Abstract
Recently developed laser-based measurement techniques are used to image the temperatures and velocities in gas flows. They require new phosphor materials with an unprecedented combination of properties. A novel synthesis procedure is described here; it results in hierarchically structured, hollow microspheres of Eu3+-doped Y2O3, with unusual particle sizes and very good characteristics compared to full particles. Solution-based precipitation on polymer microballoons produces very stable and luminescent, ceramic materials of extremely low density. As a result of the – compared to established template-directed syntheses – reduced mass of polymer that is lost upon calcination, micron-sized particles are obtained with mesoporous walls, low defect concentrations, and nanoscale wall thicknesses. They can be produced with larger diameters (~25 µm) compared to known hollow spheres and exhibit an optimized flow behavior. Their temperature sensing properties and excellent fluidic follow-up behavior are shown by determining emission intensity ratios in a specially designed heating chamber. Emission spectroscopy and imaging, electron microscopy and X-ray diffraction results are presented for aerosolizable Y2O3 with an optimized dopant concentration (8%). Challenges in the field of thermofluids can be addressed by combined application of thermometry and particle image velocimetry with such hollow microparticles.
Highlights
New hollow and multiscale structures of materials for sustainable energy consumption and other applications are very much sought after[1], but their bottom-up synthesis is challenging when self-organization[2] or Kirkendall-diffusion fails[3]
A new type of tracer particles of phosphor materials with an excellent thermal and fluidic follow-up behavior was developed for this purpose and is presented here, both in terms a) of synthesis and characterization and b) of applicability in thermometry
Of phosphor materials to be useful for further thermometry method development[19,20]
Summary
Y(NO3)3·6H2O (99.9%, 352.4 mg), Eu(NO3)3·6H2O (99.9%, 35.7 mg) and urea (99.5%) were dissolved in 25 ml demineralised water and were added slowly to the suspension via a dropping funnel, while the reaction temperature was increased up to 363 K. A sample (400 mg) of Y2O3:Eu hollow microspheres was placed on the membrane at the bottom of the chamber. For an adequate signal-to-noise ratio, the dimensions of individual particles have to be at the micrometer scale These two requirements motivated the use of hollow microspheres. For planar temperature measurements two spectral bands were monitored separately using two CCD cameras. These spectral bands were selected by appropriate bandpass filters and are denoted as “blue” (Sphblue) and “red” (Sphred) channels. For a thermographic phosphor such as Y2O3:Eu, the intensity ratio (r) of the “blue” and “red” filter band is a function (equation (2)) of the phosphorescence quantum yield φph in each channel[41]: r=
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