Shawn Goedeke

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.

Emission spectra from ZnS:Mn due to low velocity impacts

Triboluminescence (TL) is the emission of light due to crystal fracture and has been known for centuries. One of the most common examples of TL is the flash created from chewing wintergreen Lifesavers. Since 2003, the authors have been measuring triboluminescent properties of phosphors, of which zinc sulfide doped with manganese (ZnS:Mn) is an example. Preliminary results indicate that impact velocities greater than 0.5 m/s produce measurable TL from ZnS:Mn. To extend this research, the investigation of the emission spectrum was chosen. This differs from using filtered photodetectors in that the spectral composition of fluorescence can be ascertained. Previous research has utilized a variety of schemes that include scratching, crushing, and grinding to generate TL. In our case, the material is activated by a short duration interaction of a dropped mass and a small number of luminescence centers. This research provides a basis for the characterization and selection of materials for future spacecraft impact detection schemes.

In-Flight Armature Diagnostics

A feasibility demonstration is reported for a method of determining instantaneous temperature and velocity of an armature in flight. Instantaneous diagnostics such as this could be critical for achieving further improvements in railgun operation. Such activity has the potential to enable design enhancements by providing information on the state of the armature and its relationship to the rail as it proceeds down the bore. The method exploits the temperature dependence of fluorescence from a phosphor coating applied to the armature. The demonstration used both a very small-scale portable railgun and a small-scale benchtop railgun. For these tests, the output of a pulsed ultraviolet (UV) laser is delivered by optical fiber through an access port drilled into the insulator between the rails. As the armature passes, the UV light illuminates a small area of phosphor on the armature. The phosphor fluoresces and decays at a rate dependent on the temperature of the phosphor. A second optical fiber in close proximity collects the fluorescence and conveys it to a detector and associated data acquisition system. Temperature is determined from a measurement of the decay time. To provide for velocity measurement on the small-scale railgun, light from a red diode laser, delivered by fiber probe inserted into the bore, produced distinctive reflections at the leading and trailing edges of the armature as it passed. Also, two grooves cut into the armature produced fiducial pulses that enabled velocity measurement

Evidence of Annealed Proton Damage From a ZnS:Mn‐Based Phosphor Paint

Phosphors are materials that are doped with trace elements that give off visible light when excited. Many phosphors have a ceramic base and can survive and function at high temperatures. Research has shown that the fluorescence decay time can be used to measure temperatures in adverse environments, such as those found in space. Development of space‐based phosphor sensors will depend heavily upon research investigating the resistance of phosphors to ionizing radiation and the ability to anneal damage caused by ionizing radiation. Preliminary results indicate that a consistent increase in the fluorescence decay time after thermal cycling was observed at two measured 3 MeV proton fluences. This “annealing” of proton damage was observed over the entire measured temperature range. The more heavily irradiated ZnS:Mn samples did not have annealed decay times that were as large as those that received lesser radiation fluences.

Phosphor Thermometry in an Operating Turbine Engine

It is well known that phosphor thermometry may be used in environments characterized by high brightness from blackbody background or combustion flames. The present work describes measurements performed inside an operating turbine engine. For this application, two thermographic phosphors were chosen to cover the expected temperature range. The first, Yttrium Oxide doped with Europium (Y2O3:Eu) has received much attention as a temperature-indicating material for higher temperature ranges. The second, Yttrium Vandidate doped with Thulium (YVO4:Tm) is relatively new, and was chosen for several reasons. First, it exhibits a bright emission line at 475 nm, which is much bluer than Y2O3:Eu line at 611 nm. Blackbody emission will have less effect on the detection of blue emission as opposed to the red. Second, YVO4:Tm also exhibits temperature dependence from at 100 C to above 900 C in the range not covered by Y2O3:Eu. This testing proved that temperature measurements are possible in this environment, given proper selection of the emission wavelength and temperature range.

Dual-fiber optic microcantilever proximity sensor

Microcantilevers are key components of many Micro-Electro- Mechanical Systems (MEMS) and Micro-Optical-Electro- Mechanical Systems (MOEMS) because slight changes to them physically or chemically lead to changes in mechanical characteristics. An inexpensive dual-fiberoptic microcantilever proximity sensor and model to predict its performance are reported here. Motion of a magnetic- material-coated cantilever is the basis of a system under development for measuring magnetic fields. The dual fiber proximity sensor will be used to monitor the motion of the cantilever. The specific goal is to sense induction fields produced by a current carrying conductor. The proximity sensor consists of two fibers side by side with claddings in contact. The fiber core diameter, 50 microns, and cladding thickness, 10 microns, are as small as routinely available commercially with the exception of single mode fiber. Light is launched into one fiber from a light-emitting diode (LED). It emerges from that fiber and reflects from the cantilever into the adjacent receiving fiber connected to a detector. The sensing end is cast molded with a diameter of 3 mm over the last 20 mm, yielding a low profile sensor. This reflective triangulation approach is probably the oldest and simplest fiber proximity sensing approach, yet the novelty here is in demonstrating high sensitivity at low expense from a triangular microstructure with amorphous magnetic coatings of iron, cobalt, permalloy, etc. The signal intensity versus distance curve yields an approximate Gaussian shape. For a typical configuration, the signal grows from 10% to 90% of maximum in traversing from 6 to 50 microns from a coated cantilever. With signal levels exceeding a volt, nanometer resolution should be readily achievable for periodic signals.

Developing a phosphor-based health monitoring sensor suite for future spacecraft

The desire to explore the Moon and Mars by 2030 makes cost effective and low mass health monitoring sensors essential for spacecraft development. Parameters such as impact, temperature, and radiation fluence need to be measured in order to determine the health of a human occupied vehicle. A phosphor-based sensor offers one good approach to develop a robust health monitoring system. The authors have spent the last few years evaluating physical characteristics of zinc sulfide (ZnS) phosphors. These materials emit triboluminescence (TL) which is fluorescence produced as a result of an impact. Currently, two ZnS materials have been tested for impact response for velocities from 1 m/s to 6 km/s. These materials have also been calibrated for use as temperature sensors from room temperature to 350 °C. Finally, any sensor that is intended to function in space must be characterized for response to ionizing radiation. Research to date has included irradiating ZnS with 3 MeV protons and 20 keV electrons, which are likely components of the space radiation environment. Results have shown that that the fluorescence emission intensity decreases with radiation fluence. However, radiation induced damage can be annealed at small fluence levels. This annealing not only increased light intensity of the exposed sample from excitation but also TL excitation as well.

Development of a Phosphor-Based Sensor Suite for Spacecraft Health Monitoring

The current interest in returning to the Moon and Mars by 2030 makes cost effective and low mass health monitoring sensors essential for spacecraft development. In space, there are many surface measurements that are required to monitor the condition of the spacecraft including: surface temperature, radiation fluence, and impact. Through the use of phosphors, materials doped with trace elements that give off visible light when excited, these conditions can be monitored. Practical space-based phosphor sensors will depend heavily upon research investigating the resistance of phosphors to ionizing radiation and the ability to anneal or self-heal from damage caused by ionizing radiation. Preliminary investigations into these sensors have recently been performed using ZnS:Mn. This phosphor has been found to be temperature sensitive from 100 to 350 C and responsive to both impact and radiation fluence. A 3 MeV proton fluence as small as 2.28 x 10 mm was found to statistically reduce the ZnS:Mn fluorescence decay time for temperatures less than 200 °C. Reductions in decay time appear to be proportional to increasing fluence. This testing has also shown that the proton damage decreases the light emission with respect to impact energy. While this testing is not all inclusive; it does illuminate the process that can be used in the selection of appropriate sensor materials.