Noah P. Bergeron

Changes in half brightness dose due to preparation pressure for YAG:Ce

Previous research shows that certain properties, such as half brightness dose (N/sub 1/2/) and fluorescence intensity, depend on preparation pressure. Phosphor tablets composed of 50% cellulose and 50% yttrium aluminum garnet doped with cerium (YAG:Ce) powders, were created using a Carver press with an applied force of 78 kN. The average 3 MeV proton N/sub 1/2/ for the tablet samples was 11.6 and 36.6 times smaller than equivalent values for the paint and crystal forms of YAG:Ce respectively. It is quite apparent that the application of a large preparation force damages some of the YAG:Ce grains which reduces the N/sub 1/2/. The fluorescence efficiency of the tablets was also less than that measured for the other forms of YAG:Ce.

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.

Proton survivability measurements for candidate solar sail materials

Once thought to be difficult or impossible, solar sailing has come out of science fiction and into the realm of possibility. A solar sail is a thin membrane material that uses the momentum carried by photons, to propel spacecraft. These photons originate from the Sun, or can be beamed onto the sail with a laser. Any spacecraft using this method would need to deploy a thin sail that could be as large as many kilometers in extent. A perfectly reflective solar sail (R = 1) at a distance of 1 AU from the Sun experiences a light pressure of 9.1 muN/m. Radiation from a variety of sources exists in the space environment with protons and electrons primarily dominating the distribution. Practical sails must be resistant to the effects of long duration proton exposure. For this reason, sail irradiation research was initiated using 1 MeV protons because it represents the approximate upper limit for these particles in the Earths radiation belts. Tested solar sail materials include aluminized Mylar, CP1, and aluminized CP2. These materials were chosen on the basis of application, availability, and manufacturability. Sail samples were exposed to a 1 MeV proton fluence of about 1013 mm-2 using a tandem pelletron accelerator at Alabama A&M University. The Mylar samples showed no observable damage as a result of the irradiation. However, a small amount of darkening was observed on the CP1 and CP2 samples. The CP1 and CP2 samples are much thicker than any of the tested Mylar. It is likely that the darkening was caused by the deposition of large amounts of energy into the CP1 and CP2, causing dislocation damage and sooty carbon formation in the polymer

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.

Triboluminescence at Speeds Greater than 100 m/s

The emission of light due to crystal fracture, or triboluminescence (TL), is a phenomenon that has been known for centuries. One of the most common examples of TL is the flash created from chewing real Wint-O-Green Lifesavers®. This chapter discusses a unique material, zinc sulfide doped with manganese (ZnS:Mn), that can aid in the design of an impact sensor capable of discerning impacts with speeds greater than 100 m/s. To achieve these speeds, however, hypervelocity guns and firearms must be used. As such, getting expensive equipment near the impacts to measure this effect becomes complicated. This chapter discusses how these complications were overcome as well as the groundbreaking results that showed that the triboluminescent spectrum shifts as a function of impact energy and ZnS:Mn can be used as the active element in impact sensors.

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.