temperature sprays. Thin coatings will quickly assume the same temperature as the surface to which they are applied. The temperature dependence of phosphors is a function of the base matrix atoms and a small quantity of added activator or “dopant” ions. Often for high temperature applications, the selected materials are refractory and include rare earth ions. Phosphors like Y3Al5O12 (YAG) doped with Eu, Dy, or Tm, Y2O3 doped with Eu, or similar rare earth compounds, will survive high temperatures and can be configured to emit light that changes rapidly in lifetime and intensity. Recently, a YAG:Cr phosphor paint emitted fluorescence during short duration tests in a high Mach number hydrogen flame at 2,200 °C. One of the biggest challenges is to locate a binder material that can withstand tremendous variations in temperature in an adverse aerospace environment. This presentation will give research results applicable to the use of phosphors for aerospace thermometry. Emphasis will be placed on the selection of phosphor and binder combinations that can withstand high temperatures.