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Tutorial 2b

William A. Hollerman
University of Louisiana, Lafayette, USA

Tutorial Description:

The temperature-dependent characteristics of several rare-earth-doped ceramic phosphors have made these materials the focus of a major effort in the field of non-contact thermometry over the past few decades. These "thermographic phosphors", like Y2O3:Eu, have been used for remote measurements of the temperatures of both static and moving surfaces, and have performed many other tasks that standard sensors (thermocouples, thermistors, etc.) cannot. The range of usefulness of this class of materials extends from cryogenic temperatures to those approaching 2,000 °C. The instrumentation needed for this type of thermometry has followed many different lines of development, and this evolution has produced a wide variety of both field- and laboratory-grade systems that are now described in the literature. In general, the technique offers high sensitivity (± 0.05 °C), robustness (stability of the sensor sample in harsh environments), and NIST traceability. In addition, such systems have been successfully adapted to make remote-sensing measurements of pressure, heat flux, shear stress, and strain. In this tutorial, we summarize the physical mechanisms that form the basis for the technique, and then catalog and discuss the instrumentation-related aspects of several different remote phosphor-based sensor systems.

The emission of fluorescence due to mechanical excitation, 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 wintergreen Lifesavers®. For the last several years, the authors have measured the production of TL from projectile impacts over the energy range of 1 m/s to 6 km/s. A so-called 'smart' structure with embedded TL material could be capable of determining whether a particle had struck, and record the relative intensity of its impact. Through the use of specific fluor material, it is believed that sensors can be developed that will indicate the occurrence of an impact and its location. To apply TL to a damage sensor, it would need to be incorporated into some type of smart material or coating. This combination might need to have a phosphor suspended and evenly distributed in a resin that would then cover a grid of optical fiber. The optical fibers would be used to capture the emitted light and transmit it to a series of detectors. Through the careful monitoring, the entire exterior of any structure could be a passive sensor that can locate and possibly determine the impact velocity or energy of a debris strike.

The interest in returning to space 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 1013 mm-2 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.

The proposed tutorial will cover the width and breadth of phosphor-based sensors with uses in adverse environments.

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