High-frequency surface and near-surface temperature measurements of burning composite propellants

The work performed over the course of this reward revolved around the technique of phosphor thermometry, which is a semi-intrusive temperature sensing method that can allow for high spatial and temporal resolutions. In this technique, thermographic phosphors are first applied to the medium of interest for temperature measurement. This can take the form of phosphor powder coated onto a surface, embedded within a sample, or entrained within a gaseous flow. The phosphor particles are then excited by a light source, typically a laser emitting in the UV or visible region, though LEDs have also been employed in the past. This incident radiation will interact with the valence electrons of the phosphor materials, causing them to transition to higher electronic energy levels. This is immediately followed by relaxation back to their ground state which is accompanied by the emission of a photon. This radiative energy release competes with nonradiative energy release (via phonon production), the latter of which is temperature sensitive. Therefore, by measuring the induced emission from the phosphors, temperature can be inferred. Two types of methods exist for determining temperature from phosphor emission. Time-resolved methods primarily involve the measurement of the lifetime decay of the phosphor emission after excitation from a pulsed light source is ceased. Other time-resolved methods include measurement of phosphor emission rise time and phase shift from an oscillating excitation source. Time-integrated methods involve measuring two different spectral bands in the emission spectra of the phosphors. The ratio between these two spectral bands is then calculated to obtain a temperature dependent intensity ratio calibration. Over the course of this fellowship, each of these two methods have been considered toward completing the goal of this work, which is to obtain surface temperature measurements of reacting solid propellants. To this end, work initiated by first surveying available thermographic phosphors that would be most useful for this application. Desirable traits included phosphors which emitted in the blue region as this would limit interference from chemiluminescence of the propellant flame. A short lifetime decay is also desired which could again limit interferences from background signals in the flame through decreased gating times. The phosphor BaMgAl10O17:Eu (or BAM) was selected as the primary candidate for this application as it met all of these criteria. Calibration experiments were then performed to determine this phosphor’s response to temperature. The lifetime of this phosphor was considered primarily, and with the use of a high-temperature calibration furnace, the lifetime decay of BAM was measured from room temperature up to about 1100 K. In addition, the effect of laser fluence on the emission from this phosphor was studied. It was important to determine if the lifetime decay was affected by laser fluence to any degree, as this would determine whether laser fluence would need to be carefully monitored during in-situ temperature measurements. It was determined that a range of laser fluences yielded insignificant changes to BAM lifetime decay while operating at room temperature. Therefore, a fluence in this range will be used during laser excitation of this phosphor. This discovery, along with the lifetime temperature calibration of BAM, will be employed during future propellant burn experiments. The other major thrust of the work performed for this fellowship involved studying x-rays as an alternate excitation source for phosphor thermometry. Laser excitation will yield visible emission from the phosphors due to interactions within the valence band. X-rays, on the other hand, have sufficient energy to interact with electrons deeper within atoms, called inner shell electrons. An incident x-ray can eject one of these inner shell electrons, resulting in ionization of the atom within the crystal structure of the phosphor. The resulting hole will be filled by an electron from an outer shell, which is accompanied by photon emission. This results in another hole to be filled by another electron and another emitted photon, and so on. This energy cascade leads to emission of photons with energies in the x-ray region (from recombination of holes in the inner shells) and the visible region (from recombination of holes in the valence band with electrons from the conduction band). Therefore, x-ray emission and visible emission can be observed from the phosphors under x-ray excitation, termed x-ray fluorescence (XRF) and x-ray excited optical luminescence (XEOL), respectively. Both types of emission were studied for a response to temperature for various phosphors. Experiments were performed using x-ray tube sources located at Purdue University and Sandia National Laboratories as well as a beamline located at the Advanced Photon Source at Argonne National Laboratory, which is a synchrotron x-ray source. Visible emission from the phosphors was measured using a spectrometer and x-ray emission was observed using a silicon drift detector (SDD), which is a type of x-ray detector. The visible emission from these phosphors was found to remain thermographic under x-ray excitation in a similar manner to UV excitation. The same transition manifolds were observed in the emission spectra of these phosphors when comparing each type of excitation source. In addition, the characteristic x-ray emission lines from several phosphors were observed to show temperature sensitivity. These x-ray emission lines are considered characteristic as they are specific to the elements within each phosphor. Temperature sensitivity was explored within these x-ray emission lines in a similar manner to lines present in the visible emission spectra, by integrating under select lines and calculating ratios between lines to obtain temperature-sensitive intensity ratio curves. The main advantages of employing x-rays as an excitation source is to allow for excitation through obstructions in certain test environments, such as soot within flames. X-rays possess sufficient energy to pass through these opaque particles. Temperature sensing through solid boundaries without optical access is also theoretically possible when using x-rays to excite phosphors and measuring their x-ray emission, both of which could pass through these boundaries. To this end, a patent was published based on this technique jointly between Purdue University and Sandia National Laboratories. Current and future work will explore the physical processes that lead to temperature sensitivity in the x-ray emission of phosphors, as well as simultaneous in-situ temperature measurements and x-ray tomography of reacting energetics.