David L. Beshears

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.

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.

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.

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.

USE OF PHOSPHOR COATINGS FOR HIGH TEMPERATURE AEROSPACE APPLICATIONS

Phosphor thermometry has been used for many years for non -contact temperature measurements. 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 tech niques. Often the phosphor powders are suspended in binders and applied like paint or applied as high temperature sprays. Thin coatings that are less than 50 µm thick are used on the surfaces of interest. These coatings will quickly assume the same temp erature 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 Y 3Al 5O12 (YAG) doped with Eu, Dy, or Tm, Y 2O3 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 i ntensity. 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 temper ature 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 tem peratures. Evidence for light pumping for Y 2O3:Cr/YAG:Ce mixture and preliminary triboluminescence results for ZnS:Mn will also be presented. These results are the first step towards the development of a smart material damage sensor.

Hybrid Solar Lighting Provides Energy Savings and Reduces Waste Heat

ABSTRACT Artificial lighting is the largest component of electricity use in commercial U.S. buildings. Hybrid solar lighting (HSL) provides an exciting new means of reducing energy consumption while also delivering significant ancillary benefits associated with natural lighting in buildings. As more than half of all federal facilities are in the Sunbelt region (defined as having an average direct solar radiation of greater than 4 kWh/m2/day) and as more than half of all square footage available in federal buildings is also in the Sunbelt, HSL is an excellent technology fit for federal facilities. The HSL technology uses a rooftop, 4-ft-wide dish and secondary mirror that track the sun throughout the day (Figure 1). The collector system focuses the sunlight onto 127 optical fibers. The fibers serve as flexible light pipes and are connected to hybrid light fixtures that have special diffusion rods that spread out the light in all directions. One collector powers about eight hybrid light fixtures—which can i...

Tracking Systems Evaluation for the “Hybrid Lighting System”

As part of the design and development effort for the “Hybrid Lighting System,” Oak Ridge National Laboratory (ORNL) scientists have evaluated two potential candidate-tracking systems for the solar collector. The first system, the WattSun Solar Tracker, built by Array Technologies, utilizes a patented, closed loop, optical sun sensor to sense the sun’s position and track it. The second tracking system, SolarTrak Controller, built by Enhancement Electronics, Inc., is a micro controller-based tracking system. The SolarTrak micro controller-based Tracker’s sun position is determined by computing the celestial bearing of the sun with respect to the earth using the local time, date, latitude, longitude and time zone rather than sensing the relative bearing of the sun with optical receptors. This system connects directly to the mechanical system hardware supplied by Array Technologies. Both the WattSun Solar Tracker and the SolarTrak Controller were mounted on the prototype “Hybrid Lighting” mechanical system (array) hardware. A simple switch allowed independent testing of each system. Upon completion of the evaluation of the two systems we found the WattSun Solar Tracker controller to be unacceptable for use with our prototype hybrid lighting system. The SolarTrak Controller has performed well to date and provides suitable tracking accuracy for use with our prototype “Hybrid Lighting System”. After a six-month evaluation period at ORNL, the first prototype “Hybrid Lighting System” was installed at Ohio University as part of an “Enhanced Practical Photosynthetic CO2 Mitigation.” This document will highlight the results of the tracker investigation and outline the remaining issues to be addressed, to provide a suitable tracking system for our “Hybrid Lighting” collector.© 2003 ASME

Prototype Weigh-In-Motion Performance

Oak Ridge National Laboratory (ORNL) has developed and patented methods to weigh slowly moving vehicles. We have used this technology to produce a portable weigh-in-motion system that is robust and accurate. This report documents the performance of the second-generation portable weigh-in-motion prototype (WIM Gen II). The results of three modes of weight determination are compared in this report: WIM Gen II dynamic mode, WIM Gen II stop-and-go mode, and static (parked) mode on in-ground, static scales. The WIM dynamic mode measures axle weights as the vehicle passes over the system at speeds of 3 to 7 miles per hour (1.3 to 3.1 meters/second). The WIM stop-and-go mode measures the weight of each axle of the vehicle as the axles are successively positioned on a side-by-side pair of WIM measurement pads. In both measurement modes the center of balance (CB) and the total weight are obtained by a straight-forward calculation from axle weights and axle spacings. The performance metric is measurement error (in percent), which is defined as 100 x (sample standard deviation)/(average); see Appendix A for details. We have insufficient data to show that this metric is predictive. This report details the results of weight measurements performed in May 2005 at two sites using different types of vehicles at each site. In addition to the weight measurements, the testing enabled refinements to the test methodology and facilitated an assessment of the influence of vehicle speed on the dynamic-mode measurements. The initial test at the National Transportation Research Center in Knoxville, TN, involved measurements of passenger and light-duty commercial vehicles. A subsequent test at the Arrival/Departure Airfield Control Group (A/DACG) facility in Ft. Bragg, NC, involved military vehicles with gross weights between 3,000 and 75,000 pounds (1,356 to 33,900 kilograms) with a 20,000-pound (9,040 kilograms) limit per axle. For each vehicle, four or more separate measurements were done using each weighing mode. WIM dynamic, WIM stop-and-go, and static-mode scale measurements were compared for total vehicle weight and the weight of the individual axles. We made WIM dynamic mode measurements with three assemblages of weight-transducer pads to assess the performance with varying numbers (2, 4, and 6) of weigh pads. Percent error in the WIM dynamic mode was 0.51%, 0.37%, and 0.37% for total vehicle weight and 0.77%, 0.50%, and 0.47% for single-axle weight for the two-, four-, and six-pad systems, respectively. Errors in the WIM stop-and-go mode were 0.55% for total vehicle weight and 0.62% for single-axle weights. In-ground scales weighed these vehicles with an error of 0.04% (within Army specifications) for total vehicle weight, and an error of 0.86% for single-axle weight. These results show that (1) the WIM error in single-axle weight was less than that obtained from in-ground static scales; (2) the WIM system eliminates time-consuming manual procedures, human errors, and safety concerns; and (3) measurement error for the WIM prototype was less than 1% (within Army requirements for this project). All the tests were performed on smooth, dry, level, concrete surfaces. Tests under non-ideal surface conditions are needed (e.g., rough but level, sun-baked asphalt, wet pavement), and future work will test WIM performance under these conditions. However, we expect the performance will be as good as, if not better than, the present WIM performance. We recommend the WIM stop-and-go mode under non-ideal surface conditions. We anticipate no performance degradation, assuming no subsurface deformation occurs.

Fiber sensor design for turbine engines

Determination of blade temperatures in the high-speed and turbulent environment of a turbine engine is difficult using standard pyrometry techniques because of the presence of high- temperature flame and the reflective nature of the inspection surfaces. A technique utilizing thermographic phosphor compounds bonded to engine vanes and turbine blades is presented that mitigates the negative effects of blackbody radiation while potentially allowing near real- time acquisition of blade temperature information. Specialized single and dual fiber-optic probes were designed to interrogate both fixed and rotating surfaces by delivering ultraviolet light from a quadrupled Nd:YAG (266 nm) laser to phosphor coatings consisting of Y2O3:Eu, YVO4, and YAG:Tb ceramic compounds. This technique utilizes the temperature- dependent fluorescent emission of a ceramic phosphor coating to discern the temperature of the interrogated surface. By using these methods, surface temperature measurements to 1200 degree(s)C are achievable in the combustion environment.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.