Characterization of Morphological and Chemical Properties of Scandium Containing Cathode Materials A dissertation presented to the faculty of the College of Arts and Sciences of Ohio University In partial fulfillment of the requirements for the degree Doctor of Philosophy Michael V. Mroz May 2020 © 2020 Michael V. Mroz. All Rights Reserved. 2 This dissertation titled Characterization of Morphological and Chemical Properties of Scandium Containing Cathode Materials by MICHAEL V. MROZ has been approved for the Department of Physics and Astronomy and the College of Arts and Sciences by Martin E. Kordesch Professor of Physics and Astronomy Florenz Plassmann Dean, College of Arts and Sciences 3 ABSTRACT MROZ, MICHAEL V., Ph.D., May 2020, Physics and Astronomy Characterization of Morphological and Chemical Properties of Scandium Containing Cathode Materials Director of Dissertation: Martin E. Kordesch Understanding thermionic cathodes is crucial for the future development of communication technologies operating at the terahertz frequency. Model cathode systems were characterized using multiple experimental techniques. These included Low Energy Electron Microscopy, X-Ray Photoemission Spectroscopy, and Auger Electron Spectroscopy. This was done to determine the mechanisms by which tungsten, barium, scandium, and oxygen may combine in order to achieve high current densities via thermionic emission. Barium and scandium films are found to dewet from the tungsten surfaces studied, and not diffuse out from bulk sources. The dewetted droplets were found to contribute the most to thermal emission. Barium oxide and scandium oxide are also found to react desorb from the emitting surface at lower temperatures then the metals themselves. The function of scandium in scandate cathodes was determined to act as an inhibitor to oxide formation. These observations are not compatible with certain models of cathode operation, mainly the dipole and semi-conductor models. 4 DEDICATION To Eli, my parents Lisa and Michael, and my sister Kalie, And to everyone who helped light my path on this adventure. 5 ACKNOWLEDGMENTS This work was supported by the DARPA INVEST (Grant No. N66001-16-1- 4040). This research used resources of the Center for Functional Nanomaterials and National Synchrotron Light Source II, which are U.S. Department of Energy (DOE) Office of Science Facilities, at Brookhaven National Laboratory under Contract No. DE- SC0012704. 6 TABLE OF CONTENTS Page Abstract ............................................................................................................................... 3 Dedication ........................................................................................................................... 4 Acknowledgments............................................................................................................... 5 List of Tables ...................................................................................................................... 8 List of Figures ..................................................................................................................... 9 Chapter 1 : Introduction .................................................................................................... 12 History......................................................................................................................... 12 Thermionic Emission .................................................................................................. 14 Dispenser Cathodes ..................................................................................................... 16 DARPA INVEST ........................................................................................................ 20 Chapter 2 : Experimental methods .................................................................................... 24 Low Energy Electron Microscopy (LEEM) ............................................................... 24 Photoemission Electron Microscopy (PEEM) ............................................................ 27 Thermionic Emission Electron Microscopy (ThEEM) ............................................... 28 Low Energy Electron Diffraction (LEED) ................................................................. 29 Auger Electron Spectroscopy (AES) .......................................................................... 30 X-Ray Photoelectron Spectroscopy (XPS) ................................................................. 32 Samples and Preparation ............................................................................................. 35 Chapter 3 : Data analysis .................................................................................................. 46 Particle Analysis ......................................................................................................... 46 Auger Data Smoothing ............................................................................................... 48 Gnuplot ....................................................................................................................... 50 Chapter 4 : Results ............................................................................................................ 51 Overlapped Ba/Sc Depositions ................................................................................... 51 Dewetting .................................................................................................................... 67 Adsorbate Surface Structure ....................................................................................... 86 X-Ray Spectroscopy ................................................................................................... 97 Auger Spectroscopy .................................................................................................. 107 Chapter 5 : Discussion .................................................................................................... 115 Dipole Model ............................................................................................................ 115 7 Semi-Conductor Model ............................................................................................. 118 Function of Scandium ............................................................................................... 119 Emission from Barium .............................................................................................. 120 Role of Field Emission.............................................................................................. 121 Future Direction ........................................................................................................ 122 Chapter 6 : Conclusions .................................................................................................. 123 References ....................................................................................................................... 127 8 LIST OF TABLES Page Table 1.1 Work functions of relevant metals. ................................................................... 21 Table 4.1 Temperatures at which phenomena occur in the overlapped deposition. ......... 67 Table 4.2 Surface coverage of dewetted scandium films. ................................................ 75 9 LIST OF FIGURES Page Figure 1.1. Filament of an Edison bulb............................................................................. 13 Figure 1.2. Primary principle of thermionic emission. ..................................................... 15 Figure 1.3. Schematic of a generic dispenser cathode. ..................................................... 17 Figure 1.4. Cathode current density increase over time. ................................................... 19 Figure 1.5. Scanning Electron Microscope (SEM) image of scandate cathode............... 22 Figure 2.1. LEEM III at NSLS-II. .................................................................................... 24 Figure 2.2. LEEM V at CFN. ........................................................................................... 25 Figure 2.3. Simple LEEM Schematic. ............................................................................. 27 Figure 2.4. Schematic of the photoemission process. ...................................................... 28 Figure 2.5. MicroCMA on the LEEM V at CFN. ............................................................ 30 Figure 2.6. Auger electron emission process. .................................................................. 31 Figure 2.7. microCMA diagram....................................................................................... 32 Figure 2.8. NSLS/XPS setup. ........................................................................................... 33 Figure 2.9. Resulting spectrum from hemispherical analyzer. ........................................ 34 Figure 2.10. W(100) single crystal. ................................................................................. 36 Figure 2.11. Sample cartridge with loaded W(100) crystal. ............................................ 37 Figure 2.12. Heated W(100) crystal in LEEM V. ............................................................ 39 Figure 2.13. Clean 1X1 pattern of a W(100) surface. ...................................................... 40 Figure 2.14. Clean tungsten Auger spectrum................................................................... 41 Figure 2.15. Clean W(100) surface. ................................................................................