William Andrew Hollerman

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.

Cathodoluminescence emission studies for selected phosphor-based sensor materials

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 dose, and impact. Through the use of phosphors, 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. The cathodoluminescence (CL) testing was performed using the low energy electron system located at the NASA Marshall Space Flight Center (MSFC) in Huntsville, Alabama. For the materials tested, several interesting results were observed. For most materials, increases in both beam energy and current density improved the CL fluorescence yield. It was also noted that YAG:Nd,Ce has the greatest near infrared intensity for any of the tested materials. The evaluation of dopant concentration in YPO4:Nd showed minimal differences in spectral shape and intensity. While the total electron dose was small, the intention was to maximize the number of irradiated materials

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.