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X-RAY AND LASER-BASED PHOSPHOR THERMOMETRY MEASUREMENTS AND THEIR APPLICATION TOWARD ENERGETIC MATERIALS
Phosphor thermometry is a semi-intrusive remote temperature measurement technique that allows for zero-, one-, or two-dimensional temperature measurements with high spatial and temporal resolutions. The working principle of this technique involves the electronic excitation of luminescent centers within the crystal lattice of a phosphor. Relaxation follows after the excitation source is removed, which is accompanied by photons emitted by the phosphor. Interactions between the dopants and host crystal matrix lead to temperature sensitivity of this phosphor emission. Proper calibration to temperature allows for phosphor powders to be used as thermometers when coated onto a surface, seeded within a flow, or mixed within a sample.
Applied research related to phosphor thermometry described throughout this dissertation can be split into two thrusts. The first is to apply phosphor thermometry to make in-situ temperature measurements of energetic materials where conventional methods would be difficult to implement or fail completely. The phosphor BaMgAl10O17:Eu, or BAM, was calibrated for temperature sensitivity in both its temporal and spectral response. This phosphor makes for an ideal candidate for temperature measurements within these target applications due to its blue/green emission band and short lifetime decays. The effect of laser fluence on the lifetime decay and intensity ratio of this phosphor was also studied. To avoid errors in the measurement from oversaturation effects, an upper limit of approximately 15 mJ/cm2 is found when performing lifetime decay measurements. Laser fluence was observed to have little effect on the calculated intensity ratios within the range of fluences tested. This phosphor was then encapsulated within ammonium perchlorate (AP) particles and excited using a UV laser to study the effects that this energetic material has on the emission from BAM. Deviations between intensity ratios calculated with neat BAM powder and these encapsulated particles suggest that the AP preferentially absorbs or scatters certain wavelengths of BAM emission. It is hoped that, after correcting for this shift in intensity ratio calibration, this method of encapsulation will allow for in-situ temperature measurements within energetic crystals.
The second thrust related to phosphor thermometry revolves around x-ray excitation of thermographic phosphors, as opposed to the typical UV excitation. This method would provide several benefits, which include excitation through optically occluded environments (e.g., sooty flames), negate the need for optical access of the excitation source, and allow for simultaneous temperature measurements and x-ray diagnostics using a single x-ray source. The spectral response of several thermographic phosphors was characterized for temperature sensitivity during synchrotron x-ray excitation. Similar sensitivities and spectral line shapes are observed when compared to UV excitation. An exception is seen for ZnO:Zn, which exhibits the same spectral bands between UV and x-ray excitation; however, relative intensities between these spectral bands varies dramatically. It was determined that a likely explanation for this is the excitation pulse duration rather than the photon energy (i.e., x-ray vs. UV excitation). Temperature sensitivity in the x-ray fluorescence produced by several thermographic phosphors was also explored. Trends were seen with some phosphors, particularly YAG:Dy and ZnGa2O4:Mn.
A separate research project is also presented, which focused on the combustion of nanothermites rather than diagnostics via phosphor thermometry. The goal of this project was to determine the effects that confinement has on the ability to tailor substrate destruction using inkjet-printed nanothermites. Aluminum bismuth (III) oxide (Al/Bi2O3) and aluminum copper (II) oxide (Al/CuO) nanothermites were tested over a range of equivalence ratios. Destruction was determined qualitatively through fragmentation and correlated to quantitative burning rate measurements. Al/CuO demonstrated the ability to fragment a silicon substrate when confined, which was not possible in previous unconfined tests. These results demonstrate that confinement can be used as a method to tailor fracturing performance of representative substrates. Future applications include the destruction of electromechanical systems for anti-tamper systems.