Aerosol deposition of blended metal plus phosphor samples for use with a novel simultaneous X-ray tomography and thermometry technology

Thermographic phosphors provide remote, full-field temperature measurements in optically obscured environments (e.g., sooty fires). Thermographic emission follows excitation of the pure phosphors excited by an ultra-violet or X-ray source. Developing a novel X-ray tomographic technology requires phosphor particles blended into a metal matrix for high-contrast imaging. This would provide the ability to simultaneously resolve microstructure deformation and measure the internal temperature of the sample.

Aerosol deposition (AD) is a room temperature kinetic spray process that has been used to produce metallic, ceramic, polymeric, and blended coatings. This technique produces coatings through the impact consolidation of submicron particles accelerated through a converging-diverging nozzle into a vacuum chamber. As a room temperature technique, the phase and stoichiometry (and therefore the associated functionality) of the feedstock material can be maintained after deposition. Our previous work demonstrated that AD is ideal for producing thin, dense, robust phosphor coatings capable of accurate temperature measurements.

For this project we have successfully produced thick (> 500 microns) blended tantalum plus phosphor (dysprosium doped yttrium aluminum garnet) coatings using AD. Initial X-ray measurements indicate that the blended coatings do provide sufficient response necessary for X-ray tomography. But these preliminary AD coatings exhibit horizontal lines of phosphor-rich regions between each pass. An ideal coating would have a homogeneous distribution of the phosphor within the metal matrix. Therefore, we have focused our efforts on elucidating the process/structure relationship of the blended AD coatings, specifically probing the powder processing methods. The metal and phosphor powders are combined either by rolling them together (resulting in a homogeneous mixture, but individual particles of each powder) or ball milling them together (resulting in mechanically joining the metal and phosphor particles). The method of combining the metal and phosphor powders has a direct effect on the resulting coating.