Bulletin of the American Physical Society
2021 Annual Meeting of the APS Four Corners Section
Volume 66, Number 11
Friday–Saturday, October 8–9, 2021; Virtual; Mountain Daylight Time
Session K05: Characterizations of Surface Properties |
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Chair: Heinz Nakotte, New Mexico State University |
Saturday, October 9, 2021 1:00PM - 1:12PM |
K05.00001: Embedded Charge Distributions in Electron Irradiated Polymers -- Pulsed Electroacoustic Method Reproducibility and Calibration Zachary Gibson, JR Dennison, Ryan Hoffmann The pulsed electroacoustic (PEA) method has been used to measure the embedded charge distributions in electron irradiated polymers. The PEA method allows for non-destructive direct measurements of embedded charge distributions in dielectric materials. Samples of polyether-etherketone (PEEK) and polytetrafluoroethylene (PTFE) of 125 \textmu m or 250 \textmu m thickness were tested after irradiation with either a 50 keV or 80 keV electron beam. The reproducibility of the PEA method and the experimental conditions were studied by: (i) measuring each sample multiple times in a given mounting configuration, (ii) re-measuring each sample after repositioning them in the PEA test fixture, and (iii) measuring two similar samples of each of these eight different experimental configurations. For accurate absolute measurements of the charge distribution and deposition depths, calibration of charge position, charge density, and amplitude attenuation for the PEA system are required. Calibration is accomplished by measuring the speed of sound in each material and by observing the effects of applying a small DC voltage to use as a reference signal. A deconvolution of the measured waveform is performed with the reference signal to remove the effects of system response, resulting in only the charge distribution. Reproducibility of measurements before and after application of DC voltage identified any effects of the applied voltage. [Preview Abstract] |
Saturday, October 9, 2021 1:12PM - 1:24PM |
K05.00002: The Effects of Surface Contaminants on Electrostatic Breakdown Testing Megan Loveland DeWaal, Joshua Boman, JR Dennison Electrostatic breakdown of baked and unbaked thin film polyether ether ketone (PEEK) was measured under vacuum to determine the effect of surface contaminants on the electrostatic field strength. To quantify these contamination effects which are often not accounted for, half of a sample set received a vacuum bake out to 375 K for 3 days to remove water and volatile surface contaminants, while control samples did not. Sample storage in dry nitrogen minimized recontamination from subsequent exposure to air. A voltage was applied across the samples, increasing at a rate of 20 V per 4 sec in a parallel plate geometry at room temperature, until breakdown was observed as an abrupt increase in conductivity. The absence of these contaminants was found to decrease electrostatic breakdown potential and to decrease frequency of surface flashover events which is crucial to collecting accurate and reproducible breakdown data. Electrostatic breakdown tests of uncontaminated samples demonstrate significant differences which impact how highly disordered insulating materials such as PEEK react in environments such as space where they may experience prolonged heating and outgassing under vacuum. [Preview Abstract] |
Saturday, October 9, 2021 1:24PM - 1:36PM |
K05.00003: Electron Yield Measurements of Highly Insulating Granular Materials Tom Keaton, Matthew Robertson, JR Dennison Measurements of electron yield (EY) show how surface roughness and surface coverage of two different materials affect EY. Total, secondary, and backscattered EY data were taken for coverages from 0{\%} to nearly 100{\%} of homogeneous, highly-insulating, granular, 68 \textmu m sized, Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ particulates mounted on conductive graphitic carbon substrates. Trends in maximum EY, energy of maximum EY, and slopes at low and high energy limits are evident between the graphitic to Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ secondary EY data sets. EY curves at intermediate coverages are consistent with linear combinations of the constituent EY curves. Approximately 100{\%} coverage Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ data exhibited a suppressed maximum EY of \textasciitilde 11{\%} that of smooth, bulk Al$_{\mathrm{2}}$O$_{\mathrm{3\thinspace }}$or sapphire, indicating a roughness coefficient for granular surfaces. Backscattered EY showed minimal differences with surface roughness or Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ coverage. Experimental results are compared with physically motivated EY models of multicomponent samples and for roughness coefficients characterizing varying size and shape of granular materials. Understanding how surface features of multicomponent systems can affect EY has numerous applications including semiconductor and insulator charging, EY suppression, and modeling of cosmic dust in early planetary body formation. [Preview Abstract] |
Saturday, October 9, 2021 1:36PM - 1:48PM |
K05.00004: Models of Electron Yield Roughness Coefficient Trace Taylor, Matthew Robertson, JR Dennison Models to calculate the effects of roughness on electron yield (EY) were developed for several surface morphologies and electron emission energy distributions. EY, the ratio of emitted electrons to incident electrons, plays an important role in many applications such as spacecraft charging, high voltage systems, and scanning electron microscopy. Surface roughness is known to generally reduce EY of materials by reducing the critical escape angle for an emitted electron, though coupling geometry effects with emission angular distributions can complicate the issue. Four surface morphologies were considered: square, triangular, sawtooth, and sinusoidal periodic wells, each with critical angles dependent on where the incident electrons strike along the width of the surface feature. Both secondary electron (with energies \textless 50 eV) and backscattered electron (with energies \textgreater 50 eV) yields are considered. Secondary electrons and backscattered electrons have different energy-dependent angular emission distributions (Lambertian and screened Rutherford, respectively), with backscattered electrons generally having a narrower distribution; specular and isotropic distributions were also considered as limiting cases. The results are compared to experimental EY data of roughened Cu and Al samples to verify the model. [Preview Abstract] |
Saturday, October 9, 2021 1:48PM - 2:00PM |
K05.00005: A “Patch” Model of Electron Yield for Complex Materials Matthew Robertson, Trace Taylor, Tom Keaton, JR Dennison Electron yield (EY) is a material attribute which describe the ratio of emitted electrons to incident electrons when irradiated with an electron beam. EY is beam energy dependent, unique to each material determined by its chemical composition, electronic configuration and modified by several extrinsic factors such as surface roughness and contamination. Most yield models describe the yield of a single material with no consideration of extrinsic modifications. These models are also not suited to handle more complex samples made of more than one material. This research proposes a simple “patch” model to characterize complex materials and extrinsic factors in yield analysis. The “patch” model considers the electron yield contribution of each material or feature independently. The yield for a complex material is then the sum of the yield contributions of each of the individual material “patches”. Using a “unit cell” view of the surface creates a simpler “patch” model where the yield is the weighted sum of the individual components. This model works with any number of different materials and can be extended to a second layer using current two-layer yield models. Separating the yield contributions of individual components allows for greater characterization of complex materials. [Preview Abstract] |
Saturday, October 9, 2021 2:00PM - 2:12PM |
K05.00006: Preparation and Characterization of Highly Insulating Granular Samples for Electron Yield Measurements Heather Allen, Thomas Keaton, JR Dennison Methods to prepare and characterize highly insulating particulate samples for electron yield measurements were developed and evaluated. Accurate analysis methods were required to characterize particle size and shape and the magnitude and uniformity of fractional particle coverage. Optical microscopy had insufficient resolution and contrast to fully differentiate the highly-insulating, granular, 68 \textmu m mean sized, Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ particulates mounted on substrates of standard scanning electron microscopy (SEM) graphitic carbon tape with acrylic-based conductive adhesive. Modest resolution SEM images were used instead and also provided spatial information of occasional charging of Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ particles. A custom MATLAB script analyzed SEM images numerical greyscale pixel values to determine allowing global and regional fractional coverages; commercial software was less effective. Gravimetric deposition of particles suspended in deionized water onto adhesive substrates was the most successful preparation method; loose particulates were removed with nitrogen gas jets. Application of other particulate shapes, sizes, and 0{\%} to \textasciitilde 100{\%} coverages are discussed, as are less successful preparation methods. Representative electron yield measurements are presented. [Preview Abstract] |
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