Date of Award
Master of Science (MS)
This work presents a systematic study on sample preparation methods and accuracy of electron yield (EY) measurements of highly insulating, granular materials. EY measurements of highly insulating materials, especially those with high EY, are challenging due to the effects of sample charging even for very low fluence electron probe beams. EY measurements of particulates are complicated by: (i) roughness effects from particulate size, shape, coverage, and compactness; (ii) particle adhesion; (iii) substrate contributions; and (iv) electrostatic repulsion and potential barriers from charged particles and substrates. Numerous methods were explored to rigidly affix particles on conducting substrates at varying coverages for accurate EY measurements. Gravimetric deposition of particles suspended in deionized water onto standard scanning electron microscopy (SEM), aluminum backed, graphitic carbon tape with a carbon infused, acrylic-based, conductive adhesive top layer, proved the most successful method, with robust results for ranges of particle sizes, shapes, and coverages. To mitigate potential electrostatic lofting effects of charged particulates, less adhered particles were removed with dry nitrogen jets and applied high electric fields prior to EY measurements. Particle sizes were determined via laser diffractometry, while SEM measurements were used to determine fractional coverage of adhered particles. Low fluence, pulsed electron probes (3-5 μs at 1-30 nA-mm-2) used 100 to 102 electrons per pulse per particle to measure EY with minimal charging effects. Surface charge accumulation from each pulse was dissipated between pulses with 1-2 s bursts of ~4.9 eV photons from a UV LED and electrons from a flood gun; 3 to 6-hour thermal annealing of the samples at 310 to 340 K could also be used intermittently to dissipate deeper dielectric charging. Preliminary studies of highly insulating, 67±23 μm sized, angular Al2O3 polishing compound particles adhered to graphitic carbon conductive tape from 0% to ~100% coverage are presented to demonstrate the effectiveness of these methods. Results of high accuracy EY tests using these methods have important applications in lunar dust and asteroid technologies and lofting, electrostatic dust agglomeration in space, granular and aerosol coatings for spacecraft charge mitigation, and many coating, contamination and roughening issues applied to a wide variety of fields subject to charging.
Keaton, Tom, "Methods for Preparing and Characterizing Granular Materials for Electron Yield Measurements" (2023). All Graduate Plan B and other Reports. 1697.
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