Thermoluminescent microparticle thermal history sensors

While there are innumerable devices that measure temperature, the nonvolatile measurement of thermal history is far more difficult, particularly for sensors embedded in extreme environments such as fires and explosions. In this review, an extensive analysis is given of one such technology: thermoluminescent microparticles. These are transparent dielectrics with a large distribution of trap states that can store charge carriers over very long periods of time. In their simplest form, the population of these traps is dictated by an Arrhenius expression, which is highly dependent on temperature. A particle with filled traps that is exposed to high temperatures over a short period of time will preferentially lose carriers in shallow traps. This depopulation leaves a signature on the particle luminescence, which can be used to determine the temperature and time of the thermal event. Particles are prepared—many months in advance of a test, if desired—by exposure to deep ultraviolet, X-ray, beta, or gamma radiation, which fills the traps with charge carriers. Luminescence can be extracted from one or more particles regardless of whether or not they are embedded in debris or other inert materials. Testing and analysis of the method is demonstrated using laboratory experiments with microheaters and high energy explosives in the field. It is shown that the thermoluminescent materials LiF:Mg,Ti, MgB 4 O 7 :Dy,Li, and CaSO 4 :Ce,Tb, among others, provide accurate measurements of temperature in the 200 to 500 °C range in a variety of high-explosive environments. Thermometry: Feeling the heat of an explosion Micrometer-scale particles offer a means of better understanding explosions by tracking changes in temperature through time. Joseph Talghader at the University of Minnesota, United States, and his colleagues review the use of thermoluminescent materials in the creation of sensors that provide a thermal history of high-temperature events. Such sensors record the magnitude and duration of temperature excursions. Thermoluminescent materials represent one possible approach to these sensors because the population of filled traps in the materials have a strong relationship with the temperatures to which they have been exposed. After an explosion, the remaining filled traps can be excited to emit light that indicates the thermal history. Thermoluminescent microparticles are cheap to produce and can be fabricated in large quantities and embedded throughout the test area. The