Links between microstructure, erosion, and electron emission behavior in polycrystalline boron-based materials

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Date
2016
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University of Alabama Libraries
Abstract

Hall effect thrusters (HETs) are an important technology underpinning the effort to develop high impulse electric propulsion devices. The research here has addressed boron based materials that are used in these devices – namely the protective chamber wall for protecting the magnetic circuit components and the cathode that injects electrons into the plasma. The microstructural features of a post-operated chamber wall were examined and found to have three characteristic erosion features after extended plasma exposure. Conical protrusions are believed to be a result of sputter erosion whereas micro-cracking and loss of BN grains is contributed to residuals stresses created by anisotropic thermal expansion of BN within the composite ceramic. To study the effect of these topological features, particularly the protrusions, had on secondary electron emission; a series of mechanical attrition features on the length scale of microns were created in an analogue graphite material. It was found that there was an overall reduction in emission from smooth to roughened surfaces; however, a rougher surface does not necessarily yield the lowest emission. A characteristic length scale was observed where surface feature size relative to roughness appears to regulate the emission. In addition, a series of large scale topology grooves (on the order of millimeters) were created to simulate larger length erosion features noted in extended plasma exposed materials. The effect of these features on the plasma sheath floating wall potential was measured for a BN surface. As the grooves depth increased, the floating wall potential necessary to collapse the plasma sheath increased. A detailed plasma simulation revealed that trapping of electrons occurs near the entrance of the grooves which was used to explain the plasma sheath phenomenon. Finally, lanthanum hexaboride (LaB6), was fabricated as a coating using vacuum plasma spraying to determine its feasibility as a replacement for bulk cathode configurations. Excess boron was added to LaB6 powders prior to spraying and was found to improve the densification of the coating. However, the work function of these coatings was near ~ 4.3 eV, this is higher than other processes and is contributed to the incorporation of impurities in the coating.

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Electronic Thesis or Dissertation
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Materials science
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